Priming in Boilers

Priming refers to the carryover of small droplets of water along with the steam.  Prime is undesirable as it causes corrosion to the steam line and other equipment.
Priming is caused due to many reasons such as high water levels, poor boiler design and fluctuating loads.

When the load in the boiler suddenly increases, the steam pressure in the boiler drops.  This causes the level of water to surge.   This surge of water level causes priming.  Small droplets of water are thrown up into the steam above the water.  This water can carry with it dissolved substances such as chloride, silica, copper, etc.  This is known as carryover.

Priming can be prevented by operating the boiler at steady loads and by maintaining good water chemistry which prevents foaming.