Boiler feedwater
Encyclopedia
Boiler feedwater is water
used to supply ("feed") a boiler
to generate steam
or hot water. At thermal power station
s the feedwater is usually stored, pre-heated and conditioned in a feedwater tank
and forwarded into the boiler by a boiler feedwater pump
.
- Corrosive components, especially O2
and CO2
have to be removed, usually by use of a deaerator
. Remnants can be removed chemically, by use of oxygen scavenger. Furthermore feedwater has to be alkalized to a pH
of 9 or higher, to reduce oxidation and to support the forming 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 alkalic agents into the feedwater, like sodium hydroxide (caustic soda) or volatile ammonia
.
/ Sediment
s / 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 would remain in soluted form when the feedwater is evaporated have to be removed, because these would be concentrated in the liquid phase and require excessive "blow-down" (draining) to avoid that the liquid eventually becomes saturated and solid crystals fall out. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feedwater has to be demineralized/deionized water.
, tannin
, phosphate
and an anti-foaming agent.
Treatment systems have included:
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
used to supply ("feed") a boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
to generate steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
or hot water. At thermal power station
Thermal power station
A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this...
s the feedwater is usually stored, pre-heated and conditioned in a feedwater tank
Storage tank
A storage tank is a container, usually for holding liquids, sometimes for compressed gases . The term can be used for reservoirs , and for manufactured containers. The usage of the word tank for reservoirs is common or universal in Indian English, American English and moderately common in British...
and forwarded into the boiler by a boiler feedwater pump
Boiler feedwater pump
A boiler feedwater pump is a specific type of pump used to pump feedwater into a steam boiler. The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler...
.
Conditioning
The feedwater has to be specially conditioned to avoid problems in the boiler and downstream components. The water required for boiler feed purposes i.e for steam generation should be of very high quality and thus requires a lot of treatment. Untreated water, containing impurities may lead to the following problems in boilers.Boiler corrosion
CorrosionCorrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
- Corrosive components, especially O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
have to be removed, usually by use of a deaerator
Deaerator
A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and...
. Remnants can be removed chemically, by use of oxygen scavenger. Furthermore feedwater has to be alkalized to a pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of 9 or higher, to reduce oxidation and to support the forming of a stable layer of magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkalic agents into the feedwater, like sodium hydroxide (caustic soda) or volatile ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
.
Scale and sludge
DepositsDeposition (chemistry)
In chemistry, deposition is the settling of particles or sediment from a solution, suspension and mixture or vapor onto a pre-existing surface...
/ Sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s / Fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
- Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
salts and minerals that would remain in soluted form when the feedwater is evaporated have to be removed, because these would be concentrated in the liquid phase and require excessive "blow-down" (draining) to avoid that the liquid eventually becomes saturated and solid crystals fall out. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feedwater has to be demineralized/deionized water.
Locomotive boilers
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 carbonateSodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
, tannin
Tannin
A tannin is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.The term tannin refers to the use of...
, phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
and an anti-foaming agent.
Treatment systems have included:
- Alfloc, developed by British Railways and Imperial Chemical IndustriesImperial Chemical IndustriesImperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...
- Traitement Integral Armand (TIA), developed by Louis ArmandLouis ArmandFor the writer and critical theorist, see Louis Armand Louis Armand was a French engineer who managed several public companies and had a significant role during World War II as an officer in the Resistance...
- Porta Treatment, developed by Livio Dante PortaLivio Dante PortaLivio Dante Porta was an Argentine steam locomotive engineer. He is particularly remembered for his innovative modifications to existing locomotive systems in order to obtain higher performance, energy efficiency and reduced pollution. He developed the Kylpor and Lempor exhaust systems...