Beneficial Effects of Higher Pressure Testing on Redistribution of Residual Stresses and Crack Blunting

Abstract

For testing Group 4 unfired pressure vessels designed according to the European Standard EN 13445, not subject to non-destructive testing other than visual inspection, the hydraulic proof test is expected not only to prove leak tightness but also to improve the vessel’s reliability (500 full pressure cycles minimum fatigue life). In the HYDFAT research program two complementary approaches were combined to investigate the benefits of the hydraulic test pressure on the fatigue life. (1): Numerical modeling allowing for the determination of the critical weld defect size under various pressure test conditions, complemented by tests on wide plates and small-scale vessels (wall thickness 6 and 12 mm) containing weld defects. (2): Statistical investigation (multivariate analysis) of a series of experiments on thin-walled vessels (≤ 6 mm). It was concluded that the hydrotest pressure had to be as high as possible and ratios of test pressure to the maximum allowable pressure were proposed for ferritic and austenitic steels. However the maximum allowable peaking for testing Group 4 vessels should be lower than that currently allowed in EN 13445 for vessels of other testing Groups. These results have been implemented in the first edition of the European standard EN 13445 published in May 2002.