LOGOS Learning · 6
Pumps in parallel and series: which arrangement pays off
Pumps in parallel add flow at the same head (Q_total = Q1 + Q2), while pumps in series add head at the same flow (H_total = H1 + H2). The real gain depends on where the combined curve crosses the system curve H_st + hf(Q).
Switch the arrangement and the system type. The same second pump can add a lot or almost nothing.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. To run pumps in parallel it is important that they…
2. Two identical pumps in series…
3. Two identical pumps in parallel…
Why this happens
Arrows compare with one pump alone in the same system. Both pumps are identical.
Pumps in parallel
In parallel, the pumps run at the same head and add flow. The combined curve is one pump curve stretched horizontally.
Operating point
The system does not change; only the pump curve does. The new crossing climbs along the same system curve, and its slope decides how much you gain.
Flow gain
With a flat system (static head dominant), parallel pays off because head barely rises with flow. Series pays less: it adds head the system does not ask for.
Teaching pump with an illustrative curve.
Formulas in plain text
- Pumps in parallel
Q_total = Q1 + Q2 with H1 = H2- Q_total = combined flow (m3/h) · Q1, Q2 = flow of each pump (m3/h) · H1, H2 = head of each pump (m)
- Pumps in series
H_total = H1 + H2 with Q1 = Q2- H_total = combined head (m) · H1, H2 = head added by each pump (m) · Q1, Q2 = flow through each pump (m3/h)
- Operating point of the arrangement
H_arrangement(Q) = H_st + hf(Q)- H_arrangement(Q) = combined pump curve (m) · H_st = static head (m) · hf(Q) = friction loss at flow Q (m)
- Flow gain over one pump
gain = Q_arrangement / Q_1pump - 1- Q_arrangement = flow with two pumps (m3/h) · Q_1pump = flow with one pump in the same system (m3/h)
Frequently asked questions
Why do two pumps in parallel not double the flow?
More flow means more friction loss, so the system asks for more head and each pump moves back along its curve. Each pump ends up delivering less than it did alone; the gain is large only when the system curve is flat (static head dominant).
When should pumps be connected in series instead of parallel?
When the system is steep, dominated by friction or by a head that one pump cannot reach. Series adds head at each flow, which is what a steep system curve asks for.
What happens to head when pumps run in parallel?
Both pumps run at the same head, equal to the system head at the combined flow. That head is higher than with one pump, because the system curve rises with flow.
Do pumps in parallel need to be identical?
Not strictly, but unequal pumps are risky: if the system head rises above the shut-off head of the weaker pump, it is pushed to zero flow while still running. Identical pumps share the flow evenly.