Concept reference

Pumps in Parallel and in Series

C.1Plain-language definition

Parallel pumps share suction and discharge nodes, so their flows add at the same head. Series pumps pass the same flow, so their heads add. Neither rule says the installed station will deliver double: the combined curve must still intersect a rising system curve.

C.2Governing relation or decision logic

For parallel operation, Qtotal(H) = ΣQi(H). For series operation, Htotal(Q) = ΣHi(Q). These relations combine pump curves before intersection with Hsystem(Q). Unequal units require individual curves, check-valve logic and review of each pump’s operating point.

C.3The failure to name

Adding rated flows ignores the extra friction created by the combined flow. Unequal parallel pumps can drive a smaller unit toward shutoff, reverse flow or an unacceptable region; identical nameplates also do not guarantee identical field curves after wear or trimming.

C.4Field review checklist

• Combine curves at equal head for parallel or equal flow for series.
• Include common and branch losses in the correct system paths.
• Check each pump’s flow, power, NPSH and allowable region in every staging state.
• Verify check-valve behaviour, minimum flow and control transitions on start and stop.

C.5Where it fits in the course

§7.3 quantifies the missing “second-pump” flow; §4.5 treats unequal branches.