The weakest links
With the exception of turbines, the HRSG is perhaps the combined-cycle plant system that can benefit the most from a well-designed EOH-tracking program. Although HRSG suppliers typically provide limited recommendations for scheduled maintenance, HRSG users must have a formal and detailed program of inspection and cleaning in place to avoid serious O&M problems. Such problems can be caused by daily cycling, overfiring duct burners, large steam load swings, tube leaks, water chemistry upsets, short- and long-term layups, or fouling of a selective catalytic reduction system's (SCR's) catalyst (Figures 1 and 2).
1. Extreme service. This boiler tube was ruptured by cyclic fatigue. Many HRSGs designed for baseload service experience problems like this when pressed into cycling service. Courtesy: Tetra Engineering Group Inc.
2. Deposit account. These finned superheater tubes are covered by heavy deposits as a result of burning low-Btu gas with higher-than-normal H2S content. Courtesy: Tetra Engineering Group Inc.
EOH formulas for an HRSG should be developed from a review of its design and operating history and based on a straightforward assessment of the lifetimes of major components. The assessment must include not just the HRSG's pressure parts (Figure 3) but also all of its ancillary systems: main stream valves and sprays, reheaters, duct burners and supporting components, and its casing and stack. Remember to include the post-combustion emissions catalysts and controls for the SCR and CO systems.
3. Beat the drum. This HP HRSG drum had to be removed to repair damage caused by a gas explosion inside it. Courtesy: Tetra Engineering Group Inc.
An effective EOH program should also scrutinize other major systems and components of the typical combined-cycle plant, as explained below.
Valves. Large valves (bypass, stop and check, and feedwater control valves) are high-maintenance devices that must be installed correctly and are very sensitive to plant operating conditions (see “Desuperheating valves take the heat”). EOH formulations are best developed from direct experience at the plant and from the experience of similar installations at peer plants. A systematic review of maintenance and repair records usually provides invaluable insight into the effectiveness and thoroughness of previous maintenance programs.
Condensers. Steam condensers (air-cooled condensers in particular) are susceptible to corrosion, erosion, and mechanical damage at rates that depend on the plant's operating profile and location. Fuel switches, bypass steam dump operation, and changes in steam and water chemistry all take their toll on long-term reliability. Both experience and analysis are required to set EOH parameters for condensers, and that's also the case for deaerators and feedwater heaters.
Pipes. Power plant piping (main steam, reheat, and feedwater lines) is not immune to changes in unit operation. Thermal transients can fatigue and damage pipe supports, start-ups can produce water hammer, and load changes can increase flow-accelerated corrosion. Pipe stress and flexibility reports can provide basic data about thermal fatigue that EOH calculations can turn into actionable information. Separately, operations reviews and analyses can provide a wealth of data on corrosion and other damage mechanisms.
Pumps. Boiler feedpumps, condensate pumps, and circulating water and other large pumps also are affected by cycling. Here, experience and vendor guidance are the best foundations on which to develop EOH factors.
Electrical systems. Large transformers and switchgear usually come with guidance on overhaul intervals based on time, the effect of switching on load current, or other measures. These recommendations can be incorporated directly into EOH formulas.
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