Boiler water is liquid
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
within a
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, centr ...
, 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
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 ma ...
, steam condensate being returned to a boiler, or
boiler blowdown Boiler blowdown is water intentionally wasted from a boiler to avoid concentration of impurities during continuing evaporation of steam. The water is blown out of the boiler with some force by steam pressure within the boiler. Bottom blowdown used ...
being removed from a boiler.
Early practice
![Weir boiler feed water pump](https://upload.wikimedia.org/wikipedia/commons/3/36/Weir_boiler_feed_water_pump.jpg)
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.
Boiler explosion
A boiler explosion is a catastrophic failure of a boiler. There are two types of boiler explosions. One type is a failure of the pressure parts of the steam and water sides. There can be many different causes, such as failure of the safety val ...
s 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
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
caused brittle
crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
solids to crack and flake off metal boiler surfaces. Other solids were removed by
acid washing or
mechanical scouring. 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
Trial and error is a fundamental method of problem-solving characterized by repeated, varied attempts which are continued until success, or until the practicer stops trying.
According to W.H. Thorpe, the term was devised by C. Lloyd Morgan (18 ...
until chemists devised means of measuring and adjusting
water quality
Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
to minimize cleaning requirements.
Boiler water treatment
Boiler water treatment is a type of
industrial water treatment
There are many uses of water in industry and, in most cases, the used water also needs treatment to render it fit for re-use or disposal. Raw water entering an industrial plant often needs treatment to meet tight quality specifications to be of use ...
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
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 ...
, or
foaming
Foams are materials formed by trapping pockets of gas in a liquid or solid.
A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the r ...
. 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.
Within the boiler
At the elevated
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
s and
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
s within a boiler,
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
exhibits different physical and chemical properties than those observed at
room temperature and
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
. 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
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
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.
Oxygen scavengers
Sodium sulphite
Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...
or
hydrazine may be used to maintain reducing conditions within the boiler. Sulphite is less desirable in boilers operating at pressures above ; because sulfates formed by combination with oxygen may form sulfate scale or decompose into corrosive
sulfur dioxide or hydrogen sulfide at elevated temperatures.
[Perry p.9-51] Excess hydrazine may evaporate with steam to provide corrosion protection by neutralizing carbon dioxide in the steam condensate system; but it may also decompose into ammonia which will attack
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
alloys. Products based on filming
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
s such as
Helamin Chemical fouling inhibitors are products that are mixtures of fouling and corrosion inhibitors use in boiler feedwater treatment. Several of these products use aliphatic polyamines to coat the surface of pipes.
Helamin
Helamin is a boiler feedwater ...
may be preferred for corrosion protection of condensate systems with copper alloys.
Coagulation
Boilers operating at pressures less than may use unsoftened feedwater with the addition of
sodium carbonate or
sodium hydroxide to maintain alkaline conditions to precipitate
calcium carbonate,
magnesium hydroxide
Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (). Magnesium hydroxide is a common component of antacids, such as milk ...
and
magnesium silicate
Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...
.
Hard water
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
treated this way causes a fairly high concentration of suspended solid particles within the boiler to serve as precipitation nuclei preventing later deposition of
calcium sulfate
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...
scale. Natural organic materials like
starches,
tannins
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'', f ...
and
lignins
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
may be added to control crystal growth and disperse precipitates. The soft sludge of precipitates and organic materials accumulates in quiescent portions of the boiler to be removed during bottom blowdown.
Phosphates
Boiler sludge concentrations created by coagulation treatment may be avoided by
sodium phosphate
Sodium phosphate is a generic term for a variety of salts of sodium (Na+) and phosphate (PO43−). Phosphate also forms families or condensed anions including di-, tri-, tetra-, and polyphosphates. Most of these salts are known in both anhyd ...
treatment when water hardness is less than 60 mg/L. With adequate
alkalinity
Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale.
Alkalinity is the strength ...
, addition of sodium phosphate produces an insoluble precipitate of
hydroxyapatite with magnesium hydroxide and magnesium and
calcium silicates. Lignin may be processed for high temperature stability to control
calcium phosphate
The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are whi ...
scale and magnetic
iron oxide deposits.
[Kemmer pp.39-13&39-16] Acceptable phosphate concentrations decrease from 140 mg/L in low pressure boilers to less than 40 mg/L at pressures above . Recommended alkalinity similarly decreases from 700 mg/L to 200 mg/L over the same pressure range. Foaming problems are more common with high alkalinity.
Coordinated control of pH and phosphates attempts to limit caustic corrosion occurring from concentrations of
hydroxyl ion
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
s under porous scale on steam generating surfaces within the boiler. High pressure boilers using demineralized water are most vulnerable to caustic corrosion. Hydrolysis of
trisodium phosphate
Trisodium phosphate (TSP) is the inorganic compound with the chemical formula . It is a white, granular or crystalline solid, highly soluble in water, producing an alkaline solution. TSP is used as a cleaning agent, builder, lubricant, food a ...
is a pH buffer in equilibrium with
disodium phosphate and sodium hydroxide.
Chelants
Chelant
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
s like
ethylenediaminetetraacetic acid
Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes ev ...
(EDTA) or
nitrilotriacetic acid
Nitrilotriacetic acid (NTA) is the aminopolycarboxylic acid with the formula N(CH2CO2H)3. It is a colourless solid that is used as a chelating agent, which forms coordination compounds with metal ions (chelates) such as Ca2+, Co2+, Cu2+, and Fe ...
(NTA) form complex ions with
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
and
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
. Solubility of these complex ions may reduce blowdown requirements if anionic
carboxylate
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s are added to control scale formation. Potential decomposition at high temperatures limits chelant use to boilers operating at pressures less than .
Decomposition products may cause metal corrosion in areas of stress and high temperature.
Feedwater
Many large boilers including those used in
thermal power station
A thermal power station is a type of power station in which heat energy is converted to electrical energy. In a steam-generating cycle heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a stea ...
s recycle
condensed steam for re-use within the boiler. Steam condensate is
distilled water
Distilled water is water that has been boiled into vapor and condensed back into liquid in a separate container. Impurities in the original water that do not boil below or near the boiling point of water remain in the original container. Thus, di ...
, but it may contain dissolved gases. A
deaerator
A deaerator is a device that removes oxygen and other dissolved gases from liquids and pumpable compounds.
History
Before 1881, feed water heaters were used for marine applications. Two sister ships Olympic and Titanic (1912) had contact feed h ...
is often used to convert condensate to feedwater by removing potentially damaging gases including oxygen,
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
,
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 wa ...
and
hydrogen sulfide.
Inclusion of a polisher (an
Ion exchange
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
vessel) helps to maintain water purity, and in particular protect the boiler from a condenser tube leak.
Make-up water
All boilers lose some water in steam leaks; and some is intentionally wasted as boiler blowdown to remove impurities accumulating within the boiler.
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. Hard water intended for low-pressure boilers may be
softened by substituting
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
for
divalent
In chemistry, the valence (US spelling) or valency (British spelling) of an chemical element, element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, ...
cations of dissolved calcium and magnesium most likely to cause
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
and
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
scale. High-pressure boilers typically require water demineralized by
reverse osmosis,
distillation
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...
or
ion-exchange.
[Woodruff, Lammers & Lammers p.527]
See also
*
Dealkalization of water
Sources
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References
{{Boilers
Boilers
Water