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All of these components are exposed to different temperatures and pressures at different times while the HRSG is started, operated, and shut down. How well the components react to the transients and steady-state conditions determines the integrity of the entire HRSG.
Understanding Damage Mechanisms
Operators need to understand the various mechanisms that can cause HRSG damage, as well as how to control them.
According to Vogt-NEM engineers Akber Pasha and Robert Allen, the basic mechanisms affecting the life of an HRSG include:
Low-cycle fatigue - Most dominant damage mechanism in today's HRSGs; occurs at a low number of stress cycles when the strain is high.
Creep - Material exposed to high temperature and stress for a considerable amount of time will creep.
Thermal shock - Cold water or steam impinging on hot surfaces can damage the base material.
Oxidation - Oxidation and exfoliation can occur with exposure to high temperatures.
Differential expansion - Adjacent tubes or pipes at different temperatures, expand or contract unevenly, stressing both components; piping supports also stress the components.
Corrosion fatigue - Corrosion initiating a crack, results in progressive damage occurring because of fatigue and corrosion, typically at a temperature range of 300°F to 500°F.
Corrosion in tubes - Improper water chemistry results in tube corrosion.
Flow-accelerated corrosion (FAC) - Combined high flow and poor chemistry control can cause FAC in specific HRSG locations.
Corrosion product migration - Migration of corrosion products to other HRSG components may cause further corrosion or other damage.
Deposits - Temperature or water-chemistry fluctuations may result in deposits on heat-transfer surfaces.
Erosion - Transient high velocities may initiate or perpetuate erosion.
While these damage mechanisms can affect all HRSG components, Vogt-NEM engineers _caution that some components are more vulnerable because of their location, construction, or exposure. These critical components, they point out, need to be designed and monitored more closely for potential failures, including: superheater and reheater outlets; tube-to-header joints in hot sections; drum-to-downcomer nozzles in the high-pressure drum; bent portions of heat-transfer tubes; attemperators; and bypass valves.
Combating HRSG System Failures
According to Bechtel Power Corp.'s assistant chief engineer, Andy Chrusciel, thermal cycling is the key factor contributing to HRSG system failures, producing a cumulative fatigue damage that cannot be reversed. Eliminating cyclic fatigue damage, he points out, will substantially increase the HRSG life cycle.
Chrusciel, together with Bechtel engineers Justin Zachary and Sam Keith, drew upon considerable HRSG operating data (collected and analyzed by all of the major HRSG manufacturers over the past 10 years) and identified a number of actions that HRSG operators can readily implement to combat cyclic fatigue damage. These include:
Minimizing operating regimen.
Thermal fatigue damage occurs every time an HRSG is started or stopped or cycled down to part-load. According to vendors, damage from a cold-start is 20 times to 40 times greater than from warm starts. Therefore, keep the HRSG warm overnight or over the weekend by closing the stack damper, installing insulation up to the damper, sparging steam, or running the SCR ammonia vaporizer heaters.
Lowering the pressure/temperature of superheated steam.