Apuntes bombas (Inglés)

8/13/2019 Apuntes bombas (Ingls)

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Section A -- Centrifugal Pump FundamentalsA-1 HeadThe pressure at any point in a liquid can be thought of as being caused by avertical column of the liquid which, due to its weight, exerts a pressure equalto the pressure at the point in question. The height of this column is called

the static head and is expressed in terms of feet of liquid.

The static head corresponding to any specific pressure is dependent uponthe weight of the liquid according to the following formula.

A Centrifugal pump imparts velocity to a liquid. This velocity energy is thentransformed largely into pressure energy as the liquid leaves the pump.Therefore, the head developed is approximately equal to the velocity energyat the periphery of the impeller This relationship is expressed by thefollowing well-known formula

Where H! Total head developed in feet.v! "elocity at periphery of impeller in feet per sec.g! #$.$ %eet&'ec$

(e can predict the approximate head of any centrifugal pump by calculatingthe peripheral velocity of the impeller and substituting into the above formula.A handy formula for peripheral velocity is

D! )mpeller diameter in inches

V! "elocity in ft.&sec

The above demonstrates why we must always think in terms of feet of liquidrather than pressure when working with centrifugal pumps. A given pumpwith a given impeller diameter and speed will raise a liquid to a certain heightregardless of the weight of the liquid, as shown in %ig. *.


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Fig. 1 Identical Pumps Handling Liquids of Different Specific Gravities.

All of the forms of energy involved in a liquid flow system can be expressedin terms of feet of liquid. The total of these various heads determines thetotal system head or the work which a pump must perform in the system.The various forms of head are defined as follows.

'+CT) )%T exists when the source of supply is below the center line ofthe pump. Thus the 'TAT)C '+CT) )%T is the vertical distance in feetfrom 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(. )Static Suction Head*

'+CT) /0A1 exists when the source of supply is above the centerline ofthe pump. Thus the 'TAT)C '+CT) /0A1 is the vertical distance in feetfrom 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 isLocated +elo" t!e Suction 'an(. )Static Suction Head*


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'TAT)C 1)'C/A230 /0A1 is the vertical distance in feet between the pump centerlineand the point of free discharge or the surface of the liquid in the discharge tank.

TTA 'TAT)C /0A1 is the vertical distance in feet between the free level of the source ofsupply and the point of free discharge 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 the si6e, condition and type of pipe, number and type ofpipe fittings, flow rate, and nature of the liquid. %rictional tables are included in Water Data.

"0C)T7 /0A1 4hv5 is the energy of a liquid as a result of its motion at some velocity ". )tis the equivalent head in feet through which the water would have to fall to acquire the samevelocity, or in other words, the head necessary to accelerate the water. "elocity head canbe 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 factor and must be considered in low head systems.

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

The above forms of head, namely static, friction, velocity, and pressure, are combined tomake up the total system head at any particular flow rate. %ollowing are definitions of thesecombined 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 thepump suction flange plus the total friction head in the suction line. The total dynamic suctionlift, as determined on pump test, is the reading of a gauge on the suction flange, convertedto 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 atthe pump suction flange minus the total friction head in the suction line. The total dynamicsuction head, as determined on pump test, is the reading of the gauge on the suction flange,converted to feet of liquid and corrected to the pump centerline;, plus the velocity head atthe point of gauge attachment.

TTA 17A:)C 1)'C/A230 /0A1 4hd5 is the static discharge head plus the velocity

head at the pump discharge flange plus the total friction head in the discharge line. The totaldynamic discharge head, as determined on pump test, is the reading of a gauge at thedischarge flange, converted to feet of liquid and corrected to the pump centerline;, plus thevelocity head at the point of gauge attachment.

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

TDH = hd hs!"ith a suction lift#TDH = hd - hs!"ith a suction head#
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Section A -- Centrifugal Pump FundamentalsA-2 Capacity

Capacity 4? is obtained by dividing the number or foot pounds for one horsepower

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 hydraulic horsepower or outputdue to the mechanical and hydraulic losses incurred in the pump. Therefore the pumpefficiency is the ratio of these two values.

A-4 Specific Speed and Pump Type'pecific speed 4s5 is a non-dimensional design index used to classify pump impellers as totheir type and proportions. )t is defined as the speed in revolutions per minute at which ageometrically similar impeller would operate if it were of such a si6e as to deliver one gallonper minute against one foot head.

The understanding of this definition is of design engineering significance only, however, andspecific speed should be thought of only as an index used to predict certain pumpcharacteristics. The following formula is used to determine specific speed

(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 impeller outlet diameter, 1$, to the inletor eye diameter, 1i, decreases. This ratio becomes *.? for a true axial flow impeller.


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2adial flow impellers develop head principally through centrifugal force. 8umps of higherspecific speeds develop head partly by centrifugal force and partly by axial force. A higherspecific speed indicates a pump design with head generation more by axial forces and lessby centrifugal forces. An axial flow or propeller pump with a specific speed of *?,??? orgreater generates its head exclusively through axial forces.

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

"alues of 'pecific 'peed, s

Fig. , Impeller Design vs Specific Speed

A-5 Net Positie Suction Head !NPSH" and CaitationThe /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 analysisof energy conditions on the suction side of a pump to determine if the liquidwill vapori6e 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 characteristicof every fluid and increased with increasing temperature. (hen the vaporpressure within the fluid reaches the pressure of the surrounding medium,the fluid begins to vapori6e or boil. The temperature at which thisvapori6ation occurs will decrease as the pressure of the surrounding mediumdecreases.

A liquid increases greatly in volume when it vapori6es. ne cubic foot ofwater 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, wemust keep it in liquid form. 8'/ is simply a measure of the amount ofsuction head present to prevent this vapori6ation at the lowest pressure pointin the pump.

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 ofthe liquid. The 8'/ 2equired is the positive head in feet absolute requiredat the pump suction to overcome these pressure drops in the pump and


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maintain the maority of the liquid above its vapor pressure. The 8'/2equired varies with speed and capacity within any particular pump. 8umpmanufacturers curves normally provide this information.

Net Positive Suction Head (NPSH) 8'/ Available is a function of the systetm in which thepump 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 shows four typical suction systems with the 8'/Available formulas applicable to each. )t is important to correct for the specific gravity of theliquid and to convert all terms to units of 9feet absolute9 in using the formulas.

8E! Earometric pressure in feet absolute."8! "apor pressure of the liquid at maximum pumping temperature, in feet absolute.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 capacityFig. alculation of s#stem /et Positive Suction Head %vaila&le for t#pical suction conditions.

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

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

Cavitation is a term used to describe the phenomenon, which occurs in a pump when thereis insufficient 8'/ Available. (hen the pressure of the liquid is reduced to a value equalto or below its vapor pressure the liquid begins to boil and small vapor bubbles or pocketsbegin to form. As these vapor bubbles move along the impeller vanes to a higher pressurearea above the vapor pressure, they rapidly collapse.

The collapse, or 9implosion9 is so rapid that it may be heard as a rumbling noise, as if youwere pumping gravel. )n high suction energy pumps, the collapses are generally highenough to cause minute pockets of fatigue failure on the impeller vane surfaces. This action


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may be progressive, and under severe 4very high suction energy5 conditions can causeserious pitting damage to the impeller.

The accompanying noise is the easiest way to recogni6e cavitation. Eesides possibleimpeller damage, excessive cavitation results in reduced capacity due to the vapor presentin the pump. Also, the head may be reduced and&or be unstable and the power consumptionmay be erratic. "ibration and mechanical damage such as bearing failure can also occur asa result of operating in excessive cavitation, with high and very high suction energy pumps.

The way to prevent the undesirable effects of cavitation in standard low suction energypumps is to insure that the 8'/ Available in the system is greater than the 8'/2equired by the pump. /igh suction energy pumps require 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 suctionenergy pumps, when operating in the allowable operating range.

Section A -- Centrifugal Pump Fundamentals%-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 systemredesign. n the other hand, providing excessive 8'/ available mayneedlessly increase 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=??? is a reasonable value of suction specific speed. 8umps with a minimumsuction specific speed of =??? are readily available, and are not normally

subect 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.


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A related problem is in selecting a new pump, especially at higher flow, foran existing 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 apump can be run without exceeding 8'/ availableH 48'/:argin 2atio! *.Ffrom above I '.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'/ Ais constraining 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.

+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. 'uction

0nergy is defined as


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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 isa function of 'uction 0nergy and the critical nature of the application asfollows

Suction Energy NPSHMargin Ratio(NPSHA/NPSHR)Low 1.1 - 1.3

High 1.2 - 1.

!ery High 1. - 2."

'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.

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 plottedover the capacity range of the pump.

%igures F, >, L B are non-dimensional curves which indicate the general

shape of the characteristic curves for the various types of pumps. They showthe head, brake horsepower, and efficiency plotted as a percent of theirvalues at 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.


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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 axialflow pump, the head and brake horsepower both increase drastically nearshutoff as shown in %ig. B.

Fig. 0 4i5ed Flo" Pump


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Fig. %5ial Flo" Pump

The distinction between the above three classes is not absolute, and thereare many pumps with characteristics falling somewhere between the three.%or instance, the %rancis vane impeller would have a characteristic betweenthe radial and mixed flow classes. :ost turbine pumps are also in this same

range depending upon their specific speeds.

%ig. @ below shows a typical pump curve as furnished by a manufacturer. )tis a 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

A-# Affinity $awsThe affinity laws express the mathematical relationship between the severalvariables involved in pump performance. They apply to all types ofcentrifugal and 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

(hen the performance 4


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Fig. 6 omposite Performance urve

The affinity laws listed under * above will be used to determine the newperformance, with * *BF? 28: and $ ! $??? 28:. The first step is toread the capacity, head, and horsepower at several points on the *#9 dia.curve in %ig. = below. %or example, one point may be near the bestefficiency point where the capacity is #?? 38:, the head is *>? ft, and theE/8 is approx. $? hp.

This will then be the best efficiency point on the new $??? 28: curve. Eyperforming the same calculations for several other points on the *BF? 28:curve, a new curve can be drawn which will approximate the pumpsperformance at $??? 28:, %ig. =.

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


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Fig. 7A-% System Cures%or a specified impeller diameter and speed, a centrifugal pump has a fixedand predictable performance curve. The point where the pump operates onits curve is dependent upon the characteristics of the system )n which it isoperating, commonly called the 'ystem /ead Curve. ..or, the relationshipbetween flow and hydraulic losses8in a system. This representation is in agraphic form and, since friction losses vary as a square of the flow rate, thesystem curve is parabolic in shape.

Ey plotting the system head curve and pump curve together, it can bedetermined

*. (here the pump will operate on its curve.$. (hat changes will occur if the system head curve or the pump

performance curve changes.
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'TAT)C /0A1 - A %2)CT)As the levels in the suction and discharge are the same 4%ig. *5, there is nostatic head and, therefore, the system curve starts at 6ero flow and 6erohead and its shape is determined solely from pipeline losses. The point ofoperation is at the intersection of the system head curve and the pump

curve. The flow rate may be reduced by throttling valve.

Fig.1 /o Static Head %ll Friction

8')T)"0 'TAT)C /0A1The parabolic shape of the system curve is again determined by the frictionlosses through the system including all bends and valves. Eut in this casethere is a positive static head involved. This static head does not affect theshape of the system curve or its 9steepness9, but it does dictate the head ofthe system curve at 6ero flow rate.

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


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Fig. 2 Positive Suction Head

03AT)"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 to overcome the pipe frictionlosses in excess of 9/9 - the head of the suction above the level of thedischarge. )n other words, the system curve is plotted exactly as for anyother case involving a static head and friction head, except the static head isnow negative. The system curve begins at a negative value and shows thelimited flow rate obtained by gravity alone. :ore capacity requires extrawork.


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Fig. , /egative )Gravit#* Head

:'T7 )%T- )TT0 %2)CT) /0A1

The system head curve in the illustration below starts at the static head 9/9and 6ero flow. 'ince the friction losses are relatively small 4possibly due tothe large diameter pipe5, the system curve is 9flat9. )n this case. the pump isrequired to overcome the comparatively large static head before it will deliverany flow at all.

Fig. 4ostl# Lift - Little Fricition Head


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8H)draulic losses in piping s)stems are composed of pipe friction losses6 valves6el9o"s and other fittings6 entrance and e(it losse !these to the entrance and e(it toand from the pipeline normall) at the 9eginning and end not the pump# and lossesfrom changes in pipe si:e 9) enlargement or reduction in diameter

Formulas

S)m9ols

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 second

g ! #$.*> ft&sec$

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 persecond1 ! )mpeller in inchesc ! critical speedf ! shaft deflection in inches8 ! total force in pounds

! bearing span in inchesm ! constant usually between D@ and BF for pumpshafts0 ! modules of elasticity, psi - $B to #? million forsteel

;'00 S%;*DS A$D S;

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