Pumps in Series - 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 flowrates 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 flowrate 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