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.
All boilers lose some water in steam leaks; and some is intentionally wasted as boiler blow down 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 fl oating and suspended materials. Hard water intended for low-pressure boilers may be softened by substituting sodium for divalentcations of dissolved calcium and magnesium most likely to cause carbonate and sulfate scale. High-pressure boilers typically require water de-mineralized by distillation or ion-exchange.
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. 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.
Sodium sulfi te or hydrazine may be used to maintain reducing conditions within the boiler.[Sulfi te is less desirable in boiler operating at pressures above 1,000 pounds per square inch (6,900 kPa);[because sulfates formed by combination with oxygen may form sulfate scale or decompose into corrosive sulfur dioxide or hydrogen sulfi de at elevated temperatures. 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 alloys. Filming amines may be preferred for corrosion protection of condensate systems with copper alloys.
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.
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). Decomposition products may cause metal corrosion in areas of stress and high temperature.
Boilers operating at pressures less than 200 pounds per square inch (1,400 kPa) may use softened 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. The soft sludge of precipitates and organic materials accumulates in quiescent portions of the boiler to be removed during bottom blow down.