Net Positive Suction Head:

To prevent cavitation in centrifugal pumps, a pressure of the fluid at all points inside the pump have remain above saturation pressure. A quantity used to determine if the pressure of the liquid being pumped is adequate to prevent cavitation is the NPSH (net positive suction head).

The net positive suction head available (NPSH_A) is the difference among the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. A net positive suction head needed (NPSH_R) is the minimum net positive suction head essential to prevent cavitation.

The condition which must exist to prevent cavitation is in which the net positive suction head available must be greater than or equal to the net positive suction head needed. This needs could be begin mathematically as displays below.

NPSH_A ≥ NPSH_R

A formula for NPSH_A could be stated as the following equation.

NPSH_A = P_suction - P_saturation

While a centrifugal pump is taking suction from a tank or another reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure due to the elevation difference among the surface of liquid within the tank and the pump suction less the head losses because of friction in the suction line from the tank to the pump.

NPSH_A = P_a + P_st - h_f - P_sat

Where:

NPSH_A = denotes net positive suction head available

P_a = absolute pressure on the surface of the liquid

P_st = pressure because of elevation among liquid surface and pump suction

h_f = head losses in the pump suction piping

P_sat = denotes saturation pressure of the liquid being pumped