NPSH (Net Positive Suction Head)

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Net Positive Suction Head (NPSH)

A pump does NOT suck, or pull, a fluid into its inlet. A fluid has no tensile strength and therefore cannot be pulled. A pump simply lowers the pressure at its inlet and the fluid is pushed into the pump.

NPSH is simply the difference between the actual pressure of the fluid at the pump inlet and the vapour pressure of that fluid.

The vapour pressure varies with temperature, if you fall below this pressure the liquid will boil.

For water;

Centigrade	Vapour pressure (Bar) A
4.4	0.00839
37.8	0.06546
82.2	0.5179
100	1.0135
148.9	4.62

NPSH is important in determining the operation of a pump. If the NPSH is negative cavitation will occur – the liquid will boil.

For any given system there is a Net Positive Suction Head Requirement (NPSHr – a system characteristic). This is a representation of the minimum head required to prevent the liquid boiling. This value is dependent on the flow and is determined by the losses between the source and the pump suction. NPSHR represents the minimum positive head required at the pump suction, taking into account the inlet losses and the vapour pressure of the liquid being pumped, to prevent cavitation.

The pump will develop a ‘suction pressure’ at the impellor eye. This is known as the Net Positive Suction Head Available (NPSHa – a pump characteristic) and is due to the static suction head, friction losses in the pipeline and pressure acting on the surface of the liquid.

NPSHA is defined as static head + surface pressure head - the vapor pressure of liquid - loss in the piping, valves and fittings.