In an industrial gas plant, an operator was conducting a visual inspection of a 6 m³ acetylene tank without residual pressure. At around 10 a.m., 40 to 50 cm flames appeared around the neck of the cylinder from which he had just removed the valve. He alerted his co-worker and together they triggered the emergency stop and secured the installations. After analysing the situation, management called the emergency services at 10:05 a.m. The ten employees at the site relocated to the designated safe area while the site’s team of 1st responders deployed fire hoses to protect the premises and cool down the gas cylinder. The fire-fighters arrived on site at 10:25 a.m., although by that time the flames had already been put out. They continued to spray water on the cylinder until a temperature measurement was taken at around 10:50 a.m., indicating that the cylinder could be handled. Once the utilities had been put back into service, the cylinder was removed and lowered into a nearby tank of water with a lanyard to fully neutralise its contents. Operations resumed at 11:30 a.m. The operator informed the municipality, the neighbouring communities and the administration.

The investigation conducted by the operator showed that the cylinder was tipped horizontally on an easel and its valve had been removed with a power screwdriver. The valve must be removed in order to see the cylinder’s number, which is hidden underneath the testing washer. This number, however, was hidden by rust around the flange since the cylinder was stored outdoors. A pneumatic brush had been used to remove the rust. Since the tool was worn out, the operator had to apply a significant amount of pressure to be able to read the number. This caused the walls of the cylinder to heat up considerably and vaporised part of the 13 kg of the acetone containing the saturating acetylene which had remained in the porous material in the cylinder. These combustible vapours were released and ignited in contact with the air (oxidiser) and the hot wall (ignition source), as evidenced by the lack of carbon black, proof of the acetylene’s organic combustion.

The operator implemented the following measures:

• a cap is to be placed on the neck of the cylinder if difficulty is encountered when brushing the number and the use of a pneumatic brush is required,
• hand-held or automatic non-sparking tools are to be used; safety glasses are to be worn,
• the pneumatic brush head is to be replaced prior to excessive wear (indication of an acceptable level of wear),
• review of the cylinder inspection procedure to specify that manual brushing should be used for numbers masked with a small amount of corrosion.