Flash fire in gas supply equipment onboard dive support vessel

A member has reported a small explosion on a vessel, resulting in a flash fire which caused minor burns to the hand of one crew member and slight damage to some equipment, including pipework and fittings.

A life-support supervisor (LSS) was in the process of decanting oxygen to increase the percentage of an 18% oxygen kelly to 20%. An oxygen line on deck was opened by the LSS to allow oxygen down into the main gas storage/mixing flat. The line to the kelly was opened and then the panel valve on the diaphragm compressor was opened. At no time was the diaphragm compressor pump running.

On opening the needle valve to decant oxygen into the bulk gas mixing panel, the LSS a loud crack followed by a flash. He immediately closed the oxygen inlet valve and extinguished the burning remains of a hose with some water from a receptacle near by.

However, oxygen was still escaping from the back of the panel. The LSS went on deck and closed the supply valve on the quad of oxygen. He returned to the main gas storage/mixing flat, via the saturation control room, to raise the alarm. Fire detectors had already picked up the fire and the alarms were raised.

The LSS sustained superficial minor burns to his arm and hand, plus singed hair.

Following investigation, a clear direct cause for the explosion/flash fire was not found. There were three equally likely possible causes:

• a failing (crack) in a fitting or pipe, causing escape of oxygen and a flash fire from a fuel source external to the pipework;
• rapid oxidisation and a flash fire from a fuel source internal to the pipework;
• the valve being opened too quickly, causing friction/ignition within the pipework or a sudden increase in pressure on the pipework or fittings, leading to it rupturing and allowing oxygen to escape on to an external fuel source, either cause resulting in the flash fire.

The following remedial actions have been recommended:

• The panel, valves and all other pipework on the diaphragm compressor were replaced;
• video surveillance was installed in the main gas storage/mixing flat;
• a specific line for oxygen was installed, direct from the oxygen quads to the main gas storage/mixing flat;
• further non-destructive testing was conducted on parts of the diaphragm compressor to identify a failing in a fitting;
• internal swab and laboratory testing was conducted to determine the fuel source of the fire;
• signs were installed above all valves taps stating ‘open valve slowly to ease the pressure into the line’;
• a regular and documented oxygen cleaning program was instituted.