Boiler feedwater is an essential part of

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central ...

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 A closed system is a natural physical system that does not allow transfer of matter in or out of the system, although — in contexts such as physics, chemistry or engineering — the transfer of energy (''e.g.'' as work or heat) is allowed. In ...

Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sourc ...

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 significant extent, especially if the boiler was “ blown down”. It was general practice to install zinc plates and/or

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

chemicals to reduce corrosion within the boiler. Many tests had been performed to determine the cause (and possible protection) from corrosion in boilers using distilled water, various chemicals, and sacrificial metals.

Silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

can be added to feedwater samples to detect contamination by

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

. Use of lime for alkalinity control was mentioned as early as 1900, and was used by the French and British Navies until about 1935. In modern boilers, treatment of feedwater is critical, as problems result from using untreated water in extreme pressure and temperature environments. This includes lower efficiency in terms of heat transfer, overheating, damage, and costly cleaning.

Characteristics of boiler feedwater

Water has higher

heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat cap ...

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 A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...

. The

that is used is the

. 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 Boiler water is liquid water within a boiler, or in associated piping, pumps and other equipment, that is intended for evaporation into steam. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam ...

must be treated in order to be proficient in producing steam. Boiler water is treated to prevent scaling,

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

, 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 In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

. 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.

Boiler feedwater treatment

Boiler water treatment is used to control alkalinity, prevent scaling, correct pH, and to control conductivity. The boiler water needs to be alkaline and not acidic, so that it does not ruin the tubes. There can be too much conductivity in the feed water when there are too many dissolved solids. These correct treatments can be controlled by efficient operator and use of treatment chemicals. The main objectives to treat and condition boiler water is to exchange heat without scaling, protect against scaling, and produce high quality steam. The treatment of boiler water can be put into two parts. These are internal treatment and external treatment. (Sendelbach, p. 131)Sendelbach, M. (1988). Boiler-water treatment: Why, what and how. Chemical Engineering, 95(11), 127. The internal treatment is for boiler feed water and external treatment is for make-up feed water and the condensate part of the system. Internal treatment protects against feed water hardness by preventing precipitating of scale on the boiler tubes. This treatment also protects against concentrations of dissolved and suspended solids in the feed water without priming or foaming. These treatment chemicals also help with the alkalinity of the feed water making it more of a base to help protect against boiler corrosion. The correct alkalinity is protected by adding phosphates. These phosphates precipitate the solids to the bottom of the boiler drum. At the bottom of the boiler drum there is a bottom blow to remove these solids. These chemicals also include anti-scaling agents, oxygen scavengers, and anti-foaming agents. Sludge can also be treated by two approaches. These are by coagulation and dispersion. When there is a high amount of sludge content it is better to coagulate the sludge to form large particles in order to just use the bottom blow to remove them from the feed water. When there is a low amount of sludge content it is better to use dispersants because it disperses the sludge throughout the feed water so sludge does not form.

Deaeration of feed water

Oxygen and carbon dioxide are removed from the feed water by deaeration. Deaeration can be accomplished by using deaerator heaters, vacuum deaerators, mechanical pumps, and steam-jet ejectors. In deaerating heaters, steam sprays incoming feed water and carries away the dissolved gases. The deaerators also store hot feed water which is ready to be used in the boiler. This means of mechanical deaeration is used with chemical oxygen scavenging agents to increase efficiency. (Sendelbach, p. 129) Deaerating heaters can be classified in two groups: spray types and tray types. With tray type heaters the incoming water is sprayed into steam atmosphere to reach saturation temperature. When the saturation temperature is reached most of the oxygen and non-condensable gases are released. There are seals that prevent the recontamination of the water in the spray section. The water then falls to the storage tank below. The non-condensables and oxygen are then vented to the atmosphere. The components of the tray type deaerating heater are a shell, spray nozzles, direct contact vent condenser, tray stacks, and protective interchamber walls. The spray type deaerator is similar to the tray type deaerator. The water is sprayed into a steam atmosphere and most of the oxygen and non-condensables are released to the steam. The water then falls to the steam scrubber where the slight pressure loss causes the water to flash a little bit which also aids the removal of oxygen and non-condensables. The water then overflows to the storage tank. The gases are then vented to the atmosphere. With vacuum deaeration a vacuum is applied to the system and water is then brought to its saturation temperature. The water is sprayed into the tank like the spray and tray deaerator. The oxygen and non-condensables are vented to the atmosphere. (Sendelbach, p. 130)

Conditioning

The feedwater must be specially treated to avoid problems in the boiler and downstream systems. Untreated boiler feed water can cause corrosion and fouling.

Boiler corrosion

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 Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

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 Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...

( caustic soda) or

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

. Corrosion in boilers is due to the presence of dissolved oxygen, dissolved carbon dioxide, or dissolved salts.

Fouling

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

evaporate Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humi ...

d 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 Boiler water is liquid water within a boiler, or in associated piping, pumps and other equipment, that is intended for evaporation into steam. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam c ...

added to replace any losses of feedwater must be demineralized/

deionized water Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently pu ...

, unless a purge valve is used to remove dissolved minerals.

Caustic embrittlement

Priming and foaming

Locomotive boilers

Steam locomotives A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...

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 carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

sodium bisulfite Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutio ...

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

and an anti-foaming agent. Treatment systems have included: * Alfloc, developed by

British Railways British Railways (BR), which from 1965 traded as British Rail, was a state-owned company that operated most of the overground rail transport in Great Britain from 1948 to 1997. It was formed from the nationalisation of the Big Four British ...

and

Imperial Chemical Industries Imperial Chemical Industries (ICI) was a British chemical company. It was, for much of its history, the largest manufacturer in Britain. It was formed by the merger of four leading British chemical companies in 1926. Its headquarters were at ...

* Traitement Integral Armand (TIA), developed by

Louis Armand Louis François Armand (17 January 1905 – 30 August 1971) was a French engineer and senior civil servant who managed several public companies, as well as had a significant role in World War II as an officer in the Resistance. He became the f ...

* Porta Treatment, developed by

Livio Dante Porta Livio Dante Porta (21 March 1922 – 10 June 2003) was an Argentine steam locomotive engineer. He is particularly remembered for his innovative modifications to existing locomotive systems in order to obtain better performance and energy effici ...

References

{{Reflist *Shun'an, C. , Qing, Z. , & Zhixin, Z. (2008). A study of the influence of chloride ion concentration on the corrosion behavior of carbon steel in phosphate high-temperature boiler water chemistries. Anti-Corrosion Methods and Materials, 55(1), 15–19. *Sendelbach, M. (1988). Boiler-water treatment: Why, what and how. Chemical Engineering, 95(11), 127. *Characteristics of boiler feed water. (n.d.). Retrieved March 21, 2015, from http://www.lenntech.com/applications/process/boiler/boiler-feedwater-characteristics.htm

External links

Boiler Feedwater System Configuration
Power station technology Boilers Chemical process engineering Steam locomotive technologies