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?
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.
I have had a few of those, which are old and abused by previous people...
I found that if I put a couple of turns of ptfe on the bullet shaped part so that the tape "sits" between the bullet nose and the cone then it helps seal the imperfections and stops any weeping.
Make sure the threads are clean as well.
I know others say no tape or sealant needed and for new joints I absolutely agree, but for old "butchered" ones it is a different story.
This is a "union". It relies on the interface of the bullet-shaped end and the socket. There's no call for sealants as the metal is supposed to deform enough to seal. Neither tape nor thread compound are intended to seal between two parallel surfaces anyway.
I'd maybe try some metal polish or rubbing compound (if anything--it should seal as-is). Buff up the surfaces and try again. Be sure to use two wrenches* so you don't stress the pipe or solder joint, and remember that this is just copper, which is fairly soft. Don't go ape on it. (The nut is brass, but the same advice applies.)
You could put some anti-seize or dielectric grease on the threads to reduce turning friction, which could make getting a seal easier.
* In general, and here in particular, avoid adjustable wrenches and pliers. Copper will deform with the least coercion, so don't give it the chance. Use open-ended wrenches of the proper size and engage them fully. In the event that you do booger it up, switch to actual pipe wrenches, which grab harder with their teeth as you apply force, in contrast with most other turning tools.
I can think of a couple of ways to make it seal.
In a 1960s Popular Mechanics magazine, it was suggested that you place a round, thin piece of celluloid —or equivalent thin plastic— from a pack of cigarettes between the two mating surfaces. That will form a seal between them. Trim it small enough that it doesn't interfere with the threads. Tighten the union. The water pressure will break the plastic so that water will flow through the union.
In lieu of that, you might try putting a little anti-seize compound in the nut. Both the surface that contacts the mating part as well as the threads. This just might let you tighten it a little more so that the mating angled surfaces seal.
Step 1: Clean the threads. As others said, not with a grinder or sandpaper, just with a clean cloth. Hold tightly and twist.
Step 2: Put plumbing tape around the male fitting.
This works for me just about 100% of the time. It only fails if the union is damaged.
Some prefer pipe compound to plumbing tape. Personally I find it much messier and difficult to apply correctly with no obvious advantage, but whatever. I'm sure there are those who will explain to me why it's better.
If that doesn't work ... Really I'd get a new union, it's probably damaged. You could try putting an o-ring where the male and female meet. It would have to be big enough not to fall in but small enough to fit. But I'd see that as a jury rig more than a true fix.
