Flashback Arrestors

This quote is from Weld Talk at Hobart Welders.

To: Those Concerned
From: David Pryor
Date: November 21, 2008

Subject: The Use of Flashback Arrestors in an Oxy/fuel System

This letter is intended to provide an understanding of the use of flashback arrestors in
oxy/fuel systems and to provide specific reasons why Thermadyne makes certain
recommendations.

The condition known as Flashback is defined as the return of the flame through the torch,
into the hoses, and potentially into the regulators. In rare cases, it may also reach the
cylinders. This condition can possibly cause an explosion at any point in the system.

Flashback is caused by the ignition and propagation of mixed gas in one side of an oxy/fuel
system. This situation can arise by the oxygen migrating back into the fuel system (or visa
versa) (a.k.a. reverse flow) and suddenly be ignited by lighting the welding or cutting tip. The
flame will then propagate back into the fuel side of the system (in this case); sometimes
causing an explosion such as the rupturing of the welding hose.

The best way to avert flashback on an oxy/fuel system is to always follow the recommended
start-up procedure which includes purging the hoses before use. In addition, safety devices
such as reverse flow check valves and flashback arrestors can be used to reduce the
potential for flashback in the event the correct operating procedures are not followed.
Things to know about using Flashback Arrestors:

1. The safest place to locate flashback arrestors on an oxy-fuel system is at the torch
end. At this location, the welding hoses are protected from the potential hazard of an
explosive flashback. The majority of flashback incidents that occur in the field are the
result of mixed gases forming in either the oxygen or fuel hose and subsequently
exploding when all the conditions are met to produce this event. Locating the
flashback arrestors only at the regulator outlets of an oxy-fuel system does not
protect the hoses, hence, greatly limits the protection to the operator.

2. The use of flashback arrestors located at the regulator outlets and at the torch can
potentially create an additional hazard by restricting the flow of oxygen and/or fuel to
the torch tip. If the user has installed a cutting tip or heating tip that has a high flow
demand, the restrictions created by having a flashback arrestor at both the torch and
regulator may cause insufficient gas velocity at the exit end of tip. This condition can
cause backfires to occur due to overheating of the tip or nozzle. Over time, the
backfiring can cause carbon to clog the flashback arrestors on the torch which may
further compound the flow restriction problem. If the user decides to use flashback
arrestors at the torch and the regulators, a determination must be made that the flow
demand of the cutting tip or heating nozzle being used will not be restricted by the
use of both safety devices.

3. Dispelling the cut hose myth. Many operators of oxy-fuel systems believe that if the
welding hose is ever cut, the flame may travel back into one of the hoses and into the
regulator; and possibly even into the cylinder. First off, everyone should be reminded
of the triangle of combustion. In order for combustion to take place, there must be a
fuel source, an oxidizer source and an ignition source. Inside a pressurized oxy-fuel
welding hose, one hose will contain pure fuel, while the other contains pure oxygen. If
one or both hoses are cut, there is no opportunity for the flame to propagate back into
the hose toward the supply source because one leg of the triangle will be missing.
Typically, when a fuel hose is cut and ignited, the flame will attach to the point where
the fuel is being dispersed. The flame cannot propagate back into the hose because
there is no oxidizer inside the hose. In addition, the pressure in the hose is above
atmospheric pressure, meaning the ambient air cannot suddenly flow back into the
hose. It should also be mentioned that flashback arrestors mounted on the regulators
will not prevent hoses from free flowing oxygen or fuel in the event they are cut.

4. If users prefer to not use torch mounted flashback arrestors, at a minimum, regulator
mounted flashback arrestors should be used. This is with the understanding that the
hoses are not protected from explosive flashback.
Summary –

1. Locating flashback arrestors at the torch end is the safest place.
2. The use of flashback arrestors at the torch and regulator may cause flow restriction
leading to a potential hazard.
3. Flashback arrestors mounted on the regulators do not prevent hoses from being cut
and gases to free flow.
4. A flame cannot propagate in a hose with pure oxygen or fuel.
5. Users may locate flashback arrestors on the regulators with the understanding that
the hoses are not protected from explosive flashback.
David Pryor