An acetylene cylinder on fire: what to do what never to do, and why

An acetylene cylinder is not a bottle of compressed gas. It is a steel shell filled with a porous mass soaked in acetone, with the acetylene dissolved in the acetone. Heat does two things to it: it raises the pressure of the gas coming out of solution, and above a certain temperature it can start the gas decomposing — acetylene breaking into carbon and hydrogen, releasing heat, with no air needed at all.

The cylinder is designed for this. Fusible plugs in the head and foot yield between 98 and 104 °C (IS 13497:2017) and let the contents vent before the shell can fail. A venting cylinder sounds like a loud report when the plug goes and a roar afterwards; the gas burns at the plug, bright at first and yellower as acetone is carried out with it; the far end of the cylinder can frost from the rate of withdrawal. That is the cylinder working as intended. The task is to manage it, not to stop it.

The guidance below follows the Safety Bulletin of the Compressed Gas Association, which KIGL reproduces in the operation manual issued with its plants.

Clear everyone who is not needed. One trained person takes charge. Nobody stands in line with the fusible plugs — the ends of the cylinder — at any time, because that is where a plug releases and where the flame will be.

Put it out immediately. A gloved hand over the plug, a heavy cloth, wet waste or mud all work; so does dry chemical powder from a short distance. Do not use a CO₂ extinguisher: the Gas Cylinders Rules 2016 bar CO₂ on flammable-gas fires because of the static it generates. Then cool the cylinder with water, keep cooling until it stays cold with the water off, move it to a ventilated place away from other cylinders, mark it, and tell the supplier. It does not go back into service.

Do not try to put it out unless you are outdoors, or in a well-ventilated space with no source of ignition. A large flame extinguished indoors leaves acetylene pouring from the plug, mixing with air and finding the next spark; the explosion that follows is worse than the fire was. Fight it from a safe distance, behind protection from flying metal. Keep the cylinder and every cylinder near it cool with large amounts of water until the fire has burned out.

The instinct is to get it outside. Often that is wrong.

Do not move it if that means bringing it down from a second or higher floor, into a lift, from a non-combustible building with little in it to burn, through corridors, past flammable material, or loosening it from a tightly fastened position at close range.

Consider moving it if it is near other cylinders, near highly flammable material, or in a combustible building that is hard to protect. If so: extinguish any small flame from a distance; if the cylinder is cool, close the valve; if it is hot or a plug has released, cool it with a hose from a distance; allow a burning cylinder to burn down until the flame is less than a foot long, put it out, and move it promptly. Never move a cylinder showing signs of internal heating or with flame playing on its plugs.

A cylinder that has been hot is suspect even if it never vented, because decomposition can be under way inside it. Keep it cool, keep it quarantined and upright, and tell the supplier so it can be taken for inspection. Fire damage, torch or arc burns and fire-affected valves are each on KIGL's list of reasons a cylinder is refused for filling.

Almost every cylinder fire starts at a leak or a flashback. The defences are the ordinary ones: flashback arrestors at the regulator and the torch; never more than 0.62 bar g free acetylene at the point of use; the cylinder upright and secured, away from heat, valve closed when not in use; soap solution for leak tests, never a flame; and a store ventilated at high level, since acetylene is lighter than air and collects under the roof. The handling guide sets these out in full.