STEEL BOILER DESIGN

This topic has 9 replies, 7 voices, and was last updated 28 February 2026 at 21:24 by duncan webster 1.

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26 February 2026 at 19:30 #839514
noel shelley
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@noelshelley55608
Whilst the 9F is a slow job, a quick project is to complete a De Winton in 7.25″ – I know it’s a locomotive but steel boilers are more common in the bigger traction engines. The loco I have has never been assembled, is more or less complete BUT the boiler whilst tested is of a very poor design and has no paperwork, sadly it must be considered scrap !

I need advice on who to speak to ? As to how to get a design approved, the type of steel for the boiler shell and tubes Etc. I have a design in mind and have the gear to be able to prepare all the components for welding by a coded welder of pressure vessels. I take it that nothing has changed as far as welding the boiler ones self goes ?

It will be a vertical multi tube boiler. Shell 273mm od 6.3mm wall, firebox 219mm od 6.3mm wall. Crown sheet and smokebox tube plates 10mm thick, foundation ring 10mm , 34 off 20mm x 2.5 wall tubes and 2 off 25mm x 2.5 wall super heater flues. Tubes expanded in. Firebox height130mm, tubes about 300mm long. Overall height including ash pan and smoke box about 500mm.

Some simple calculations indicate that the above design will be good for a working pressure of 120 psi. Is there a boiler of about this size approved if so where ?

I am a member of a local club who are happy to inspect steel boilers. Noel.
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26 February 2026 at 21:33 #839524
Nigel Graham 2
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@nigelgraham2
Your club may be happy about testing steel boilers, but likely only if the boiler is of proven design, assembled from certified plate by a coded welder, etc. I imagine most of we club boiler admirers (I am one for my club) encounter steel boilers only as commercially-made ones, which makes life relatively simple for us.

Having the design approved may be the hardest point, with obtaining the steel second. The reason for the design question is that calculating the strength of the inner firebox, even though cylindrical, is by no means easy, much harder than for the outer shell.

Do you have a welder prepared to do the work? If so he may be able to sort those two points out.

Looking at the sizes it doesn’t seem much different from ones made for the various 7-1/4″g narrow-gauge outline locos about with marine-type fireboxes. They are horizontal but probably around similar overall dimensions.

Examining the MELG’s The Boiler Test Code 2018, it does not exclude constructing one’s own welded-steel boilers, but the obstacles are the design if it is not an existing, known one; and obtaining the steel of correct grade (with certificate of conformity – bureaucracy more than engineering, but we still have to have them, for steel boiler materials).

The boiler inspector may wish to verify the joints preparation before welding, and here it may be best to ask the welder’s advice on the chamfering, etc.

Do you, or your club, have a copy of that MELG book – the latest, 2018, edition that is?

What do your own club’s boiler-examiners say?

You may be better in the long run having it made professionally, so it will come with all one bit of paper that keeps boiler inspectors happy: the maker’s test certificate for it. If you do that the manufacturer will likely verify your design and modify it if necessary but at least all the requirements will be met. It’ll just make a big hole in your bank balance.
27 February 2026 at 00:29 #839533
duncan webster 1
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@duncanwebster1
I have 2 books about steel boiler design by a chap called Alan Haigh who was the chief engineer of a firm of professional boiler makers, so he knew what he was talking about.

ISBN 978 1 901056 47 1

ISBN 978 1 901056 39 6

They are aimed at full size, but the same principles hold. Your biggest problem could well be arriving at an acceptable corrosion allowance.

I the Australian model engineers might have a design guide but not sure. The inner firebox is fairly short compared to its length, and has stiffening hoops both ends, so I’d be very surprised if implosion was an issue, but it’s not that difficult a sum anyway. If you have to go down that route send me your email by pm and I’ll scan thd relevant bit of Roark for you.

I doubt you’ll get away without material certs and welder qualifications, but if you find a friendly fabricator he might agree to you doing all the machining.

I’d be looking at expanding the tubes, much easier to replace than welded
27 February 2026 at 02:06 #839534
Simon Collier
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@simoncollier74340
The AMBSC Code 2 for Steel Boilers. Most recent edition 2024.
27 February 2026 at 07:04 #839542
JasonB
Moderator
@jasonb
Haining’s Seninel is a bit lager at approx 14″ and his Caradoc smaller at 8.8″ but both use 1/4″ nominal wall thickness tube. pressures of 100 anf 95psi respectively. Fireboxes similar thickness with no additional stiffening. I did run your 273mm tube size through the spreadsheet that several of the commercial builders use and it was well within safety factor.

You could ask one of the commercial steel boiler suppliers to look over your design and work with you to get one that would pass which they may be willing to do if they get the welding job at the end.

Back when we had that long thread about Yield Point the guy who posted a reply in MEW was able to do design verification for the Steam Boat Association, so might be able to do it. I did look at the designs on the assoc web site but all a bit larger but worth a look to see the general layouts. https://steamboatassociation.co.uk/SBAS-BoilerDesignLibrary
27 February 2026 at 12:56 #839602
Martin Johnson 1
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@martinjohnson1
Have sent you a p.m. Noel.

Martin
27 February 2026 at 13:54 #839626
noel shelley
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@noelshelley55608
Gentlemen, thank you for your various replies. Duncan, I will send a PM. Nigel, whilst If funds were unlimited I would just pay up and look big, the lead times some are talking of call into question the viability whole project – I may not last long enough to see the boiler. My club inspectors are happy to certify, so long as all the various boxes are ticked, eg certified plate, tubes etc and a coded welder. Jason, yes both Caradoc and Sentinel of J Haining are in the ball park, I have already spoken to the SBA, their VFT 101 design is 50% larger than what I need, the GA gives enough detail to show that my proposed design is sound and as you say running a few numbers indicates that it would be safe at 120psi.

The design I have in mind is a simple standard vertical multi fire tube boiler and by reference to numerous published designs it can be shown that the various criteria eg water volume, tube spacing, plate and tube thicknesses, fitting positions Etc are to the various designs and collectively the design is safe ?

I’m awaiting a call from the SBA and a chat with one of their people. My intention is to get my design OKd, or find a design that is accepted, of the size needed. Purchase certified steel (with paperwork ). Find a company or welder who will in consultation with me on the preping needed then weld the the parts I have made together and provide the relevant paperwork showing their coding. The tubes will be expanded in and the whole then pressure tested.

The engine is a design by K.N.Harris of 1913 based on 2 Stuart 5A cylinders with slide valves operated by Stevensons link motion, not the Dendy – Marshall valve gear as originally published, in The Engineer on the 20-6-1913. Does anybody have a copy ? Many thanks to all. Noel.
27 February 2026 at 14:41 #839633
David Jupp
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@davidjupp51506
Noel, you may already be aware – the ‘coding’ of welders is specific to particular joint designs, materials, orientations etc. Hopefully you can find someone who has been examined for weld types that cover all your needs, without having to produce mock-up welds, pay for extra NDT and destructive tests.

I’d hope that a welder from pressure vessel or heat exchanger fabrication would be suited to this.
27 February 2026 at 15:34 #839637
noel shelley
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@noelshelley55608
David, thanks for that, the boiler is of modest proportions and simple design so the boiler can be manipulated to avoid vertical or overhead welds. Kings Lynn has several engineering companies involved in the production of pressure vessels for the food industry, 2 I have dealt with and have contacts from whom I will be seeking advice if not help. As I write this the phone rang and the issue of coded welders seems solved. I await confirmation. Noel.
28 February 2026 at 21:24 #839748
duncan webster 1
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@duncanwebster1
Noel, have sent relevant bit of Roark
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