Q. Use of High temperature material in Superheaters?
Typically superheaters and reheaters provide service lives of approximately 15 years (i.e. half the expected service life of the boiler itself) due to increased metal temperatures in the finishing legs of the tubes. Since the metal temperatures are the greatest in this vicinity, it may be that the fire-side oxidation and corrosion potentials are also the highest.
Typical materials of construction for the superheater and reheater sections of the boiler include carbon steel for the inlet where steam temperatures may be £ 800°F (425°C), and Cr-Mo steels for steam temperatures of up to 950°F (510°C) and where metal temperatures may be 1075°F (580°C). Austenitic stainless steels may be required at the outlets where steam temperatures can be over 1000°F (540°C) and metal temperatures may be as high as 1200°F (650°C).
Several suggestions for increasing the useful service life of superheaters and reheaters are as follows:
- Drainable design for tubes would allow ease of chemical cleaning which may reduce the effects of internal scaling on tube-metal temperatures.
- The use of higher alloys in an effort to lower the oxidation limit of the metal, (e.g. substitution of SA 213-T11 for SA 210-A1, T-22 for T-11, and 304 or 321 stainless steel for T-22 if thermal cycling is expected).
- Allowance of a larger pressure drop through the tubes may improve the balance of steam flow from tube to tube. Increasing the pressure drop would reduce the imbalance in the steam flow and the tube wall temperature gradient.
- During startup procedures, the reheater can see no steam flow and is therefore the most vulnerable to high temperature damage. Bypass steam from the drum can be used to cool the reheater during startup.
- Turbine blades may be eroded by scale particles from the superheater and reheater. Bypass of the steam away from the turbine during startup and periodic cleaning of the superheater and reheater, could minimize erosion damage.