NPSHA

NPSHA, or Net Positive Suction Head Available, is a measure of the margin of suction pressure that a pump has to avoid cavitation in the suction line. It reflects the energy in the liquid at the pump eye that can be converted into pressure, above the liquid’s vapor pressure, and is typically expressed in terms of head of liquid.

NPSHA can be defined by the equation NPSHA = (P_s − P_v)/(ρg) + (V_s^2)/(2g) + z_s − h_f, where P_s is the absolute suction pressure at the pump eye, P_v is the vapor pressure of the liquid at operating temperature, ρ is liquid density, g is gravitational acceleration, V_s is the velocity of the liquid in the suction line, z_s is the suction port elevation, and h_f represents friction losses in the suction piping. When the suction source is open to atmosphere, P_s is approximately P_atm, and the formula becomes NPSHA = (P_atm − P_v)/(ρg) + (V_s^2)/(2g) + z_s − h_f.

NPSHA is compared to NPSHR, the Net Positive Suction Head Required by the pump for a given flow rate, as specified by the manufacturer. Cavitation risk arises if NPSHA falls below NPSHR. Practically, engineers seek to maintain NPSHA greater than NPSHR under all operating conditions.

Factors affecting NPSHA include the elevation of the suction source, the length and diameter of suction piping (which influence h_f and V_s), the liquid’s temperature (through P_v), and any air entrainment or blockages that reduce suction pressure or increase losses. Methods to improve NPSHA include increasing reservoir head, reducing suction lift, shortening or enlarging suction piping, and adjusting operating conditions to lower vapor pressure.