General engineering orientation to help you specify and enquire with confidence. Guidance only — selection, configuration and scope are confirmed against your project specification at quotation.
What NPSH means
Net positive suction head (NPSH) describes the margin between the pressure at the pump suction and the vapour pressure of the pumped liquid. NPSH available (NPSHa) is a property of the installation — suction level, pipework losses, atmospheric pressure and liquid temperature. NPSH required (NPSHr) is a property of the pump, established by the manufacturer by test for each model and duty point.
Cavitation
If NPSHa falls to or below NPSHr, the liquid flashes to vapour at the impeller inlet and the vapour bubbles collapse violently as pressure recovers. The result is noise, vibration, loss of performance and progressive erosion of the impeller. Persistent cavitation shortens pump life significantly.
Margin practice
Good practice keeps NPSHa comfortably above NPSHr at every operating point, not only the rated duty. The appropriate margin depends on pump type, size, energy level and duty pattern, and is agreed at selection stage against the project specification. Hot liquids, high-altitude sites and long or throttled suction lines all reduce NPSHa and deserve particular attention.
What to provide with an enquiry
- Suction arrangement — flooded or suction lift, with levels
- Liquid, temperature and vapour pressure where known
- Suction pipework size, length and fittings
- Site altitude
- NPSHa calculation if already performed
These are buyer-input prompts only and are confirmed against your project specification. Send them with your enquiry for engineering review.