An explosion occurred in a rolling stock plant at around 3:30 p.m. while an acetylene pipe was being dismantled. The contractor performing the work sustained burns to his back and neck.

A hot-work permit had been issued and the contractor had followed the procedure given below:

• close the pipe’s acetylene supply valve;
• disconnect the flange between the pipe to be dismantled and the valve;
• flush the pipe with nitrogen;
• request a hot-work permit and check the flange with an explosimeter.

The explosion was caused by the mechanical action of the sabre saw used and the presence of either soot (carbon dust) or acetylene in the pipe. The possible reasons why acetylene was inside the pipe despite the precautions taken beforehand are:

• the supply valve had not been sufficiently locked out (flange unbolted but not completely removed);
• the network had not been sufficiently flushed with nitrogen.

The operator noted that there was no nitrogen-flushing procedure. It proposed that nitrogen flushing be carried out by a specialist firm and that the firm provide a design calculation and a report. The operator also proposed to revise its lockout-tagout procedure. This will be carried out with specialised partners. The procedure for measuring flammable gases with an explosimeter is also being reviewed, particularly in terms of the number of points to be measured in order to approve hot-work permits.