Fire in a hydrogen pipe

A leak occurred on a 4 inch hydrogen main gas line which ran along a pipe bridge at the east end of a first stage compression plant building. The relased hydrogen ignited.
The source of ignition was hot welding sparks arising from work being carried out by contract personnel whilst installing a new chlorine line above the hydrogen lineon the pipe-bridge.

Before the welding work was carried out, an explosivity test had been carried out in the area and a 0 explosive gas result had been reproted in the work permit.
An investigation into the incident found that the removed section of the hydrogen main revealed an elliptical hole 2 centimetres x 1 and a half centimetres. It was only when the lagging was disturbed that the hydrogen leak became obvious.The area around the hole in hydrogen main was very pitted suggesting that the failure was due to localised corrosion.

The INITIATING CAUSE was an elliptical hole of 2 centimetres x 1.5 centimetres.
It was very probably due to localised corrosion.
The leak was not detected during the explosivity test because of a build up of scale sealing the hole, possibly dislodged later by the activities of the contractors in the area. The technical reason for the corrosion is not provided or known.
Neverhtless, ROOT CAUSE could be possibly attributed to a lack of effective inspection, not able to detect the progress of the corrosion.

Separation equipment

Welding works were ongoing, but the plant had not been shutdown (pesene of flammabe gases had been excluded by measures).

The welder saw the hole appeared as if the lagging on the pipe had caught fire and an unsuccessful attempt was made to put it out using carbon dioxide fire extinguishers and then water.