Combined-cycle plant operators have always recorded how long and often their gas and steam turbines have run and used that data to schedule overhauls and maintenance. Whether the metric used is equivalent operating hours (EOH) or equivalent starts/hour, tracking turbine use to monitor the cumulative effects of wear and corrosion has become the established method for scheduling major maintenance. It only makes sense to extend this strategy to the combined-cycle plant's third key system—the heat- recovery steam generator (HRSG).
Scheduling upkeep according to key operating parameters such as turbine operating hours or starts, HRSG drum pH, or total fuel flow since the last overhaul enhances both maintenance and system reliability. A plant that is started up hundreds of times annually will have different preventive maintenance (PM) intervals for its pumps, valves, and turbine and HRSG components than a plant that runs continuously. If PM service intervals don't reflect this reality, an equipment failure will always be a surprise—a very unpleasant surprise if it brings down a plant and prevents it from making a big profit on a peak demand day when spot market prices skyrocket. The PM approach may be operator-friendly, but addressing plant reliability by waiting for a failure to occur and then repairing its cause is not an economically viable strategy.
Who should apply?
Tracking the EOH of systems to correlate cumulative wear and corrosion with accumulated run time began in the aerospace industry. Most turbine and HRSG manufacturers now use the process to predict how long their systems will perform reliably with timely maintenance, and how quickly they will fail without it. A combined-cycle plant that would benefit from implementing an EOH tracking program is one that:
- Is routinely cycled or experiences large load changes.
- Uses lots of supplemental firing to handle large steam load swings.
- Switches the fuel of its gas turbines or duct burners.
- Duct-fires its HRSG(s) with low-quality fuels.
- Is often laid up for extended periods of time.
Tracking system EOH allows PM activities to remain consistent with plant operation. PM intervals are shortened when the plant is run harder, and extended when duty is less demanding. Even developing an EOH program is beneficial: The process identifies areas where maintenance can be improved, and those areas can then be emphasized by other management planning programs. However, the results of developing an EOH program are most important because they specify a consistent set of data collection processes suitable for plantwide use.
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