NPSH of pump is an initialism for Net Positive Suction Head. In any cross-section of a generic hydraulic circuit, the NPSH parameter shows the different between the actual pressure of a liquid in a pipeline and the liquid’s vapor pressure at a given temperature.
NPSH is an important parameter to take into account when designing a circuit: whenever the liquid pressure drops below the vapor pressure, liquid boiling occurs, and the final effect will be cavitation: vapr bubbles may reduce or stop the liquid flow, as well as damage the system.
Centrifugal pumps are particularly vulnerable especially when pumping heated solution near the vapor pressure, whereas positive displacement pumps are less affected by cavitation, as they are better able to pump two-phase flow(the mixture of gas and liquid), however the resultant flow rate of the pump will be diminished because of the gas volumetrically displacing a disproportion of liquid. Careful design is required to pump high temperature liquids with a centrifugal pump when the liquid is near its boiling point.
The violent collapse of the cavitation bubble creates a shock wave that can literally carve material from internal pump components(usually the leading edge of the empeller) 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 pump operation, two aspects of this parameter are called repectively NPSHA or Net Positive Suction Head available and NPSHR or Net Positive Suction Head required or NPSH-3, where NPSHA is the suction pressure at the pump inlet port, and NPSHR is the suction pressure limit at which the pump’s total differential head performance is reduced by 3% due to cavitation. Cavitation occurs at suction pressure levels below the NPSH-3 level and pump damage can occur from cavitation even though the pump may continue to provide the expected hydraulic performance.
Values of NPSHR are generally determined experimentally. During a typical NPSH test, the pump is operated at a constant capacity and speed while its suction pressure is gradually reduced until the total pump head drops. The value of NPSHA corresponding to a specified(frequently 3 percent) reduction in total head is cosidered to be the pump’s NPSHR at the test capacity. and this is NPSH-3. This test is repeated at various capacities and a curve of NPSHR versus capacity can be obtained. Values of NPSH head required usually increase with capacity. It should be noted that the bubbles can first begin to form under NPSHA that are significantly greater than that result in a 3 percent drop in total pump head.
