Boiler water carryover is the contamination of steam with boiler water solids. There are several common causes of boiler water carryover:

• Bubbles form on the surface of the boiler water and leave with the steam. This foaming can be compared to the stable foam of soap suds.
• Spray or mist is thrown up into the steam space by the bursting of rapidly rising bubbles at the steam release surface. This phenomenon is like the effervescence of champagne. No stable foam forms, but droplets of liquid burst from the liquid surface.
• Priming is a sudden surge of boiler water caused by a rapid change in load. (Uncapping a bottle of charged water produces an effect like this.)
• Steam contamination may also occur from leakage of water through improperly designed or installed steam-separating equipment in a boiler drum.