[kbinkster]

I know that this is an old thread, and maybe I should start a new one, but this thread is being discussed on another thread and there are some things that I would like to add.

Flashbacks are more likely to happen on premix torches, but they can certainly happen on surface mix torches given the right set of cirumstances. For a flashback to occur on a surface mix torch, there has to be a mix of oxygen and fuel inside one of the chambers of the torch. When it ignites, there is an explosion. Sometimes, the fire from the explosion travels upstream out of the torch and up one of the lines. It usually travels up the oxygen line.

It's important to understand how fuel and oxygen can mix inside the torch. There are at least two ways that this can happen.

The most common cause of the premixing that can cause a flashback is a leak inside the torch. If there is a leak inside the torch, gas from one chamber can enter another chamber and mix with its gas. Since we usually turn on the fuel first, it is most common for the fuel to leak into the oxygen chamber, instead of the other way around. When the torch is lighted and the oxygen turned on, there is a very loud "POP." That is the explosion inside the torch and it could continue to burn and go upstream and out of the torch. If you follow the POOP procedure (Propane, Oxygen, Oxygen, Propane) for turning on and shutting off your torch, the explosion will have happened in the oxygen chamber of your torch. This is part of the reason, I believe, that flashbacks tend to go up the oxygen line and not the fuel line.

But a leak is not the only way a flashback can occur. You can have a premix inside a surface mix torch even if there are no leaks. It is very rare that this can happen, but it can and does. If fuel somehow travels back into the torch, you can have a premix. A change in line pressures can sometimes cause this, too. Another thing to look out for is shutting off your valves all the way. If you ever turn your fuel valve off almost all the way but not quite all the way, a tiny stream of fuel will still bleed out of the torch. Sometimes, it finds its way back into the torch, going up in through the oxygen ports. If it does this, then it can pool inside the oxygen chamber of the torch and when you go to light up, you will have fuel in the oxygen chamber and this will cause a premix and possible flashback.

How do you know if the premix was caused by a leak or by the fuel bleeding back into the torch? Well, if it is a leak, it will happen repeatedly. If it never happened before, and has not happened since, then, the premix was likely caused by not turning the fuel off completely.

So, what about the role of flashback arrestors(FBAs)? Well, where ever the FBA is installed, that is where the FB will be stopped. If the FBAs are installed at the torch, that would keep the flashback from traveling further up the lines to the source of the oxygen (most likely path) and fuel (less likely, but I guess still possible). Some people insist on putting the FBAs at the regulators for added protection against line failure, but I don't agree with this.

The way that I understand FBAs to work is that the explosion from the premix that causes the flashback triggers the valve in the FBA to close. It's the pressure change (the burst from the explosion) that triggers the FBA to shut. If you burn through a line, the pressure actually drops in the line and the gas still flows. Unless you have a FBA that has the added feature of closing due to overheating, the FBA valve will not be triggered shut and it will do you no good.