bleeding gas lines on shut down

anchorman · Feb 9, 2011

I have a little dispute with a guy I work with, he thinks it is extremely important to bleed the lines on the mig welder when it is being shut down. I think this is a waste of time.

his argument is that the pressure (which is at most 20-25 psi) is hard on the lines. he says this is what he was taught, and that it also goes along with what he learned with scuba diving. but I can't find any info that agrees with this.

my argument is that I have never seen a failure of the gas hose or the regulator due to just shutting off the tank valve and calling it a night, and I had never heard this from ANYONE until he mentioned it. I have seen and worked with a number of machines, some that were close to 20 years old with all the original equipment that have ONLY been shut down by turning off the tank valve and leaving everything else set. It just seems like extra work and a waste of welding wire and time to me.

any thoughts?

aametalmaster · Feb 9, 2011

I was taught to bleed them 33 years ago, then one day we ran short on gas and my dad chewed my butt. Never bled them again on any mig welder i have run in any shop...Bob

slpslp123 · Feb 9, 2011

I never bleed them and no problems for the last 20+ years

Chris999 · Feb 9, 2011

Bleeding the lines because it's "hard on them" seems rather bizarre.

May have been pertinent 60 years ago when the hoses were marginal quality but not today.

Argon or CO2 are inert so they are not going to react with the hose material and 20psi or so is hardly going to stress the hose, which is probably rated for 300psi... If you were running the hoses very close to their pressure rating it might be a valid point but that's not the case.

Nothing is more aggravating than someone trying to tell you something because "that's what I heard" and not being able to explain why. Ask him to explain what he is saying.
Or counter with "I was taught it's ok to leave them pressurized as long as you wipe them down weekly with tail light oil..."

my 2 cents

Chris

dave powelson · Feb 9, 2011

The gas solenoid valves are not truly pressure tight and will bleed down all by themselves without human intervention.

I doubt anybody has seen a supply hose burst at 25-50 # pressure.

Matt@RFR · Feb 9, 2011

Maybe tell him that keeping your inert gas charged in the line will protect the line, solenoid valve and regulator from atmoshperic conditions.

Walter A · Feb 10, 2011

I have never heard of bleeding the line but rather than argue just tell him that he either has to leave a couple of residual pound of pressure or install a backflow preventor to keep the atmosphere out of the lines. Tell him to pick it at the local plumbing supply house.

Walter A.

adama · Feb 10, 2011

Scuba stuffs a bit different as the regulator is in the mouth piece, the lines are at full bottle pressure, hence a bit of a different scenario.

Johann Ohnesorg · Feb 10, 2011

Before pressure and age will affect the gas line someone will run over it with a car, a forklift or a truck and ruin it / the wire guide. Then you have to buy a new one anyhow. :bawling:

anchorman · Feb 10, 2011

thanks for the info guys... especially the part about the scuba hoses being at full tank pressure as opposed to the lower pressure post regulator. I will point out a number of these things and see where that leads.

Sea Farmer · Feb 10, 2011

I bleed the lines of flammable gasses (propane, acetylene) as a safety measure. Don't see the point with inert gas. BUT, sometimes it's just good to develop a habit and stick to it, especially if you are absent-minded, like me.

And if it's a shop with both types of gasses, and more than one person uses them, maybe a simple rule like "always bleed lines" will be safer.

anchorman · Feb 10, 2011

I'm with you on that, sea farmer, but there is a certain point where too many specific rules don't actually promote safety or good work habits. because eventually they will come across a situation where there are no set rules and not be able to reason their way to a safe way of doing things. It seems like with too many rules in place people end up not developing critical thinking skills, which is what we are in the business of teaching. The fact that stuff gets made is almost incidental to the fact that students learn to think critically and to act with autonomy in the world.

I wrote an email to someone at miller and he had this to say regarding mig/tig welder gas lilnes:

"There is no reason to bleed the line out when not in use, I’m usually lucky to see a customer who knows what a flowmeter is so you are on the correct path with your thoughts."

RFD · Feb 15, 2011

great post Anchorman, all too true.

kurcules · Feb 15, 2011

The only related failure I've ever seen is a leaky regulator that overloaded the low-pressure gauge because of a slow leak on the HP seat.

That said I don't bleed MIG, seems like a waste of gas. But I do blow down oxy-acetylene even over lunch because I don't want to risk unexpected release of even a small amount of acetylene.

TFPace · Feb 15, 2011

This has been an interesting read. I've never heard of bleeding a line on MIG application or any other process come to think of it.

Avanti · Feb 19, 2011

Hey, I still have most of my fingers!

kurcules said:
But I do blow down oxy-acetylene even over lunch because I don't want to risk unexpected release of even a small amount of acetylene.

I have never understood this. You want to bleed a tiny amount of acetylene gas all at once so it won't leak out slowly overnight. The amount of gas left in the regulators and hoses is so small it won't even make much of a pop, much less be a safety hazard. I know this because I have been doing acetylene balloons for the 4th of July for 40 years.

Certainly any leak of acetylene is not a good thing, as evidenced by the exploding van photos all over the internet, but it takes a huge amount of acetylene to do that. It takes a surprisingly large amount of acetylene to even blow the windows out of a car. BTDT (You can read all about it in the Aug '02 issue of Liquor & Guns magazine)

Walter A · Feb 19, 2011

True Story but a little OT

In the late 60s I worked as a new machinist in a machine/fabrication shop that at that time was nearly 100 years old. Since the surrounding main roads had widened and became higher the shop frequently flooded during heavy rains. The entire machine shop was actually a collection of buildings joined together with connecting walls and different height floors. The back shop was a sheet metal shop that one day really began to smell bad. All the gas cylinders and natural gas lines were checked and found to be ok but the smell persisted. The local fire department was called but even they could not find the source of the smell. A few of the guys started sifting through the collection of stuff under benches looking for maybe a dead rat when they discovered a rusted can of calcium carbide sitting in a puddle left over from a recent downpour.

In this shop nothing was every disposed of so this can had been sitting under the bench for many years since the last acetylene gas generator was long stored away. I'm not sure there was enough gas to cause much damage but it sure did stink the place up.

Over the years we did find a few dead rats trapped under some old discarded machine parts but no more carbide.

Walter A.

John Madarasz · Feb 23, 2011

anchorman said:
I have a little dispute with a guy I work with, he thinks it is extremely important to bleed the lines on the mig welder when it is being shut down. I think this is a waste of time....

my argument is that I have never seen a failure of the gas hose or the regulator due to just shutting off the tank valve and calling it a night, and I had never heard this from ANYONE until he mentioned it. I have seen and worked with a number of machines, some that were close to 20 years old with all the original equipment that have ONLY been shut down by turning off the tank valve and leaving everything else set. It just seems like extra work and a waste of welding wire and time to me.

any thoughts?

it may not be a big deal to shut off the tank at night, and reopen the tank upon the next use, but that's only ONLY if the back pressure in the hoses and on the regulator diaphragm remains the same...but if anyone has bled the lines after the tank has been closed, or there is a slow leak in any of the fittings that bleeds the lines without the regulator screw being backed off, and 1000+ lbs of tank pressure is introduced on a regulator diaphragm where the main screw hasn't been backed off, there is serious risk of damaging and shortening the life of a regulator. I know because I've done it...and damaged an o2 regulator diaphragm

fwiw

Stuart Caruk · Feb 28, 2011

I use, and repair CNC Oxyfuel/Plasma cutting machines on a daily basis and in many, many years, I have yet to see anyone bleed down any of the Oxygen, Actylene, propane, Natural Gas, or Nitrogen whether from a regulated supply or from a set of pony bottles on any of these machines, and oddly enough I have yet to see any damage come from it. I'm not saying it's not possible, becasue I understand the theory just fine. About the worst I see are blown Oxygen hoses form systems where they failed to install flashback protectors and a flashback occurred bursting the Oxygen hose. Which makes a cool sound if your aroun d by the way...

metalmagpie · Mar 3, 2011

I like keeping my system pressurized after shutdown overnight. It lets me see if I have any leaks the next day.

I have learned the hard way to soap-test all shielding gas connections. Amazing how expensive a tiny leak can be.

metalmagpie

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