What causes foaming in Boilers ?

Foaming refers to the formation of froth or bubbles in the boiler.  Foaming is caused by high concentration of dissolved solids in boiler water.  When a bubble forms in water that has high levels of dissolved solids, the dissolved solids surround the bubble and make it tougher.  The bubble does not break.  When more bubbles collect, foam is formed. 

Foaming can be prevented by maintaining good water quality.  The level of dissolved solids should be maintained as low as possible. 

The presence of oil in the water can also cause foaming.

Anti-foaming agents are available which prevent the formation of bubbles and foam.