Sample and analysis panels are on the front lines of the constant battle to detect power plant water chemistry problems. However, at many plants, operators have low confidence in the reliability of on-line analyzers, so they “default” to wet tests alone to monitor and control steam cycle chemistry. That's risky. Relying exclusively on wet tests significantly reduces the number and type of water chemistry problems that can be detected and solved. That, in turn, puts plant equipment in danger. Just because you can't see damage to steam generator tubes doesn't mean there isn't a problem.
The best intentions
Many new plants start off on the right foot by establishing maintenance schedules for sample panels and training operators in their use and maintenance procedures. But often, these schedules are abandoned early in commercial operation because keeping air-quality analyzers and instrumentation and control (I&C) systems on-line is given a higher priority, or perhaps because operators' experience with early-model analyzers was poor. Facing problems elsewhere in the plant, I&C techs sometimes put off maintaining sample panels that they are unfamiliar with and whose problems they haven't been trained to fix. (See this month's Focus on O&M story, “Solving common analyzer problems,”) Regardless of why these instruments don't get the respect they deserve, it may be time for a short refresher course in why on-line analyzers are critical to reliable plant operations and why good maintenance practices might just restore your trust in them.
Ignoring your analyzers usually reduces the efficiency of steam cycles and limits the effectiveness of the panels themselves. The negative effects are most pronounced at cycling plants, where it's hard to keep on-line analyzers operating accurately and reliably. Starts and stops of sample flow allow the probes to dry out and rewet, increasing their wear and shortening their useful life. In addition, iron and other contaminants liberated during cycling can clog or foul sample lines, pressure-reducing valves, rotometers, and probes. Unfortunately, most sample panel maintenance tasks can be performed only when the plant is operating and producing sample flows. And it's hard to develop trust in an instrument that is not properly maintained.
For these reasons, water-quality analyzers are often abandoned or ignored while sample panels are relegated to obtaining only grab samples. The readings they produce may still be entered into the plant's distributed control system (DCS), but if alarms aren't active, the readings are ignored.
One way to illustrate the importance of sample panels to water chemistry analysis is to review two case studies where perceived monitoring problems resulted in a forced outage or steam cycle damage. In both cases, the downtime and damage could have been eliminated or minimized if the sample panel had been functional or the readings trusted.
Case study # 1: “Those new analyzers won't work.”
The following series of incidents occurred over several months at a new 3 x 1 combined-cycle cogeneration plant in a southern state. After its initial start-up, the plant routinely performed wet chemistry tests but did not maintain its sample panels. Several water chemistry upsets occurred during the first several months of operation, including both low- and high-pH events. Data from the plant's new on-line analyzers (which operators didn't trust) clearly showed all of both kinds of events, but many of them were not detected by the wet tests. None of the sample panel analyzers was alarmed to the plant's DCS.
The root cause of the events, which were relatively short in duration, was diagnosed as instrument condensate contamination. Operators got into the lazy habit of feeding caustic when drum pH was low and securing chemical feed and blowing down when the pH was high. They had several opportunities to use the new on-line analyzers to detect problems, but because the instruments were poorly maintained and untrusted, their readings were disregarded. The lack of functional alarms contributed to the lack of situational awareness—only two of the six on-line pH analyzers were in service.
The six pH analyzers monitored the high-pressure (HP) and intermediate-pressure (IP) drums of the plant's three heat-recovery steam generators (HRSGs). The two operating analyzers clearly showed severe pH depressions or elevations as host condensate quality varied. Because the plant was relying only on wet test data, operators neither saw these excursions nor appreciated their significance.
As you'd expect, the tubes of this plant's HRSGs began to overheat and fail after only six months of operation. But the tube failures were only the first signs of a serious water chemistry problem. Another was the formation of iron deposits in the lower bends of the tubes (Figure 1), detected by an initial inspection. This particular plant is especially vulnerable to iron deposition because it uses high levels of duct firing on its HRSGs, which increases the heat flux across their tubes. Subsequent inspections also showed significant material loss throughout the tubes (Figure 2), not just at bends.
1. Ironed out. Iron deposition in HRSG tubes reduces the tubes' life expectancy as well as unit generating capacity. Courtesy: Nalco
2. Going deep. Abnormal loss of material from an HRSG tube, detected by a boroscope inspection. Courtesy: Nalco
Because four on-line analyzers were out of service and the other two were not trusted by the operators, the plant addressed the problem's symptoms, rather than its root cause. It did so by chemically cleaning the HRSG tube sections that had remained unaffected and replacing those with holes or heavy deposits.
Those repairs cost millions of dollars, and during the outages needed to make repairs, the plant lost millions more in production revenue. Equipment life was considerably shortened because the analyzers didn't perform due to slack O&M training and procedures plus poor alarm management—not because “those new analyzers didn't work.”
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