Net positive suction head availability

To express the quantity of energt available in the lit]iiid entering into the pump, the unit of measure for NPSH is feet of head or elevation in the pump suction. The pump has its NPSHr, or Net Positive Suction Head Required. The system, meaning all pipe, tanks and connections on the suction side of the pump has the NPSHa, or the Net Positive Suction Head Available. There should always be more NPSHa in the system dian the NPSHr of the pump. Let s look at them, beginning with what the pump recgiires ... 

The net positive suction head available (NPSHavail) is the pressure at the pump suction, above the vapour pressure of the liquid, expressed as head of liquid. 

NPSH avan Net positive suction head available at the pump suction L... 

Net positive suction head available (NPSH) is the difference between the total absolute suction pressure at the pump suction nozzle when the pump is running and the vapor pressure at the flowing liquid temperature. All pumps require the system to provide adequate (NPSH). In a positive-displacement pump the (NPSH)a should be large enough to open the suction valve, to overcome the friction losses within the pump hquid end, and to overcome the liquid acceleration head. 

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-... 

To avoid pump cavitation, the net positive suction head available must be greater than the net positive suction head required. 

Net positive suction head available is the difference between the pump suction pressure and the saturation pressure for the liquid being pumped. 

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. 

Net Positive Suction Head Available (NPSHIA) in ft, (See section FV.3 for details.)... 

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

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