Table 12-3. - Chemicals Used by NAVFAC for Internal Boiler Water Treatment in Shore-Based Boilers - Continued
solid crystals (scales) that come out in increasing are somewhat softer. A scale consisting chiefly of amounts as the water gets closer to becoming steam.
The scale-forming salts stay dissolved in the water and in the cooler parts of the boiler, but when the water reaches the hot tubes, these salts start forming solid particles that come out of the water and stick to the hot metal parts as scale deposits. These deposits are highly objectionable because they are poor conductors of heat, actually reduce efficiency, and are frequently responsible for tube failures. Some of the principal scale- forming salts to be considered in most cases are listed as follows:
Scale is made up of three main parts: calcium sulfate, calcium carbonate, and silicates of calcium and magnesium. Scales that are principally calcium sulfate or chiefly of the aforementioned silicates are very hard; those scales that are principally calcium carbonate with little silicate calcium carbonate may appear only as a thin, porous, soft scale that does not build up in thickness.
Scale can be prevented by the intelligent use of proper water treatment, and that is one of the objectives of the boiler water test and treatment program.
Scale-forming substances cannot always be prevented from entering the boiler, but they can be made to form a fluid sludge. The problem then is simply one of proper chemical treatment and blowdown.
The selection of chemicals for internal treatment is determined by many factors: the kind of feedwater hardness (whether carbonate or sulfate); the ability of feedwater to build up required causticity; the type of external treatment, if used; the pH and percentage of condensate returns; the location of chemical feed injection; and the cost and availability of chemicals.
The first two chemicals to be considered for boiler water treatment of shore-based boilers are caustic soda and sodium phosphate (table 12-3).Continue Reading