When one pump cannot do the duty, the instinctive fix is a second pump. What the second pump adds depends entirely on how it is plumbed: in series it adds pressure, in parallel it adds flow. Confusing the two is an expensive mistake.
Series: head adds up
Pumps in series feed each other. The first pump's discharge becomes the second's suction, and the pressures stack while the flow stays the same. This is the principle inside every multistage pump. In the field, series arrangements are rare outside boostering, because a multistage unit usually does the same job in one casing. The trap: the downstream pump must be rated for the inlet pressure it will see, and control must keep both machines in step.
Parallel: flow adds up
Pumps in parallel share a common suction and discharge. At any given head, the flows sum, which is why booster sets and municipal stations run pumps side by side. Two equal pumps do not give you twice the flow on a real system curve, because the head rises as flow rises, but they give you most of it, plus redundancy: one pump can hold the service while the other is serviced.
When to choose which
- Need more pressure at the same flow: series, or simply a multistage pump.
- Need more flow at similar pressure, or resilience through duty-and-standby: parallel.
- Need both: you have sized the pump wrong; go back to the system curve.