Boiler water is liquid
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
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, central h ...
, or in associated piping, pumps and other equipment, that is intended for evaporation into
steam
Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
. The term may also be applied to
raw water
Raw water is water found in the environment that has not been treated and does not have any of its minerals, ions, particles, bacteria, or parasites removed. Raw water includes rainwater, ground water, water from infiltration wells, and water from ...
intended for use in boilers, treated
boiler feedwater, 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 explosions 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 solids to crack and flake off metal boiler surfaces. Other solids were removed by
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
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 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
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number ...
,
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 engine ...
, 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 reg ...
. 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 measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
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, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
exhibits different physical and chemical properties than those observed at
room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
and
atmospheric pressure. 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 wel ...
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 or
hydrazine
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, 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
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
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 atoms have been replaced by a substituen ...
s such as
Helamin 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
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 ...
or
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 alkali ...
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.
Hard water 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 scale. Natural organic materials like
starches
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
,
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 anhydrou ...
treatment when water hardness is less than 60 mg/L. With adequate
alkalinity, addition of sodium phosphate produces an insoluble precipitate of
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
with magnesium hydroxide and magnesium and
calcium silicate
Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite.
...
s. Lignin may be processed for high temperature stability to control
calcium phosphate scale and magnetic
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
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 ions 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 is a pH buffer in equilibrium with
disodium phosphate
Disodium phosphate (DSP), or disodium hydrogen phosphate, or sodium phosphate dibasic, is the inorganic compound with the formula Na2HPO4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as forms with 2, 7, 8, ...
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 (EDTA) or
nitrilotriacetic acid (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 to ...
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 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, 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 transpar ...
,
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 was ...
and
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
.
Inclusion of a polisher (an
Ion exchange 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 locomotive
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 ...
s 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 iso ...
for
divalent cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s 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 g ...
and
sulfate scale. High-pressure boilers typically require water demineralized by
reverse osmosis
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pre ...
,
distillation
Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...
or
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, ...
.
[Woodruff, Lammers & Lammers p.527]
See also
*
Dealkalization of water
The dealkalization of water refers to the removal of alkalinity ions from water. Chloride cycle anion ion-exchange dealkalizers remove alkalinity from water.
Chloride cycle dealkalizers operate similar to sodium cycle cation water softeners. Like ...
Sources
*
*
*
*
*
*
References
{{Boilers
Boilers
Water