Net Positive Suction Head Required (NPSHR) of Pump

Pump which is used for power plant can be called as Boiler Feed Pump. Boiler feed pump serves to supply feedwater into steam boiler. Pump science is the interesting knowledge to be learned. One of them is about Net Positive Suction Head (NPSH) of pump.

The lowest pressure within the pump can usually be found at a point near the side entrance impeller blades in that place, the pressure is lower than the pressure at the suction pump hole. This is due to head losses in the suction nozzle, the flow velocity increases due to cross-sectional area narrowing, and increased flow velocity due to thickness of local blade.

To avoid evaporation of liquid, the pressure on pump inlet is reduced with a decrease in pressure within pump should be higher than steam pressure of liquids. A large pressure head equal to pressure drop is called as NPSHR / Net Positive Suction Head Required. The amount of Net Positive Suction Head Required is different for each pump. For a particular pump, NPSH required changes according to capacity and rotation.

In order for the pump can work without experiencing cavitation, it must meet NPSH available is greater than the NPSH required. NPSH required value must be obtained from the pump manufacturer. But for a rough assessment, the NPSH required can be calculated by the following equation:

σ = H_svN / H_N

Where:

σ          = coefficient of Thoma cavitation

H_svN     = NPSH required (m)

H_N       = total head pump at the point of maximum efficiency (m).

The suction specific speed (S) can also be used as a replacement for coefficient of Thoma cavitation in calculating NPSH required. The correlation can be seen in the following equation:

H_svN = (n/S)^4/3 . Q_N^2/3

Where:

H_svN     = NPSH required (m)

n          = pump rotation (rpm)

Q_N       = pump capacity (m³/min)

S          = the suction specific speed (m / min).