multiple pump operation

7/25/2019 Multiple Pump Operation

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Multiple Pump Operationby Ross Mackay

One of the challenges that face pump users quite frequently, comes into play when more thanone pump is required to operate at the same time on the same system. With such multiplepump systems, there are two arrangements where the operating characteristics of the dierentpumps can be considered to provide a single combined performance curve.

One arrangement is a Series Operation, where one pump discharges directly into the suction ofthe second pump, with both pumps delivering the same ow rate, but sharing in thedevelopment of the combined pressure.

Another arrangement is the arallel Operation where each of the two !or more" pumps ta#es itssuction from a common header, and discharges into another common header, thus sharing theow while operating at the same head.

Series Operation$his arrangement is frequently used where a larger pump cannot operate with the %S& beingmade available from the system. A smaller pump is therefore installed upstream of the largerone to boost the Suction ressure to the larger pump.

't should be noted that in the Series Operation, the only essential similarity between the twopumps is that they must both be able to operate at the same ow rate. $he pumps can delivertotally dierent levels of &ead as long as they operate at the same capacity.


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$he ultimate e(ample of Series Operation is the multistage pump where the )rst impellerpumps into the second and then the third, etcetera. $his results in a high pressure pump withall the impellers operating at the same capacity.

*et+s et ractical. 'n some pumping arrangements we can have multiple pumps that operateon two systems that are closely combined, but are not in a true Series Operation and cannotbe considered as such. One such e(ample would be when one pump is supplying the ow andpressure for one system, while the second pump is bleeding o the )rst system to deliver alesser ow to another system. While one pump is indeed discharging into the suction of thesecond pump, it is doing so at a dierent ow rate. -onsequently, they cannot be treated as

though they were operating in series.

arallel Operation'n the more common arallel Operation, ban#s of pump are combined in order to handle a highuctuation of ows in a common system. $his arrangement is widely used in the Water

$reatment business, where the potable water being supplied to a subdivision from thetreatment plant, will e(perience huge uctuations in demand from one time of day to another.

$he use of multiple pumps on the same system allows the pumps to be switched on and o asrequired to meet the varying demand.


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7ariable Speed 8i3cultiesAnother practical problem with two pumps in parallel can occur when one of the pumps has avariable speed driver. $his arrangement is also frequently used to uctuate the total output ofboth pumps.

9nder the condition when one pump is slowed to a speed lower than the other, thecombination of head and capacity of both pumps will involve the slower pump only after theshuto head of that pump is reached. At that point only, the capacity from the slower pumpwill be added to the capacity of the faster pump to provide a combined output.

A similar situation can occur if two pump styles with dierent performance curves are installedand operated in a parallel arrangement.

&owever, if the system curve moves to a steeper slope, where it cuts the :combined: curve ata head which is higher than the shuto head of the slower pump, then that slower pump willnot be contributing any ow to the system. At that point it becomes essential that the slowerpump has a nonreturn chec# valve on it+s discharge, or the faster pump will be reversing theow bac#wards from the discharge header into the discharge no;;le of the slower pump. $his


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can impose a signi)cant reverse torque on the impeller of the slower pump with severe results.

iping Arrangements4ultiple pump arrangements provide a great temptation for the system designers to ignore therules of piping...... and with serious results.

oor piping arrangements are almost normal in pac#aged systems where multiple pumps areused.

't is essential that proper piping practices are maintained for each pump in the arrangement,such as times the pipe diameter to the suction no;;le both sets of piping to be, at least, one si;e larger than the pump no;;le use of an eccentric reducer on the suction side, with the at side up

With multiple pump arrangements it is essential that both the hydraulic and mechanicalaspects of system design be considered in order to ensure a high degree of reliability.

multiple pump operation

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