Pumps in Series and Parallel: Combined Curves and the Duty Point
Series adds head at the same flow. Parallel adds flow at the same head. How to intersect a quadratic system curve and when unequal pumps idle.
Key takeaways
- Series: same Q, heads add — for high static lift.
- Parallel: same H, flows add — for high capacity.
- Duty is the intersection with Hs = Hs0 + k Q².
- Unmatched parallel pumps can idle or run back through a check valve.
A single catalogue curve is H(Q) for one pump. Stations combine two or more machines. The rule is simple: series shares the flow and adds the heads; parallel shares the head and adds the flows. The station then intersects the system curve.
Quadratic models
| Arrangement | How to combine | Typical use |
|---|---|---|
| Series | H = H_A(Q) + H_B(Q) at the same Q | High-rise, long transmission |
| Parallel (identical) | Q = 2 Q_one(H) at the same H | Duty/standby capacity |
| Parallel (mixed) | Q = Σ Q_i(H) if H < each H0 | Retrofits — check idle |
Finding the duty
Build H_station(Q), set it equal to Hs0 + k Q², and solve. For two identical parallel pumps the combined curve is “twice the flow at each head.” The duty is not twice a single-pump duty unless the system curve is a vertical line (pure static head). Friction makes the pair operate at higher head and less than 2× the single-pump flow.
Open solver: Single-pump system curveNPSH still applies to each machine
Parallel pumps share a suction header. Velocity in that header is the sum of the flows. Check NPSHa at the combined Q, and NPSHr of each pump at its share. Series pumps: the second machine sees discharge pressure, but its own suction is the first pump’s discharge — a different NPSH problem.
Open solver: NPSH at the duty pointFrequently asked questions
Only if their head ranges overlap. If the large pump’s head at low Q exceeds the small pump’s shutoff, the small unit contributes nothing (or reverse-flows without a check).