Intertek specialists have finally identified the damage mechanism long suspected in Grade-91 tubing (T-91). They presented their findings at the February 2018 HRSG (heat recovery steam generator) User’s Group Conference.
If reheater tubes reach a high enough temperature (1,150F), spinel oxide scale begins to build up on the interior surfaces. When the scale reaches sufficient thickness, layers of it break off, a process dubbed “exfoliation.” The spinel then travels downstream and forms deposits on steam turbine stop valves or other components that experience high steam velocities. The process repeats over and over, and is accelerated by flow abnormalities. Intertek recommends employing a continuous monitoring system just upstream of duct burners, to ensure they are not raising the temperature of the tubing.