Net positive suction head available NPSHa

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

The quantity used to determine if the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHa) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSHr) is the minimum net positive suction head necessary to avoid cavitation. 

V.3 Net positive suction head available (NPSHA) and required (NPSHR)... 

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