Pump Selection & NPSH

Quick pick (TL;DR)

Normal flow & head → centrifugal (end-suction the default).
Very high head → multistage centrifugal.
Low flow / very high head, or metering / viscous → positive displacement.
Size the duty point near the pump's best efficiency point (80–110% of BEP).
Compute shaft power = ρgQH/η.
Then the safety-critical check: NPSH available ≥ NPSH required with margin (≥ 1 m or 1.3×NPSHr) — otherwise the pump cavitates.

Step 1 — Choose the type

Duty	Pump type	Why
Normal flow/head	Centrifugal	Default
Very high head	Multistage	High head
Low flow/high head	PD	Flow ∝ speed
Metering / viscous	PD	Precise

Step 2 — Power & NPSH

Condensate, 100 m³/h, 60 m

Given Q = 100 m³/h = 0.0278 m³/s H = 60 m , ρ = 985 kg/m³ , η = 0.75 Step 1 — Hydraulic power Ph = ρ·g·Q·H Ph = 985 × 9.81 × 0.0278 × 60 Ph = 16.1 kW Step 2 — Shaft power Ps = Ph / η = 16.1 / 0.75 = 21.5 kW → motor ~30 kW (next frame + margin) Step 3 — NPSH check NPSHa = 7.5 m (from suction system) NPSHr = 4.0 m (from pump curve) margin = 7.5 − 4.0 = 3.5 m need ≥ 1 m and ≥ 1.3×4.0 = 5.2 m 7.5 ≥ 5.2 → no cavitation Answer → centrifugal, ~21.5 kW shaft, NPSH OK

Ph=ρgQH ÷ η = shaft power. NPSHa > NPSHr + margin or vapour bubbles erode the impeller.

Pump power & NPSH calculator

Shaft power + NPSH margin

Power + NPSHa (suction system) vs NPSHr (pump curve). The margin decides survival.

Flow Q (m³/h)

Head H (m)

Density ρ (kg/m³)

Efficiency

Suction P (bar a)

Vapour pressure (bar a)

Static suction head (m)

Friction loss (m)

NPSHr (m)

Static head +ve if liquid above pump. Screening — confirm with pump & system curves.

Decision flow

1. Shaft power = ρgQH ÷ η → motor

2. NPSHa = (Ps−Pv)/ρg + Hs − Hf

NPSHa ≥ NPSHr + marginOK — no cavitation

If shortRaise level / cut loss / sub-cool

Operating point80–110% BEP; min flow

Suction lift makes the static head negative — the most common NPSHa killer.

Step 3 — Operating point

Duty point at 80–110% of BEP.
Respect minimum continuous flow.
Raise NPSHa if margin is tight.

Common mistakes

Ignoring NPSH.
Oversizing → runs off BEP.
No minimum-flow protection.

Requisition & datasheet

Fluid, flow, density, Pv, head.
Type, power, NPSHa/NPSHr, min flow.

Frequently asked questions

What should I know about Pump Selection & NPSH?

Normal flow & head → centrifugal (end-suction the default). Very high head → multistage centrifugal. Low flow / very high head, or metering / viscous → positive displacement. Size the duty point near the pump's best efficiency point (80–110% of BEP). Compute shaft power = ρgQH/η. Then the safety-critical check: NPSH available ≥ NPSH required with margin (≥ 1 m or 1.3×NPSHr) — otherwise the pump cavitates.

Which standards or codes apply?

API 610 / ISO 13709 (centrifugal pumps), API 674/675 (PD pumps), ANSI/HI (Hydraulic Institute), ISO 9906 (performance tests).

Standards — API 610 / ISO 13709; API 674/675; ANSI/HI; ISO 9906.