1. The filling of a centrifugal-type pump with the process liquid to be pumped by displacing residual air or vapour. It is carried out manually or, in some cases, remotely by activated control valves. Centrifugal pumps are not self-priming.

2. The entrainment of boiler water in steam produced in a boiler. It may be caused by excessively high water levels or foaming caused by excessive salts in the water.