Page 3 of 5
HRSG superheaters can become supercoolers during shutdown or hot/warm restarts. The condensate flow out of a hot superheater is substantial when the gas turbine rolls back in load, resulting in the exhaust temperature cooling the finned tubes. However, lowering the pressure/temperature of the superheated steam, following the ramp down of gas turbine exhaust temperature, will minimize condensate formation in the lower headers upon shutdown.
Sizing all superheater drains generously and locating the drains strategically.
These should be headered together under the HRSG and along the side of the HRSG to the blowdown or flash tank. Make sure all bottom headers can be drained and all lines slope in the proper direction in the hot condition.
Making sure all vents and drains, required for startup and shutdown, have automatic DCS control (with manual override).
Protecting hot economizers from cold water.
For warm restarts, condensate supply must match economizer temperature within 25°F to 50°F.
Minimizing Thermal Stress
Chrusciel emphasizes it is absolutely essential that operators understand, and positively control and guide, thermal expansion within the boiler. He cites the following measures that HRSG suppliers recommend operators implement to minimize thermal stress:
Use lower cycle pressures and higher-grade materials.
This will permit minimum thickness, high-pressure drum and headers. As an alternative, a once-through HRSG design would eliminate the high-pressure drum altogether. Superheater tube material of choice is T91.
Specify (and pay for) the vendor to perform its own HRSG analysis, based on ASME Section VIII and BS2223.
Results of such an analysis will provide justification for full penetration/full strength welds, where required. HRSG manufacturer's studies indicate fatigue life of full penetration welds is at least 10 times greater than that of partial penetration welds. In many applications, the critical area will be the riser connection at the high-pressure steam drum.
Verify with the HRSG vendor that all components, subject to thermal expansion, are adequately designed to preclude thermal binding, galling, or other negative expansion effects.
Make sure to include tube offsets, harp support systems, liner panels, and the tube wash collection system, among other components.
Reduce all component stresses.
Heat-treat alloy tube bends to reduce residual stresses; avoid construction techniques that result in stress concentrations; use integral reinforced drum nozzles, full penetration nozzles, and contour nozzle corners, as well as blended tube-to-drum and other pressure-boundary welds to reduce stress discontinuities.
Additional Mitigation Steps
Bechtel's team further recommends a number of other measures that power plant operators can take to mitigate potential HRSG problems and failures:
Keep corrosion fatigue in check.
Vent completely, the system's economizers.
Eliminate wet corrosive surfaces in the back-end of HRSG units that turn down at or near the acid or water dew point.
Coat the inside of the main stack breaching and the inside portion up to the damper.
Maintain the temperature above the acid and water dew point past the HRSG's last row of tubes.
Use a low-pressure economizer, or condensate preheater bypass system, or a recirculation system; and consider upgrading material on the last several rows of tubes (stainless steel is allowed for a condensate preheater/feedwaer heater, since it is not a Section 1 component).
Prevent steaming in the intermediate and low-pressure economizer sections, when operating the HRSG under part load, to prevent acceleration of local internal erosion.
Provide a remote means of venting the offending economizer section directly to the drum, or locate the drum level control valve downstream of the economizer section to make sure adequate pressure is always present in the economizer.