NPSH Calculation

In addition to the total head, capacity, power and efficiency requirements, the condition at the inlet of a pump is critical. The system on the inlet side of the pump must allow a smooth flow of fluid to enter the pump at a sufficiently high pressure to avoid cavitation.

This is called the Net Positive Suction Head, generally abbreviated NPSH. Pump manufacturers supply data about the net positive suction head required by their pumps (NPSHr) for satisfactory operation. 

When selecting a pump it is critical the net positive suction head available (NPSHa) in the system is greater than the net positive suction head required by the pump. A simplified way to look at NPSHa. is to imagine a balance of factors working for (static pressure and positive head) and against (friction loss and vapor pressure) the pump. 

The value of NPSHa in the system is dependent upon the characteristic of the fluid being pumped, inlet piping, the location of the suction vessel, and the pressure applied to the fluid in the suction vessel. This is the actual pressure seen at the pump inlet. It is important to note, it is the inlet system that sets the inlet condition and not the pump.

Net Positive Suction Head Required, NPSHR, Net Positive Suction Head Available, NPSHA

Net Positive Suction Head Required, NPSHR:-

As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH required is the positive head (absolute pressure) required at the pump suction to overcome these pressure drops in the pump and maintain the liquid above its vapor pressure.

Net Positive Suction Head Available, NPSHA :-
Net Positive Suction Head Available is a function of the system in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction,