Welding compressed air piping - will a MIG welder work?

You seem not to be sure of this. Actually, you are using FCAW rather than MIG. Big difference.

Penetration is not an issue. Fusion, and porosity are issues.

What is the pipe? What schedule?

If you don't know MIG from FCAW, don't know the difference between penetration and porosity, and don't know what schedule pipe you are using, I'd pass if I were you. You might be opening a big bag of snakes.

Keep us posted.

Steve

You are asking for advice on welding what is in effect a pressure vessel. In this case if you need to ask you should pass on the job. Compressed air contains an enormous amount of stored energy and should be recognized as a hazard. The questions you should ask are "if my work fails who/ what gets hurts?" and "What is my liability". Personally I would see if the pipe can be replaced with threaded fittings, if not I would pass, or find a pressure vessel certified welder to bring in. I 've considered jobs that stretch my abilities or experience, over the years I'm getting better @ saying no because of the painful memories.

Enjoy

Andrew

You are right, I do need to look at this more. As much the welding, but the pipe I will be installing.

The pipe will be schedule 40, A106, black steel. Flanges and couplings rated at 150 pounds. I may not be familiar with the proper terminology, but not afraid of the work. The work is outdoors, so I am looking at the flux core rather than gas shielded.

My first question was gonna be, before I saw that Andrew beat me to it, why not use threaded fittings? They are designed for this purpose, and your assumption that the piping is only going to see 125 psi is a leap of faith... I routinely see pressure regulators set at well above the normal system operating pressure, but well within the ability of the system to handle it - maybe. You never know. What's the piping for? r/ Steve

The main work on this job is painting 1200 feet of 2" pipe that carries compressed air for running pneumatic tools. It runs on top of and down the length of a pier over salt water. There are two legs that go under the pier to provide air manifolds to the opposite side of the pier. These see direct exposure to salt water. These two legs are severely rusted, typical for the pipe location and lack of maintenance. It's a bugger to reach, high tide only and from some sort of floating platform. Because of the salt water exposure I want to put up something that will last a fair amount of time and that is why I am choosing welded fittings. No threads or thinner spots to rust through. Threaded fittings are allowed, but this is my choice.

As far as hazard goes, the worst thing that could happen if one of my joints fails is some fish would get scared and I'd have to fix it.

Another hazard is that the work would stop until someone is around at low tide to fix these pipes, which could be quite an expense. Also, if they start developing pinholes, a lot of air could be lost due to them and result in extra work/wear and ear on the compressor.

How about you do some experimenting, take a few realistic pieces of pipe (preferably with the right quantity of rust), and try to weld it, then try to find out the strength of the welds by various methods.

I am afraid that you would have some unpleasant surprises.

i

Why not just use threaded pipe?

What about using hose rated for chemicals and UV exposure? There are a million types of hose - look in McMaster for something that won't corrode in the salt. You can get them with bronze fittings.

Of course you would use steel fittings intended to be welded to the pipe? You also might consider including backing rings and leaving a gap around

1/16th at the root. That way you could ensure you had proper fusion at the root. A thin backing ring would not restrict the air flow too much and make your welding easier. Shop around for some light tubing such as automotive tubing that would match the inside diameter of the 2 inch pipe. You could cut one inch rings and they would help with alignment as well as acting as backing. Welding an open root with a flux core wire is not a normal thing to do. The reason is that the flux gets mixed up in the gap and ends up on the inside of the pipe as well as the outside and anywhere else it decides. Open roots are usually done with a process that does not use flux or E 6010 stick because the flux left behind is minimal and if used properly does not contaminate. By using a backing ring you are depositing a weld bead that will have flux on the outside where it is supposed to be. You have to clean thoroughly after each pass. To do a self test on practice pieces make up some joints and weld them up. Cut through them at random locations and inspect for slag inclusions or porosity. In most cases you will know if you put a bad pass in the joint. Don't think you can just weld over it. Grind out the bad stuff before doing your next weld pass. Make sure your cover passes are proper without undercut or too high a weld bead. If you can, do all your welding on horses while rotating the pipe. Limit the in- position welding to just the ones you absolutely have to do that way. Randy

"Wally" wrote in message news: snipped-for-privacy@4ax.com... I"m looking at a job that will replace about 50' of 2" black steel pipe that handles 125 PSI compressed air. There is 110 power at the site that would run my little Lincoln SP 100 just fine. I've got .030 flux core wire to run in it, so the penetration should be fair. I'd like to hear some opinions on whether this will do a satisfactory job. The only inspections will be visual and it can't leak. Thank you for your time.

Well, then it wouldn't be welding, it would be something for sci.engr.joining.threaded.

Bwahahah, I couldn't help myself ...

I would suggest using stick welding...e6010 root pass and e7018 for the rest.At least that how we do it at pipefitters local 72.