Scale Control —
Scale formation involves the deposition of insoluble salts on heat transfer surfaces. The most common method of controlling scale is to precipitate potential scaleforming ions as non-adhering solids, or sludges, or as looselyadhering scales. Calcium ions are preferably precipitated as calcium hydroxyapatite [3Ca3(PO4)2 · Ca(OH)2]; magnesium is preferably precipitated as serpentine [2MgSiO3 · Mg(OH)2 · H2O]. These sludge are more flocculent or fluid when precipitated
at a pH above about 9.5. Caustic soda, soda ash, or a blend of phosphates can be fed to provide this alkalinity if there is inadequate natural alkalinity in the feedwater. Proper control of phosphate and silicate residuals avoids the formation of magnesium phosphate (a sticky precipitate) and calcium silicate (normally a dense, hard scale).
In higher pressure boilers and some lower pressure boilers with high purity feedwater, coordinated phosphate-pH control is practiced. This control method provides both the phosphate residual and the pH desired in the boiler by feeding a combination of disodium and trisodium phosphates. Its purpose is to avoid the presence of free hydroxide, thus eliminating the potential for caustic attack of boiler surfaces. Chelating agents provide an alternative approach to scale control that may be attractive for some low pressure boiler systems. These chemicals form soluble complexes with ions such as calcium and magnesium. Some chelating agents will
also solubilize iron and copper ions. Chelating agents should be supplemented with an antifoam agent and an oxygen scavenger. The boiler feedwater must be low in hardness (1-2 ppmw or less) for chelating agents to demonstrate an economic advantage over the precipitation scale control methods. Chelating agents have not been successfully utilized in high
pressure boilers.
at a pH above about 9.5. Caustic soda, soda ash, or a blend of phosphates can be fed to provide this alkalinity if there is inadequate natural alkalinity in the feedwater. Proper control of phosphate and silicate residuals avoids the formation of magnesium phosphate (a sticky precipitate) and calcium silicate (normally a dense, hard scale).
In higher pressure boilers and some lower pressure boilers with high purity feedwater, coordinated phosphate-pH control is practiced. This control method provides both the phosphate residual and the pH desired in the boiler by feeding a combination of disodium and trisodium phosphates. Its purpose is to avoid the presence of free hydroxide, thus eliminating the potential for caustic attack of boiler surfaces. Chelating agents provide an alternative approach to scale control that may be attractive for some low pressure boiler systems. These chemicals form soluble complexes with ions such as calcium and magnesium. Some chelating agents will
also solubilize iron and copper ions. Chelating agents should be supplemented with an antifoam agent and an oxygen scavenger. The boiler feedwater must be low in hardness (1-2 ppmw or less) for chelating agents to demonstrate an economic advantage over the precipitation scale control methods. Chelating agents have not been successfully utilized in high
pressure boilers.
Sludge Conditioning — Various organic materials are often used to condition the boiler precipitates or sludges to make them fluid or free-flowing for easier removal by blowdown. These are usually derivatives of tannin or lignin, synthetic materials, or, in some cases, derivatives of seaweed. Starch is sometimes used in high silica waters. Sludge conditioners are frequently combined with phosphates and chelating agents. Antifoam materials, for smoother boiler operation, are sometimes also incorporated in these formulations. Foam Control — Foaming can cause entrainment of boiler water with the steam although this carryover may also be the result of poor boiler design, ineffective steam-separating equipment, or high water levels. Foaming can be caused by high levels of dissolved solids, suspended solids, alkalinity, or by the introduction of foaming-promoting materials into the boiler, for example, by the use of oil-contaminated steam condensate. Although effective antifoam agents are available to suppress foam formation, it is usually more economical to reduce or eliminate the problem by adjusting boiler water treatment (external and/or internal), increasing boiler blowdown, eliminating foam-promoting contaminants from recycled steam condensate, etc.
Corrosion Mitigation — Corrosion in boiler and steam/steam condensate systems is usually due to the effect of either low pH or the presence of oxygen. Low pH (below neutral pH = 7) is usually caused by dissolved carbon dioxide and the resulting corrosion is normally of a general nature over the entire metal surface. This acidic corrosion can be mitigated by raising the pH. Soda ash (Na2CO3) and caustic soda are often used for this purpose in boiler feedwater systems. In boilers, the proper water treatment for sludge and scale control will normally result in a satisfactorily high pH of 10-11. Pretreatment of the boiler feedwater to reduce carbonate alkalinity will result in an equivalent reduction in the potential carbon dioxide content of the steam and the carbonic acid content of the steam condensate. Filming amines, which form a thin protective layer on metal surfaces, and neutralizing amines, which react with carbon dioxide and raise the pH of the steam condensate to a sufficiently high level (8.5 to 9.5), are frequently used to mitigate corrosion in steam condensate systems. Filming amines, which tend to decompose at higher temperatures, are often fed to the steam headers at a rate sufficient to form and maintain the desired corrosion-resistant film. Filming amines will also protect the steam condensate system from corrosion due to oxygen. Neutralizing amines, which are quite stable, are usually added to the boiler feedwater at a rate proportional to the carbon dioxide content of the steam. Neutralizing amines can be used in high temperature, high pressure steam systems, but these amines will not protect against oxygen attack and are usually not economical in steam systems containing high concentrations of carbon dioxide. Ammonia is sometimes substituted for neutralizing amines; however, it should not be used in systems containing copper or most copper alloys. Oxygen may be present in the makeup water or may result from air leaks into the steam/steam condensate systems; the resulting corrosion is generally in the form of pitting. Oxygen-related corrosion can be mitigated by deaeration of boiler feedwater, the use of chemical oxygen scavengers, and the addition to the steam/steam condensate systems of a filming amine.
Caustic Embrittlement — Caustic embrittlement is intercrystalline cracking of boiler steel which may occur in the presence of all of the following factors:
· The metal must be subjected to a high level of stress. · There must be some mechanism (a crevice, seam, leak, etc.) permitting concentration of the boiler water on the stressed metal.
· The concentrated boiler water must possess embrittling characteristics and chemically attack the boiler metal. Of these three factors, the embrittling characteristics of the boiler water generally can best be shown to be present or absent in a boiler. An Embrittlement Detector developed by the U.S. Bureau of Mines can be used to determine this water characteristic. As an alternative, since there are no simple chemical tests to measure embrittlement and there is always the possibility of embrittlement occurring, a chemical embrittlement inhibitor, generally sodium nitrate, is often added to the boiler. A definite ratio of sodium nitrate to caustic alkalinity in the boiler water is required for inhibition according to the formula,
· The concentrated boiler water must possess embrittling characteristics and chemically attack the boiler metal. Of these three factors, the embrittling characteristics of the boiler water generally can best be shown to be present or absent in a boiler. An Embrittlement Detector developed by the U.S. Bureau of Mines can be used to determine this water characteristic. As an alternative, since there are no simple chemical tests to measure embrittlement and there is always the possibility of embrittlement occurring, a chemical embrittlement inhibitor, generally sodium nitrate, is often added to the boiler. A definite ratio of sodium nitrate to caustic alkalinity in the boiler water is required for inhibition according to the formula,
