Non-return valve or flashback arrestor? Why an acetylene system needs both and where each one goes

A non-return valve and a flashback arrestor are often sold in the same body and often confused, but they do different jobs. The non-return valve stops gas flowing the wrong way; the flashback arrestor stops a flame. An acetylene system needs both, because each one covers the failure the other cannot. This page sets out what each device does, why a check valve is not flashback protection, and where each belongs.

A non-return valve, also called a check valve, uses a spring-loaded or gravity-loaded disc or ball to let gas pass in one direction only. It stops oxygen being pushed back into the acetylene hose, or gas from one cylinder flowing into another, or a high-pressure line back-feeding a low-pressure one. A flashback arrestor contains a quenching element of sintered metal, packed grains or water that extinguishes a flame travelling back up the line. The C.P.I.'s 1968 code drew the line clearly. A device built to stop flashbacks must halt a flame before it reaches the accessories and generator upstream, and if it does not also stop other gases entering, a separate non-return valve is needed. Of dry non-return valves it said they "do not necessarily stop flashbacks" (C.P.I., Safety in the Production and Use of Acetylene, Paris 1968).

The ISO split makes the distinction formal. ISO 5175-1:2017 covers devices incorporating a flame arrestor; ISO 5175-2:2017 covers those without, including bare non-return valves, pressure relief valves and excess-flow cut-off valves. Part 2 includes a clause titled flashback resistance, but a Part 2 device by definition has no flame-arrestor element, so at most it can claim to survive a flashback, not to stop one.

Because it was never designed to quench a flame, and a flame moves faster than a valve closes. The American Welding Society puts it in one line: "Reverse flow check valves alone will not stop a flashback in the system" (AWS Fact Sheet 28). An Oregon OSHA interpretation adds that a backflow device only stops gas flowing back into the torch and hoses and does not reliably stop a flashback reaching hoses, regulator or cylinders.

The incident record says the same. In the hose fire investigated by IMCA (Safety Flash 02/14), a welder lit an oxy-acetylene cutting torch, there was a bang, and flame came from a coil of hoses on deck. The hoses split in three places, all at in-line connection points. The torch's non-return valve was suspected of being inoperative, the hose was perished along its length, and the connections had not been leak-checked. IMCA recommended arrestors on both the oxygen and fuel lines near the regulators, and at both torch and regulator for long hose runs.

Because the commonest route to a flashback is gas in the wrong hose. Oxygen is supplied at a higher pressure than acetylene. When a nozzle is obstructed, or the joint between the torch head and handle leaks, oxygen can climb the acetylene duct and fill the fuel hose with an explosive mixture; ignition at lighting-up then sends a flame back through a hose already full of fuel. The C.P.I. described both routes from experience. A non-return valve stops the mixing; the arrestor stops the flame if ignition happens anyway.

The acetylene literature gives a striking demonstration of why mixing, not pressure, is the danger. In experiments recorded by S. A. Miller (1965), nitrogen shock waves at 90 atm failed to explode acetylene at 3 atm, but air at 90 atm, or oxygen at only 18 atm, did, and a 65 atm oxygen shock initiated acetylene at pressures as low as 0.5 atm absolute. The difference was the driver gas mixing into the acetylene. That is why oxygen and acetylene must never be cross-connected and why the non-return valve sits on every fuel line.

The British Acetylene Association's 1962 code, 'Design and Installation of Acetylene Supply/Distribution Systems for Flame Processes', stated the manifold rule that still underlies practice: a non-return valve between each cylinder and the main header on the high-pressure side, and one or more flashback arrestors after the regulator on the low-pressure side. The non-return valves stop cylinders feeding into each other; the arrestors catch a flame arriving from the workstations. In IS 15190 (Part 2):2002 cl 6.7.4, flashback arrestors fitted with non-return valves at bends and joints can replace wall reinforcement, but not at the end of a pipe, where an arrestor serves no purpose. More on manifolds is in cylinder manifolds and bulk supply.

IS 11006:2011 cl 11.5 tests five samples. Gas is first passed forward for 5 seconds; the valve is mounted in its most disadvantageous position, with gravity acting to open it; back-pressure is then raised slowly at 6 mbar per minute to 10 % of maximum working pressure, and separately from zero to maximum operating pressure within 1 second. Leakage during the test and for one minute after it must stay within the cl 10.2 limit of 50 cm³/h for bores under 11 mm. Production testing under cl 12 applies the reverse-flow test to every device made. In service, a check with 1–2 bar of dry nitrogen applied to the outlet is the usual field method, covered in testing and replacement.