NET POSITIVE SUCTION HEAD (NPSH)

NET POSITIVE SUCTION HEAD (NPSH)

The term NPSH is very commonly used selection of a pump. The minimum suction conditions are specified in terms NPSH.

It is defined as the absolute pressure head at the inlet to the pump minus the vapour pressure head plus velocity head.

NET POSITIVE SUCTION HEAD (NPSH)
NET POSITIVE SUCTION HEAD (NPSH)

The right hand side of the above equation is the total suction head. Hence NPSH is equal to the total suction head. Thus NPSH may also be defined as the total head required to make the liquid flow through the suction pipe to the pump impeller.

For any pump installation, a distinction is made between the required NPSH and the available NPSH. The value of required NPSH is given by the pump manufacturer. This value can also be determined experimentally. 

For determining its value the pump is tested and minimum value of NPSH is obtained at which the pump gives maximum efficiency without any noise. (i.e. cavitation free). The required NPSH varies with the pump design, speed of the pump and capacity of the pump.

When the pump is installed, the available NPSH is calculated from the above equation (2). In order to have cavitation free operation of centrifugal pump, the available NPSH should be greater than the required NPSH.

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