Boiler calculations, end plates

[JasonBModerator @jasonb]

Same with the valve gear on such a small engine a slip eccentric would do the job if there is a actual need to reverse it or Stephensons gear if you really must.

Is theengine intended to drive anything or is it just for display? even less than 30psi would do for display

Either way steam consumption is not going to be much and with that size boiler you would get a decent run time without the need to refill. 5/16 is only Mamod size and they run a smaller boiler, even allowing for two cylinder sit won't need much.

[Benedict WhiteParticipant @benedictwhite51126]

I have not decided what it will drive, yet.

The purpose of the exercise is to learn a bit in the process whilst working out a few things that interest me, including Joy valve gear (which is why it is getting it).

I calculated that it will consume about 1 cubic inch of water a minute running at 1,000 RPM and 75PSI.

I wanted to work out where to put all the bits like water gauge, steam takeoff clacks etc.

As for power, no, it will not have much of that, but I do have a 3' length of 3" pipe that will make a much more substantial boiler when I get to it and have worked this one out.

Hopefully after this project I can then make a boiler for an 0-4-0 contractors loco for which I have a rolling chassis and the original drawings.

[duncan webster 1Participant @duncanwebster1]

I know it's been done on published designs, but I see no merit in having a bigger central fire tube. It would be less effective at heat transfer than the smaller tubes, and would take more than its fair share of the flue gasses. I'd also put a lot more tubes in. If you do that the end plates will be well supported, and you'll get away with the being quite thin. Your best option would be to take an existing design and modify it. There is one on p157 of KNHarris book Model Boilers & Boilermaking. It also has loads of info on construction techniques.

If you intend to run in public you'll need approval of a club boiler inspector (assuming you're UK based). Others have reported on here that meths or gas fired vertical multitude boilers don't work too well unless they have forced draft.

[Benedict WhiteParticipant @benedictwhite51126]

Many thank for the thoughts Duncan on tubes. I have KN Harris's book as well as access to our club library.

If I modify a boiler I still have to do the boiler calculations. Some one sent me a PM with a suggestion to look up the bass physics formulars which in the absence of something in the literature is what I will have to do in order to get a boiler certificate which will then make running it insurable.

Above the boiler I was going to put a cap with a chimney in it and provide forced draft by means of exhaust.

[noel shelleyParticipant @noelshelley55608]

This boiler is to be 2.54" od, the 3' of 3" tube, is this to be the shell of your more substantially boiler or is it to be the centre flue pipe of a much bigger boiler ? Noel.

[Benedict WhiteParticipant @benedictwhite51126]

Posted by noel shelley on 16/09/2023 11:30:10:

This boiler is to be 2.54" od, the 3' of 3" tube, is this to be the shell of your more substantially boiler or is it to be the centre flue pipe of a much bigger boiler ? Noel.

Completely different boiler. It allows me to make a much more substantial engine at a later date.

[duncan webster 1Participant @duncanwebster1]

Posted by Benedict White on 14/09/2023 14:20:18:

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.

The Australian code includes formula for working out different thicknesses.. However I think you'll find 0.04 is a bit on the thin side,. If you fit 13 tubes on a 1/2" hexagonal pitch and use 16g material I reckon your boiler inspector will be a happy man, but you'll have to draw it out to scale convince him, showing the tubes, the shell and the stay circles. I still think 3/8" tubes are on the big side for gas/meths, but the same principle applies.

Personally I'd sacrifice some of your 3" tube and copy KNH. You can build a lot of boiers out of 3ft of tube

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks Duncan. I will have to produce both a drawing and calculations, hence the desire for a ready made formula.

I fully intend on making boilers from the 3" pipe, It is the reason why I bought it from a scrapyard!

[SillyOldDufferModerator @sillyoldduffer]

Posted by Benedict White on 16/09/2023 17:55:24:

Many thanks Duncan. I will have to produce both a drawing and calculations, hence the desire for a ready made formula.

…

If a freelance boiler is to be steamed at a club, i.e. in public, then the club's inspector has to be persuaded the thing is safe. It's about establishing confidence: design, materials, and construction methods all shown to be in good order. Building a well-known successful boiler from an existing plan helps because there's a proven track record. All the inspector has to do with a trusted design is make sure appropriate materials have been correctly assembled. The list of 'appropriate materials' may be short – in the UK easier to assure a silver soldered copper boiler than welded steel because not many inspectors have the experience or equipment needed to inspect steel boilers. Best ask about materials and methods before building anything!

Formula are helpful, but need to be understood in context, especially if the goal is a light, efficient, high-pressure boiler. Construction methods have to match the formula's assumptions, for which reason heavy inefficient low-pressure over-engineered boilers are easier to make! Design from scratch is quite difficult, typically involving lots of sums, much thought – 'what could possibly go wrong' – and brutally testing several expensive prototypes to flush out bugs.

Another approach is to model the item in 3D-CAD and run a Finite Element Analysis on it. The Finite Element Method doesn't use textbook formula: instead the structure is triangulated and a force applied to the resulting mesh. The effect of the force on all the triangle legs is calculated. As the mechanical properties of the material are known, the effect of the force throughout the physical object is modelled, and can be colour coded:

Blue indicates areas of low stress, rising through green, yellow and orange to high (red). Simply, blue indicates parts of the model that might be reduced because they are over-strong, whilst red highlights sections that might need beefing up.

FEM is a generalised approach that doesn't require particular formula and their application to be understood, Accuracy depends on the density of the triangulated mesh, and substitutes a computer doing millions of calculations for the analytically derived formula found in textbooks.

Stress concentrations are presented visually, and CAD allows the design to be altered quickly. The method makes it possible to fix many structural problems before building anything for real.

No such thing as a free lunch! The engineer still has to understand the numbers behind the colours – red means 'area of high-stress' rather than 'this will break'. And the limitations of his model. For example, FEM is likely to assume perfect construction, resulting in a model with joints considerably stronger than real ones. The designer has to make practical allowances, including adding substantial safety factors. And if the inspector isn't familiar with FEA, it may be asking too much to expect him to approve a FEM model.

Bottom line, however the boiler is designed and built, keep the inspector happy at all stages of the process.

Dave

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks SillyOldDuffer,

I do not have the equipment for finite element analysis and despite working in IT don't get on with CAD. However I do get on with maths, so that is the way I will go.

I am still waiting to see how much the Australian code costs but unless it shows where the formula comes from I may use various equations for deflection and or burst pressure for plate to work it out then run some tests to confirm the results match what the formula suggests.

[duncan webster 1Participant @duncanwebster1]

This is getting OTT. FE analysis to design a simple boiler. I think not. If the OP is not prepared to pay for the Aussy code then he could go to a large central library and look at the relevant BS, but some interpretation would be needed as it won't mention copper or silver solder. It does give relevant equations for stay spacing. I did this many years ago for my own boiler design and got something very similar to the Aus code. I tried to interest SFED as it was at the time, but got no response.

Having mentioned SFED, I find it a bit arrogant that they have changed their name to THE Federation…… The Northern Fed still exists, but I suppose we northerners should be used to it by now.

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks duncan webster

I have emailed the people who write the Australian code and asked some technical questions and how much the code would cost. I had an answer to the technical questions but not on pricing.

Which British standards apply? Seems to me that would be the place to start.

The Southern Fed and Northern Fed produce joint seminars as far as I can tell, certainly do now. It is a shame that they did not take up your offer.

[SteambuffParticipant @steambuff]

You can buy the code books here :- **LINK**

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks Steambuff

I emailed the AMBSC, and am awaiting there reply. That link offers what appears to be an out of date copy.

I am hoping to find out which British standard might be applicable.

[David JuppParticipant @davidjupp51506]

Posted by Benedict White on 16/09/2023 17:55:24:

I fully intend on making boilers from the 3" pipe, It is the reason why I bought it from a scrapyard!

What will you do if the boiler inspector demands to see the material certificate?

It's worth finding out in advance if this is an issue – I still come across commercial projects where people end up scrapping expensive finished equipment because they didn't get the paperwork sorted out at material purchase.

[Benedict WhiteParticipant @benedictwhite51126]

Posted by David Jupp on 20/09/2023 09:54:24:

Posted by Benedict White on 16/09/2023 17:55:24:

I fully intend on making boilers from the 3" pipe, It is the reason why I bought it from a scrapyard!

What will you do if the boiler inspector demands to see the material certificate?

It's worth finding out in advance if this is an issue – I still come across commercial projects where people end up scrapping expensive finished equipment because they didn't get the paperwork sorted out at material purchase.

That isn't a problem, it has been discussed. It is new copper pipe as well.

[duncan webster 1Participant @duncanwebster1]

I used BS5500, unfired fusion welded pressure vessels. Why, because I had a copy as a result of having to design a PV in the day job (no longer). I know that boilers are fired, but one of the pressure vessel guys at work advised that as far as stays are concerned there is no difference, at least until you start talking about flexible stays (don't go there). It is mainly aimed at steel, but there is a aluminium supplement, and using a bit of logical thinking you can come up with an acceptable stress for soft copper. My figure agreed well with the Aussy figure. It also agrees reasonably as regards stay pitch.

I think BS5500 Is now known as PD5500. I have no intention of publishing my ruminations in detail, not checked by a SQEP.

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks Duncan. I will look those up. What is an SQEP?

[duncan webster 1Participant @duncanwebster1]

Suitably qualified and experienced person

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks Duncan. When coming up with such a calculation, given we don't make many steam boilers, who would count as suitably qualified and experienced.

As far as I can tell, you would need to confirm by experiment. I am fortunate that I can rig up a test rig to test end plates and buy scrap (unused offcuts) of copper from a scrapyard though.

[duncan webster 1Participant @duncanwebster1]

Experiment is not required. The design codes are there. They are based on a mix of theory and experience gained in making and using many thousands if not millions of pressure vessels. They might need a bit of judgement to suit our particular needs, and this interpretation must then be checked, no-one is infallible.

[Benedict WhiteParticipant @benedictwhite51126]

If you are trying to produce a formula for something not accepted in the literature I can't see it becoming widely accepted without proof. Besides which hydraulic testing end plates is not difficult, you just need a suitably thick walled steel tube to solder test copper plates to to pressure test till it breaks. Be fun too!

[SillyOldDufferModerator @sillyoldduffer]

Posted by Benedict White on 20/09/2023 00:15:13:

…

I do not have the equipment for finite element analysis and despite working in IT don't get on with CAD. However I do get on with maths, so that is the way I will go.

…

Yeah, I'm the other way round – comfy with CAD and hopeless at maths!

The best book in my library for pressure vessel formula is Machinery's Handbook. Mine is the 20th Edition (1976) which is good for US, UK Imperial, and Metric. Highly recommended – most mechanical engineering facts are covered!

The 6th Edition (1924) is available free online, and might get you started. Pressured cylinders are in the Strength of Materials section, try page 404.

To me the most obvious problem is that pressure vessel design has long been decided by Codes Of Practice aimed at large boilers. These codes contain 'best practice' derived from a mix of theory and decades of experience, and aren't obviously useful for small amateur boilers: scale matters. The guides put out by the Clubs and Federations are more relevant, but they don't encourage experimental work. And maybe new ideas aren't needed? I suspect it's difficult to get a modern small boiler to outperform older designs because they're already nearly as good as it gets.

Whatever the reason, model loco boiler construction in the UK hasn't changed much for a century. Could be time for a change!

Dave

[Benedict WhiteParticipant @benedictwhite51126]

Many thanks SillyOldDuffer

I have a Machinery's handbook, can't remember which year. I will have a look in that. Not sure why I didn't look.

I'm not trying to shake anything up, if the books had a rule of thumb that went down to 0.04" I would have just worked from that, but it stops at 1/16, or 0.0625 which is close… but not close enough. I can get some 1/16" if I need to but would rather work from what is to hand and make sure it is safe.

[Clive Brown 1Participant @clivebrown1]

Whatever the reason, model loco boiler construction in the UK hasn't changed much for a century. Could be time for a change!

Dave

Why, many thousands of perfectly satisfactory model boilers have been made over many years in home workshops Their safety record is first class.

From the point of view of stress analysis, I see very little to be gained by a mathematical approach.Typical boilers have a lot of flat plate in their design, very difficult to model mathematically. Also copper, the most usual material, has in its highly annealled state an extremely low elastic limit and considerable plastic ductility.

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