Flash Back -- Reverse Flow Devices Question

[Boucher]

I had these devices on all my torches when I was in business. OSHA carried a big stick in these parts for a while. Now That I am retired what are the pros and cons of their use?

[ahidley]

If your torch pops and the flame goes up inside the hose, probally the O2, it will burn up into the tank and that would be really bad. A fire could also be caused by using a cutting torch and a melted ball(spark) rolls on the floor and melts through the hose. Thats a good reason to keep the hose off the floor. I use 4 arrestors on mine, two on each hose. One loacted right after the torch which prevents flash backs when the flame pops and goes out. This will save the hose. The second one is right after the regulator. This prevents the tank from exploding if the hose is broken or melted through causing a rupture.

Cons are I notice alittle restriction when iniatally turning on the gas flow. But once its flowing (check valve open, ball off the seat?) I see no differance.

[torker]

Our welding college here has probably the best collection of O/A created disasters anywhere.
They've got a display of blown up O/A tanks/hoses/torches from all over BC. Each one has a story behind it...many have fatalities attached to the story.
Without a doubt...O/A outfits are the single most dangerous things in a welding shop.
Anyone who discounts this as something trivial is a fool.

There are NO cons worth worrying about.
Russ

[radkins]

Don't get back-flow valves and flash arresters confused, the back-flow valve attached at the torch will not prevent backflash as it's purpose (and a very important one) is to prevent the higher pressure gas from leaking back into the lower pressure hose and thus causing mixing of the fuel and Oxygen inside the hose. This usually is the Oxygen leaking back into the fuel hose but it certainly can be the other way if the fuel hose pressure becomes momentarily higher than the Oxygen. Flash arresters however are usually located on the regulator and will sense the pressure change as the "flash" travels back along the hose and will shut off the flow and block the spark from the tank before the flame reaches it. Both of these items should always be used since the back-flow valve is there to prevent mixing of the gases inside the hose and help prevent backflash in the first place and the flash arrester to keep the flame from reaching the tank if a hose backflash does occur. Due to the fact that back-flow valves are usually quite a bit cheaper than flash arresters I have seen people use them at the regulator in the mistaken belief they will prevent backflash from reaching the regulator but this is not the case and a true flash arrester is required at the regulator while the much smaller, and different purpose, back-flow valves should be used at the torch for a complete system.

[pressurerelief]

Several incorrect statements here.

"If your torch pops and the flame goes up inside the hose, probally the O2, it will burn up into the tank and that would be really bad."

Oxygen does not burn, never has, never will, it is an oxidizer and aids in combustion but you can not have a fire without a fuel. Oxygen is not a fuel. The fire tetrahedron shows this.

"Flash arresters however are usually located on the regulator and will sense the pressure change as the "flash" travels back along the hose and will shut off the flow and block the spark from the tank before the flame reaches it."

Flash arrestors do not shut off the flow of gas. A flash arrestor is a heat sink that absorb the heat of the flame propagating through a system and quenches the flame by absorbing the heat. The most prevelant flash arrestor is the mesh screen you see in an "UL" approved safety can for gas, diesel etc. A flash arrestor is designed for a particular gas, pressure and flow. The more gas that is flowing or the lower the ignition temp of the gas the larger the surface area that is required for the arrestor. Some modern day flash arrestors/ flash back preventers have an integral check valve built in and are combination units but not all. Consult your local supplier when purchasing.

A check valve works on pressure differential between the two sides of the valve. A preset pressure is usually identified on the valve as the cracking pressure of the valve. A pressure differential above this number must be maintained in order to keep the flow of gas constant. When the pressure differential drops below this value the valve closes via a spring arrangement. When downstream pressure is higher than the supply side pressure the valve will remain shut due to the pressure overriding the spring. What all this means is if you are flowing gas and a flame propagates up the system, the check valve will remain open and let the flame pass the valve as long as you have adequate gas flow.

If you have a failure at your downstream of your regulator and have an abnormal flow of gas you need and "Excess Flow Check Valve". This is a third type valve that has a ball that gravity keeps seated until the flow of gas overrides gravity and seats the ball vertically in the valve thus stopping gas flow. You find alot of these on larger containers of flammable gases, liquiefied flammable gases, etc.

If you want to assemble a safe system you should have a flashback preventer/flash arrestor and check valve combination on both lines, and an excess flow check valve on your fuel line.

PressureRelief

[radkins]

"Flash arresters however are usually located on the regulator and will sense the pressure change as the "flash" travels back along the hose and will shut off the flow and block the spark from the tank before the flame reaches it."

Flash arrestors do not shut off the flow of gas. A flash arrestor is a heat sink that absorb the heat of the flame propagating through a system and quenches the flame by absorbing the heat. The most prevelant flash arrestor is the mesh screen you see in an "UL" approved safety can for gas, diesel etc. A flash arrestor is designed for a particular gas, pressure and flow. The more gas that is flowing or the lower the ignition temp of the gas the larger the surface area that is required for the arrestor. Some modern day flash arrestors/ flash back preventers have an integral check valve built in and are combination units but not all. Consult your local supplier when purchasing.

A check valve works on pressure differential between the two sides of the valve. A preset pressure is usually identified on the valve as the cracking pressure of the valve. A pressure differential above this number must be maintained in order to keep the flow of gas constant. When the pressure differential drops below this value the valve closes via a spring arrangement. When downstream pressure is higher than the supply side pressure the valve will remain shut due to the pressure overriding the spring. What all this means is if you are flowing gas and a flame propagates up the system, the check valve will remain open and let the flame pass the valve as long as you have adequate gas flow.

If you have a failure at your downstream of your regulator and have an abnormal flow of gas you need and "Excess Flow Check Valve". This is a third type valve that has a ball that gravity keeps seated until the flow of gas overrides gravity and seats the ball vertically in the valve thus stopping gas flow. You find alot of these on larger containers of flammable gases, liquiefied flammable gases, etc.

If you want to assemble a safe system you should have a flashback preventer/flash arrestor and check valve combination on both lines, and an excess flow check valve on your fuel line.

PressureRelief


You and I said the same thing just in a different way except for one item, the flash back or flame arrester. I said the flash back arrester will block the spark/flame and I guess from the way I had it worded it sounded like I meant that it does so by blocking the gas flow but I meant that it blocks the flow in addition to stopping the spark/flame. Now here is where you have me puzzled, you state the arrester does not stop the gas flow but I have never seen one (regulator mounted) that does not also stop the flow of gas when actuated, indeed they have to be reset before flow can be restarted.:confused: The only flash arrester I have seen that does not stop the flow is the liquid type used for manifolds that have the gas passing through a liquid as the means of blocking the flame. As far as the back-flow or check valve that is exactly they way I meant it I just never took the time to explain how they worked.

[pressurerelief]

Radkins,

There are all types of flash back preventers/arrestors, check valves, etc. I tried to keep my comments to welding gas cylinders. The simplest flash back arrestor is a bucket of water. Bubble the gas through it and fire will not propagate back to the source. Not a very applicable appliance for welding though.

I design and build the gas systems we use at work to decommission gas cylinders and vessels. We deal with just about anything they want to put into a cylinder from Uranium Hexafluoride to Fluorine to Silane.

P/R

[radkins]

I had no doubt as to your qualifications as it is obvious you know what you are talking about I was just puzzled as to why you said I had made incorrect statements? I was simply trying to point out the differences between a flash-arrester and a back flow valve, or check valve if you prefer, and point out a common misunderstanding between the two that has been the cause of misuse of the back flow or check valve. I have seen numerous times where people have placed check valves at the regulator thinking they were a cheaper way of accomplishing the same thing as a flash arrester which is simply not true. I know well how they work as I have worked with the things for nearly forty years and where I erred was in not taking the time to explain how they work, which you did and in a way that is very easy to understand.

[Boucher]

Thanks Guys for the explanations. I will now go see what I have. Some of the ones on the Torches have been removed. There are some that were never installed. They appear to be different in length. I will just have to check it out. I do appreciate the explanations. The local welding supply had a ruptured accetylene tank setting by the door where you walk into their showroom. Don't remember the story but I would not have wanted to be present when it occurred.