How pumps add in parallel — flows add at equal head
Parallel pumps share a common discharge header, so they run at the same head and their flows add. For two identical pumps the combined curve is simply the single-pump curve stretched twice to the right — every point (Q, H) becomes (2Q, H). The duty point is where this combined curve crosses the system curve \(H_{sys} = H_s + k\,Q^2\), and the anchor identity this page draws for you: at the single-pump duty head H₁, the combined curve passes exactly 2×Q₁ (the hollow marker on the chart). Note the flow per pump in parallel operation is smaller than the single-pump duty flow whenever the system curve is not flat — the operating point slides up the system curve as the combined flow grows.
The pump curve model
The stable form is the textbook parabola through (0, H₀) and (Qb, 0). The unstable option adds a low-flow hump: head climbs from shut-off to a local peak of ≈ 1.06·H₀ near 0.28·Qb before falling — the drooping (rising) characteristic of high-efficiency low-nq or axial machines, drawn here as a schematic. Any duty point on that rising segment is unstable: a momentary drop in system demand raises pump head, which pushes flow down further — the feedback loop runs the wrong way.
Hunting — the red-light criteria
| Condition | Verdict | What happens |
|---|---|---|
| Delivered curve and system curve cross more than once | 🔴 Hunting | Flow and head oscillate between the intersections; the low-flow crossing sits on the rising branch where restoring forces are absent. Typical with humped curves against nearly flat systems — press the preset button to reproduce it. |
| Duty point lies on the rising branch (dH/dQ > 0) | 🔴 Unstable duty | Any transient moves the point away instead of back; in parallel operation one pump may capture the whole flow while the other runs at shut-off. |
| Any running pump below 30 % of Qb | 🟡 Minimum flow | Suction recirculation, vibration and temperature rise; provide a recycle line or trim the impeller. |
| Single crossing on the falling branch | 🟢 Stable | The system curve restrains the duty point — normal parallel operation. |
Symbol table
| Symbol | Meaning | Units |
|---|---|---|
| H₀ | shut-off head (zero flow) | m / ft |
| Qb | flow at zero head (parabola reference) | m³/h / gpm |
| Hs | static head of the system | m / ft |
| k | resistance coefficient, defined by the friction head at the reference flow | m/(m³/h)² |
| Q*, H* | duty point — total flow and head at the intersection | — |
Worked anchor (self-check)
Two identical stable pumps, H₀ = 50 m, Qb = 200 m³/h, system Hs = 20 m with 25 m friction at 300 m³/h. Pump 1 alone meets the system at Q₁ ≈ 140 m³/h, H₁ ≈ 25.5 m. With both pumps running the combined curve passes (2 × 140, 25.5) — the on-chart hollow marker — and the new duty point settles near Q* ≈ 225 m³/h at H* ≈ 34 m: well below the naive 2 × 140 = 280, exactly because the system curve climbs with Q. Flip to the hunting preset (both pumps unstable, Hs = 51.5 m, friction 1 m) and the intersection count jumps to two — the red light.
Engineering criteria applied
- Parallel construction — flows added at equal head; for identical pumps the combined curve is the exact ×2 stretch, and duty points are solved in per-pump q-space (Hpump(q) = Hsys(n·q·Qb)) so no intersection is missed.
- Hunting screen — multiple intersections or positive dH/dQ at duty raise the red light; the 30 % Qb minimum-flow screen adds the amber watch.
- Curve fidelity — the unstable hump is a schematic of the drooping characteristic; real vendor curves should be digitised before final stability analysis.