Pipe union fitting leaks slowly. How to seal


Newbie to plumbing. I have this pipe fitting that is leaking very slowly through the top of the hex nut. I tried to tighten the hex nut with a pipe wrench but it didn't help.

I'm in the US. The pipe diameter is about 1 inch.

What is this type of fitting called?

How do I seal this fitting properly?

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Best Answer

That's a "pipe union", or just a "union fitting." It exists to provide an easy way to adjust pipe position or direction, or provide a way to insert and remove a fixture like a meter, without having to dismantle all the surrounding pipe work.

This particular model works by the threaded nut compressing the two parts together. The "shoulder" inside the nut pulls the upper part down, while the threads of the nut lift the lower part up.

The quality of the seal produced by the union depends on the condition of the mating surfaces being compressed together. So if you have a leak, I suggest you first aggressively clean the lips of the upper and lower part where they will go together. Note: I said "clean." Not "polish" or "grind" or "abrade" or in any other way damage. Just make them as clean as you can using a piece of clean cloth.

If that works (and it it likely to work if the parts of the union haven't been damaged by other activities) then you're done.

If that doesn't work, you might try "polishing" the surfaces with mild sandpaper or steel wool. Again, you don't want to be too aggressive, because if you score the sealing surfaces, they won't seal. If your abrasive puts score lines in the copper, each of those score lines is what they call, in the plumbing trades, a "leak." Do NOT put score lines in the sealing surfaces.

Finally, if the fixture will not re-seal, you can remove it and replace it. It is connected via sweat fitting, which joins the union to the straight pipe coming out above and below it. These are standard fittings, available in your local big box store. If you do not have the torch and other equipment to replace the fitting, or if you are not confident in being able to do the replacement without damaging a nearby fixture that the union is protecting, ask one of your neighbors -- it is quite likely that you could let the fixture leak for 50 years and not equal the cost of a professional plumber's charge to perform the replacement.

A reminder: unions are put into plumbing to enable inserting/removing some other fixture, like a meter or filter or appliance, especially with "sweat" and "glue/weld" type connections that permit no rotation of the pipes afterwards. So you need to look up or down to determine what the "other" thing is that the union was added to protect. Anything you do that might affect that thing must be guarded against. So, for example, if there is a "near the thing" and an "away from the thing" part to the union, you should sweat the "away from the thing" connection first, to get that much more practice before sweating the "near the thing" side.