Only a few years ago, it was considered foolhardy in the United States to operate high-pressure electric utility drum boilers with free caustic in the boiler water. It was assumed that free caustic in the boiler would lead to caustic gouging of boiler tube metal. It became evident that many electric utility boilers in other countries were operating successfully on caustic treatment programs. Many U.S. electric utilities have converted their drum boilers to caustic treatment and eliminated problems such as phosphate hideout. This paper will discuss the basics of caustic treatment and present some case histories of successful operation in a caustic boiler water treatment regimen.
In looking back at the history of boiler water treatment, chemical treatment has been designed to control either deposit accumulation (e.g. “scale”) or corrosion. The original phosphate treatment programs used in boilers were scale prevention programs using high levels of phosphate. As boiler pressures increased tube damage attributed to caustic attack was experienced and phosphate levels were decreased.
Phosphate treatment has long been the staple for treating drum boilers. Before the advent of ion exchange makeup water treatment, phosphate provided both pH control and protection against the ingress of scale forming ions. Before the development of ion exchange technology there were no “high pressure” boilers (by today's standards) and “raw” untreated or partially softened waters were used for makeup. Since these waters contained calcium and magnesium, the formation of mineral scales was a serious problem. Internal “softening” treatments were used to prevent the formation of the least desirable scales such as calcium sulfate and silica scales. Caustic soda and soda ash (sodium carbonate) were commonly used. The so-called “carbonate cycle” caused calcium carbonate to precipitate preferentially over calcium sulfate (a more adherent scale). The high boiler water pH also protected boiler surfaces from corrosion. Current Phosphate Treatment Concepts Once acid phosphate corrosion was identified, the use of Congruent Phosphate Treatment (CPT) in high pressure boilers was greatly reduced. Equilibrium Phosphate Treatment (EPT) replaced CPT in many cases. However, EPT has been associated with the occurrence of hydrogen damage. EPRI has now developed the concept of a “phosphate continuum” where the phosphate concentration is a function of feedwater and steam purity. EPT as such no longer exists in the EPRI phosphate continuum. An important aspect of the EPRI recommendations is that only trisodium phosphate is used rather than mixtures containing disodium or monosodium phosphate and that polymers not be used.3 The phosphate continuum establishes a minimum phosphate concentration of 0.2 ppm PO4. The treatment range is bounded by the Na to PO4 molar ratio line of 3.0 and the TSP+1 ppm NaOH line. The phosphate continuum provides for a range of phosphate levels depending on feedwater purity. At low phosphate levels, feedwater purity requirements are the same as the All Volatile Treatment (AVT) programs. High phosphate levels are used in response to poorer feedwater quality (e.g. 0.3 µS/cm cation conductivity).
