Parallel Pump Combination Total Flow
Parallel Pump Combination Total Flow
Parallel pumping is the usual answer when a system needs more flow than one pump can deliver, or when redundancy is required so the system keeps running with one pump down for maintenance. Two or more pumps draw from a common header and discharge into a common header, so each pump sees the same discharge head while their flows add. This is the complementary case to series staging: parallel pumps add flow at constant head instead of adding head at constant flow. Because the combined pump curve flattens as more units are added, blindly assuming flows simply double when a second identical pump is added can overestimate the actual gain once the steepening system curve is accounted for.
For pumps in parallel at the same head, the combined flow is the sum Q_total = Q1 + Q2. where Q_1p is the flow delivered by the first pump and Q_2p is the flow delivered by the second pump, both at the shared discharge head.
Inputs
Q_1p := 0.03 m^3/s=0.03 m^3/s
Q_2p := 0.025 m^3/s=0.03 m^3/s
Because both pumps discharge into the same header at the same head, their individual flow contributions simply add.
Q_total := Q_1p + Q_2p=0.06 m^3/s
Results
The combined flow here is noticeably less than double either pump alone, reflecting that the two units are not identical; even with matched pumps, the true combined flow at the actual operating head is usually less than the arithmetic sum once the system curve is overlaid, because a steep system curve limits the benefit of adding parallel capacity. This is why parallel staging gives diminishing returns in high-friction systems and works best when the system curve is relatively flat.
Assumptions & limitations
- Both pumps discharge against the same head (parallel arrangement).
- Assumes each pump curve is evaluated at the shared operating head, not rated head.
- Neglects interaction losses at the common header.
- Real combined flow is typically less than the arithmetic sum once the system curve is applied.