Release of hydrocarbons mixture from a pressure vessel

A release of contents of a pressure vessel occurred when a longitudinal weld tore open. The vessel had been shut down and had just been put into use again when the incident occurred.
The gas mixture contained hydrocarbons with 30% hydrogen. It had reached its working pressure of 31 bar, but was only at -26 degrees C, instead of the working temperature of -73 degrees C, no liquid was present.
An investigation found a crack, 1.6 metres long, had formed near the upper end of the weld.

The recommendations issued after the incident, suggest that the ROOT CAUSE could be a lack of proper inspection, and/or inspection performed with inadequate tools

weld, vessel

The vessel had been shut down and had just been put into use again when the incident occurred.

PROBABLE PROCESS
Although not specified by the source, the event occurred very probably at a vessel used to distillation and gas separation processes of the refinery, for example in what is commonly called a ‘cold box’. This assumption is based on temperature data provided. Propane, for example, liquifies at -42 C, it is therefore still gaseous at 31 C (the temperature at which the release occurred) and liquid at -73C, corresponding to the liquid nitrogen temperature, which probably was used as coolant vector,

Inspection procedures had not been developed for many operations, including measuring temperature from outside field panels underneath the reactors. This hints at a poor process hazard assessement, that did not even reflect the actual equipment degradation and instrumentation used in the process.
The following recommendations were made:
1. All vessels of similar construction to be tested for incipient cracks on the inner surface by using a dye penetration test.
2. If the interior is inaccessible, welds and impact zones are to be tested by ultrasonic methods.