Recommended pipe size?

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hefyjefy
Posts: 3
Joined: Wed Apr 21, 2021 4:19 pm

Recommended pipe size?

Post by hefyjefy » Wed Apr 21, 2021 5:33 pm

Hello everyone,

I am not a complete newbie at this and have built a couple of successful small steam engines and a very small boiler. However I am planning on another somewhat larger boiler to run future engines so I am having to get serious about pipe sizes and fittings. The boiler is a 3" dia upright design similar to the one that MyfordBoy describes in his series of YouTube videos. https://www.youtube.com/watch?v=MIFHILK256Q&t=14s

I believe a suitable pipe size for the output would be 3/16" however I don't have any MTP taps or dies and am little confused about sizes and method of connection to the boiler. What methods would anyone suggest and what types or fittings, taps and dies would be correct for 3/16" pipe?

Thanks for any thoughts!
Geoff

James Powell
Posts: 401
Joined: Fri Oct 28, 2011 10:42 pm

Re: Recommended pipe size?

Post by James Powell » Wed Apr 21, 2021 5:54 pm

So, the first choice/decision is pipe or tube... because if it were I, I would be using tube not pipe for connecting something like that. I have roughly that size of plant- the Monarch plant that I have. It uses tube, in the form of 3/16" OD tube for the steam to and from the engine, (7/8" by 7/8" single), and 1/8" for the water supply from the hand pump.

If you want to use pipe rather than tube, I can't advise you- sorry, not our gig. But tube will work, and should still be readily available in the form of copper tube in 25' coils from somewhere that will supply for brake line work, or for fridge work. Silver solder on the nipples, and do up the union nuts, and it's all good :). To get fancy, buy a tube bender to make the bends not look like dogs bollox.

We'd typically be using 5/16"*26 TPI Brass for the union nuts if we were making them, though there is a wide assortment of choices of threads that will work. (3/8 NF or NC would work, you need something with a root of > .186 so that you can put the pipe through the root of the thread...) 1/4"-40 ME will "just" work for 3/16" tube, but there isn't much give left.

I don't know where you are in the world, so can't advise on which choice is going to be best- it somewhat depends on which thread series you have taps and dies for now, and what hobby suppliers around you will use. (basically, if you are in the US then it's different than ROW which tends to be either Metric or UK based ME/Brass for fittings- there are 3 "good" choices out there)

hefyjefy
Posts: 3
Joined: Wed Apr 21, 2021 4:19 pm

Re: Recommended pipe size?

Post by hefyjefy » Fri Apr 23, 2021 12:27 am

Thank you James,
I had not even thought about the difference between pipe and tube, can you explain? Happy to take your advice and go with tube, I am in the US (California). I only have a rather motley selection of taps and dies right now so looking to purchase something.
Geoff

jscarmozza
Posts: 486
Joined: Mon Mar 03, 2014 5:09 pm

Re: Recommended pipe size?

Post by jscarmozza » Fri Apr 23, 2021 7:13 am

Check out P M Research, they have a selection of MPT, UST and straight taps and dies, they also have valves and fittings with MPT and UST threads. Keep in mind they are not all interchangeable.

James Powell
Posts: 401
Joined: Fri Oct 28, 2011 10:42 pm

Re: Recommended pipe size?

Post by James Powell » Fri Apr 23, 2021 8:29 am

pipe is measured by the inside, tube by the outside (roughly!). Tubing typically doesn't have threaded connections on the tube, but relies on union nuts with cones of some sort on the tubing. Think classic brake lines, swageloc fittings, ect.

Pipe has threads on the outside, and is screwed together, with pipe dope used on tapered threads to make a seal between each section. Think black iron pipe

For a steam plant, in real life it will be pipe, often joined by flanged connections. For a model, it is often easier to use tube rather than pipe in smaller sizes, because the copper tube typically used is going to be thinner than iron pipe would be, and the alternative of brass pipe can mean lots of work to route. If you are building an "engine room" style of display piece more than one you intend to run, then pipework can look lovely.

ImageDSC_0005 by Peach James, on Flickr

Is done with tube, and is probably a bit bigger than what you are building- the boiler shell is 4.5" OD. (THING typically has lived behind the woodstove...and might end up being the first loco run on CPS II, given that at least its feed pump works...). Note: I'm not advocating the style of looping tubes that Thing has for aesthetic value- it has all the aesthetics of a brutalist building !

In the real world, we typically would use tube up to 3/8" or perhaps 1/2", and pipe beyond that. Mostly it has to do with the ends, as in what they are connecting to, and how hard plumbed something is. More modern practice is welding tube into socket weld fittings, when the fridges were redone @ work (from R12>R575, IIRC) nearly all the pipework was done that way with a TIG welder.

As for purchasing taps and dies- it is cheaper (almost always) to purchase a set vs loose pieces. That being said, if you only need 3 taps & 1 die, then a set is a bit of overkill.

James

hefyjefy
Posts: 3
Joined: Wed Apr 21, 2021 4:19 pm

Re: Recommended pipe size?

Post by hefyjefy » Sat Apr 24, 2021 1:38 am

OK many thanks for all the info! The fog is starting to clear regarding fitting sizes, the tube v pipe thing explains why I was having problems understanding the way fittings are described.
Geoff

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