Boiler feedwater is water used to supply ("feed") a boiler to generate steam or hot water. At thermal power stations the feedwater is usually stored, pre-heated and conditioned in a feedwater tank and supplied to the boiler by a boiler feedwater pump.
History of Feedwater treatment
During the early development of boilers, water treatment was not so much of an issue, as temperatures and pressures were so low that high amounts of scale and rust would not form to such a high amount, especially if the boiler was cleaned and/or “blown down”. It was general practice though, to install zinc plates and/or alkaline chemicals to reduce corrosion within the boiler. Many tests had been preformed to try and determine the cause and possible protection from corrosion in boilers using distilled water, various chemicals, and sacrificial metals. Use of lime for alkalinity control had been mentioned as early as 1900, and was used by the French and British Navies up until about 1935. In modern boilers though, treatment of boiler feedwater is extremely critical, as many problems can result from the use of untreated water in extreme pressure and temperature environments; this includes lower efficiency in terms of heat transfer, overheating, damage, and high costs of cleaning.
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The feedwater must be specially treated to avoid problems in the boiler and downstream systems. Untreated boiler feed water can cause corrosion and fouling.
Corrosive compounds, especially O2 and CO2 must be removed, usually by use of a deaerator. Residual amounts can be removed chemically, by use of oxygen scavengers. Additionally, feed water is typically alkalized to a pH of 9.0 or higher, to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkaline agents into the feed water, such as sodium hydroxide (caustic soda) or ammonia. Corrosion in boilers is due to the presence of dissolved oxygen, dissolved carbon dioxide, or dissolved salts.
Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile salts and minerals that will remain when the feedwater is evaporated must be removed, because they will become concentrated in the liquid phase and require excessive "blow-down" (draining) to prevent the formation of solid precipitates. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feedwater must be demineralized/deionized water, unless a purge valve is used to remove dissolved minerals.
Priming and foaming
Steam locomotives usually do not have condensers so the feedwater is not recycled and water consumption is high. The use of deionized water would be prohibitively expensive so other types of water treatment are used. Chemicals employed typically include sodium carbonate, sodium bisulfite, tannin, phosphate and an anti-foaming agent.
Treatment systems have included:
- Alfloc, developed by British Railways and Imperial Chemical Industries 
- Traitement Integral Armand (TIA), developed by Louis Armand
- Porta Treatment, developed by Livio Dante Porta 
- Lyon,Frank. Hinds, A.W.. Marine And Naval Boilers. (1912). The Lord Baltimore Press.
- Osbourne, Alan. Modern Marine Engineers Manual. (1965). Cornell Maritime Press, inc.
- Bane, M. (11 December 2006). "Porta Treatment Internal Boiler Water Treatment for the 21st Century". "Developments in Modern Steam Traction for Railways". York, UK. Retrieved 31 December 2013.
- Bane, Martyn. "Modern Steam Glossary". Martyn Bane's steam and travel pages. Retrieved 31 December 2013.
- "Porta Treatment: Advanced Internal Boiler Water Treatment". 18 October 2007. Retrieved 31 December 2013.