The operating staff must prepare proper records of all maintenance to ensure that any problems with the boiler are known. Examination methods such as nondestructive testing are then used to evaluate the remaining life of the boiler component in question.
The condition of the steam drum is a critical component to evaluate. There are two types of steam drums:
1. The all-welded design used predominantly in utility boilers, where the operating pressure exceeds 1800 psi.
2. Drums with rolled tubes, such as the two-drum industrial-type boiler
Since the steam drum operates at saturation temperature, the steam/water temperature is less than 700°F (see Table C.1), and therefore, the drum is made of carbon steel and is not subject to significant creep.
Creep is the slow deformation of continuously stressed metal over time, which could lead eventually to a fracture.
Damage to a drum is primarily due to internal metal loss. This can be caused by corrosion and oxidation, which can occur during extended outages if proper precautions are not taken. Damage also can occur from mechanical and thermal stresses on the drum, which concentrate at nozzle and attachment welds. These stresses occur often in boilers that are cycled frequently in an on/off mode of
operation.
The feedwater penetration area has the greatest thermal differential because incoming feedwater can be several hundred degrees below the drum temperature. Steam drums with rolled tubes (e.g., a two-drum boiler) have problems with tube seat leakage, where there is a slight seeping of water through the rolled joint.
Caustic embrittlement can result if the leak is not stopped. In lower drums of industrial-type boilers, large thermal differential or mechanical stresses are not present; however, rolled-tube seat leakage can occur, with similar problems resulting as with the steam drum.
Superheater and reheater tubes are affected by both erosion and corrosion. In addition, the high temperature results in increased stress on the tubes. These factors lead to tube cracks and eventual leaks.
Water-cooled tubes such as the furnace walls, boiler bank, and economizer operate at or below saturation temperature and therefore are not subject to significant creep. Proper water chemistry is important in maintaining tube life, and if necessary, water-side deposits can be cleaned by chemicals when required.
Erosion and corrosion are the primary problems of the tubes, assuming that good water chemistry is practiced. If not, deposits form on the inside of the tubes, and heat transfer is affected, which can result in high metal temperatures and eventual failure of the tube.
These and other components and auxiliaries of the boiler must be examined and maintained properly during planned outages. This is proper operation, which will lead to high plant availability.
