Difference Between Priming and Foaming in Boiler

Foaming and Priming in Boilers

This is a term used in boiler engineering power generation.

Foaming: Causes High TDS (total dissolved solids), High suspended solids, detergent type contamination, and high alkalinity. The effects of foaming can cause carry over to turbines and downstream damage.

Primming: Causes high water level, sudden steam demand, and forcing a boiler, the effects are carried over and downstream damage.

When a boiler is steaming (i.e., producing steam) rapidly, some particles of the liquid water are carried along with the steam. This process of 'wet steam' formation is called priming.

Priming is caused by;

  • the presence of  a large number of dissolved solids;
  • high steam velocities,
  • sudden boiling ;
  • improper boiler design
  • a sudden increase in steam-production rate.

Foaming is the production of persistent foam or bubbles in boilers, which do not break easily. Foaming is due to the presence of substances like oils (which greatly reduce the surface tension of the water).

Priming and foaming, usually, occur together. They are objectionable because;

(i) dissolved salts in boiler water are carried by the wet steam to the super-heater and turbine blades, where they get deposited as water evaporates. This deposit reduces their efficiency, 

(ii) dissolved salts may enter the parts of other machinery, where steam is being used, thereby decreasing the life of the machinery; (iii) actual height of the water column cannot be judged properly, thereby making the maintenance of the boiler pressure becomes difficult.

Priming can be avoided by:

(i) fitting mechanical steam purifiers;

(ii) avoiding rapid changing steaming rate; 

(iii) maintaining low water levels in boilers, and 

(iv) efficient softening and filtration of the boiler-feed water.

Foaming can be avoided by: 

(i) adding anti-foaming chemicals like castor oil, or 

(ii) removing oil from boiler water by adding compounds like sodium aluminates.

Boiler water carry-over is the contamination of the steam with boiler-water solids. Bubbles or froth actually build up on the surface of the boiler water and pass out with the steam. This is called foaming and it is caused by the high concentration of any solids in the boiler water. It is generally believed, however, that specific substances such as alkalis, oils, fats, greases, certain types of organic matter, and suspended solids are particularly conducive to foaming. In theory, suspended solids collect in the surface film surrounding a steam bubble, making it more challenging. The steam bubble, therefore, resists breaking and builds up foam. It is believed that the finer the suspended particles the greater their collection in the bubble.

Priming is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the superheaters and in the turbines. Priming may be caused by improper construction of the boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler water.

Some mechanical entrainment of minute drops of boiler water in the steam always occurs. When this boiler water carryover is excessive, steam-carried solids produce turbine blade deposits. The accumulations have a composition similar to that of the dissolved solids in the boiler water. Priming is a common cause of high levels of boiler water carryover. These conditions often lead to superheater tube failures as well. Priming is related to the viscosity of the water and its tendency to foam. These properties are governed by alkalinity, the presence of certain organic substances, and by total salinity or TDS. The degree of priming also depends on the design of the boiler and its steaming rate.

The most common measure to prevent foaming and priming is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also help. Very often contaminated condensate returned to the boiler system causes carry-over problems. In these cases, the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foaming and anti-priming agents, mixtures of surface-active agents that modify the surface tension of a liquid, remove the foam, and prevent the carry-over of fine water particles in the stream, can be very effective in preventing carry-over due to high concentrations of impurities in the boiler water.

Difference Between Priming and Foaming


1 When a boiler is producing steam very rapidly some particles of the water are carried along with the steam. This process of “Wet Steam” formation is called priming.

2 Priming is caused by dissolved salts, high steam velocity, sudden heating, improper design, etc.

3 Priming can be prevented by improving the boiler design, adding a mechanical purifier, maintaining a low water level, and decreasing the salt concentration.

4 Priming reduces the boiler efficiency and life of machinery parts.

5 Due to priming one can’t judge the water level properly.


1 Foaming is the persistent formation of bubbles or foam in the boiler.

2 Foaming is caused by the presence of oily substances in water.

3 Priming can be prevented by the addition of antifoaming agents such as castor oil, Gallic acid,

Tannic acid, sodium aluminate, etc. 

4 Foaming reduces the boiler efficiency and damages machinery parts. 

5 Due to foaming boiler pressure can’t be maintained and water level also can’t be judged

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