Pump Intro

8/13/2019 Pump Intro

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A BRIEF INTRODUCTION TO

CENTRIFUGAL PUMPS

Joe Evans, Ph.D

This publication is based upon anintroductory, half day class that I presentedmany years ago. It is designed to providethe new comer with an entry levelknowledge of centrifugal pump theory andoperation. Of equal importance, it will makehim aware of those areas that will requireadditional study if he is to become trulyproficient.

You will notice references to our Puzzler.These brain teasing discussions investigatethe specifics of pumps, motors, and theircontrols and can be downloaded from theeducation section of our web site. Thisintroduction, on the other hand, is muchmore structured and sticks to the basics.

Some of the figures you will see have beenreduced in size substantially. If you havedifficulty reading them, just increase their sizeusing the Acrobat scaler. If you have

comments or suggestions for improving oureducational material, please email me [email protected].

INTRODUCTION

Definition & Description

By definition, a centrifugal pump is a

machine. More specifically, it is a machinethat imparts energy to a fluid. This energyinfusion can cause a liquid to flow, rise to ahigher level, or both.

The centrifugal pump is an extremely simplemachine. It is a member of a family knownas rotary machines and consists of two basicparts: 1) the rotary element or impeller and 2)the stationary element or casing (volute). The

figure below is a cross section of a centrifugalpump and shows the two basic parts.

VOLUTE

IMPELLER

Figure 1

The centrifugal pumps function is as simpleas its design. It is filled with liquid and theimpeller is rotated. Rotation imparts energyto the liquid causing it to exit the impellersvanes at a greater velocity than it possessedwhen it entered. This outward flow reducesthe pressure at the impeller eye, allowingmore liquid to enter. The liquid that exits theimpeller is collected in the casing (volute)where its velocity is converted to pressure

before it leaves the pumps discharge.

A Very Brief History

The centrifugal pump was developed inEurope in the late 1600s and was seen in theUnited States in the early 1800s. Its widespread use, however, has occurred only in thelast seventy-five years. Prior to that time, thevast majority of pumping applicationsinvolved positive displacement pumps.

The increased popularity of centrifugal pumpsis due largely to the comparatively recentdevelopment of high speed electric motors,steam turbines, and internal combustionengines. The centrifugal pump is a relativelyhigh speed machine and the development ofhigh speed drivers has made possible thedevelopment of compact, efficient pumps.


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Since the 1940's, the centrifugal pump hasbecome the pump of choice for manyapplications. Research and development hasresulted in both improved performance andnew materials of construction that havegreatly expanded it's field of applicability. Itis not uncommon today to find efficiencies of

93%+ for large pumps and better than 50%for small fractional horsepower units.

Modern centrifugal pumps have been built tomeet conditions far beyond what wasthought possible fifty to sixty years ago.Pumps capable of delivering over 1,000,000gallons per minute at heads of more than 300feet are common in the nuclear powerindustry. And, boiler feed pumps have beendeveloped that deliver 300 gallons per minuteat more than 1800 feet of head.

THEORY

In operation, a centrifugal pump slingsliquid out of the impeller via centrifugal force.Now centrifugal force, itself, is a topic ofdebate. Although I will not go into detailhere, it is considered by many, includingmyself, to be a false force. For our purposeshere, we will assume that it is a real force.Refer to the False Force Puzzler for moreinformation.

Centrifugal Force

A classic example of the action of centrifugalforce is shown below. Here, we see a pail ofwater swinging in a circle. The swinging pail

Figure 2

generates a centrifugal force that holds thewater in the pail. Now, if a hole is bored inthe bottom of the pail, water will be thrownout. The distance the stream carries (tangent

to the circle) and the volume that flows out(per unit time) depends upon the velocity ( inft/sec) of the rotating pail. The faster the pailrotates the greater the centrifugal force andtherefore the greater the volume of waterdischarged and the distance it carries.

The description above could be consideredthat of a crude centrifugal pump (sans voluteof course). It demonstrates that the flow andhead (pressure) developed by a centrifugalpump depends upon the rotational speed and,more precisely, the peripheral velocity of itsimpeller (pail).

Peripheral Velocity & Head

Gravity is one of the more important forces

that a centrifugal pump must overcome. Youwill find that the relationship between finalvelocity, due to gravity, and initial velocity,due to impeller speed, is a very useful one .

If a stone is dropped from the top of abuilding it's velocity will increase at a rate of32.2 feet per second for each second that itfalls. This increase in velocity is known asacceleration due to gravity. Therefore if weignore the effect of air resistance on the fallingstone, we can predict the velocity at which itwill strike the ground based upon its initialheight and the effect of acceleration due togravity.

The equation that describes the relationship ofvelocity, height, and gravity as it applies to afalling body is:

v2 = 2gh

Where:

v = The velocity of the body in ft/sec

g = The acceleration due to gravity @ 32.2

ft/sec/sec (or ft/sec2)

h = The distance through which the body falls


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For example if a stone is dropped from abuilding 100 feet high:

v2 = 2 x 32.2 ft/sec2x 100 ft

v2 = 6440 ft2/sec2

v = 80.3 ft/sec

The stone, therefore, will strike the ground ata velocity of 80.3 feet per second.

This same equation allows us to determinethe initial velocity required to throw the stoneto a height of 100 feet. This is the casebecause the final velocity of a falling bodyhappens to be equal to the initial velocityrequired to launch it to height from which it

fell. In the example above, the initial velocityrequired to throw the stone to a height of 100feet is 80.3 feet per second, the same as itsfinal velocity.

The same equation applies when pumpingwater with a centrifugal pump. The velocityof the water as it leaves the impellerdetermines the head developed. In otherwords the water is thrown to a certainheight. To reach this height it must start withthe same velocity it would attain if it fell from

that height.

If we rearrange the falling body equation weget:

h = v2/2g

Now we can determine the height to which abody (or water) will rise given a particularinitial velocity. For example, at 10 Ft per Sec:

h = 10 ft/sec x 10 ft/sec / 2 x 32.2 ft/sec2

h = 100 ft2/sec2/ 64.4 ft/sec2

h = 1.55 ft

If you were to try this with several differentinitial velocities, you would find out that thereis an interesting relationship between theheight achieved by a body and its initial

velocity. This relationship is one of thefundamental laws of centrifugal pumps andwe will review it in detail a little later. As afinale to this section, let's apply what we havelearned to a practical application.

Problem: For an 1800 RPM pump, find the

impeller diameter necessary to develop ahead of 200 feet.

First we must find the initial velocity requiredto develop a head of 100 feet:

v2 = 2 gh v2 = 2 x 32.2 ft/sec2x 200 ft

V2 = 12880 ft2/sec2

v = 113 ft/sec

We also need to know the number ofrotations the impeller undergoes each second:

1800 RPM / 60 sec = 30 RPS

Now we can compute the number of feet apoint on the impellers rim travels in a singlerotation:

113 ft/sec / 30 rotations/sec = 3.77 ft/rotation

Since feet traveled per rotation is the same asthe circumference of the impeller we cancompute the diameter as follows:

Diameter = Circumference /

Diameter = 3.77 Ft / 3.1416

Diameter = 1.2 Ft or 14.4 In

Therefore an impeller of approximately 14.4"

turning at 1800 RPM will produce a head of200 Feet.

It just so happens that water, flowing fromthe bottom of a tank, follows the same rules.See the Up and Down Puzzler tounderstand how.


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THE PERFORMANCE CURVE

Once a pump has been designed and is readyfor production, it is given a complete andthorough test. Calibrated instruments areused to gather accurate data on flow, head,

horsepower, and net positive suction headrequired. During the test, data is recorded atshut off (no flow), full flow, and 5 to 10 pointsbetween. These points are plotted on a graphwith flow along the X axis (abscissa) and headalong the Y axis (ordinate). The efficiency,brake horsepower, and Net Positive SuctionHead Required (NPSHR) scales are alsoplotted as ordinates. Therefore, all values ofhead, efficiency, brake horsepower, andNPSHR are plotted versus capacity andsmooth curves are drawn through the points.

This curve is called the Characteristic Curvebecause it shows all of the operatingcharacteristics of a given pump. The curvebelow is an example.

Figure 3

For publication purposes, it is much moreconvenient to draw several curves on a singlegraph. This presentation method shows a

number of Head-Capacity curves for onespeed and several impeller diameters or oneimpeller diameter and several differentspeeds for the same pump. This type ofcurve is called an Iso-Efficiency or CompositeCharacteristic Curve. You will note that theefficiency and horsepower curves arerepresented as contour lines. Also theNPSHR curve applies only to the Head-Capacity curve for the full size impeller.

NPSHR will increase somewhat for smallerdiameters. The curve below is a typicalComposite Curve.

Figure 4

The method of reading a performance curveis the same as for any other graph. Forexample; in Figure 3, to find the head,horsepower and efficiency at 170 GPM, youlocate 170 GPM on the abscissa and read thecorresponding data on the ordinate. Thepoint on the head / capacity curve that alignswith the highest point on the efficiency curveis known as the Best Efficiency Point or BEP.In Figure 3, it occurs at approximately 170GPM @ 125TDH.

The composite curve shown in Figure 4 isread in much the same manner. Head is readexactly the same way; however, efficiencymust be interpolated since the Head-Capacitycurve seldom falls exactly on an efficiencycontour line. Horsepower can also beinterpolated as long as it falls below ahorsepower contour line. If the Head-Capacity curve intersects or is slightly abovethe horsepower contour line, brake

horsepower may need to be calculated.NPSHR is found in the same manner as head.We will discuss NPSHR in detail a little later.

Calculating Brake Horsepower (BHP)

Usually, if a specific Head-Capacity point fallsbetween two horsepower contour lines, thehigher horsepower motor is selected.Sometimes, however, we may need to know


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the exact horsepower requirement for thatpoint of operation. If so Brake BHP for acentrifugal pump can be calculated as follows:

BHP = GPM X Head / 3960 X Efficiency

For example, in Figure 4, the BHP required at

170 GPM for the 5 1/2" impeller is:

BHP = 170 X 90 / 3960 X .74

BHP = 15300 / 2860.4

BHP = 5.22

Many operating points will fall between thevarious curves, so it is important that youunderstand a composite curve well enough tointerpolate and find the approximate values.

A pump is typically designed for one specificcondition but its efficiency is usually highenough on either side of the design point toaccommodate a considerable capacity range.Often, the middle one third of the curve issuitable for application use.

Different pumps, although designed forsimilar head and capacity can vary widely inthe shape of their Characteristic Curves. Forinstance, if two pumps are designed for 200GPM at 100' TDH, one may develop a shut offhead of 110' while the other may develop ashut off head of 135'. The first pump is said tohave a flat curve while the second is said tohave a steep curve. The steepness of thecurve is judged by the ratio of the head atshut off to that at the best efficiency point.Each type of curve has certain applications forwhich it is best suited.

Operation in Series (Booster Service)

When a centrifugal pump is operated with apositive suction pressure, the resultingdischarge pressure will be the sum of thesuction pressure and the pressure normallydeveloped by the pump when operating atzero suction pressure. It is this quality of acentrifugal pump that makes it ideally suitedfor use as a booster pump. This quality alsomakes it practical to build multi-stage

(multiple impeller) pumps. A booster pumptakes existing pressure, whether it be from anelevated tank or the discharge of anotherpump, and boosts it to some higher pressure.

Two or more pumps can be used in series toachieve the same effect. The figure below

shows the curves for two identical pumpsoperated in series. Both the head andhorsepower at any given point on thecapacity curve are additive. Capacity,however, remains the same as that of eitherone of the pumps.

Figure 5

Parallel Operation

Two or more pumps may also be operated inparallel. The curves developed during paralleloperation are illustrated below.

Figure 6


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Pumps operating in parallel take their suctionfrom a common header or supply anddischarge into a common discharge. AsFigure 6 illustrates, the flows and horsepowerare additive. You will notice that, while headdoes not change, flow is almost doubled atany given point.

THE AFFINITY LAWS

The Centrifugal Pump is a very capable andflexible machine. Because of this it isunnecessary to design a separate pump foreach job. The performance of a centrifugalpump can be varied by changing the impellerdiameter or its rotational speed. Eitherchange produces approximately the sameresults. Reducing impeller diameter is

probably the most common change and isusually the most economical. The speed canbe altered by changing pulley diameters orby changing the speed of the driver. In somecases both speed and impeller diameter arechanged to obtain the desired results.

When the driven speed or impeller diameterof a centrifugal pump changes, operation ofthe pump changes in accordance with threefundamental laws. These laws are known asthe "Laws of Affinity". They state that:

1) Capacity varies directly as the change inspeed or impeller diameter

2) Head varies as the square of the change inspeed or impeller diameter

3) Brake horsepower varies as the cube ofthe change in speed or impeller diameter

If, for example, the pump speed or impellerdiameter were doubled:

1) Capacity will double

2) Head will increase by a factor of 4 (2 tothe second power)

3) Brake horsepower will increase by a factorof 8 (2 to the third power)

These principles apply regardless of thedirection (up or down) of the speed orchange in diameter.

Consider the following example. A pumpoperating at 1750 RPM, delivers 210 GPM at75' TDH, and requires 5.2 brake horsepower.

What will happen if the speed is increased to2000 RPM? First we find the speed ratio.

Speed Ratio = 2000/1750 = 1.14

From the laws of Affinity:

1) Capacity varies directly or:

1.14 X 210 GPM = 240 GPM

2) Head varies as the square or:

1.14 X 1.14 X 75 = 97.5' TDH

3) BHP varies as the cube or:

1.14 X 1.14 X 1.14 X 5.2 = 7.72 BHP

Theoretically the efficiently is the same forboth conditions. By calculating several pointsa new curve can be drawn.

Whether it be a speed change or change inimpeller diameter, the Laws of Affinity giveresults that are approximate. The discrepancybetween the calculated values and the actualvalues obtained in test are due to hydraulicefficiency changes that result from themodification. The Laws of Affinity givereasonably close results when the changes arenot more than 50% of the original speed or15% of the original diameter.

Here, we have only described the affinitylaws and applied them to a few examples.For an in depth discussion and proof of theirvalidity, see the Affinity Puzzler.


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SPECIFIC GRAVITY AND VISCOSITY

Specific Gravity

(or why head is usually expressed in feet)

The Specific Gravity of a substance is the ratioof the weight of a given volume of thesubstance to that of an equal volume of waterat standard temperature and pressure (STP).Assuming the viscosity of a liquid is similar tothat of water the following statements willalways be true regardless of the specificgravity:

1) A Centrifugal pump will always developthe same head in feet regardless of a liquidsspecific gravity.

2) Pressure will increase or decrease in directproportion to a liquids specific gravity.

3) Brake HP required will vary directly with aliquids specific gravity.

The Figure below illustrates the relationshipbetween pressure (in psi) and head (in ft) forthree liquids of differing specific gravity.

Figure 7

We can see that the level in each of the threetanks is 100 feet. The resulting pressure at the

bottom of each varies substantially as a resultof the varying specific gravity. If, on theother hand we keep pressure constant asmeasured at the bottom of each tank, thefluid levels will vary similarly.

A centrifugal pump can also develop 100' of

head when pumping water, brine, andkerosene. The resulting pressures, however,will vary just as those seen in Figure 7. If thatsame pump requires 10 HP when pumpingwater, it will require 12 HP when pumpingbrine and only 8 HP when pumpingkerosene.

The preceding discussion of Specific Gravityillustrates why centrifugal pump head (orpressure) is expressed in feet. Since pumpspecialists work with many liquids of varying

specific gravity, head in feet is the mostconvenient system of designating head.When selecting a pump, always rememberthat factory tests and curves are based onwater at STP. If you are working with otherliquids always correct the HP required for thespecific gravity of the liquid being pumped.

The Effect of Viscosity

Viscosity is a fluid property that isindependent of specific gravity. Just asresistivity is the inherent resistance of aparticular conductor, viscosity is the internalfriction of a fluid. The coefficient of viscosityof a fluid is the measure of its resistance toflow. Fluids having a high viscosity aresluggish in flow. Examples include molassesand heavy oil. Viscosity usually varies greatlywith temperature with viscosity decreasing astemperature rises.

The instrument used to measure viscosity is

the viscometer. Although there are many, theSaybolt Universal is the most common. Itmeasures the time in seconds required for agiven quantity of fluid to pass through astandard orifice under STP. The unit ofmeasurement is the SSU or Seconds SayboltUniversal.

High viscosity can gum up (pun intended) theinternals of a centrifugal pump. Viscous


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liquids tend to reduce capacity, head, andefficiency while increasing the brake HP. Ineffect this tends to steepen the head - capacityand HP curves while lowering the efficiencycurve.

The performance of a small Centrifugal

Pump, handling liquids of various viscosities,is shown graphically in the figure below.

Figure 8

Normally, small and medium sizedcentrifugal pumps can be used to handleliquids with viscosities up to 2000 SSU. Below50 SSU the Characteristic Curves remainabout the same as those of water; however,there is an immediate decrease in efficiency

when viscosity increases over that of water.Viscosities over 2000 SSU are usually bettersuited for positive displacement pumps.

FRICTION AND FRICTION HEAD

Friction occurs when a fluid flows in oraround a stationary object or when an object

moves through a fluid. An automobile andan aircraft are subjected to the effects offriction as they move through ouratmosphere. Boats create friction as theymove through the water. Finally liquidscreate friction as they move through a closedpipe.

A great deal of money has been spent on thedesign and redesign of boats, aircraft, andautos to reduce friction (often referred to asdrag). Why? Because friction produces heatand where there is heat there is energy,wasted energy that is. Making these vehicles"slippery" reduces friction therefore reducingthe energy required to get them from point Ato point B.

As water moves through a pipe its contact

with the pipe wall creates friction. As flow (ormore correctly velocity) increases, so doesfriction. The more water you try to cramthrough a given pipe size the greater thefriction and thus the greater the energyrequired to push it through. It is because ofthis energy that friction is an extremelyimportant component of a pumping system.

Laminar flow describes the flow of a liquid ina smooth pipe. Under conditions of laminarflow, the fluid nearest the pipe wall movesmore slowly than that in the center. Actuallythere are many gradients between the pipewall and the center. The smaller the pipediameter, the greater the contact between theliquid and the wall thus the greater thefriction. The figure below shows laminar flowthrough two cross sections of a pipe. Thevector lengths in the right hand drawing areproportional to the velocity of the flowingliquid.

Figure 9


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Friction or Friction Head is defined as theequivalent head in feet of liquid necessary toovercome the friction caused by flow througha pipe and its associated fittings.

Friction tables are universally available for awide range of pipe sizes and materials. They

are also available for various pipe fittings andvalves. . These tables show the friction lossper 100 feet of a specific pipe size at variousflow rates. In the case of fittings, friction isstated as an equivalent length of pipe of thesame size. An example is shown below.

Figure 10

It is evident from the tables, that frictionincreases with flow. It is also evident thatthere is an optimum flow for each pipe size,after which friction can use up adisproportionate amount of the pump output.For example, 100 GPM for short lengths of 2"pipe may be acceptable; however, over

several hundred feet the friction loss will beunacceptable. 2 1/2" pipe will reduce thefriction by 2/3 per 100 feet and is a muchbetter choice for a 200 foot pipeline. Forsystems that operate continuously 3" pipemay be the appropriate choice.

Many friction tables show both friction lossand fluid velocity for a given flow rate.Generally it is wise to keep fluid velocityunder 10 feet per second. If this rule isfollowed, friction will be minimized.

The friction losses for valves and fittings canalso add up . 90 degree turns and restrictivevalves add the most friction. If at all possiblestraight through valves and gentle turnsshould be used.

Consider this problem: What head must apump develop if it is to pump 200 GPMthrough a 2.5" pipe, 200 feet long, and to anelevation of 75'? Use the table in Figure 10.

Elevation = 75'

Friction 2.5 Pipe = 43' / 100ft or 86'

Total head = 75' + 86' = 161' TDH

Use of 3" pipe in the above problem willreduce friction head by 30% and although itwill cost more initially, it will pay for itself inenergy savings over a fairly short period oftime.

The Hot and Cold Puzzler delves deeperinto fluid friction and its consequences.

SUCTION CONDITIONS

Suction conditions are some of the mostimportant factors affecting centrifugal pumpoperation. If they are ignored during the


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design or installation stages of an application,they will probably come back to haunt you.

Suction Lift

A pump cannot pull or "suck" a liquid up itssuction pipe because liquids do not exhibit

tensile strength. Therefore, they cannottransmit tension or be pulled. When a pumpcreates a suction, it is simply reducing localpressure by creating a partial vacuum.Atmospheric or some other external pressureacting on the surface of the liquid pushes theliquid up the suction pipe into the pump.

Atmospheric pressure at sea level is calledabsolute pressure (PSIA) because it is ameasurement using absolute zero (a perfectvacuum) as a base. If pressure is measured

using atmospheric pressure as a base it iscalled gauge pressure (PSIG or simply PSI).

Atmospheric pressure, as measured at sealevel, is 14.7 PSIA. In feet of head it is:

Head = PSI X 2.31 / Specific Gravity

For Water it is:

Head = 14.7 X 2.31 / 1.0 = 34 Ft

Thus 34 feet is the theoretical maximumsuction lift for a pump pumping cold water atsea level. No pump can attain a suction lift of34 ft; however, well designed ones can reach25 ft quite easily.

You will note, from the equation above, thatspecific gravity can have a major effect onsuction lift. For example, the theoreticalmaximum lift for brine (Specific Gravity = 1.2)at sea level is 28 ft.. The realistic maximum is

around 20ft. Remember to always factor inspecific gravity if the liquid being pumped isanything but clear, cold (68 degrees F) water.

In addition to pump design and suctionpiping, there are two physical properties ofthe liquid being pumped that affect suctionlift.

1) Maximum suction lift is dependent upon

the pressure applied to the surface of theliquid at the suction source. Maximumsuction lift decreases as pressure decreases.2) Maximum suction lift is dependent uponthe vapor pressure of the liquid beingpumped. The vapor pressure of a liquid is thepressure necessary to keep the liquid from

vaporizing (boiling) at a given temperature.Vapor pressure increases as liquidtemperature increases. Maximum suction liftdecreases as vapor pressure rises.

It follows then, that the maximum suction liftof a centrifugal pump varies inversely withaltitude. Conversely, maximum suction liftwill increase as the external pressure on itssource increases (for example: a closedpressure vessel). The figure below shows therelationship of altitude and atmospheric

pressure.

Figure 11A pumping application located at an elevationof 5000 feet will experience a reduction inatmospheric pressure of approximately sixfeet. This will result in a reduction in NPSHA(discussed in the next section) by the sameamount. Elevation must be factored into apumping application if the installation is morethan a few hundred feet above sea level

The maximum suction lift of a liquid varies

inversely with the temperature of the liquid.The higher the temperature, the higher thevapor pressure and thus suction lift isdecreased. If a centrifugal pump is used topump a liquid that is too hot the liquid willboil or vaporize in the pump suction. Thiscondition is called cavitation and will bediscussed in more detail later.


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The figure below shows the relationshipbetween vapor pressure and temperature forclear,cold water.

Figure 12

At a temperature of 70 degrees F, a pressureof only one foot is required to keep water inthe liquid state. As its temperature rises,however, more and more pressure isrequired. At about 210 degrees, a pressure of34 feet or, sea level atmospheric pressure, isrequired. As it rises to 212 degrees the waterwill boil unless some additional pressure isapplied. When pumping liquids at elevatedtemperatures, the liquids vapor pressure atthat temperature must be included in theNPSHA calculation.

Capacity and Suction Lift

The suction lift of a centrifugal pump alsovaries inversely with pump capacity. This is

illustrated in the figure at the top of theadjoining column. Figure 13 shows how thehead - capacity curve falls off quickly atvarious suction lifts. You will notice thatmaximum suction lift increases as pumpcapacity decreases. For this reason pumpsused in high suction lift applications areselected to operate in a range considerably tothe left of their peak efficiency.

Figure 13

Net Positive Suction Head (NPSH)

Net Positive Suction Head Required (NPSHR)is a function of a specific pump design. Insimple terms it is the pressure, measured atthe centerline of the pump suction, necessaryfor the pump to function satisfactorily at agiven flow. Although NPSHR varies withflow, temperature and altitude have no effect.

Net Positive Suction Head Available (NPSHA)is a characteristic of the system in which thepump operates. It depends upon theelevation or pressure of the suction supply,friction in the suction line, altitude of the

installation, and the vapor pressure of theliquid being pumped.

Both available and required NPSH vary withthe capacity of a given pump and suctionsystem. NPSHA is decreased as the capacity isincreased due to the increased friction lossesin the suction piping. NPSHR increasesapproximately as the square of capacity sinceit is a function of the velocities and friction inthe pump inlet. NPSHA can be calculated asfollows:

NPSHA = Ha + Hs - Hvp

Where:Ha = Atmospheric pressure in feetHs = Total suction head or lift in feetHvp = Vapor pressure in feet


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For example a pump installed at an altitude of2500 ft and has a suction lift of 13 ft whilepumping 50 degree water. What is NPSHA?

NPSHA = Ha + Hs - Hvp

NPSHA = 31 - 13 - .41

NPSHA = 17.59 ft

Often a two foot safety margin is subtractedfrom NPSHA to cover unforeseencircumstances. When selecting a pump forthe conditions above, the NPSHR as shownon the pump's characteristic curve should be15.59 ft or less (17.59 - 2).

Working in the opposite direction, we have apump that requires 8 ft of NPSH at 120GPM.

If the pump is installed at an altitude of 5000 ftand is pumping cold water at 60 degrees,what is the maximum suction lift it can attain?

NPSH = Ha + Hs - Hvp

8 + 2 = 28.2 - Hs - .59

Hs = 28.2 - 8 - 2 - .59

Hs = 17.61 ft (Including the 2 ft margin ofsafety)

The preceding has dealt only with water. Thesame general principles apply to other liquids;however, vapor pressure must be factoredinto the equations. For roughapproximations where the vapor pressure isunknown, a pump will usually operatesatisfactorily if the NPSHA is equal to orgreater than that required for water undersimilar conditions. This method may be usedonly when the viscosity of the liquid is

approximately the same as water.

Some of the illustrations used in thisintroduction were adapted from CentrifugalPump Manual by F.E. Myers, a member ofthe Pentair Pump Group.

7 6 1 A h u a H o n o lu l u , H I 9 6 8 1 9

Ph o n e 8 0 8 . 5 3 6 . 7 6 9 9 Fa x 8 0 8 . 5 3 6 . 8 7 6 1

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