A subcontractor was preparing to use an oxy-acetylene torch on the back deck of a vessel. The oxygen and acetylene cylinders were located on the quayside with the hoses rigged to the vessel. On lighting the torch, a ‘pop’ sound was heard, and a small fire was observed from a rupture of the acetylene hose just ahead of the flash-back arrestor.
What went wrong?
The actual cause of this incident could not be established. However, flash-back can occur when a flammable mixture remains in the blowpipe or hoses when the torch is lit. Ignition can travel towards the cylinder but is normally stopped by the flash-back arrestor, which prevents decomposition of the acetylene or an explosion.
What actions were taken?
Our member took the following actions:
- Regularly check the condition of oxygen and acetylene hoses and ensure they are replaced as required in any planned maintenance system (PMS) or supplier instructions;
- Keep torches in good condition to avoid turbulent gas flow and so reduce the risk of flash-back;
- As a minimum there should be a non-return valve at the torch end of each hose and a pressure or temperature variation activated flash-back arrestor at the cylinder end;
- Only use regulators, flash-back arrestors, hoses and blowpipes which are designed for either acetylene or oxygen and are clearly marked and manufactured to the correct standard;
- Ensure that flash-back arrestors are being replaced at least every 12-months;
- Welding gas hoses should be correctly connected, and jubilee clips should never be used;
- For cutting and purging operations the oxygen and acetylene hoses should be separately purged to remove any flammable mixtures before lighting the blowpipe;
- Monitor and confirm the subcontractor’s compliance to the above requirements.
What lessons were learned?
Our member gave the following advice to crew for what to do if a flash-back occurs and it is safe to do so:
- Immediately close the oxygen nozzle valve and then the acetylene nozzle valve (NB: this is the opposite of the normal closing down sequence);
- Close both oxygen and acetylene cylinder valves;
- If the flame cannot be put out at once, evacuate the area and raise the alarm. For onshore sites or for a vessel which is alongside alert the emergency services;
- Once the situation is secured, the equipment must be quarantined until it can be fully examined for damage and either confirmed safe for reuse or safely disposed of;
- Never attempt to move or vent a cylinder but monitor for any heat generation;
- If a hotspot is detected on the acetylene cylinder evacuate the area, raise the alarm and where appropriate, notify the emergency services, if safe to do so apply cooling water.