Boiler water

Boiler feed water pump

Impurities in water will leave solid deposits as steam evaporates. These solid deposits thermally insulate heat exchange surfaces initially decreasing the rate of steam generation, and potentially causing boiler metals to reach failure temperatures.[1] Boiler explosions were not uncommon until surviving boiler operators learned how to periodically clean their boilers. Some solids could be removed by cooling the boiler so differential thermal expansion caused brittle crystalline solids to crack and flake off metal boiler surfaces. Other solids were removed by acid washing or mechanical scouring.[2] Various rates of boiler blowdown could reduce the frequency of cleaning, but efficient operation and maintenance of individual boilers was determined by trial and error until chemists devised means of measuring and adjusting water quality to minimize cleaning requirements.

Boiler water treatment is a type of industrial water treatment focused on the removal or chemical modification of substances potentially damaging to the boiler. Varying types of treatment are used at different locations to avoid scale, corrosion, or foaming.[3] External treatment of raw water supplies intended for use within a boiler is focused on the removal of impurities before they reach the boiler. Internal treatment within the boiler is focused on limiting the tendency of water to dissolve the boiler, and maintaining impurities in forms least likely to cause trouble before they can be removed from the boiler in boiler blowdown.[4]

At the elevated temperatures and pressures within a boiler, water exhibits different physical and chemical properties than those observed at room temperature and atmospheric pressure. Chemicals may be added to maintain pH levels minimizing water solubility of boiler materials while allowing efficient action of other chemicals added to prevent foaming, to consume oxygen before it corrodes the boiler, to precipitate dissolved solids before they form scale on steam-generating surfaces, and to remove those precipitates from the vicinity of the steam-generating surfaces.[5]

Many large boilers including those used in thermal power stations recycle condensed steam for re-use within the boiler. Steam condensate is distilled water, but it may contain dissolved gases. A deaerator is often used to convert condensate to feedwater by removing potentially damaging gases including oxygen, carbon dioxide, ammonia and hydrogen sulfide.[15] Inclusion of a polisher (an Ion exchange vessel) helps to maintain water purity, and in particular protect the boiler from a condenser tube leak.

All boilers lose some water in steam leaks; and some is intentionally wasted as boiler blowdown to remove impurities accumulating within the boiler.[16] Steam locomotives and boilers generating steam for use in direct contact with contaminating materials may not recycle condensed steam. Replacement water is required to continue steam production. Make-up water is initially treated to remove floating and suspended materials.[17] Hard water intended for low-pressure boilers may be softened by substituting sodium[18] for divalent cations of dissolved calcium and magnesium most likely to cause carbonate and sulfate scale.[19] High-pressure boilers typically require water demineralized by reverse osmosis, distillation or ion-exchange.[20]

• Dealkalization of water

• Babbitt, Harold E. & Doland, James J. (1949). Water Supply Engineering. McGraw-Hill.
• Betz Laboratories (1976). Handbook of Industrial Water Conditioning (7th ed.). Betz Laboratories.
• Kemmer, Frank N. (1979). The NALCO Water Handbook. McGraw-Hill.
• Linsley, Ray K. & Franzini, Joseph B. (1972). Water-Resources Engineering. McGraw-Hill. ISBN 0-07-037959-9.
• Perry, Robert H.; Chilton, Cecil H.; Kirkpatrick, Sidney D. (1963). Chemical Engineers' Handbook (4th ed.). McGraw-Hill.
• Woodruff, Everett B.; Lammers, Herbert B.; Lammers, Thomas F. (1984). Steam-Plant Operation (5th ed.). McGraw-Hill. ISBN 0-07-071732-X.