Analysis of the power boiler superheater strain-and-stress state under creep conditions

The paper presents a procedure for assessing the condition of the current structure of boiler tubes. First, a modified version of the Garofalo equation is proposed to model the strain-and-stress state under creep conditions in Super304H steel. In parallel with the creep model development, the microstructure is examined using SEM and TEM techniques. The Super 304H steel structure is tested for a new material and for the superheater material after 100,000 h of operation. According to the presented description of the tested steel degradation, it is possible to operate the material for another 100,000 h if the design conditions are met. The results show that the conventional approach based on European standards does not exclude the superheater safe operation. However, a more detailed analysis, taking account of additional loads from the pipeline and using the proposed creep equation, proves that the conditions for the superheater safety are not satisfied.