Analysis Report On Pumps Introduction Fluid flow is a very important phenomenon in industrial sectors and residence. So transfer of fluids has been of manâ€™s interest from the ancient ages. The pumps are the oldest devices known to mankind which are used to lift or transfer fluids from low pressure region to high pressure region. It converts electrical or mechanical energy to the pressure or velocity gradient of a fluid. There are various tactics for fluid flow through a conduit such as action of centrifugal force, volumetric displacement, mechanical impulse, transfer of momentum from another fluid, electromagnetic force, gravity etc. Of these centrifugal force is most effective. Pumps are the devices which use centrifugal force to transfer a large amount of momentum into the fluid that cause to fluid flow. Various kinds of pumps are used in different industries for different purposes. It is essential for an engineer to have some basic knowledge about the classification, construction, advantages and disadvantages of different kinds of pumps. Also he should know several specific characteristics of the pump. These are the capacity, the energy or head supplied to the fluid, the power required to run the pump and the efficiency of the unit. These characteristics help an engineer to select the right worthy pump for industrial and commercial purposes. When choosing or designing a pump for a definite purpose the designer must know about some general problems in operating the pump such as priming and cavitations. Above all, it is necessary to have a clear concept about the physical phenomena and operating system to select a suitable pump for the definite purpose. Pumps: Pumps are fluid motive device and used to transfer liquid from low-pressure zones to high pressure zones. Pumps are also used to move liquids from a low elevation into a higher elevation, and to move liquids from one place to another. It converts electrical or mechanical energy into pressure or velocity gradient. Classification: Pumps may be classified on the basis of the application they serve and more basic system of classification first defines the principle by which energy is added to the fluid, goes on to identify the means by which the principle is implemented, and finally delineates specific geometries commonly employed. Under this system all pumps may be divided into two major categories: 1) Positive displacement pumps 2) Kinetic pumps Positive displacement pumps are of two types a. Reciprocating b. Rotary Kinetic pumps are of three types a. Centrifugal
b. Axial c. Mixed flow The classification of pumps is given in the following page in a tabular form. Pumps
Positive displacement pumps (PDPs)
Kinetic/momentum changes pumps
Axial Figure 1: Classification of pumps Mixed flow
Positive displacement pumps A positive displacement pump causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe or a positive displacement pump has an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation.
Figure 01: A positive Displacement pump
A positive displacement pump can be further classified according to the mechanism used to move the fluid: a. Reciprocating pumps This type of pump adds energy to a fluid system by means of piston acting against a confined field. Reciprocating pumps are of two kinds – • Plunger pumps • Diaphragm pumps The advantages and disadvantages of this pump are discussed here: Advantages Practically well suited for pumping viscous fluids because the high rate of shear acting on the cylinder walls serve as an additional packing. It is good for attaining high pressures. This is used for metering fluids because of its positive displacement characteristics. Disadvantages • •
Liquids containing abrasive solids should not be pumped with a reciprocating pump because of damage to the machine surface. This type of pump has odd size, high maintenance and high initial cost.
b. Rotary pumps Positive displacement rotary pumps are pumps that move fluid using the principles of rotation. The vacuum created by the rotation of the pump captures and draws in the liquid. Rotary pumps are very efficient because they naturally remove air from the lines, eliminating the need to bleed the air from the lines manually. This is a kind of positive displacement pumps, which traps a quantity of liquid and moves it towards the discharge point. Rotary pump operate best on clean, moderately viscous fluids such as light lubricating oil. Positive displacement rotary pumps can be grouped into three main types – Gear pumps Screw pumps Moving vane pumps The advantages and disadvantages of this type of pump are discussed here Advantages • This pump is capable of delivering a nearly constant capacity against all pressures within the limits of the pump design. • This is capable of pumping fluids of all viscosities. • They typically have larger flow capacity and operate at low pressures. • The diaphragm contacts the fluid, eliminating contamination from the drive elements.
Disadvantages • The fluid must be free of abrasive materials to avoid the damage of machine parts. • They deliver a pulsating flow to the output because each functional element moves a set, captured volume of fluid from suction to discharge. Kinetic pumps Kinetic pumps or Rotodynamic pumps (or dynamic pumps) are a type of velocity pump in which kinetic energy is added to the fluid by increasing the flow velocity. This increase in energy is converted to a gain in potential energy (pressure) when the velocity is reduced prior to or as the flow exits the pump into the discharge pipe. This conversion of kinetic energy to pressure can be explained by the First law of thermodynamics or more specifically by Bernoulli's principle. Dynamic pumps can be further subdivided according to the means in which the velocity gain is achieved. These types of pumps have a number of characteristics: Continuous energy Conversion of added energy to increase in kinetic energy (increase in velocity) Conversion of increased velocity (kinetic energy) to an increase in pressure head One practical difference between dynamic and positive displacement pumps is their ability to operate under closed valve conditions. Positive displacement pumps physically displace the fluid; hence closing a valve downstream of a positive displacement pump will result in a continual build up in pressure resulting in mechanical failure of either pipeline or pump. Dynamic pumps differ in that they can be safely operated under closed valve conditions (for short periods of time). It can be classified into two categories• Special effect pumps •
Special effect pump This type of pump is of four classes; those are jet or ejector type, gas lift, hydraulic ram, and electromagnetic type. The jet pumps are used for household water systems, are composed of a centrifugal pump along with a jet or ejector assembly. The gas lifter is used for lifting gas from deep. The hydraulic ram depends on the hydrodynamic action of the propeller blades to lift and accelerate the fluid axially along a path parallel to the axis of the propeller. The electromagnetic pumps have no moving parts so that no seals of any type are required. It works on the same principle of induction motor. Now some advantages and disadvantages of this type of pump will be discussed here: Advantages In the jet pump the jet issuing from the nozzle creates a vacuum behind it, which causes well water to be drawn up along with the jet. Liquid metals having high electrical conductivity are pumped with this device.
Disadvantages When the gas-lifting pump is used we must take care of that the gas pressure is in control, otherwise it could be harmful. Centrifugal pump Centrifugal pumps are most widely used pumps in the chemical and petroleum industries. In a centrifugal pump pressure is generated dynamically. The impeller of the pump is coupled with a motor or driver. Fluid entering the pump gains additional energy from the rotation of the impeller. Later on, this centrifugal energy is converted to pressure energy at the pump discharge.
Figure 02: A centrifugal pump Advantages It is simple in construction and can, therefore, be made in a wide range of materials. There is a complete absence of valves in this pump. It operates at high speed (up to 100 Hz) and therefore can be coupled directly to an electric motor. In general, the higher the speed the smaller the pump and motor for a given duty. It gives a steady delivery. Its maintenance costs are lower than for any other type of pump. No change is done to the pump of the delivery line becomes blocked, provided it is not run in this condition for a prolonged period.
It is much smaller than other pumps of equal capacity. It can, therefore, be made into a sealed unit with the driving motor and immersed in the suction tank. Disadvantages The single stage pump will not develop a high pressure. The multistage pumps will develop greater heads but they are very much more expensive and readily are made in corrosionresistant material because of their greater complexity. It is generally better to use very high speeds in order to reduce the number of stages required. It operates at a high efficiency over only a limited range of conditions. This applies specially to turbine pumps. It is not usually self-priming. If a non return valve is not cooperated in the delivery f suction line, the liquid will run back into the suction tank as soon as the pump stops. Very viscous liquids cannot be handled effectively. Comparison between positive displacement pumps and kinetic pumps Kinetic pump generally provide a higher flow rate than PDPs and a much steadier discharge but are ineffective in handling in high viscosity liquids. Kinetic pump also generally need priming. On the other hand PDP is self priming for almost any application. A PDP is appropriate for high pressure rise and low flow rate while a dynamic pump provides high low rate with low pressure rise. Pressure rise or head increase
Positive displacement pump High Îź Low Îź Dynamic pump
Figure 03: Comparison of performance curves of dynamic and PDPs at constant speed Table 01: A table on Comparison among different pumps
Parameter Centrifugal Pumps Reciprocating Pumps Optimum Flow and Medium/High Low Capacity, Pressure Applications Capacity, High Pressure Low/Medium Pressure Maximum Flow Rate (gallon per minute) Low Flow Rate Capability Maximum Pressure (pound per square inch) Requires Relief Valve Smooth or Pulsating Flow Variable or Constant Flow Self-priming Space Considerations
Rotary Pumps Low/Medium Capacity, Low/Medium Pressure 10,000+ GPM
No Requires Less Space
Yes Requires More Space
Lower Initial Lower Maintenance Higher Power
Higher Initial Higher Maintenance Lower Power
Yes Requires Less Space Lower Initial Lower Maintenance Lower Power
Suitable for a wide range including clean, clear, non-abrasive fluids to fluids with abrasive, high-solid content. Not suitable for high viscosity fluids Lower tolerance for entrained gases.
Suitable for clean, clear, non-abrasive fluids. Specially-fitted pumps suitable for abrasive-slurry service. Suitable for high viscosity fluids Higher tolerance for entrained gases
Requires clean, clear, non-abrasive fluid due to close tolerances Optimum performance with high viscosity fluids Higher tolerance for entrained gases
Selection of pumps Although centrifugal pumps have been standardized, the standards leave enough freedom for each pump to be specified and designed to meet an individual requirement. Consequently, selecting a centrifugal pump can be a complex problem for many engineers not yet briefed on how to relate all the pertinent data. This problem is resolved simply by organizing the data according to the relation of the dependent and independent variables. With centrifugal pumps, the relation follows the order: characteristics curves, impeller design, and number of stages, net positive suction head, volute design, diffuser design, and mounting. Proper pump selection can reduce start-up and operating problems, as well as future maintenance cost. The keys to pump selection are: 1) A detailed listing of fluid characteristics is usually the first step in selection. The most important characteristics are:
PH, dissolved oxygen concentration. Historic data acquired during previous handling. Absolute viscosity of pumping temperature and kinematic viscosity. Specific gravity of fluid. Pumping temperature during normal operation and vapor pressure of the fluid measured at the pumping temperature. A number of items that make up the ‘personality’ of the fluid. These include toxicity, explosive nature, crystallizing or polymerizing tendencies, and the presence of entrained gases. 2) After the nature of the fluid has been recorded, the hydraulics of the system must be studied. Pertinent factors are: Pressure. Static head. Velocity head. Suction lift. Total discharge head. Head losses. 3) Other factors are: The type of power source (electric motor, diesel engine, steam turbine, etc.). Space weight and position limitations. Environmental conditions. Cost of pump purchase and installation. Cost of pump operation. Governing codes and standards. Centrifugal pumps A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through piping. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits into the downstream piping. Centrifugal pumps are used for large discharge through smaller heads. Types of Centrifugal Pumps Centrifugal Pumps have been classified in many types. Some of them are worth mentioning. • • • • •
End Suction Pump In-Line Pump Double Suction Pump Vertical Multistage Pump Horizontal Multistage Pump
• • • •
Submersible Pumps Self-Priming Pumps Axial-Flow Pumps Regenerative Pumps
Depending on the construction centrifugal pumps may be classified into two major classes:
Single-stage centrifugal pump Multi- stage centrifugal pump
Single-stage centrifugal pump A centrifugal pump consisting of only one impeller is referred to as single-stage centrifugal pump. It is constructed of single-stage pedestal mounted unit with a single impeller and a single packing box. There are different types of single-stage centrifugal pumps which are briefly discussed below: Double suction single-stage pumps are of different operating units made of iron or bronze to flow liquids that are non-corrosive to iron or bronze are being handled. Closed couple single-stage pump equipped with a built-in electric motor or sometimes steam-turbine driven (i.e., with a pump impeller and driver on the same shaft). Canned motor pumps are closed couple units in which the cavity housing the motor rotor and the pump casing are interconnected. Multi-stage centrifugal pump While two or more impellers are arranged in such away that the discharge from one impeller enters the eye of the next impeller, it is referred to as multi-stage centrifugal pump. These pumps are used for services requiring heads (pressure) higher than can be generated by a single impeller. That means, if the total head-capacity combination to be developed is greater than that which can be developed from a single impeller, multi-stage operation is used. Multi-stage pumps may be thought of as being several stages pumps on one shaft with flow in series. Multi-stage pumps employ several impellers on a common shaft and the casing directs the discharge from the periphery of one impeller to the suction of the next stage where the discharge pressure of the first stages preserved. The fluid after entering the second stage will have added to it the pressure energy developed in the stage and so on. Impeller type of the multi-stage centrifugal pump: Diffuser type impellers are used in the multi-stage centrifugal pumps. Multi-stage impeller pumps have substantially increased heads over single-impeller types. There are five rotating impellers in sequence, with the intake from the lower hole on the left and the outlet at the lower right. The upper two holes connect to the lower holes, balancing intake and outlet pressures on the shaft.
The liquid swirls (not shown here) with the rotation of the pump. As many as 12 separate impellers are often connected together to increase flow rates and head. In many models the
fluid enters at either end and works towards the middle, or enters one end and immediately after the first impeller, is sent to the opposite end to the second impeller, then back to the first end, and so on, exiting in the middle. The advantages of such a system are improved pressure balance on the shaft and relatively low pressure around the seals. Figure 04: Sketch of a single-stage centrifugal pump (semi sectional view)
Figure 05: A cut way view of six-stage centrifugal pump Uses of different types of Centrifugal pump Double suction single stage centrifugal pump: These pumps are used for general water supply and circulating services and for chemical services when handling liquids that are noncorrosive to iron or bronze. Canned motor centrifugal pump: They are widely used for handling organic solvent, organic head transfer liquids and light oils as well as many clean toxic or hazardous liquids or for installation in which leakage is an economic problems.
Closed coupled centrifugal pump: These pumps are extremely compact and are suitable for a variety of services where standard iron and bronze materials are satisfactory. Multi stage centrifugal pump: These pumps are used for services requiring higher heads (pressure), and then can be generated by single stage pumps. Such services include higherpressure water supply pumps, fire pumps, boiler feed pumps and change pump for refinery process. Working principle of a centrifugal pump A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes are occurred by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy. Liquid enters the pump inlet i.e., eye of the impeller, gets energized by virtue of centrifugal impeller action and is then discharged via the pump outlet. The energy imparted upon the liquid is a direct result of the impeller action which is then translated into an increase in velocity of the fluid. It is to be noted that: a) With a centrifugal pump the outlet can be briefly closed while the pump is running and no damage to the pump will result. b) Depending upon the pumping arrangement a check valve must be installed to prevent siphoning. c) No pressure relief valve is necessary to protect the pump and piping system. It is noteworthy that, all of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head. Generation of centrifugal force The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller, a low-pressure area is created causing more liquid to flow towards the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string. Conversion of kinetic energy to pressure energy The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid. This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulliâ€™s principle.
The figure shows diagrammatically how the liquid flows through a centrifugal pump. The liquid enters axially at the suction connection, station (a). In the rotating eye of the impeller, the liquid spreads out radically and enters the channels between the vanes at station
Figure 06: Flow of fluid in a centrifugal pump 1. It flows through the impeller, leaves the periphery of the impeller at station 2, is collected in the volute and leaves the pump discharge at station b. The performance of the pump is analyzed by considering separately the three parts of the total path: first, the flow from station (a) to station 1; second, the flow through the impeller from station 1 to station 2; and third, the flow through the volute from station 2 to station b. Vector diagram The vector diagram in the following page represents the various velocities at the station 1 and station 2 at the entrance and exit of the vane, respectively. Considering the vectors at station 2, by virtue of the design of the pump the tangent to the impeller at its terminus makes an angle β2 with the tangent to the circle traced out by the impeller tip. Vector v2 is the velocity of the fluid at point 2 as seen by an observer moving with the impeller and is therefore a relative velocity. Two idealizations are now accepted. It is assumed, first, that all liquid following across the periphery of the impeller is moving at the same speed, so the numerical value (but not the vector direction) is v2 at all points; second, it is assumed that the angle between the vector v 2 and the tangent is the actual vane angle β2. This assumption in turn is equivalent to an assumption that there are an infinite number of vanes of zero thickness at an infinitesimal distance apart. This ideal state is referred to as perfect guidance. Point 2 at the tip of the blades is moving at peripheral velocity u 2 with respect to the axis. The vector v2 is the resultant velocity of the fluid stream leaving the impeller as observed from the ground. It is called the absolute velocity of the fluid. The angle between V2 and u2 is denoted by α2. A comparable set of vectors applies to the entrance to the vanes at station 1 as shown in figure-4(a). In the usual design α1 is nearly 90o and vector V1 can be considered radial. Figure- 4 is the vector diagram for point 2 that shows the relations between the various vectors in a more usual way. It also shows how the absolute velocity vector V2 can be resolved into components, a radial component denoted by Vr2 and a peripheral component denoted by Vu2
The basic equations:
(a) Vectors and Vanes
(b)Vector Diagram at Tip of Vane
Figure 07: Velocities at entrance and discharge of vanes in centrifugal pump The basic equations relating the power, developed head, and capacity of a centrifugal pump are derived for the ideal pump from fundamental principles of fluid dynamics. Since the performance of an actual pump differs considerably from that of an ideal one, actual pump are designed by applying experimentally measured corrections to the ideal situation. 1) Power equation of a centrifugal pump The power input to the impeller, and therefore the power required by the pump can be calculated from the angular momentum equation for steady flow. Then,
Tg e = m r2VU 2 − r1VU1
The momentum correction factors are unity in view of the assumption of perfect guidance. Also, in radial flow, where α = 90o, Vu = 0. At the entrance, therefore, r1VU = 0 , the second 1
term in the above equation vanishes and so,
Tg c = mr2VU 2
Since, P = Tω, the power equation for an ideal pump is,
Pfl g c = m ω r2VU 2
Where, the subscript ‘fl’ denotes a frictionless pump.
2) Head-flow relations for an ideal pump For an ideal pump, And therefore, Since, Then,
P = Pfr = m ∆H
r2Vu 2ω gc ωr2 = u 2 uV ∆Η = 2 u 2 gc ∆Η =
From Figure (3b),
Vu 2 = u 2 −Vr 2 / tan β2
∆Η r =
u 2 (u 2 − Vr 2 / tan β 2 ) gc The volumetric flow rate qr through the pump is given by, q r =Vr 2 A p
Where Ap is the total cross-sectional area of the channel around the periphery. u 2 (u 2 − q r / A p tan β 2 ) ∆Η r = Then, gc
Since u2, Ap, and β2 are constant, the relation between head and volumetric flow is linear. The slope of the head-flow rate line depends on the sign of tanβ2 and therefore varies with angle β2. If β2 is less than 90°, as is nearly always the case, the line has negative slope. Flow in a piping system may become unstable if the line is horizontal or has a positive slope. 3) Head-work relation for an ideal pump The work done per unit mass of liquid passing through an ideal pump is, Pfr ω W pr = o = r2Vu 2 g c m A Bernoulli equation written between station 1 and 2, assuming no friction, neglecting Z a-Zb and assuming perfect guidance gives p 2 p 2 V 22 V12 ω r2Vu 2 = − + − gc ρ ρ 2g c 2g c Also, Bernoulli equation written between station a and 1 and b and 2, respectively, are p a α a V a2 p V2 + = 1+ 1 ρ 2g c ρ 2g c p α V2 p 2 V22 + = b + b b ρ 2gc ρ 2g c
Adding three Equations gives,
p b α b Vb2 p α V2 ω + = a + a a + r2Vu 2 ρ 2g c ρ 2g c gc
ω r2Vu 2 = W pr gc ∆H=∆Hr and Wp=Wpr.
Equation can be written in the form ∆H r = H b − H a = For a frictionless or ideal pump, where, η=1, Actual performance of a centrifugal pump:
The developed head of a centrifugal pump is considerably less than the calculated from the ideal pump relation. Also the efficiency is less than unity and the fluid horsepower is greater than the ideal horsepower. General components of centrifugal pump A centrifugal pump has two main components: 1) A stationary component comprised of a casing, casing cover, and bearings. 2) A rotating component comprised of an impeller and a shaft.
Figure 08: Different Parts of a Centrifugal Pump The main components are discussed in brief below.
1. Stationary components â€˘
Centrifugal pump casings may be of several designs, but the main function is to convert the velocity to the fluid by the impeller into useful pressure energy. In addition to this, the casing serves to contain the fluid and to provide an inlet and outlet for the pump. Casings are generally of two types: volute and diffuser. The impellers are fitted inside the casings. Volute casings build a higher head; circular casings are used for low head and high capacity. A volute is a curved funnel increasing in area to the discharge port. As the area of the crosssection increases, the volute reduces the speed of the liquid and increases the pressure of the liquid.One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturer's recommended capacity. Running volute-style pumps at a lower capacity than the manufacturer recommends can put lateral stress on the shaft of the pump, increasing wear-and-tear on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial thrusts become significant at reduced capacities.
. Figure 09: Volute casing inside a centrifugal pump Diffuser casing have stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Diffusers are interposed between the impeller discharge and the casing chamber. Conventionally, the diffusers are applied to multi-stage pumps. The casings can be designed either as solid casings or split casings. Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. Split casing implies two or more parts are fastened together. When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller. â€˘
Suction and Discharge Nozzle
The suction and discharge nozzles are part of the casings itself. They commonly have the following configurations. 1. End suction/Top discharge - The suction nozzle is located at the end of and concentric to, the shaft while the discharge nozzle is located at the top of the case perpendicular to the shaft. This pump is always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the impeller eye. 2. Top suction / Top discharge nozzle -The suction and discharge nozzles are located at the top of the case perpendicular to the shaft. This pump can either be an overhung type or between-bearing type but is always a radially split case pump. 3. Side suction / Side discharge nozzles - The suction and discharge nozzles are located at the sides of the case perpendicular to the shaft. This pump can have either an axially or radially split case type. â€˘ Seal chamber and Stuffing box Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control. Gland: The gland is a very important part of the seal chamber or the stuffing box. It gives the packing or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports. Throat Bushing: The bottom or inside end of the chamber is provided with a stationary device called throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller. Throttle Bushing refers to a device that forms a restrictive close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland. Internal circulating device refers to device located in the seal chamber to circulate seal chamber fluid through a cooler or barrier/buffer fluid reservoir. Usually it is referred to as a pumping ring. â€˘ Bearing housing The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water. 2. Rotating Components
Impeller The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. The number of impellers determines the number of stages of the pump. A single stage pump has one impeller only and is best for low head service. A two-stage pump has two impellers in series for medium head service. A multistage pump has three or more impellers in series for high head service. There are different types of impellers: • • •
Straight vane single-suction closed impeller Double-suction impeller Nonclogging impeller
• • •
Semi open impeller Open impeller Mixed-flow impeller
Closed impeller Closed impellers require wear rings and these wear rings present another maintenance problem. The fluid enters the eye of the impeller where the vanes add energy to the fluid and direct it to the discharge nozzle. There is no impeller to volute or back plate clearance to set. Wear rings restrict the amount of discharge fluid that recalculates back to the suction side of the impeller. When this wear ring clearance becomes excessive the ear rings must be replaced. Closed impeller can compensate for shaft thermal growth, but if there is too much axial growth the vanes may not line up exactly with the discharge nozzle. It is good for volatile and explosive fluids because the close clearance wear rings are the parts that will contact if the shaft displaces from its centerline. This type of impeller is initially very efficient, but loses its efficiency as the wear ring clearance increases. No impeller adjustment is possible. Once the wear rings clearances doubles they have to be replaced. This means the pump had to be disassembled just to check the status of the ear rings. The impeller can clog if solids or "stringy material" are pumped. It is difficult to clean out these solids from between the shrouds and vanes. The impeller is difficult to cast because the internal parts are hidden and hard to inspect for flaws. The closed impeller is a more complicated and expensive designs not only because of the impeller, but the additional wear rings are needed. The impeller is difficult to modify to improve its performance. Again, the specific speed choices (the shape of the impeller) are limited.
Figure 10: Closed impeller of a centrifugal pump Open impeller The open impeller has all the parts visible. The pump is less costly to build with a simple open impeller design. Open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal re-circulation. The impeller to volute or back plate clearance must be adjusted when the pump is at operating temperature and all axial thermal growth has occurred. You would have to use soft, non-sparking materials for the impeller and that is not very practical. Efficiency can be maintained through impeller clearance adjustment. The impeller can be adjusted to compensate for wear and stay close to its best efficiency. No pump disassembly is necessary. The open impeller is less likely to clog with solids, but if it does, it is easy to clean. The vanes can easily be cut or filed to increase the capacity. There is a greater range of specific speed choice. L shows the leading edge or higher-pressure side of the impeller. T describes the trailing edge of the impeller. In the straight vane single-suction closed impeller the surfaces of the vanes are generated by straight line parallel to the axis of rotation. The double-suction impeller is, in effect, two single suction impellers arranged back to back in a single casing. For handling liquids containing stringy materials and soft solids, these two impellers are likely to become clogged because of restricted flow passages. A non-clogging impeller is designed to have large flow passages to lessen the possibility of clogging. Open impellers have vanes attached to a central hub and are well adapted for pumping abrasive solids.
The semi open impeller has a single shroud and the closed impeller has shrouds on the both sides of the vanes. The semi closed impeller has pump-out vanes located on the back of the shroud whose purpose is to reduce the pressure at the back hub of the impeller. The mixed-flow impeller is a design in which there are both a radial component and an axial component of flow. Auxiliary Components Auxiliary components generally include the following piping systems for the following services: • Seal flushing, cooling, quenching systems • Seal drains and vents • Bearing lubrication, cooling systems • Seal chamber or stuffing box cooling, heating systems • Pump pedestal cooling systems Auxiliary piping systems include tubing, piping, isolating valves, control valves, relief valves, temperature gauges and thermocouples, pressure gauges, sight flow indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and all related vents and drains. Operating Problems of Centrifugal pump Several problems may arise while operating the Centrifugal Pump. These problems are discussed below in short. a) Cavitation When a centrifugal is operating at a high-speed capacity, low pressure may develop at the impeller eye or vane tips. When this pressure falls below the liquid vapor pressure, vaporization may occur at these points .The bubbles of vapor formed move to a region of high pressure and collapse. This formation and collapse of vapor bubbles is called cavitation. There are two types of cavitation: Suction cavitation: Suction Cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. Discharge cavitation: Discharge cavitation occurs when the pump discharge is extremely high. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater similar to what occurs in a venturi and turns the liquid into a vapor. Cavitation can occur in all types of pumps and it can create a serious problem. Severe cavitation may be quite destructive to the pump and result in pitting of impeller vanes. Since any pump can be made to cavitate, care should be taken in selecting the pump for a given system and planning its installation. Pump manufacturers specify the net positive suction
head required (NPSHr) for the operation of a pump without cavitation. Pump cavitation can be avoided by assuring that the net positive suction head available (NPSH a) is always greater than that required (NPSHr) by the pump. b) Priming Centrifugal pumps usually are completely filled with the liquid to be pumped before starting. Pumps have been developed to start with air in the casing and then be pumped. This is an unusual procedure with low specific speed pumps but is sometimes done with propeller pumps. In many installations, the pump is at a lower elevation then the supply and remains primed at all times. This is customary for pumps of high specific speed and all pumps requiring a positive suction head to avoid cavitation. As the centrifugal pump throws liquid out from the eye of the impeller, the volute design creates a low pressure area where the liquid used to be. At that point either atmospheric pressure, gravity, or a combination of the two will fill up the low pressure area with either more liquid or additional air. The problem with centrifugal pumps is that a given impeller diameter and speed will throw all fluids (either a liquid or a gas) to the same height. Since air qualifies as a fluid it will throw air to the same height as water. That height is not enough to overcome atmospheric pressure, so the centrifugal pump has to have all of its air removed before it will pump a liquid, and that is priming of the pump. Actually priming is required for the continuum of fluid. There are several methods which can be used to remove air from a centrifugal pump: • • •
The pump and suction pipes are filled with liquid and start all over again. A priming pump can be attached to the discharge side of the pump to remove any air in the pump and suction piping. A foot valve can be installed at the end of the suction piping to insure that the fluid will not drain from the pump and suction piping. These valves seldom work out because, like all check valves, they leak.
c) Net Positive Suction Head (NPSH) Required Net Positive Suction Head (NPSHr) The required net positive suction head (NPSH r) is the amount of energy required to prevent the formation of vapor-filled cavities of fluid within the eye of the impeller. The formation and subsequent collapse of these vapor-filled cavities is called cavitation and is destructive to the impeller. The NPSHr to prevent cavitation is a function of pump design and is usually determined experimentally for each pump. The head within the eye of the impeller, also called net positive suction head available (NPSHa), should exceed the NPSHr to avoid cavitation. Available Net Positive Suction Head (NPSHa) Net positive suction head available (NPSHa) is the absolute pressure of the water at the eye of the impeller. It is atmospheric pressure minus the sum of vapor pressure of the water,
friction losses in the intake pipe, and suction head or lift. Since any variation of these four factors will change the NPSHa, NPSHa should be calculated using the following formula: NPSH a = BP - SH - FL - VP
BP SH FL VP
= = = =
barometric pressure m (ft) suction head or lift m (ft) friction losses in the intake pipe m (ft) water vapor pressure at a given temperature m (ft)
Suction head (SH) must be added instead of subtracted if the water source is located above the eye of the pump impeller (submerged pump). An accurate determination of NPSH a is critical for any centrifugal pump application. The NPSHr (net positive suction head required) is a measure of the head necessary to transfer water into the impeller vanes efficiently and without cavitation (see the discussion of cavitation in a later section of this lecture). The NPSH r required by a specific centrifugal pump depends on the pump design and flow rate. It is constant for a given head, flow, rotational speed and impeller diameter. However, it changes with wear and different liquids since it depends, respectively, on the impeller geometry and on the density and viscosity of the fluid. For a given pump, NPSHr increases with increases in pump speed, flow rate, and water temperature. The value of NPSH r is provided by the manufacturer for each specific pump model and it is normally shown as a separate curve on a set of pump characteristic curves. To avoid cavitations NPSHa must always be equal to or greater than NPSHr. For any pump, the manufacturers specify the minimum value of the net positive suction head (NPSH) which must exist at the suction point of the pump. The NPSH is the amount by which the pressure at the suction point of the pump, expressed as the head of the liquid to be pumped, must exceed the vapor pressure of the liquid. For any installation this must be calculated, taking into account the absolute pressure of the liquid, the level of the pump, and the velocity and friction heads in the suction line. The NPSH must allow for the fall in pressure occasioned by the further acceleration of the liquid as it flows on to the impeller and for irregularities in the flow pattern in the pump. If the required value of NPSH is not obtained, partial vaporization if liable to occur, with the result that both the suction head and delivery head may be reduced. The loss of suction head is more important because it may cause the pump to be starved of liquid. If the vapor pressure of liquid is P v, the NPSH is the difference between the total head at the suction inlet and the head corresponding to the vapor pressure of the liquid at the pump inlet.
Where, Pv is the vapor pressure of the liquid being pumped.
Figure 11: Schematic diagram for relation between total head and NPSH. If cavitations and loss of suction head does occur, it can sometimes be cured by increasing the pressure in the system, either by alteration of the layout to provide a greater hydrostatic pressure or a reduced pressure drop in the suction line. Sometimes, slightly closing the valve on the pump delivery or reducing the pump speed by a small amount may be effective. Generally a small fast-running pump will require a larger NPSH than a larger slow-running pump. Characteristic curves of Centrifugal pump Pump action and the performance of a pump are defined in terms of their characteristic curves. These curves correlate the capacity of the pump in unit volume per unit time versus discharge or differential pressures. The fluid quantities involved in all hydraulic machines are the flow rates (Q) and the head (H), whereas the mechanical quantities associated with the machine itself are the power (P), speed (N), size (D) and efficiency (Îˇ). Although they are of equal importance, the emphasis placed on certain of these quantities is different for different pumps. The output of a pump running at a given speed is the flow rate delivered by it and the head developed. Thus, a plot of head and flow rate at a given speed forms the fundamental performance characteristic of a pump. In order to achieve this performance, a power input is required which involves efficiency of energy transfer. Thus, it is useful to plot also the power P and the efficiency Îˇ against Q. Head Capacity relation curves
Head Reduction Due to Circulatory Flow Shock Loss
Volumetric Flow Rate
Figure 12: Head-capacity relation curves
In the above figure-12, the theoretical head flow rate (often called ‘head-capacity) relation is a straight line, the actual developed head is considerably less and drops precipitously to zero as the rate increase to a certain value in any given pump. This is known as the ‘zero-head flow rate’; it is the maximum flow that the pump can deliver under any conditions. The rated or optimum operating flow rate is, of course less than this. The difference between the theoretical and actual curves results primarily from circulatory flow. Other contributing factors to the head loss are fluid friction in the passages and channels of the pump and shock losses from the sudden change in direction of the liquid leaving the impeller and joining the stream of liquid traveling circumferentially around the casting. This curve relates head produced by a pump to the volume of water pumped per unit time. Generally, the head produced decreases as the amount of water pumped increases. The shape of the curve varies with pump's specific speed and impeller design. Usually, the highest head
Total power input Bearing Losses Disc Friction
Volumetric Flow Rate
is produced at zero discharge and it is called the shut-off head. Power curve Figure 13: Power curve Typical curves of fluid power ‘Pf’ and total power ‘PB’ versus flow rate is shown in figure13. The differences between ideal and actual performance represents the power lost in the pump; it results from fluid friction and shock losses both of which are conversion of mechanical energy into heat, and by leakage, disk friction and bearing losses. Leakage is the unavoidable reverse flow from the impeller; this reduces the volume of the actual discharge
from the pump per unit of power expanded. Disk friction is the friction between the output surface of the impeller and the liquid in the space between the impeller and the inside of the casting. Bearing losses constitute the power required to overcome mechanical friction in the bearing and stuffing losses or seals of the pump. Efficiency curve Figure 14: Efficiency curve The pump efficiency is the ratio of fluid power to the total power input. The curve shows that the efficiency rises rapidly with flow rate at low rates, reaches a maximum in the region of
Volumetric Flow Rate
the rated capacity, and then falls as the flow rate approaches the zero-head value. This is defined as η. Theoretically the efficiency remains same with increase of flow rate, which is a horizontal line as shown in the figure-14. Efficiency increases with the increase in volumetric flow rate, reaches a maximum value and then drops with the increase of volumetric flow rate. Efficiency,
Total power input − Fluid power η= Total power input Net Positive Suction Head Required Versus Pump Capacity One of the curves typically published by manufacturers is the net positive suction head required, NPSHr, versus capacity, Q. For a typical centrifugal pump the NPSH r steadily increases as Q increases. To assure that the required energy is available, an analysis must be made to determine the net positive suction head available NPSH a which is a function of the pumping system design.
Figure 15: Net Positive Suction Head Required Versus Pump Capacity Curve After calculating NPSHa the NPSHr versus Q curve can be used. The NPSH a must be greater than NPSHr at a given Q to avoid pump cavitation. A typical curve representing NPSHr versus capacity Q is shown in Figure 15. Operating point of a centrifugal pump A centrifugal pump can operate at a combination of head and discharge points given by its HQ curve. The particular combination of head and discharge at which a pump is operating is called the pumpâ€™s operating point. Once this point is determined brake power, efficiency, and net positive suction head required for the pump can be obtained from the set of pump curve.
Figure 16: Head capacity curve presenting operating point The operating point is determined by the head and discharge requirement of the irrigation system. A system curve, which describes the head and discharge requirements of the irrigation system, and a head-discharge characteristic curve of the pump are used to
determine the pump operating point (Figure 16). The operating point is where the headdischarge requirements of the system are equal to the head-discharge produced by the pump. Discussion Various kinds of pumps are used in different industries for different purposes. It is essential for an engineer to have some basic knowledge about the classification, construction, advantages and disadvantages of different kinds of pumps. The engineers need to know several specific characteristics of the pump like capacity of pumps, the energy or head supplied to the fluid by a pump, the power required to run the pump and the efficiency of the unit. These characteristics help an engineer to select the right pump for industrial and commercial purposes. When choosing or designing a pump for a definite purpose the designer or engineer must know about some general problems in operating the pump such as priming and cavitations. Above all, it is necessary to have a clear concept about the physical phenomena and operating system to select a suitable pump for the definite purpose.