Phosphate

Boiler water
From Wikipedia, the free encyclopedia
Boiler water is the liquid phase of steam within a boiler. The term may also be applied to raw
water intended for use in boilers, treated boiler feedwater, steam condensate being returned to a
boiler, or boiler blowdown being removed from a boiler.
Storage tank of water for locomotive boilers.
Contents
 1 Early practice
 2 Boiler water treatment
 3 Within the boiler
o 3.1 Oxygen Scavengers
o 3.2 Coagulation
o 3.3 Phosphates
o 3.4 Chelants
 4 Feedwater
 5 Make-up water
 6 See also
 7 Sources
 8 Notes
Early practice
Boiler feed water pump
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. [1] 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 caused brittle crystalline solids
to crack and flake off metal boiler surfaces. Other solids were removed by acid washing or
mechanical scouring.[2] 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 to minimize cleaning
requirements.
Boiler water treatment

Boiler water treatment is a type of industrial water treatment focused on removal or chemical
modification of substances potentially damaging to the boiler. Varying types of treatment are
used at different locations to avoid scale, corrosion, or foaming.[3] External treatment of raw
water supplies intended for use within a boiler is focused on 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.[4]
Within the boiler

Main article: Superheated water
At the elevated temperatures and pressures within a boiler, water exhibits different physical and
chemical properties than those observed at room temperature 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
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.[5]
Oxygen Scavengers
Sodium sulphite or hydrazine may be used to maintain reducing conditions within the boiler. [6]
Sulphite is less desirable in boilers operating at pressures above 1,000 pounds per square inch
(6,900 kPa);[7] because sulfates formed by combination with oxygen may form sulfate scale or
decompose into corrosive sulfur dioxide or hydrogen sulfide at elevated temperatures. [8] Excess
hydrazine may evaporate with steam to provide corrosion protection by neutralizing carbon
dioxide in the steam condensate system;[9] but it may also decompose into ammonia which will
attack copper alloys. Products based on filming amines such as Helamin may be preferred for
corrosion protection of condensate systems with copper alloys.[8]
Coagulation
Boilers operating at pressures less than 200 pounds per square inch (1,400 kPa)[10] may use
unsoftened feedwater with the addition of sodium carbonate or sodium hydroxide to maintain
alkaline conditions to precipitate calcium carbonate, magnesium hydroxide 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, tannins and lignins may be added to control
crystal growth and disperse precipitates. [11] The soft sludge of precipitates and organic materials
accumulates in quiescent portions of the boiler to be removed during bottom blowdown.[8]
Phosphates
Boiler sludge concentrations created by coagulation treatment may be avoided by sodium

phosphate treatment when water hardness is less than 60 mg/L. With adequate alkalinity,
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 scale and magnetic iron oxide deposits.[12]
Acceptable phosphate concentrations decrease from 140 mg/L in low pressure boilers to less
than 40 mg/L at pressures above 1,500 pounds per square inch (10,000 kPa). 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.[8]
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 and
sodium hydroxide.[13]
Chelants
Chelants like ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) form

complex ions with calcium and magnesium. Solubility of these complex ions may reduce
blowdown requirements if anionic carboxylate polymers are added to control scale formation.
Potential decomposition at high temperatures limits chelant use to boilers operating at pressures
less than 1,500 pounds per square inch (10,000 kPa).[12] Decomposition products may cause
metal corrosion in areas of stress and high temperature.[14]
Feedwater
Main article: Boiler feedwater
Many large boilers including those used in thermal power stations recycle condensed steam for
re-use within the boiler. Steam condensate is distilled water, but it may contain dissolved gases.
A deaerator is often used to convert condensate to feedwater by removing potentially damaging
gases including oxygen, carbon dioxide, ammonia and hydrogen sulfide.[15] 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. [16] 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. [17] Hard water intended for low-
pressure boilers may be softened by substituting sodium[18] for divalent cations of dissolved
calcium and magnesium most likely to cause carbonate and sulfate scale.[19] High-pressure
boilers typically require water demineralized by distillation or ion-exchange.[20]

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Boiler water

From Wikipedia, the free encyclopedia

Storage tank of water for locomotive boilers.

Boiler water treatment

Within the boiler

Boiler sludge concentrations created by coagulation treatment may be avoided by sodium

Chelants like ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) form

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