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Leaking union joint

Posted by thomas_e (My Page) on
Fri, Jul 27, 12 at 19:07

Leaking union joint

I re-plumbed a new pressure regulator but...

I am having trouble (leaking a few drops per minute) with the bottom union joint of this new pressure regulator.

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Both unions use a rubber seal. The bottom union leaks ever so slightly, perhaps a drop or two per minute ?, in spite of what seems to me extreme tightening.

The main water supply comes from the bottom in 1" PVC. The top union joint is ok, though I had to tighten that one too pretty well with a pipe wrench before it stopped leaking.

Bottom union is at 100 PSI water pressure, top is at 55 PSI. The branch to the left after the pressure regulator goes to irrigation subsystem. The pipe to the top continues and becomes the main water supply for the house.

The unions are new and the rubber seals are in good shape (I took apart a few times to confirm that I had not accidentally damaged them).

The whole setup is also pretty well aligned. My first re-plumbing attempt resulted in slight angle misalignment and I had trouble getting the union joints to seal, so I replumbed again by exposing about 3 feet of the PVC main line (now buried in the picture), cutting the PVC and regluing the PVC with the regulator tight in place to achieve a stress free union alignment. Alas, still leaking a bit.

Any advice greatly appreciated.


Follow-Up Postings:

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RE: Leaking union joint

Are you using two wrenches? One to hold the valve, other to turn the nut. Also, might want to smear a smidge of plumber's grease on the threads (only the threads).


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RE: Leaking union joint

My guess is that I have the 1" union joints tightened to something like 50 lb-ft torque but without any plumber's grease (is that the same as teflon paste?). I'm reluctant to tighten rubber seals a lot, unless I hear it's ok. I'm afraid an overtightened rubber seal may fail soon. This is my first time dealing with union joints so I have no experience as to how much they are typically tightened. I tend to think that it should not take much compression to get rubber to seal.

There seems to be friction in the nut threads, perhaps the machining is not that great on these parts, hence some grease might help?

What was surprising (to me at least) is that there was a difference in tightening resistance with the water pressure on vs. pressure off. When I turned on the bottom ball valve, the union joint nuts became much more difficult to turn. I would have not thought a mere 50-100 PSI would make a difference.


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RE: Leaking union joint

Code requires a backflow preventer on that top horizontal line to the irrigation system.


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RE: Leaking union joint

Can't the backflow preventer be placed further downstream? Also, all the irrigation circuits downstream have anti-syphon valves.


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RE: Leaking union joint

Well, after a few days, without any intervention, the bottom union has almost stopped leaking. It now leaks so little that the water evaporates before running down the pipe - as you can see the soil is dry. Yet there is still a tad bit of moisture to the touch (no it is not condensation). So I'd like to apply some plumber's grease to the threads see if I can get the nut to turn some more.

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Is this the appropriate plumber's grease ? It should be something that does not harden. I don't plan to apply it to the rubber seal, but I should use something that is harmless to the seal nonetheless.

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I went to Lowes and asked for some lubricant to apply to the threads of a union joint (for the purpose of lubrication, NOT sealing) and they gave me Teflon pipe joint compound. I believe it also does not harden completely but I don't think that is what I need.


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RE: Leaking union joint

They heard "threads" and gave the automatic response of teflon tape. But not for the nut on a union. That would just interfere with properly tightening the union. (Same goes for those braided water supply lines under sinks - they don't get teflon tape either (the seal is made by the washers), but a tiny dab of plumber's grease helps to hand-tighten the nut before you use your wrench for the extra twist.)


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