[DCALLWELD]

I have a 2'' sch 40 gas line that has been in use for about 2 years now, I can open from flange just after the regulator and open a relief down the line to push a purge gas threw it .
Can I use argon for a purge gas?
Would I want to purge with air first?
How long should I purge with argon?
How mush natural gas contaminates the wall of the pipe and if so is there a threat?

Let me know what you think ?
Carbon steel pipe by the way

[gamble]

I have zero input on this, I'm just subscribing to see what others say. Hopefully from someone that has done it before.
Pics would be a must on this one.

[MinnesotaDave]

Do not purge with air - air contains oxygen, which is needed for combustion - do you want that mixed with natural gas?

I'm puzzled why you would be tasked with welding this pipe without training.

This is the method I use (for propane and gasoline, I haven't worked with any natural gas):

Full document: https://hssestorage.blob.core.windows.n ... urging.pdf

[Otto Nobedder]

Some more detail would help, but, generally,

Use (cheap) compressed nitrogen to clear the line of gas. Not air, as you don't want to introduce oxygen (for obvious and less obvious reasons). You CAN, frankly, weld on this line with nitrogen as the backpurge. There are many topics of discussion here that support this. There's virtually zero hazard welding this as long as you exclude oxygen from the inside of the pipe.

Also, what Dave posted about pressure purge is quite good. I use this method frequently, though I'm usually purging from "atmosphere" to "nitrogen". I've occasionally done it to change a pure hydrogen hazardous atmosphere to nitrogen.

Steve S

[noddybrian]

Interesting thread - I realize this may sound strange but since we have some gas specialists here may I ask - can liquid gas become impregnated into either steel or adhering contaminants in a pipe or vessel - the reason I ask & have cautioned others is I've on occasion had to weld such things ( liquid propane mostly ) & my initial purge due to cost here is CO2 - it also has the advantage of being cheap -( I don't always have nitrogen available ) - I never normally have issues using it & it has the advantage that you can " pour " it into things as liquid & allow it to keep expanding so replenishing what escapes - I try to use nitrogen if the weld is 100% penetration but the one in question was only adding some reinforcement to a branch fitting that had a suspected hair line crack from impact damage - I welded the piece without incident - but part way through welding after I'd put a fair amount of heat in the hydrocarbon detector I had inserted in a nearby fitting started going nuts - it had registered nothing on any fitting close & had been inserted in the pipe the entire time - after re-puging it quietened down but it appeared as a layman that there was initially no combustible gas present & the CO2 entirely filled the piece yet after some heating gas was present - amount unknown as the hydrocarbon monitor while very sensitive is not calibrated - the only conclusion being it was trapped in the metal & the heating action caused it to come out - sounds unlikely - but I had no other theory & have no formal training for such work ( yes redneck / potential Darwin award winner accepted ! ) is this the reason for the pressurized multi purge suggest by Dave ? or was it something I missed ?

[Braehill]

Yes gas ( CH4 ) will in fact impregnate the metal. You should weld anything that has had any flammable gas in it with a constant purge. Argon would be the first choice, but on Carbon steel you can use N2 or CO2.

If you have a flange at the regulator you should use a blank to be sure nothing gets by it. If you can break a flange further down line, blank it also.


Dave's procedure is dead on if a constant purge is not possible. The cost of the purge gas is cheap in comparison to a person being injured or worse.

Len


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[noddybrian]

Thanks for the info - I knew aluminum will absorb all sorts but I was'nt sure about steel - though it seemed the only logical explanation - if using CO2 I usually leave a small hole for venting & dump it in as liquid - that way as long as " fog " is coming out logically there must be close to 100% purge - if welding on flammable gas parts I do check with an audible alarm hydrocarbon detector so if the tone changes I know to stop & re-check things - I'm intrigued why you suggest solidly blanking both ends of something - I understand isolating the source - but in a similar situation I close the other end off with duct tape on the grounds it will retain the purge gas but would give any sudden expansion / flame / explosion somewhere to vent to at little or no pressure rather than trying to contain it thereby creating the chance for a very high pressure event.

[Otto Nobedder]

Our OP on this topic didn't state whether this repair was on a spool he could remove and take to the bench, or something that must be repaired in place

It's called "double-block, and bleed", and Len's brief but accurate description lacked some of the fine details.

When a flange cannot be moved, the gasket is removed and a "blank", either of soft metal like low-alloy aluminum, or steel/ss with gasketing is inserted in place.

The blinds are to prevent accidental introduction of the process material from either end of the pipe. The "bleed" (actually, "bleeds" in this case) provide a path to purge into and vent from the part being serviced.

If you google "double-block and bleed" you'll get very many results, for many purposes such as a valve changeout or an instrument calibration, or the welding purge this topic is about, but the general idea is the same.

I'm not sure I explained it adequately, so have a google of it...

Steve S

[Braehill]

Brian,
Sorry was checking out the forum while I was suppose to be welding a light standard. Bosses aren't usually around but today he ventured out my way just as I was typing.

I didn't, mean to seal it off completely, just to prevent anything from migrating back from down line. You will still need a weep hole for your purge gas.

There's nothing better than to have a constant monitor on your weep hole, whether it be an LEL or a hydrocarbon monitor. Good insurance.

Len


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[Superiorwelding]

Now these are the discussions I REALLY like!!
-Jonathan

[noddybrian]

Many thanks for your wisdom Len / Steve - in the scenario I was remembering I was lucky enough to be able to rotate the downstream pipe at a bend enough to separate the flanges - I'd blanked the downstream side with plate to guard against sparks so I had only to purge the piece on which I was welding & was also able to leave one end taped that could let pressure go - I see your point if the piece your welding needs isolating but can't be separated from a continuous line - it's probably a good case for don't take on things you are not qualified for - but over the years I've had various jobs that no one wanted to try & with a fair degree of common sense have done without incident ! been a good post - also worth pointing out most stuff mentioned here would apply to anything that had contained gasoline - I doubt many members / lurkers reading this will attempt welding on gas pipelines - but the same potential for accidents applies to a quick repair on a fuel tank & many will have done this - most will not have the luxury of a hydrocarbon monitor so are playing Russian roulette !

[Dblcorona]

Some good posts already. I really can't add much because it's already been said. Just backing up the prcedures that others have mentioned. We have always purged with Nitrogen and we would meter the pipe before any welding is to be done. Never really had issues welding the pipe after the all clear on the purge.

[DCALLWELD]

Thanks to all for the thoughts , as some one ask I can not take the section down it is in the ceiling of a building. What I'm thinking I can brake lose just after regulator where it comes jn building in mechanical room. There is a blender just after that I will cap the end that comes from regulator and hook up argon to purge nipple. I can shut off valve to second story and bleed out of a nipple down the line.
Question it branches off in to. 12 small runs. I will need to bleed them all out so no natural gas gets trapped in the end of a run correct. ?
The runs are all down the line from the area of repair.
Kinda like bleeding brakes on a car ?
Sounds elementary I know just looking for options.

[DCALLWELD]

Also I I'll be using a LEL meter .
Will pressure up to 15 psi 4 times the keep a constant 10 psi purge wile welding

[Otto Nobedder]

Question it branches off in to. 12 small runs. I will need to bleed them all out so no natural gas gets trapped in the end of a run correct. ?
The runs are all down the line from the area of repair.

Preferrably. But if all can be blocked (LOTO on the valves), the pressure/vent procedure is sufficient.


Steve S

[Otto Nobedder]

Also I I'll be using a LEL meter .
Will pressure up to 15 psi 4 times the keep a constant 10 psi purge wile welding

Good practice.

I assume you meant 10 CFH, as 10 psi would make the weld hard to close...

The presence of combustibles in the line is relatively minor, as by this point you've reduced them below the lower explosive limits, and the purge eliminates oxygen making combustion impossible. I speak as someone who welds on piping and vessels for liquid hydrogen service, with the widest "explosive limit" range there is.

Steve S

[forrestderp]

I weld on propane pipe for my current job, I typically either vent the line and blow it out with compressed air, or do a burn down of the gas with a burner before welding with 6010 root and 7018 cap on SCH80. The other day I had to add a T and a valve into a vapor line on a propane tank, the owner came out and asked the guy I work with how we were going to weld with propane in it, we closed off that line vented it, nothing more no explosions or funny business. Although not saying the guys I am working with do things proper in the least bit.
-Sean

[Braehill]

@sean,
Two things come to mind after reading your post. I'm never flying with Malaysia Air and I'm never working on your crew, just sayin'.

Len

[DCALLWELD]

Otto yes not psi lol my mistake thanks for the input welded a lot of pipe just not many repairs on inservice lines,
Thanks

[noddybrian]

@ Braehill
What are your saying ? Malaysian air lines are just a bit unlucky ( OK 2 in a year is REALLY unlucky ! ) - but Sean's crew sounds like the business - all that wasted purge gas they saved & the cost of monitoring equipment - they're saving the bosses thousands & if they do have a mishap he's probably got a bunch more cousins that would be glad of the work ! no good letting the safety nazis run the show - maybe they'll get on You Tube someday ( in the Darwin award section ! ) there's already a natural gas pipeline that some guy dug up with a backhoe - though he won't be doing it again.

[MinnesotaDave]

@sean,
Two things come to mind after reading your post. I'm never flying with Malaysia Air and I'm never working on your crew, just sayin'.

Len

@ Braehill
What are your saying ? Malaysian air lines are just a bit unlucky ( OK 2 in a year is REALLY unlucky ! ) - but Sean's crew sounds like the business - all that wasted purge gas they saved & the cost of monitoring equipment - they're saving the bosses thousands & if they do have a mishap he's probably got a bunch more cousins that would be glad of the work ! no good letting the safety nazis run the show - maybe they'll get on You Tube someday ( in the Darwin award section ! ) there's already a natural gas pipeline that some guy dug up with a backhoe - though he won't be doing it again.

+1 and +1

[Otto Nobedder]

+1, here, as well.

Don't "half-ass" anything where combustibles are involved.

Steve S

[forrestderp]

You guys would be very surprised at all the hack shit that is in place in the field. I know the guys i work for are idiots, but it is a job and it pays, I did however just have an interview with a mining operation company and a weld test lined up for next week, until I have another guaranteed job offer I am stuck with these monkeys. But honesty there are no codes nor inspections for welds on the piping attached to the tanks, hell you can use threaded fittings, and the amount of old systems that do leak is astonishingly high, as in I have yet to encounter one that doesn't leak a little somewhere. That's just the real world, just go out to the oil fields, people burn off tons of natural gas right off the separators, mainly because nat gas isn't worth much.
Anyway around it I don't see myself with this company for too much longer, I am a multi process 6G derp

[Otto Nobedder]

Nothing in the field surprises me. I worked industrial construction and the shutdown circuit for a few years.

On one Ethanol plant, my small crew worked Saturdays, but the primary crew (and their site safety people) did not. Needless to say, Saturday was when we did all the stuff the safety man would run you off for.

On another job with a different outfit, the safety man was drunk in the morning. Another one, the safety man wouldn't piss test you if you'd accept first aid, shut up about it, and go back to work.

Steve S

[winston weldall]

what are the best ( and least expensive while still maintaining a high level of sensitivity) hydro carbon detectors/monitors?

i assume these have an audible alarm?

thanks.

mike