Create closed loop with threaded pipe

Question

Context: DIY shelf with steel threaded pipes and fittings. (NOT plumbing! -- I guess such a construct does not make much sense there.)

Consider e.g. this example, if you look at it from the side, it has some "closed loops", where I define closed loop as: T-connector #1 -- pipe A -- T-connector #2 -- pipe B -- T-connector #3 -- pipe C -- T connector #4 -- pipe D -- T-connector #1. See also schema below:

         A
#1 ├------------┤#2
   |            |
 D |            | B
   |            |
#4 ├------------┤#3
         C

Question is: what is the way of building such a "loop"?

Of course, it is trivial to build one semi-loop with only 3 pipes (T-connector #1, pipe A, T-connector #2, pipe B, T-connector #3, pipe C, T-connector #4). But then, how do you insert pipe D?

Another idea would be to build first the sections T#1 - D - T#4 and T#2 - B - T#3 separately. And then somehow screw in pipes A and C simultaneously. However, (unless I'm mistaken) if you tighten pipe A in connector #1, then you are loosening it in connector #2 and vice-versa.

So it seems impossible to build such a loop, but obviously there is a way, as the example in the link (and many others) demonstrate.

Answer 1

The link shows a hidden trick. The front section and the rear sections are built separately, there are no closed loops in each piece. Then the short horizontal pieces are cut in half and screwed into the tees. The front and rear sections are joined by sliding the cut pieces into larger diameter horizontals. After a dry fit, the front and rear can be glued together with a small amount of adhesive. If you can find tees with smaller thread on the crossbar, then you can use a smaller half-sleeve which would be covered by the correct size horizontal pipe. It is easy once you find the right parts.

Answer 2

Plumbing-oriented:

Classically, for threaded pipes, you use a union, or two unions.

Modernly, it's trivial with push-to-connect fittings.

It's also classically trivial with soldered connections. Sometimes brazed rather than soldered, depending on the "industrial" nature of the process.

(for the example shown in the link, given no need to hold water, probably the side connection pipes are loose in the threads from having been partially unscrewed to make the connection; or else the threads have been drilled out on one side.)

For considerable investment in tooling, you could use left-handed and right handed threads on opposite ends of a pipe nipple so that it can be screwed into both ends at the same time to make tight connections. For a manufacturer who's found a market for things like you show in the linked picture, that could pan out; for a DIY project the costs would not make much sense.

Answer 3

Using iron pipe with threaded fittings and ends, the "trick" used is to put inline joints at B and D so the pipework can be taken apart.

This is also done in real systems so that changes and repairs can be made easily. This is the type of connection, called an inline joint or coupling :

And it has a flat rubber type washer which is compressed by the nut. There are various sizes available.

Answer 4

If you want to make a loop, or repair a hole you drilled in a pipe without undoing all of it, there are unions, but those are large, and not legal to use inside walls (or so I’ve been told). The other clean option is a left/right nipple and a left/right coupling.

https://www.plumbingsupply.com/leftright.html

These are fittings with left handed threads so when you turn the nipple, it tightens both sides at the same time like a turnbuckle.

Answer 5

An alternative to threaded plumbing connections is to use T's and elbows (and corners) with set screws. Called 'structural pipe fittings' they actually have a threaded screw perpendicular to the pipe, and its tightened to keep pipes in place. I suspect they don't have the weight capacity of threaded or bonded (cemented) pipe.

source: lowes.com (entire webpage of structural fittings)

They are commonly available in a variety of metal types with a black or silver finish. They certainly won't look as 'industrial' as common black iron gas pipe though.

Answer 6

If you are looking to pipe this as a closed loop, such as a configuration for a heating loop, all you need to do is start with one long pipe and add an elbow at one end, next is a nipple, then the next elbow, then the next long pipe and so on. this will give you a closed loop. IF this is not what you want, please explain what you want the end product to be and what you will be using it for.

Answer 7

There is also a slimline union method. You can get a straight iron (gas barrel) connector which is completely threaded on the inside. You cut (parallel) threads on one of the pipe ends to be connected so you can wind a locknut, and connector, completely onto it.

You thread the normal amount (tapered) of the end of the other pipe to be connected, apply tape or sealant, and back the connector off the pipe it is on, and onto the other pipe, until it seals on the taper.

You then tape and seal around the other end of the connector, and back the locknut up to it enough to seal that end too. It's not a great seal because the thread isn't tapered, but iron pipe fills any gaps with rust in a day or two.

Answer 8

Most threaded pipes an fittings for water use are tapered so that the fit gets tighter as you thread the pieces together.

If you get pipes that have straight threads for "A" and "C", you can first thread one end in farther than you need to, and then as you thread in the opposite end, it will unscrew, but not all the way.

Unfortunately, pipes and fittings with straight threads will probably be more expensive and harder to obtain than tapered.

Comparison of threads (from Parker Hannifin blog.)

Answer 9

You cheat

You make a saw cut halfway down the least observable part of the pipe. Now the 2 halves of that pipe can rotate independently.

How do you keep them physically aligned? Install inside it, a pipe or rod whose OD is slightly smaller than the ID of the pipe. This sleight-of-hand pipe won't have any strength in tension, so don't depend on it for that. I suppose you could drill holes and drive shear pins through it if you needed that.