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Thread: Cast iron no hub couplings show leakage... why?

  1. #1
    DIY Senior Member DavidTu's Avatar
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    Default Cast iron no hub couplings show leakage... why?

    We just did our pressure test for the DWV system. It is a two-story house w/ basement and there is a lot of 4" cast iron. We filled up through the roof w/ water and it was holding for over 2 hours awaiting the inspector. Anyway, by the time we were done he noticed a coupling at the bottom of the stack was leaking. Didn't seem too concerned about it and didn't flag it on the inspection, which he signed off on. However, when I looked more carefully (we were using a sausage and so a little water at the base was expected from "sausage drip") the coupling was indeed leaking. I tightened it up again w/ the 60 lb torque wrench and it stopped. However, I noticed that three couplings above also had some leakage.

    So, now I am wondering what is going on. Again, it's all sections of straight fittings here, all the couplings were originally tightened to 60 lbs. We have run the test twice before without any leakage noticed. The stack was assembled a long time ago... as much as a year prior! Trouble is that part of it is now framed-in a bit and cannot easily access with the torque wrench. I am wondering if the couplings just relax a bit over time. It always seems that the torque wrench applied to any previously tightened coupling will tighten it up a bit before the wrench clicks.

    So if we have a water column through the roof w/ a lot of 4" for 2 hours time there is a lot of pressure. I can make the couplings accessible and tighten them up, if need be... but is there any point? Are they not just gonig to relax again enough to allow this high pressure test to leak again... if not immediately then in a year's time? I really do think these connections are very clean and the couplings in good condition. Am I justified in thinking this is just due to time and in actual usage we will NOT see any leakage, but only with the high pressure of the test, after the stack is sitting so long? Or is that nonsense?

    Thanks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    A typical household sewer line isn't full of water. And, what's there is (hopefully) flowing so there's little to no pressure on things. The casting may have been a little rough. If it was mine, and I know I tightened everything up to spec upon first installation, I probably wouldn't worry about it. Hopefully, one of the pros will chime in to confirm or deny my thought. It's possible that over that year, things moved a little through expansion/contraction cycles and maybe settling. If I had access, I'd consider just retorquing things, but if I couldn't, I wouldn't lose sleep over it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    The rubber could have just "adjusted" to the pressure of the band over that period of time. We usually assemble the system and test it within a matter of days so we do not experience the situation you have. Also, in most cases, we try to limit the pressure head to 10' which is all that is required. The only time it would EVER be subjected to that pressure again would be if the drain got clogged and you were somehow able to back the water up to the roof top.
    Licensed residential and commercial plumber

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Also I think if you keep checking torque past a click, I think you end up over-torquing it. I am not saying to loosen it, but don't necessarily think it wasn't tight to begin with.

    Chances thermal expansion/contraction and over-pressurization caused your troubles. Small weeps might even plug themselves with solids over time too.

    Jason

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    DIY Senior Member Smooky's Avatar
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    That type of coupling is designed for a maximum working pressure of 4.3 psi and that converts to around 10 foot of head, so you probably had a lot more than that from the basement to the roof.

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    DIY Senior Member DavidTu's Avatar
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    Well these are the sort of replies I was hoping for! Thanks everyone.... was afraid I was fooling myself into thinking there wasn't a problem when maybe there was.

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