The use of a simplified analytical expression for metastable thermal stress analysis and its application to creep-fatigue damage of a 2.25Cr 1Mo thick walled component

摘要

Thick walled pressure vessels are of considerable importance in a wide range of industries. The evaluation of stresses is necessary not only from a design point of view but also for fitness for service analysis of age-ing infrastructure. The accumulation of creep-fatigue damage over time is the principal damage mecha- nism which will eventually lead to crack initiation in critical high temperature fossil plants. Many power stations are being subjected to two-shift operation due to changes in demand and competition from cheaper energy sources, and in the future from added carbon taxes. To assess high temperature compo-nents for creep-fatigue damage for example, under faster ramp rates and additional cycles, as a first pass it would be useful to explore the feasible operational envelope using simplified calculations. These are, however, generally not available and more complex finite element analysis is necessary. This paper uses a simplified closed form solution for metastable thermal stresses in thick walled pressure vessels. This form of solution can if necessary be used with either stress concentration factors or superposition of polynomials for more complex components derived from FEA analysis, such that the closed form solution can be used to estimate any ramp rate on the unit. In this case the ramp rates are considered to provide sufficient time to become metastable. Many existing units rely on heavy section 2.25Cr 1 Mo steel (P22) pipework and tubing, and hence for two shifting can be subjected to high levels of cyclic strain. Based on the simplified expression developed, an operational envelope is explored for thick walled cylinders con-structed using P22 steel. Creep-fatigue damage is calculated based on the R5 methodology. The analysis shows that for thick walled components with minimal stress concentrations, creep will dominate the life of the component. However, complex interaction between base rupture, onset of significant cycling, creep, and fatigue dictates the upper bound on feasible ramp rates, as a result it is possible to construct screening curves based on the effective elastic stress intensity range.