Selecting the Proper Pump

SRAC Publication No. 372

December 1994

J. David Bankston, Jr., and Fred Eugene Baker*

Before you can select a pump that against increases. This is sketched A cost analysis of pumping will will fit your needs, you must as curve B. The operating point of consider initial cost of capital know four things: 1) the total the system occurs when the two investment, annual fixed cost and head or pressure against which it curves cross. operating cost. All three costs are must operate, 2) the desired flow In order to obtain a pumping sys- somewhat dependent on each rate, 3) the suction lift, and tem that will meet your require- other. The type of pumping 4) characteristics of the fluid. ments, and meet them in an effi- equipment, size of pipelines, size Aquaculture operations imply cient manner, you must match the of pumps and type of water sup- that you will be pumping water, pump to the piping system and ply affect not only the initial cost but you also need to know such required flow rate. Manufacturers but also the fixed cost as well as things as the temperature range, should be able to supply a pump the operating cost. For example, the corrosiveness (is it salty curve which shows the perfor- piping systems using large pipes water?), and how much and what mance of the pump and the allow- may cost more but could allow kind of trash or debris such as able operating ranges. Do not the use of smaller horsepower sand, dirt, leaves, fish or twigs plan to operate outside of this rec- pumps which cost less, require may be in the water. With this ommended range outside this smaller power sources and cost knowledge, you and your dealer may damage the pump. Your less to operate than a piping sys- can select the proper pump. dealer may also be able to help tem with small diameter pipe. The total head, suction lift and you analyze or plan your piping The lowest priced system is not flow rate are dependent upon the system. Information on piping always the best buy, especially if piping system and the pump’s systems is also available in the the lower price means less effi- characteristics. The piping system SRAC Publication No. 373, Piping cient pumps. To get the most effi- and the pump interact to deter- Systems. cient pump, an analysis should be mine the operating point of the made of all pumping require- pump – flow rate and pressure. ments. Key points to consider are: The pump cannot independently ● net positive suction head control these parameters. As the (NPSH) flow rate is increased the work to ● move each unit of water or total priming dynamic head the pump must ● flexibility produce increases as shown in ● corrosion curve A in Figure 1. A pump will ● useful life typically have reduced capacity as ● maintenance the pressure or head it is pumping ● Figure 1. Pressure as a function of flow quantity pumped *Louisiana Cooperative Extension Service, rate for a typical piping system (A) and a ● pumping head Louisiana State University Agricultural typical centrifugal pump (B). ● power source Center. ● economics. The NPSH is the amount of ener- may look like an impeller, but the Operating a pump under a lift gy in the water at the pump’s force imparted to the water is par- larger than it was designed for inlet. The required NPSH is a allel to the propeller shaft. may cause cavitation. Cavitation characteristic of the pump and is the formation of vapor (steam) depends on pump design, size Operating principles bubbles when the pressure drops and operating conditions. The below the vapor pressure of the required NPSH is determined by To operate properly, a centrifugal water. This results in inefficient the pump manufacturer. The pump must be submerged in a operation and will usually dam- available NPSH must equal or source of water or be filled with age the pump. A pump that is exceed the required NPSH. The water and connected through a cavitating may have a rattling available NPSH is a characteristic water-filled, airtight pipe to the sound. of the piping system. For the sys- water source. In the latter case, tem shown in Figure 2, the NPSH water is pushed into the pump by Horizontal centrifugal may be calculated as: the difference between atmospheric pressure and the pumps (for surface supply and shallow (atmospheric pressure - vapor pressure) pressure at the pump - h - friction loss in pipe entrance. (The efficien- wells) 2.3 cy and capacity of the Horizontal centrifugal pumps are pump is extremely sen- frequently used if the source of The vapor pressure in this equa- sitive to any leak in the inlet pip- water is a surface supply, such as tion is dependent upon water ing.) After the pump is started, a lake, stream, canal or pond, or a temperature and both it and the the impeller throws the water it shallow well. A shallow well, as atmospheric pressure are contains outward by centrifugal opposed to a deep well, is one in expressed in pounds per square force and creates an area of low which the water level in the well inch while h and friction loss are pressure at its center. This pres- is high enough to permit the vacu- expressed in feet. sure is lower than atmospheric um at the pump to lift the water pressure and is commonly called a and keep it flowing at an accept- vacuum. Since atmospheric pres- able rate. As the name implies, sure is pushing down on the sur- horizontal centrifugal pumps nor- face of the source of supply, the mally have a horizontal shaft. water is forced through the suc- This type of pump is usually sub- tion pipe to the lower pressure divided into two groups, single area at the center of the impeller suction (end suction) and double Figure 2. Simple pump system. to replace the water being thrown suction (often called split case). outward. Thus, there is a continu- Either of these may be single or Types of pumps ous flow of water through the multistage; that is, they may have pump. only one impeller or they may Pumps used in irrigation or aqua- have two or more impellers. culture often are a form of the Theoretically, if a pump could be These impellers are so constructed centrifugal pump. Two basic designed to produce a perfect vac- that the water, in passing through types of centrifugal pumps are uum at its center and was being the pump, is conducted from the horizontal and vertical. As the operated at sea level, the atmos- discharge of one impeller to the name implies, centrifugal pumps pheric pressure of 14.7 pounds per suction of the second; thus, the use centrifugal force to move square inch acting downward on total head is that developed by a water from one point to another the surface of the water source single impeller multiplied by the and to overcome resistance to its would be capable of forcing water number of impellers in the pump. flow. In its simplest form, this up the suction line to the pump a pump consists of an impeller vertical distance of 34 feet. In The most common pump and the fixed on a rotating shaft within a practice, this is impossible, lowest in cost is the end suction, volute-type (spiral) casing. Water because a perfect vacuum cannot single stage (Figure 3). Available enters at the center of the impeller be created at the center of the sizes vary with the manufacturers. and is forced to the outer edge at impeller and because there are a high velocity by the rotating losses caused by friction created Jet pumps impeller. The water is discharged by the flow through the suction A jet pump is often used for very by centrifugal force into the cas- line. There are also losses caused low capacity requirements (5 to 20 ing where the high velocity head by turbulence at the entrance to gpm), such as a home water sys- is converted to pressure head. the suction line and at the entrance to the impeller. Usually, tem. This pump consists of a The axial flow propeller pump is a vertical suction lift of about 15 small centrifugal pump located at another type that is used. It is feet is considered the maximum ground level connected to a jet often used in low-lift, high-vol- for reasonably efficient operation. installed below the water level in ume situations. The propeller the well (Figure 4). By circulating

2 Axial flow propeller pumps bly in the well, provides a discharge for water being deliv- Axial flow propeller pumps are ered, and also accommodates designed to operate efficiently for the driver for the pump. aquacultural, irrigation or drainage pumping at low head 4) Driver - this may be either an and high volume (more than 500 electric motor or a right-angle gpm). Their efficiency is high, gear for connection to a power especially when the total head is unit. When an electric motor is in the range of 8 to 20 feet. The used, the usual type is a verti- pumping element of an axial flow cal hollow-shaft design that propeller pump consists mainly of permits the pump shaft to Figure 3. Diagram showing construction of a revolving propeller in a station- come up through its center for the most commonly used pump and the ary bowl which contains vanes securing at the top. The right- lowest in cost. above and below the propeller. angle gear also is usually the Water enters the pump through hollow-shaft design type for part of the water from the pump the intake bell. It is discharged the same reason and has a hori- back through the jet, water is into the distributor section and zontal shaft for connection to forced up to the impeller in the then out the discharge elbow. the engine drive or power take- pump, and a continuous flow at Flowing in essentially a straight off. The internal gears are reasonable pressure is provided. line along the pump axis keeps available in various ratios to Shallow-well jet pumps operate friction and turbulence to a mini- accommodate any engine with on the recirculation principle, but mum. The propeller of an axial an operating speed different the jet is installed above ground flow pump must be submerged in and the allowable lift is limited to the source of water (Figure 5). about 22 feet. Deep-well jet One of the advantages of this pumps, however, have a maxi- pump is that it will handle some mum lift of about 65 feet. debris. Deep well vertical turbine pumps For a deep well, the most widely used pump is a vertical centrifugal, commonly referred to as a “deep well turbine.” Basically, this is a centrifugal pump designed to be installed in a well. It will not handle debris. It consists of four major components: 1) Bowl assembly - this contains one or more impellers, each in its own housing. 2) Column and shaft assembly - Figure 4. Parts of a jet pump. This pump is this consists of the pipe to sus- used most often for low capacity require- pend the bowl assembly in the ments, such as a home water system. well and carry the water to the surface. Inside this pipe (or column) is the shaft that con- Jet pumps are designed for home nects the impeller shaft to the water systems, and their capaci- driver located at ground level. ties are seldom adequate for aqua- The shaft may be either water cultural purposes. Also, the jet lubricated or oil lubricated. pump requires about twice the horsepower that a submersible 3) Discharge assembly - this requires to deliver the same often is called the “head or amount of water from the same base.” It is normally cast of depth. iron and designed for installation on a foundation. It sup- ports the column and shaft Figure 5. Diagram showing parts of an assembly and the bowl assem- axial flow propeller pump.

3 from the desired pump-operat- Water-lubricated pump column includes a tube which encloses the ing speed. assembly is provided with fluted shaft. Inside the tube are bronze Because of the limited diameter of rubber bearings to permit the shaft bearings threaded on the its impellers, each impeller devel- water being pumped to lubricate outside to serve as couplings. ops a rather low head, and it is the shaft. If the pump is to be Alternatively the tube may have necessary in the average applica- operated at less than about 2,200 oil-impregnated redwood bear- tion to stack several impellers in rpm, these bearings, which are ings. Lubricating oil is fed into the series one above the other with fixed in the column pipe coupling, top of the tubing and passes by each in its own bowl or diffuser are usually placed at 10-foot inter- gravity over surfaces of the bear- housing. This is called staging. vals. For higher speeds, the bearings. At the bottom of the column Thus, a four-stage bowl assembly ings are on 5-foot spacings. is an opening that lets oil flow out contains four impellers, all Water-lubricated turbine pumps (Figure 7). attached to a common shaft are simpler, cheaper and more To maintain the pump, drain and through the separate housing or commonly used. If more than clean the oil reservoir each year bowls. The bowl shaft is attached four or five of the rubber shaft and then fill the reservoir with the to the line shaft through the center bearings are above the water level proper turbine oil specified by the of the pump column pipe and and become dry when the pump individual pump supplier. Take must be long enough to locate the is not operating, some means of care that the oil reservoir contains bowl assembly below the level of pre-lubrication, such as a small enough liquid to lubricate the the water in the well when pump- pre-lub tank from which water ing at required capacity. can be spilled For any given capacity and speed, over the bearings each impeller develops a certain before starting amount of head. For example, the pump (Figure assume that a well has a 10-inch 6), is required. casing and a static water level 60 With smaller feet below the surface and that a pumps, a foot pump is needed to deliver 1,500 valve can be gpm. In this example the static installed below water level is 60 feet below the the bowl assem- surface. When water is being bly to keep the pumped from the well, the water column pipe full level will fall. This distance the of water. Because level falls when pumping is called of friction loss, it drawdown. Drawdown varies is impractical to with localities, the formation into use a foot valve which the well is drilled and the for applications amount of water pumped. requiring large Assuming in this case the draw- flows. down is 40 feet (at a pumping rate When the water of 1,500 gpm), then the total dis- level is very tance to the surface becomes 100 deep, oil lubrica- feet. In addition we have pipe tion is normally friction and inlets and outlets used. Although which add 21 feet of head. There- there is no defi- fore, the total head the pump nite point at must develop is 100 feet plus 21 which it becomes feet, or 121 feet. If an impeller necessary, it is were available that delivered usually recom- 1,500 gpm at a head of 22 feet, a 6- mended for stage pump would be required. depths of more than 150 to 200 Optional vertical turbine feet. pump features Oil-lubricated pump column You can get most pumps with assembly construction features to suit indi- includes a tube vidual preferences and particular which encloses Figure 6. This diagram of a water-lubricated pump column assembly applications. the shaft. Inside shows the parts of the surface discharge head and the pump bowl assembly. Water-lubricated 4 face (Figure 8). the vanes and the bowl seat is This type is avail- increased, and water is allowed to able in a wide circulate through this area. This range of capacities makes possible a variation in how for 4-inch wells the pump performs at any given and larger. speed. Thus, by adjustment of the Most submersi-ble impeller clearance, a specific per- pumps used for formance can be maintained even aquaculture though there is a change in the require three- water level. With enclosed phase electrical impellers, performance and effi- service. ciency are not affected by small differences in their position, and Enclosed vs. constant performance can be maintained. semi-open impellers In an enclosed impeller, vanes are covered on both top and bottom edges; in a semi-open impeller, only the top edge of the vanes is covered. Figure 9 shows top and bottom views. The bottom edge Figure 9. Views of impellers. of the vanes runs with close clear- Choice of impellers ance with the The impeller of the pump has a pump bowl. By wear ring that must match with a raising the setting similar wearing surface in the Figure 7. An oil lubricated pump column assembly showing parts of the semi-open bowl of the pump. It is necessary of the surface discharge head and the pump bowl assembly. impellers, the to maintain the proper clearance clearance between between these two surfaces and to pump at all times. A drip adjust- allow for the stretch of the drive ment controlling the amount of oil shaft within the pump. Periodic allowed to flow into the pump adjustment of the impeller clear- should comply with the manufac- ance is essential for high efficiency turer’s specifications. Do not operation. Contact your pump over-lubricate the pump or leave installer at least once every five the oil flowing when the pump is years to make the adjustments. not operating. Too much lubrica- Pumpers who periodically adjust tion flowing into the oil tube will the thrust nut to get impeller cause additional friction loss that clearance should take care to could cause the tube to plug and make the proper adjustment. bearings to wear. Nonreverse ratchet Submersible pumps At a small additional cost, either The submersible pump consists of an electric motor or a right-angle a multistage vertical turbine gear drive can be provided with a pump connected directly to an ratchet to prevent the pump’s electric motor designed to operate rotating in reverse. Some electric Figure 8. Parts of a submersible pump under water. Both the pump and motors will operate in either below ground connected by a pipe to the motor are suspended in the well direction. If it should be operated surface. below the water level by a pipe in the wrong direction because of that conducts the water to the sur- phase reversal in the power sup-

5 ply, the pump may be damaged. A horsepower is defined as the pressure gauge. The gauge read- Reverse spin can also occur after power required to raise a weight ing translated into feet registers stopping because the water flow- of 33,000 pounds a vertical dis- the height to which the pump is ing back down the pump column tance of 1 foot in 1 minute. The capable of elevating water. This is through the impellers turns them rate of work performed by a known as the “shut off head.” like a water wheel. Undue wear pump (in horsepower) is propor- If the valve is slowly opened, the is caused by the shaft’s being tional to the weight of the water pressure gauge reading will fall as turned in its bearings without it delivers per minute multiplied the flow increases, and this will water for lubrication. by the total equivalent vertical continue until some point of max- distance in feet through which it imum flow and minimum head is Basic considerations is moved. For example, a pump reached. If the total head being delivering 396 gallons per minute developed at any given rate of If you must lift water, reducing at a total head of 10 feet is per- the lift will improve the overall flow is plotted against the quanti- forming work at the rate of 1 ty of water being delivered, the efficiency of operation. Surface horsepower. If it were possible to sources of water usually require result will be a performance curve achieve 100 percent efficiency, it for this particular pump at this much less lift than pumping from would only be necessary, in this wells. Two common types of particular speed. If you observe instance, to apply 1 horsepower the power required to turn the pumps designed primarily for to the pump shaft. In practice the low-lift operations are the pro- pump during this process, you energy input must be greater will note that the power is at a peller axial flow pump and the than 1 horsepower. horizontal PTO-driven centrifugal minimum for this typical centrifu- pump. Axial flow propeller Some of the energy losses that gal pump when there is no water pumps have very high efficiencies result in lower efficiency are fric- being discharged from the pump and are capable of pumping large tion in the bearings that support and that the power required will volumes. Horizontal centrifugal the pump shaft, friction between gradually increase as the rate of PTO-driven pumps are less effi- the shaft and the packing in the flow increases and the head cient but still maintain the capa- stuffing box, unavoidable leakage decreases. The maximum efficien- bility of pumping large volumes between areas of high pressure cy will be about midway between of water. They also are portable and adjacent areas of low pres- zero flow and maximum flow. A and often fit into a flexible man- sure inside the pump case, and typical turbine pump perfor- agement plan for aquaculture the friction caused by the water mance curve is shown in Figure production. moving across the metallic sur- 10, a typical axial flow propeller faces in the pump. There are also pump curve in Figure 11. Pump efficiency other losses of a more complex A pump designer often must sac- nature. rifice some degree of efficiency to All segments of our economy, achieve some other desirable including aquaculture and agri- Determining pump characteristics to provide a unit culture, must make the most effi- efficiency with maximum usefulness. cient use of available energy Available pump efficiencies vary sources. Selecting a correct The efficiency of a pump is deter- with the pump size, type, etc., but pumping plant not only will con- mined by actual tests. Referring usually should be between 65 per- serve valuable energy supplies to the example just mentioned: If cent and 85 percent. However, but also will reduce total annual 1.25 horsepower must be applied this does not mean that a pump pumping costs. Inefficient pump- to the input shaft when the pump used in an aquaculture applica- ing plants can increase costs dra- is doing work equivalent to 1 tion is either capable of or oper- matically. horsepower, the pump efficiency ates at that range of efficiency. will be 80 percent (1 divided by For example many pumps used in Meaning of efficiency 1.25). crawfish production in Louisiana The efficiency of a pump is a During a test, the total head that had efficiencies of less than 30 measure of the degree of its a centrifugal pump can develop percent. If you do not have a hydraulic and mechanical perfec- is a function of the speed at pump curve for the pump or tion. Pump efficiency is the ratio which the impeller is turned and haven’t determined its operating of the output water horsepower the diameter of the impeller. If a point, you are just guessing or to the input shaft horsepower pump impeller is being turned at gambling on its performance. expressed as a percentage: its rated speed and a valve on the Where practical, select a pump discharge side of the pump is which will operate near the point GPM X Total Head X 100 closed, it will develop a certain of its curve where maximum effi- Pump Efficiency = maximum head. Under these ciency is reached. This is referred Input HP X 3,960 conditions, this head is read on a to as the “design point.”

6 ing that adequate water is available to flow into the well and that encrusted wells are not the problem, then the surge is normally caused by the pump not being submerged enough to provide water for intake. Where the pump is located some distance from the bottom of the well, it is often possible to lower the pump and reduce the amount of surging. This can require more power since full water flow may be obtained along with higher head. This may overload the existing motor. Ask your pump installer to evaluate the capability of the existing pump to provide the required water. In some instances Figure 10. A typical performance curve of an 8-inch turbine pump 1,760 rpm. you may have to add another stage and change the motor to one of greater horsepower. When the pump is set near the bottom of the well and it is impossible to lower the pump to mini- mize surging, consider other alter- natives. Long-term solutions resulting in higher efficiency include pulling the pump and trimming the impeller or replac- ing the pump with a different pump of a smaller capacity. It might also be possible to decrease the pump speed and thus decrease the amount of water which is pumped, but this could result in a sizable decrease in efficiency. These short-term solutions for a single pumping season will result in a decrease in efficiency; adjustment of the impellers upward, throttling of the discharge by clos- ing a valve on the discharge side, and, for belt-driven pumps, the exchange of pulleys in such a way to decrease rpm. Figure 11. A typical axial flow propeller pump curve. Consult your individual pump Remember, the point at which the Pumping from a well installer for the best solution to pump operates depends upon the your problem, realizing that year- pump and upon the piping Changes in pumping heads by-year fluctuations in the water arrangement. The pump’s operat- because of different operating sys- table may cause the problems. ing range and environment tems usually require a change in Does your well pump sand? If should also conform to the manu- the pumps to maintain greatest you observe sand in the discharge facturer’s recommendation to pre- efficiency. after the initial period of develop- vent damage. Does your pump surge? If you ment, remember that this sand is observe a surge in the discharge, wearing the pump and that con- the pump may be having difficul- tinuous maintenance and replace- ty getting enough water. Assum- ment will be necessary to provide

7 the desired capacity and overall to get maximum capacity from a Pumping plant efficiency efficiency. Sand in the water given well size. Maximum per- wears the impeller and the bear- missible speed depends upon a A pumping plant is a combination ings, and the pump must be number of factors, but for 4-inch, of a pump and power unit. pulled periodically and worn 6-inch and even 8-inch pumps, a Overall efficiency is a product of parts replaced. Sand is usually speed of 3,600 rpm is not uncom- both the pump’s and the power caused by poor well design mon. For larger sizes, however, unit’s efficiency. Table 1 lists and/or poor well screen selection. this speed is not advisable. Since obtainable efficiencies of the com- If you cannot correct the problem nominal electric motor speeds ponents of a pumping plant. you will need to be particularly used in pumping are either 1,760 For electric motors, the efficiency concerned about loss of efficiency rpm or 3,450 rpm, intermediate ranges from about 85 percent to 92 and capacity. If your well is speeds may be achieved with percent. Typical thermal efficiency pumping sand, monitor its perfor- right-angle gears of suitable ratio of an internal combustion engine mance or ask your pump supplier or with belt and pulley drives. ranges from 5 percent to 37 per- to determine the overall efficiency. The following are considered nor- cent. Typical efficiency of an indi- If you can measure the appropri- mal maximum capacities expected vidual pump will vary between 25 ate quantities (flow rate, total head from deep wells: and 85 percent. Thus the greatest and input horsepower) you can theoretical efficiency for a good determine efficiency from the SIZE UP TO pumping plant seldom exceeds 70 equation for pump efficiency. percent. 4 - inch 90 gpm Even without measurement you Tests conducted in the field indi- may be able to determine from the 6 - inch 400 gpm cate that pump efficiencies could pump’s performance that it is 8 - inch 600 gpm vary from less than 10 percent to operating inefficiently. If efficien- 10 - inch 1,000 gpm about 75 percent, but average 50 cy is low, pull the pump and make percent to 60 percent. Pumps the needed repairs. You may also 12 - inch 2,000 gpm operating at an efficiency of 60 to consider whether a different type 70 percent seldom need major of pump could handle the sand. These figures are not limiting. adjustments or major repairs, but Much depends on the water level, they can be improved, usually by Well size and capacity yield characteristics of water-bear- adjusting the impeller and follow- limits ing formation and the pressure to ing recommendations for adjust- be developed, but they serve as a ments and maintenance. Pumps There are definite capacity limita- general guideline. In any case, with an overall efficiency of less tions for a given diameter of well evaluate carefully the overall than 50 percent usually require casing. To obtain this limit, the installation. For instance, major repairs and should have pump must have sufficient capaci- although 1,000 gpm may be immediate attention to reduce ty. The capacity of a centrifugal obtained from a 10-inch pump pumping costs. pump varies directly with its with reasonably good efficiency, a speed of operation. It may be nec- 12-inch pump may be better con- essary to increase the pump speed sidering overall cost.

Table 1. Pumping equipment efficiency and useful life Type Attainable Efficiency Percent Useful Life* (Yrs) Centrifugal-type pump 75-85 15 Right-angle pump drive (gear head) 95 15 Automotive engines - gasoline 20-26 9 - LPG 20-26 14 Light industrial engine (diesel) 25-37 14 Electric motors 85-92 25 *Based on 2,000 hours per year of use. With proper maintenance and fewer hours of annual use, the useful life could be increased.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 89-38500-4516 from the United States Department of Agriculture.