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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?

codechimp
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2 Answers2

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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.

Ed Beal
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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.

Greg Hill
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