Net Positive Suction Head
Net Positive Suction Head or NPSH for pumps can be defined as the difference between the suction pressure and the saturation pressure of the fluid, expressed in terms of height of liquid column. NPSH is used to measure how close a fluid is to saturated conditions. Lowering the pressure at the suction side can induce cavitation. If cavitation occurs, the violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as “pumping gravel”. Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.
In general, there are two suction heads defined in hydraulics:
- NPSH Available (NPSHa): The absolute pressure at the suction port of the pump. NPSHa is a function of water temperature. As the inlet temperature increases NPSHa decreases, because the saturation pressure decreases.
- NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSHa is not a function of water temperature.
NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. During operation the available NPSH must be maintained at a level greater than the NPSH required by the pump manufacturer. It has been found that cavitation rates increase rapidly with the increase in the volume flow rate. This can be seen from the picture, as volume flow rate increases, NPSH required increases, but the available NPSH decreases.
How to increase NPSH available?
To avoid suction cavitation, NPSH available must be increased as much as possible. The only way to increase NPSH available is to increase the pressure at the pump inlet:
- Lower the pump level
- Raise the reservoir level
- Reduction of motor RPM if possible
- Reduce minor losses upstream of the pump
- Reduce major losses upstream of the pump
- Shorten the length of pipe
- Use a smoother pipe
- Increase of the diameter of the eye of the impeller
- Use of a booster pump to feed the principal pump.