Question just about the order of pipe dope sealant and teflon tape

I should also note that the original fittings certainly had Teflon tape on them, presumably from the professional installers.

I did have leaks in the two inlets to the pool pumps though; but who knows why (hopefully they're fixed with the new fittings).

I didn't know what people use!

Thanks for that advice.

Caryn:

"NPT" means "National Pipe Thread", and NPT threads are a tapered thread, like this:

A waterproof seal is achieved by some soft material (like teflon tape or pipe dope) being compressed between the male and female threads as the joint is tightened.

Other threads, like the threads on a bolt or a light bulb are not tapered:

In plumbing, you can tell which threads to put teflon tape, pipe dope or thread sealer on by just asking yourself the question: "In this connection, is the water tight seal made by the threads themselves?" If so, then you need pipe dope, teflon tape or thread sealer on those threads. If not, then you shouldn't put anything at all on those threads.

So, in the case of the union shown here:

The two threads where pipes screw in at each END of the union are NPT threads and would need pipe dope, teflon tape or thread sealant on them because it's the threads themselves that make the water proof seal at those joints.

However, the coarser thread on the collar of the union is not a tapered NPT thread and doesn't need pipe dope, teflon tape or thread sealant on it because it doesn't make a water proof seal. The water leakage path through those collar threads is prevented by the O-ring shown inside that union (as seen in the picture). It's that O-ring that prevents water leakage through the collar threads, not the threads on the collar wedging themselves together as the joint is tightened like NPT threads do.

So, with the above union, you would put pipe dope, teflon tape or thread sealant when screwing the union onto the pipes it's meant to connect. But, you wouldn't put anything on the collar threads when connecting the two halves of the union together. The teflon tape, pipe dope or thread sealant would prevent leakage through the pipe threads, and the O-ring prevents leakage though the collar threads.

Hope this helps.

Pipe wrenches have the wrong jaws, too. They're curved, have a moving jaw, and have teeth, all of which are designed to bite into a round pipe. None of which is required for unions and certainly isn't wanted for PVC.

" professional installers" ??? yeah, right! from the layout, they look like hacks

Part of the problem is taking pictures without getting the goop on the camera, so I tried to use only one hand for putting on the goop while the other hand held the camera ...

:)

Danny D:

From what I can see, you're putting on way too much sealer on those threads. I don't think it will do any harm, but unless you're catching what comes dripping out of that joint as you tighten it, then you're just wasting the stuff.

I would put a much lighter coat of thread sealant on your male threads.

And, of course, I'd try to be sloppy about it.

Depends on when it was built but with that hidden cover storage & automatic cover.... 200k ain't far off.

Actually, two things work in our favor.... threading machines have automatic stops & molded parts are consistent (and hopefully the tooling was "proofed").

Of course there is always some variability of the parts & how much tape (that's dope is better for plastic) but the number of turns to "make up" after finger / hand tight is pretty consistent.

Pipe threads have a standardized / designed thread form.....well, unless they come from China :)

Seriously pipe threads are very consistent unless made by a total hack shop...which is happening less & less as 3rd world mfrs learn (ie get parts rejected)

IIRC pipe threads might even have torque limits for "tight joints"...too lazy to look it up.

cheers Bob

Missed this...

Are the union threads NPT also?

No, threads on unions are some sort of straight thread. The purpose of the union thread is to provide a means to compress the seal; o-ring, brass insert or union material depending on design of union.