handling the equipment discovered the leak and raised the alarm. The unit was shut down while the gas spread and then ignited in contact with a welding station located approximately 20 m from the leak. Four people were burned, one of them seriously. The site’s internal firefighters stopped the release and were able to put out the fire after 25 minutes. Approximately 900 kg of isobutane was released.

Error during disassembly

The leak happened when a technician turned the valve’s control rod with a wrench. In order to open the isobutane circuit, the technician actuated its handwheel. Seeing that it was jammed, he disassembled the manual control system, including the handwheel and a gearbox. During this operation, instead of removing the system from its support, he dismantled the assembly. To accomplish this, he removed the support’s four retaining bolts, which also happened to be the bolts used to secure the valve’s bonnet to the body. The pressurised gas escaped as soon the rod was operated.

Organisational and human factors not taken into account

The accident analysis points to several failures in risk management:

• blockage of this type of manual control mechanism was recurrent;
• the dismantling of these mechanisms, to operate the valve rods directly with the wrench, was common practice;
• the ergonomics of these mechanisms, the support of which is mounted on the pressurised part of the valve, is confusing;
• the technician was not trained to perform this task;
• this task was not the subject of a written procedure.

After the accident, the operator identified 15 valves equipped with the same control mechanism. Depending on the configurations, they were either replaced or modified in order to dissociate the mounting of the mechanism support and the mounting of the valve bonnet. A group-wide audit was conducted to detect other incorrect attachments on pressurised vessels.