Boiler calculations, end plates

I acquired a short piece of copper pipe, suitable for the making of a small boiler to run a twin cylinder engine at 5/16 bore.

I have found a formula for the barrel and am satisfied that it will suffice but can't find anything but rules of thumb for the end plates and a calculation for stay dimensions and pitches.

Does anyone have a source for plate thickness strength calculations?

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:

https://fmes.org.uk/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

Many thanks for your reply.

I have spent quite a bit of time searching, though not on here.

Oddly the first thread is of what is probably exact the same type of pipe.

However no post mentions calculating end plate thickness or burst pressure.

I have read K N Harris and am working though Martin Evans book, also I have read the FMES guidance on boiler calculations. So far I have not had sufficient sight of the Australian code to know if that helps.

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.

Many thanks Noel, however I am working from a bit of pipe not in a design.

It is going to be a multi tube boiler which will reduce the need for stays, but it is also 40 thou thick so what I was looking to do was confirm what thickness would be acceptable. The literature does have tables but all start at 1/16 or there abouts including the tables in the Australian code.

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.

It has an ID of 2.54" (like the one in the thread mentioned above:

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

And from calculations using formulas in K N Harris's book as well as the latest guidance that should be happy at 75 PSI.

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

JasonB, does the spreadsheet you have say anything about end plate thickness?

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

Many thanks Jason. Does it have anywhere to put in the end plate thickness or does it assume barrel thickness?

If so, does it suggest any stay dimensions in terms of spacing and 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)

JasonB, given that the tube end plates on top and bottom of this proposed boilers are equivalent to firebox plates, could you put in 0.040" as the firebox plate thickness and see what it says for stay spacing and thickness please?

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.

Many thanks JasonB.

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.

Many thanks Duncan. I asked in that other thread because someone in this thread suggested I look there.

It will be a vertical multi tube boiler at 75 PSI. How many tubes is yet to be decided, I have a lot of 10mm tube with a 0.7 mm wall thickness and as going to put 6 in a circle followed by a larger tube in the middle, either 15 or 22 mm.

If I typed 5 PSI somewhere, that was a typo.

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.

Many thanks for the cautionary advice Jason.

I have a little experience and was planning to build on it. I do have around 60' of tube to play with.

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.