Feedwater

Boiler feedwater is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the main condenser. From the condenser it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere. This cycle is known as a closed system or Rankine cycle.

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 performed to try to determine the cause and possible protection from corrosion in boilers using distilled water, various chemicals, and sacrificial metals.Silver nitrate can be added to feedwater samples in order to detect contamination by seawater. 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.

Water has higher heat capacity than most other substances. This quality makes it an ideal raw material for boiler operations. Boilers are part of a closed system as compared to open systems in a gas turbine. The closed system that is used is the Rankine cycle. This means that the water is recirculated throughout the system and is never in contact with the atmosphere. The water is reused and needs to be treated to continue efficient operations. Boiler water must be treated in order to be proficient in producing steam. Boiler water is treated to prevent scaling, corrosion, foaming, and priming. Chemicals are put into boiler water through the chemical feed tank to keep the water within chemical range. These chemicals are mostly oxygen scavengers and phosphates. The boiler water also has frequent blowdowns in order to keep the chloride content down. The boiler operations also include bottom blows in order to get rid of solids. Scale is precipitated impurities out of the water and then forms on heat transfer surfaces. This is a problem because scale does not transfer heat very well and causes the tubes to fail by getting too hot. Corrosion is caused by oxygen in the water. The oxygen causes the metal to oxidize which lowers the melting point of the metal. Foaming and priming is caused when the boiler water does not have the correct amount of chemicals and there are suspended solids in the water which carry over in the dry pipe. The dry pipe is where the steam and water mixture are separated.

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Wikipedia