Writing an emergency response plan for an acetylene plant: stop the gas vent the high-pressure side, deal with hot cylinders, and inspect before restart

An acetylene plant holds a gas that can burn without air, compressed to more than twenty atmospheres on its filling side, beside a store of calcium carbide that makes more gas when wet. Its emergency plan cannot be a generic fire plan. It has to say, in order, how the supply of acetylene is stopped, how the high-pressure system is emptied, what is done with a cylinder that is heating, which extinguishing media go where, and what must happen before the plant runs again. This page sets out those elements from the codes that define them — the C.P.I. code of 1968, EIGA Doc 123, NFPA 51A, IS 14814 and the Gas Cylinders Rules 2016 — and from the engineering lessons of incidents KIGL has investigated.

NFPA 51A-1984, the US charging-plant standard (international practice, not Indian law), opens its fire chapter with the doctrine in one paragraph: the major fire hazard is acetylene escaping from equipment, piping or cylinder fittings; it may or may not ignite; in either case every attempt consistent with personnel safety is made to shut off or remove the source; fire is not normally extinguished in any other way; and cylinders exposed to fire are kept cool with water so that their safety devices do not release more gas. An acetylene flame put out while gas still escapes leaves an expanding cloud with a 2.5 percent lower explosive limit (IS 14814:2023) and an ignition energy of about 0.02 mJ (EIGA Doc 123). The plan is built round that sequence.

The same code required means to lower the pressure in the high-pressure circuit rapidly when needed, and recommended quick-opening discharge valves installed at several places, preferably near the exit doors of the halls, with outlet pipes ending in the open air where the gas cannot form an explosive mixture or cause damage if it ignites, and where the outlet cannot be obstructed, especially by ice. The siting near the exits is the point: the operator opens the vent on the way out. Filling-manifold cylinder valves are closed last at the end of charging and first in an emergency only where it is safe to reach them; NFPA 51A notes that cylinder valves are opened first and closed last in normal charging. The physics of why emptying the system matters is on the acetylene decomposition page.

The C.P.I. code required devices to warn the works staff of fire and standing orders for calling the fire brigade and other public services; it also required staff to know what to do "in the course of the first few minutes preceding the arrival of aid from outside". The plan should name the alarm, the assembly point upwind and outside the 15 m plant boundary, the person who calls the fire service and what they tell it — that the site holds dissolved acetylene cylinders and calcium carbide, and that water must not be used on the carbide store. A written emergency procedure and periodic fire drills are required by NFPA 51A, and EIGA Doc 123 asks for a drill at least annually.

EIGA Doc 123 clause 7.3 sizes the deluge at 10 l/m² per minute, with 90 minutes' supply normally and 12 hours for a hot-cylinder incident, and requires a test every three months. Hydrants go outside the buildings with adequate pressure and delivery (C.P.I. 1968); hose stations and deluge valves are placed so they can be worked from outdoors or at an exit (NFPA 51A). Form E requires dry chemical powder extinguishers at strategic locations and states that carbon dioxide extinguishers are not to be used on flammable-gas fires because they generate static; IS 14814 clause 9.2 allows CO₂ on an acetylene flame. The safe reading is dry powder for gas fires and CO₂ for electrical equipment. Carbon tetrachloride is never used; it forms phosgene in a flame.

A DA cylinder that has been in a fire, has had a flashback into it, or has a top that feels warm may have decomposition running inside it. The published advice divides on the valve, and the plan must choose; all sources agree on sustained water cooling, which the Indian filling-licence conditions require.

The reasoning behind Miller's caution is the data he compiled: of five decomposing cylinders without safety devices, two burst — one with its valve kept closed, one vented only sporadically. A partly blocked valve, or a leak beside a closed valve, produces exactly the slow venting he warned against. Whichever procedure the site adopts, the plan should say so, and should say that if the chosen procedure is producing slow venting, the operator changes to the other — and that if neither is possible, the area is cleared. A cylinder cooled under water spray is not safe until it stays cool with the water off; EIGA's 12-hour deluge supply is consistent with blow-down times of 8 to 10 hours for a large full cylinder. Under Rule 24(2) of the Gas Cylinders Rules 2016, a DA cylinder damaged by fire is condemned and destroyed. Cylinder-level guidance for customers is on the acetylene cylinder fire page.

A generator that overheats, a hopper that ignites or a carbide drum that heats are plant-specific emergencies the plan must cover; Form E requires written procedures for carbide storage and transport, carbide fires, lime spillage and hot cylinders. The common thread in the generator incidents KIGL has investigated across several operators is that each began in the screw feed or hopper, and in most the ignition source was tramp metal or stone in the carbide rubbing at a running clearance of a few millimetres. In those events the wet flashback arrestors and relief valves contained the decomposition where the arrestors had water in them, and did not where they had been allowed to run dry. The emergency response is to stop the feed, vent the generator to its safe vent, admit nitrogen, keep water away from carbide, and leave the hopper closed until it has cooled.

The C.P.I. code set a deliberately low trigger: after an accident in which an explosion has occurred, or is believed to have occurred, in any part of the installation, the installation is carefully inspected and tested before it is put into service again. That means soot checks in generator, arrestors and pipelines — soot is the signature of decomposition — relief-valve inspection, arrestor strip-down, pressure testing with nitrogen, and a documented restart authorisation. Rule 67 of the Gas Cylinders Rules 2016 requires notice of an accident to the Chief Controller by fax or e-mail, with a letter of particulars within twelve hours.