Pumps :Basic fundementals

8/12/2019 Pumps :Basic fundementals

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the weight of the liquid, as shown in %ig.*.

Fig. 1 Identical Pumps Handling Liquids of Different Specific Gravities.All of the forms of energy involved in a liquid flow system can be expressed in terms of feet of liquid. The total of thesevarious heads determines the total system head or the work which a pump must perform in the system. The various forms ofhead are defined as follows.

'+CT) )%T exists when the source of supply is below the center line of the pump. Thus the 'TAT)C '+CT) )%T is thevertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.


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Fig. 2-a Suction Lift S!o"ing Static Heads in a Pumping S#stem $!ere t!e Pump is Located %&ove t!e Suction 'an(. )StaticSuction Head*

'+CT) /0A1 exists when the source of supply is above the centerline of the pump. Thus the 'TAT)C '+CT) /0A1 isthe vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.

Fig. 2-& Suction Head S!o"ing Static Heads in a Pumping S#stem $!ere t!e Pump is Located +elo" t!e Suction 'an(. )StaticSuction Head*

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'TAT)C 1)'C/A230 /0A1 is the vertical distance in feet between the pump centerline and the point of free discharge orthe surface of the liquid in the discharge tank.
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TTA 'TAT)C /0A1 is the vertical distance in feet between the free level of the source of supply and the point of freedischarge or the free surface of the discharge liquid.

%2)CT) /0A1 4hf5 is the head required to overcome the resistance to flow in the pipe and fittings. )t is dependent upon thesi6e, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. %rictional tables areincluded in(ater 1ata.

"0C)T7 /0A1 4hv5 is the energy of a liquid as a result of i ts motion at some velocity ". )t is the equivalent head in feetthrough which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate

the water. "elocity head can be calculated from the following formula

The velocity head is usually insignificant and can be ignored in most high head systems. /owever, it can be a large factorand must be considered in low head systems.

820''+20 /0A1 must be considered when a pumping system either begins or terminates in a tank which is under somepressure other than atmospheric. The pressure in such a tank must first be converted to feet of liquid. A vacuum in thesuction tank or a positive pressure in the discharge tank must be added to the system head, whereas a positive pressure inthe suction tank or vacuum in the dis-charge tank would be subtracted. The following is a handy formula for converting inchesof mercury vacuum into feet of liquid.

The above forms of head, namely static, fr iction, velocity, and pressure, are combined to make up the total system head atany particular flow rate. %ollowing are definitions of these combined or 91ynamic9 head terms as they apply to the pump.

TTA 17A:)C '+CT) )%T 4hs5 is the static suction lift minus the velocity head at the pump suction flange plus thetotal friction head in the suction line. The total dynamic suction lift, as determined on pump test, is the reading of a gauge onthe suction flange, converted to feet of liquid and corrected to the pump centerline;, minus the velocity head at the point ofgauge attachment.

TTA 17A:)C '+CT) /0A1 4hs5 is the static suction head plus the velocity head at the pump suction flange minusthe total friction head in the suction line. The total dynamic suction head, as determined on pump test, is the reading of thegauge on the suction flange, converted to feet of liquid and corrected to the pump centerline;, plus the velocity head at thepoint of gauge attachment.

TTA 17A:)C 1)'C/A230 /0A1 4hd5 is the static discharge head plus the velocity head at the pump discharge flangeplus the total friction head in the discharge line. The total dynamic discharge head, as determined on pump test, is thereading of a gauge at the discharge flange, converted to feet of liquid and corrected to the pump centerline;, plus the velocityhead at the point of gauge attachment.

TTA /0A1 4/5 or TTA 1ynamic /0A1 4T1/5 is the total dynamic discharge head minus the total dynamic suction heador

TDH = hd hs!"ith a suction lift#TDH = hd - hs!"ith a suction head#

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Section A -- Centrifugal Pump Fundamentals

A-2 Capacity

Capacity 4


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Section A -- Centrifugal Pump FundamentalsA-3 Power and EfficiencyThe work performed by a pump is a function of the total head and the weightof the liquid pumped in a given time period. The pump capacity in gpm and theliquid specific gravity are normally used in the formulas rather than the actualweight of the liquid pumped.

8ump input or brake horsepower 4bhp5 is the actual horsepower delivered tothe pump shaft. 8ump output or hydraulic horsepower 4whp5 is the liquidhorsepower delivered by the pump. These two terms are defined by thefollowing formulas.

The constant #=>? is obtained by dividing the number or foot pounds for onehorsepower 4##,???5 by the weight of one gallon of water 4@.## pounds.5

The brake horsepower or input to a pump is greater than the hydraulichorsepower or output due to the mechanical and hydraulic losses incurred inthe pump. Therefore the pump efficiency is the ratio of these two values.

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Section A -- Centrifugal Pump FundamentalsA-4 Specific Speed and Pump Type'pecific speed 4s5 is a non-dimensional design index used to classify pumpimpellers as to their type and proportions. )t is defined as the speed inrevolutions per minute at which a geometrically similar impeller would operateif it were of such a si6e as to deliver one gallon per minute against one foothead.

The understanding of this definition is of design engineering significance only,however, and specific speed should be thought of only as an index used topredict certain pump characteristics. The following formula is used todetermine specific speed
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(here ! 8ump speed in 28:< ! Capacity in gpm at the best efficiency point/ ! Total head per stage at the best efficiency point

The specific speed determines the general shape or class of the impeller as

depicted in %ig. #. As the specific speed increases, the ratio of the impelleroutlet diameter, 1$, to the inlet or eye diameter, 1i, decreases. This ratiobecomes *.? for a true axial flow impeller.

2adial flow impellers develop head principally through centrifugal force.8umps of higher specific speeds develop head partly by centrifugal force andpartly by axial force. A higher specific speed indicates a pump design withhead generation more by axial forces and less by centrifugal forces. An axialflow or propeller pump with a specific speed of *?,??? or greater generates itshead exclusively through axial forces.

2adial impellers are generally low flow high head designs whereas axial flow

impellers are high flow low head designs.

"alues of 'pecific 'peed, s

Fig. , Impeller Design vs Specific Speed

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Section A -- Centrifugal Pump FundamentalsA-5 Net Positie Suction Head !NPSH" and Caitation

The /ydraulic )nstitute defines 8'/ as the total suction head in feetabsolute, determined at the suction no66le and corrected to datum, less thevapor pressure of the liquid in feet absolute. 'imply stated, it is an analysis ofenergy conditions on the suction side of a pump to determine if the liquid willvapori6e at the lowest pressure point in the pump.

The pressure which a liquid exerts on its surroundings is dependent upon itstemperature. This pressure, called vapor pressure, is a unique characteristic


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of every fluid and increased with increasing temperature. (hen the vaporpressure within the fluid reaches the pressure of the surrounding medium, thefluid begins to vapori6e or boil. The temperature at which this vapori6ationoccurs will decrease as the pressure of the surrounding medium decreases.

A liquid increases greatly in volume when it vapori6es. ne cubic foot of water

at room temperature becomes *B?? cu. ft. of vapor at the same temperature.

)t is obvious from the above that if we are to pump a fluid effectively, we mustkeep it in liquid form. 8'/ is simply a measure of the amount of suctionhead present to prevent this vapori6ation at the lowest pressure point in thepump.

8'/ 2equired is a function of the pump design. As the liquid passes fromthe pump suction to the eye of the impeller, the velocity increases and thepressure decreases. There are also pressure losses due to shock andturbulence as the liquid strikes the impeller. The centrifugal force of theimpeller vanes further increases the velocity and decreases the pressure of

the liquid. The 8'/ 2equired is the positive head in feet absolute required atthe pump suction to overcome these pressure drops in the pump and maintainthe maority of the liquid above its vapor pressure. The 8'/ 2equired varieswith speed and capacity within any particular pump. 8ump manufacturerscurves normally provide this information.

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Net Positive Suction Head (NPSH) 8'/ Available is a function of thesystetm in which the pump operates. )t is the excess pressure of the liquid in feet

absolute over its vapor pressure as it arrives at the pump suction. %ig. D showsfour typical suction systems with the 8'/ Available formulas applicable toeach. )t is important to correct for the specific gravity of the liquid and to convertall terms to units of 9feet absolute9 in using the formulas.


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8E! Earometric pressure in feet absolute."8! "apor pressure of the liquid at maximum pumping temperature, in feetabsolute.8 ! 8ressure on surface of liquid in closed suction tank, in feet absolute.s! :aximum static suction lift in feet./! :inimum static suction head in feet.hf! %riction loss in feet in suction pipe at required capacity

Fig. alculation of s#stem /et Positive Suction Head %vaila&le for t#picalsuction conditions.

)n an existing system, the 8'/ Available can be determined by a gauge onthe pump suction. The following formula applies

(here3r ! 3auge reading at the pump suction expressed in feet 4plus if aboveatmospheric, minus if below atmospheric5 corrected to the pump centerline.hv! "elocity head in the suction pipe at the gauge connection, expressed infeet.

Cavitation is a term used to describe the phenomenon, which occurs in apump when there is insufficient 8'/ Available. (hen the pressure of theliquid is reduced to a value equal to or below its vapor pressure the liquidbegins to boil and small vapor bubbles or pockets begin to form. As thesevapor bubbles move along the impeller vanes to a higher pressure area abovethe vapor pressure, they rapidly collapse.


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The collapse, or 9implosion9 is so rapid that it may be heard as a rumblingnoise, as if you were pumping gravel. )n high suction energy pumps, thecollapses are generally high enough to cause minute pockets of fatigue failureon the impeller vane surfaces. This action may be progressive, and undersevere 4very high suction energy5 conditions can cause serious pitting

damage to the impeller.The accompanying noise is the easiest way to recogni6e cavitation. Eesidespossible impeller damage, excessive cavitation results in reduced capacitydue to the vapor present in the pump. Also, the head may be reduced and&orbe unstable and the power consumption may be erratic. "ibration andmechanical damage such as bearing failure can also occur as a result ofoperating in excessive cavitation, with high and very high suction energypumps.

The way to prevent the undesirable effects of cavitation in standard lowsuction energy pumps is to insure that the 8'/ Available in the system is

greater than the 8'/ 2equired by the pump. /igh suction energy pumpsrequire an additional 8'/ margin, above the 8'/ 2equired. /ydraulic)nstitute 'tandard 4A')&/) =.>.*5 suggests 8'/ margin ratios of from *.$ to$.F times the 8'/ 2equired, for high and very high suction energy pumps,when operating in the allowable operating range.

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%-0 /PSH and Suction Specific Speed)n designing a pumping system, it is essential to provide adequate 8'/available for proper pump operation. )nsufficient 8'/ available mayseriously restrict pump selection, or even force an expensive system redesign.n the other hand, providing excessive 8'/ available may needlesslyincrease system cost.

'uction specific speed may provide help in this situation.

'uction specific speed 4'5 is defined as

(here ! 8ump speed 28:38: ! 8ump flow at best efficiency point at impeller inlet 4for double suctionimpellers divide total pump flow by two5.8'/2! 8ump 8'/ required at best efficiency point.

%or a given pump, the suction specific speed is generally a constant - it doesnot change when the pump speed is changed. 0xperience has shown that


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=??? is a reasonable value of suction specific speed. 8umps with a minimumsuction specific speed of =??? are readily available, and are not normallysubect to severe operating restrictions, unless the pump speed pushes thepump into high or very high suction energy.

An e(ample'

%low $,??? 38:G head >?? ft. (hat 8'/Awill be requiredH

Assume at >?? ft., #F?? 28: operation will be required.

A related problem is in selecting a new pump, especially at higher flow, for anexisting system. 'uction specific speed will highlight applications where8'/A may restrict pump selection. An example

0xisting system %low $??? 38:G head >?? ft.G 8'/A #? ft.G 'pecific 3ravity*.?G 'uction o66le > in. - (hat is the maximum speed at which a pump canbe run without exceeding 8'/ availableH 48'/:argin 2atio! *.F from aboveI '.0. ! *B# x *?>5

2unning a pump at this speed would require a gear and at this speed, thepump might not develop the required head. At a mini-mum, existing 8'/ A isconstraining pump selection.

'ame system as *. )s a double suction pump practicalH%or a double suction pump 1e! .BF x >9 ! D.F'.0. ! D.F x #FF? x =??? x *.?'.0. ! *#> x *?>4/igh '.0.5

%or a double suction pump, flow is divided by two.


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+sing a double suction pump is one way of meeting system 8'/ andobtaining a higher head.

The amount of energy in a pumped fluid, that flashes into vapor and thencollapses back to a liquid in the higher pressure area of the impeller inlet,determines the extent of the noise and&or damage from cavitation. 'uction0nergy is defined as

Suction &nerg) = De( $ ( S ( Sg

Where D e = *mpeller e)e diameter !inches#Sg = Specific gravit) of li+uid !Sg - ,. for cold "ater#

/igh 'uction 0nergy starts at *>? x *? > for end suctabtion pumps and *$? x*? > for hori6ontal split case pumps. "ery high suction energy starts at *.Ftimes the /igh 'uction 0nergy values. %or estimating purposes you cannormally assume that the impeller eye diameter is approximately =?J of thesuction no66le si6e, for an end suction pump, and BFJ of the suction si6e fora double suction split case pump.

According to the /ydraulic )nstitute, ans 8'/ margin is required above the8'/2of the pump to supress incipient cavitation. The amount of margin is afunction of 'uction 0nergy and the critical nature of the application as follows

Suction Energy NPSHMargin Ratio(NPSHA/NPSHR)Low 1.1 - 1.

Hig! 1." - 1.#

$ery Hig! 1.# - ".%

'uction specific speed =,???, pump speed #FF? 28:, suction no66le si6e >inch, specific gravity *.?, and the pump type is end suction.

De/ 0 ( 12 = 342Suction &nerg) = De( $ ( S ( Sg= 34 ( 533. ( 06... ( ,.

= ,75 ( ,.1

'ince *B# x *?>K *>? x *?>, this is a /igh 'uction 0nergy pump.


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Section A -- Centrifugal Pump Fundamentals%- Pump !aracteristic urvesThe performance of a centrifugal pump can be shown graphically on acharacteristic curve. A typical characteristic curve shows the total dynamichead, brake horsepower, efficiency, and net positive 'uction head all plotted

over the capacity range of the pump.%igures F, >, L B are non-dimensional curves which indicate the generalshape of the characteristic curves for the various types of pumps. They showthe head, brake horsepower, and efficiency plotted as a percent of their valuesat the design or best efficiency point of the pump.

%ig. F below shows that the head curve for a radial flow pump is relatively flatand that the head decreases gradually as the flow increases. ote that thebrake horsepower increases gradually over the flow range with the maximumnormally at the point of maximum flow.

Fig. 3adial Flo" Pump

:ixed flow centrifugal pumps and axial flow or propeller pumps haveconsiderably different characteristics as shown in %igs. > and B below. Thehead curve for a mixed flow pump is steeper than for a radial flow pump. Theshut-off head is usually *F?J to $??J of the design head, The brakehorsepower remains fairly constant over the flow range. %or a typical axial flowpump, the head and brake horsepower both increase drastically near shutoffas shown in %ig. B.


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Fig. 0 4i5ed Flo" Pump

Fig. %5ial Flo" Pump

The distinction between the above three classes is not absolute, and there aremany pumps with characteristics falling somewhere between the three. %orinstance, the %rancis vane impeller would have a characteristic between the

radial and mixed flow classes. :ost turbine pumps are also in this same rangedepending upon their specific speeds.

%ig. @ below shows a typical pump curve as furnished by a manufacturer. )t isa composite curve which tells at a glance what the pump will do at a givenspeed with various impeller diameters from maximum to minimum. Constanthorsepower, efficiency, and 8'/2 lines are superimposed over the varioushead curves. )t is made up from individual test curves at various diameters.


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Fig. 6 omposite Performance urve

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Section A -- Centrifugal Pump FundamentalsA-# Affinity $awsThe affinity laws express the mathematical relationship between the severalvariables involved in pump performance. They apply to all types of centrifugaland axial flow pumps. They are as follows

*. (ith impeller diameter 1 held constant

(here

< ! Capacity, 38:/ ! Total /ead, %eetE/8 ! Erake /orsepower ! 8ump 'peed, 28:

$. (ith speed held constant


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(hen the performance 4


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This will then be the best efficiency point on the new $??? 28: curve. Ey performing the same calculations for several otherpoints on the *BF? 28: curve, a new curve can be drawn which will approximate the pumps performance at $??? 28:, %ig.=.

Trial and error would be required to solve this problem in reverse. )n other words, assume you want to determine the speedrequired to make a rating of #D# 38: at a head of $?= ft. 7ou would begin by selecting a trial speed and applying the affinitylaws to convert the desired rating to the corresponding rating at *BF? 28:. ( hen you arrive at the correct speed, $??? 28:in this case, the corresponding *BF? 28: rating will fall on the *#9 diameter curve.

Fig. 7

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Section A -- Centrifugal Pump FundamentalsA-% System Cures%or a specified impeller diameter and speed, a centrifugal pump has a fixed and predictable performance curve. The pointwhere the pump operates on its curve is dependent upon the characteristics of the system )n which it is operating, commonlycalled the 'ystem /ead Curve. ..or, the relationship between flow and hydraulic losses;in a system. This representation is ina graphic form and, since friction losses vary as a square of the flow rate, the system curve is parabolic in shape.
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Ey plotting the system head curve and pump curve together, it can be determined

*. (here the pump will operate on its curve.

$. (hat changes will occur if the system head curve or the pump performance curve changes.

'TAT)C /0A1 - A %2)CT)As the levels in the suction and discharge are the same 4%ig. *5, there is no static head and, therefore, the system curvestarts at 6ero flow and 6ero head and its shape is determined solely from pipeline losses. The point of operation is at theintersection of the system head curve and the pump curve. The flow rate may be reduced by thrott ling valve.


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Fig.1 /o Static Head %ll Friction8')T)"0 'TAT)C /0A1

The parabolic shape of the system curve is again determined by the friction losses through the system including all bendsand valves. Eut in this case there is a positive static head involved. This static head does not affect the shape of the systemcurve or its 9steepness9, but it does dictate the head of the system curve at 6ero flow rate.

The operating point is at the intersection of the system curve and pump curve. Again, the flow rate can be reduced bythrottling the discharge valve.

Fig. 2 Positive Suction Head03AT)"0 432A")T75 /0A1)n the illustration below, a certain flow rate will occur by gravity head alone. Eut to obtain higher flows, a pump )s required toovercome the pipe friction losses in excess of 9/9 - the head of the suction above the level of the discharge. )n other words,the system curve is plotted exactly as for any other case involving a static head and frict ion head, except the static head isnow negative. The system curve begins at a negative value and shows the l imited flow rate obtained by gravity alone. :orecapacity requires extra work.


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Fig. , /egative )Gravit#* Head:'T7 )%T- )TT0 %2)CT) /0A1The system head curve in the illustration below starts at the static head 9/9 and 6ero flow. 'ince the friction losses arerelatively small 4possibly due to the large diameter pipe5, the system curve is 9flat9. )n this case. the pump is required toovercome the comparatively large static head before it will deliver any flow at all.

Fig. 4ostl# Lift - Little Fricition Head

;/ydraulic losses in piping systems are composed of pipe friction losses, valves, elbows and other fittings, entrance and exit


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losse 4these to the entrance and exit to and from the pipeline normally at the beginning and end not the pump5 and lossesfrom changes in pipe si6e by enlargement or reduction in diameter.

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Formulas S)m8ols

38: ! gallons per minuteC%' ! cubic feet per secondb. ! pounds/r. ! hoursEE ! barrel 4D$ gallons5'p.3r. ! specific gravity/ ! head in feetpsi ! pounds per square inch)n. /g. ! inches of mercuryhv! velocity head in feet" ! velocity in feet per secondg ! #$.*> ft&sec$4acceleration of gravity5A ! area in square inches).1. ! inside diameter in inchesE/8 ! brake horsepower0ff. ! pump efficiency expressed as a decimals! specific speed ! speed in revolutions per minutev ! peripheral velocity of an impeller in feet per second1 ! )mpeller in inchesc ! critical speedf ! shaft deflection in inches8 ! total force in pounds ! bearing span in inchesm ! constant usually between D@ and BF for pump shafts0 ! modules of elasticity, psi - $B to #? million for steel

;'00 ')1' A1 '+22)0'%2 '+227 %2:+A'ext 'ection
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Pumps :Basic fundementals

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