For students, it is just a topic but it is a key term for pump manufacturers. First of all, let’s discuss that how a pump works. Pumps operate by creating low pressure at the inlet which allows the liquid to be pushed into the pump by atmospheric or head pressure. Even with a perfect vacuum at the pump inlet, atmospheric pressure limits how high the pump can lift the liquid. With liquids lighter than mercury, this lift height can increase, but there’s still a physical limit to pump operation based on pressure external to the pump. This limit is the key consideration for Net Positive Suction Head.
Net Positive Suction Head (NPSH)
NPSH can be defined as two parts:
NPSH Available (NPSHA):
The absolute pressure at the suction port of the pump.
NPSH Required (NPSHR):
The minimum pressure required at the suction port of the pump to keep the pump free from cavitation.
NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA must be greater than NPSHR for the pump system to operate without cavitation. Put another way, you must have more suction side pressure available than the pump requires.