“Suitable safety devices to prevent reverse gas flow and to arrest a flashback must be installed on each hose in an Oxy-fuel system, between the torch and the regulator.”
–Work Safe BC regulation 12.120 Flashback prevention
Click here for the publication from WorkSafe BC
There are three different types of burn backs:
1. Backfire: The flame goes back into the torch tip. You hear a pop sound. This is usually NOT a safety concern. When we are testing a torch after it has been repaired, we will cause it to back fire. The backfire only happens in the tip.
2. Sustained Backfire: The flame goes past the torch tip and into the torch itself. You will hear a hissing or squealing sound and/or a pointed flame.
3. Flashback: The flame is going past the mixer on the torch and/or into the hoses. If the flame reaches the hoses it will explode and rupture or separate the hoses. If there is nothing to stop the flashback before entering the cylinder, it could lead to an explosion.
The instigators of the flashback are the mixture of oxygen and fuel gases burning in the same tube. You can prevent this mixing by making sure the gas hoses are purged before and after every use. A second contributor to a flashback can be the flashback arrestor. In situations where high volumes of gas are needed, (e.g. heating steel) the arrestor (by design) will restrict the volume. This can cause the gases to creep back into the torch and possibly create a flashback. In these cases, it is better to put high volume arrestors on the regulator.
A check valve can only prevent the reverse gas flow. However, a Flash Back Arrestor prevents reverse gas flow AND arrests the flashback. A check valve cannot arrest a flash back. This could allow the flame to get past the torch and into the hoses.
After a flashback or a backfire, the arrestor cannot be reset – it must be replaced. Check valves are a spring-loaded valve, which allow the flow of gas one way but prevent it flowing in the opposite direction. If dust and dirt get on the valve seat than it can weaken the effectiveness of the check valve. Because these torches are in areas where they can easily accumulate dust and dirt, we replace the check valves on every repair. When the torch goes out, it has plastic plugs on the end to keep dust and dirt out.