"Union" fittings

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SL Ethel
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"Union" fittings

Post by SL Ethel » Sun Jun 04, 2017 2:26 pm

I've been watching Keith Appleton's Youtube videos of model steam engine repair, and I love the "unions" he uses for pipework. One side threaded, the other side with a "union nipple" that looks like it functions as an upgraded compression fitting, silver soldered onto the piece of tube. It looks very tidy when finished, and appears to be good for disassembly and re-assembly.

I've found these unions only in model sizes. Does anyone know if they're available (or appropriate) for tube sizes up to 1/2" or even 3/4"?

And a related question: my understanding is that compression fittings with copper tube are inappropriate for steam service as they may come loose over time. Would it address the problem if one silver soldered the compression ring to the tube, after first tightening it up once to create the usual mechanical joint?

I am looking for options here for a steam canoe power plant, where pressure will be 150 psi or less, mildly superheated, and the main steam line will likely be only 3/8". Any suggestions would be welcomed.

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Re: "Union" fittings

Post by Mike Rometer » Sun Jun 04, 2017 4:30 pm

SL Ethel wrote:
I've found these unions only in model sizes. Does anyone know if they're available (or appropriate) for tube sizes up to 1/2" or even 3/4"?
The most common kind of fitting in model steam work. You want bigger? Make 'em! Easy enough, the angle on the nipple is usually 45degs (matches a Slo-come taper), but any close matching pair will seal. They also work if cut square but I'm not so keen on those, too easy to knock an edge when fitting. I've had commercial ones up to 10mm (3/8"). Usually brass but occasionally bronze.
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Re: "Union" fittings

Post by SL Ethel » Sun Jun 04, 2017 4:53 pm

That's sort of what I was thinking about compression fittings. Start with the standard compression fitting and upgrade it by brazing the compression ring to the tube, or alternately, by making up the union nipple piece from stock to go where the tube end and compression ring would normally go. The latter seems more solid and workmanlike.

A compression fitting has a curved female side that the ring normally seats on. If I were going to make my own union nipple to mate to it, would you suggest re-profiling the female side to create a straight taper, or grinding up a special tool to profile the nipple to mate with the existing curved profile?

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Scott
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Re: "Union" fittings

Post by Mike Rometer » Sun Jun 04, 2017 5:08 pm

No need, I have done just that, successfully. The contact band is quite thin so the pressure needed to get a seal is light.
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Re: "Union" fittings

Post by DetroiTug » Sun Jun 04, 2017 5:08 pm

I'm a big fan of Keith's videos as well. Those are nice unions he uses, I don't know of anything available for water or steam in larger sizes like that. Compression fittings as I understand it are only meant for one time use, in other words, once the ring has crushed and deformed in the joint, reinstalling and achieving a reliable seal is not likely. That is why they are not approved here in the US for any sort of flammable gas or liquids.

Flare fittings for copper tubing are the best alternative. Piping up a system is a lot of work, I can't imagine having to silver solder it all together, would double or triple the build time. It would look nicer though.

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Re: "Union" fittings

Post by cyberbadger » Mon Jun 05, 2017 2:23 am

I also have starting watching Keith and am a fan. I like his dry British humour.

I have used Flare fittings (both 37 and 45 degree) in both copper and stainless with good success for steam. The copper you just have to worry about accidental bendage a bit more...

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Re: "Union" fittings

Post by Lopez Mike » Mon Jun 05, 2017 3:20 am

For normal launch size pipe, say, 1/4" and up, visit your local hydraulic equipment shop. They sell great unions in either cad plated or S.S. for economical prices. Called 37 degree JIC. They are rated for thousands of pounds per square inch.

Here's a typical male: http://www.hydraulicsdirect.com/Product ... asil%20New

And here's a sample of a female: http://www.hydraulicsdirect.com/Product ... asil%20New

I strongly advise staying away from compression fittings even with attempts to fix their basic failings with silver solder. Use flare fittings. The flaring tools are available in any hardware store.
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Re: "Union" fittings

Post by fredrosse » Mon Jun 05, 2017 5:58 am

My 2 cents worth, use compression fittings with 2-piece furrells, the best is Swagelok brand, but Parker and others make similar compression fittings. These are reliable to several thousand psi, and can be taken apart several times with never a leak. The down side is that they are expensive. I agree that the single furrell compression fittings found at Home Depot are not for high pressure/steam service.

Swagelok makes them in stainless steel as well as brass, and carbon steel. I have used them within the furnace of boilers in both stainless and carbon steel with never a leak, however I would not recommend the brass ones in a furnace environment. For a copper economizer coil, where gas temperatures are down below 1000F, the brass Swagelok fittings will do well so long as the economizer is kept flooded with water. The ASME Boiler code does allow copper alloy piping and fittings in service to 406F maximum, corresponding to 250 psi saturated steam pressure.

A pic of a coil boiler I made with Swagelok fittings, stainless steel, 5000 psi test, 200 psi operating pressure.
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Re: "Union" fittings

Post by Lopez Mike » Mon Jun 05, 2017 3:25 pm

I don't have any problem with using S.S. tubing and Swagelock fittings but they ares a little thin on the ground at the corner hardware store where most of us steamers get our piping.

Your stuff looks like NASA construction. Mine runs more to a hillbilly still.
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Re: "Union" fittings

Post by barts » Wed Jun 07, 2017 2:51 am

I've had good luck w/ Swagelok fittings on stainless tubing. They're steam tight, and the joints can be reassembled several times w/o any problems.
Regular compression fittings will leak w/ steam, sooner or later.

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