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Series and Parallel combination of Pumps In this article, we will study the need for a combination of pumps and its net effect on overall head and discharge rate. Basically we need to combine one or more pumps together either to increase the flow rate or to achieve more head which the single pump alone can’t do.

Pumps in Series: So here are pumps in series it means the discharge of the first pump is connected to the suction of the second pump. Suppose we have a flow rate coming in and we have two pumps Pump A and Pump B in series and since they are in series the mass of water entering the pump A per second has to be equal to the mass of water exiting from pump B according to the conservation of mass i.e.                                    QA=QB Now talking about the energy conservation, since both the pumps are pumping the fluid together in a similar direction so the resultant head will be the sum of the individual heads, i.e.                                       Heq=Ha+Hb

we can see that the head of the two pumps add up to increase the net head though the net flow rate is not the point of interest here. Centrifugal Pumps are connected in series to overcome the head loss in larger systems where a single pump can’t handle the head alone. If the pumps are identical, Then Heq=Ha+Hb=2Ha.

Generally, multistage centrifugal pumps are used. Limitations of using pumps in series: Since the suction of the second pump is connected to the discharge of the first pump so it is important that the second pump should be of higher suction and discharge pressure rated. Pumps failure often happen if not taken care, if the second pump fails then the first pump will suddenly feel high system resistance which leads its flow rate close to zero. If the first pump failed, then the suction at second pump becomes low which would result in cavitation and the second pump might get failed too If not stopped immediately after the failure of the first pump.

Pumps in Parallel: Unlike pumps in series here, we have two independent connections for the suction of fluid tough the suction conditions are similar but will have common discharge connection. Suppose two pumps 1 and 2 are connected to similar suction condition with flow rate Q1 and Q2 respectively and with heads H1 and H2 respectively.

Now applying conservation of mass, The net mass of water flowing out per second is the sum of the mass flow rates of the individual pumps,

i.e. Qeq=Q1+Q2

and the net head gained overall is same for both the pump.

i.e. Heq=H1=H2

This combination is required where we need to move water more quickly which an alone pump can’t do efficiently. If there are multiple pumps in parallel then

Qeq=Q1+Q2+…+Qn

While combining pumps in parallel we must ensure that all the pumps are equally rated head wise so that all are running at the same speed with the same diameter of their impellers. Due to the increase in friction in pipes the net flow rate produced by the two pumps in parallel will be less than twice the flow rate if individual pump flow rate. Centrifugal pumps in parallel are used to overcome the larger flow of volume which alone one pump can’t handle.