Some facts you should know about Phosphate Treatment for Boilers
- One whole class of boilers (the nuclear PWR) experienced such massive failures that many had to be replaced; others had sleeves installed on thousands of boiler tubes to replace lost wall thickness. Today, less than half a dozen PWRs still operate with phosphate chemistry.
- Wide-spread failures of fossil-fired boilers initiated an extensive R&D program to find ways to curb the problem. In December of 1994, EPRI released their Cycle chemistry guidelines for fossil plants: Phosphate treatment of drum units, EPRI TR-103665.
- The charts used to control the operation of a coordinated or congruent phosphate boiler treatment were developed under laboratory conditions using glass equipment. No industrial boiler is constructed from glass. The interactions between phosphate and iron are an important term that must not be overlooked.
- Almost every book or report on boiler chemistry shows the graph of tri-sodium phosphate solubility vs. temperature. This curve has a complex shape due to the presence of the various crystalline materials that form under different temperature conditions. Each of these materials behaves differently within a boiler.
- Blending caustic soda, tri- and di-sodium phosphate to get the correct pH and sodium-to-phosphate ratio is a common practice. There is growing evidence that those plants where such blends are used may be more likely to experience tube failures.
- Most specifications, for pH, sodium and phosphate, originate from outside sources and many boilers, if left to seek their own level, are unable to operate within them. Continuous addition of phosphate to keep in spec may actually be forcing a boiler to function in a region that may actually be dangerous to it's operation and result in increased hideout (more frequent cleaning) or acidic phosphate corrosion (sudden failures with costly downtime).
- Many failures have appeared after years of successful operation. In many cases, it has been
difficult to properly identify the underlying cause, e.g., some failures previously attributed to caustic attack have now been shown to have resulted from acidic-phosphate attack.
What should you be doing? First ask your boiler what limits it can tolerate. If you find yourself trying to operate to specifications beyond these limits and/or you must continuously add disodium phosphate to keep within the chosen block on the control chart, you may be in trouble. Get away from that traditional congruent phosphate program quickly and switch to an Equilibrium Phosphate program
that can be made fully compatible with your system. Stop trying to follow specifications derived from other plants or organizations. They may be industry standards, but they are also capable of damaging your plant. If you didn't know how to ask your boiler the right questions and listen to its answers, you also have a monitoring and control problem to address.
PO4 vs. pH Diagram. Click for full image.
The Phosphate vs. pH Diagram shows how the control ranges for Congruent Phosphate and Equilibrium Phosphate Treatments differ. The initial Coordinated Phosphate Treatment used the Na/Phosphate ratio of 3.0 as the control range. This corresponds to the ratio in pure trisodium phosphate and control was based upon keeping below this line. Congruent Phosphate Treatment or CPT went further and recommended keeping the ratio below 2.6 and at the same time keeping the pH high enough to maintain corrosion protection. The new concept of EPT adds only TSP and lets the system seek its own equilibrium. As the diagram shows, the CPT and EPT control ranges are incompatible. EPT was initially developed at Ontario Hydro by Jan Stodola.
Click here to read our paper at the 1996 Internet Corrosion Conference
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This page last updated 2003-June-09