Building a steam engine..

I'm toying with the idea of building a steam engine as a winter project. Nothing huge, just the size of one of those mamod engines. I was thinking of using an old mig welding gas bottle as a boiler (Obviously modified to include a pressure relief valve etc). One of the small bottles with some stubs of tubing welded on would suffice.
However, always aware of the risks of such things I have had another idea.
I could place a coiled tube, filled with water, over a burner and produce steam that way. Bit like a condensing coil but in reverse. The advantage would be that there would only be a small quantity of boiling water and thus the risk of getting scalded would be reduced if the thing went pop.
The problem I have, however, is how to ensure a continuous flow of water into the boiling coil. I'd need to feed water in one end whilst the steam produced would come out of the other. Obviously the tube would be at pressure so the cold water would need squirting in under pressure..but how? Has anyone any ideas?
Harry

Try googling for 'flash boiler' 'flash steam' etc.

Nick H wrote:
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Thanks! That gives me some stuff to work on. (Initial thoughts is that it's going to be a bit too complex)
Harry

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You've got to be very careful with boiler material selection. Some materials can't be used - stainless is a good example. If you want to get a boiler ticket on the thing you'll need to conform to standard practices, Mig bottles will be a no-no I suspect. I've got a full size steamer, all material is steel boiler plate, and any new material needs a release certificate. Copper is OK I think.
You really should seek expert advice here before starting off, I have very little detailed knowledge. It would be a shame to produce a lovely model then find you can't get a boiler ticket and hence the public liability insurance needed for display in a public place.
Julian.

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Is there a size and/or pressure limit below which a boiler cert is not required - I can't imagine all those little Mamod etc engines one sees in the model tent of most rallies have been inspected?

Nick H wrote:
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Most/all are run off compressed air for demonstration purposes, I assume for this very reason.
FWIW, you *can* make a boiler from stainless steel, however, it's got to be welded by someone certified to make stainless steel boilers.
Reason I know this is that a friend was planning to build a ride-on sized steam loco and had all the stainless steel machined to make a boiler with firetubes in it etc. It was only then that he discovered he'd have to pay ££££££s to get it welded to be able to insure it.
25 years on and the bits are still sat in his garage!
Harry

snipped-for-privacy@freeuk.com wrote:
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try uk.rec.models.engineering
Guess what they know about :-)
BugBear

On 25 Jul 2006 02:09:18 -0700, snipped-for-privacy@freeuk.com wrote:
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If you have the slightest interest in building boilers, then get yourself a copy of the ancient Keith Harris "Model Boilers and Boilermaking" book (blue cover, MAP press, probably available from Camden books). It's good on techniques and on the many varied designs.
First of all, make yourself a Cornish boiler. This is because it's simplest and the time you spend making it will soon be repaid in savings if you make anything more complicated.
Take some big pipe. Put a medium pipe up the middle of it. Seal the ends with doughnut plates. You're done! Old duffers in brown dustcoats will mock you, but it's a simple source of steam and it's easy to fire with a gas or paraffin burner. For boats it's great, for stationary engines it's historical (especially when surrounded by a firebrick repro box flue). Just don't let the gricers get to you.
For a bit more sophistication, make a Lancashire boiler. Put small waterpipes (Galloway tubes) across the medium firetube. Big ones had two firetubes, but don't go that far. This (with a propane burner) really is the best boiler design for nearly all practical models.
Boilers need to be made of the same base metal throughout if you're not to run into corrosion problems (and joining problems). So for small stuff, bite the bullet, use copper and silver-solder or braze it. Welded steel boilers are a _lot_ more complex, particularly when it comes to the small bits - the shell is the easy part.
I'm almost tempted to make a titanium boiler. I've got plenty of Ti tube scrap lying around and it's one of the easiest metals to TIG weld. It's really not such a daft idea...
Pressure testing and inspection is easier than is made out - especially if it's not a locomotive boiler, and it's not embedded inaccessibly in a locomotive!
Don't go anywhere near locomotive (water jacketed firebox and many small firetube) boilers. They're a complete pain in the backside to make and their gas dynamics scale down really badly. They just don't even work right at these scales, let alone the complexity.
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If the tube is heated empty, then water is pumped in and "flashes" into steam, then you have a flash steam boiler. Neat idea (popular pre-war for racing boats and hydroplanes) but it's another level of complexity to worry about.
If you keep the tubes full of water and have a steam drum above, then you can build either a steam road vehicle boiler (big steam drum) or a marine watertube boiler (tiny steam drum volume). Both of these have lots of parts, but they're conceptually simple and also scale down well to small boilers. You even avoid most of the expansion problems that make them difficult at larger sizes. Watch out for sludge deposits in those narrow tubes though. Harris has good design sketches for steam wagon boilers. A vertical Sentinel or similar design would be a good use for a "fire extinguisher" steel drum - keep the original bolted flanges top and bottom and it even demounts for cleaning.

Andy Dingley wrote:
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From another world comes similar advice.
"The quickest way to make a 6 inch telescope mirror is to make a 4 inch telescope mirror first."
BugBear