For pumps to start the transport of a medium, they have to be completely filled with the medium. The decisive factor here is the suction characteristics of the pump. While positive displacement pumps are generally self-priming and suction the required volume flow themselves, centrifugal pumps are often not self-priming. For this reason, they have to be filled with liquid prior to operation. If the suction characteristics for self-priming pumps is insufficient or if air enters the suction line of the pump during operation, it can lead to cavitation.
An important parameter for assessing the suction characteristics is the pump-specific NPSH value (Net Positive Suction Head). In the past, this value was also designated as retaining pressure head. To avoid cavitation, a certain pressure is required at the suction side of the pump. In this case, a distinction must be made between the available NPSH of the installation (available, therefore NPSHA) and the pump (required, therefore NPSHR). To avoid cavitation, the available NPSH should be higher than the NPSH required by the pump.