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merchant pipe couplings

TONY_22 Member Posts: 28
That it was a gas company substation where some contractor accidentally used several of these couplings. It was a 55# gas install where small leaks at these coupled joints leaked to the point of setting off leak sensors. The subsequent investigation found merchant couplings as the culprit with 1 1/2" threads grounding out before sealing could be achieved.

This got all the way back to the pipe manufacturer as the Gas Company saw potential liability with similar gas lines ran using merchant couplings instead of tapered threaded couplings. I am sure there are tons of installs where they have been used. Something else to keep contractors up at night.

Rich K.


  • realolman
    realolman Member Posts: 513
    I wonder

    if someone would be kind enough to explain what "merchant" pipe couplings are good for, or more importantly, what they are not good for. They have no taper, but are threaded straight through.

    Every piece of galvanized steel pipe we get comes with one of these on it's end, and most of the couplings we have been getting are this type. They seem to be very common.

    One of the things I am puzzled about is that Mc Master Carr sells conduit couplings for $0.92 and merchant pipe fittings for $2.48. They appear to me to be exactly the same.
  • Tekkie
    Tekkie Member Posts: 58
    Merchant vs conduit fittings.

    Well, just guessing but conduit fittings just connect conduit while I think any kind of pipe fitting would have to meet some strength or fitness rating for the purpose.
  • TONY_22
    TONY_22 Member Posts: 28

    Cheap thread protectors on t and c pipe is all I can think of or for pipe handrails ??????
  • realolman
    realolman Member Posts: 513
    you mean

    that they put a coupling onthe end of every pipe that you're not supposed to use for anything?
  • Bob Sweet
    Bob Sweet Member Posts: 540
    Thread savers

    like Rich said, inspectors will not allow them to be used as couplings around here. Have used them in pinch but dont like the running thread idea.

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  • Tony Conner_2
    Tony Conner_2 Member Posts: 443
    Merchant Pipe Couplings Are...

    ... thread protectors for shipping & handling purposes. Nothing more. They are most certainly NOT pipe fittings. Take them off, and throw them in the scrap bin.
  • In Canada

    we call them "turned couplings", and Rich is right.

  • realolman
    realolman Member Posts: 513
    if you buy

    them seperately they cost about 2.50.

    Why do they put them on the pipe? Why not just a plastic cap and lower the price of the pipe?

  • They actually do have a use. They are used in a gas keyed-shutoff extension kit. If they are only on for thread protection, why are they only protecting one end?

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  • JohnNY
    JohnNY Member Posts: 2,972

    Those couplings cost the pipe manufacturers less than a dollar a piece.
    They act as an insurance policy that they won't get that length of pipe back due to a damaged thread or ovalled end.

    Some guys use them, but without the taper, they are certainly a substandard fitting when applied to a plumbing system.

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  • They can also

    be fabricated up for "nipple chucks" too, if a guy was stuck.

  • Ron Gillen
    Ron Gillen Member Posts: 124
    Emergency Nipple Chuck

    Great for when you need to make a short nipple in a pinch. Put a running thread on the end of a pipe, screw the thread protector on leaving about 3/8" female thread showing, screw in short nipple, thread the other end, screw the protector on to the pipe till the nipple falls out. Good for nothin' else.
  • Bob D._2
    Bob D._2 Member Posts: 34
    ASTM A865

    "Merchant" couplings are really legitimate steel pipe fittings for specific applications when they actually comply with ASTM A865. The problem historically, which gets them a bad rap, is that they do tend to deform when overtightened. Most fitters have grown up being told to "take 'em off and throw 'em away", but they really do have a use. You still have to be careful, because the Chinese (and others) made fittings might look the same, but not meet the Standard. Unless you're sure of source, use the "regular" fittings you are comfortable with.
  • Bill Clinton_5
    Bill Clinton_5 Member Posts: 38
    merchant couplings

    Been a while since I did much gas piping, but when I did, I always used those couplings that came with the pipe. Never a problem. American pipe fittings are for some reason built on the theory that the taper is needed to insure a tight fit and therefor a leak free joint. Somehow we forgot to tell the europeans because they neglect to taper their threads and somehow do just fine.

    I've worked with a fair amount of non-tapered thread European stuff and had no problem. It's actually easier to work with.

    As I see it, it is not the tight metal to metal force fit that comes from tapered threads that makes joints tight: Its the pipe dope or tape or string that does the job. Try making up a tapered thread joint with just a little oil for lubricant. Wrench it together good and proper and then pressure test it. Betcha it leaks. It's the dope, tape, or string that does the job, not the taper.

    None of this is to defend substandard couplings I have occasionally seen on pipe. Toss the junk, but junkiness is not defined by the taper of the threads.

  • realolman
    realolman Member Posts: 513
    we get

    pipe and fittings from who knows where, and after seeing guys put on silicone dope and then on top of that put tape dope, I gotta agree with you that seals are being made with pipe dope.

    I don't think that that is how it should be, though. I think seals should be made by metal on metal with the dope as a lubricant to get the metal tighter.

    But hey, if the stuff don't fit, which often these days it does not, you gotta do what you gotta do.

    I am so glad that some more of you folks responded. It's not a very glamorous subject, but it seems to me to be absolutely vital and basic.

    Truth is, I didn't realize that the fittings that came on pipe weren't tapered. I would much prefer to use the merchant couplings... I don't really know why... I think they look better on the finished job.

    My questions started in an electrical forum where it seemed to be consensus that rigid conduit did not exclude water very effectively because of the running threads on the couplings. I always thought that threaded rigid conduit would be the best means of sealing out water.

    Well, then I found out that the merchant pipe couplings are running threads also... for 2.5 times the price... and are included on every pipe, and now people say they're mostly useless, but we're paying for them on every piece of pipe????

    Now I'm thoroughly confused.
  • realolman
    realolman Member Posts: 513
    what kind

    of uses are they good for, please. I would rather use them, myself.
  • Tony Conner_2
    Tony Conner_2 Member Posts: 443
    If They Get Used In...

    ... piping systems in Canada that fall under an ASME piping or pressure vessel code, and they aren't registered as a fitting, then they're not to be used. I believe you'll find the same requirements in fuel utilization codes. Anybody using these things should verify that they are in fact actual FITTINGS, and suitable for the installation.
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