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Discussion Starter · #1 ·
i've never used a 3/4" union joint before (water). most places i've read/ watched about their installation, the joint doesn't need any sealant, yet i've seen some where a small amount of sealant was applied.


both places i've used them are leaking. i've already switched one place to a regular straight coupling and it's doing ok, but for the other spot, a union joint would work so much better.


should i try some sealant, or are machined (no gasket) union joints inherently leak prone?


thanks...gary
 

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Ive never had one leak but I've never had to deal with foreign made pipe fittings.


Try this: teflon tape the threads and oil ( any oil ) or grease the threads, the tapered surface of the male half and the back side of the male half flange and assemble with the male taper pointing in the flow direction ( not normally required but this isn't a normally ). Then tighten he!! out of it, you can't hurt it with anything less than a 4 ft. cheater pipe. and a 18" pipe wrench
 

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Discussion Starter · #3 ·
these were from McMaster-Carr. not exactly cheap. could you point me to a better product? within reason, i can deal with a better quality part if i knew where to find one. thanks.
 

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Ive never had one leak but I've never had to deal with foreign made pipe fittings.


Try this: teflon tape the threads and oil ( any oil ) or grease the threads, the tapered surface of the male half and the back side of the male half flange and assemble with the male taper pointing in the flow direction ( not normally required but this isn't a normally ). Then tighten he!! out of it, you can't hurt it with anything less than a 4 ft. cheater pipe. and a 18" pipe wrench



How are you tightening them?
 

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Those look like good quality unions from what I can see here. My guess is that if you could get them a little tighter they would probably seal up. The problem is that you often need 3 arms and 3 pipe wrenches to tighten them up, so if you can find a helper and another pipe wrench it might be a good thing.
 

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Unions are inherently prone to leaking especially in pneumatic systems.. I use them all the time and always apply some RTV ect to the mating surfaces. The unions with o-rings are great but more expensive. :wink2:
 

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i've never used a 3/4" union joint before (water). most places i've read/ watched about their installation, the joint doesn't need any sealant, yet i've seen some where a small amount of sealant was applied.
Ayuh,..... Are you Sure they're not leakin' from the pipe threaded connections, rather than the union connection,..??..??
 

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You need to tape and paste the NPT connections just like any other NPT threaded connection. This must be done first. Now for the union itself you can smear a little teflon paste onto the threads and seal. It is not acting as sealant here, just lubrication so the joint tightens a little easier.
You only need 2 good wrenches to tighten a union. If you need three you are doing wrong and your NPT connections are not tight.


Mix up a little dish soap and water 50/50 and brush it on the union. This will help you find which connection is actually leaking.


Last, the actual union tightening does take a bit of force. Put your 2 wrenches on opposing each other about a pie slice apart and squeeze them together.
 

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Discussion Starter · #11 ·
appreciate the input. from the comments i have a feeling i have not been tightening the unions hard enough and i'll also try applying a thin layer of Teflon sealant as it also seems logical that it would help mate the surfaces.
 

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appreciate the input. from the comments i have a feeling i have not been tightening the unions hard enough and i'll also try applying a thin layer of Teflon sealant as it also seems logical that it would help mate the surfaces.
That should work, and they almost have to be what I call gorilla tight. That should be self explanatory. :wink2:
 

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Ayuh,..... Are you Sure they're not leakin' from the pipe threaded connections, rather than the union connection,..??..??
The machined union surfaces are what seals the joint not the threads. The threads of unions are ONLY intended for tightening purposes, not sealing. :wink2:
 

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The machined union surfaces are what seals the joint not the threads. The threads of unions are ONLY intended for tightening purposes, not sealing. :wink2:
The OP's union per pic has 2 NPT ends that can also leak.
Bondo was just pointing out that maybe one of the NPT ends are leaking and not the union itself. :wink2:
 

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The OP's union per pic has 2 NPT ends that can also leak.
Bondo was just pointing out that maybe one of the NPT ends are leaking and not the union itself. :wink2:
OK but I never have an issue with the pipe threads so my natural thought was(and I've experienced this first hand)some people with pneumatic systems snoop the threads and think the union threads are leaking when the mating surfaces are but make it look like the tightening threads are leaking and this happens with fluids also.
I use loctite 569 sealant, and have never had a leak. It's very expensive but it works all the time and you can pressurize the system right away, it's about $40 for a 1.69oz bottle.:surprise:
 

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Have hundreds of pipe unions all over the chemical plant I work, in every size up to 3". They don't require sealant and sealant won't really help seal them. They do need to be tightened with a decent amount of force using two pipe wrenches.

What also helps a lot is to put a little anti seize on the threads but also where the top side of the nut pulls up against the fitting. The anti seize will lube the nut which makes tightening easier and also keep rust from locking it together in the future. Most pipe unions I deal with are outside, either instrument air, cooling water or steam fittings. If someone fails to anti seize the union we'll often end up having to split the nut with a cut off tool to get the fitting apart the next time.
 
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