Concept reference

NPSH Available and Required

C.1Plain-language definition

Net positive suction head measures how far the liquid at the pump suction remains above its vapour-pressure condition, expressed as head of the pumped liquid. NPSH available belongs to the system; NPSH required comes from a pump test and criterion. Having NPSHa merely equal NPSHr is not a universal guarantee of quiet, damage-free service.

C.2Governing relation or decision logic

At the pump suction reference, NPSHa = ps,abs/(ρg) + Vs²/(2g) − pv/(ρg), in m (ft) of pumped liquid. From an open source it can be rearranged from atmospheric head, source level, suction losses and vapour-pressure head. Compare values at the same flow, fluid temperature and pump datum.

C.3The failure to name

Using gauge pressure in the absolute-pressure formula removes atmospheric head and can make the answer meaningless. Cold-water vapour pressure, sea-level atmosphere, clean strainers or normal wet-well level can each overstate the available margin.

C.4Field review checklist

• Use absolute pressure and vapour pressure at maximum design temperature.
• Check minimum source level, site atmospheric pressure/altitude and maximum suction loss.
• Read NPSHr at every operating flow from the correct pump curve and test basis.
• Apply a documented project or manufacturer margin rather than inventing a universal allowance.

C.5Where it fits in the course

§6.5 assembles NPSHa and margin; §8.3 separates it from air entrainment.