Going Against the Flow: Better Chemistry at Combined Cycle and Co-Generation Plants

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Going Against the Flow: Better Chemistry at Combined Cycle and Co-Generation Plants - Part 2

By District Energy posted 09-24-2018 16:58

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Brad Buecker, ChemTreat, Power Engineering

Summary

Author's Note: Part 1 of this article provided an overview of several of the most important water/steam chemistry issues that presently face steam generation operating and technical personnel. In this Part 2 of the article, we will examine three of the most critical steam generation chemistry topics, with cooling water issues to be evaluated in Part 3 next month.

Minimizing FAC

As reported by the Electric Power Research Institute (EPRI) and the International Association for the Properties of Water and Steam (IAPWS), flow-accelerated corrosion (FAC) continues to be the top corrosion concern in HRSGs around the world. In a number of instances over the last three decades, FAC-induced failures of carbon steel piping and tubing have injured or killed plant personnel. Yet, as this author can attest from over seven years of experience reviewing water/steam chemistry specifications for combined cycle power plants, most continue to call for oxygen scavenger, aka reducing agent, feed to the condensate/feedwater system of the HRSGs. Oxygen scavenger treatment is the single greatest contributor to FAC, and unless the feedwater system contains copper alloys (virtually no HRSGs do), this practice has been completely discredited. Rather, the correct treatment, as developed by EPRI, is the program known as all-volatile treatment oxidizing [AVT(O)], in which the (normally) small amount of oxygen that enters via condenser air in-leakage is allowed to remain, such that the feedwater dissolved oxygen (D.O.) content is controlled within a range of 5 to 10 parts-per-billion (ppb). This may require closing of the deaerator vents for units that have deaerators, and supplemental oxygen injection at the low-pressure (LP) economizer and boiler feed pump suction in HRSGs. Ammonia continues to be the feedwater pH-conditioning chemical of choice, where the recommended pH range may be between 9.2 and 10.0 depending upon HRSG design. A key requirement of AVT(O) is maintaining cation conductivity, aka conductivity after cation exchange (CACE), at ≤0.2 µS/cm, as otherwise oxygen corrosion would result.