Stripped of all nonessential details, a centrifugal pump consists of an impeller attached to and rotating with the shaft and a casing that encloses the impeller. In a centrifugal pump, liquid is forced into the inlet side of the pump casing by atmospheric pressure or some upstream pressure. As the impeller rotates, liquid moves toward the discharge side of the pump. This creates a void or reduced pressure area at the impeller inlet. The pressure at the pump casing inlet, which is higher than this reduced pressure at the impeller inlet, forces additional liquid into the impeller to fill the void
If the pipeline leading to the pump inlet contains a noncondensable gas such as air, then the pressure reduction at the impeller inlet merely causes the gas to expand, and suction pressure does not force liquid into the impeller inlet. Consequently, no pumping action can occur unless this noncondensable gas is first eliminated, a process known as priming the pump.
With the exception of a particular type of centrifugal pump called a self priming centrifugal pump, centrifugal pumps are not inherently self priming if they are physically located higher than the level of the liquid to be pumped. That is, the suction piping and inlet side of centrifugal pumps that are not self-priming must be filled with noncompressible liquid and vented of air and other noncondensable gases before the pump can be started. Self-priming pumps are designed to first remove the air or other gas in the suction line, and to then pump in a conventional manner.
If vapors of the liquid being pumped are present on the suction side of the pump, this results in cavitation, which can cause serious damage to the pump.