A Commentary On Backfires, Flashbacks and Flashback Arrestors

The safety equipment industry in the U.S. has flourished for the last several years as manufacturers have increased efforts to promote safety awareness and education for end-users.  As part of that, manufacturers of oxy-fuel cutting and heating apparatus have also been successful in raising awareness of safety issues related to their equipment.  This is a positive trend that should continue.  However, even safety equipment, if used improperly, can create hazardous conditions, which increase the risk of accidents and/or injury.

Many years of field experience have shown that when used properly per the manufacturer’s instructions, oxy-fuel torches are safe and reliable without the use of accessory safety devices such as check valves and flash arrestors. That said, it must be emphasized that it takes training and diligence on the part of the end-user to operate oxy-fuel equipment safely. In today’s fast paced workplace, it is easy to skip certain safety practices such as purging hose lines when starting up/shutting down or making sure that all gas passages are clear and free of dirt, oil and debris. 

Realizing this, most manufacturers, at a minimum, will install check valves on torches and some are even making check valves and flash arrestors integral to the torch.  All of this is well and good, but manufacturer’s efforts to promote equipment safety can never replace the user's responsibility to employ safe operating techniques.  If an end-user neglects or ignores the manufacturer’s stated or published safety and maintenance recommendations, that person is an accident waiting to happen and no accessory safety device will prevent all possible accidents. As one who has handled accident litigation cases for the past several years, I can state unequivocally that most oxy-fuel accidents are caused not by equipment malfunction, but by user neglect, impatience or lack of experience in using equipment safely.

Flashback Arrestors

Flashback arrestors (FBA’s) are commonly used safety devices that stop or impede the progress of a flame front upstream of the insertion point.  Probably 90% of all FBA’s used on oxy-fuel cutting and heating torches are designs which have sintered stainless steel elements. The remainder are designs which use coiled tubing to quench the heat associated with a flashback flame. These elements quench the flame front as it propagates back upstream toward the torch and/or regulator.  FBA’s are generally reliable in performing the intended function so long as the device is used according to the manufacturer’s instructions.

Flashback arrestorsThere are two types of sintered element arrestors.  One is mounted onto the outlet of regulators and the other is mounted onto the inlet of torches. Regulator arrestors are typically larger and have a higher flow capacity than torch models because the sintered element has more surface area allowing for more flow.  The sizes of torch mount arrestors are limited so as not to make the torch too cumbersome to handle.  Torch arrestors are preferred because their location is ideal for preventing reverse flow flashbacks into the hoses.  If regulator mount arrestors are used, flashbacks can still occur in the hoses with a high potential for injury.  Regulator mount arrestors are used when higher flows are needed and these will protect regulators and cylinders from the effects of flashback.

Flashbacks and Sustained Backfires
Oxy-fuel cutting and heating torches are subject to three types of phenomenon: Backfires, Sustained Backfires and Flashbacks.  It is important to understand these terms and to be able to identify these reactions if they occur.  Unfortunately, manufacturers and distributors of oxy-fuel torches have not done a thorough job in explaining to their customers exactly what these terms mean.  The result is a general misunderstanding among end-users.  

Backfire.  A backfire is defined as the momentary retrogression of the flame into the torch tip. The user hears a “pop” and the flame is extinguished.  The flame may be re-ignited if an adjacent ignition source is present or the use of a manual igniter may be necessary.  This is not normally a safety concern and, in fact, many manufacturers induce backfires during design and production tests to insure flame integrity of torches and tips.

Sustained Backfire.  This is defined as a sustained burning of the flame back inside the torch, usually at the mixer, but could also happen further upstream under the right conditions.  This is often accompanied by a hissing or squealing sound and/or a smoky, sharp pointed flame.  The user should immediately close all torch valves to avoid damage or injury.  If a sustained backfire continues to burn without closing torch valves, severe damage to the torch as well as an increased risk of fire would result.  Most sustained backfires can be contained inside the torch until the user can shut off the flow of gas and will not breach to the outside.  There are, however, some torch designs with thin construction walls where the flame will breach the wall and spew fire and molten metal in a short time.

Flashback.  A flashback is a momentary or sustained retrogression of the flame upstream of the mixer usually in the torch or hoses.  This is a potentially dangerous situation, particularly if the flame reaches the hoses where an explosion will result causing a rupture or separation of the hose.  A flashback is generally caused by the reverse flow of gases upstream into the hoses or other equipment.  This reverse flow is usually the result of 1) improper shutdown and/or startup procedures or 2) by allowing cylinder pressures to become too low or 3) by a check valve that is not working properly. 

Flashback arrestors are normally very adequate in preventing reverse flow flashbacks, however, they will not prevent a sustained backfire, nor were they intended to function in that way.  In fact, FBA’s can actually contribute to the likelihood of sustained backfires.  Here’s how:  Oxy-fuel gas mixers are designed to distribute appropriate volumes of oxygen and fuel gas into a mixing passage or tip.  In addition, a properly designed mixer will insure adequate exit velocities of the mixed gas at the tip to prevent a sustained backfire.  This all happens without incident when normal safety precautions are taken and the pressures are set as recommended by the manufacturer.  FBA’s which incorporate sintered elements create dramatic pressure drops across the sintered element and into the torch.  This means that the pressures at the mixer are lower also.  Lower pressures at the mixer means lower exit velocities of the mixed gas at the tip or nozzle.  This could result in a sustained backfire if the torch gets too close to the workpiece.  Heavy heating applications are especially vulnerable to this event since heating tips use large volumes of gas and are highly susceptible to backfires, which could become sustained, if mixer pressures are too low. 

One way to avoid the hazards associated with sustained backfires is to compensate for the pressure drop when using flashback arrestors.  Most arrestor manufacturers supply pressure drop charts along with their products (see Fig. 1).  These charts should be closely followed when setting pressures in systems using flashback arrestors.  The charts are read as follows:

Determine the flowrate and the recommended pressure required based on the tip or nozzle used (this can be obtained from contacting the manufacturer or through the manufacturer’s literature).

Since most charts use air or nitrogen as a test gas, correct the flow rates for the density of the actual gas used (i.e. oxygen and acetylene, propane, natural gas, etc.).

Find the pressure drop that would be expected at a given flowrate and pressure on the chart.  Here is an example based on the chart below: if the tip or nozzle required 400 scfh at 80 psig, this would fall on the p=10 psig curve and the pressure at the regulator should be adjusted up to 90 psig.

If these pressure drop charts are adhered to by end users, this will insure that adequate pressures are maintained at the tip and decrease the likelihood of sustained backfires.  There are, however, some heavy heating applications where the flows are so high that it would be impractical to use torch mount flashback arrestors.  For example, some heavy heating tips manufactured by Harris Calorific require up to1600 scfh oxygen and 400 scfh propane.  The pressure drops would be so dramatic on these tips that torch mounted arrestors should not be used.  Rather, high flow regulator mount arrestors should be used making sure that the appropriate pressures are compensated to deliver adequate flows to the tip. 
For additional information on the use of flashback arrestors as related to oxy-fuel equipment, the Compressed Gas Association has two (2) publications; SB-16 Use of High Flow Oxy-Fuel Gas Heating Torch Apparatus and TB-3 Hose Line Flashback Arrestors available on their website (www.cganet.com). 

In summary, users should always follow the manufacturer’s recommended safety and maintenance procedures.  No one knows how to safely operate and maintain the equipment more than the manufacturer.  Also, do not think that an accessory safety device makes the equipment immune from all accidents.  This can create a false sense of security and lead to carelessness.  And finally, be extremely careful with flashback arrestors in heavy heating applications.  If used, it is very important that the arrestor be a high flow model, probably regulator mount, and that the appropriate pressure compensation charts are followed.  This should result in safe and accident-free use of your oxy-fuel equipment.
