CENTRIFUGAL PUMP SYSTEMS TIPS

This is a list of ideas or DOS AND DON'TS for pump systems. You may not of thought of some of

these and they will help you design and trouble-shoot pump systems and select the proper pump. Also

there is information here that is hard to find elsewhere. You can think of this list as GUIDELINES for

the pump system designer.

1. Flow and pressure relationship of a pump

When the flow increases, the discharge pressure of the pump decreases, and when the flow

decreases the discharge pressure increases

2. Do not let a pump run at zero flow

Do not let a centrifugal pump operate for long periods of time at zero flow. In residential

systems, the pressure switch shuts the pump down when the pressure is high which means there

is low or no flow.

3. Use pressure gauges

Make sure your pump has a pressure gauge on the discharge side close to the outlet of the pump

this will help you diagnose pump system problems. It is also useful to have a pressure gauge on

the suction side, the difference in pressure is proportional to the total head. The pressure gauge

reading will have to be corrected for elevation since the reference plane for total head

calculation is the suction flange of the pump.

4. Do not let a pump run dry, use a check valve

Most centrifugal pumps cannot run dry, ensure that the pump is always full of liquid. In residential

systems, to ensure that the pump stays full of the liquid use a check valve (also called a foot valve) at

the water source end of the suction line. Certain types of centrifugal pumps do not require a check

valve as they can generate suction at the pump inlet to lift the fluid into the pump, see

http://www.watertanks.com/category/43/. These pumps are called jet pumps and are fabricated by

many manufacturers Goulds being one of them.

Make use of check valves to isolate pumps installed in parallel.

5. Suction valves

Gate valves at the pump suction and discharge should be used as these offer no resistance to flow and

can provide a tight shut-off. Butterfly valves are often used but they do provide some resistance and

their presence in the flow stream can potentially be a source of hang-ups which would be critical at the

suction. They do close faster than gate valves but are not as leak proof.

6. Eccentric reducer

Always use an eccentric reducer at the pump suction when a pipe size transition is required. Put the flat

on top when the fluid is coming from below or straight (see next Figure) and the flat on the bottom

when the fluid is coming from the top. This will avoid an air pocket at the pump suction and allow air to

be evacuated.

7. Use a multi-stage turbine pump for deep wells

For deep wells (200-300 feet) a submersible multi-stage pump is required. They come in different sizes

(4" and 6") and fit inside your bore hole pipe. Pumps with different ratings are available, see

http://www.webtrol.com/waterwell%20homepage.html

8. Flow control

If you need to control the flow, use a valve on the discharge side of the pump, never use a valve on the

suction side for this purpose.

This is an excellent treatment of the types of control systems for a centrifugal pump. Thanks to Walter

Driedger of Colt Engineering a consulting engineering firm for the petro-chemical industry in Alberta,

Canada.

9. Plan ahead for flow meters

For new systems that do not have a flow meter, install flanges that are designed for an orifice plate in a

straight part of the pipe (see next Figure) and do not install the orifice plate. In the future, whoever

trouble-shoots the pump will have a way to measure flow without the owner having to incur major

downtime or expense. Note: orifice plates are not suitable for slurries.

10. Avoid pockets and high points

Avoid pockets or high point where air can accumulate in the discharge piping. An ideal pipe run is one

where the piping gradually slopes up from the pump to the outlet. This will ensure that any air in the

discharge side of the pump can be evacuated to the outlet.

11. Location of control valves

Position control valves closer to the pump discharge outlet than the system outlet. This will ensure

positive pressure at the valve inlet and therefore reduce the risk of cavitation.

When the valve must be located at the outlet such as the feed to a tank, bring the end of the pipe to

the bottom of the tank and put the valve close to that point to provide some pressure on the discharge

side of the valve making it easier to size the valve, extending it's life and reducing the possibility of

cavitation.

12. Water hammer

Be aware of potential water hammer problems. This is particularly serious for large piping systems such

as are installed in municipal water supply distribution systems. These systems are characterized by long

gradually upward sloping and then downward sloping pipes. Solutions to this can involve special

pressure/vacuum reducing valves at the high and low points or additional tanks which provide a buffer

for pressure surges (see http://www.ventomat.com/default.asp).

For pumps 500 gals/min or larger use semi-automatic manual valves at the discharge that are

controlled to open gradually when starting the pump. This will avoid water hammer during the initial

start and damage to the piping system.

13. The right pipe size

The right pipe size is a compromise between cost (bigger pipes are more expensive) and excessive

friction loss (small pipes cause high friction loss and will affect the pump performance). Generally

speaking, the discharge pipe size can be the same size as the pump discharge connection, you can see if

this is reasonable by calculating the friction loss of the whole system. For the suction side, you can also

use the same size pipe as the pump suction connection, often one size bigger is used . A typical velocity

range used for sizing pipes on the discharge side of the pump is 9-12 ft/s and for the suction side 3-6

ft/s.

A small pipe will initially cost less but the friction loss will be higher and the pump energy cost will be

greater. If you know the cost of energy and the purchase and installation cost of the pipe you can select

the pipe diameter based on a comparison of the pipe cost vs power consumption,

14. Pressure at high point of system

Calculate the level of pressure of the high point in your system. The pressure may be low enough for

the fluid to vaporize and create a vapor pocket which will be detrimental to the performance of the

system. The pressure at this point can be increased by installing a valve at some point past the high

point and by closing this valve you can adjust the pressure at the high point. Of course, you will need to

take that into account in the total head calculations of the pump.

15. Pump pressure rating and series operation

For series pump installations make sure that the pressure rating of the pumps is adequate. This is

particularly critical in the case where the system could become plugged due to an obstruction. All the

pumps will reach their shut-of head and the pressure produced will be cumulative. The same applies for

the pressure rating of the pipes and flanges.

16. Inadequate pump suction submersion

There is a minimum height to be respected between the free surface of the pump suction tank and the

pump suction. If this height is not maintained a vortex will form at the surface and cause air to be

entrained in the pump reducing the pump capacity.

17. Pump selection

Select your pump based on total head (not discharge pressure) and flow rate. The flow rate will depend

on your maximum requirement. Total head is the amount of energy that the pump needs to deliver to

account for the elevation difference and friction loss in your system

Pump selection starts with acquiring detail knowledge of the system. If you are just replacing an existing

pump then of course there is no problem. If you are replacing an existing pump with problems or

looking for a pump for a new application then you will need to know exactly how the systems is

intended to work. You should have the P&ID diagram and understand the reasons for all the devices

included in your system. You should make your own sketch of the system that includes all the

information on the P&ID plus elevations (max., min., in, out, equipment), path of highest total head,

fluid properties, max. and min. flow rates and anything pertinent to total head calculations.

The next figure is a typical example:

Typical example of flow schematic used for total head calculations.The control method is important

(on-off, control valve, re-circulating, variable speed) as it may affect your selection. Besides the system

sketch, that you can use to record some of the data.

CENTRIFUGAL PUMP SIZING

Service: Pump number:

Fluid WATER Suction side of pump

Viscosity (cSt) 1.13 Pump suction elevation (ft)

Temperature (f) 70 Suction fluid surface elevation min. (ft)

Specific gravity 1.0 Suction fluid surface pressure (ft fluid)

Discharge side of pump

Discharge surface elevation max. or

discharge pipe end elevation (ft)

Atmospheric pressure (ft water) 32.8 Surface pressure in discharge tank or

pipe end pressure (ft fluid)

Vapor pressure (ft water) 22

Pipe roughness RMS (ft) .00015

1. Design flow (USgpm) 750 Comments:

2. Flow contingency %

(applied on 1.)

0

3. Flow depreciation %

(applied on 1.)

0

4. Total (USgpm) 750

5.Discharge static head (ft fluid) 40

6. Suction static head (ft fluid) 10

7. Total static head (ft fluid) 20

Calculations

8. Pipe and fittings friction (ft

fluid)

(calc. on 1.)

Comments:

9. Equipment friction (ft fluid)

(based on 1.)

10. Velocity head (ft fluid)

11. Sub-total (ft fluid)

(7+8+9+10+11)

12. Total head contingency (ft

fluid)

(applied on 8, 9, 10)

13. Total head depreciation (ft

fluid)

(applied on 8, 9, 10)

14. Total head (ft fluid)

Depending on the industry or plant that you work in, you will be forced to either select a certain type of

pump or manufacturer or both. Manufacturers are normally a very good source of information for final

pump selection and you should always consult with them, do your own selection first and confirm it

with the manufacturer. They can help you select the right type, model, and speed if you have all the

operating conditions and if not they will rarely be able to help you

PUMP SELECTION DATA

Pump Manufacturer

Pump Model

Type

Suction dia. (in)

Discharge dia. (in)

Impeller speed (rpm)

Operating head (ft)

Operating Flow (USgpm)

Pump efficiency (%)

Specific speed

Suction specific speed

Fluid type

Viscosity (cP)

Temperature (F)

Specific gravity

Vapor pressure (psia) at ______(F)

Brake horsepower (hp)

Selected horsepower (hp)

Motor frame

Motor speed

Direct drive (yes/no)

Pump shut-off head (ft)

System high point (ft) zhigh z1

NPSH required (ft abs.)

NPSH available (ft abs.)

Max. impeller size (in)

Min. impeller size (in)

Selected impeller size (in)

Note: units for specific speed and suction specific speed are 75.0

5.0

ft

gpmrpm

Aside from the normal end suction pump, vertical turbine and submersible pumps, there is a wide

variety of specialized pumps that you should consider for your application if you have unusual

conditions.

SPECIALTY PUMPS Jacques Chaurette p. eng.

www.lightmypump.com October 2004

Synopsis

My intention in this article is too highlight some specialty pumps with characteristics that

are unusual and helpful in particular situations. Some of these pumps are mentioned in

the Hydraulic Institute classification of centrifugal pumps by mechanical type which you

can view at http://www.lightmypump.com/pumpdatabase/hydraulic_institute-chart.htm.

1.0 JET PUMPS

This pump is used frequently for domestic water supply. It is a typical centrifugal pump

with the difference that the suction is augmented by a venturi which creates a vacuum allowing water to be lifted from a deep well. It is an ingenious use of the pump own

discharge pressure and flow to provide pressure water at the inlet of a venturi which is located in the pump suction. The jet of water is accelerated in the small diameter of the

venturi which creates a low pressure or vacuum, this vacuum is used to assist in lifting the water in the well to the suction compartment of the pump (see Figure 1b). One big

advantage is there is no need to use a foot valve (i.e. check valve) at the end of the suction pipe, this reduces the maintenance on this item and potential plugging.

Figure 1a A typical jet pump by Goulds, see http://www.goulds.com/master.asp?id=3

Figure 1b The venturi action of a jet pump.

1.1 VISCOUS DRAG PUMP

The impeller of this pump is a flat disc that accelerates the fluid by shearing the fluid. The ability of the fluid to resist this shear force (this is the definition of viscosity) means

that a certain quantity of fluid will follow the disc and be accelerated towards the pump casing (see Figure 2). As in a normal pump, the velocity energy of the fluid is converted

to pressure energy when the fluid hits the stationary casing. The advantage of this pump is that it can handle large quantities of air or gazes and still perform which is not the case for traditional centrifugal pumps. Because the discs are open there are no tight

passages as in traditional curved impeller vane passages and therefore solids can be handled effectively. These pump are available from the Discflo Corporation in a variety

of sizes.

Figure 2 The discflo pump by Discflo

Corp. see http://www.discflo.com/

1.2 DOUBLE VOLUTE PUMPS

A double volute pump is one where the immediate volute of the impeller is separated by a partition from the main body of the casing. The result is that the impeller is subjected

to equal forces that are generated at the cutwater positions (see Figure 3a) and

therefore is balanced hydraulically. A single volute pump always has a net hydraulic

force that acts on the impeller causing wear and tear on the rotating components, the

force gets larger the further one operates away from the optimum flow at the Best

Efficiency Point (B.E.P.).

Figure 3a Schematic of the double volute design (source:

Pumps & Systems magazine June 2004 The double volute pump is therefore more robust and will require less

maintenance, however it is less efficient and more expensive.

Double volute pumps are available in the medium to large size pumps from most

manufacturers (see Figure 3b)

Figure 3b Availability of double volute pumps (source: Pumps & Systems magazine June 2004

1.3 CHOPPER PUMPS

This type of pump has a serrated impeller edge which can cut large solids and therefore

prevent clogging (see Figure 4). It is used for municipal waste handling and would no

doubt be very useful for handling slurries containing many different type of solids such

as in the pulp and paper industry.

Figure 4 The Chopper pump by The Vaughan Co. Inc. see http://www.chopperpumps.com

1.4 ROTATING CASING (PITOT) PUMPS

This pumps specialty is low to medium flow rates at high pressures. It is frequently

used for high pressure shower supply on paper machines.

Figure 5a The rotating casing Roto-Jet pump by

Weir specialty Pump see http://www.rotojet.com/

Roto-Jet pumps are designed with only two working parts, a stationary pick-up tube (pitot tube) and a rotating casing (see Figure 4b). These pump come with a built in

recirculation line with an orifice which can bleed high pressure fluid from the discharge

to the inlet to avoid damage due to running the pump with a closed discharge valve. An alternate choice to this pump is a multi-stage centrifugal pump such as the Goulds

model 3355 which can be seen at

http://www.gouldspumps.com/cat_pumps.ihtml?pid=602&lastcatid=86&step=4

Figure 5b A sectional view of the rotating casing Roto- Jet pump.

1.5 RECESSED IMPELLER

This pump is a frame-mounted, back pull-out, end suction, recessed impeller, tangential

discharge pump designed specifically to handle certain bulky or fibrous solids, air or

gas entrained liquids or shear sensitive liquids (see Figure 6). For example, certain bulky or fibrous solids like some long denim fiber or recycle stock can clog or abrade

parts of conventional process pumps. In addition, shear sensitive liquids like latex are

degraded when pumped at high velocities through process pump casings. Last, if air or

gas binding is a problem, the recessed impeller is the answer, it can also handle liquids with up to 5% entrained air or gas.

Figure 6 A sectional view of a recessed impeller pump by Goulds see http://www.mackpump.com/CV3196.htm

1.6 SELF-PRIMING PUMPS

Reliable Self-Priming Operation - Before any centrifugal pump will perform, it must

first be primed; that is, air or gases expelled from the suction and impeller eye area,

and replaced with liquid. This is no problem when the pump is submerged (submersible or vertical sump

pumps) or when liquid supply is above the pump. However, when suction pressure is

negative, air must be evacuated to accomplish pump priming. The self-priming pump is designed to insure that a sufficient quantity of liquid to

reprime is always retained in the priming chamber (see Figure 7b).

Figure 7a Outside dimensions for a self-priming pump by Goulds see

http://www.gouldspumps.com/cat_pumps.ihtml?pi

d=226&lastcatid=76&step=4

Figure 7b Priming and pumping action of a self-

priming pump. 1.7 SLURRY PUMPS

The slurry pump is a rugged heavy duty pump intended for aggressive or abrasive slurry solutions with particles of various sizes. It achieves this by lining the inside of the pump

casing as well as the impeller with rubber (see Figure 8). All though rubber does

eventually wear, the elasticity of its surface allows the hard mineral particles to bounce off thereby reducing what would be otherwise very aggressive erosion. These pumps

are used wherever abrasive slurries need to be pumped, especially in the mining

industry. The NPSH requirement for these types of pumps is typically higher than

comparative standard centrifugal pumps.

Figure 8 Slurry pump by Warman see http://www.warman.co.za/

1.9 LOW FLOW HIGH HEAD PUMPS (RADIAL VANE OR PARTIAL EMISSION)

This radial vane or partial emission pump (see Figure 10) is a frame mounted, end

suction, top centerline discharge, ANSI pump designed specifically to handle

corrosive chemicals at low flows. By low flows we mean:

Flows outside the recommended operating range of typical end

suction pumps. Low flows which require users to throttle end suction pumps to

operating conditions well below their best efficiency point. Flows that increase mechanical vibration, decrease bearing and seal

life, increase maintenance costs, and decrease the pump life of end

suction pumps. In other words, the radial vane pump is designed to operate where standard end suction

pumps operate poorly - at throttled low flows.

Figure 10 Low flow vane pump by Goulds

see http://www.mackpump.com/LF3196.htm 1.9 LOW N.P.S.H. PUMPS (LOW FLOW, HIGH HEAD) This type of pump is used where the available N.P.S.H. is low (see Figure 10). It is

specifically designed for low flow and high head requirements and offers good efficiency

even under these conditions (see the article published by Industrial Technology magazine

at http://www.industrialtechnology.co.uk/1998/may/impeller.html).

Figure 10 Low N.P.S.H. pump (model CPX) by Flowserve see http://www.fpdlit.com/cms/results_detail.asp?ModelID=10

1.10 SLUDGE PUMP

Certain types of sludges tend to settle very quickly and are hard to keep in suspension. The

Lawrence pump company has solved this problem by putting an agitator in front of the pump

suction (see Figure 11). For more info see the Pumps & systems magazine, issue March

2004 at http://www.pump-zone.com/.

Figure 11 Lawrence Series 5100 submersible

pump with sludge agitator,

In the selection process, you will be trying to match your flow rate with the B.E.P. of the pump. It is

not always possible to match the flow rate with the B.E.P. (best efficiency point), if this is not possible,

try to remain in the range of 80% to 110% of the B.E.P..

Desirable selection area for impeller size for centrifugal pumps.

Operating outside this range will lead to excessive vibration, recirculation and cavitation, see the next

two figures. The first one from the Pump Handbook from McGraw-Hill which shows how the axial

force increases with the distance in terms of percent flow from the B.E.P. and the second from Goulds

essentially shows the same information but in terms of vibration.

Radial force vs. % flow of BEP

Vibration level vs. flow Electronic pump curves have been created for many (over 50) manufacturers, . They have all been

developed by Engineered Software located in Lacey Washington USA of which I am a representative.

Their pump sizing software PUMP-FLO can help find the best pump for the application, it can select

the closest one to the B.E.P. for you and do all kinds of searches based on NPSHR, efficiency, size, etc.

When you order your pump make sure that the motor is installed with spacer blocks so that the next

largest motor frame can be installed.

18. Air in pump reduces capacity When air enters a pump it sometimes gets trapped in the volute, this reduces the capacity, creates vibration and noise. To remedy, shut the pump down and open the vent valve to remove the air. If the pump is excessively noisy do not automatically assume that the problem is cavitation, air in the pump creates vibration and noise. Cavitation produces a distinct noise similar to gravel in a cement mixer. If

you have never heard the sound of cavitation here's a recording of it in WAV format, courtesy of my friend Normand Chabot, water hammer specialist here in Montreal. 19. Effect of viscosity on pump performance Viscosity is the main criteria which determines whether the application requires a centrifugal pump or a positive displacement pump. Centrifugal pumps can pump viscous fluids however the performance is adversely affected. If your fluid is over 400 cSt (centiStokes) in viscosity consider using a positive displacement pump.

20. Avoid running pump in reverse direction Avoid running a pump in reverse direction, pump shafts have been broken this way especially if the pump is started while running backwards. The simplest solution is to install a check valve on the discharge line. 21. Minimum flow rate Most centrifugal pumps should not be used at a flow rate less than 50% of the B.E.P. (best efficiency point) flow rate without a recirculation line. If your system requires a flow rate of 50% or less then use a recirculation line to increase the flow through the pump keeping the flow low in the system, or install a variable speed drive.

How is the minimum flow of a centrifugal pump established (answer from the Hydraulic Institute http://www.pumps.org/content_detail.aspx?id=2138) The factors which determine minimum allowable rate of flow include the following: * Temperature rise of the liquid -- This is usually established as 15F and results in a very low limit. However, if a pump operates at shut off, it could overheat badly. * Radial hydraulic thrust on impellers -- This is most serious with single volute pumps and, even at flow rates as high as 50% of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage. * Flow re-circulation in the pump impeller -- This can also occur below 50% of BEP causing noise, vibration, cavitation and mechanical damage. * Total head characteristic curve - Some pump curves droop toward shut off, and some VTP curves show a dip in the curve. Operation in such regions should be avoided. There is no standard which establishes precise limits for minimum flow in pumps, but "ANSI/HI 9.6.3-1997 Centrifugal and Vertical Pumps - Allowable Operating Region" discusses all of the factors involved and provides recommendations for the "Preferred Operating Region".

22. Three important points on the pump characteristic curve The performance or characteristic curve of the pump provides information on the relationship between total head and flow rate. There are three important points on this curve

1. The shut-off head, this is the maximum head that the pump can achieve and occurs at zero flow. The pump will be noisy and vibrate excessively at this point. The pump will consume the least amount of power at this point. See also the pump glossary. 2. The best efficiency point B.E.P. this is the point at which the pump is the most efficient and operates with the least vibration and noise. This is often the point for which pumps are rated and which is indicated on the nameplate. The pump will consume the power corresponding to its B.E.P. rating at this point. 3. The maximum flow point, the pump may not operate past this point. The pump will be noisy and vibrate excessively at this point. The pump will consume the maximum amount of power at this point. Sometimes the characteristic curve will include a power consumption curve. This curve is only valid for water, if the fluid has a different density than water you cannot use this curve. However you can use the total head vs. flow rate curve since this is independent of density.

Typical centrifugal pump characteristic curve. If your fluid has a different viscosity than water you cannot use the characteristic curve without correction. Any fluid with a viscosity higher than 10 cSt will require a correction. Water at 60F has a viscosity of 1 cSt.

23. Normal, flat and drooping characteristic curves There are three different characteristic curve profiles for radial flow pumps. Figure 4 shows the various vane profiles that exist and the relationship between them. This tip is related to the radial vane profile which is the profile of the typical centrifugal pumps.

Pump vane profiles vs. specific speed. There are three different curve profiles shown in the next figure: 1. Normal, head decreases rapidly as flow increases 2. Flat, head decreases very slowly as flow increases 3. Drooping, similar to the normal profile except at the low flow end where the head rises then drops as it gets to the shut-off head point.

Different types of radial pump characteristic curve profiles. The drooping curve shape is to be avoided because it is possible for the pump to hunt between two operating points which both satisfy the head requirement of the system. This is known to happen when two pumps are in parallel, when the second pump is started it may fail to get to the operating point or hunt between two points that are at equal head. Thankfully not to many pumps have this characteristic, here are a few:

Drooping curve (Goulds).

Drooping curve (Sundyne). A flat curve is sometimes desirable since a change in flow only causes a small change in head, for example as in a sprinkler system. As more sprinklers are turned on the head will tend to decrease but because the curve is flat the head will decrease only a small amount which means that the pressure at the sprinkler will drop only a

small amount, thereby keeping the water velocity high at the sprinkler outlet. The National Fire Prevention Association (N.F.P.A.) code stipulates that the characterictic curve must be flat within a certain percentage. This code can be purchased at ANSI. The normal curve can be more or less steep. A steep curve can be desirable from a control point of view since a small change in flow will result in a large pressure drop. The steepness of the curve depends on the number of vanes and the specific speed.

24. Suction piping Many people are way to CONSERVATIVE about suction piping design. The usual advice you get is make the piping as straight, as big and short as possible. I have seen a suction line 300 ft long, now that's not short. I believe the important considerations are: - by all means make the pipe as short and straight as possible, particularly if the fluid has suspended solids which may cause plugging or hangups. - make sure there is sufficient pressure at the pump suction (this means check the NPSHA against the NPSHR); - make sure that the stream flow lines are coming in nice and straight at the pump suction. This generally means having 5 to 10D straight pipe ahead of the pump inlet. Avoid the use of filters at the pump inlet if at all possible. Their maintenance will often be neglected and the pump will suffer from poor performance and perhaps cavitation. Use a 90 or 45 elbow at the pumps inlet pipe end. This will allow almost complete drainage of the tank and is especially useful in the case of fluids that can not be readily dumped to the sewers. It also provides additional submergence reducing the risk of vortex formation.

Also be careful of elbows that are too close to the pump suction. 25. The meaning of specific speed If you are having trouble with a pump or want to check whether the new pump to be installed is appropriate, check the specific speed and the suction specific speed of the pump. The specific speed provides a number which can help identify the type of pump (for example radial or axial flow) that is best suited for your application. The specific speed of the pump type selected (see Figure 4) should be close to the specific speed calculated for your application. The suction specific speed will tell you if the suction of the pump is likely to cause problems in your application.

26. Different types of centrifugal pumps

There are many different types of pumps available other than the standard end suction, submersible or

vertical multi-stage pump. In this article, you will see a number of pumps that are specialized and may

suit a particular need.

27. Unusual aspects of pump systems

This article discusses unusual aspects of pump systems: variation in pressure throughout the system

and effect of fluid properties.

Synopsis

There is a number called the specific speed of a pump whose value tells us something about

the type of pump. Is it a radial type pump which provides high head and low flow or an axial or

propeller type pump which provides low flow but high head or something in between. If you are worried whether you have the right type of pump or not this number will help you decide.

The article gives you an example of how to calculate this number. Also if you are worried that

your pump may be cavitating there is another number related to specific speed called suction specific speed that will help you diagnose and avoid cavitation. There is a multitude of pump designs that are available for any given task. Pump designers have needed a way to compare the efficiency of their designs across a large range of pump model and types. Pump users also would like to know what efficiency can be expected from a particular pump design. For that purpose pump have been tested and compared using a

number or criteria called the specific speed (NS) which helps to do these comparisons. The

efficiency of pumps with the same specific speed can be compared providing the user or the

designer a starting point for comparison or as a benchmark for improving the design and increase the efficiency. Equation [1] gives the value for the pump specific speed, H is the pump total head, N the speed of the impeller and Q the flow rate.

NS

N(rpm) Q(USgpm) [1]

H( ft fluid)0.75

Figure 1 Specific speed values for the different pump designs.

(source: the Hydraulic Institute Standards book, see www.pumps.org) Pumps are traditionally divided into 3 types, radial flow (see Figure 2), mixed flow (see Figure

3) and axial flow (see Figure 4). There is a continuous change from the radial flow impeller,

which develops pressure principally from the action of centrifugal force, to the axial flow

impeller, which develops most of its head by the propelling or lifting action of the vanes on

the liquid.

Many pump types have been tested and their efficiency measured and plotted in Figure 5. Notice that larger pumps are inherently more efficient. Efficiency drops rapidly at specific

speeds of 1000 or less.

Figure 2 Radial flow Figure 3 Mixed flow Figure 4 Axial flow

pump cross-section,

pump cross-section, pump cross-section,

(source: Hydraulic

(source: Hydraulic (source: Hydraulic

Institute

Institute Institute

www.pumps.org).

www.pumps.org). www.pumps.org).

Figure 5 Efficiency values for pump with different specific speeds (source: The Pump Handbook published by McGraw Hill)

The following chart provides the efficiency data for pumps of various types vs the

flow rate and maybe easier to read than Figure 5. However some corrections are

required (use the chart in the upper left corner of Figure 6) to the values predicted.

Figure 6 Efficiency values for pumps of different types (source: The

Hydraulic Institute www.pumps.org).

28. Predict pump efficiency Save time in the initial phase of the project and calculate power requirement prior to the final

pump selection using a chart that predicts the efficiency of standard end suction centrifugal

pumps Alternatively, compare the efficiency of the final pump selection with the industry

average.

You will notice that efficiency increases with specific speed, this means that a pump with a

higher speed (rpm) that meets your requirements will be smaller and more efficient and therefore

cost less to operate,