Foaming is the thick layer of steam bubbles on top of water surface inside boiler. It occurs due to -

1. High concentration of impurities.
2. By the animal or vegetable fats in feed water carry over from the oil heaters.
3. Increase in level of dissolved and suspended solid TDS level.
4. Increase in water level.

To prevent foaming, surface or scum blow down should done frequently to expel any floating impurities for the boiler and no lube oil should be allowed to enter the boiler.