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water JOURNAL OF THE AUSTRALIAN WATER &WASTEWATER ASSOCIATION

FEDERAL SECRETARIAT PO Box 388, Artarmon 2064 Telephone (02) 413 1288 Facsimile (02) (02) 413 1047 Office Manager - Margaret Bates

FEDERAL PRESIDENT Peter Norman, Phone (08) 226 2249

EXECUTIVE DIRECTOR Peter Hughes, Phone (02) 413 1288 Facs imi le (02) 948 1746

FEDERAL SECRETARY Greg Cawston, Phone (042) 29 0236

FEDERAL TREASURER John Molloy, Phone (03) 615 5991

BRANCH SECRETARIES Canberra, ACT Peter Cox, PO Box 306, Woden 2606 Phone (062) 498 522 New South Wales Nick Apostolidls, GCEC, 39 Reg en t Street Rai lway Square 2000 Phone (02) 699 9922 Vi ctoria John Park, Cl- Water Training Centre, PO Box 409, Werribee 3030 Phone (03) 741 5844 Queensland Don Mackay, PO Box 412, West End 41 01 Phone (07) 840 4844 South Australia Neil Palmer, Cl- State Water Laboratori es, E&WS Private Mai l Bag, Salisbury 5108 Phone (08) 381 0268 Western Austral ia Steve Gibson, CMPS, 200 Adelaide Terrace Perth 6000 Ph one (09) 325 9366 Tasman ia Annette Ferguson, GPO Box 503E, Hobart 7001 Phone (002) 28 2757 Northern Territory Lind say Monteith, PO Box 351, Darwin 0801 Phone (089) 81 5922

EDITORIAL CORRESPONDENCE E.A. (Bob) Swinton, 4 Pleasant View Crescent, Glen Waverley 3150 Office Phone-Fax (03) 560 4752 Home (03) 560 9306

ADVERTISING Ann Sykes-Smith, Appita, 191 Royal Parade, Parkvi lle 3052 (03) 347 2377 Fax (03) 348 1206

ISSN 0310-0367

Volume 18, No. 5, October 1991

CONTENTS 3 4

5 11 12

My Point of View President's Message It Seems to Me Association News IAWPRC News Industry News

Seminar Reports 15 NSW Branch Regional Conference

Features - Instrumentation, Control, Automation 17 Instrumentation Development for Lake Mixing Investigations 19 21 22 26 31 33 36 39 44 48 49 51 52

J. lmberger Intelligent Interface: Remote Sensing Through Satellites N. Goldie, I. Johns and I. Durham Telemetry Project for Gosford/Wynong G. Burton Automation of Chemical Watering Pumps P. T. Burlew The Principles of FIOYJmeter Selection: BS7405 RA Furness A New Technoloigy Water Meter D.J. Lomas Measurement of Turbidity in Wastewater Treatment G. Schrank and J. Lane Solids Measurements Improve Performance of Wastewater Treatment Plants S. Valheim Four-beam Turbidimeter for Low NTU Wastes K. King Tighter Chlorination System Control D. Little and S. Bienak Valve Actuation Solves Many Needs G.L. Delaney Plant, Products, Equipment Book Review Conference Calendar

OUR COVER

Our picture shows the fine scale water quality probe developed by the Centre for Water Research being lowered into the Swan River from the Resea rch vessel, Djinnang. Thi s probe, which measures temperature, conductivity, oxygen and pH, as a function of depth, is a high accuracy, high resolution, general purpose, fine scale profiler. The system has undergone numerous major revisions and is now eq uipped with a very low noise, highly accurate, data acquis ition system which also offers the opportunity for extensive data management. This system has been installed by the Mell:)ourne Board of Works, the Marin e Biological Station, Piran, Yugoslavia and a further system will probably be delivered to Lake Biwa Research Institute in Japan, this year. See story this issue.

~r

PUBLICATION Water is bi-monthly. Nominal distribution times are !he lh lrd weeks of February, April , ,.k.JOe, August , October, December.

IMPORTANT NOTICE

PRODUCTION EDITOR John Grainger, Appita, 191 Royal Parade, Parkville 3052 (03) 347 2377 Fax (03) 348 1206

The views expressed by the contributors are not necessarily endorsed by the Australian Waler and Wastewater Association. No reader should act or fail to act on the basis of any material contained herein . No responsibility is accepted by the Association , the Editor or the contributors for the accuracy of Information contained In lhe text and advertisements. The Australian Water and Wastewater reserves the righl to alter or lo omit any article or advertisement submitted and requires indemnity from advertisers and contribu tors against damages which arise lrom material published. All material in Water Is copyright and should not be reproduced wholly or In J:)ilrt without th e wrillen permission of the editor.

WATER October 1991


INSTRUMENTATION DEVEWPMENT FOR LAKE MIXING INVESTIGATIONS by J. IMBERGER SUMMARY Since 1983 the Centre for Water Research at the University of Western Australia has been investigating the physics of water quality problems in lakes and reservoirs and coastal seas. The focal areas are the Centre for Environmental Fluid Dynamics, the Hydraulics and Geographical Fluid Dynamics Laboratory and the Centre for Limnological Modelling. Instruments have been developed which cover the range from microstructure to basin-scale motions.

Jorg Emberger is a professor in the Department of Civil and Enviro nmental Engineering at the University of Western Australia and is Director of the Centre for Water Research and the Special Research Centre/or Environmental Fluid Mechanics.

MIXING IN WATER BODIES Most inland lakes, estuaries and coastal waters are characterized by a well defined density stratification of the water column . This .stratification may be introduced by a riverine buoyancy flux or it may be due to solar surface heating. In most Mediterranean climates destratification persists for a large part of the year, stabilizing the water columns and suppressing mean vertical motions and active turbulence. Surface stresses introduced by the wind are thus insufficient to completely mix the water column. In general, as a lake or coastal sea is exposed to a wind, the wind imparts both momentum and turbulent kinetic energy to the water surface. The turbulent kinetic energy propagates downwards forming an actively mixing surface layer. On the other hand, the introduced momentum causes the surface layer to move and if there are boundaries, this movement will lead to large, basin scale, internal waves being established. In the ocean, these waves are often influenced by the rotation of the earth, whereas in smaller water bodies such as lakes, the waves are purely gravitational. The motion associated with this seiching leads to an actively turbulent, benthic boundary layer on the bottom and also, in cases of severe seiching, to sporadic, isolated turbulent mixing events within the water column. Most recently, it has also been found that immediately below the surface layer, there is a secondary, strongly stratified, turbulent layer, energized by internal waves directly forced by the turbulence in the surface mixing layer. The study of mixing in lakes and the coastal regime must, therefore, recognize that there are essentially three classes of motions within these water bodies. First, we have large scale, internal waves and mesoscale eddies being generated by global adjustments of the water within the basin as a whole. These are called basin scale motions. Second, there is a whole spectrum of internal waves ranging in wavelengths from a few centimetres to kilometres, which are ubiquitous and continually propagate in all directions throughout the stratified part of the water body. The frequency of these waves ranges from that of the buoyancy frequency (usually a few minutes) to the times of the basin scale motions (many hours) and very often, there are no spectral gaps. Third, there are small scale turbulent, mixing motions located in the surface mixing layers, the benthic layers and in turbulent patches energized by the basin scale shear combining with the shear from the continuous spectrum of internal waves to cause gravitational instabilities which may be either shear-driven or advective instabilities. The mixing associated with these turbulent events causes both a readjustment of the momentum within the water body and also the mass fluxes which redistribute the density and all the water quality parameters such as oxygen, nutrients and pollutants. T he work at the Centre for Water Research has shown that the global dispersion of these quantities is, therefore, governed by dispersion within the active turbulent regions followed by slow, gravitational adjustments spreading the material horizontally; mixing, therefore, disperses the material vertically and gravitational adjustments spreads it laterally. This delicate scenario is now well established.

RANGE OF PARAMETERS In summary, there are, therefore, four scales for experimentalists to contend with. First the basin scale motion and oscillations. Second, the fine scale motion consisting of internal waves and intrusions setting the stage for an entrainment event. Third, entrainment motions which cause inter-leaving .o f water with

different properties and fourth, scales associated with the actual mixing of mass, momentum and energy. Similarly, there is a hierarchy of time scales. Characteristic time scales for basin or shelf scale motions are measured in hours. Fine scale velocities have time scales of minutes and entrainment motions have a life of not much longer than a few seconds, while the mixing time scales are measured in tens of a millisecond. The task of the water quality engineer is thus formidable. In order, for instance, to establish the oxygen flux path in the lake, the investigator must establish the causal relationships for the physical dynamics by measuring velocity, temperature, conductivity, oxygen, pH and other water quality parameters on a space scale from kilometre (to measure the energy input) to fractions of a millimetre (to measure at what rate the properties are being mixed) and allow for a temporal variability from many hours to milliseconds. Perhaps the greatest difficulty in such a measurement programme is, howe~er, brought about by the non-homogeneity, the intermittency and the non-stationality of the mixing filed at a particular point; single point moorings contribute little towards answering tqe global mass and momentum transfer questions. However, it must be stressed that in a strongly stratified fluid, the active turbulence is nearly always confined to narrow fronts, gradient regions or isolated patches. The bulk of the fluid is either in a quiescent non-turbulent state or in a state of gentle undulation. Mixing tends to be isolated and mostly non-interacting between patches, and distinct from the coherent motions in an active uniformly turbulent field. Thus, there is some hope of isolating and identifying a particular entrainment motion and to measure or model the subsequent mixing.

MEASUREMENT The mixing measurement system may be conveniently thought of as consisting of four components. First, the basin scale motion and meteorological forces are measured using in-situ weather stations, thermistor conductivity chains, acoustic current meters, as well as a portable, fine scale probe mounted from a small dinghy. Second, intense fine scale work using the new fine scale profiler directly from the research vessel is used to define the fine scale. Third, entrainment and mixing events are synoptically identified in the field with acoustic sensing. Fourth , microstructure profilers, which are able to measure the turbulent flux within the water column, are used to define the energetics of the mixing . In order to maximize the likelihood of success in isolating and measuring properties of the entrainment motions, the facility is designed to allow on-line processing. All data from the peripheral equipment is sent to the central computing system on the research vessel where it is combined with data collected from the fine scale and microstructure instruments. The computer then analyses this data and displays a series of diagnostics allowing the investigator to get an immediate, on-line overview of the experimental results. This allo.ws. the person to update the sampling strategy and, therefore, opt1m1ze the data return. Lastly, by bringing relatively powerful c.omputers onto the vessel , the investigator is able to run, in real time, numerical simulations of the situation that is being investigated. By overlaying the results from the numerical WATER October 1991

17


simulations and the data that is streaming in, in real time, the investigator is able to differentiate the host of phenomena, which are in the numerical model, from those which cause departures fron:i the predictions; this is a further diagnostic tool allowing the investigator to optimize the deployment of the equipment.

INSTRUMENTATION The suite of instruments consists of the following: Meterological Stations

The meteorological stations are normally mounted on a guide tower or spar buoy. Apart from the standard variables, most of the stations are also equipped with thermistor chains which are used to collect time series of the water temperature at different depths.

presently, is undergoing a further major redesign in terms of computer hardware. The present Djinnang (Figure 2) is a 13 metre twin hull fibreglass craft, chosen for its stability and high speed. The whole body and the air-conditioned cabin are made of fibreglass for lightness, durability and lack of electrical connectiveness. The vehicle is equipped with two 250 HP outboard motors and a 7KVA Onan diesel generating unit. The Djinnang forms the central unit to the instruments discussed above and is used as a central data acquisition analysis and data management platform. A flexible system architecture has been devised to facilitate the interfacing of a wide variety of instruments and computers.

Fine Scale Probe

The fine scale of F-Probe is equipped with depth, temperature, conductivity, pH and oxygen sensors. To this may also be added turbidity, chlorophyll and rhodamine sensors. The F-Probe is mechanically the most flexible and when used from the research vessel may be deployed in three distinct modes. First, in the free fall mode allowing vertical profiles to be taken in a matter of minutes. Second, towed at a predetermined depth with time as a conditional channel, yielding horizontal contours and third, in a yo-yo pattern yielding a vertical section through the water column. (see front cover and Figure I)

At present, the computing system is being updated so that portable MSDOS computers can be used as data acquisition units and portable UNIX based laptops phased in for data processing and computer simulations. Computers are interconnected via Ethernet links. The major investments in this research facility may conveniently be divided into a number of categories: a. Software for the data management and file structure management b. Signal enhancement techniques c. Static and dynamic calibration of the sensors d. Fast running numerical algorithms for hybrid modelling The Djinnang, in its various stages, has now been in service for about eight years and has Jed to a major improvement in our understanding of mixing in lakes and coastal seas. It forms the base facility for the Special Research Centre, the Centre for Environmental Fluid Dynamics and has been the tool for over 50 papers on mixing in lakes and coastal seas.

CONCLUSIONS Acoustic Imaging System

The acoustic imaging system has been primarily designed to assist in the optimum deployment of the instrument and provides a real time image of the water column, detailing changes in the acoustic impedance arising from bubbles, turbulence, particle matter and changes in stratification. These images allow the investigator to obtain a synoptic overview of the mixing patterns underneath the research vessel.

The instruments developed by the Centre for Water Research have in common both spatial and temporal, resolution, which has been carefully matched to the variability of the signal to be measured. This had yielded instruments which cover the whole range from microstructure scales (mm's), to fine scale (!O's of centimetres), to basin scale motions (kilometres). The application of these new tools is co-ordinated through the research vessel, Djinnang, although each is a stand-alone system.

The Portable Flux Probe

A recent development has been the mm1aturization of the microstructure flux profiler. This new instrument will allow the detail measurement of two components of velocity, three temperatures, three conductivities and one oxygen. Once again, the instrument is a free flight, vertically rising profiler which is able to measure the above parameters at a scale of one millimetre. This information can be used to directly compute most of the important properties of mixing. Acoustic Doppler Current Meter

This is a broad band acoustic current meter which is presently under development. The aim of the instrument was to construct a low cost system which can be placed at the bottom of the study site and which will profile one component of velocity in a continuous high resolution fashion. Present indications are that this instrument will be able to resolve to a distance of tens of metres at a resolution of a fraction of a centimetre. Initial tests of the prototype are most encouraging. (Figure 3).

THE DJINNANG The research vessel, Djinnang (Aboriginal word to discover) was specifically designed to allow the undertaking of a measurement program to assess the flux paths within the coastal regimes and in lakes. The design has evolved over three major revisions and 18

WATER October 1991

Together, these systems have revealed the fundamentals of mixing processes in a stratified water body. This knowledge has allowed the Centre for Water Research to formulate water quality control strategies which find application in controlling salinity in reservoirs, improving water quality (Mn, Fe, DO2 , Chlorophyll) in lakes and which result in a better design of ocean outfalls. The underlying principle of these strategies is that, for given inputs, mixing determines water quality. Further, by removing the variability caused by the randomness of mixing from a water quality signal, the modelling of that parameter becomes a more straightforward matter.


INTELLIGENT INTERFACE: REMOTE SENSING THROUGH SATELLITES by N. GOLDIE, I. JOHNS and I. DURHAM SUMMARY MODAC (MOBILESAT Data Acquisition and Control) is a facility for the transfer of field data to a personal computer in the office, laboratory, or works depot. MODAC, comprising the INMARSAT-C terminal and Intelligent Interface connects directly to the data acquisition equipment currently used by water authorities and environmental agencies worldwide. This breakthrough in low cost remote monitoring technology is the brainchild of the CSIRO Division of Water Resources, part of the Institute of Natural Resources and Environment. Essentially, the new technology involves a marriage between existing data loggers or remote location sensors and the INMARSAT-C satellite communications system, by way of MODAC fntelligent Interface. CSIRO scientists in Canberra worked for two years to perfect what they call, familiarly, "I-squared". MODAC has attracted international interest. This Australian development is unique because it provides connection of INMARSAT-C satellite communications to existing equipment. INMARSAT-C has geostationary satellites over the Atlantic, the Pacific, and the Indian Oceans, so that a user can have access to data from remote sites anywhere in the world.

Nick Goldie is a Communications Officer for the CS/RO Division of Water Resources, Canberra Laboratory. The development of the MODAC Intelligent Interface has been the prime responsibility of Ian Johns, Manager, and Ivor Durham, Engineering Supervisor of the Instrumentation Research and Development Group. A conference/ workshop on MODAC systems was held in Canberra, 28-30 October.

I. Durham

I. Johns

N. Goldie

It was this which led to the development of MODAC Intelligent

REMOTE MONITORING

Interface.

INMARSAT started life as a London-based marine communications system involving 56 countries and the United Nations; more recently INMARSAT have extended their operations to land-based communications. Used with the latest INMARSATC transceivers, the MODAC Intelligent Interface has eliminated the need for an operator at remote sites. The high costs of land lines and radio telemetry systems, when used over great distances, meant that it was necessary for an expedition to go to the site, plug a computer into the data recorder, and so collect the information. Intelligent Interface takes the place of the operator, and send the data home, back to a personal computer by way of an X.25 pad or X.32 modem. The Australian water industry has made limited use of low cost satellite telemetry since 1978, originally via the French ARGOS Data Collection Service. Oceanographers and meteorologists have used ARGOS since the early 1970s, but there were some major limitations, notably the uplink-only capability, the maximum message bits per transmission being 256, and the gap time between passes which did not allow continuous real-time data recovery. The introduction of INMARSAT-C has also substantially cut costs in actual bits-per-dollar charges, as compared to the fixed cost per day charged by ARGOS (see Figure 1). AUSSAT Pty Ltd briefed the Instrument R & D Group of the CSIRO to design an interface to the INMARSAT-C satellite system, suitable for data acquisition and control. DOLLARS PER K/blt (APPROX. ONLY)

10.----------------------, •

THE FIRST INSTALLATION The first commercial installation of Intelligent Interface was at Weeli Wolli Creek, in the remote mineral-rich Pilbara region of Western Australia, which was commissioned cm January 31st, 1991. The site is a priority metering station for hydrographers working at Karratha. Seven sensors measure river levels, rainfall, temperature, humidity, solar radiation, and wind speed and direction . Information from the sensors is rec~rded every two hours. After six hours three sets of readings are automatically transmitted over INMARSAT's Pacific Ocean satellite to an electronic mailbox at the new Perth Land Station operated by INMARSAT's Australian signatory OTC. Hydrographers at Karratha use their PCs to dial the OTC mailbox to review or recover data using a software program called Hydsys, which recovers the data from OTC and processes it to a telemetry data base. According to Ian Tite of the Western Australian Water Authority, the system has worked well, with no loss of transmissions.

THE SYSTEM Components of the INMARSAT-C SCADA (supervisory command and data acquisition) system would typically include sensors, a data logger, Intelligent Interface, and the INMARSAT-C transceiver, together with a solar panel and battery (see Figures 2 and 3). Data is transmitted on L band (1.5-1.6 GHz) to the satellite, and on C band to the CES (Coastal Earth Station), and from there by landline to its destination. The system cycles through three modes of operation on a pre-set time interval. These modes are: 1. sleep

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WATER October 1991

19


mode; 2. stand-by mode; 3. transmit mode. For the majority of the time, the system exists in the 'sleep' mode, when activity is limited to maintaining an internal clock, and· monitoring for changes in state at the interface. The system can be brought out of sleep either when commanded by the internal clock, or by the input of an alarm. At this point, the system enters the 'stand-by' mode, and monitors the time division multiplex (TDM) channel generated by the coast earth station, and prepares packets for transmission . In the 'transmit' mode, the system powers up the transmitter to send one or more packets of data. Similarly, there are three modes of access to the MODAC system. In 'pre-assigned reporting' mode, the field equipment initiates communication by transmitting a request-for-access at the commencement of the 'transmit' mode. In 'on demand' mode, the customer's premises may command the system to transmit a realtime reading. In 'alarm mode', the system initiates communication upon sensing an external reportable condition, such as a flash flood in a river. INTELLIGENT INTERFACE

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Unlike ARGOS, INMARSAT-C can be used to send instructions to the Intelligent Interface as well as receiving data. This capacity may be extended to include real-time bi-directional systems, for example remote control and monitoring of pumping stations, water dosing, control valves and gates. Real-time flood warning systems and remote calibration of sensor systems will also be possible. MODAC Intelligent Interface requires a small omni-directional antenna; the whole system would fit into a suitcase, with the heaviest single item being a twelve volt battery. Intelligent Interface itself weighs 700gms. As well as being convenient, this small size adds to the security of the system when it is unobtrusively in place in a remote location. Expeditions to the site need only take place when the system itself indicates that something is amiss and that it is operating in its backup mode.

APPLICATIONS AND COSTS Typical applications for Intelligent Interface would include water quality monitoring, flood warning, gas and oil pipeline monitoring, earthquake monitoring, and meteorology. The system allows asynchronous serial data transfer, in a store and forward manner, between a field unit (e.g. river height station) and the customer premises. Costing depends to a large degree on the situation. Intelligent Interface itself is relatively inexpensive, being under $2000 (Australian) at the time of writing. The transceiver is more expensive, being provided by one of fifteen commercial manufacturers worldwide, and the sensors and data loggers depend on the individual application. Intelligent Interface does offer an enormous saving to any operation involving remote monitoring; in the first place, because of the elimination of the direct costs of site visits in vehicles, personnel, and infrastructure; secondly, there are great indirect savings due to the streamlining of data collection and the savings in time. Another attractive feature of Intelligent Interface is that it is a low powered, low energy user. The transceiver is automatically turned off when it is not in use. A normal application might be a data transmission twice every day; but this could be weekly, daily, or even every five minutes. 20

COMMERCIALISATION A company, MODAC Systems, has been established in Canberra to manufacture and market the new product, which is being built and assembled in Australia. The export value of the system has been estimated at between A$24m and A$90m. There is the possibility of manufacture in other countries under license. During 1990, the system was evaluated by a number of Australian state water authorities, and Ian Johns says, "the technology is now advanced to the stage where no obstacles remain to the adoption of MODAC Intelligent Interface as a standard, routine method of monitoring." Although the system was designed to cope with Australia's great distances and isolated areas, there would be a place for MODAC Intelligent Interface anywhere in the world where there isn't a telephone.

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A radio network is being developed ~ Ivor Durham as an 'optional extra'. This will enable Intelligent Interface to receive data from a group of sensors individually, and transmit their various messages through the same transceiver. As the transceiver is the most expensive item in the package, this is more than just added efficiency.

WATER October 1991

Weeli Wolli Creek is near Torina Springs in the Pilbara region of Western Australia. When streamflow and floods occur, it is necessary for staff of the Water Authority of Western Australia to spend several days at the site to measure river flows and water qualities. Access is by helicopter, as roads quickly become impassable in flood times. The catchment is arid and uninhabited, remote from any weather reporting sites, and significant rainfall is rare and brief. It has been difficult to predict if and when the required flow conoitions might occur. To overcome this, MODAC Intelligent Interface was installed for a three month trail period. It connected the Unidata data logger at the site to a Thrane & Thrane INMARSAT-C transceiver. Data was relayed by the Pacific Ocean INMARSAT satellite to a mailbox at the Land Earth Station in Perth. The equipment worked reliably and all data was delivered successfully for the three month period. The following graph is an example of data collected.

Water Authority of Western Australia WNk Plot hm OCkCNUOA>4/1 111 to OCtO(L1 7..o4/ti91

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14/04/1911


Telemetry project for Gosford/Wyong by G. BURTON SUMMARY The Public Works Department of New South Wales has begun the implementation of a major water and wastewater telemetry project for Gosford and Wyong. This project will provide the central coast cities with the ability to centrally monitor and control their respective water assets. This article describes the central monitoring facility and the telemetry system.

PROJECT SCOPE Gosford sources water from Mangrove Creek and Mooney Mooney Dams via pump stations to a balance tank and water treatment plant. The treated water is then pumped separately to the Gosford area and the Woy Woy area into storage reservoirs located near major population centres. Wyong sources water from weirs at Lower Wyong River and ¡ourimbah Creek and collected at Mardi Dam. The dam water is treated by the Wyong Water Treatment Plant before being distributed separately to the Kanwal and The Entrance distributed reservoir systems. The Gosford Wastewater System is divided into two separate systems, Gosford and Woy Woy. Both systems have kerb side pump stations pumping to collection wells at the major pump stations. The major pump stations then deliver to Kincumber or Woy Woy Treatment Plant either directly or via intermediate pump stations. Pump station control is automatic. Wyong Shire has six separate wastewater systems which feed Treatment Plants at Wyong South, Bateau Bay, Toukley, Mannering Park, Gwondalan and Charmhaven. Kerb side pump stations pump to collection wells at major pump stations which in turn pump to their respective Treatment Plant. Pump station control is automatic. The Gosford system will initially telemeter 23 reservoirs, 19 pump stations and flow meters, 28 minor and 4 major sewerage pump stations, two sewerage treatment plants and one water treatment plant. The Wyong system will initially telemeter 14 reservoirs, 15 pump stations and flow meters, 53 sewerage pump stations, 6 sewerage treatment plants and one water treatment plant.

CENTRAL MONITORING FACILITY Each Council wilt have a water and wastewater information system consisting of a concurrent standby computer system based on 486 platforms and running the SCO ODT/ UNIX operating system. SCADA software will be the Monitor/ UX host and workstation software package from TUSC Computer Systems, Melbourne with application-specific software written and configured by Email Electronics Systems Group. As the Councils have a need to share water, a wide area network wilt provide workstation access and selected data transfer between the Councils. Two workstations wilt initially be provided at each Council CMF and will consist of Intel 386SX personnel computers fitted with 19" colour monitors and Ethernet ThinLAN connection to the host. The host computers are Intel DX401, 486 tower computers fitted with 24 Mbyte of RAM, 380 Mbyte hard disc and 8 serial data ports. The host application software, Monitor/ UX is a comprehensive and flexible supervisory control system running under UNIX. The software has six key components. a. Configuration database which carries all definition information about the process sub-systems, 1/0 points, operators and workstations. b. Process 1/0 sub-system which interfaces any process control or 1/0 device to Monitor/ UX. A specific device handler interfaces to the Email Microtran telemetry system. c. Event handler which examines incoming data to determine whether any alarm or significant events have occurred; it then schedules the appropriate event handler to process that event. d. Presentation sub-system which looks after the processing of operator commands and the presentation of information to the operator (such as alarm lists, schematics, trends etc).

Graham Burton is Product Manager of the Systems Group at Email Electronics. He has many years experience in design and project management in telemetry and data acquisition.

e. Data and event logger which records selected or significant events and data for subsequent analysis as well as handling data compression over time. f. Historian and analysis sub-system which provides mechanisms for the automated retrieval of historical data and the transfer to other analysis systems. Features provided with the system include an after-hours alarm paging facility, and remote terminal access to the CMF via laptop personal computers over Telecom PSTN. The concurrent standby configuration of two 486 computers provides Monitor/ UX concurrently running on both hosts. The duty host interrogates the duty front end processor for current analog and digital field data as well as time tagged events which have been stored at remote telemetry units in between system poll requests. The hosts are joined by an Ethernet LAN for high speed data file transfer from duty to standby machine, and also joined by a low speed serial connection. The low speed serial connection forms a heartbeat circuit through which the standby host continually checks the status of the duty host. Should the duty host fail, the standby host wilt take over the duty host's tasks automatically and continue until the other host is manually reinstated. Both the duty and standby telemetry front end processors connect to both hosts thus providing an automatic changeover facility for the telemetry interface.

TELEMETRY SYSTEM Each Council system uses a centrally located UHF duplicated repeater through which the CMF telemetry front end processor polls local telemetry units which are located at water and wastewater assets. Remote locations communicate with a data concentrator telemetry unit in their area via a separate UHF radio network . The data concentrators are connected to the UHF repeater by either a 900 MHz link or store and forward telemetry unit. The telemetry equipment at the front end processor, repeater, data concentrator and asset sites is Email Electronics Microtran. This single board unit incorporates a 16 bit Intel rnicrocontroller to perform data acquisition, communications and limited local control functions at the different types of sites in this project. The basic board measures 210mm x 250mm and provides 16 digital inputs, 7 analog inputs and 4 control outputs. An onboard VG modem interfaces to either radio or two wire leased line depending on version. A spare V 24 port is available for connection to modem or PLC as required . Configuration of the Microtran is done via an additional Test Port, which may be interrogated by either PC or ASCII terminal. Expansion is available by adding 1/0 expansion modules, and a dual serial port expansion is also available (and is used in this project in the front end processor configuration). A unique feature of this system is the local pumping controls which are to be implemented in several areas of both Gos ford and Wyong. Water from the Treatment Plant is available for distribution to various service reservoirs by pumping stations. A distribution reservoir will typically have a level transmitter whose analog signal is transmitted to the telemetry front end processor during a regular RTU poll. The controlling processor will check the level of the reservoir against a table of duty levels. If the level is below the first duty level, the controller will instruct the pump station to start pumps appropriate to the duty level. If demand on the reservoir is such that the first duty is insufficient to bring up the level, another Continued on page 24 WATER October 1991

21


Automation of Chemical Metering Pumps by P. J. BARWW

INTRODUCTION Chemical metering pumps are extensively used in all facets of industry to control the rate at which a volume of fluid is injected into a process. Generally speaking, metering pumps are capable of high accuracy and may be adjusted during operation to vary the flow rate. With correct pump selection and hydraulic design, reliable and accurate chemical metering is achievable with varying suction, discharge and process conditions. Metering pumps referred to in this paper would generally be considered as positive displacement devices where a relatively proportional output may be achieved with various automation or control systems. Metering pumps operate by various pumping mechanisms, the two largest categories being reciprocating pumps and rotary pumps. Reciprocating pumps include mechanical diaphragm, piston pumps, hydraulic piston diaphragm and hydraulic tubular diaphragm pumps. Rotary pumps include helical rotor, vane, lobe and peristaltic designs. The following concepts on automation of metering pumps are applicable to any of these pump styles, however most of the comments made will relate to reciprocating diaphragm pumps, as these are the most widely used chemical dosing pumps utilised in industry. In many chemical processing applications, it is desirable to select a metering pump whereby the output may be varied via manual or automatic adjustment of the displacement mechanisms. With reciprocating pumps this is achieved by either regulating the swept volume of the diaphragm/ piston (stroke control) or by varying the reciprocating speed (motor speed).

PRE-REQUISITES FOR AUTOMATION With modern instrumentation and electronic control equipment, very accurate and reliable automation of chemical metering pumps is achievable. Obtaining accurate and reliable electronic control signals for automation of metering pumps is often quite simple, however obtaining the desired variations of chemical injection to the process depends on the actual metering pump being able to accurately respond to the control signal applied. Therefore, prior to designing and implementing an automation system the correct selection and design of hydraulic conditions external to the chemical metering pump is of significant importance. There are many factors which influence the linear output accuracy of a chemical metering pump, such as varying pump suction conditions, varying discharge conditions, reciprocating pulsation effect, pump output linearity with relation to stroke variation, etc. Detailing all of these aspects is beyond the scope of this paper, however careful consideration should be given to the ability of the chemical metering pump to achieve the accuracy and physical output variation being requested by the automation system.

Philip 1 Barlow is the Technical Director for Watertec Engineering Pty. Ltd, the Australian distributors for the Al/dos range ofchemical metering pumps and associated equipment. He is responsible for development and marketing of metering equipment and engineered system design. He has been involved in the Water Treatment industry jor 25 years.

for varying demands of the chemical being added or for other variations such as process flow rate. There are two basic methods of controlling chemical additions with this type of automation, one being where a pump simply injects chemical into a process stream whenever the process is in fact operating. The second method is where a chemical metering pump is operated from a controlling instrument, such as a pH controller, whereby the pump is started when the process reading has drifted from the desired set-point, and is stopped when the desired setpoint has again been reached. A typical example of this type of automation is for acid additions to a swimming pool. The limitations of this type of automation is that a constant dose rate of chemical is being applied, therefore no capacity is provided for increased demand requirements for the chemical often resulting in either overdosing or underdosing of the chemical. 2. Proportional Control. (Figures 1, 2) This type of automation is utilised where the metering pump output is increased or decreased in direct proportion to a process control signal, irrespective of any other influences. A typical example would be where chlorine is injected into a water pipe line, where a flow meter provides a signal to the metering pump. This signal directly varies the output of the pump according to the actual flow rate in the pipe.

..

-------7 I

I I

CONTACT HEAO 1'ÂŁTER

I I I

I

,------PULSE

I

1 _ _ _ _ _ _ _ _ _ ....JI

-------- --- -

-

2!.0v 50Hz POWER

"

ELECTRON( PULSE CON1ROLLED

HOT!ll DRIVEN ME][RING PUMP

METERING PUMP AUTOMATION TECHNIQUES There are several basic methods of automating chemical metering pumps, the generally used methods being; 1. On/ Off Control 2. Proportional Control 3. Proportional Control modulating around a set-point. 4. Proportional and integral control modulating around a set-point. 5. Compound Loop control. The degree of automation utilised in any specific process is very dependent on the process variables that will be applicable, and the degree of accuracy required for chemical injection. The following information will provide a basic guide as to the different forms of automation, with some relevant comments on automation methods. 1. On / Off Control. This is obviously the most simple method of automation of a chemical metering pump, and is often quite adequate to achieve the necessary results. The dosing pump is adjusted to a pre-set output and simply stopped and started as required according to the process. This method does not compensate 22

WATER October 1991

Fig. 1 -

Proportional dosing with pulse controlled metering pump

Proportional control is very effective and simple, however it does not compensate for variation in chemical demands. With this type of automation the accuracy or linearity of the metering pump output during automation is crucial. For example, under certain conditions a diaphragm metering pump could have an error of greater than 10% of full output, when operating at 20% of its rated output. In this instance, although the flow meter signal is requesting an injection rate of 20% of the full pump flow, the actual metering pump is not capable of providing the correct chemical injection, therefore resulting in inaccurate chemical injection. 3. Proportional Control Modulating around a set-point. A chemical residual control instrument, such as a pH controller, would normally have an adjustable set-point. Once the desired set-point has been


..

I I I

..

I

VSQ~

4·21>mA -------7

---------- -----7 4~20mA

:

240v CR 415v SOHz

POWER

VARIABLE FREOJENCY

I l !w

OOM

i

-1

I

I

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- v•·

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I

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7

:

:

2f.Ov CR415v - --SO Hl POWER

.

r

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I _J

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w

J

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STROKE POSITIO'-l

COIITROllER

Fig. 2 - Proportional dosing with motor driven pumps - speed or stroke control

-exceeded, a signal will be provided to start the chemical metering pump. Proportional control may be provided, whereby the pump output will increase as the process reading deviates further from the desired set-point. This type of control is generally used in recirculating systems, such as a swimming pool, whereby chemical (chlorine) demand changes may suddenly vary. In this instance as the chlorine residual decreases rapidly, the analyser reading will drop accordingly. The chlorine metering pump will then increase output towards its maximum dose rate. As the chlorine residual increases towards the desired set-point, the pump output will proportionally decrease until the set-point has been reached, at which time the pump will stop. 4. Proportional Plus Integral (Pl) Control Modulating around a set-point. (Figure 3) This type of control is extensively used in the water and waste water industry, and is primarily used for oncethrough systems where corrective dosing is required. Corrective dosing is proportional to the deviation from the set-point of a residual controller. This method of control is most suitable where a relatively constant process stream flow rate is provided, however the actual demand or dose rate of the chemical will vary. For example, where chlorine is added to a flowing pipeline the residual analyser will provide a signal to a PI Controller, whereby both the requirements for proportional dosing and the integral time is accommodated. The integral function allows for the time lapse between dosing and the analyser measuring the dosed solution and compensates for varying process demands for the chemical being dosed. This type of instrumentation does not normally automatically compensate for varying integral time parameters associated with process flow changes. Some controllers utilise a derivative action, i.e. PID, which provides a once only change of pump output, to compensate for an abnormal process condition. This function is not often required in simple water treatment applications.

r-----, I I I I I I

I I I I I

Fig. 3 -

Proportional control modulating around a set point · - speed and stroke control

Fig. 4 -

Compound loop control -

speed plus stroke

5. Compound Loop Control. (Figure 4) This method of automation will regulate the output of a chemical dosing pump from two individual process parameters, such as varying pipeline flow rates and varying chemical demands. Using modern control instrumentation, two process signals (flow and residual) may be connected as inputs to a process controller, which will then carry out the necessary computations to supply a single output signal to the chemical metering pump for final injection rate automation. Alternatively, one instrument (flow) may be used to automatically vary the pump stroke length, while the second instrument (residual) can vary the pump speed or vice versa. There are many adaptations to these basic control techniques, however it is beyond the scope of this paper to cover them in further detail.

.

METERING PUMP AUTOMATION METHODS There are two basic methods of altering the volumetric output of reciprocating metering pumps, being; 1. Stroke Control 2. Reciprocating Speed Control. , 1. Stroke Control. As the name implies, the stroke length of a metering pump may be manually or automatically regulated to limit the swept volume of the diaphragm or piston and therefore the volumetric output of the pump. There are many methods of mechanically achieving this variation, however they all limit the actual reciprocating movement of either a piston and/ or diaphragm. The output accuracy of a metering pump from stroke control is very dependent on the stroke adjustment mechanism being placed accurately in the desired position. Automation of stroke control is generally achieved via an electric or pneumatic stroke position actuator. These actuators are designed to accept a process signal (normally 4-20mA) whereby the stroke length of the metering pump is varied proportionally to the input signal. Although some metering pumps, such as hydraulic piston diaphragm, are able to provide a 20:1 accurate turndown on stroke adjustment, once an automatic stroke positioner is utilised, the turndown is normally reduced to a maximum of 10:1. This is primarily due to the errors introduced with the mechanical operation of the stroke actuators, and the stroke position feedback mechanism. The main advantage of automating the stroke length of a metering pump is that the reciprocating speed or stroking rate is maintained at maximum . This aspect is important when an even injection of chemical is required to the process stream. When selecting whether to utilise stroke control or pump speed control, the degree of mixing required in the actual process stream is of great importance. 2. Reciprocating Speed Control. A metering pump output may be varied by regulating the speed of the drive motor. Traditionally, DC drive motors and motor controllers have been utilised to vary metering pump speeds. Over recent years significant advances have occurred in the development of AC variable speed drives (VSD) or variable frequency controllers. These controllers are now readily available from a number of suppliers and are a valuable tool for automation of chemical metering pumps. In simple terms these WATER October 1991

23


controllers vary the frequency and voltage to an AC drive motor, which regulates the motor speed. VSD's offer an economical and a simple means of automating chemical metering pumps, however, consideration must be given to the fact that they are positive displacement, and therefore require a similar torque at low speed as they do at high speed. One problem associated with VSD's is that their torque significantly reduces as the frequency is decreased or increased from 50Hz. The main limitation of utilising motor speed variators for automating metering pumps is that the reciprocating rate, or stroking rate, of the pump is reduced. This must be considered if consistent dosing of chemical into a flowing pipeline is important. However, the correct use of pulsation dampeners will effectively remove up to 980Jo of the pulsation effect of this style of metering pump. Even at very slow speeds or slow stroke rates, pulsation dampeners can prove to be very effective. One of the main advantages of utilising motor speed for automating metering pumps is that the manual stroke adjustment is then available for altering the volumetric output per stroke, whilst full automation on speed is being utilised. This is of benefit when the actual dose rate of chemical is likely to vary for different process .conditions or seasonal conditions. For example, consider a chemical being added to a pipeline at a dose rate of 10mg/L, whereby the speed of the pump is controlled on a proportional basis from a flow meter. Should a dose rate of 5mg/ L be required, then the manual stroke adjustment may be set at 50%. The pump speed and therefore output will be regulated via the signal provided from the water flow meter, however the required 5mg/ L dose would be maintained. When using VSD's for automation of metering pumps several factors must be considered when selecting the metering pump, drive motor and actual VSD unit. Should an automated pump turndown of 10:1 be required, then simply installing a VSD on a standard pump may not prove to be successful. As detailed earlier metering pumps are generally positive displacement, and require normal drive torque at low speeds. Should a standard pump utilising a 4 pole motor be selected, then at 10:1 turndown the motor will be operated at 5Hz or approximately 140rpm. At this speed the motor fan is operating too slowly to provide effective cooling of the motor, however the motor current draw will be such that heat dissipation is of significant importance. This of course, often results in drive motors overheating and possibly failing. As a general rule, if a VSD is to be utilised on a metering pump then the following steps should be taken, where possible. A . Increase the kilowatt rating of the drive motor, which will improve the heat dissipation capabilities of the motor itself. For example if a pump is normally supplied with a 0.55kw motor then this should be increased to a 0.75kw. B. Select a pump gear ratio so that the drive motor may be operated at a higher frequency than 50Hz, to achieve the desired pump output. Preferably operate the pump at lO0Hz speed, at which level the motor fan will be generally operating at a range to provide adequate cooling. It should be noted that simply increasing the speed of a normal pump is not always the answer, as one must carefully consider the stroking rate and the hydraulic capabilities of the pump unit. Therefore, the pump stroking rate should not exceed the maximum rate recommended by the pump manufacturer, for a given pump model. Most modern VSD's have the facility to limit the minimum speed that will be permitted, thereby eliminating the possibility of reducing the pump speed to a level where overheating and damage .to the drive motor may occur. These drives can also be set up, or programmed, so that the optimum starting and running criteria is provided for a given metering pump under normal operating conditions.

PUMP SELECTION One of the most important considerations when selecting a chemical metering pump, particularly where automation is to be applied, is to size the volumetric output of the pump correctly. Far too often the maximum output of the metering pump is significantly overstated, to ensure that adequate capacity is available to meet the expected requirements of a particular application. This often results in the actual pump being oversized, whereby the pump operates below its recommended maximum turndown range when automation is applied. Where output requirements of the pump are in question consideration should be given to being able to alter the pump output by simple liquid concentration and/or gear ratio 24

WATER October 1991

changes. When selecting chemical meter~g pumps one should establish, as accurately as possible, the output range that will be required, then select the pump so that it will have the capabilities of achieving the range desired.

SUMMARY Providing process signals, such as from a flow meter, chemical residual analyser etc, is a well proven technology and may be achieved with a large selection of equipment available on the Australian market. Actually utilising these signals and automating a chemical metering pump, to provide reliable and accurate injection of chemical, is not always a simple matter. There are many aspects regarding the physical capabilities of a metering pump which must be considered when accurate automation is required. Some of these have been discussed in this paper, however, only general comments have been made. The primary considerations include the degree of automated turndown required of the metering pump, the accuracy required, the degree of flexibility needed to compensate for varying seasonal and process conditions, and varying hydraulic conditions external to the chemical metering pump, but to name a few. The utilisation of chemical metering pumps is a specialised field, therefore all relevant information pertaining to the conditions under which a metering pump is to be utilised should be discussed with the equipment supplier, so that the most appropriate pump is selected. Far too often chemical metering pumps are not selected correctly and therefore misapplied, resulting in unsatisfactory performance or inadequate chemical control in the final process. Each application for selecting and automating a chemical metering pump should be carefully evaluated, to enable equipment and control systems to be the most appropriate chosen whereby the treatment process is optimised. With the correct selection of metering pumps, automation equipment and overall equipment installation, very effective and reliable chemical control is easily achievable.

1

G. Burton Continued from page 21 duty is initiated. Before any pump station controls are sent, the controller checks the level of the source reservoir so as to avoid draining the source reservoir. The control strategy for each of these systems is set up in the PLC attached to the telemetry front end processor. The set points required for each control algorithms are downloaded from the host computer, via the front end processor to the PLC. The PLC reads field transmitter levels, plant status etc, from the front end processor comparing these values with the programmed set points, and then putputs the appropriate control action through the telemetry system. When manual control is required, for example when topping up a reservoir, the control is changed from auto manual at the CMF automatically down loading a command to the PLC to stop the automatic control. Other control systems provided by the telemetry include the automatic control of a motorised valve to avoid draining the Gosford Clearwater Tonk, and again the control decisions are made by the telemetry front end processor with data received from other remote locations through the radio communication system.

IMPLEMENTATION This project is being implemented by Email Electronics, a division of Email Limited. The Systems Group has been set up to co-ordinate specialised project work and to offer a further enhanced capability to deliver custom designed systems based on the wide range of computer, analytical and manufacturing expertise within the Email Electronics organisation.


THE PRINCIPLES OF FWWMETER SELECTION: BS 7405 by R. A. FURNESS SUMMARY With so many different flowmeters available from so many sources of supply, flowmeter selection is becoming increasingly difficult. This paper looks at the principles of flowmeter selection and lists the factors which effect the choice of technology and even design variants within a particular technology. The new BS 7405 classifies closed conduit flowmeters into 10 major groups and this grouping was used in the basic layout of the standard. More than 45 variables were identified as the most important factors in selection and these have also been grouped into five major areas. These five areas can be considered in turn, depending on which is the most important, against the required application specification. The paper looks at the mechanics of flowmeter selection and highlights the importance of specification before the elimination process is begun . The standard only allows selection of a flow metering type and does not give guidance or recommendations on suppliers.

FWWMETER TYPES The starting point in drafting the selection procedure was the identification of the techniques of metering and this uncovered in excess of 100 designs available from over 200 different suppliers. The operating principles formed the most convenient basis for classifying the various types and indeed this also formed the basis for the document layout. Open channel meters were listed in the general classification, even though they and solid types are specifically excluded from the guide. There are 12 major classes, 11 closed conduit types and the open channel types, and are listed below: Group I 2 3 4 5 6 7 8 9 IO II 12

Description Conventional differential pressure types Other differential pressure types Positive displacement types Inferential types Fluid oscillatory types Electromagnetic types Ultrasonic types Direct and indirect mass types Thermal types Miscellaneous types Solids types Open Channel type

Over 70 of the more important meters are listed and discussed in detail in the body of the document.

FACTORS IN THE SELECTION OF FWWMETERS Even before starting to obtain meter information on which to base the selection, the first question is, "Do I need a meter at all"? In many industrial applications the user may often merely wish to know whether the fluid in the line is moving slowly, rapidly or not at all. If this is the case, then what is 26

WATER

October 1991

really needed is a flow indicator, readily available from flowmeter vendors at a fraction of the cost of the most simple flowmeter. If alarm limits for high or low flow are required, indicators can be fitted with micro switches. If something a little more sophisticated is needed, say an indication of flow to within 10%, it may still be unnecessary to purchase a flowmeter. As an example, many installations have changes of section or bends somewhere in the system. By putting pressure tappings at convenient points, the purchase of a differential pressure transmitter will turn the pipework into a crude venturi or elbow meter. If a calibration can be performed under these conditions a reasonable accuracy, around 50Jo, could be obtained . If better accuracy is required or the signal is being used to control the process, then the difficult task of meter selection needs to be undertaken. In selecting a meter the primary consideration is to obtain the optimum measuring accuracy at minimum cost, regardless of the throughput. Before even starting selection an application specification is required. The user must know or have a good estimate of the following: 1. Range of physical and chemical fluid properties including: a. Viscosity b. Flowing density c. Vapour Pressure d. Composition (if a mixture) e. Corrosive or abrasive nature f. Presence of foreign material g. Lubricating characterisitics 2. Range of flow rates expected or required. 3. Fluid temperature and pressure ranges to be covered. 4. Ambient temperature range expected. 5. Duration of operation (continuous or intermittent). 6. Location of meter or metering station (whether station is attended, unattended, local or remotely controlled). 7. Accessiblity for maintenance. 8. Required accuracy. 9. Money available to spend on the meter. There are other considerations which should be examined once several techniques have been selected for the duty. These may include end connections, size and weight of meter, materials of construction and the update capability as technology advances. Figure 1 shows that selection is really an iterative process, with progressive elimination of the many techniques until a short list is obtained against which the requirements can be compared in detail. Other factors, however, cannot be conveniently handled, such as in-house preferences, because a particular company may have large investments in say, orifice meters, and would prefer to stay with this principle even though the process engineer

Dr. Richard A. Furness is currently the Business Director for ABB Kent-Taylor Ltd at Stonehouse, Gloucestershire. A graduate in both chemical and mechanic.at engineering, he has been involved on the theoretical and practical side offlowmetering for some 18 years in R & D, in industry and in the academic worlds. This paper is based on a presentation to a seminar organised by the National Engineering Laboratory in London on 18 September.

---------

Hf.TU SH(CTION

Fig. 1 -

Flowmeter selection procedure

may recommend a positive displacement meter for a new part of the plant.

SELECTING FWWMETERS The real meat of the standard, BS 7405, is section 2, the General Selection Procedure. Five broad areas have been identified as being crucial and within each area, a number of factors that need consideration are listed. The broad areas are: a. performance considerations (eight factors discussed) b. fluid property considerations (14 factors discussed) c. installation considerations (11 factors discussed) d . environmental considerations (five factors discussed) e. economic considerations (nine factors discussed). This is not intended to be an exhaustive list, but gives the major factors when considering a flowmeter specification. The selection procedure is presented as a number of figures and tables so that a logical sequence can be followed. The method has been made "user friendly" for the


inexperienced user of meters but it also provides a very useful checklist to those people experienced in buying and applying f!owmeters. Figure 1 shows the actual selection procedure. For each of the boxes shown on this figure, there is a table of data which can be used to eliminate those meters not suitable. It is essential at the outset that the application specification is known, otherwise meters may by erroneously eliminated or included so that selection is poor. The first area of consideration is the application . Table 1 (Table 2.1 in the guide) shows 18 of the most common applications, and representative flowmeters from each of the main groups are matched against the applications. This will allow the elimination of those types clearly not suitable. Once applicable operating principles have been established then more detailed selection can .begin .

Under the five major areas shown in Figure 1 a number of variables can be listed and discussed. These are: Performance Considerations Accuracy Repeatability Linearity Rangeability (turndown) Pressure drop Output signal characteristics Response time Uncertainty Installation Considerations Orientation Flow direction Up/ down stream pipework Line size Location for servicing Effects of local vibration Location of valves Electrical connections Provision of accessories Hazardous atmosphere Effect of pu lsations/ unsteady flow

Economic Considerations Purchase price Installation costs Operation costs Maintenance costs Calibration costs Meter life Spares cost and availability Pumping power and headloss Technical optimisation Environmental Considerations Ambient temperature effects Humidity effects Safety factors Pressure effects Electrical interference

Table 1 Broad areas of application Application UquJde

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B

D

Group

Type

A

1

Orifice

I 7 I I I I I I I I I I I I I I I I I I

Venturi Nozzle 2

3

4

5

6

7

VA Target Awraging pitot ~nic nozzle Sliding vane Oval gear Rotary piston Gas diaphragm Rotaey gu

B

F

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Turbine Pelton Mechanical meter Insertion turbine

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I 7 I

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Vortex Swirlmeter Insertion wrtex Electromagnetic Insertion electromagnetic Doppler

I I ? I I I I I 7 7 ?

?

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Coriolil (direct)

K

L

I I I I I I I I I I I I I I I I I ? I

7 7

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I I

7

B

7 7

7

I

I I I I I

7

p

7 7 7 7

Q ll s r I 7 7 7 I 7 7 7 I 7 7 7

I I

7 7 7 ?

?

? ?

7 7

,I

I I I

?

7

I I I I I I ? ? I I I

• 7 •, I , • , I I 7 ?

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Tramit time 8

C

Miloell

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1l

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7

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?

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7

7

, ?

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7 7 7

I I I ? I I I

? 7 I 7 I 7 7

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Twin rotor (indirect)

9

Anemometer

10

The.imal mue Tracer Laser

7 7

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1

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• ,

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Fluid Property Considerations Liquid or gas Temperature and pressure Density Specific gravity Viscosity Lubricity Chemical properties Surface tension Compressibility Abrasiveness Pressure of other phases Presence of other components Real gas effects

I

, •

Comprehensive tables of data are given for each of the five areas, these being compiled from commercial literature, text books and other rep u ta bl e so u rces. Examp les of these and ins ta ll ati on constraints are shown respectively. Other relevant data is given, such as the typical performance distribution and line size availability which are shown in Figures 2 and 3. Cost comparison factors can be assessed more completely than before. Each of the 48 factors listed is discussed in general terms so that this section is a stand alone part of the whole work. If public opinion and committee decision do go the way of smaller sections then it is probable that section 2 of the general selection document with supporting sections will be published separately.

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,

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lwr

,...enll, applicable ? ii worth canaiderinr, oomeumea applicable

I ia suit.able;

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Typical performance distribution of flowmeter groups

I ia worth cowoclerinc, limited availability ar tenda t.o be upenaiw A blank indicatu W>IUit.able; 110t applicable NOl'E 1. Liquid applicau01>1 11ft indicated bJ the mllow':.ic: A General liquid application (< l50 cP) (< 0.05 Pa·&) B Low liquid llo,n (< 0.12 mS/h) (< 2 I/min) C Luce liquid llo,n (> 1000 m 3/h) (> 1.7 x 10' I/min) D Lt.rs• water pipea (> 500 mm bore ) E Bot liquida (tempentwu > 200 'C) F V'ucoua liquid& (> l50 cP) (> 0.05 Pa·&) G Cr}'osenic liquida H H)'iienic liquid,

NOI'E 2. Gu applicatian.t , n indicated J Genen.l pa applicalima X Low cu llowo (< 150 m3/h) L Lt.rs• cu 11o,.. (> 5000 m3/h)

-·· ....

bJ the i>l1owiac:

M Bot ruu (tempentuna < 200 'C) NS!Mm NOI'E 3. Miace!IIUOUI applicauona 11ft indicated by the f'Dllowinc: P Slurrieo and particle !lOWI Q Liquic!Aiqaid mixture& R Liquid/cu mmun,o s eom,.;..,. liquid& T Corro,,;ve cuee

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Fig. 3 -

,00

lt .000

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Size distribution of flowmeter groups

WATER October 1991 27


As shown in Figure 1, it may be necessary to consider some areas more than once due to the influence of one or more of the other areas modifying the original shortlist. Meter selection is often a compromise, and rarely is the ideal meter actually used. The five areas are highly interactive and it is very frequently the case that trade-offs between the various areas have to be made. For example, high accuracy may be specified but the funds available may not permit the higher cost, high-performance meters to be considered so the performance expectation is often lowered. By the same token however, all users want the best performance for the lowest price! It may be that the initial specification may have to be modified as a result of the first iteration of the selection process. It is VERY important not to request a characteristic or feature which is difficult to achieve as this may over-ride all other ~onsiderations. Once Table 2 has been used as a coarse filter to eliminate those techniques clearly not applicable, the actual process of selection (or should I say elimination) can begin. Tubles 2 to 6 (Tubles 2.3 to 2. 7 of the guide) are the base data tables for the major areas of consideration, aided by the purchase price comparisons of Figures 4 and 5 (note that these are in £ sterling). For example, if the order of priority for a chemical application is performance, safety, space, fluid and cost, then these Tables are used in the order 2, 5, 4, 3, and 6. If cost is the first priority then clearly Tobie 6 is the first to be considered. When using each of the figures in whichever order is deemed to be the most important, the features listed are ranked against the requirements for each meter type applicable, so that techniques can be given points for each feature and an order of preference can be drawn up.

10•

l-----+-----+---+--t----+---+----+----+--4--4,..---b---4--~ T1Kbffle I~ Sliding ,.-;;,....... /

'!)po

u ....,o,c•>

.,....bi:Ut;r (110

Orilia,

'

' ' '

"8,nturi Noalo

\viable uea

2

3

~ A,vqincpit.ot

NS

Sonicnoule

*0.26 .,_

Slidlnc-

±0.1 R t.o ±o.3 .,_ R

o..i...,

*0.26 .,_ R ±0.6 R t.o ±1 .,_ R

Row,, piat.on Oudiapbrqm Row,, p1

• OPctll

.,, '----'----'--~--'----'----,':--~--~-----'------';--~ 1

I

10• r-----r---r--+--t---r--+- - t--t/-l::::>"....1'--so+--+---+----'r---1 __.-;;---is

25

6--

. . . . . . ..!JO.

ZS

=e

,ol

=

2----tr;-----

R to ±0.2 .,_ R

1

.._...

---~ - --

. ...... .....

3 or 4:1 3 or 4:1 3 or .f:l 10:1 3:1

314 2

2/3

'

100:l

3 3 1/2 314

10 t.o 20:l

4/6

4

R R R R R v. R

' ' '

T T T T T

> 0.5 • > 0.5 I > 0.5 I > 0 .6 1 > 0.5 I 5me\o25ma 6mato26m1

No data NS

'

NS

4 t.o 40:1

3

R

0.61 minimum

NS

10 to 30:1 16 t.o 30:1

3

1nMrtioa "°rt.ex

< t2 .,_ R t2 .,_

NS 611>1

B--t.ic

±o.6 .,_R t.o t1 .,_R

Doppler

8

9 10

v.

t2 .6 .,_R t.o 4 .,_R

±o,l .,_R t.o *-0.2 .,_FS 10 t.o 100:1 *O ,l .,_R 10:1

I I

R v. R v,

No date *0.I R to±.,_ R

to.2 .,_FS ±o.2 .,_R tot\ .,_FS

6 t.o 26:1 10 t.o 300:1

I I

v_R R

I

6011>1 G ma co 26

> 0.2

m1

I

NS

Coriolia

NS

*-0.1 R t.o *0.25 .,_ R

10 t.o 100:1

216

R

0.02 e t.o 120 1 0.1, to

Twin rotor

No date

No date

10 t.o 20:1

314

R

3600. 6011>1

Anemometer "lbermal ....

No date to.6 FS t.o t2 .,_ FS

*-0.2 .,_FS to.2 .,.,stot1 "'R

10 t.o 40:1 10 t.o 600:1

2 2

R

n-.c...

No data No data

No data to.6 .,_R

Up t.o 1000:1 Up to 2600:1

I 1

1-

R»U.e..-. TilU..ftll-a..

v.;.11._......,_ 11 1ia l5Mqh

28

WATER October 1991

Vp 1a U.. paiaL ..&ocilt 11,Ril\be,......a-n.t.

•nwu..,__....w-i.

;;--';,-- r--x,o-- --;,

,oJ

10'

Figs. 4, 5 - Selection by economic factors - purchase price comparison

*-0.IR t.o±! .,_R

Trvlllit time

·200 . . . . . J)O

HQ)QmUffl flov rote. m> /h

elednma,netic 7

l)()

10'

1)

t1 .,_R

±o,l .,_ R

iio'

10''----'----'----'---'-----'----',--'----..__ _,.,_ ___._ ____....__ _.,__ _,

R R R

--

6

. . ·1~ . • ,, _.

Eltefro.tl09"ttic,r-... ·- ·7( ·-X.:,1-----l---4'!.,,-'=-f.""'-'--..... ,, = =.;;i.c,.=;.,I0-1--~-1-----1-- - 1 - - -+---+---+---I

3 4 3 U2

--·

.

S '---+-----+---+--+Oopplrr / l-----+---+----t--+---+---+--1----1 rullrosoric

,l

*0.01 R t.o ±0,06 .,_ R t0 .06 R to to.I .,_ R t 0 .2R No date

~

-·~

Transit litnt

6 t.o 10:1 4 t.o 10:1 10 to 280:1 10 to 40:1

1-rtion turbine

.....

uHrosonic \.

i

No date ±0.26 R t.o i6 .,_ R

Mecbanical meter

L---1--

\2

to.2"' •0.02 R io t0.6 .,_ R *-0.1 R t.o ±0 ,26 .,_ R t1 .,_FS ±o.l R t.o ±2 .,_ R

No data t1"' t0.16 R t.o t1 .,_ R •0.26 R io to.2 .,_ R

I

Cotiolt aou

4/6 2 2

llolt.on

6

'

10'

Haxilf!Yffl flowratt , 1111 /h

*-0.6 .,_FS t.o ±1 .,_FS NS ±0.06 .,_ ±0.1 .,_

,0 1

,0 1

,0

10 t.o 260:1 100:1 26:1

'lwt,ine

4

' ' .,_ FS t.o ±6 .,_ FS ±1

300

Orifiu plate

flow

1

/

.; ----,-- --m-- --;- --200

Table 2 Performance factors in meter selection

"'-

"""'

v,

v. v.

No date 0.12,to7,

No data No data

·-~-~tial~~ NSioclica1-not,peci.A.S

At the end of the process a short list should be obtained with meter types in a preferred order. This may need to be repeated if the specification is not complete. On pages 35 and 36 of the guide two examples of the use of the data tables are given. The first example uses a comprehensive specification for the batch production of HCI to arrive at a suitable short list. The second example for steam flow in a plant shows how a very poor specification leads to no selection because of lack of forethought at the specification stage.

ADDITIONAL FACTORS IN SELECTION BS 7405, although very comprehensive, does not address the problem of source of supply even when a particular type of flowmeter has been chosen. There are several sources of help in meter selection, such as independent consultants, trade associations and test houses. The cost of seeking independent help may be quickly recovered if consultants are used. It is also useful to study the subject and speak to as many people as possible to gain opinions on particular aspects. This is one of the benefits of conferences and seminars, in that they allow users' experience and suppliers' data to be discussed. When the appropriate meter types have been identified, it is useful to obtain several


__ ..,,

Table 3 Selection by fluid property constraints G""'P

I

----) ---

T),pe

Orifice 'h:nturi

Nonle Variable are.a

2

Target Avtraging pitot Sonic noule Sliding vane

3

Oval gear Rotary piston a.. diaphn.gm Rotary,.. 4

5

100 100 170 200 100

\\>rt.ex

260 100 70

-200 to +430 - 40 to +UO -3 0 to +150

electromagnetic

Uqald(lJ

S

.

}0 4

No data 3 X !0 4 10'

2.6

X

10 4

lo' 10' 10' 2.6 X 10 2 lo'

} 2

X

10'

10 4

No data 6 x 10 3 No limit No da!.a

p

-240 to +400 -240 to +360

10' 10'

Ariemometer

20 300

-200 to +400 0 to +100

No data No data

No data

No data No data

No limit No limit

L L L, L, L, L,

Tracer

l..ueT

.

N N N

L, G L, G L, G L L

390 400

N N N

Orieotadoa

Orifice Venturi

H, VU, VD, I U,B H, VU, VD, I u H. VU, VD, I u

3onic nonle Sliding vane

Oval gar Rotarypi,ton

a.. diapbngm Rotary gu Turbine Pelton Mechanical meter lnJertion turbine

\\>rt.a Swirlmeter

tn.ertion wrtex 6

Electromarnetic lneertion electromagnetic

7

Doppler 'n-anaittime

8

Coriolis Twin rotor

9

Anemometer Therma!JDUI '!'nicer

10

Dlncdoa

'!)po

'hriable azu 'larget Averaging pitot

6

Luu

u

vu

H. VU, VD, I u H, VU, VD, I H, VU, VD, I H, VU, VD, I H H. VU, VD, I H H, VU, VD, I H. VU, VD, I H, VU, VD, I H, VU, VD, I H. VU, VD, I H. VU, VD, I H. VU, VD, I H, VU, VD, I H. VU, VD, I H, VU, VD, I

• •

A NA

A A

'

•NA A A A NA NA

A A A A A A

Rowygu

4 4 4

'

Turbine Pelton MechWca.l meter Lnsertion turbine

3 3 3 3

A A A A

5

¼rt.ex Swirl.meter ln.&ert:fon vortex E lectromagnetic Insertion electromagnetic Doppler 'lranmtime Coriolis Twin rotor Anemometer

2 2

A A A A A

6

8 9 10

I

I 314 3/4

I 2 3

Tbenna.1 mua

'!'nicer

I 1

Luer

N

I

A

Rotary piston

'

a.. diaphragm

I 3 2 1/2 1/3 1/S 1/3 1/3 1/3

•• I

• •

N

3 3

A N

3 3

• • •

A A AINA No data NA A N NA

A NA A No data NA A N NA

__ --- - --

1/2 1/2 1/2

4

2 2

I

R i.t ~a1dtd Nia DOt - - . r y A i.t .....Uable NAilDO'l...-.ilablt I i.t kpu,deot Oti Ue:n:nta.J prurun meuurancmt

u 1 i.t lo" 5 i.t high

., .........

......,_

ru,.,

l!DISOD 2D/8D 0.6D/29.6D IJ) 6D/80D

N N

OD

OD

p

6D/20D

3.5D/UD

U, B

2D/26D

2D/4D

N p

u u u u u

>5D

>OD

N

OD OD OD OD

OD OD OD OD

R R R

U, B

OD/IOD

OD/SD

R

N

U, B

SDl20D

3D/10D

p

u u

6D

6D

SD/IOD

R R

U,B

!ODISOD

u u u

ID140D

ID/SD SD/IOD

.....

Table 6 Selection by economic factors -

mm 6 I<> 2600

>6

2 to 600 12 to 100 >26 ~6 25 to 250 4 to 400 6 to 1000 20 to 100 60 to 400

Gn>ap

T),pe

I

Orifice Venturi Noule

2

6 to 600 4 to 20 12 to 1800 >76 12 to 200 12 to 400 >2 00

4

N N N N N

2 to 3000 > 100

5

N N

>25

6

>4

p

U,B U, B

OD/IOD 25D

6D

H. VU, VD, I U,B

!OD 0D/60D

6D

OD

OD

N N

6 to 160 6 to 150

7

20D

R R

>26 2 to 300

8

N p

Unlimiud

9

H, VU, VD, H. VU, VD, H. VU, VD, H, VU, VD, H, VU, VD, H. VU, VD, H, VTJ, VD,

I I I I I I I

SD 20D

U,B

u u

2D/5D

U,B

10D/40D

u

No data

6D No data No data

'OD

•OD

U,B U,B

3 4

¼rt.ex

Doppler CorioliJ T-win rotor Anemometer Thermal mue Tra<tt

l ia low Sil hip

NOTE. F« pw-m.a.M pict -

--

........

quotes from different suppliers. The larger flowmeter suppliers have more extensive back-up facilities and more in-house expertise, but this has to be paid for. Also in-house manufactured equipment may be less expensive than an equivalent imported instrument purchased through an agent. Thus the cost of say, electromagnetic meters, may vary widely. It may be worth paying a little more to a reputable manufacturer than buying the cheapest 'equivalent'. WATER October 1991

3

I 5·

• 4

2

I 3 2 I

4 4

5 6

3 2 3

'

'

4 3 3 3

4

2

3 2 3

3 3 2 2

3 2

3 3 2

3 4 3

3 3 2

3 3 3

3 2

3

I 2

3

3

3

3 2

1/3 1/3

I 3

I I

3 3

2 2

4

4 3

3 3 3

3

s 3 2

5

(a)

2

3f

I 3 2

3

3 3

Luer

1/4 3

Sp.,,_CNta

3 3 4

3

Swirlmete:r Insertion vortex Electromagnetic ln&e.rtion electromagnetic

p

N

I 3 2

Turbine Pelton Mechanical meter ln5ertioo turbine

l iai.ad.inedfl -

...,

2 2 2 3/4

3

,i..~~

B

2 3 3

3

• _...,

~w Oow

............ .....

3 2 2

Rotary gu

VU UI upward \IIU'tic.l flow ~

.....

3 3 3

Transit tune

10

Opendoo

I

Sliding vane Oval geu Rotary piston a.. diapbrq:m

u

lmi-4irtetional flow b.i-dinctiooa.1 Cow

c.si,..doa

1/3 3 2 2

Averaging pitot Sonic noule 3

.....

2/4 4 3

'!Mi••

general data table

.....

lut&natfOD

VA

Riahori.&otilalfl'"'

VD u

EMI Rl1 .«edll

aplocloapf'OOI

,'

4

6D ID SD OD/SD

""'

30

3 3 3

,.,

Table 4 Selection by installation constraints .,

Noz.zle

4

s

.......

""'

•i.,d•per,de,;,t=lh•nti.D.cdtbeplpe-.11

3

Variable area 'Duiet Averaging pitot Sonic nonle

--- -··.......

Oval gear

7

N N p

4 3 3

...

A A A A A

N

G G G G

Venturi Nou.le

Sliding vane

3

NIP p

Nitootni1.&blie

2

Orifice

2

s

SM~

I

I

S'P

p Ml poMi.hl.

..,_

effea 11

N N

N N N N

Coriolia Twin rot.or

10

s

.,...,..,.

fypo

N

L, G L,G L, G L, G

L L, G

G-p

N

N N

5 X !OS 6 X 10•

,.,,

Twoormon

G

-2 0 to +80 -200 to +250

Thermal mus

...... p p

L, G L, G L, G L, G L, G L, G G L L L G

10' 2 X 104

-60 to +220 +6 to +26

300 20

X

200

Doppler Ttansit time

9

< +64 0 < +66 0 -3 0 to +200 -Hi to +290 -40to +l7 0 -SO to +200 -4 0 to +160 -2 68 to +63 0 -225 to +630 -2 6 t o +200 -6 0 to +4SO

Electromagnetic Inaertion

. 8

-,a to +120

3500 3500 600 70 250

ln eel"Uon vortex

7

- 80 to +400

700 100 400 400

Turl>ine Pell<>n Mechanical meter lnaertion turbine

Swirlmeter 6

GN:<G)or

(-C)

< +65 0 < +66 0 < +650

400 400 400

Table S Selection by environ mental conswaints

3 2

-' -

I 3

I 2 4 4

4

2 3

3 3

s

3

• 2

3 3 4

6

6

a.od (bJ fli 6fl,U"'t U

Other publications, such as the Redwood Flowmeter Guide (Furness and Heritage 1989), or trade directories such as the IMC Instrument Engineers Yearbook (Institute of Measurement and Control 1991) give information on actual sources of supply, with contact addresses and lists of products that are available. The selection of the 'correct' company is often, in my experience, more difficult than the selection of the me.tering technology. For example, there are

more than 200 suppliers in the UK alone, so the market is very fragmented with no major dominant companies in each of the flowmeter product groups. There are 15 coriolis mass meter suppliers, 34 different magnetic flowmeter suppliers and over 60 turbine type meter suppliers. Design variations exist within each meter group and the data tables in section 2 of BS 7405 only give a broad approach to the characteristics of each group. Con tinued on page 52


A NEW TECHNOWGY WATER METER byD. J. WMAS INTRODUCTION Water is an increasingly expensive and scarce commodity throughout the world and yet in most distribution systems over 250Jo of all water is designated as "unaccounted for". An indication of the financial significance of this lost water is given for example by the UK where the electrical pumping cost and chemical cost of the "unaccounted for" water is estimated at £38m per annum. The "unaccounted for" water is primarily due to leakage. It is vitally important therefore that leakage is reduced both for significant financial reasons and to avoid having to build new treatment works and locate new water sources. Many UK water companies are consequently now employing an active leakage control policy. Active control means finding leaks and repairing them before their presence is identified by consumers. One of the most effective methods of leakage control is through <;listrict and waste metering. District metering involves installing flowmeters at strategic points in the distribution system to enable the total consumption of up to 5000 properties to be monitored . Combined district and waste metering sub-divides the distribution network further by monitoring consumption of typically 2000 properties. Statistical analysis of the readings is then used to guide leakage control teams to areas of suspected leakage. Opening and closing valves in the area and diverting flow will then help to further refine the location of the potential leak. Flow measurement is critical to the success of district and waste metering both with regard to accuracy and, more particularly, flow range. Analysis of night-time flow rates is particularly important since any leakage is a greater percentage of the over-all flow and hence more identifiable. As a consequence, one of the UK water companies perceived the need for a new technology dual mode district and waste flowmeter as a cost effective alternative to the traditional meters then being used .

THE NEW METER The key requirements identified for the new meter were: • very wide flow range to enable accurate measurement of low night flows; • forward and reverse flow measurement capability; • reduced over-all cost of ownership; • local sensor intelligence; • preferably non-invasive in order to minimise sensor fouling and deterioration. The ABB Kent-Taylor Aquamag was the new meter designed to satisfy these requirements. The Aquamag flowmeter system (Figure 1) comprises a flanged sensor which fits into the pipeline, a sophisticated microprocessor transmitter and a power source. The output from the transmitter is then fed to a data logger or telemetry system.

David Lomas is Sales & Marketing Manager, ABB Kent-Taylor, Flowmetering Products, and has been involved with flow measurement applications and systems for over 15 years and served on numerous national and international standards committees. He is currently a member of the Institute of Measurement & Control editorial committee.

The sensor was based on the successful Kent-Taylor VTC/ VUC electromagnetic flowmeters. These meters do not have any moving components and are totally non-invasive. Consequently they have zero pressure loss and problems due to sensor fouling are also eliminated. The meters are tolerant of poor hydraulic conditions and this facilitates location in typical distribution pipe systems.

INSTALLATION Installation of traditional mechanical meters in below-ground pipelines is difficult and expensive due to the necessity for a meter chamber or pit. This requirement has been designed out and completely eliminated with the Aquamag sensor. It is submersible to IP 68 rating and its construction is such that it provides a mechanically rugged and strong structure suitable for underground installation without any form of pit or chamber. The meter is fitted into the pipe, the inter-connection cables are routed to the electronic unit and the hole in the ground is then merely backfilled. The associated electronic unit is also fully submersible as standard and features robust, military plug and socket connectors for easy installation. As a result of these characteristics the instailation cost of the Aquamag is greatly reduced. Although the purchase cost of the Aquamag can be more expensive than traditional meters, the over-all installed cost, ie, unit purchase cost plus installation costs, is significantly lower. The actual difference depends on meter size, labour rates, pipe position, etc but users have reported savings of 40-600Jo in over-all cost by using Aquamag. The absence of moving, wearing components coupled with the fact that all components in the sensor are passive, ensures a long, maintenancefree life with no routine requirement to access the sensor. Bypasses which are often fitted around mechanical meters to enable repair and/or routine maintenance to be performed, are also eliminated with the Aquamag for the same reasons as stated previously. Likewise there is absolutely no need for a filter in the system since there is nothing inside the sensor to become blocked or damaged. The wetted materials of construction selected were stainless steel and injection moulded PVDF/ EPDM. The lining materials, in addition to providing excellent abrasion and wear resistance, are approved for use in potable water.

BATTERY OPERATION

Fig. 1 -

The Aquamag Flowmeter

The associated microprocessor-based transmitter incorporates many radical design innovations. The cost of provision of mains power to a large number of metering points can be prohibitive and, consequently, an essential requirement for the new meter was that it had to be capable of operating without mains power. In the past power consumption has been a major constraint in consideration of electromagnetic meters in distribution networks. The combination of bi-polar pulse operation and a unique energy recovery system, ensures that the power consumption of the standard Kent-Taylor electromagnetic flowmeters is a mere 6 watts. Even with this low power consumption, mains power is nevertheless normally required and battery operation would only be possible for short periods. Consequently, although the 6 watts power consumption is extremely small in conventional terms, it was unacceptably high for district/waste metering. The novel approach adopted to overcome this problem was flash powering. WATER October 1991

31


With this technique the field coils are only powered for short periods (eg, 6 seconds every 15 mins), consequently the overall system power consumption is dramatically reduced, thus enabling a battery life in excess of one year to be achieved with one small submersible 12 volt alkaline manganese battery. In most water distribution systems the flow rates are very stable and change relatively slowly. Consequently the powering interval is selected by the user to optimise battery life and performance, and can be any value from O(ie, continuous) to 255 minutes. Output from the intelligent electronics is continuous, but it is only updated to a new flow value at the flash powering interval. Continuous flow measurement is essential when investigating leakage, in order to minimise the time involved when assessing the ~ffect of opening and closing valves. Consequently the Aquamag can be set to a continuous mode of operation by means of a magnetic wand. The microprocessor electronics unit incorporates a 4 digit LCD display of flow rate and 8 digit LCD display of total volume flow. The transmitter is an intelligent unit and incorporates a 19 way input/ output connection providing forward and reverse frequency outputs for data logger, connections for hand-held or remote terminals for programming a wide range of parameters such as time constant, volume units, intermittent interval, pulses per unit volume, etc (Figure 2).

Fig. 2 -

Hand-held terminal

APPLICATIONS It is the performance of the new Aqua mag meter which has had the most impact on the water industry and has been of most benefit to the users. As a result of the unique microprocessor sensor the Aquamag provides a usable flow range of over 400:1 with a minimum start-up flow rate of 1/1000 of full scale maximum. This rangeability, coupled with an accuracy of Âą 1% of reading or Âą2mm/sec and excellent repeatability, is applicable in both the forward and reverse directions with separate outputs for each direction. Any flow reversals are thus measured accurately and recorded on the associated data logger. The significance of the Aquamag's performance can be best gauged by comparing it with ISO requirements (see Figure 3), both with regard to flow rangeability and accuracy. As a result of this performance BS ISO EEC Specification +5 +4

e<:

~

':f!. 0

+3 +2 +l 0 -1

-2 -3 -4 -5

Fig. 3 -

32

qmin

Specification

WATER October 1991

ABB KENT -TAYLOR AQUAMAG specification

specification the Aquamag can be considered as a dual mode meter, ie, a general purpose district meter, which acturately measures peak flows in the day and a waste flowmeter which also measures the critical low night flow rates. As stated before, the night flows are particularly revealing and the additional data provided by Aquamag ensures better identification of leakage. This advance in flow metering technology has thus helped to improve the effectiveness of district and waste metering and has contributed to reducing leakage and saving water. One use has in fact cut "unaccounted for" values by over 200Jo through district and waste metering.

DATA RETRIEVAL Retrieval of the data from the flowmeter depends on the water company's standards and requirements. Traditional methods of data retrieval have been centred around manual reading of local indicators/ integrators. Such a method, though still widely applied in distribution metering, is being rapidly superseded with systems offering the enhanced functionality necessary to cater for many of the metering categories. Until relatively recently such a system would have necessitated the use of some form of telemetry using leased line or radio link communication strategies. However, the recent arrival on the market place of outstations with programmable software-based intelligence and abundant on-board memory has allowed the development of a more cost effective communications medium. Another important and relevant development has been the battery powered outstation. At locations where mains power is provided this will be of little consequence. However, where this is not the case, significant cost savings can be achieved if mains power is not provided. A number of products have now arrived on the market place that will operate well in excess of a year from quite small battery sources. Provision of such technology still has appreciable cost implications both in the provision and maintenance of the equipment and the communication medium. Many UK water companies have for some time been using data logging equipment in conjunction with meters. This logging equipment can be programmed to operate at various times and for a specified time interval (eg, from 2300 hours to 0600 hours daily) enabling information to be obtained on water consumption at certain times of the day/week/month, etc. The loggers are normally powefed from an internal lithium battery giving around five years operation per battery. For flow measurement the input signal is normally a volt free pulse from a pair of electrical contacts, or the semi-conductor equivalent (ie, open drain, or open collector transistor). Multichannel loggers are also available so that combinations of flow and pressure or forward and reverse flow data can be recorded. When the logger has been in use for the required time period it can be removed and the stored information down loaded into the computer and statistical/graphical information produced. In line with development in data logging a variety of software packages have now become available, both bespoke and general purpose, to enable the user to brief/de-brief and analyse resultant data on a stand-alone desktop micro-computer ensuring access to operational data in the shortest possible time.

WIDER APPLICATIONS The proven performance of the Aquamag for district and waste metering applications has led water companies to consider its use for bulk revenue applications, ie, measurement of water to factories, hospitals, etc. for revenue payment. Frequently the pipe feeding water to the factory is over-size and the traditional meter is often working well down its flow range even at peak flows during the day. Water authorities have identified that many large users continue to use small volumes of water at off peak times. Often this low flow usage is below the minimum registration point for existing meters and these flows are consequently not registered and hence are not billed. The Q max. of Aquamag is up to three times higher than for mechanical meters and yet it still satisfied the most stringent accuracy requirements at low flows, ie, EEC class C (ISO norm 4064/ 1). A smaller diameter Aquamag can consequently be fitted with appropriate upstream/downstream cones. This enables minimal off peak flows to be measured and the customer billed for this water with resulting additional revenue for the Authority. This additional revenue is potentially very significant with large users and the pay-

Contiuned on page 38


Measurement of Turbidity in Wastewater Treatment by G. SCHRANK and J. LANE

Turbidity measurement is one of the most common methods used to monitor the efficiency of water treatment processes for municipal and industrial application such as boiler water, beverages, beer, potable water. Turbidity measurement can also be an important parameter for process control in waste water treatment plants. This paper covers the theoretical background of this parameter and describes where and how to use it for a more stable and efficient process. It treats three major applications, the final clarifier, final effluent and filtration processes for advanced treatment, and shows the good correlations between suspended solids concentration and turbidity. Data from different plants in different countries are presented to show the results and good performance of these instruments.

INTRODUCTION Over the past few decades a wide variety of instruments for measuring suspended solids concentrations and turbidity has been developed by different manufacturers. The measuring instruments, designed initially for laboratory application, have undergone constant adaptation to industrial requirements, so that the instruments now on the market represent the "state of the art" and are used in many different applications. As these developments, most of which have taken place in USA, Switzerland and Germany, were originally not subject to standard specifications, there are often differences in the results from different measuring instruments in the same sample. This is unsatisfactory, especially when standardized quality values such as those applying to brewing or potable water purification have to be adhered to. Often this difference also leads to the claim that individual measuring instruments are defective or at least that the precision characteristics are inapplicable. The fundamental physical principles illustrate why the measured values are bound to deviate from one another in different measuring instruments.

TURBIDITY MEASUREMENT Turbidity in liquids is caused by the presence of particulate matter. In the case of undissolved, finely dispersed matter, the turbidity may be determined by measuring the attenuation of a radiant flux as it passes through the liquid or by measuring the intensity of diffused radiation 1• The best technique for measuring low to moderate turbidity is the scattered light method. The absorption method (attenuation principle) is suitable for moderate to highly turbid samples 2 • To distinguish between these concentration ranges, the term "turbidity" is generally used for low to moderate concentrations (up to approx. 0.1 C1fo suspended solids concentration). Beyond this concentration, the term "suspended solids concentration" is appropriate. . Undissolved matter can broadly be classified as: • settling matter (particle size > 1000nm) • non-settling matter (particle size 0.01 to 1000nm) - macromolecules and colloids (particle size 1 to 1000nm) - molecules and low-molecular matter (0.01 to 1nm). On this basis, solutions can be classified as: • genuine solutions (particle sizes < 1nm); • colloidal solutions (particle sizes 1 to 100nm); • suspensions (particle sizes > 100nm). As the light is scattered on the surface of the particles, it is clear that the size of the particles must have an influence on scattered light properties. Other influencing factors are the colour of the particles, their surface properties (as a reflection plane) , the nature bf the carrier fluid and the measuring principle. The scattered light effect is virtually independent of particle size in the case of very small particles ( <0.0lµm). In the range of a particle size between 0.05 and lµm, the scattering increases sharply. The scattered light intensity then undergoes another distinct drop with increasing particle size. The scattered light intensity is also influenced by the wavelength of the light source used. In the case of a source with monochromatic

Guenter Schrank is General Manager of BTG Anlagentechnik GmBH, which is the German manufacturing and sales subsidiary of the international BTG group. Guenter has Diplomas in Chemical Engineering and Economical Engineering from Dortmund, and eight years experience in product development and marketing of instrumentation and control in the chemical industry and wastewater treatment.

John Lane is General Manager of BTG Australia Pty Ltd. John has a Degree in Chemical Engineering from RMIT. Prior to his current position, he had seven years experience specialising in process control within the Petrochemical, Pulp and Paper and Wastewater Treatment Industries.

light of wavelength 400nm, the intensity of the scattered light is 16 times greater than with a monochrnmatic light source of wavelength 800nm. As the particle sizes occurring in practical application are within the spectral range of the light source used, dependence on the wavelength of the light source is evident. Measuring techniques using infrared (800nm) light evaluate colloidal turbidity particles less distinctly than larger particles (eg, bacteria). Therefore, larger particles can be measured in the presence of colloids without unwanted signals being recorded . On the other hand, if the colloidal particles ar! to be measured reliably, it is advisable to use white or olue (400nm) light. Sharp-edge, irregular particles scatter light quite differently from spherical particles. The colour of the particles is of significance in that the energy of the irradiated light is partially absorbed at the surface. An influencing factor of considerable importance is the measuring angle of the instrument being used. The light irradiated into a sample is scattered in all spatial directions by the suspended matter. The intensity of the scattered light is, however, not distributed uniformly throughout the measured volume but, quite characteristically, is dependent on the influencing factors referred to above. Figure 1 shows typical scattered light curves and their correlation with particle size. The traced curves are the envelope curves representing the range of constant scattered light intensity. DIAMETER / WAVELENGTH = 0.1 ,1.0 ,10.0

) Fig. 1 -

Scattered light intensity profiles for a range of Diameter to Wavelength ratios3

Microparticles display a uniform forward and backward scatter pattern. The larger the particles become, the more markedly the proportion of forward scatter is emphasized. For example, the scattered light intensity with a constant particle size of 50nm and a measuring angle of 25° is about 100 times higher than with a measuring angle of 90°. Although the scattered light intensity is distinctly stronger with small measuring angles (close to the transmitted light axis), it must also be taken into account in selecting WATER October 1991

33


the measuring instrument that the level of interference light increases as measuring angles decrease. In the limited case of the 0° measuring angle, the scattered light will be completely blanketed by transmitted light. Comparable measured values can be obtained with different instruments only if the instrument-specific characteristics are identical. This has led to the development of standard methods and specifications.

MEASURING TECHNIQUE Many of the measuring instruments manufactured today use the measuring methods prescribed in various standards. As these are not standardized on an international basis it is difficult to directly compare the measured values of different measuring instruments. For instance, the many different measuring instruments currently on the market use a variety of light sources and measuring angles. This results from an endeavor to do justice to the varying demands of industrial application . Other reasons, however, certainly date back to earlier technical development at the production plants.

LABORATORY MEASURING INSTRUMENTS Turbidity meters currently on the market can be divided into laboratory and field instruments. In the case of the laboratory instruments, the suspension to be tested is poured into cells and placed in the light beam of the measuring instrument; it is essential that these glass vessels are clean, colorless and unscratched. In many cases they have to be calibrated together with the measuring instrument, ie, the position of the cells must be fixed within the instrument. Internal standards are often used for calibration. These are sample vessels either filled with a defined fluid or made of opaque glass. This kind of calibration is very simple, though it does involve the risk of the instruments not being adjusted with adequate precision when the characteristics of these standards undergo changes. Practical examinations have shown repeatedly that measuring instruments are not correctly calibrated because scratches or unclean glasses are used or the applied standards have lost their set values. These standards too must be regarded as items subject to wear and replaced from time to time. The other category comprises continuously operating field measuring instruments, where an essential distinction can be made with regard to their adaptation to the process.

The electronics are separated from the measuring cell and connected to it by a cable. This permits the ~struments to be used in critical applications, eg, at high temperatures and pressures or in explosion-proof zones. To prevent direct contact between the medium and the optical parts of the measuring instrument, instruments are used in which the turbidity measuring is performed in a free-flowing water jet or on the surface of a free-flow stream of fluid (surface scatter). Whilst this undoubtedly has advantages with respect to primary contamination of the measuring window, there is a risk of soiling due to splashing, foaming or bubbling. As the casing of the measuring cell is constantly exposed to moisture from the medium, corrosion presents an additional problem.

TURBIDITY METERS WITH IMMERSION OR INSERTION PROBES The installation, handling and maintenance systems using insertion probes is more convenient. Figures 3 and 4 show two insertion probes that can be installed in the process line either with a ball valve or with special-purpose installation fittings. The probe (Figure 3) can also be used inside open basins or channels. The advantage in this case is that there is no need for complex bypass installations or additional pumps to counteract freezing, soiling or additional energy requirements.

Fig. 3 -

Turbidity measurement with an immersion probe. (90° scatter)5

MEASURING INSTRUMENTS WITH FWW-THROUGH PROBES Flow-through measuring instruments have their measuring cells installed directly in a process line, provided this is permitted by the diameter and flow conditions. In most cases, however, they are installed in a bypass. The advantage of this is that the instruments are easier to service or to inspect without any process interruption. One variant of bypass installation is charging the instrument with the fluid to be measured by means of an additional pump or the pressure head of a process line, with the necessary flow being kept low and the fluid discarded after the measuring operation. In some of the flow-through turbidity meters, the medium comes into direct contact with the optical components. Care must therefore be taken to ensure that soiling is prevented as far as possible or that the windows of the measuring cell are easy to clean. The technical structure of a typical flow-through probe operating on the forward scatter principle is shown in Figure 2.

Fig. 4 -

Turbidity measurement with an insertion probe. (180° backscatter) 5

The measuring probe illustrated in Figure 3 operates on the principle set out in the ISO standard for 90° measuring using infrared light. The probe is equipped with a self-cleaning device for automatic soiling compensation. The design of this probe and the associated electronic unit comply in full with the demands of water treatment technology, so that its main application is in the field of wastewater treatment plants and water monitoring or purification. Figure 4 shows a probe that operates on the 180° backscatter principle. The use of optical fiber cables permits application at high temperatures and in explosion-proof zones. Another advantage of this special steel probe is that it is very simple to use in applications where stringent demands are made with respect to sterilizability and CIP (cleaning in place), eg, in the food processing industry or in pharmaceutical fermentation processes.

TURBIDITY MEASURING AS A GAUGE OF CONCENTRATION Fig. 2 34

Turbidity measurement with a flow through probe 4

WATER October 1991

Modern measuring equipment is claimed to have an instrumentspecific accuracy of 1OJo or better. Deviations and major measuring


errors are due in most cases to changes in the particles or the carrier fluid. If all process parameters apart from concentration could be kept constant, modern turbidity meters could be quite precise. The accuracy of the correlation of turbidity measurement to actual solids concentration depends largely on the process and the respective application. The accuracy of the correlation in the application of drinking water purification is very high as the measured values as well as limiting values are recorded as turbidity units (related to formazine or AEPA-1). The accuracy is then only related to the precision and stability of the measuring instrument. Some of the influencing factors can often be excluded in industrial processes. The type of particle, for example, hardly varies in this case, so that the refraction coefficient differential remains constant. The shape of the particles is also virtually constant. For this reason, good accuracy figures can be expected in such applications. The use of such measuring instruments in waste water treatment plants is a different matter. As the nature of the sludge floccule may change significantly, deviations are to be expected. The function of turbidity measuring in this case is continuous measurement of the suspended solids discharge or qualitative registration of the undissolved components at the different process stages, so that precise analytical evaluation can be eliminated in most cases. Figures 5-8 illustrate the correlations between measured turbidity and laboratory-determined suspended solids concentrations in different plants . The zero off-sets in the correlations indicate the variations that can occur between the field laboratories and the instruments which are calibrated in standards-approved laboratories.

[mo/ i] 40 0 r - - - - - - - - - - - -- - - - - --

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130 140

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These correlations may be poorer in individual cases. Heavy rainfall or seasonal variations may affect measuring accuracy. The assessment of opto-electronic measuring instruments for registering high suspended solids concentrations is very similar. In this case, good correlations are obtained between measured values and concentrations. The measuring instruments operate, as already stated, on the absorption (attenuation) principle, and are used successfully in a wide range of processes. In the field of wastewater purification in particular, the Four-Beam Alternating Light system 8 , has gained an excellent reputation worldwide. These instruments are used primarily to determine concentrations in activated-sludge basins, in return or surplus sludge flows and in the sludge thickening and dewatering processes.

An ever-increasing awareness of our responsibility towards the environment has resulted in increasing endeavours to ensure that the waste water discharge into streams,- rivers or lakes is as clean as possible. For this reason, statutory limiting values are becoming more and more stringent. Although it is primarily dissolved components on which attention is focused in the purification process, turbidity measurement is also a 'valuable parameter for measuring pollutants such as phosphorus, a substance that plays a major role in the eutrophication, as up to 50% of phosphorus is attached to solid matter. Continuous turbidity measurement can supply reliable information on tlfe functioning of a waste water treatment plant.

In addition, the turbidity measuring method can be used to register the sludge level in the final settling basin. The simplest method is to use portable turbidimeters, which operate far more accurately (reproducibly) than the "Sludge Judge" or "Secchi Disk". A correlation between turbidity (measured with an MET- P manual measuring instrument) in a final settling basin and visual depth measured with a Secchi Disk is shown in Figure 9. The correlation coefficient in this case is 0.96. A measuring instrument of somewhat different design but also developed for mobile application shows the sludge level directly. It is only reliable information on the sludge level that enables the operating personnel to prevent the settling basin from

·

Wastewater purification plant discharge (R=0.97) 6

Continued on page 43

.

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FINAL SETTLING BASINS, SEDIMENTATION BASINS

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Fig. 8 -

Inflow of a water treatment plant (R=0.94) 6

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Turbidity and visual depth in a final settling basin 9 WATER October 1991

35


SOLIDS MEASUREMENTS IMPROVE PERFORMANCE. OF WASTEWATER TREATMENT PLANTS by S. VALHEIM INTRODUCTION With running costs of treatment plants rising along with public demand for a cleaner environment, the wastewater treatment industry is striving to optimize the process by improving performance while cutting costs. This is a complex combination of reducing both effluent load and energy consumption, as well as minimizing the sludge to be disposed. A number of wastewater treatment plants in Australia are on the road to meeting the objectives with a well planned control strategy which includes accurate and continuous measurements of .suspended solids concentration and sludge levels. The flow-on benefits from continuous measurements - better performance at a lower cost - increase according to the sophistication of the control options installed. In the two main control areas, primary clarification and aeration/ secondary clarification, a range of solutions is possible. The most appropriate solution for an individual plant depends on the specific problems encountered in the plant, the required effluent quality, as well as the expected return on investment.

PRIMARY CLARIFIERS Control objectives The main objective for a primary clarifier is to remove as much solid as possible so that a minimum is carried over into the aeration stage and subsequent treatment phases. This makes it essential that the sludge level in the primary clarifier be maintained below a critical level. At the same time, from a cost point of view, the sludge being discharged needs to be as concentrated as possible. When the sludge density is too low, a lot of energy is wasted because the digester becomes overloaded. Solutions

Minimum. A single instrument is installed in the primary clarifier which closes an alarm when the sludge blanket exceeds a certain level. Desludging will then start automatically, and continue for a set time (Figure I).

(:

.. Fig. 1 -

High level control -

WATER October 1991

(: : ~

• Fig. 2 -

..

High/ low level control with ~fluent alarm -

primary

Compared to the minimum solution, which has only a single measuring point, this strategy meets many of the objectives for efficient control of primary clarifiers. However, it fails to solve the problem of poor sedimentation in the primary clarifier. One step further is necessary for optimum performance to be achieved. Full Control. Full control of the primary clarification stage can be achieved by adding an instrument to continuously monitor sludge density (Figure 3). This measurement of sludge density now becomes the main control parameter. Sludge withdrawal will only take place when the sludge density is within preset limits .

primary

While relatively inexpensive to install, this simple solution has a couple of limitations. There will be frequent sludge carry-over into the aeration stage because there is no measurement of sludge density. Also, low density sludge will be transferred to the digester. Intermediate. A more efficient control strategy would include two additional measuring points: one to monitor low sludge level and the other to monitor high effluent concentration (Figure 2). 36

Svein Valheim graduated in Chemical Engineering from the Norwegian Institute of Technology, then worked at the BTG R&D facility in Saffle, Sweden. He is currently an Applications Engineer with BTG Australia Pty Ltd.

Fig. 3 -

..

Optimum control -

primary


The high level alarm will automatically initiate the transfer of sludge into the digester. When the density has reached a predetermined minimum, it will automatically stop. This can be overridden by the low level alarm, as effluent will get no residence time in the clarifier, and pumping should be stopped. Due to these controls, there will be energy savings in the desludging operation, and there will be a smoother operation of the aeration/ secondary clarifier at reduced effluent load.

AERATION/SECONDARY CLARIFIER Control objectives

The first objective in this subsequent stage of the process is to keep the level of activity in the aeration tank constantly at optimum. This can be achieved by keeping the concentration of suspended solids as consistent as possible. The second objective is to keep the effluent turbidity as low as possible, so that turbidity levels in excess of the licensed level will be avoided. To do this, the sludge level in the secondary clarifier must be kept within preset limits. Solutions

Minimum Control. Two continuous measurements are required here for a minimum level of control: suspended solids concentration in the aeration tank and turbidity of the final effluent (Figure 4). With these measurements, the density of both return and excess sludge can be controlled. The major benefit will be a constant

-

Fig. 6 -

BTG's suspended solids analyzer

Fig. 7 - BTG's self-tracking sludge level detector

CONCLUSION This article highlights some of the improvements that can be made in a wastewater treatment plant by comprehensively measuring and controlling suspended solids concentration and sludge level. The immediate benefits to the plant are optimized operation resulting in reduced effluent loading, reduced energy consumption and a reduced amount of sludge to be disposed, all at a minimum cost.

D.J. Tomas Continued from page 32

Fig. 4 -

Minimum control -

aeration/ secondary

concentration of suspended solids in the aeration tank, which is needed to keep the amount of dissolved oxygen at an efficient level. Full Control. To further improve the control of this stage, two more measurements are required. The concentration of the return activated sludge (RAS) should be used for feed-forward control of the solids concentration in the aeration basin. The return flow should be controlled so that the concentration in the aeration basin will be kept as close as possible to set point (Figure 5). Continuous measurement of the sludge level in the secondary clarifier will ensure that the effluent loading is kept at a minimum, and that the concentration of return and excess sludge is within acceptable limits. This control should be overridden when the turbidity in the effluent goes too high. It then may be required to lower the sludge level set point or to add sedimentation additives, eg, flocculants. By installing such a control strategy, a constant concentration and activity in the aeration basin will be achieved. Hence, both excess sludge and effluent discharge will be minimised.

Fig. 5 -

38

Optimum control -

WATER October 1991

aeration/secondary

back period for the water supplier can be remarkably short. The pressure loss through the reduced diametet Aquamag is still less than that of a pipe-size mechanical meter. For bulk revenue applications the Aquamag operates in a continuous mode and is powered from mains supply through an ac/dc converter. The Aquamag tran'smitter is typically mounted in a polyester box together with a mains switch with mini circuit breakers and an EEC approved small electricity consumption meter. The system incorporates battery back-up and in the event of mains failure the system automatically changes over to the battery. Once the power supply is re-established the system automatically reverts from the battery to the ac/ dc supply. The low power consumption of the Aquamag also makes it ideal for powering from solar panels and trickle charge batteries.

CONCLUSION Aquamag and its concept are bot~ completely new, but in the short time that the meter has been on the market it has become firmly established in the UK water industry and has been the subject of numerous user technical papers and presentations. All such presentations have focused on the same key benefits, namely the 400:1 flow range with high accuracy and repeatability; the typical 40% reduction in over-all installed cost as a result of the meter sensor being both submersible and buriable, and the attendant elimination of metering pits and chambers; the possibility of operating for over one year from one small battery and the overall flexibility of the system. Aquamag has satisfied the original design objectives set for the meter and is now playing a significant role in helping to reduce leakage and provide better and more efficient use of available water resources. AQUAMAG IN AUSTRALIA The Aquamag flowmeter has been successfully launched in Australia, and is currently being used by several water authorities in applications such as revenue flowmetering , water leak detection, and wide range flowmetering. All Aquamag flowmeters can be provided with a N.A.T.A. approved flow test certificate. These flow tests are carried out in the ABB Kent-Taylor Flow Laboratory at Caringbah NSW.


FOUR-BEAM TURBIDIMETER FOR WW NTU WATERS by K. KING SUMMARY As the need to measure increasingly lower levels of turbidity has evolved over the past 20 years, shortcomings of older turbidimeter designs and measurement standards have become evident. This paper discusses the appropriateness of international standard ISO 7027-1984(E) and presents the research and testing involved in the development of a new modulated four-beam turbidimeter.

INTRODUCTION From the 1970s to the present, turbidimeter design has been strongly influenced by (USA) EPA Method 180.1 (Storet 00076), which recommends criteria for nephelometers (90° scatter) used for turbidity determinations of potable water, typically below 40 · NTU. Another guiding influence has been the Safe Drinking Water Act. These US Federal regulations required turbidity levels of drinking water to be at or below 1.0 NTU. Consideration to reduce the legal limit of 1.0 NTU to 0.1 NTU has been reviewed (EPA 40 CFR Part 141) and resulted in a compromise limit of 0.5 NTU.

OLDER TURBIDIMETERS The need to provide instrumentation for stable and repeatable measurement at this new low level has prompted manufacturers to evaluate existing designs which suffer from some or all of the following problems: • Zero stability is inadequate at low turbidity levels. • Cu~ettes can scratch and are sensitive to orientation, leading to maJor errors. • Complex flow bodies are difficult to clean, leading to errors at low turbidities. • Reading varies with internal sample level (flow rate) where there is an air/water interface. • Unsealed sensor electronics are vulnerable to splashing. • Accurate calibration is difficult because the reading is sensitive to sample level and tilt of calibration cyclinder. • Response time is either very fast but without means of bubble rejection, or is very slow. • Distance between sensor and analyzer/readout is limited to a few feet. • Microprocessor-based electronics are prone to memory loss during power line fluctuations. • ~nalytically signific~nt turbidity information is masked by f1ltenng of bubble reJect in the software. • Secondary calibration/near-zero standards are unstable or inaccurate. • Identical samples yield different values when measured in instruments of different design. The instrument design recommendation of EPA Method 180.1, Section 5.4 reads: 5.4 Differences in physical design of turbidimeters will cause differences in measured values for turbidity even though the same suspension is used for calibration. To minimize such differences, the following design criteria should be observed: 5.4.1 Light source: Tungsten lamp operated at a colour temperature between 2200-3000° K. 5.4.2 Distance traversed by incident light and scattered light within the sample tube: Total not to exceed 10cm. 5.4.3 Detector: Centered at 90° to the incident light path and not to exceed ±30° from 90°. The Detector, and filter system if used, shall have a spectral peak response between 400 and 600nm.

Karl King, Manager of Development Engineering for Great Lakes Instruments, Inc, Milwaukee, WI, attended graduate school at the University of Illinois. He has worked for over 20 years in the field of electrochemical and optical sensors for chemical process and pollution control. He holds patents relating to measurements of pH and dissolved oxygen. Great Lakes Instruments is represented exclusively in Australia by the Combined Instruments Group Pty Ltd who organised the submission of this paper.

Inst~uments which are suitably accurate at 1 NTU can be designed followmg the_recommendation of EPA Method 180.1. However, the commercially available turbidimeters with simple designs in accordance with EPA Method 180.1 are not stable at lower levels of turb~d~ty for t~e :easons given below. Furthermore, at any level of t~rb1d1ty, turb1d1meter~ of different design do not necessarily prov_1de comparable readings on the same sample, as stated in Sect10n 5.4 of EPA Method 180.1. This occurs because the recommendations on important aspects of the hardware, such as photode_tector c~a~acteristics, are not exact. There are also problems comparing turb1d1meters with only 90° scatter with those which ratio 90 ° scatter to scatter/transmission at other angles. The most serious problem with EPA Method 180.1 is that it calls for an incandescent light source at a given colour temperature. I~candescent_ lamps are known to vary their output with time. See Figure 1. This makes frequent recalibration of instrument span necessary. Most commercial turbidimeters drive the lamp at a constant voltage so that colour temperature is not controlled. Incandescent lamp life is seldom more than 10 000 hours. This ca~ses more recalibration and the need for replacement mamtenance. The most commonly used lamps are driven under steady-state conditions because they are most stable in that mode. ~owev~r, this makes simple modulation of the light source 1mposs1ble except by expensive means that are not cost-justified for applications at 1 NTU and above. Because the lamps are not mod~lated, the ph_otodetector signal processing electronics operate on direct current signals. Consequently, the output signal includes all DC offset errors, electronic drift errors, and lamp aging error factors. Tests below 0.1 NTU have shown that commercial turbidimeters designed to recommendations of EPA Method 180.1 ~ave such an unstable zero that weekly, if not daily, recalibration 1s needed.

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Some products following the design recommendation of EPA Method 180.l are represented in their literature as being "EPA approved". This suggests that the EPA tested and approved these products in the same way as instrument approval agencies such as UL or CSA. The EPA is not such an agency, making this claim irrelevant and inaccurate.

88 86 20

40

60

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SERV\CE HOURS

Fig. 1 - Lamp intensity aging WATER October 1991

39


NEWER TURBIDIMETERS An existing international standard - ISO 7027- 1984(E) removes the fundamental constraints of EPA Method 180.1. (Refer. to Figure 2.) The ISO 7027-1984(E) apparatus specification, Section 9.1 reads: 9.1 Apparatus Any apparatus may be used provided that it complies with the following requirements: a) the wavelength, \ of the incident radiation shall be 860nm; b) the spectral bandwidth, ~).., of the incident radiation shall be less than or equal to 60nm; c) there shall be no divergence from parallelism of the incident radiation and any convergence shall not exceed 1.5°; d) the measuring angle, 8, between the optical axis of the incident radiation and that of the diffused radiation shall be 90 ± 2.5°; e) the aperture angle, w8, shall be between 20 and 30° in the water sample.

NOTE - According to recent investigations, it is preferable to have an angle of less than 20°. The narrow definition of the light source makes it unnecessary to specify the spectral sensitivity of the photodetector, which is important when the light source is broad-band as specified in EPA Method 180.l. This tighter specification makes for better comparison of readings on different turbidimeters. The ISO 7027 - 1984(E) standard clearly permits a superior turbidimeter design in terms of instrument-to-instrument comparison. However, it does not suggest any way to remove DC offset drifts, which can be a serious problem when measuring low turbidity levels, or the aging effects of the light source and other components. Modulating the light source allows the signal processing electronics to be designed to remove long term offset drifts, greatly improving stability at low NTU levels. Aging effects and other span errors can be removed by using a four-beam method. A new turbidimeter design which implements a modulated, fourbeam method in accordance with ISO 7027-1984(E) has been proven to have zero and span stability far superior to single-beam or ratiometric designs which follow the recommendation of EPA Method 180.1. Turbidimeters that measure high suspended solids with a four beam method have been available since the 1970s from several manufacturers. These instruments have demonstrated excellent span stability, even in the fouling conditions of sewage treatment plants where they monitor suspended solids.

THE MEASUREMENT TECHNIQUE A modulated, four-beam method is illustrated in Figure 3. The measurement phase is performed in two parts. Initially, Source A , a modulated infrared LED, is turned on and Source B, also a modulated infrared LED, is turned off. Readings are taken with Detectors l and 2. Ia = intensity of Source A T1 = tran smittance of window on Ta = transmittance of window on Detector I Source A G1 = sensitivity of Detector I cp = turbidity of suspension V la = output of Detector I when

(3 = absorption coefficient Lai = optical distance from

Source A to Detector I

Source A is on - etc. for B,2 -

8

DETECTOR

7

=O

4= SOURCE B

DETECTOR 2

i

'--J

ft

SOURCE A

Fig. 3 - Modu lated, four-beam method

For the direct path with Source A on: Y1a = Ia · Ta · [exp(- {3 · Lal · <t>)] · T1 · G1 For the 90° scatter path with Source A on: V2a = Ia · Ta · [<7> • exp(- {3 · La2 · <7> )] · T 2 · G2 For the direct path with Source B on: Y2b = lb· Tb· [exp(-{3 · Lb2 · <t> )] · T2 · G2 For the 90 ° scatter path with Source B on: Y1b = lb· Tb· [<t> · exp(-{3 · Lb1 · <t>)] · T 1 · G 1 Compute the ratio R 2 R 2 = (Y2a · Y1b / Y1a · V2b) = ¢ 2 · exp(-{3 · t.L · <t>) Where t.L = La2 - La1 + Lb 1 - Lb 2. Geometrically, t.L = 0 but optically it is not zero because the scattered light has alternate paths to the detector. In practice, t.L is < 0. Taking the square root, R = <t> exp(-{3 · t.L / 2 · <t>) (see Fig. 4) This curve is essentially linear when NTU < I. Above 1 NTU there is nonlinearity which is removed by a linearization function when the signal is processed by the turbidimeter's microprocessor. The major advantage for this method is that all terms relating to light source intensity, window transmittance and detector sensitivity cancel from the ratio. Therefore, the effects of lamp aging, non-homogeneous window fouling and photodetector aging all cancel. This provides the span stability associated with this method. The effects of attenuation due to sample color are not included above, but they also cancel, making the turbidimeter selfcompensating for color. The conventional four-be;im method does not inherently cancel offset errors. These are removed by 1) modulated light sources that permit an electronic design to eliminate all electronic offset errors, and 2) a light baffle in the sensor's sample chamber that absorbs virtually all stray light, allowing stable and repeatable measurement down to the value of minimum turbidity water (0.012 NTU for this instrument) .

TURBIDIMETER TESTING EPA protocols for lab test methods are unsuitable for on-line turbidimeters. Consequently, an in-house test stand was built and SQUARE ROOT OF' RA TIO 100

860 nM

/

90

/

80

bendwidth 60 nM

/

70

/ /

60

V

,o •o

90° +/- 2 .5°

7

PERCENT JO

I/

/

20

~I,..-/ 10 ~

----

10

20

30

40

50

60

NTU

Fig. 2 40

ISO optical apparatus

WATER October 1991

Fig. 4 -

Characteristic curve for modulated, four-beam method


used to run tests over a period of months on the modulated fo urbeam turbidimeter and four other turbidimeters designed to the recommendation of EPA Method 180.1. These instruments were tested in various combinations by recirculating turbidity standards· through them. Turbidity values were changed by dosing the test stand with concentrated primary standard suspension. Instrument outputs were recorded by a strip chart recorder and by a digital Data Acquisition System. Flowing samples were preferred over static samples because they more closely stimulate actual use conditions. The test stand connections were made with Tygon tubing. Standard suspension was introduced into a plastic tank for mixing. The tank outlet was connected to a centrifugal pump with motor speed control. The outlet of the pump was connected to valves and filters, allowing the fluid to be filtered or unfiltered. The fi lters, with a 0.2 and 0.04 micron pore size, were used to remove particles so that Minimum Turbidity Water (MTW) could be produced and measured. The turbidimeters being tested were connected in series. Fluid from the last turbidimeter was returned to the mixing tank. The Data Acquisition System was controlled by a PC. The strip chart recorder was monitored by the test stand operator to ensure that all turbidimeter readings had stabilized before another addition of standard was made. · Formazin stock suspensions, made in accordance with Standard Methods formulas, were kept refrigerated until used in testing. Because there is considerable variation ( ± 1"lo) between batches of formazin, the same stock suspension was used for calibrating and testing the turbidimeters. APS copolymer suspensions were also used in testing. They were stored and used as directed by the manufacturer. Formazin suspensions are known to be unstable with time 1• This was a complication in the testing. The decay rate had to be known and taken into account. A computer program which compensated for the decay rate was used to calculate the doses needed to obtain desired turbidity values. Tests were run on the stand for I, 10 and 40 NTU formazin suspensions to obtain decay rate values (Fig. 5).

Some testing was also done with static samples, but the time instability of formazin caused serious operational problems. Styrene divinylbenzene beads were also used and found to be stable with time, and therefore, a preferred calibration suspension , but only for static samples.

RESULTS The modulated, four-beam turbidimeter in accordance with ISO 7027-1984(E) was found to be superior or equal in every performance category to designs in accordance with EPA Method 180.1. 4 BEA M 'iS NTU

100

/

/

4BEAM

PERCENT FVU.-SCA.1...£

/

I/

/

I.

10

I

I

/

/

0. 1

I

II

I

I

II

I

NTU

Fig. 6 -

Conformance, four-beam reading vs NTU LOW NTU

0,14

L,o

0. 12 0. 10

~ i:::--~ :------.....

10

-...-...

------

"-

60

r--r---_

r-----.....

,0

"'

~------ ---

-- ---- --

10

12

14

o~/

o.oe

..,Im,

0.06

~

0.04

I RATIO

- - - - · Fl\/

Ill

18

~ ./" , ./ ,,,..

I

0.00 0.00

20

0.02

0.04

o.oe

0.06

HOURS

0. 10

0 12

NTU

Formazin decay rates Fig. 7 -

The test stand was filled with filtered, deionized water with a typical turbidity of 0.1 NTU. The water was circulated to remove entrained air and through the filters to obtain MTW. Turbidimeters with an adjustable zero were adjusted to display the manufacturer's suggested value of MTW on that instrument (0.012 NTU, for example). Suspensions were prepared with MTW using class A volumetric glassware, and pipetted into the test stand with class A laboratory glassware. Automatic addition methods were found unsuitable because of the tendency of formazin to settle in dead spots in the delivery system. To characterize and test a turbidimeter, the stand was dosed to the following turbidity values. • 0-0.1 NTU range: 0.01 to 0.1 in steps of 0.01 NTU • 0-1 NTU range: 0.1 to 1.0 in steps of 0.1 NTU • 0-10 NTU range: 1 to 10 steps of 1 NTU • 0-100 NTU range: 10 to 100 in steps of 10 NTU After each test, the Data Acquisition System files were plotted for inspection and statistical analysis was done. The performance was judged by: conformance to the predicted NTU values, error at very low turbidity, repeatability from one run to another, range tracking, response time, stability, and sensitivity to fouling. Also of interest were: simplicity of operator interface, ease of calibration, ease of maintenance, stability of secondary standards, response to over-range conditions, effects of bubbles in the suspension, sensitivity to flow rate, and sensitivity to power interruptions. 42

_/

,,-

4BEAM

0.02

v _.,, ·-

.v ./

J

NONRATIO

4 8EAM

JO

Fig. 5 -

100,0

10,0

,-

100

,o ,o

/

10

WATER October 1991

Low NTU Conformance, four-beam reading vs NTU

I\

-'

"'

-

-- -- ,-,_

-

" ~u

'9;

'

""

"

., Fig. 8 -

.., "°'"" roo f--"'1 i--

-

~

"

~

~

"

Fouling effect on turbidity reading

~

,_,....., ~

'

70

EE

I~ .

0. 14


Conformance to predicted NTU values is shown in Figure 6. Conformance in the range below 0.1 NTU is shown in Figure 7. All instruments were calibrated as per manufacturer's instructions. The four-beam instrument was tested against a single-beam instrument to determine the effects of fouling (Fig. 8). Light sources were reduced by means of neutral density filters. The single-beam instrument was directly effected according to the transmission of the fi lter. According to the mathematics of the four-beam instrument, the filters should have no effect even if the filtering is non-homogeneous. This was verified by the testing - a 90% loss of light strength yielding only a 4% error in reading. A Jong-term (6 months) stability test was run with two four-beam units, a single-beam unit and a ratiometric lab unit. The units using incandescent lamps drifted downward by about 1OJo of reading per month at higher turbidity values. The four-beam unit showed no drift over the six months. On MTW, non-modulated systems showed the effects of electronic drift and offset. When ambient temperatures were uncontrolled (15 to 28 °C), the offset of the single-beam ratiometric turbidmeter was unstable and had to be adjusted frequently. The single-beam, non-ratiometric unit drifted randomly from 0.018 to 0.032 over the period. The modulated four-beam units did not drift over the period. Repeatability was ± 0.001 NTU throughout the test.

serves as a bubble deflector to remove the bulk of entrained air before reaching the measuring area. The smaller flow chamber permits a low residence time. Sensor electronics are totally sealed. The sensor can be mounted up to 100 metres from the analyzer/ transmitter without affecting performance. The separate analyzer's display autoranges across three decades: 0.012 to 1.000 NTU, 1.00 to 10.00 NTU, and 10.0 to 100.0 NTU. Signal filtering includes bubble rejection software that does not obscure valid turbidity data. The analyzer provides relays for alarm and control, analog outputs for transmission, a simple user interface, and a set of diagnostic messages. The diagnostics indicate when the system needs cleaning. They also identify failure of items such as light sources, detectors and the signal processor. The memory is protected from power loss or fluctuations. A stable secondary calibration standard is available.

REFERENCE I. Ontario Ministry of Environment, Rexdale, Ontario, Canada: "Calibration Control

for Tu rbidity Measurements", Sept. 12, 1985, p.1 ; J. Crowther and J. Thrush.

SECONDARY STANDARD Secondary standards have long been used for calibration but have had questionable success because of non-permanency. A new standard for use with the four -beam turbidimeter has been developed . It is a glass/ ceramic material in the form of a cube. Light passing through the cube is scattered by the ceramic particles inside the glass (not a surface phenomenon). The number and size of the particles is constant in time provided the materials is not heated above 600 °C. The cube is mounted in a holder which protects it from inadvertant touching or scratching. In calibration, the instrument's flow chamber is emptied, the secondary standard inserted and the reading checked. The cube simulates a turbidity of approximately 17.8 FTU. Thi': assembly is keyed to ensure repeatable positioning with each use. Long term tests (continuing) have verified the stability of the glass/ ceramic material.

CONCLUSION The modulated, four-beam turbidimeter is more than an evolution of turbidimeters whose de signs follow the recommendation of EPA Method 180.1. It provides the capability to hold its calibration for much longer time periods, along with the stability and accuracy to measure turbidities at 0.5 NTU and below. Consequently, it is better suited for turbidity applications in existing and future ·water plants. This particular design solves the problems inherent to existing designs listed earlier. The sensor body of the modulated, four-beam turbidimeter is a flow-through type with plug-in light sources and detectors which eliminate cuvettes and air/ water surfaces. Structure within the sensor is open and cleaning is simple. The sensor's stray light baffle also

G. Schrank and J. Lane Continued from page 35 overflowing and thus the turbidity in the discharge water from rising. In addition, information on the turbidity in the inflow and discharge water of settling basins (possibly backed up by sludge level monitoring), permits control strategies to be developed to ensure that these separation systems operate reliably.

FILTRATION PLANTS Filtration plants are being used increasingly in waste water purification. Their reliability and their functioning can be monitored efficiently with turbidimeters. One example 10 , is where a set of 10 filters provide the core of the water treatment plant. The quantity and quality of the treated water depends to a marked extent on the efficiency of this treatment stage. For this reason the filters are equipped on the filtrate side with turbidimeters. The limiting value triggering an alarm was set at 1.0 FTU. Filter failures are thus signaled without delay and the filter concerned cut out of the network immediately. Each filter has to be backflushed once a day by forcing filtered water upwards through the filter. This process takes 20 to 30 minutes, depending on the degree of soiling. Controlling and thus shortening this backflushing operation is advisable for three reasons: energy and product water are saved, and the operation can be interrupted when a specific residual turbidity is present in the backflushing water. Adjustment to optimum capacity is consequently much faster in the next filtering operation . In this one practical application, the operator claimed that the amount of energy and water saved by the backflushing system is about 35%. (The positive experience gained with this measuring technique has resulted in turbidimeters being installed in the inflow to the waste water treatment plant for flocculant dosage purposes. The turbidity value of the inflowing water multiplied by the flow yields the total impurity content in the untreated water. The flocculant is fed in automatically in proportion to the total impurity content).

CONCLUSION Turbidity measurement is a reliable parameter with which to supervise and control processes 11 • What is important is not to compare measuring instruments based on different measuring methods. As long as there is no international standard, differences between measuring instruments will always be found . The primary assumption cannot, however, be that measuring instruments are defective but that the characteristics of the suspended solids influence the light scattering profile thus providing a variation in the results obtained by measuring instruments using different optical geometries. Fundamentally, however, the user now has at his disposal reliable, low-maintenance instruments whose accuracy is quite adequate to the tasks concerned.

REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO. 11.

ISO 7027 (1984), Water quality - Determination of turbidity International Organization for Standa rdization. Bon fi g, K.W. , Kramp, E. (1986) . "Grundlagen and Probleme der Trubungsessung". C LB Chemie fur Labor und Betrieb, 37, No. 4, 158- 61. Simms, R.J. (1971) , Industrial Turbidity Measurement , ISA Transactions II , No. 2, 147- 154. Datasheet , product information, Monitek . Product Information, BTG. Mecrin, D. , Millot. , Audie, J.M . (1989), "Etude del'automatisation d'un reacteur physico-chimique a un biofilter", Proceedings AQTE 12th Symposium on Wastewater Treatment, Montreal (November 1989). Reinemann, L., Schemmer, H ., Tippner, M . (1982), "Turbidity Measurement for Determining Suspend ed Solids Concentration" (in German) , Deutsch e Gewasserkundliche Mitteilungen 26, No. 6, 167-174. Schrank , G. (1990), "Mebtechnik in der Abwasserbehandlung Fesmoffgehaltsund Trubungsmessung", I. Hungarian/ German Conference on Environmental Protection, Budapest (November 1990). BTG (1990), Internal test results of MET- P. Schrank, G. (1990), "Regelungssystem fur die Phasenseparation", Proceedings 4. Karlsruher Flockungstage, Karlsruhe (November 1990). Schrank, G. (1989), "Polymer dosage control for a higher efficiency of solid-liquid separation processes", Proceedings AQTE 12th Symposium on Wastewater Treatment. Montreal (November 1989).

WATER October 1991

43


TIGHTER CHLORINATION SYSTEM CONTROL by D. LITTLE and S. BIENIAK INTRODUCTION Public demand for consistently high quality drinking water, public awareness of possible harmful effects of excess chemical residue in drinking water, new regard for clean discharges to the environment and the need to minimise the wastage of chemicals to reduce plant ongoing operating costs has resulted in a requirement for improved methods of water disinfection systems control. New technology in the form of the Fischer & Porter MC5000 programmable process control station allows the above demands to ¡be met at a low installed cost in small to medium size water treatment plants.

DISINFECTION METHODS Water disinfection involves specialised treatment for the destruction of harmful and of nuisance organisms which are usually present in raw water accumulations. Chlorine is a strong disinfecting agent and has been applied in a number of forms throughout the developed world to ensure that safe drinking water is available for human consumption. Improvements in chlorine disinfection methods and chemical dispensing equipment have contributed to the widespread adoption of chlorination for the disinfection of town drinking water and waste water. The demand for chlorine is a function of both the rate of flow through the disinfection plant and the quality of the water being treated. Small but varying quantities of undesirable substances which may be either suspended in or chemically combined with the water will react with the chlorine when it is dosed into the water supply. The amount of chlorine necessary to provide a satisfactory level of disinfection will therefore vary with the type and nature of contaminents present at any point in time. Such variation is later referred to as water quality.

METHODS OF CHLORINE CONTROL Adequate disinfection requires that the chlorine introduced be in contact with the water for a period of time to allow the chemical reactions to take place. To ensure that sufficient chlorine is provided, a residual quantity of a few parts per million must be present after the necessary contact period. Effective chlorination plant control therefore requires the chlorine feed rate to vary with changes in water flow rate and water quality whilst ensuring that the desired level of residual chlorine is present at the end of a satisfactory period of contact with the water. There are several methods of introducing chlorine into water supplies including liquid injection of chlorine dioxide or sodium hypochlorite, contact with the solid powder calcium hypochlorite and injection of chlorine gas. 44

WATER October 1991

Most municipal chlorination systems utilise the chlorine gas system and there are several methods of controlling the operation of chlorine gas feeders, three of which require automatic controllers and are described below. Automatic Proportional Control (Flow Pacing) This mode of operation utilises a simple electronic controller which automatically adjusts the chlorine gas feed rate in proportion to the water flow rate, thus providing a constant, pre-established part per million dosage for all rates of water flow. No automatic adjustment is made for changes in water quality and as a consequence the residual chlorine level is not guaranteed.

David Little is National Engineering Manager for Fischer & Porter, Australia. Studied Electronics at Gordon Institute of Technology and has since studied Computer Science at North Eastern University, Boston, USA and also holds a Post Graduate Diploma in Business from Chisholm Institute of Technology. He has worked in the field of Industrial Control Engineering for 17 years, both in the USA and Australia. Stephen Bieniak is Manager of Control Systems for Fischer & Porter in the UK. Graduated from Caulfield Institute of Technology as an Electronics Engineer. Since graduation, he worked with Fischer & Porter, Australia as a Control Systems Engineer for 8 years.

Automatic Residual Control This method utilises a closed-loop control system including an on-line analyser to measure the chlorine residual value which is then compared to a set target residual value. The dose rate is automatically varied by a simple controller to adjust the measured residual value to match the required target. This system will work only if flow variations are very small and occur slowly because of the delay between dosing and resultant residual analyser readings due to the water contact time required by the chlorine to complete disinfection. A delay of up to 30 minutes can exist before a change in dosage is registered as a change in residual chlorine value and, as a consequence, a substantial change in flow rate or water quality may result in periods of either over or under chlorinated water passing into the supply system. Combined Automatic Flow and Residual Control This method is the most effective and includes the functions of items 1 and 2 to maintain a preset chlorine residual by adjustment of the dose rate for changes in both water flow rate and water quality thus giving tight control of residual chlorine levels to prevent under or over dosing and resultant cost savings through lower chlorine usage. Fig. 1 -

APPLICATIONS New microprocessor-based controllers (see Figure 1), through their programming power and versatility, allow the implementation of complex control strategies. This capability is required to implement a combined automatic flow and residual chlorination control system. Chlorine is commonly dosed either into contact tanks, or directly into pipelines. Each requires a slightly different control strategy to meet the need for tight on-line control of the quantity of chlorine used. These methods are described in the following section.

A modern microprocessor-based controller

CHLORINE DOSING INTO A CONTACT TANK Contact Tanks are designed to allow optimum water residence time for the chlorine reaction to take place. Figure 2 shows a typical contact tank configuration. The capacity of the tanks ensure that demand flow rate changes have no effect on the disinfection process as the time between a change in dose rate and the consequent change in residual chlorine concentration is quite long. The ideal method of control is


Town Water Supply

STORAGE & CONTACT TANK

ON-LINE ANALYZER

TARGET CHLORINE RESIDUAL LEVEL

Fig. 2 -

(Proportional plus Integral plus Derivative) residual feedback control loop as wide ranges of flow through the pipe make the fixed controller tuning settings established for one flow rate unsuitable for other rates. Periods of over or under chlorination result and a cycling effect can occur which may prolong the periods of operation outside of target chlorine concentration. Figure 3 is a diagram of a Chlorine Disinfection System installed in a Victorian country town water supply. Table l is a list of lag times which were measured at the site and used to establish dynamic tuning the F & P controller which was specifically programmed to precisely dose chlorine over the wide range of flow rates shown:

Contact tank chlorination

for periodic changes of dose rate to be made in proportion to the calculated deviation between a set target and the actual residual -chlorine concentration. The time between dose variations is the time required for a change in dose rate to be detected by a downstream residual chlorine analyser. Such a system may operate as follows: 1. A reading is taken from the residual analyser and compared to a target value by a microprocessor based controller. 2. If the measured residual has deviated from the target value, the dose rate is altered by an amount proportional to the error. 3. Further adjustment is delayed until a preset time has elapsed. Step l is then repeated. An improvement can be made to the above system so that the time between error calculations becomes proportional to the volume of water passing through the tank, i.e. large flow rates through the tank will result in more frequent updates. Additional chlorine may also be needed to compensate for the reduced contact time to ensure that a predetermined formula for residual level and contact time is met. 1 The flow signal can also be used to immediately trim the dose rate in response to flow changes thus avoiding the delay period for the residual feedback control loop to operate. The inputs signals needed by the microprocessor based controller are: 1. Residual chlorine measurement from an on-line residual analyser. 2. Water flow rate from a flowmeter. The Outputs generated by the Controller are: 1. Dose rate signal to chlorine dosing valve. 2. Optional re-transmission of chlorine dose rate to recorder for permanent record purposes. Digital information such as a low flow switch can also be incorporated into the controller for further refinement of the control strategy. Low and high Residual alarms generated by the controller can be transmitted to alarm systems via digital outputs.

Table 1 - Dynamic Lag Times of a Town Water Chlorination System Flowrate Main Pipe Delay Sample Line Delay Total Delay MLPD Minutes Minutes Minutes

2.20 2.65 3.31 4.41 6.62 13.32 26.46

60 50 40 30 20

10 5

4.50 4.95 6.61 6.71 8.92 15.53 28.76

Magnetic Flowmeter ChlorlnaUon Bulldlng

l

Main Line Data Diameter .75 Melres Maximum Flow • 60 MLPD

Untl1ng1h • 70Mt1rH Pipe0iamt11r. 15mm • 6LPM

SampltFlow

Dislanee of Dose to sample Point:

SO Metres to allow adequate mixing time

Sample Point

Fig. 3 -

Chlorine dosing into a pipeline

FLOW MLPD 60

40

20

CHLORINE DOSING INTO A PIPELINE The flow rate of water through a supply pipeline can vary quite quickly over a large range. Such changes in flow rate cannot be handled effectively by a standard P.I.D.

2.30 2.30 2.30 2.30 2.30 2.30 2.30

The wide range m process lag times made a residual-only feedback control loop unsuitable for this application. Since water quality generally does not change rapidly, flow rate is used as the primary source for the determination of chlorine dose rate. The controller sets the dose rate at the target proportion of the measured flow rate and uses an on-line residual chlorine analyzer signal to trim this value to the required target residual concentration to compensate for changes in chlorine demand due to water quality variations. As water flow rate variations occur, the controller responds with rapid changes to the dose rate by-passing the relatively slow residual feedback control loop. The typical flow profile over a 24 hour period is shown in Figure 4. This system operates as follows: 1. A chlorine dose rate is calculated from the measured flow rate and a preset target for concentration (i.e.: it acts as flowpaced controller). 2. The residual controller then varies the calculated dose rate in order to reach the target residual chlorine concentration. The changes are maintained within set limits, usually between 0.5 and 2.0 times the target dose rate to prevent excessively high or low chlorine concentrations. Figure 5 shows a block diagram of the control strategy. The key -to tight control and operating cost savings is the programmable capabilities of a process control station such as the Fischer & Po;ter MC5000 which is necesary to enable the loop tuning constants to be automatically recalculated with variations in, water flow rate in order to compensate for the changes in the time taken for chlorinated water to travel from the dosing point to the analyzer. Should the residual analyzer be taken out of service, the controller mode can be simply switched to manual to allow the controller to work as a Flow Paced Only controller. The Inputs to the Controller are: 1. Residual chlorine concentration measurement from an on-line residual analyzer. 2. Water flow rate measurement from a flowmeter

0

16

8

24

HOURS MIDNIGHT

Fig. 4 -

Town water flow profile

WATER October 1991 45


ENHANCE~FUNCTIONAL POSSIBILITIES

Target ConcentraUon

-Water

Flowr•te

~

Flow Raw Input

Dooo

- ~ Celcul•tlon Rate

Chlorln•tor

Delay Time Calculation

-

Manual

Tuning Conatant Calculation

--

Ruldual Chlorine Input

-

Ruld1.1al Cono•nlratlon Alarm

Ch•oll

a--8

II

Auto

Target Conoenlratlon

Chlorine

lo

~

L..._.

Ru ldual

Output

Multiplier

--

P+l+D

A••ld1.1•I Control

-

Limit

Check A••ld1.1al Cono•nlr•llon Alarm

Output lo

Recorder DLL2(33)

Fig. 5 -

L..._.

-

Flow pacing with residual trim chlorine control strategy using F & P MCSOOO programmable control station

The Outputs generated by the Controller are: I. Dose rate signal to chlorine dose valve 2. Re-transmission of chlorine dose rate to recorder. Displays on the Controller faceplate can include: I. Measured chlorine residual value

2. 3. 4. 5. 6.

Target chlorine residual Chlorine dose rate Water flow rate Total water treated Trend graphs of flow rates and residual concentration 7 . Annunciation of fault conditions.

New microprocessor-based controller technology has brought with it further possibilities including telemetry system connection to enable a central depot to communicate by either telephone line or radio link to monitor the performance of remote unmanned chlorination sites. In addition to the primary controller measurements, auxiliary inputs i ncluding remaining weight of chlorine in containers, pump status and gas leak detection monitor status can be connected to controllers to give a single source of information for all site conditions to be monitored by the central depot. Control of pumps and of auxiliary equipment is also possible from the controller.

CONCLUSION A combination of modern programmable micro-processor controllers and automatic chlorinators can provide an effective method of accurately controlling the amount of chlorine fed into water to achieve a consistent level of disinfection. The results are fewer consumer complaints, reduced consumption of expensive chlorine and reduced health risks.

REFERENCES I. US EPA/ 625/ 4-89/ 023 Technologies for Upgrading Existing or New Drinking Water Treatment Facilities.

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Minimise capital and civil costs by reducing the need for long, deep and large gravity mains. Our complete range of capacities and reputation for reliability leads to straightforward installation and operation.

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46

WATER October 1991

MONO WASTE·TEC DIVISION Phone Sydney (02) 521-5611 OFFICES IN ALL STATES.


Valve actuation solves many needs by G. L. DELANEY

The world of valves and actuators is full of paradox . On the one hand, developments in the field have resulted in control valves and actuators reaching hitherto unattainable levels of reliability, economy and accuracy. On the other hand, the ordinary user often knows little about actuators and lacks confidence to specify them correctly - so he avoids the use of actuated valves. An example of this dichotomy is the paper manufacturer who installed pneumatically operated valves in a tall oil plant. These actuated Saunders Valves were cycling from open to closed every 10 seconds or so. The plant engineer was extremely happy with their performance. In the same room were about a dozen manually operated valves at roof height. When it was suggested that in view of his satisfaction with the actuated valves that he might retrofit actuators to these valves also, he was genuinely taken aback. "It ¡ never occurred to me," he said. "We don't operate them very often." He seemed to have missed the point. Obviously, these valves were not used very often, but just imagine the inconvenience caused when they had to be operated. Again, what would happen in an emergency! It is estimated that only 40/o of the valves in industry are actuated, and of these only 200/o are actuated because they are out of reach. These figures may be surprising, but any user who reflects upon them even in their own situation will find that the figures are not too far out. Why is a lever or wheel fitted to a piece of equipment which noone can reach? One piece of faulty reasoning could be: "We know the valves will be difficult to operate if we ever need to do so, but since we seldom, if ever, need to operate the valves, the difficulty doesn't come into it and the extra cost of fitting an actuator is not justified." This argument is faulty because if the valve genuinely is never operated, it is not needed at all. If a valve is th_ere, the presumption must be that it will need to be operated sometime and therefore that same thought must be given to the method of operation when choosing or specifying the valve. Another flaw in the argument against actuation is that it leaves out the safety aspect. Retrofitting an actuator to a manually operated valve, or choosing and fitting an actuated valve in the first place improves safety in at least four ways: speed, reliability, remote control and maintenance. If it is necessary to operate any valve to prevent, reduce or extinguish a fire, speed is vital. In terms of reliability, pneumatically operated valves normally store enough energy at the valve to close or open in an emergency, either by switching off the actuating air or by interrupting it as a result of the emergency. The ability of actuated valves to be remotely controlled can greatly increase safety, particularly in an emergency. If a plant is handling hazardous, toxic or flammable material, the only certain way to avoid injury to people is to remove them t? a safe location from whence the plant can be controlled. Actuation avoids the need to reach the valve. In many cases, operators need not be climbing ladders or balancing on pipework to close or open seldom-touched manual valves. It is for all of the above reasons that Saunders Valves is taking a pre-eminent position in providing plants with valve-actuated packages, or retrofiting actuators to existing valves. Because of Saunders' modular approach to its diaphragm valve business, Saunders actuators in most cases will fit its valves in pipework installed 10 to 20 years ago. There is no need to replace them. Saunders has the actuators to automate its range of diaphragm valves - from DN 8 right up to DN 200, failsafe closing or opening, as well as double acting. The actuated valve package is becoming more popular in the water industry as engineers become more familiar with actuators and understand the role they can play in automating their process. One such plan is the Engineering & Water Supply (E&WS) installation at Glenelg in South Australia which has just undergone an extensive upgrade to improve its efficiency, cut operating costs and improve the working conditions for its plant staff, and promote safety. 48

WATER October 1991

Glenn Delaney is General Sales Manager, Saunders Valves Australia Pty Ltd. He graduated in Metallurgy from University of Woollongong, NSW.

Saunders valves have been used on a variety of services throughout the plant including raw sewage, effluent, scum, sludge, air and water duties. For over 30 years, maintenance has been minimal except for the replacement of rubber diaphragms, and there was very little evidence of erosion of the Type KB bodies of the valves, despite the presence of sand in the lines. In the upgrade, designed skilfully by the site engineers, some 30 Saunders valves and actuators were used . Additional safety was provided in hazardous areas with add-on electrics such as explosionproof micro-switches and solenoid valves. With the upgrading program underway for this and other treatment plants to improve their efficiencies, those equipped with diaphragm valves and particularly those with actuators, are finding the overall cost far lower than those with other types of valves. There are two pressing reasons to upgrade these older plants environmental and costs. Automation brings about environmentally safe handling methods. It eliminates human error as well as ensures the environment is kept safe for workers. Automation also cuts down manual handling of various proces~es and this in tum cuts down costs. Also, as discussed earlier, there is much to be gained by taking a pro-active stance for those valves in hard-to-reach locations, or isolated areas. Occupational Health & Safety pressure may be the catalyst needed to examine a plant and highlight key danger areas. With better performing materials now available and used in the present generation of Saunders diaphragm valves compared to 30 years ago, these valves are expected to play their vital role in the treatment cycle for the extended lifetime of these plants for many more years to come.

Simple retrofit: ES Actuators can be installed in-situ.

Automated Valves in Glenelg aeration tanks, Inset: Double Acting ES Actuator.

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Water Journal October 1991  

Water Journal October 1991