Prevention of brittle fracture in pressure vessels: A review of the design rules of EN 13445 Annex B and BSI PD 5500 Appendix D

Abstract The selection of materials with appropriate levels of toughness is critical for the avoidance of brittle fracture in ferritic steel pressure vessels and is usually achieved by specifying a test temperature at which the material's Charpy energy meets a certain energy criterion, typically 27 J ( T 27J ) or 40 J ( T 40J ). This paper traces the history of the toughness requirements in two pressure vessel codes – the European pressure vessel standard EN 13445-2 and the UK legacy code PD 5500 (previously BS 5500), which was one of several national standards that influenced the development of EN 13445-2. Since publication of Issue 1 of EN 13445-2 in 2002, the toughness requirements for certain categories of European pressure vessels (in particular, those made from lower-strength steels in the as-welded condition) have been relaxed considerably, leaving a substantial disparity (which can be more than 60 °C) in Charpy test temperature requirements ( T 27J ) between PD 5500 and EN 13445:2014. The reasons for the discrepancy between the codes are examined, and the large-scale tests used to validate the PD 5500 requirements re-interpreted in terms of both the current EN 13445-2 rules and the fracture mechanics-based flaw assessment procedure BS 7910:2013. The tests used to validate PD 5500 do not support the EN approach, suggesting an urgent need for further examination. Finally, two historical pressure vessel failures (which preceded both codes) are examined in the light of whether they could have been avoided by the use of either of the codes. In both cases, use of the PD 5500 procedure would have led to a sufficiently high level of defect-tolerance that failure could have been avoided. Use of the EN procedure for materials selection, however, would not have avoided failure. Whilst there is considerable room for opinion about what constitutes an appropriate level of defect-tolerance (recognising improvements in fabrication methods, inspection techniques and Quality Assurance systems since the failures occurred), this work underlines the need for a re-examination of the basis of Annex B of EN 13445-2, in particular its application to lower-strength steels.