Pumps A pump is a machine that uses energy transformaons to increase pressure of a liquid (gure 1) Liquid out (high pressure)
Energy in
;rier
Pump
Liquid in (low pressure)
[Electricity steam !il high pressure hydraulic hydraulic "uid #ompressed air$ %igure (1) Energy in (using sha&'impeller) inec energy (gain in elocity) (using e*pansion) Pressure energy (gain in pressure) +he reasons for increasing liquid pressure is to, 1) increase stac eleaon -) oercome friconal losses .) increase pressure /) increase elocity0 elocity0 elaon 2etween pressure and head, pressure (psi)3 [head (&)*45$'-0.1 #lassicaon of pumps energy is connuou connuously sly added to the liquid to increase increase its elocity elocity00 6hen the liquid liquid elocity is Kinetc, energy su2sequently reduced this produces a pressure increase0 periodically ly added to the liquid 2y the direct direct applica applicaon on of a force to Positve displacemen displacemen , energy is periodical one or more moa2le olumes of liquid0 #entrifugal pumps A centrifugal pump consists of an impeller a7ached to and rotang with the sha& and a casing that encloses the impeller0 8n a centrifugal pump liquid is forced into the inlet side of the pump casing 2y atmospheric pressure or some upstream pressure0 As the impeller rotates liquid moes toward the discharge side of the pump0 +his creates a oid or reduced pressure area at the impeller inlet0 +he pressure at the pump casing inlet which is higher than this reduced pressure at the impeller inlet forces addional liquid into the impeller to ll the oid0 8f the pipeline leading to the pump inlet contains a noncondensa2le gas such as air then the pressure reducon at the impeller inlet merely causes the gas to e*pand and sucon pressure does not force liquid into the impeller inlet0 #onsequently #onsequently no pumping acon can occur unless this noncondensa2le gas is rst eliminated a process 9nown as priming the pump0 6ith the e*cepon of a parcular type of centrifug centrifugal al pump called a self:prim self:priming ing centrifuga centrifugall pump pump centrifug centrifugal al pumps pumps are not inherentl inherently y self: self: priming if they are physically located higher than the leel of the liquid to 2e pumped0 +hat is the sucon sucon piping piping and inlet inlet side side of centri centrifug fugal al pumps pumps that that are are not self: self:pri primin ming g must must 2e lled lled with with
noncompressi2le liquid and ented of air and other noncondensa2le gases 2efore the pump can 2e started0 4elf:priming pumps are designed to rst remoe the air or other gas in the sucon line and to then pump in a conenonal manner0 8f apors of the liquid 2eing pumped are present on the sucon side of the pump this results in caitaon0 !nce it reaches the rotang impeller the liquid entering the pump moes along the impeller anes increasing in elocity as it progresses0 A typical elocity and pressure changes in a centrifugal pump as the liquid moes through the "ow path of the pump is illustrated in the gure 2elow (%igure -),
Pressure !utlet p of impeller ane
8nlet p of impeller ane
;ischarge %igure (-)
A centrifugal pump operang at a *ed speed and with a *ed impeller diameter produces a di=erenal pressure or di=erenal head0 +he amount of head produced aries with the "ow rate or capacity deliered 2y the pump as illustrated 2y the characterisc head–capaciy curve (%igure .)0 As the head of the pump decreases the capacity increases0 Alternaely as the pump head increases the "ow decreases0 4lip Posie displacement > #entrifugal
? %igure (.)
+he centrifugal pump casing is one of seeral types0 A single olute casing (%igure /) has a single cutwater where the "ow is separated0 As the "ow leaes the impeller and moes around the olute casing the pressure increases0 +his increasing pressure as the liquid moes around the casing produces an increasing radial force at each point on the periphery of the impeller due to the pressure acng on the pro@ected area of the impeller0 4umming all of these radial forces produces a net radial force that must 2e carried 2y the sha& and radial 2earing system in the pump0 +he radial 2earing must also support the load created 2y the weight of the sha& and impeller0 +he radial 2earing loads generated 2y a pump also ary as the pump operates at di=erent points on the pump performance cure with the minimum radial force 2eing deeloped at the 2est eciency point (BEP) of the pump (%igure C)0 4ymptoms of e*cessie radial loads include e*cessie sha& de"econ and premature mechanical seal and 2earing failure0 #onnuous operaon of the pump at too low a minimum "ow is one of the most common causes of this type of failure0 Bearing life is inersely proporonal to the cu2e of the 2earing load0
%igure (/)
4ingle olute adial load ;ou2le olute ;i=user
>
BEP
? %igure (C)
A di=user casing (%igure D) is a more comple* casing arrangement consisng of mulple "ow paths around the periphery of the impeller discharge0 +he liquid that leaes the impeller anes rather than haing to moe completely around the casing periphery as it does with the single olute casing merely enters the nearest "ow channel in the di=user casing0 +he di=user casing has mulple cutwaters eenly spaced around the impeller as opposed to the one cutwater found in a single olute casing0 +he main adantage of the di=user casing design is that this results in a near 2alancing of radial forces (%igure C) thus reducing sha& de"econ and eliminang the need for a heay:duty radial 2earing system0 +he dead weight of the rotang element must sll 2e carried 2y the radial 2earing 2ut oerall the di=user design minimies radial 2earing loads compared with other casing types0 +his is usually only found to 2e the case in mulstage high:pressure pumps0
%igure (D)
3<-'-g and elocity at impeller p is <3[rpm * ; (in)$'--I0 +hus >3[rpm * ;$ -'[.0.JC * 1GD$
4tac head the total eleaon change that the liquid must undergo normally measured from the surface of the liquid in the supply essel to the surface of the liquid in the essel where the liquid is 2eing deliered0 Kote that for a pump in a closed loop system the total stac head is ero0 %ricon head aries as the square of liquids elocity (assuming fully tur2ulent "ow)0 +o determine fricon head for a piping the designed capacity and a chosen pipe sie are used0 f3F* Mf* [L'd$ * <-'-g >f3 * <-'-g
Pipes Nngs
[Mf from fricon factors chart$ [9 is resistance coecient$
Pressure head is the head required to oercome a pressure or acuum in the system upstream or downstream of the pump0 8f the pressure in the supply essel from which the pump is pumping and the pressure in the deliery essel are idencal (e0g0 if 2oth are atmospheric tan9s) then there is no required pressure head ad@ustment to pressure head0 Li9ewise there is no pressure ad@ustment to pressure head for a closed loop system0 8f the supply essel is under a acuum or under a pressure di=erent than that of the deliery essel a pressure head ad@ustment is required0 +he pressure or acuum must 2e conerted to feet0 g) is conerted to feet of head 2y the formula, g * 101..'45 8f the sucon essel is under acuum the amount of acuum (equialent to gauge pressure conerted to feet) must 2e added to the deliery essel gauge pressure (also conerted to feet) to get the total pressure ad@ustment to total head0 8f the sucon essel is under posie pressure (2ut di=erent from the pressure of the deliery essel) then the sucon essel pressure (conerted to feet) should 2e su2tracted from the deliery essel pressure (conerted to feet) to get the pressure ad@ustment to total head0 3< -'-g Performance #ure !nce the pump conguraon and rang (capacity and head) hae 2een determined as descri2ed in the three preceding secons the ne*t step in the selecon process is to decide which pump speeds should 2e considered0 +he aaila2le motor speeds for standard alternang current (A0#0) electric motors are 2ased on the following formula, pm3J-GG'K at DG h where K is num2er of poles0 pm3DGGG'K at CG > [Europe$ !nce the pump speeds to 2e considered hae 2een determined a centrifugal pump selecon can 2e made0 Oost centrifugal pumps hae the capa2ility to operate oer an e*tended range of head and "ow 2y trimming or cuNng the impeller diameter from its ma*imum sie down to some predetermined
minimum sie0 +hus for a gien pump speed a centrifugal pump produces an enelope of headcapacity performance (%igure J)0
Oa*imum impeller diameter >
Oin impeller diameter
? %igure (J) +he conenon for designang the sie of a centrifugal pump is as follows, 4ucon sie Q ;ischarge sie Q Oa*imum impeller diameter >orsepower and Eciency >orsepower refers to the amount of energy that must 2e supplied to operate a pump0 6ater horsepower (6>P) refers to the output of the pump handling a liquid of a gien specic graity with a gien "ow and head0 +he formula for 6>P is, 6>P3? * > * 45'.IDG [? in gpm > in feet$ 6>P3IJIJ * ? * > [? in cu2ic meter'sec > in meters$ Bra9e horsepower (B>P) is the actual amount of power that must 2e supplied to the pump to o2tain a parcular "ow and head0 8t is the input power to the pump or the required output power from the drier0 +he formula for B>P is, B>P3 ? * > * 45'.IDG * R [R is pump eciency$ 6ire:to:water horsepower is required power input into the drier and is found 2y diiding B>P 2y the motor eciency0 8n the case of a pump using a aria2le:speed deice or other au*iliary driing equipment such as a gear 2o* B>P is diided 2y the com2ined eciency of all of the drier components to o2tain the wire:to:water horsepower0 B>P is greater than 6>P 2ecause of the fact that a pump is not a perfectly ecient machine due to the following reasons, 10 >ydraulic losses due to the fricon in the walls of liquid passageways around impeller and olute or di=user -0
K4P> and #aitaon
