Boiler Design – issue 4765

[MEinThailandParticipant @meinthailand]

I know I keep droning on about this, but the Australian standard is extremly clear and is referenced back to national standards. The article in MEW was not clear, in places was wrong. Why reinvent the wheel.

Duncan many thanks for your observations. My colleague and co-author of the Yield Point Method has the AMBSC Code for copper boilers and we have studied it. There are several questions arising which we would like to resolve, but can’t find an avenue to do so. Can you help?

With regards to your statement “[it – the AMBSC Code] is referenced back to national standards” I would be grateful if you could expand on this. Reference back to what national standards. How and where is this referencing back reported?

Also I would be pleased to clarify any point where you say that “The article in MEW was not clear”, if you could identify those for me, please.

And if you could point out where “The article in MEW…, in places was wrong” I’d be grateful for your further explanation. The article did have a mistake in the calculations for Example 2 which we have corrected on this forum but apart from that I know of no other area where the article is “wrong”. If you could please elucidate, I’d be glad to clarify.

Why reinvent the wheel.

• We are led to believe that the first wheels were made from stone in the stone ages and had a hole in the middle for a wooden axle.
• Then, so we are told, came wooden wheels.
• Next they added a steel hoop on the perimeter.
• Then the spoked wheel was invented and some kind of metallic bearing was introduced.
• Along came steam traction engines and then the motor car and a rubber rim was added.
• Then the pneumatic tire was invented. With an inner tube.
• Next came tubeless tires and steel wheels running on proper axle bearings.
• Cross-ply tires came next with increased grip when cornering.
• Then alloy wheels. Bearings were now of the roller type.
• Now we have low-profile tires with inbuilt pressure sensors.

That’s why the wheel has continuously been reinvented.

 

[Charles Lamont]

On Charles Lamont Said:

On Paul Lousick Said:

Deformation of the boiler when undergoing testing is a sign of weakness in the design or manufacture of the boiler and is one of the first things that is checked when doing a hydraulic test and is one of the requirements in our design code for model boilers in Australia.

A small amount of permanet movement in a copper boiler when first pressurised is not necessarily a sign of weakness.

I know the article is only talking about hoop stress in the cylindrical shell, but someone is going to misinterpret this as applying to, for example, flat stayed surfaces as well.

(I also have a B.Sc.Eng.)

Where do I get copies of the AMBSC codes?

Charles, if I may be permitted to respond to your comments:-

“A small amount of permanet movement in a copper boiler when first pressurised is not necessarily a sign of weakness.”

So you are in disagreement with Paul Lousick who said:-

“Deformation of the boiler when undergoing testing is a sign of weakness in the design or manufacture of the boiler and is one of the first things that is checked when doing a hydraulic test and is one of the requirements in our design code for model boilers in Australia.” ?

“I know the article is only talking about hoop stress in the cylindrical shell, but someone is going to misinterpret this as applying to, for example, flat stayed surfaces as well.”

Charles, I do not understand this. I assume that you have read the YPM article because you correctly confirm that it states that – as you say – “the article is only talking about hoop stress in the cylindrical shell”. So if this is your (correct) understanding of the article, why would another reader not also come to the same conclusion? I’m baffled by your comment. We clearly state in the article, and I quote:-

“No consideration is given to flat plates such as tube plates and firebox plates.”

“(I also have a B.Sc.Eng.)”

And an MSc? And a PhD? And formerly a Chartered Engineer, (C.Eng) a member of the Institution of Mechanical Engineers (MIMechE) and a member of the Chartered Institution of Building Services Engineers (MCIBSE)?

 

[Luker]

On Luker Said:

On MEinThailand Said:

Luker, just in case readers are not familiar with your many articles in Model Engineer (now MEW) magazine, can I say that you are a prolific contributor and ardent supporter of new methods and new trends in engineering that can be and are being increasingly adopted by progressive model engineers?

I am therefore disappointed and perplexed why you should chose here to apparently slander the Yield Point Method article when by your own admission you have not even read it!

To quote your post:-

“Unfortunately I have not read the article in question”

“an article that is fundamentally flawed will dissuade builders making their boilers.”

I acknowledge that you are not directly saying that the YPM article is “fundamentally flawed” but it could be inferred by the average reader that that is what you meant.

I would be grateful if you could clarify whether you think that the YPM article is “fundamentally flawed” or not and give your reasons.

I have permission from the Moderator JasonB to give you access to the Yield Point Method article as posted on the ModelEngineeringInThailand.com forum and look forward to your positive comments when you have read and digested the article.

I apologise if my initial post was a little brash, and I should have sourced your article before commenting, so again I do apologise. Having said that; from the OP I had a very good idea of what methods were being employed and unfortunately I could see the potential for a ‘zero deflection boiler test’ to cause ‘confusion’ with already fickle boiler inspectors. I have subsequently read through your article, and Jason has forwarded links to another forum where I skimmed through some of the comments.

The yield point of a material has a defined strain, typically 0.2% but this varies depending on the standards used. Fundamentally; for a zero deflection model you should be using the elastic limit which is very difficult to measure accurately for copper (the YP and UT point scatter is related). The fact is, you will get some deflection when testing an annealed boiler either with the UTS method or a YP method. From practical experience, copper boilers are in a fully annealed state. Even the TIG welded copper boilers I’ve made could be dented by pushing on the shell with my finger (the required interpass temperature is enough to anneal the copper). The silver brazed boilers are worse.

Any new idea or innovation should improve the status quo.  My concern is, that, the boiler code writers would see this method and incorporate it into the code because it makes copper boilers ‘safer’ (not making boilers at all is even safer, but nobody wants that!). And just like that, numerous designs are void and many people cannot complete their builds that have been lingering under a bench, when in fact the supplied materials would have been fine. Then there’s the cost involved (which is significant) and the added heat input requirement (already tricky) for silver soldering etc. etc.

Practically you can test nearly all boilers that were designed (and properly made) using the UTS method at 2XWP, allow the deformation… have some coffee… release the pressure, then call the boiler inspector for the test. The chances of further deformation are small due to the work hardening of the copper.

On a related matter, but from the other forum, you cannot use a ductile failure model for calculating the stresses in a stainless boiler. The mode of failure is not that simple. If you have a look at my stainless boiler designs they seem ridiculously overdesigned, but this is intentional to limit the surface potential for SCC and CCC. I had to use FEM (the element type is critical for accurate results) for these boilers to make sure I had a safe design. You can use austenitic, but not 304, you need to source 316 or higher (low carbon). This limits delta ferrite and improves weldability.

Please don’t stop innovating and contributing!

Luker many thanks for your reply and your apologies which are greatly appreciated and respected.

Your response is positive and encouraging. I think we are on the same page when it comes to bringing new technologies and ideas to model engineering. My own personal goal is to increase the level of professionalism in model engineering not only with regards to the physical construction of our models but also to the manner in which we communicate our ideas.

Once again thank you for your positive comments – this forum could do with more like this.

[MEinThailand]

On MEinThailand Said:

Charles said

“(I also have a B.Sc.Eng.)”

And an MSc? And a PhD? And formerly a Chartered Engineer, (C.Eng) a member of the Institution of Mechanical Engineers (MIMechE) and a member of the Chartered Institution of Building Services Engineers (MCIBSE)?

 

As I posted earlier

I’m sure the members would also be interested in past experience in this field not just the calculations but what boilers you have both previously made, were they to published designs/methods and did you find that they distorted and if so by unacceptable amounts.

maybe I’m a bit sceptical and regulars here will know I’m more a man for practical matters than theories but although you have the letters after your names I wonder how relevant the MSc and PHd are as they do not really relate to engineering or show a background in pressure vessels which several of the respondants here do have.

This is the Bio that would seem not to have made it into the article.

As the G1 Loco is electric and the Case not made that only leaves the Little Samson Boiler as one you may have made or was it done by a third party as most here would do with a steel boiler?

 

Original article for anyone following along who is not a subscriber

https://modelengineeringinthailand.com/forum/index.php?threads/the-yield-point-method-full-original-article.47/

 

The thread that Luke R and I have mentioned in this thread

https://modelengineeringinthailand.com/forum/index.php?threads/thickness-of-copper-boiler-shells.6/

[DiogenesParticipant @diogenes]

Thanks, the full context will make it much easier to follow..

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for making the article more widely accessible

Although I have no personal interest in the design/manufacture/use of small steam boilers, I am encouraged to see some discussion of the test processes.

… I could, after-all be the innocent bystander who gets injured at some event.

As an “intelligent layman” I found the article interesting and informative … and not particularly contentious … It is helpfully peppered with caveats, and it thereby exposes the fundamental weakness of any and all boiler testing in the Model Engineering domain.

Whether you test by YP or any other protocol … Unless you test to failure you will never know for sure !

Keep up the good work … there’s still a long way to go.

MichaelG.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I remember doing tensile testing during my Tech college days, using a Hounsfield Tensometer although we didn’t test copper, the results showed the elastic range, the yield point, the ultimate tensile strength and breaking point. designs were considered to be within the elastic range.

Regards Nick.

[JasonB]

On JasonB Said:

On MEinThailand Said:

Charles said

“(I also have a B.Sc.Eng.)”

And an MSc? And a PhD? And formerly a Chartered Engineer, (C.Eng) a member of the Institution of Mechanical Engineers (MIMechE) and a member of the Chartered Institution of Building Services Engineers (MCIBSE)?

 

As I posted earlier

I’m sure the members would also be interested in past experience in this field not just the calculations but what boilers you have both previously made, were they to published designs/methods and did you find that they distorted and if so by unacceptable amounts.

maybe I’m a bit sceptical and regulars here will know I’m more a man for practical matters than theories but although you have the letters after your names I wonder how relevant the MSc and PHd are as they do not really relate to engineering or show a background in pressure vessels which several of the respondants here do have.

This is the Bio that would seem not to have made it into the article.

As the G1 Loco is electric and the Case not made that only leaves the Little Samson Boiler as one you may have made or was it done by a third party as most here would do with a steel boiler?

 

Original article for anyone following along who is not a subscriber

https://modelengineeringinthailand.com/forum/index.php?threads/the-yield-point-method-full-original-article.47/

 

The thread that Luke R and I have mentioned in this thread

https://modelengineeringinthailand.com/forum/index.php?threads/thickness-of-copper-boiler-shells.6/

Dear JasonB, the short-form Bios you have posted above were submitted to the Editor at Model Engineering & Workshop magazine as part of the Yield Point Method article.

The Editor chose not to include this information and I assume that as Editor he had good reason not to so. And for reasons that you will probably never understand I agree with his editorial decision.

You have now usurped the Editor’s editorial decision by posting it yourself, presuming that you know better than the Editor.

As I have repeatedly said the scope of the Yield Point Method article ends at the output of a theoretical calculated figure for the minimum safe thickness of a copper boiler and compares that value to one obtained by the traditional (UTS) method.

We, the authors of the Yield Point Method article, have not ventured either in the article nor on this forum into the realms of boiler construction beyond the establishment of boiler shell thickness calculations and I will reiterate the statement in our article:-

“There may be operational factors acting on the boiler that will increase the shell stress but that cannot be readily determined. A boiler for a model traction engine for example will have additional stresses derived from its own weight when travelling over rough terrain.

Due to the wide range of uses and environments a steam boiler can be used in, the authors have not added any allowance for such environmental conditions and leaves it to the boiler designed to account for such factors in the design.”

We will respond to technical questions relating to the YPM article but feel that we have no responsibility to go further than the stated limits of the article.

By the way, since you have posted our short form Bios on the forum (did you get the Editor’s permission to do that, since he previously decided not to) how about posting your own?

PS Spelling mistakes in your post:-

PHd should be PhD
respondants should be respondents

 

 

[MEinThailandParticipant @meinthailand]

Yes, that’s correct. As you say “designs were considered to be within the elastic range.” ie stresses should not exceed the yield point and that is the whole point of the Yield Point Method.

[JasonBModerator @jasonb]

Only compressive testing for me at College and that was on concrete though did learn to do structural steel calcs as well as ones for reinforced concrete. Though I have always engaged the services of a structural engineed when needed as their experience is far greater and I doubt Building control would approve my calcs but are happy to with known local engineers and probably don’t look over the calcs in much detail.

[duncan webster 1Participant @duncanwebster1]

Criticising someone’s spelling really is scraping the bottom of the barrel.

[JasonBModerator @jasonb]

I have actually posted the long form as well as the short as that gave details of what subjects the qualifications were in.

You yourself posted the link to it earlier in this thread so  not sure why there is a problem with me reposting the link or a screen shot of what is on your forum that you were happy to post the link to.

I would imagine that Neil’s main reason for not including it was simply one of space. You have what is in the short version included in the title photo and the rest would not really have added anything to the content of the article.

 

Before I go any further I detect a bit of a Sock puppet here. Are the two of you posting as one member?

I will also repeat that I am sorry for my minor Dyslexia most people would make allowance for it once aware.

[duncan webster 1Participant @duncanwebster1]

If I might be allowed to widen the scope a little, I’m thinking about making a gas fired boiler with a horizontal shell, horizontal firetubes and the fire in a box under the shell, as described by KNHarris in his book on model boilers. It is tempting to just cram in as many tubes as possible, but the tubes will be hotter than the shell, and so there will be differential expansion. Full size boiler designs allow for this ‘breathing’ by keeping the outermost tubes away from the shell, but I no longer have access to BS at work, and they are expensive to buy. Anyone care to come up with a figure for tube temperature? I’d guess it won’t be more than a few degrees above water temperature or you’d get film boiling, but we’re getting a bit above my pay grade.

[SillyOldDufferModerator @sillyoldduffer]

An technical article isn’t proven correct just because the author has engineering qualifications, and nor is it proven wrong because it disagrees with the readers past practical experiences: experience is unreliable.  Rather technical content stands or falls on it’s merits, not opinion.

I welcome this interesting article and it’s stimulated debate.   Touches on a problem, which is a small-c conservative inspection system that makes it difficult for model boiler makers to innovate in the UK.  Unfortunately, challenging the status quo always upsets people. Here, there’s a rift between modernisers and those who want live steam to be done in the time honoured ways they understand.  The rift might lead to an emotional exchange of schoolboy irrelevances as opposed to a quality discussion.

This is about engineering, not ego.   We’re supposed to be reviewing the article, not rubbishing the author or readers.   Polite review allows mistakes to be corrected.   And if the words and music have been misunderstood, the author is here to explain.  We all benefit from a civilised review, where the rules of debate are followed:

1. Simply announcing an article is wrong isn’t good enough.  Critics have to show why it’s wrong. Experience alone does not explain “why” because experience could be out-of-date, sub-optimal or irrelevant.
2. Don’t criticise authors, criticise what they say.
3. Concentrate on important points, not trivia.  Though it’s easy to get hung up on them, typos rarely matter!

Dave

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, my understanding of these replies is about deformation or not in the relative ME&W article. The point being that the yield point of the material is the deciding factor, but of course, that should be of the weakest material in the design. So the design is best if it’s use is in the lowest part of the elastic range in the weakest material. Once a material reaches the yield point, there will be permanent deformation, once that happens, there is no second elastic limit, and deformation will continue to the eventual breaking point, just as the Tensometer graph above shows. I think the science shows why things are wrong.

Regards Nick.

[noel shelleyParticipant @noelshelley55608]

Nick Farrs graph brought back fond memories of happy days. I read the article and didn’t spot any mistakes, I found it interesting as indeed I did the article on FEA some time ago. I’m not a metallurgist, this is a simple question. If the copper shell is annealed and there is minor deformation under a hydraulic test at 2 x WP then will the movement not cause the copper to become work hardened and regain it’s strength ? With safety factors of 6 or 8 does it make any REAL difference if the shell is .112″ or .122″ or .125″ ? If only .118″ (3mm) is available then just drop the WP if need be ?

If the aim of the authors was to increase the level of professionalism in model engineering, then having seen work by many that is truly stunning over several decades I have to question the need for such. That one seldom if ever hears of any sort of boiler failure even in use, within the fraternity would seem to indicate that the existing designs, construction methods and testing work. I have no wish to stand in the way of progress but change for changes sake, or little or no benefit I see as pointless.

As to the chronology of the development of the wheel, roller bearings predate alloy wheels by decades !  Noel.

[Neil WyattModerator @neilwyatt]

I anticipated that this article would generate debate.

Just to say, the magazine is not a peer-reviewed journal, and biographies of authors (unless part of the point of an article) are not normally used.

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Noel, I understand what you are saying, and I’m not a metallurgist either, but they did go into it in detail in my Welding Institute course. The main thing’s were producing a design, choosing materials that will cope with all the pressures, loads, temperatures etc. and then add on the safety factors required. Testing would be done only up to maximum permissible loading, where upon no permanent distortion or other failures would present themselves. If you need a short unsustained period of overload; that should be included in the design.

Regards Nick.

[Charles LamontParticipant @charleslamont71117]

As the person you can all blame for starting this thread, can I request a bit of cool. 20 posts yesterday and 16 already today (at the time I started writing this one )is a lot to digest. Restraint, respect, and sticking somewhere near the point, please.

Trying to address some of these:

I am not a traditionalist reactionary. Any proposed change of design criteria needs to to be thoroughly explored.

Right now I do not have access to the AMBSC codes so I don’t know what they say about permanent set in copper boilers. The AMBSC codes are not something with which I would lightly take issue.

The debate is not about the details of a rather trivial calculation, but about the design philosophy for a structure made with annealed copper.

Annealed copper does not behave exactly like mild steel under stress, so comparisons should be treated with caution. In particular, copper work hardens and mild steel does not.

Nick Farr – In a tensile test, stressing a ductile material beyond yield and into plastic deformation does not lead to failure. That only happens if you keep on increasing the load. If you take the load some way beyond yield and into the plastic region, and the relax the load, the test piece will relax through the elastic range, but it will have some permanent deformation. If you increase the load again, to the same as before, then there will be no further plastic deformation. The test piece will stretch elastically to the same total amount of deflection as before.

“Deformation of the boiler when undergoing testing is a sign of weakness in the design or manufacture of the boiler… “   What does ‘sign of weakness’ mean? As has already been said, most builders will have conducted their own pressure tests on a boiler before submitting it to the inspectors, or is that not allowed? If we are talking about the first ever pressurisation after construction of a copper boiler, then yes, I would say that some permanent deformation may well be expected and acceptable, and is not necessarily a sign of weakness. Can anyone tell me that a bit of  work hardening is bad or dangerous?

In many instances yield point, or proof stress design is perfectly adequate, but it should be remembered that it is still a relatively crude method full of assumptions – but then so is designing on the basis of a UTS that seems to be ill-defined.

Credentials: I graduated in mechanical engineering from Imperial College, London, in 1975, and am an A.M.I.Mech.E. (In my subsequent engineering career I never needed to go for full professional status.) I have little practical experience of making model boilers. I am not a metalurgist.

The clarity and precision of an article, with all the riders and caveats, does not mean that people will not misunderstand it. You only need to look around this forum to see people answering questions they obviously did not properly read!

 

[noel shelleyParticipant @noelshelley55608]

Charles, surely mild steel DOES work harden ? Thats why after bending it is difficult to straighten where it was bent. Noel.

[Bob WorsleyParticipant @bobworsley31976]

Just to keep the pot boiling, does copper exhibit the same strength difference as iron when tested along or across the grain? I have never seen this mentioned. Because if using a drawn tube for the barrel then the drawing will, possibly, increase the strength along and not around, exactly the opposite of what is needed.

As for cramming as meny tubes in a boiler, why? I did post some observations on this and got well and truely flamed. After a lot of work I found that a steam bubble is about 1/4″ diameter, so putting tubes less than this is pointless. As LBSC said, you can’t scale nature, but even he never seemed to consider a clearance between tubes of 1/16″ to 1/8″. Likewise half the steam generation comes from the firebox, snag is the clearance between the inner and outer firebox is frequently only 1/4″, so how does the cold water flow down the gap to be boiled. You see boilers with sloping throatplates, the only real reason seems to be the increase in water space.

Seems to me that the firebox is redundant in a model boiler. What would be better is for the inner firebox to be a piece of 1/4″ copper plate, or bar, taken through the throatplate and continued along inside the boiler. Copper is a good conductor, why not use it? Removing all the staying from the flat sides would automatically inprove the safety of the boiler, staying isn’t easy to do.

 

 

[noel shelley]

On noel shelley Said:

Charles, surely mild steel DOES work harden ? Thats why after bending it is difficult to straighten where it was bent. Noel.

Yes. What was I thinking?

[Michael GilliganParticipant @michaelgilligan61133]

Much of it is probably superfluous to the present debate, but here’s a free 96 page document from an authoritative source:

https://www.copper.org/publications/pub_list/pdf/copper_tube_handbook.pdf

… surely worth a look if you work with copper tube

MichaelG.

[Nigel Graham 2Participant @nigelgraham2]

Bob –

What sort of boiler do you envisage? You need put the fire somewhere, whether it burns coal, oil or gas!

The sloping throat-plate is likely matched by a sloping tubeplate, or to accommodate a combustion-chamber, and was used in full size on some of the larger locomotives, such as I think the biggest BR Standard classes. On a miniature boiler a sloping throatplate and vertical tubeplate may help the water circulation – your question on that is valid, but such boilers all work even with all-vertical plates.

 

I don’t understand your replacement for the firebox. No-one says fitting stays is easy, but if you suggest omitting them I can only assume you have never seen – and I was also party to having to condemn – a small locomotive boiler, brand-new but ruined by such lack. (The fault lay entirely with the drawings, not the unfortunate builder. This would have been his first model-engineering project and he’d followed the “design” faithfully. We asked, but he refused to tell us the anonymous drawing’s source.)

 

Are you thinking of the Briggs pattern boiler, which has a dry-wall firebox with a coiled water-tube wall? Or the marine-type boiler with cylindrical firebox used on some narrow-gauge locomotives?

I have a copy of ME&E* carrying a description of the original Briggs boiler. The shell is a simple cylinder with a right-angled step formed in the back end to be the tube-plate and firebox crown. The assembly is still a round-topped locomotive-type by external appearance. It was for a freelance traction-engine of about 3 to 4- inch scale, judging by the photograph, so the right shape. For a locomotive you could disguise it as a Belpaire, LMS or LNER pattern by suitably shaped cladding! It still has a firebox for a real, living fire, though.

 

*ME&E… that dates it!

 

[Michael Gilligan]

On Michael Gilligan Said:

Thanks for making the article more widely accessible

Although I have no personal interest in the design/manufacture/use of small steam boilers, I am encouraged to see some discussion of the test processes.

… I could, after-all be the innocent bystander who gets injured at some event.

As an “intelligent layman” I found the article interesting and informative … and not particularly contentious … It is helpfully peppered with caveats, and it thereby exposes the fundamental weakness of any and all boiler testing in the Model Engineering domain.

Whether you test by YP or any other protocol … Unless you test to failure you will never know for sure !

Keep up the good work … there’s still a long way to go.

MichaelG.

MichaelG many thanks for your positive feedback. It’s refreshing to hear from an “intelligent layman”  that you found the article “interesting and informative … and not particularly contentious”.

Also I find your “fundamental weakness of any and all boiler testing in the Model Engineering domain.” interesting.

Model copper boiler design is indeed fraught with difficulties and misunderstandings. Only by keeping an open mind and following proper mechanical engineering design processes can one begin to unravel the mysteries from 100+ years ago that have survived and still continue to be applied today, without question.

 

Thank you again for your positive post.

[Author]

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