How to combine pumps in parallel and in series
When identical pumps are grouped together, the connection decides what adds up. In parallel the discharge lines merge into one header, so the flows add while the head stays the same as a single pump. In series the discharge of one pump feeds the suction of the next, so the heads add while the flow stays the same as a single pump. Short on gallons per minute, go parallel; short on pressure, go series.
The combined values shown here are the ideal sum with no system resistance. In a real plant the system curve rises with the square of flow, so two pumps in parallel rarely deliver exactly twice the flow — 80% to 95% of the ideal sum is typical. Enter a lower real-world factor, such as 90, to see that derated result.
Water horsepower is the power actually delivered to the liquid, calculated as GPM × ft ÷ 3,960 for water with a specific gravity of 1.0. Motor sizing is always larger because pump efficiency, drive losses and a service margin still have to be added. Missing check valves on a parallel header and casing pressure limits on the downstream pump in a series set are common field failures, so have a mechanical engineer and the manufacturer's performance curves confirm the final design.
Frequently Asked Questions
Pipe friction rises with the square of flow, so the operating point moves up the system curve. Actual delivery of 80% to 95% of the ideal sum is normal for a parallel pair.
It is not recommended. The pump with the lower shutoff head can be pushed back by the header pressure and deliver nothing or even backflow, so parallel sets should use identical pumps with matching curves.