Understanding NPSH (Net Positive Suction Head) for Metering Pumps

Before discussing net positive suction head, we first need to understand what cavitation is. Cavitation is the process in which a liquid, flowing through the components of a pump at a given temperature, vaporizes because the pressure drops. This vaporization can do significant harm to both the equipment itself and its performance. For this reason, cavitation must always be avoided during pump operation.

Net positive suction head is usually expressed as NPSH (Net Positive Suction Head). It is the total head of the fluid at the pump inlet minus

the vapor head of the fluid. In plain terms, it is the amount by which the pressure at the pump inlet exceeds the vapor pressure of the liquid, and it is usually converted into a difference in liquid level (in meters) for expression.

For metering pumps, NPSH is further divided into NPSHR and NPSHA. NPSHR is the net positive suction head required by the pump. The value of NPSHR is determined mainly by the structure of the metering pump itself, which means that once a metering pump is selected, its NPSHR is fixed; the specific value can be determined by testing and is provided by the metering pump manufacturer. NPSHA is the net positive suction head that the metering pump's supporting piping system can provide. The value of NPSHA is determined by factors such as the length of the pump's inlet piping, the liquid level, the diameter of the inlet piping, and the number of valves and fittings in the inlet piping. To avoid cavitation when a metering pump is in service, you must ensure that NPSHA > NPSHR when selecting the pump — that is, the net positive suction head the system can provide must be greater than the net positive suction head the metering pump requires.

Because the required net positive suction head NPSHR of a metering pump is a fixed value, ensuring normal operation of the equipment requires designing the inlet piping sensibly in order to increase NPSHA and guarantee that the pump runs properly.

For metering pump net positive suction head, the NPSHA calculation formula is as follows: NPSHA = absolute head + static head – vapor head – piping losses – acceleration head loss

In the formula, the absolute head is the gas pressure (absolute pressure) above the liquid, the static head is the height of the liquid in the tank above the pump inlet, the vapor head is the saturated vapor pressure of the liquid at the current temperature, the piping losses are the pressure losses as the liquid flows through the pump inlet piping, and the acceleration head loss is the pressure loss in the inlet liquid caused by the metering pump's flow pulsations.

To increase the NPSHA value, you can raise the gas pressure above the liquid, raise the liquid level, or lower the saturated vapor pressure of the liquid. The way to reduce piping losses is to shorten the length of the pump inlet piping and reduce the number of valves and fittings in the inlet piping. The way to reduce acceleration head loss is to shorten the pump inlet piping, increase the diameter of the inlet piping, and lower the reciprocating stroke frequency of the metering pump. For metering pumps with long inlet piping in particular, acceleration head loss is the main factor causing cavitation, and it must be given sufficient attention.