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Steam Pipe Size

What size pipe from boiler to simple oscillator.

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James Hall 325/01/2021 23:30:36
36 forum posts
8 photos

Have my simple boiler up and running and next step is to connect to my simple oscillator (both from Stan Bray's book 'Making Simple Model Steam Engines) but the book omits to give any guidance on size of steam pipework.
I've assumed 3mmOD as I recall this from childhood Mamods, but would hate to put a lot of work in on fabricating unions etc. for the wrong sized pipe.

The engine (a Slim Sam) has a bore and stroke of 10x11mm and is a very basic oscillator.

Guidance would be much appreciated, as usual.


Jeff Dayman26/01/2021 00:58:10
2068 forum posts
45 photos

I'd say 3 mm OD pipe is about right for that engine (which looks great BTW).

You might consider mating bronze flanges at each end of the pipe, with 3 or 4 screws securing them. They are easy lathe turning and drilling and can be designed with a groove to hold a small o-ring for sealing. A gasket can also be used between the flanges. If you use a short length of pipe out of the boiler with a flange soldered on it, and same out of the engine, the pipe between just needs the flanges soldered on aligned so the screws line up. Extreme accuracy is not needed usually as the pipe can be tweaked a bit to rotate the flanges a few degrees to align them if need be. Such flanges are used on full size steam plants almost exclusively, in sizes above 2" dia pipe.

For fastening, if you use bolts and nuts or studs and two nuts no tapping is required and lineup/assy is simpler.

Be sure to drill the holes accurately so that any flange fits the pattern on any other and no matching is needed.

Link below is to a video showing some model flanged pipe work and some good ideas on simple ways to make them.

JasonB26/01/2021 07:05:07
19961 forum posts
2178 photos
1 articles

Yes 3mm or 1/8" will be Ok on that. If its going to be a general use pipe that you intend to use for other engines in the future then run in 5mm or 3/16" and just reduce down at the engine.

I'd probably go with nuts and nipples for joints as they tend to line themselves up and no need for gaskets or sealants.

Jeff Dayman26/01/2021 10:50:31
2068 forum posts
45 photos

5 mm OD or 3/16" OD pipe will work just as well as other sizes to pass steam of course , just as Jason said. But if the larger pipe is used the surface area is much larger which means faster/more condensation of steam until everything is warm. this means a much longer time of sputtering and spewing water when starting. 100 mm of 3 mm OD pipe has a surface area of 706 mm squared roughly, but 100 mm of 5 mm OD pipe has a surface area of 1963 mm squared roughly.

The sputtering may not matter to you at all, of course. The pipe could also be insulated with heavy cotton string secured with white glue, or silicone tube, to reduce heat loss.

James Hall 326/01/2021 11:16:07
36 forum posts
8 photos

Thanks JasonB and Jeff.

I'll probably stick with 3mm pipe for this setup as that's what I've got. Joining with flanges looks good and I'd certainly use them for authenticity on any future larger or replica models - but will stick with nuts and nipples for the present as less fiddly - union nuts are fast and simple to make. I'm currently soldering an appropriately sized washer to the end of a pipe sealed to the nipple with a sandwiched washer or o-ring but soldering this on square at this size is very fiddly. I'd rather use cones but have no idea where to obtain them for 3mm pipe (for 1/8" would do) - any suggestions?

Just to clear up any confusion over terminology, I use 'nipple' to refer to the fitting onto which the union nut screws and 'cone' to refer to the tapered fitting soldered on to the end of the pipe which is then held onto/into the nipple by the union nut. This comes from my one-time plumbing background, but if wrong in the ME world will happily be put right.

James Hall 326/01/2021 11:27:56
36 forum posts
8 photos

As a matter if interest, I had to make a second cylinder/port-face assembly for my Slim Jim - the first, made in accordance with the drawings, had a design error - you can see what it is in the pic below. A valuable lesson to check plans/drawings carefully before proceeding - it should have been obvious to me, but I naively assumed published drawings must be right.

Problem simply solved by using a wedge-shaped port block so that ports were still covered at 'full swing' and at my embryo stage it's all good practice. Spotted the problem on first assembly, second assembly with modification ran immediately on compressed air without tweaks or fiddles so I was very happy.


James Hall 326/01/2021 11:40:25
36 forum posts
8 photos

Tony Wright 1 - I certainly will in future.

Andrew Johnston26/01/2021 11:43:38
5929 forum posts
666 photos
Posted by Jeff Dayman on 26/01/2021 10:50:31:

But if the larger pipe is used the surface area is much larger which means faster/more condensation of steam until everything is warm. this means a much longer time of sputtering and spewing water when starting. 100 mm of 3 mm OD pipe has a surface area of 706 mm squared roughly, but 100 mm of 5 mm OD pipe has a surface area of 1963 mm squared roughly.

I don't agree with that. For a cylinder:

Surface Area = pi*d*h

Volume = pi*r^2*h

Ignoring wall thickness, for a 3mm tube and 100mm long surface area is 942mm^2 and volume is 707mm^3. For a 5mm tube the surface area is 1571mm2 and the volume is 1963mm^3. The volume goes up more quickly than the surface area. A larger volume means more steam. If we assume that the amount of heat in the steam is proportional to the volume of steam then the larger pipe will have more heat per unit surface area than the small pipe, so cooling will be slower, not quicker.


Jeff Dayman26/01/2021 14:33:56
2068 forum posts
45 photos

I apologize for the math error on the area calcs, I used the wrong formula.

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