Superheater designs are usually referred to as convection, radiant or combination types. A convection superheater is placed in the path of hot gas. Radiant-type units are located in or near the furnace and receive their heat by radiation.
Many superheater designs in modern boilers are of the pendant, nondrainable type. In such systems, whenever the boiler is shut down, the lower bends of each pass retain the condensate produced from the residual steam in the superheater.
On startup, this condensate must be carefully re-evaporated and the steam released through a vent before the turbines are driven. During layup periods or boiler cleanings, these nondrainable superheaters must be filled with high purity water containing a volatile neutralizer and oxygen scavenger to prevent corrosion.
Alternatively, the superheater can be purged with an inert gas such as nitrogen. Under no circumstances should a nondrainable superheater be filled with water containing nonvolatile dissolved solids; these solids will deposit on the superheater tube surfaces and will be almost impossible to remove by flushing or chemical cleaning.
Where reheaters are employed, the same precautions apply as previously discussed.
Depending on the final steam system temperature, various steel alloys are used that are more resistant to oxidation by the steam at the higher tube metal temperature.
Current practice is to use carbon steel tubing up to 800°F superheated steam temperature; chrome-molybdenum steel up to 950°F and stainless steel, type 321; or Croloy up to 1050°F. The alloy selection is also governed by fireside deposit corrosivity.
Whether carbon steel or alloy steel is used, it is critically important to maintain proper steam flow in each superheater tube to avoid overheating. Operation at steam temperatures above design for the tube metal employed, even though not sufficiently high to result in tube failure, can result in excessive iron oxide formation inside the tube.
This oxide tends to spall from changes in temperature during startup and shutdown and may cause severe abrasion of a steam turbine's nozzle block and first stage blades.
It is equally important to keep tube surfaces clean internally and externally, and to minimize carryover. Otherwise, internal deposits of boiler water solids become a distinct danger resulting in overheating and possible failures.