In a centrifugal pump, the pressure of the fluid at all points within the pump must remain above saturation pressure. Otherwise, cavitation occurs in the pump. Net positive suction head (NPSH) determines the adequate pressure of the fluid to avoid cavitation. The total suction head in meters absolute determined at the suction of the pump called NPSH of the pump. Thus, Net positive suction head (NPSH) can be described as, “an analysis of total energy at the suction side of the pump to determine if the liquid will vaporize at the lowest pressure point in the pump”. If vapors pressure within the fluid reaches to pressure of the surrounding medium then fluid starts to vaporize. The temperature at which vaporization occurs will decrease as the surrounding medium pressure decrease.
There are two types of NPSH consider for pump installation.
- Net Positive Suction Head Available
- Net Positive Suction Head Required
Net Positive Suction Head Available (NPSHA):
The difference between the pressure at the suction of the pump and the saturation pressure of the pumped liquid is called NPSHA. It is the available pressure head at the suction of the pump. Net Positive Suction Head Required (NPSHA) formula can describe as:
NPSHA = Psuction – P saturation
Psuction = Pressure at the suction of the pump
P saturation = Saturation pressure of the pumped liquid
When centrifugal pump taking suction from a tank or other reservoir the pump suction pressure is the sum of absolute pressure at the surface of the liquid in the tank plus pressure due to the elevation difference between the surface of liquid in the tank and the pump suction less the head losses due to friction in the suction line from the tank to the pump.
NPSHA = Pa + Pst – hf – Psat
Pa = Absolute pressure on the surface of the liquid
Pst = Pressure due to elevation between the liquid surface and pump suction
hf = Head losses in the suction piping of the pump
Psat = Saturation pressure of the pumped liquid
Net Positive Suction Head Required (NPSHR):
Net Positive Suction Head Required (NPSHR) is the minimum head required to avoid cavitation. It is the function of pump design. The positive head in meters absolute required at the pump suction to overcome pump internal head losses called NPSHR. These internal head losses are instance turbulence as the liquid strikes the impeller, losses created in suction passage and losses occurred by the liquid passing through the vanes and also maintain the liquid above its vapor pressure. NPSHR varies with speed and capacity within any specific pump.
To avoid cavitation, the net positive suction head available must be greater than or equal to the net positive suction head required. This statement can describe mathematically as:
NPSHA ≥ NPSHR
For successful pumping, NPSHA is greater than NPSHR. However, to maintain a minimum liquid level in a tank, it is to be assumed that NPSHA is equal to NPSHR. Generally, the pump manufacturer provides the information along with the pump characteristic curve.