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What is the risk of having pipe dope in natural gas line? Not blobs of dope, but a thin layer or bead on inner (female) thread.

Most instructions on using pipe dope on gas pipe says to put pope dope on male threads with no reference to female thread. While I understand pipe dope serves primarily to fill gaps in thread engagement, it seems like additional fillet of pipe dope at leading edge of male end, as fantasized in attached figure, would serves as additional sealant.

inner bead of pipe dope

Of course a big issue is overapplying and post-install inspection, but, provided the application technique is sound (big if), is there a risk to pipe dope or gas itself if there is residual pipe dope inside the pipe? Is it a violation of code anywhere?

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  • What sound technique would reliably eliminate obstruction at the joint and downstream? You'd have to have extremely precise application.
    – isherwood
    Jun 8, 2022 at 18:21
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    Because doing it that way is a doping scandal :P
    – Machavity
    Jun 9, 2022 at 14:17
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    What would having "additional sealant" be good for? Either the threads seal properly with a nice solid metal-to-metal contact that you can rely on for decades (the pipes themselves aren't going anywhere, if it doesn't leak right from the start, it never will), or they don't. If they don't, you want to know and fix it, not rely on "additional sealing" by means of a layer of a viscous liquid which can get disturbed at any point in the future and cause your house to go boom.
    – TooTea
    Jun 10, 2022 at 9:10
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    Additional reason is that iron pipe often has burrs on the cut threads. Applying sealant with your fingers is a risk. You should use a brush applicator, and that is hard to do well inside the fitting, much easier to see complete cover and no drips on the outside of the pipe.
    – Paul_Pedant
    Jun 10, 2022 at 16:49

2 Answers 2

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41

The pipe dope on the inside threads would be mostly pushed out of the threads into the fitting.

Now that blob ends up migrating to regulators and valves causing failures.

If only applied on the male thread the excess is outside the fitting. When completed none needed inside and nothing to foul regulators and burners.

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  • I think you meant to say "...is mostly pushed in," as the male end advances inward, which may end up partially reducing the capacity of the pipe.
    – Suncat2000
    Jun 10, 2022 at 14:54
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    @suncat2000 yes it could be said several different ways but the dope is pushed out of the threads yes it is migrating inward but I mentioned that also, we all look at things differently no matter how worded someone would suggest a change.
    – Ed Beal
    Jun 10, 2022 at 16:50
  • I'm going to mark this a "answer" over my own objection because the gist of this answer is "you will never NOT get a blob".
    – codechimp
    Jun 15, 2022 at 14:10
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The idea that a fillet of pipe dope as shown in the figure would enhance sealing is flawed. The pipe dope is a semi-liquid; it won't stay in place unless held there by some mechanical force. The pressure of a fluid (gas) inside the pipe should act to displace the fillet of dope, probably causing it to spread into a thinner and wider layer. Friction from the flow of gas should also tend to smear the fillets.

I'll admit that this is a little bit hypothetical in the case of fuel gas in a residential context because the operating pressure and flow velocity are so low - but there do exist higher pressure gas systems, and systems for other high pressure fluids too, and the same people with the same basic training work on them all.

Consistency of methods is important to reduce errors on the part of tradespeople making installations. Because pipe dope accumulated on the inside of the pipe won't help with sealing, and does create risk both in fuel gas and in liquid systems, the practice is to simply never apply pipe dope there.

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