The operators of a new combined-cycle plant being run in two-shift mode (on 16 hours, off 8) had become used to the cation conductivity sample of the condensate pump discharge being in alarm for a while following each day’s start-up. Normally, after a few hours of operation, the reading dropped to an acceptable limit. So, when cation conductivity didn’t drop as usual and the alarm never cleared, a problem wasn’t caught in time. Scratching their heads over the high reading, the operators guessed, “Maybe the cation column needs to be replaced.”
Weeks later, what had been a small condenser tube leak suddenly became big enough to contaminate the plant’s heat-recovery steam generator (HRSG) and steam turbine. Although the large leak was quickly detected and the plant shut down, it took nearly three months to clean up and recondition the turbine. Despite the repairs, the long-term damage done to the expensive new turbine is still really anybody’s guess. Had the operators not ignored the high cation conductivity reading at the condensate pump discharge, they would have known they had a problem. Locating and plugging the leaking condenser tube would have been easy and would have avoided the repair costs and millions of dollars in lost generation revenues.
Unfortunately, this kind of story has become commonplace. A change in cation conductivity has been the first indication of many problems, but it has been ignored until major contamination occurs. Nevertheless, some operators have said that it is unnecessary (and, in some cases, impossible) to strictly follow ASME, EPRI, or a turbine manufacturer’s guidelines for cation conductivity. Their comments might suggest to some that the parameter is no longer important.
To be sure, some conventional fossil fuel–fired boilers and combined-cycle plants have operated for many years with the cation conductivity of feedwater or steam at levels well above what is considered normal. Indeed, one combined-cycle plant has been run for more than 15 years with cation conductivity more than an order of magnitude higher than the recommended limit of 0.25 µS/cm (microSiemens per centimeter) without causing corrosion or cracks in its turbine. Here’s why this plant has avoided problems. Because its water supply has significant concentrations of naturally occurring organic compounds, it adds both organic amines and a carbon-containing oxygen scavenger (a reducing agent) when preparing makeup.
Another case of unexpected immunity occurred at an older plant that recently purchased its first cation conductivity analyzer. Following installation, the unit immediately indicated a high level in feedwater, and the cation resin columns were exhausting in a few days. This plant’s condensers have stainless steel tubing that has never leaked. I have regularly inspected the boilers, deaerators, and other equipment at this plant for several years. The steam drum and tubing are in excellent condition. Every time the turbine is inspected, there are no signs of cracking or pitting. As at the other plant, the additions of amine and the CO2 picked up by the condensate drive feedwater and steam cation conductivity high.
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I believe if this instrument, and the ORION chloride analyzer can be refined to meet the demands of once-through cycle chemistry, 1 million of each could be sold over night. It is important that the industry stay on top of these developments, as these can revolutionize the way cycle chemistry is viewed in the electrical power industry.
Thank you again for an excellent article.