Net Positive Suction Head

Net positive suction head is the absolute head required at the inlet of the pump to prevent cavitation. Similar to net positive inlet pressure (NPIP).

Net positive suction head can be noted with the suffix “A” (NPSHA) for available NPSH from the system or "R" (NPSHR) for NPSH required by the pump. Similar to Net Positive inlet pressure (NPIP).

NPSH_A is a function of your system and must be calculated, whereas NPSH_R is a function of the pump and must be provided by the pump manufacturer. NPSH_A MUST be greater than NPSH_R for the pump system to operate without cavitating. You must have more suction side pressure available than the pump requires or cavitation will occur.

Pump cavitation occurs when the pressure at the pump inlet drops below the vapor pressure of the liquid. Vapor bubbles form at the inlet of the pump and are moved to the outlet where they collapse, often taking small pieces of the pump with them.

Cavitation is often characterized by:

1. Loud noise often described as a grinding or “marbles” in the pump
2. Loss of capacity (bubbles are now taking up space where liquid should be)
3. Pitting damage to parts as material is removed by the collapsing bubbles

To learn more, watch this video about cavitation.