Priming (steam locomotive)

If boiler water is condensed and re-used, any oil or grease must be extracted as this form of contamination is particularly likely to give trouble.

In districts where the feed water is unsuitable, blowdown valves ("scum valves"), either continuously working while the regulator is open or operated in conjunction with the boiler feed, are fitted.

Valves at water level reduce surface scum; those towards the bottom of the boiler help remove precipitated solids.

Other forms of prevention include the chemical treatment of water before it enters the boiler.

Once occurring, the problem can affect the level indicated in the boiler's gauge glass and for this reason is difficult to put right without reducing the water level to the extent that the firebox crown becomes dangerously exposed.