Potential guidelines for design and fitness for purpose for carbon steel components subject to repeated thermal shock

Abstract

Thermal shock cracking is an important damage mechanism for many pressurised components. The growth of thermal shock cracks is influenced by a combination of several factors including geometry, the severity of the thermal shocks, the applied mechanical loads and the environment. In the work reported here it is shown that thermal shock cracks will arrest if the above-mentioned factors are below certain limits. This work seeks to determine the relevant levels of those factors. This paper presents an analysis of the crack growth mechanisms involved, including an assessment of the environmental effects. Proposals are presented as to the guidelines which might be used for design and operation of vessels to prevent the initiation of thermal shock cracking. The guidelines presented here are much more flexible than the highly restrictive guidelines published by EPRI in 1989 for economiser headers. The work also includes a study of the nature of crack growth. Most of the cracks are found to grow in conditions leading to arrest. This presents the possibility of fitness for purpose assessment of the cracking when it is found in service.