A Study of Integrity Changes in Structures During Exploitation

Abstract

Spherical and cylindrical pressure vessels are manufactured as welded structures, where cracks could be initiated/propagated during fabrication, pre-service hydro-test, service itself, service welding repair, or during the service hydro-test. In this case, fracture mechanics approach is necessary. This paper analyzes the stress state around cracks in the bottom head which is one of the main components in a cylindrical pressure vessel (radius of 1056 mm, wall thickness of 92 mm, pressure of 15.5 MPa and temperature of 454°C). Sizes of several detected cracks at the inner surface are determined by the NDE methods. Finite element analysis is performed to determine the stress zones in the vicinity of the most critical crack. Such analysis is done before and after the hydro-test. It will show the influence of the hydro-test on propagation of the existing cracks and fracture behavior of repaired cracks through the stress state analysis. Changes in material properties were analyzed. Results will be used to assess the pressure vessel integrity and estimate its useful life.