What "in series" means hydraulically
Two pumps in series share the same flow path. Whatever leaves the first pump's discharge enters the second pump's suction. Heads add; flow stays the same.
Q_series = Q_pump_1 = Q_pump_2
H_series = H_pump_1 + H_pump_2Compare to parallel, where flows add and head stays the same. Series exists for the high-head, modest-flow regime — the place a single-stage centrifugal pump runs out of head.
The construction: combined pump curve
For each flow value Q, plot a single point at H = H1(Q) + H2(Q). Repeat across the curve range. The result is a steeper curve sitting above either single pump's H-Q line.
With identical pumps: the series curve is just the single curve doubled vertically. With different pumps: add headway-by-headway. The smaller pump's runout limits the series curve. Above that flow, the smaller pump can't contribute and the series effectively becomes single-pump.
Worked example
Two identical pumps, each delivering up to 200 ft TDH at shutoff and 100 ft at 600 gpm. System needs 600 gpm at 180 ft TDH.
A single pump at 600 gpm produces 100 ft — short by 80 ft. Adding a second pump in series produces 200 ft at 600 gpm — comfortable margin over the 180 ft system requirement.
The system operating point lands wherever the series curve crosses the system curve. With static lift dominating (160 ft static + 20 ft friction at 600 gpm), the system curve is steep and stable.
When series beats single high-pressure pump
Three scenarios where series-of-two is the right answer:
1. Off-the-shelf availability. Two standard 100-ft pumps are cheaper, faster to procure, and easier to maintain than one custom 200-ft single-stage pump. 2. Maintenance flexibility. Service one pump while the other handles backup-pressure (with bypass) or part-load operation alone. 3. Future expansion. Install one pump now, add the second when system head requirements grow.
When series goes wrong
The classic failure: the upstream pump's discharge becomes the downstream pump's suction. That suction sees:
- Pump-1 discharge pressure (good — high NPSHa for pump 2)
- Plus pump-1 suction temperature (water heats slightly as it goes through pump 1, possibly raising vapor pressure)
- Plus any solids carried over
Common pitfalls:
Air binding the upstream pump. If pump 1 loses prime, pump 2 sees vacuum on its suction. Not the cavitation kind — the actual atmospheric vacuum. Pump 2's mechanical seal pulls in air and fails within hours. Mitigation: prime-loss-detection on pump 1 + auto-shutdown of both.
Pump 2 protected by pump 1. Engineers sometimes spec pump 2's casing pressure rating only for the discharge head minus the suction head. But pump 1 can fail (or be intentionally shut off) leaving pump 2 to see suction static only. Spec pump 2's casing for the full system pressure assumption.
Mismatched pump curves. If pump 1 has a steeper curve than pump 2, at high flows pump 1 can fall below the system pressure while pump 2 still pushes — pump 1 effectively backs flow. Add check valves between pumps OR match curve characteristics.
Single-pump fallback impossible. Some series installations have no single-pump bypass. If one pump fails, the system is offline. Always include a single-pump bypass valve (often a check-valve-and-pipe arrangement around the failed unit).
Series + parallel together
Large pump stations often combine both. Two parallel "trains" each containing two pumps in series. This gives:
- Capacity scaling via parallel
- Head capability via series
- Fault tolerance: lose one pump in one train, the other train still runs
The hydraulic analysis: build the series curve for each train, then construct the parallel curve from those train curves. Same logical workflow as single-pump-curve analysis, just nested.
Multi-stage pumps as built-in series
A multi-stage centrifugal pump is functionally many radial impellers in series within one casing. Same flow through each stage, head adds across stages. The same pump curve interpretation applies — except the pump is a single unit on one shaft, simplifying hydraulics + mechanicals.
When the duty calls for >250 ft TDH and modest flow, a multi-stage pump is usually a better choice than two single-stage pumps in series. Less plumbing, single seal package, single motor, smaller footprint. The catch: pump-internal stage failures aren't fixable in the field — the whole pump goes back to the OEM for rebuild.
Series vs. parallel decision matrix
| Need | Choose | |---|---| | Higher flow at same head | Parallel (or single bigger pump) | | Higher head at same flow | Series (or multi-stage) | | Operating flexibility (variable load) | Either, with VFD | | Standby redundancy | Parallel (1 of 2 sized for full duty) | | Critical-system high-pressure | Multi-stage with N+1 pumps in parallel |
The math test: if the ratio Hrequired / Hsingle > 1.5 you're in series territory. If Qrequired / Qsingle > 1.5 you're in parallel territory. Both > 1.5 means you need a different pump class entirely (mixed-flow, axial-flow), not series-parallel of small radial pumps.
How the calculator handles it
The Headloss Calculator's pump panel includes a "Multiple pumps" toggle with three options:
- Single
- Series (n identical, default 2)
- Parallel (n identical, default 2)
Series mode constructs the combined curve via vertical addition and re-solves the system intersection. The output panel shows:
- Combined operating point (Q, H)
- Per-pump operating point (Qeach = Q, Heach = H/n)
- Per-pump efficiency at duty
- Per-pump distance from BEP
- A flag if any pump in series operates outside its AOR
References
- Karassik, I. J., et al. *Pump Handbook,* 4th ed. McGraw-Hill — multi-pump operation chapter.
- Hydraulic Institute. *ANSI/HI 1.3 — Rotodynamic Centrifugal Pumps for Design and Application.*
- Lobanoff, V. S., and Ross, R. R. *Centrifugal Pumps: Design and Application,* 2nd ed.