Long Term Overheating in Steam Boiler

Long term overheating can occurs in all of pressure part in steam boiler such as water wall tubes, header, superheater, reheater, economizer, steam drum, water drum, roof tubes, screen tubes, generating bank, etc. Long term overheating may affect failure in pressure part. Almost 90% failure can occurs in superheater, wall tubes near with the combustion process, reheater and rarely occurs in economizer and floor tubes.

Long term overheating can caused by deposits attached on inner surface of tubes, thus insulate the tubes from cooling by water resulting in reduced heat transfer and make excessive heat input, so affect subsequent failure. Definition of long term overheating is a condition in which pressure parts have metal temperature higher than temperature design in long term or even exceed many months without doing routine maintenance. The higher temperature will lower the allowable stress in material (table allowable stress material based on temperature can be checked in ASME BPV Section II Part D).

Material which has more alloying element such as molybdenum and chromium will increase maximum allowable stress. Alloy material is often used as superheater and reheater. To protect the tubes from overheating, wall tubes and floor tubes usually use refractory tiles and refractory castable and make sure feed water before supplied into steam boiler is treated well, free from scales and deposits.