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I'm an electrical engineer, working on repairing a somewhat complicated pneumatic system, but I figured this would probably be the best place to ask. This is probably a very basic question to anyone who works in plumbing, but I couldn't find anything about it online.

When you have right-angle pipe fittings, the kind with NPT threads that you screw together, how do you control what angle they screw in to? If you just screw two things together, the relative angle between them will be essentially random as it depends on the exact angle of where the thread starts and how far the threads engage. But we need to put in pipe at specific angles to align fittings to other parts of the system, so how does one actually do that?

Hearth
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    I had this question myself - I think the crux of your question is, how tight does a fitting need to be, and can I still adjust the angle at that tightness? Maybe this will help- https://diy.stackexchange.com/questions/12670/how-tight-should-threaded-brass-pipe-fittings-be – Aww_Geez Mar 31 '21 at 13:56
  • I don't understand the question. Tapered threads (any steel threads, really) self-align as you begin assembly. You'll find that there's a lot of rigidity in the components once they're even loosely assembled. – isherwood Mar 31 '21 at 13:59
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    @isherwood If that's the case, then there is no way to control the angle a right-angle fitting will be pointing when it bottoms out. I think the OP assumes the fitting needs to be bottomed out, and would like clarity on that. – Aww_Geez Mar 31 '21 at 14:01
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    @isherwood I'm talking about rotational angle. Imagine using two 90° fittings to offset a pipe by a little bit--how do you ensure that the pipes on both ends are parallel to each other? – Hearth Mar 31 '21 at 14:01
  • Consider making an [edit] to add a picture of the area in question. – Ecnerwal Mar 31 '21 at 20:31

2 Answers2

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There is a range between "tight enough" and "too tight" - most alignment for NPT threads is done by using that.

Then again, there are unions and compression fittings (Swagelok, and lesser versions) that remove the connection between the threads and the sealing surface. The vast majority of "complicated lab gas manifolds" I did many moons ago were done almost entirely with Swageloks.

Ecnerwal
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  • This, I'm afraid, is not a lab gas manifold, but a piece of specialized equipment that needs a valve replaced. There's not room for anything more than exactly what was in there before, which was a right angle fitting, a short length of copper tube, another right angle fitting, and a quick-disconnect that plugs into a desiccant column. No room for anything other than that. – Hearth Mar 31 '21 at 14:03
  • The first paragraph covers that. Thread compound or PTFE tape create a cushion of sorts that allows at least a full rotation of adjustability. Watch any video on black iron pipe assembly for clarification. – isherwood Mar 31 '21 at 14:14
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    i.e. The fitting has to be tight enough to not leak. Once you hit that point, you should have nearly a full 360° of rotation left before you hit "won't turn any further". Therefore, once sealed, you keep rotating until it's in the desired direction and all is good. – FreeMan Mar 31 '21 at 14:16
  • @FreeMan How can you determine when it's tight enough to not leak? I can't attach it while under pressure, because it can't be under pressure until it's fully assembled; the pressure path just doesn't give any way to connect it to pressure without also fully assembling it. – Hearth Mar 31 '21 at 14:43
  • One could, @Hearth, tighten it until it won't go any further, then back it off until it lines up. Based on isherwood's comment, there should be about 360° of rotation, so you should get it to lined up in any direction before you've backed it off a full revolution, and it should remain sealed. Testing for leaks prior to putting it into full service would seem reasonable, especially if one has to back the fitting off an uncomfortably large amount. – FreeMan Mar 31 '21 at 14:51
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    @Hearth With some practice one learns the feeling of "it's almost as tight as it'll go." When I get to that point I make a judgement call: if I can get the fitting clocked at the angle I want with up to a half-turn of additional tightening I'll continue tightening. Otherwise I'll loosen it up to half a turn. One can also loosen or tighten the fitting at the far end of the pipe, rather than the present fitting, to make up some of the desired clocking/angle. You might be surprised to find that a joint holds pressure well before reaching impossible-to-turn tightness. – Greg Hill Mar 31 '21 at 14:59
  • when you are replacing a worn out or broken device or fitting or any component in an already installed piece of equipment it is sometimes very hard to get every thing to tighten and align as it did before so that there are no leaks at the threaded joints. Depending on who or what machine cut the threads into a fitting or pipe, of either the male or female thread, will determine how tight the joint has to be. One extra turn will change the dimension. This is why in many instances the last few fittings/piping is done with a flexible material that can be cut to size and fitted into the device. – d.george Mar 31 '21 at 15:09
  • Ran out of space. In your line of work, If you need to add a component or move it's location slightly and it is piped with rigid conduit the installer may need to use Liquid -tight, BX, or MC flex to make the change. The only other alternative would be to replace the conduit to gain a few inches. Hope this helps. – d.george Mar 31 '21 at 15:20
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    @d.george I'm afraid you have my line of work wrong--i'm an electrical engineer, not an electrician! I do circuit boards, not conduit. That makes sense though. Unfortunately, I don't think that's an option here. – Hearth Mar 31 '21 at 15:26
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    @FreeMan I would have thought backing off would ruin the seal, honestly. Since the teflon has already been compressed into shape, and it's not elastic enough to return to fully seal the thread? – Hearth Mar 31 '21 at 15:27
  • @Hearth this is my interpretation of the answer and the comment referenced. Fortunately, I've not had to deal with a whole lot of (read: any) threaded connections.I'd hope that isherwood or Ecnerwal will pipe back in. (pun not intended!) – FreeMan Mar 31 '21 at 15:32
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    I prefer teflon dope to teflon tape these days. I prefered teflon tape to old-school pipe dope in earlier days. You either have to get someone who knows how it feels when pipe is tight enough, or become someone who knows how it feels, and there's always the possibility that you pressure it up, apply soapy water, pressure it down, and take another turn. How's the copper tube connected? That may be another degree of freedom. But assuming this is a "work" project, far more cost effective for work to hire a plumber/pipefitter to help the EE than to pay an EE to become a plumber.... – Ecnerwal Mar 31 '21 at 15:38
  • @Ecnerwal It's also screwed on with NPT connectors. And that might be the case, but the particular way this thing is broken (it's equipment built in 1990, from designs that date to the early 80s at the latest, and it hasn't been turned on in at least a decade, and the company that made it no longer exists), we keep finding new things wrong with it as we work on repairing it. We'd have to call a plumber in every other day... Some of the problems are also a combination of electrical and pneumatic, too. – Hearth Mar 31 '21 at 16:12
  • @Hearth It's a common belief that teflon tape seals the joint. IMHO it's really only a lubricant; the tight fit of the tapered threads is what makes the seal. Like Ecnerwal I prefer pipe thread compound (aka pipe dope) with PTFE rather than dry tape. Apply only on the male threads and don't get any inside the pipe. The pneumatics might not like that. – Greg Hill Mar 31 '21 at 17:04
  • Copper pipe vitually always screws on by means of an adaptor - it's not directly threaded. Red brass might be confused for copper, and be threaded. Copper pipe will have a fitting to convert to threads. – Ecnerwal Mar 31 '21 at 18:21
  • @Ecnerwal It may well be brass, I haven't done a chemical assay of it. – Hearth Apr 01 '21 at 00:30
  • @GregHill In my limited research, it looks like sealing without the tape is possible with NPTF, but not NPT connectors. – Hearth Apr 01 '21 at 00:33
  • Could you post some pictures of the device you have questions about, and explain again just what you it is you have questions about. A picture is worth a thousand words or "so they say.". You could also have a plumber or someone who works with threaded pipe and fittings, better explain what you are inquiring about. My 2 cents – d.george Apr 01 '21 at 10:09
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An extra wrap or two of PTFE tape around the pipe threads can help the fitting to line up snugly in the direction you want.

Andrew Schulman
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