Have you tried 'model boiler calculations' in a search engine?

Given its focus, could it be possible that such a question has been asked on this forum in the past?

Please start with these:

https://www.model-engineer.co.uk/forums/postings.asp?th=75370

https://www.model-engineer.co.uk/forums/postings.asp?th=170551

The SFMES link in the above is now incorrect:

Pressure Vessels and Testing

Edit:

https://www.homemodelenginemachinist.com/threads/model-boiler-design-information.8176/

Edited By DC31k on 14/09/2023 13:21:01

An accepted or published design is one way to go – basing your design on it ! There are many books on the subject, many have designs in them. Authers like K.N.Harris, Martin Evans, E.L.Pearce Etc. If you want the boiler to have a club certificate then your boiler inspector is the man to talk to. If on the other hand you just want it to be safe you can conduct your own tests. As To end plates, I have just had alook at K.N.Harris and it would seem that A) you use the same thickness as for the shell and if a multi tubular fire tube boiler then the tubes act as stays, for unsupported plate then the stay calculations are given. You do not say what type of boiler you are considering, but saftey factors of 8 are normal and IF the boiler end is hemispherical reqires little or no stays. Noel.

PS I have just looked at my 6"vertical and the plates are as above.

Right ! You WILL only be building a small boiler with 1mm thick sheet ! or it will have a VERY low working pressure. What sort of boiler O/D ? Noel.

The spreadsheet I have (used by a boiler maker) would also make 75psi quite safely.

No just barrel and firebox sheet thicknesses, stay dia and spacing. certainly would not go below barrel thickness

Nowhere to enter end plate thickness

No suggestions enter dia and spacing and it tells you max and how that relates to the minimum Calculated factor of Safety needed. As it doe swith teh barrel both drawn and Rolled & Welded (tig)

I had entered 0.040" for both. It does not suggest what to use for stays but entering 5/32" or 4mm stays at 3/4" spacing gives a sililar figure above the minimum that the 0.040" plate/tube does.

Is this a vertical boiler, or a horizontal, if vertical is it lots of fire tubes or one big one?

It would make life a lot easier if you stuck to one thread at the moment this is spread across 2. The Aussy code doesn't go as low as 1mm thick plates, but it givbes the required formulae for working out the stay spacing for any thickness of plate. You will find that you need a forest of stays for flat ates 1mm thick at 75 psi, or is it 5 psi as mentioned somewhere.

Going down to 0.7mm wall tubes may just be pushing it too far.

Have you any experience silver soldering? I know of novice builders overheating and damaging tubes of far thicker wall so the risk is a lot higher with 0.7mm (22swg) For what you are going to exspend on solder, time, etc you may a swell spend out on some more substantial tube.

You would be best practicing with some of that tube first. The problems start if you need to remove and replace an overheated tude as it requires a higher temp to remelt the solder and you also have to get the assembled boiler completely clean as if you don't fresh solder won't take and you will tend to apply excessive heat trying to get it to flow which burns out more tubes.

For such a small boiler and engine, had you considered a horizontal boiler with water tubes ? 5 longtitudal stays, or reduce pressure to say 40psi and less stays and 3 or 4 water tubes, flanged ends. You do not mention a fuel type ? What water feed do you intend to use ? or run on one fill. If you build a hand feed water pump this can also be used to conduct a simple hydraulic test for safety. Good luck Noel.

With a bore of only 5/16" X 2 ( as in O/P ) you will have very little power for much above the friction levels. I would save the crank driven feed pump for a much bigger engine. At this size meths is a good fuel, it's combustion is something you need to think about, pre-empting your answer is why I had suggested a horizontal with water tubes. It will give you more room for burners and also the air feed they will need, it will be able to supply more steam than you will want. 30psi will be more than adequate and will simplify boiler making. Small injectors are not easy to make, often temperamental in operation and unsuitable for a boiler of this size. Gain experience making a simple unit that will work well first, then move on to bigger projects that with current metal prices will be expensive. Good luck. Noel.