Analyzing the State of Phosphate Water Chemistries in High-Pressure Drum Boilers

Abstract

An analysis is performed of the phosphate water chemistry of a high-pressure drum boiler. In Russia, water chemistries with purely phosphate alkalinity and phosphate-and-alkali water chemistry are now mainly used at power plants equipped with drum boilers. One of the main quantitative parameters determining the maintenance of phosphate water chemistries is the ratio of sodium and phosphate concentrations. Calculated dependences of the ratios of pH, the concentration of phosphate, and the sodium-to-phosphate concentration are given. A relationship is found between such ratios and the domains where acid–phosphate corrosion, the hydrogen embrittlement of metal, and alkali cracking occur. It is shown that at concentrations of phosphate below 2.5 mg/dm³, the chloride and sulfate concentrations in boiler water must be monitored to avoid the hydrogen embrittlement of metal. Dependences are presented for the pH and sodium-to-phosphate concentrations at different temperatures. Results are presented from industrial tests of purely phosphate alkalinity water chemistry during the startup and normal operation of a boiler. Analysis of the chemistry of a high-pressure drum boiler water shows that the concentration of phosphate in the pure compartment of a drum has almost no effect on the pH, but the concentration of phosphate in the drum’s salt compartment affects it strongly. Attention should therefore mainly be given to the pH prescribed by the relevant standard when managing the water chemistry in the pure compartment. It is shown that phosphate hideout is often observed when starting power units equipped with high-pressure boilers, so mono- and disodium phosphate solutions are used to maintain the pH and concentrations of phosphate. An analysis of the quality of boiler water during a startup shows there was a drop in the concentration of phosphate in the boiler water and a rise in the sodium-to-phosphate concentrations, so a hideout occurred. The possibility of identifying deviations when monitoring phosphate water chemistry is thus demonstrated, based on an analysis of sodium-to-phosphate ratios of concentrations.