It is common practice in taking a boiler off the line to do no more than let the fire burn out and allow the water in the unit to assume its natural level. This practice is frequently adopted so the boiler can be placed in service again on short notice. However, if immediate need for the boiler is not anticipated, it is preferable to prepare the unit so that corrosion cannot occur during the idle period, when the boiler is in standby.
Recognizing standby corrosion and distinguishing between it and corrosion occuring during operation are often difficult tasks and require considerable experience on the part of the boiler inspector or plant operator. Specific locations in the boiler are especially susceptible to attack during standby, but under certain conditions any area in the unit may be affected.
A number of case histories are cited to illustrate the complexity of the problem. These describe damage to boilers, superheaters and turbines—vital units of power plant equipment. Reference is made to the pertinent points which establish the fact corrosion occurred during periods of standby.
A number of practical preventive measures are described which have proved effective in practice. These include both dry and wet procedures. For boilers and superheaters the dry method provides the most certain protection. Dry standby entails draining the boiler and keeping it dry during the idle period. When the wet method is used it is essential that the unit be completely filled with properly conditioned water at all times and that circulation be supplied at intervals to insure against dilution resulting from leakage of feedwater or steam condensate into the unit.
Standby protection in the case of turbines will usually involve guarding against steam in-leakage and prevention of siphoning action through drain lines.
Basically, prevention of standby corrosion requires keeping all surfaces of equipment free from contact with moist air or water containing dissolved oxygen. While operating requirements of industrial power plants will vary, suggested procedures will nevertheless serve as guides in preventing standby corrosion.
