A recent failure on a UK refinery involved an 8" diameter vertical relief line, approximately 5m in length, which catastrophically failed during normal operation part way along its length, releasing approximately 75 tonnes of extremely flammable material at elevated temperature and pressure. Connected to an insulated process header and vessels, the failure involved complete separation of the relief line (which was not insulated).
Subsequent examination of the line revealed excessive internal thinning which was very local to the failure point, but relatively normal wall thickness elsewhere along its length. The inspection regime included regular thickness testing at four locations, but not at the area local to the failure. Notably, an adjacent relief line was also found to have very similar localised and severe internal corrosion, which had also gone undetected.
The incident is similar in nature, and on related plant, to an incident in 2003 at another UK oil refinery. In that instance, internal corrosion resulted in thinning along a horizontal section of a relief line, which again went undetected and resulted in catastrophic failure. Liquid partially filling the line resulted in a particularly aggressive interface, whilst condensation local to pipe supports was also a factor. In this and the recent incident, harm was avoided because personnel weren't in the vicinity, and escalation was avoided because the resulting vapour cloud dispersed before finding a source of ignition.
Operators of process plant should ensure that their pipework examination regime takes account of dead-legs, and in particular that: