Priming usually follows a sharp jump in steam demand: pressure drops, the water flashes and swells, and the level surges into the steam outlet. High water level, undersized steam space or high solids make it worse. Slugs of water traveling at steam velocity cause severe water hammer and can break fittings and damage equipment downstream. In heating systems, priming is behind wet steam and spitting radiator vents.
Boiler primed when they opened the big header valve too fast, you could hear the water hit the elbows all the way to the plant.
Related terms
Carryover
Boiler water and the solids dissolved in it leaving with the steam instead of staying in the boiler.
Foaming
A stable layer of bubbles on the boiler water surface that makes level hard to read and carries water out with the steam.
Bouncing glass
A gauge glass water level jumping up and down instead of holding steady, a warning sign of surging or dirty water.
Spitting
Describes a steam radiator vent that throws out water and steam instead of just air.