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LIFE CYCLE COST ASSESSMENT OF ONE SMALL AREA OF PROCESS. This site has three 2400m3 Mesophilic digesters which are fed 24 hours a day with raw sludge, typical dry solids content (D/S) of between 6 & 8%. These are heated using spiral heat exchangers with a primary heat source of around 60 deg C. Pumps then re circulate sludge through the heat exchangers to maintain a constant heat within the digesters. This temperature should be maintained at 35 deg C ± 1 deg C for process requirements and must not fall below 32 deg C to ensure HACCP compliance. The previously installed screw centrifugal pumps frequently failed due to the amount of rag and grit in the sludge. Seal failure was common with one pump being replaced on average every 4 - 6 weeks. The rag would also cause blockages within the heat exchangers. These required weekly cleaning to remove trapped rag from within. This caused operational process issues, trying to maintain the required throughput of the site. If the temperature on any digester dropped below the Critical Control Point, this would inhibit the digester feed, reducing the site throughput by one third. Monthly maintenance costs on these pumps were around £2K each. Plus labour costs for contractors and overtime payments to staff around £1K.

With this high level of breakdowns, it was necessary to have a fourth digester recirculation pump as a spare. Even with this there were times when more than one pump would fail, resulting in panic changing pump of positions as the temperatures within the digesters fell. This would incur even higher costs, due to callout of Engineers and Electricians, day or night. P&M Pumps, of Sawtry, were asked to look at a possible solution to this issue. The immediate recommendation was to install a HF4P6 15 KW Vaughan Chopper Pump, the

objective, to ensure the required flow, yet would be able to cope with the rag and grit within the system. The decision was taken to install the first pump in December 02. Immediately the site operators reported how quiet the Vaughan was in comparison to the other pumps. As a maintenance process, operators continued to open up the heat exchangers. However they soon found that build up of rag was almost zero. It was also very quickly recognized, the digester temperature stabilised and remained so. This being now totally due to the ability of the heat exchangers to transfer the heat more efficiently, without blockages or pump failures. Heat exchanger operational maintenance now consists of an annual inspection. There are no issues with rag building within these units. The second and third pumps have also been installed and have now been running for over 30 months with no requirement for reactive maintenance. Costs for this application - Pre and Post. Historic costs: Maintenance costs for Digester Recirculation Pumps

£2,000.00 per month

Labour costs on callouts for repairs

£1,000.00 per month

Weekly cleaning of Digester heat exchangers

£80.00 per month Total

Total per annum

£3,080.00 per month £36,960.00

The tables below show Life Cycle Costs (LCC) for the screw centrifugal pumps against the Vaughan pumps retrofitted on this application over a two year period.

Cost of Purchase plus maintenance requirements on 3 Screw centrifugal pumps Purchase price on Screw centrifugal pumps x 3, £9,000, 11%

Reactive maintenance costs year 2, £36,960, 45%

Reactive maintenance costs year 1, £36,960, 44%

Project costs:

Purchase price on Screw centrifugal pumps x 3 Reactive maintenance costs year 1 Reactive maintenance costs year 2

Cost of HF4P pump

£8,000.00 each

Less discount of 15%

£6,800.00 each x 3

Cost for installation £5K per pump for electrical and pipe work modifications

£15,000.00 total

Total approximate project cost


Payback period less than one year for this project. Savings against previously installed Screw pumps in the same 2 year period. £38,520, 52%

Retrofit Project Costs

Installation Costs, £15,000, 20%

Purchase cost of 3 X HF4P Vaughan Pumps

Purchase cost of 3 X HF4P Vaughan Pumps, £20,400, 28%

Installation Costs

Zero reactive maintenance costs, Savings against previously installed Screw pumps

The above shows how accurate costing of plant ownership can be used to identify cost effective retrofits for problem areas with good payback periods. We expect there to be minimal maintenance requirements on the Vaughan pump for the first 3 years. Thereafter up to year 5 we would be happy to use a maintenance factor of approximately 25% of purchase price. This would allow for the replacement of the main wearable parts within the Vaughan pump.

These are:

Impellor Cutter bar Upper cutter Cuter nut

£1288.00 £1154.00 £290.00 £48.00



All of these prices are subject to discount off list. Therefore total cost £2363.00 Payback on this project in year 1

Expected budget savings in years 2 to 5 including spares:


These pumps have been running now for over 24 months with no maintenance issues at all. With this in mind this company has already seen a cost benefit to the sum of over: £38,000.00 We see no reason why this should not continue through years 3 to 5 and beyond. As we mentioned earlier this site is always under pressure to ensure it is able to process sludge for its catchments. One of the ways this site has been able to operate in excess of its design throughput when required, is to ensure the digester feed is above 7% D/S. At 8% D/S this can result in 114% of the sites target for throughput.

The Belt Thickeners were able to produce this but the discharge pumps were unable to pump this without excessive wear resulting in extremely high maintenance costs.

The Aqua belts are used to thicken sludge pre digestion. The discharge pumps were one of the factors that reduced throughput due to reduced capacity caused by frequent wear to the stators and rotors. To maintain the required though put on site these had to be rebuilt with new stators and rotors on average every 2 months (At an approx cost of £2.8K ea). In addition to this, the discharge lines suffered from fat build up on the walls of the pipe work. These had to be jetted every 6 to 8 weeks. This consisted of removal of lengths of the pipe work and then using high pressure jetting equipment. This would require 3 men for 4 hours to complete this task. The pipe work would reduce in size from 4� to 1.5� in about 10 days due to the fat adhering to the pipe walls. Twice these PC pumps had been upgraded to try and prolong the periods between replacement of stators and rotors. As the pumps wore, the ability of them to pump the sludge at this D/S reduced, until the site operators were unable to process the required throughput. To ensure this site was able to run effectively the decision was made to upgrade the belt discharge pumps and pipe work, including the control systems, against all historical method, to trial a Vaughan Chopper Pump, a centrif pump, not a positive displacement. For the selected pump , P&M made simple recommendation to ensure the Vaughan pump would operate correctly, the discharge hopper would have to increase in size. This ensured there was sufficient sludge available for the pump to maintain the constant flow being processed by the Aqua belts. The new system was fully automated, incorporating a Variable Speed Drive, an ultrasonic level gauge to control the speed of the pump was also installed. This removed the requirement for the manual variable speed gearboxes which were also very high maintenance cost items. The first of the two HF3L Vaughan pumps was installed and commissioned in early December 04. Since then, to February 06, these have been operating, with no mechanical issues and no change in the discharge pressure. This led us to look at the fat issues within the pipe work as it had run for 16 weeks with no pressure change. The old system would

have been jetted twice and the pump would have been rebuilt at least once in the same period. Inspection of the pipe showed that there is fat building on the pipe wall, but the consistency is completely different than previously. Now it is very soft. Historically it used to be very hard and difficult to remove. Now the site operators can simply clean using hoses on the pipe work. This is caused by the pump emulsifying the fat within the sludge. They are now running trials on a 20 week run and are considering putting heat trace on the first three metres of pipe as this is the only area that seems to be affected. We feel that if we could get to the stage where they are only cleaning this pipe work every six months this would be very successful.

Site operations now spend very little time nursing this process. They previously were locked into considerable time adjusting pump speeds, constantly trying to optimize the process. This pump is now self-optimizing on level control. They are able to spend time completing other tasks to improve the site. Costs for this application Pre and Post. Historic costs:

Pump rebuilds on average every 6 weeks

£2,800.00 ea

Cleaning of discharge pipe work 3 men 4 hours

£240 each

At £20 per hour per man. Total maximum cost per machine every 6 weeks Total per annum Project costs:

Cost of Vaughan HF3L Pump

£3,040.00 £24,320.00 £6,000.00

Cost of other equipment plus contractors charge £13,000.00 Not all of these installation costs were as a direct result of installing the Vaughan pump. They include

pipe work modifications requested by the client.

Total cost of upgrade

£19,000.00 ea

This installation using these costs had a payback on investment period of approximately 9 month’s. This gave operational savings on the budget of £5,320.00 per machine in the first year. Total budget saving in year one:

£5,320.00 per machine

On every month thereafter an additional minimum budgetary saving of £2,000.00 should be seen. We expect there to be minimal maintenance requirements on the Vaughan pump for the first 3 years. Thereafter up to year 5 we would be happy to use a maintenance factor of approximately 25% of purchase price. This would allow for the replacement of the main wearable parts within the Vaughan pump. These are:



Cutter bar


Upper cutter


Cutter nut




All of these prices are subject to discount at 15% off list. Therefore Total Cost £1,647.30

There are other financial factors that I am unable to calculate, although they are real savings that have increased the cost effectiveness of this installation.

Some of these are: 1. Increased availability of equipment. Now duty standby, not duty assist. 2. Minimal downtime of the equipment. 3. Ability to pump sludge at the required D/S and throughput (up to 9% @ 90m3h). 4. Reduced callouts of M&E staff for reactive breakdowns. 5. Operational staff being able to optimize the process with minimal supervision. Expected budget savings in year 1


Expected budget savings in years 2 to 5


Total expected savings in 5 years plus factors 1-5 above


Looking at the detail within this paper it is clear that if we strategically place the Vaughan systems supplied by P&M Pumps, we are able to see many improvements: 1. Improved availability. 2. Reduced breakdowns in process equipment. 3. Ability to process sludge containing rags and other solids effectively. 4. Allows other process equipment to operate more efficiently. 5. Reduces operators intervention, cleaning etc. 6. Effective and efficient mixing using dual zone bi-rotational mixing. 7. Cost savings to budgets.

The more commercial the approach to our process, the more aware we have been to profit. After all, the process of sludge through any dedicated area is a business. The way in which equipment is chosen is changing, life costs are a real factor in today’s environment. It is no longer acceptable to choose equipment based on purchase price only. If it consequently costs more to run and maintain, then this equipment is not cost effective. As we have seen within this document those areas that are high cost in maintenance and, process, can become very cost effective by using the correct equipment in the correct areas. This is where a working partnership with P&M Pumps has created a profitable sludge process regime.

Millbrook data final paper  

The rag would also cause blockages within the heat exchangers. These required weekly cleaning to remove trapped rag from within. This caused...

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