Leakage from a valve in a heavy-oil hydro-desulphurisation unit

The incident occurred during the start-up after a routine repair of the refinery's heavy oil hydrodesulfurization system. A valve at the inlet pipe of a high-temperature and high-pressure separation tank ruptured, resulting in oil and gas spillage.
The valve was part of a pressure gauge instrument. It failed at the welding, due to hydrogen and hydrogen sulphide attack. Its weld material had an excessively hardness due to insufficient heat treatment after welding.

This conclusion was based on the findings of the post-incident investigation:
(i) The fracture was approximately in the middle of the butt weld between the valve body and the sealing.
(ii) On the inner circumference of the fracture surface, a small weld bevel remained, indicating incomplete penetration of the weld.
(iii) The hardness at the weld fracture was high.
(iv) Both the base metal and the welded metals were normal and in accordance with the specified standards.
(v) From a microstructural point of view, both the welded metal and the welding heat-affected zone of the damaged part were showing hardened structures such as those observed at the time of quenching. It points out at an insufficient post-welding heat treatment. There were no signs of decarburization with hydrogen.
(vi) Observations by scanning electron microscope revealed a pseudo-sheared fracture surface, characteristic of delayed fracture by hydrogen.

The INITIATING CASUE was the failure of a welding between a pipe and a pressure gauge.
The ROOT CAUSE was shortcoming in the manufacturing of the weld.

valve

The unit was restarting after a routine repair.

The incident highlights the complexity and the possible pitfalls of the relationship between suppliers of components and the plant operators.
From the point of the valve manufacturer (the supplier), this incident was the results of a manufacturing error, the failure to execute a weld respecting the specification. It could be as well be attributed to lack of quality control.
From the point of the user (the plant operator) the incident could be classified as a case of ‘force majeure’, because the user choose the supplier and specified the quality/application standard, but was not able to assess the quality of the weld and the compliances to the standard.