NPSH and Cavitation: What the Suction Line Must Deliver
NPSHa is a system number, NPSHr is a pump number. Build the suction energy budget, include fittings K, and see why hot water and altitude eat the margin.
Key takeaways
- NPSHa belongs to the system; NPSHr belongs to the pump curve at that Q.
- Hot liquid and high altitude destroy NPSH faster than extra pipe length.
- Keep suction short, fat, and simple — fittings K often beats wall friction.
- Leave 0.5–1.0 m of margin after NPSHr unless the manufacturer says more.
Cavitation starts when the local pressure at the impeller eye drops to vapor pressure. The catalogue number that protects you is NPSHr. The number you actually have is NPSHa, and only the piping, the liquid temperature, and the site altitude can change it.
The energy budget
| Term | Typical water value | What hurts it |
|---|---|---|
| P_atm/ρg | 10.3 m at sea level | Altitude (~1 m per 800 m elevation) |
| z_s | +2 m flooded / −3 m lift | Suction lift, empty tank |
| h_fs | 0.3–2 m | Small D, long suction, strainer, globe valve |
| P_v/ρg at 20 °C | 0.24 m | Hot water: ~2 m at 60 °C, ~7 m at 90 °C |
Design habits that actually help
- Suction velocity often below 1.5 m/s for water; lower for hot or flashing liquids.
- One long-radius elbow, not a tight screwed 90° plus a strainer in the same metre.
- Flooded suction whenever the layout allows. A metre of lift costs a metre of NPSH plus extra friction.
- Compare NPSHa to NPSHr at the duty Q, not only at BEP.
- Leave 0.5–1.0 m of margin after NPSHr. HI and manufacturer notes differ; the physics does not.
Worked sketch: Patm = 101 kPa, ρ = 998 kg/m³, z_s = +2 m, h_fs = 1.1 m, P_v = 2.3 kPa (20 °C). Atmospheric head ≈ 10.3 m, vapor head ≈ 0.24 m, NPSHa ≈ 10.3 + 2 − 1.1 − 0.24 = 11.0 m. If NPSHr = 3.5 m you have a healthy margin. Raise the water to 80 °C and P_v/ρg jumps toward 5 m — the same piping may fail.
Open solver: Suction fittings K-factorOpen solver: Vapor pressure vs temperatureOpen solver: Pump system curveFrequently asked questions
It is a head (metres or feet of the pumped liquid). Convert with ρg if you think in kPa. Using metres of water for a hydrocarbon with SG = 0.7 will lie.