Net Positive Suction Head (NPSH)
Net positive suction head (NPSH) is the margin between the absolute pressure at the pump suction and the vapour pressure of the liquid, expressed as head of liquid. NPSH available (NPSHa) is a property of the system, calculated from suction vessel pressure, static elevation, suction line friction losses and vapour pressure at pumping temperature. NPSH required (NPSHr) is a property of the pump, established by the manufacturer through testing and conventionally defined as the NPSH at which total head drops by 3% (NPSH3). To prevent cavitation, NPSHa must exceed NPSHr by an adequate margin, specified as a ratio or a fixed head allowance depending on the service. Guidance on NPSH margins is given in ANSI/HI 9.6.1, and API 610 covers NPSH testing of process pumps. NPSH is especially critical for hot liquids, volatile hydrocarbons and suction-lift installations.
Examples
- Checking NPSH margin for a boiler feedwater pump
- Evaluating suction conditions for LPG transfer
- Assessing a pump drawing from a below-grade sump