Series and Parallel Pump Curves Calculator

Why connect pumps in series or parallel?

When one pump is not enough there are two options. In series the liquid passes through the pumps one after another: the flow stays the same and the heads add. It suits lines that need a high lift or pressure. In parallel the pumps discharge into a common header: the head stays the same and the flows add. It is the usual choice for variable demand and for standby capacity.

How is the combined curve built?

For series, add the heads of the pumps at each flow: Hseries(Q) = H1(Q) + H2(Q). For parallel, add the flows at each head: Qparallel(H) = Q1(H) + Q2(H). The operating point is where the combined curve crosses the system curve H = Hs + k × Q².

Example

A pump with a straight curve, 40 metres at shut-off and 80 m³/h at zero head, delivers 28.2 m³/h on its own into a system H = 10 + 0.02 × Q². Two of them in parallel deliver 33.0 m³/h, a gain of only 17 percent. In series they deliver 39.2 m³/h at a total head of 40.8 metres. Because this system curve is steep, series gives the larger gain here.

Limits

Two pumps in parallel do not double the flow. The gain depends on how steep the system curve is. The calculator never extrapolates the curves you enter and joins points with straight lines. Check the casing pressure of the second pump in series, and NPSH and motor power for every arrangement. Final selection belongs with the manufacturer's curves.