Pumps - Parallel vs. Serial Arrangement

Pumps can be arranged and connected in serial or parallel to provide additional head or flow rate capacity.

Pumps in Serial - Head Added

When two (or more) pumps are arranged in serial their resulting pump performance curve is obtained by adding their heads at the same flow rate as indicated in the figure below.

Centrifugal pumps in series are used to overcome larger system head loss than one pump can handle alone.

• for two identical pumps in series the head will be twice the head of a single pump at the same flow rate - as indicated with point 2. 

With a constant flowrate the combined head moves from 1 to 2 - BUT in practice the combined head and flow rate moves along the system curve to point 3.

• point 3 is where the system operates with both pumps running 
• point 1 is where the system operates with one pump running

Note that for two pumps with equal performance curves running in series

• the head for each pump equals half the head at point 3
• the flow for each pump equals the flow at point 3

Operation of single stage pumps in series are seldom encountered - more often multistage centrifugal pumps are used. 

Pumps in Parallel - Flow Rate Added

When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding the pumps flow rates at the same head as indicated in the figure below.

Centrifugal pumps in parallel are used to overcome larger volume flows than one pump can handle alone.

• for two identical pumps in parallel and the head kept constant - the flow rate doubles compared to a single pump as indicated with point 2

Note! In practice the combined head and volume flow moves along the system curve as indicated from 1 to 3.

• point 3 is where the system operates with both pumps running 
• point 1 is where the system operates with one pump running

In practice, if one of the pumps in parallel or series stops, the operation point moves along the system resistance curve from point 3 to point 1 - the head and flow rate are decreased.

Note that for two pumps with equal performance curves running in parallel

• the head for each pump equals the head at point 3
• the flow for each pump equals half the flow at point 3

Note - for parallel operation

• zero flow or “shut valve” heads must match for the pumps
• unstable pump curves must be avoided
• steeper pump curves are preferred