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Old 10-28-2011, 01:46 PM   #1
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Resealing of black iron union connection


I would like to rotate the plumbing downstream of a black iron union by loosening the large nut of the union and then rotating the downstream piping. The upstream plumbing will not be moved.

After rotating the pipe and then retightening the large union nut, will the connection be sealed properly? If not, what must I do to seal the connection from gas leaks?

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Old 10-28-2011, 04:49 PM   #2
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Resealing of black iron union connection


All you have to do is re-tighten it. Be sure to test it for leaks afterwards.

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Old 10-28-2011, 10:05 PM   #3
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Resealing of black iron union connection


I like to put a very thin skim coat of pipe dope on the mating surfaces even on new unions. Sometimes they aren't perfect, and when they're old, they're even less perfect.
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Old 10-28-2011, 10:52 PM   #4
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Resealing of black iron union connection


Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.


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I like to put a very thin skim coat of pipe dope on the mating surfaces even on new unions. Sometimes they aren't perfect, and when they're old, they're even less perfect.
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Old 10-29-2011, 12:31 AM   #5
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Resealing of black iron union connection


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Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.
I will keep that in mind for my own personal judgement. But if my boss tells me to put dope on the mating surface, I will.
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Old 10-29-2011, 12:49 AM   #6
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Resealing of black iron union connection


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I will keep that in mind for my own personal judgement. But if my boss tells me to put dope on the mating surface, I will.
I've done it the same way you do for the last 15 years. The master I apprenticed under has been doing it for about 25 years, and he learned it from a guy who did it for 40 years and so on, and so on...

I'm trying to figure out why pipe dope is OK on the threads, but not on the union. The only reason to put pipe dope on the threads is to fill in the imperfections in the threads - the same rationale for using it on the mating faces of a union.
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Old 10-29-2011, 02:15 AM   #7
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Resealing of black iron union connection


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I'm trying to figure out why pipe dope is OK on the threads, but not on the union. The only reason to put pipe dope on the threads is to fill in the imperfections in the threads - the same rationale for using it on the mating faces of a union.
It's because the threads of a union aren't what create the seal.
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Old 10-29-2011, 10:02 AM   #8
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Resealing of black iron union connection


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It's because the threads of a union aren't what create the seal.
But what creates a seal on a coupling?
I put pipe dope on all my unions too. As well as compression ferrules and nuts, ground joints on p traps. A little dab is all.
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Old 10-29-2011, 11:27 AM   #9
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It's because the threads of a union aren't what create the seal.
He's talking about : Why is it ok to put pipe dope on a threaded connection, but not on the mating surface of a union.
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Old 10-29-2011, 01:55 PM   #10
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Resealing of black iron union connection


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It's because the threads of a union aren't what create the seal.
We were talking about putting pipe dope on the mating surfaces of a union. And you said, "Pipe dope on unions, and any other similar fittings, is a HUGE no-no". Why?

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Old 10-29-2011, 04:32 PM   #11
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Resealing of black iron union connection


Re: Ishmael's response: Pipe dope is a no-no because it dries out and/or hardens over time. Unions are a purely metal on metal seal, just like a flare fitting, and as such do not require dope. If they do, something is wrong. Do any of you use pipe dope on flare fittings? I sure hope not, although I have seen dope and even tape on a flare fitting, which I red tagged. The only substance that should be used on these sorts of fittings is oil or vaseline, which doesn't dry or harden, and enables the union to be removed later on as it will prevent seizing. You guys can do whatever you want with your own fittings, but I posted here to let DIYers know that it is not the proper way to do things. The fact that some have been doing it for decades makes no difference, it's wrong.
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Old 10-29-2011, 04:57 PM   #12
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Resealing of black iron union connection


Is this by anychance for gas? If so, I hope this union is not under the house or in a wall. It's my understanding that any union for a gas pipe has to be outside.

As for the union....the mating surface is a machined match and I believe one side is usually a slightly softer material so that it conforms to the other side for a better seal.

Also understand that pipe dope and teflon tape are not what is doing the sealing....they help a little...but their real purpose is to lubricate the threads so that you can get more turns on the pipe and better compression of the thread connection.

Case in point...see how many turns you can get with a dry thread....then do it with teflon or pipe dope. Also, see how hard it is to break that dry connection loose vs the dope or teflon connection. Same thing with stainless steel connections....you need to use antiseize compound, otherwise, the SS connection will never come loose....and will most likely leak...
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Old 03-26-2012, 10:26 AM   #13
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Resealing of black iron union connection


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Originally Posted by hvac benny View Post
Pipe dope on unions, and any other similar fittings, is a HUGE no-no. One of the problems with pipe dope is that it dries out, and when it does, you'll have a leak. If a union leaks, throw it out. Also, never mix union parts, ie. the female fitting goes with the male fitting and neither can be replaced by a male or female end from another union. I work on gas systems up to 60 psig, and the only thing I use on unions is vaseline to ensure that it can be taken apart later, if necessary.
They are machined in pairs?

I think I found my leak!

I put the female side on, and when I went to install the male side the threads were junk. So I grabbed another male and put it together, without changing it's mate on the other side.

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Old 03-27-2012, 07:55 AM   #14
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Resealing of black iron union connection


They are called Ground joint unions for a reason. A little oil or Vaseline OK. Nothing else.

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