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View Full Version : Flash Arrestors vs. Check Valves?



yjweldor
03-03-2005, 08:48 PM
so what's the difference between flashback arrestors, and reverse flow check valves? like on my torch set up currently i have long flashback arrestors on the regulators, and shorter check valves at the torch....so what's the difference? they both do the same thing.....right? b/c there's a big difference in price between the two.

cnslmva
03-03-2005, 09:26 PM
Yes and no. Flashback arrestors will stop a backflow of gasses and they will also stop the much more serious flashback ( flame actually travelling back with the gasses). The check valves will only stop a backflow of gasses. If choosing between them, I'd go with flashback arrestors at the torch, Victor has them made in their torches. :)

Scuppers
03-03-2005, 09:54 PM
No, they don't serve the same function. And for safety's sake, one should have both.

Check valves simply stop reverse flow of the gases. Check valves, however, typically cannot stop a flame front traveling back through the torch and the hoses -- flashback arrestors serve that function. A flashback constitutes a serious safety hazard. If the flame reaches the regulators (it'll likely ruin them) and the cylinders, an explosion might occur. A prolonged flame inside the torch handle can actually melt it.

One may install flashback arrestors either at the torch or at the regulators, and check valves typically at the torch. If the torch lacks these safety devices, one may add them although, typically, you'd have to compensate for them when adjusting the regulators. Victor makes a style of torch, called Vanguard, that has a patented check valve-flashback arrestor combination.

Roger
03-04-2005, 12:32 AM
When I learned gas welding never saw flash back arresters or check valves on gas welding rigs. Now OSHA requires them and manufactures trying to prevent law suits include them with new outfits. Flash back arrester is a screen or sintered metal filter that flame can't easly pass through. Some but not all flash back arresters include check valves. Check valves prevent reverse flow of gas. This prevents mixing of gas in hoses preventing flame traveling up hose. Flash back arrester should be mounted at torch. Check valves can be mounted at torch or regulator but not both to prevent excessive flow restriction. Flash back arresters should be replaced if exposed to flash back.

yjweldor
03-04-2005, 06:44 PM
cool now i know the difference...now what would you guys say CAUSES a flash back in a torch? also, someone said something about a prolonged flame in a torch handle can melt it....well check valves OR flash arrestors would do nothing to prevent that....but would you say that operator error is usually to blame when it comes to flashbacks? i have never once experianced this...which i figure is b/c i half *** know how to use a torch....(actually i really do know the proper ways of using a torch etc..)...

trackwelder
03-04-2005, 07:47 PM
At work all torch setups must have check valves on the torch and arrestors on the regulators. Once I saw a fifty foot hose burn right to the regulators and am glad the company requires and checks to make sure they are in use.

bcraytor
03-04-2005, 08:09 PM
Flashback can occur when the fuel pressure drops suddenly or when the tip of the torch becomes plugged and the torch is hot. Oxygen, generally at a higher pressure, especially with a cutting tip, will overcome the fuel pressure and feed back into the hose. Acetylene, Oxygen and heat can cause the mixture to ignite, and a flame can travel up the hoses, through the torch body etc. Carbon deposits can be left in the torch, hoses etc from the enclosed flame. These carbon deposits can plug tips etc. This is an explanation of a portion of the result of flashback. Results may vary, void where prohibited.

moody
03-04-2005, 08:10 PM
hmm

ive never knew this

thats how those huge forced air heaters work with out blowing up

i always wondered why the flame didnt go back through the pipe and explode
i thought it had somthing to do with the pressure of the fues comming out of the vents

so Check valves and flash arrestors are on just about anything that runs off gan and burns, (stoves, grills, gas heaters, ect.)

you learn somthing new every day

hankj
03-04-2005, 08:25 PM
My GOD Moody! You scare the **** out of me sometimes!

We're talking about oxy/acetylene welding equipment, not BBQ grills! Why would you need a check valve on a grill? It only has ONE fuel source - the propane bottle, and NO pressurized oxygen supply! It draws the atmospheric air it needs for combustion by the venturi effect in the burner! What's to flashback????

Check valves prevent the higher pressure oxygen from blowing into the acetylene hose, which is under much less pressure in a cutting operation.

"Flashbacks" don't occur in heaters, grills, stove, weed burners, or ANY fuel-air appliance.

Awake, my son. Join the party.... :p :p

Hank

Mike W
03-04-2005, 08:44 PM
Hank, you just made me laugh again. :D

hankj
03-04-2005, 08:50 PM
Mike,

Did you get the e-mail I sent via the forum?

Hank

enlpck
03-04-2005, 09:40 PM
cool now i know the difference...now what would you guys say CAUSES a flash back in a torch? also, someone said something about a prolonged flame in a torch handle can melt it....well check valves OR flash arrestors would do nothing to prevent that....but would you say that operator error is usually to blame when it comes to flashbacks? i have never once experianced this...which i figure is b/c i half *** know how to use a torch....(actually i really do know the proper ways of using a torch etc..)...

Color yourself lucky. Flashbacks happen to the best of us on occation. You deal with it.

In ALL oxyfuel torches, the fuel gas and oxygen mix in the torch at the head (ya, ok, there are a few oddball exceptions, but I doubt anyone reading this ever uses one) This mix can ignite in the head if the velocity of the gas at the jet(s) is lower than the flame propagation velocity. The flame will then burn into the orifice faster than the gas pushes it out. This is accentuated by dirty/damagd tips, as a dirty orifice will have a lower gas velocity than a clean one, but can occur soley with too low gas pressure/too low gas flow. First time rosebud users learn this real fast.

You can see the effect of gas velocity versus flame propagation: Set your torch to a neutral flame. Up the acetylene flow (with the valve on the torch) and reset the neutral flame. The flame base will be farther fromthe tip. The base will be at the point where the gas expansion has lowered the velocity to equal the flame propagation velocity. If you ower the flow, the flame base will get closer and closer to the tip. Just below the point where the base touches the tip, you begin to get flashback.

hankj
03-04-2005, 10:16 PM
enlpck,

**** good explanation!

Even though Victor equipment deos not mix gases at the head, the flashback phenomenon occurs for exactly the same reason. Sometimes, flashback is so subtle that you don't realize it happened, especially with weldig tips. When you pull your welding tip off the Victor handle, look in the mixer. If it's full of soot, you've had a fire in there, and never knew it! One of the common causes of this in Victor stuff is turning off the acetylene first when shutting down.

Hank

moody
03-04-2005, 10:25 PM
Hank, you just made me laugh again. :D

LOL

that makes two of us.

i have no idea how O/A equipment works.


now i understand everything.

Zrexxer
03-04-2005, 10:59 PM
i have no idea how O/A equipment works.That, I believe.
now i understand everything.This, I don't.

yjweldor
03-04-2005, 11:51 PM
LOL now THAT was TRUE!

moody
03-05-2005, 12:02 AM
ok ok i still dont really understand O/A cutters but i do understand the propane

i thought it could happen to propane bottles because once i heard somone sprayed WD40 with a lighter infront of the tip and it blew up the can and then i heard somthing about the flame going back into the can causeing the explosion

but i guess this cant happen

Scuppers
03-06-2005, 09:30 PM
Color yourself lucky. Flashbacks happen to the best of us on occation. You deal with it.

In ALL oxyfuel torches, the fuel gas and oxygen mix in the torch at the head (ya, ok, there are a few oddball exceptions, but I doubt anyone reading this ever uses one) This mix can ignite in the head if the velocity of the gas at the jet(s) is lower than the flame propagation velocity. The flame will then burn into the orifice faster than the gas pushes it out. This is accentuated by dirty/damagd tips, as a dirty orifice will have a lower gas velocity than a clean one, but can occur soley with too low gas pressure/too low gas flow. First time rosebud users learn this real fast.

You can see the effect of gas velocity versus flame propagation: Set your torch to a neutral flame. Up the acetylene flow (with the valve on the torch) and reset the neutral flame. The flame base will be farther fromthe tip. The base will be at the point where the gas expansion has lowered the velocity to equal the flame propagation velocity. If you ower the flow, the flame base will get closer and closer to the tip. Just below the point where the base touches the tip, you begin to get flashback.
And hence the safer practice of using a smaller tip, rather than lowering the pressure/flow supplied a larger tip. "Starving" the tip tends to promote flashback, as does getting too close to the work (e.g., while welding in corners), a sooty tip (which tends to raise its temperature), etc.

Roger
03-09-2005, 09:29 AM
This is off topic about a 3rd safety device called a flow fuse not commonly used with welding equipment.
Flow fuse stops flow when flow is excessive or pressure difference is excessive between down stream side and up side. If your filling gasoline tank at "gas station" and hose breaks gasoline will flow out of broken hose until pump is turned off or flow fuse stops flow.