fang
Seasoned Member
Posts: 100
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Post by fang on Jul 18, 2014 16:26:31 GMT
Not sure if this has been asked on here before, but is it acceptable to build a riveted boiler?? I did think that if I decided to use copper I could effectively caulk the joints afterwards with a solder of some sort.
On a slightly separate issue, why is a minimum of 6mm generally used for steel boilers for models? If you calculate the pressure you could safely use in these boilers it is a pretty large number!! I can understand the added thickness for corrosion but 6mm seems quite excessive to me, doing calculations for various diameters of boilers it seems that more like 1 - 1.5mm is what is needed, so adding on a corrosion allowance 6mm still seems excessive.
Wondering if it is possible to build a boiler from approx 4mm steel plate and rivet it rather than weld regarding the boiler regulations
They do it on full size boilers, but can't recall seeing any riveted copper or steel model boilers
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jackrae
Elder Statesman
Posts: 1,333
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Post by jackrae on Jul 18, 2014 18:36:41 GMT
Basically, you can build what you want for use in your own garden (insurance liability ignored), but problems will arise when you present it for certification for use on a club track.
Questions such as these (and others) will be asked :-
To what previously accepted design detail did you build
If not to a previously published or "approved" design can you provide a complete set of stress calculations justifying your design
Do you have material certificates for your build materials
Did you get the club boiler inspector to approve the design prior to commencement of construction
Did you involve the club boiler inspector in progress inspection of the build.
If the answer to any question is "no" then you are literally on your own, as no club inspector will entertain the product.
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fang
Seasoned Member
Posts: 100
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Post by fang on Jul 18, 2014 18:54:02 GMT
I have read the code of practice for boilers, but cannot see any specific guidelines for methods of construction, minimum thickness of material etc.
Obviously if you calculated the thickness required and it came out at 0.5mm you would more than likely use something thicker. What I'd like to know is are there any requirements that need to be adhered to regarding construction methods or material thicknesses, or is what appears to be a fairly standard 3mm copper/6mm steel rule for boilers only done because that's the way it always has been done??
I did notice on my air receiver tank (200L) it has a plate showing material thickness and maximum corrosion allowance, I know it is not a steam boiler, but it still has water sat in it most of the time corroding it away, and it is only made of 1/8" steel!!
Obviously I want to be safe and everyone else to be safe, but equally I want to make highly detailed models, so would rather question why something has to be a certain way
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fang
Seasoned Member
Posts: 100
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Post by fang on Jul 18, 2014 19:00:54 GMT
Quick calculation from the figures in Martin Evans book
Copper 25,000 psi UTS heat factor x 0.8 equals 20,000psi
Steel 60,000 psi UTS
So 1mm thick steel has the same UTS as 3mm copper, so if you were to use 3mm steel you have 2mm allowance for corrosion before it gets to the same strength of the full 3mm copper!!
Agreed 1mm is rather thin to get down to, but to me this suggests that something more like 4mm steel would ok??
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jul 18, 2014 21:02:58 GMT
hi fang,
with the greatest of respect you are ignoring the excellent advice from jackrae. it's not a matter of what you think is acceptable, but what our insurers consider acceptable.
cheers,
julian
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Post by andyhigham on Jul 18, 2014 22:00:47 GMT
Insurers know nothing about engineering, they are basically bookies. But their word is law.
Same with motor insurance.
It costs me about £450 fully comp to insure my Audi A6 avant 2.7
For my very powerful 150MPH Suzuki 1100 it costs about £220 fully comp
For a crappy little Pugeot van with a 1.9 engine the fully comp cost was over £1000
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Post by ejparrott on Jul 21, 2014 17:14:48 GMT
I'm sure RSA would be pleased to hear you say 'Insurers know nothing about engineering'. The man I spoke to about boiler design and acceptance knew more about it than I did. You need to do your homework.
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Post by andyhigham on Jul 21, 2014 18:22:21 GMT
Maybe I should have said underwriters
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fang
Seasoned Member
Posts: 100
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Post by fang on Jul 21, 2014 19:11:23 GMT
Maybe the green book should include a section compiled by the insurers and the people who compile the green book so that at least there are certain guidelines, both to people building boilers so that they can ensure what they build is acceptable to the underwriters, and also so the boiler inspectors know what the underwriters will accept. It seems like many things the vagueness of the green book is up to the interpretation of the individual inspecting the boiler. I'm sure by now what the number of boilers that have been built something sensible could be drafted.
I also note that Martin Evans book does quote thickness of boiler tubes for mild steel as thin as 16 sag
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jul 21, 2014 21:58:31 GMT
this is a bit of a can of worms, and your quote from martin evans i cannot find - all he states in his 'model locomotive boilers' book is that "steel boilers are generally used for 2" scale and larger". no mention of 16swg.
when you have a collection of MEs going back 60 plus years as i have you can usually quote a previous design that ive based a new design on with just very small differences/improvements. i am quite able to do the calculations if required. however there are huge discrepancies! martin evans favoured a large pitch for stays of large diameter from gunmetal that i think is very iffy, and then advocated monel metal which johnson matthey then advised wont silver solder well! don young always advocated closer pitched stays. martin evans dispensed entirely with crown stays connected to the outer firebox wrapper on lots of his designs. don young never did. i beefed up my BOXHILL boiler by substituting the butt joint to barrel throatplate with a double flanged throatplate, substantially increased the number of stays, and increased the thickness of the inner wrapper and width of all flanged joints, and made the crown girder stays attached to the outer wrapper. my club boiler inspector took the view (knowing perhaps i have a certain level of judgement and experience in such matters) that although i had departed from the drawings what i had done constituted improving upon the original 1964 design. but someone building the (what i consider to be suspect 1964 design) will get his boiler approved and many have been built to the 1964 design which have been long lasting and perfectly safe.
there are lots of older designs where problems with the design are well known and extra stays or a different method of construction will be advised by the club boiler inspector.
the problems occur when 'tyros' decide (due usually to work experience) that the designs can be simplified or cheaper materials substituted, without any previous experience of making a miniature boiler previously! dont dismiss out of hand the club boiler inspector's judgement and experience! as well as passing many well made boilers they will have failed many badly made boilers for very good reason.
a well made copper silver soldered boiler will last at least 50 years. initially expensive but in the long run much better for thermal efficiency and longevity than a steel boiler.
there are no short cuts in this lark! even the TIG specialists have to use special deoxidised copper!
cheers,
julian
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Tony K
Elder Statesman
Posts: 1,573
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Post by Tony K on Jul 22, 2014 7:51:59 GMT
Fang, I think Julian and jackrae sum it all up pretty well.
We have a system which has been proven to work. The insurers have input to the testing regime in place and it, as far as I have seen, is carried out very responsibly.
Yes, the green book could be expanded into a bible followed to the letter by inspectors, with no room for judgement, and constantly updated with yet more "clarity," but it is unnecessary.
Getting insurers involved more would just complicate matters and bring the day nearer when we have to use costly professional boiler inspectors with all the nonsense involved.
It works - leave it alone.
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Post by Jim on Jul 22, 2014 9:04:20 GMT
I couldn't agree more with Tony and Julian's comments. The various codes, be they in the UK or here in Australia, are the result of much dedicated hard work by fellow modellers determined to get a set of accepted standards that allow modellers to construct safe boilers to accepted safety standards. It's a system that works for us as modellers. As Tony says, leave it alone.
Jim
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Post by ejparrott on Jul 22, 2014 9:32:46 GMT
I don't exactly agree that updating the code will bring about the day we have to use professional testers, I think there's nowhere near enough for them to worry about. Testers may have to go on a course perhaps yes.
However, I have been campaigning for clarity on boiler design for years, along with several others, and every time we are ignored. The Aus code covers it, why can't we have a standard set of formula's in the book which every designer, builder and tester can refer to, then we all know the boiler will be accepted by the governing body first time. The Aus code has a very simple table that says tube plates must be 12mm thick for boilers up to 300mm diameter, 16mm from 300 to 350, simple! We've got nothing in our code, so a boiler tester code look at a 6mm plate, say yes it'll take the pressure, go ahead, whereas another might say no, in the Aus code, it's defined, there's no discussion!
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Post by Jim on Jul 22, 2014 10:23:32 GMT
My apologies Ed, I'd assumed your codes were as specific as ours in allowing you to design your own boiler within the parameters of the code and supervised by the club boiler inspector.
Jim
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Post by ejparrott on Jul 22, 2014 10:38:34 GMT
we're allowed to design yes, and the inspectors can supervise, but there are so many methods of designing it that I can't see how the inspectors can know them all - we are volunteers after all! What I want is a set of codes in the green book, that say 'the stay pitch shall be calculated by this formula', then the inspector knows the formula is correct, can check the maths, and approve or not.
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Post by Jim on Jul 22, 2014 10:54:02 GMT
we're allowed to design yes, and the inspectors can supervise, but there are so many methods of designing it that I can't see how the inspectors can know them all - we are volunteers after all! What I want is a set of codes in the green book, that say 'the stay pitch shall be calculated by this formula', then the inspector knows the formula is correct, can check the maths, and approve or not. As you say Ed that is exactly the detailed information you find in the Australian Miniature Boiler Standards Committee's (AMBSC) codes. |
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Post by ejparrott on Jul 22, 2014 11:44:04 GMT
It would just be that's that, there it is, and it is it. simple! I do the role voluntarily, as all do, and I don't think it's right that it isn't defined. I can't be expected to know all the stress calcs there are in the world, imperial and metric equivalents, stay pitch equations etc. How am I supposed to know that the guy that just brought me some calcs hasn't made them up? I can check the math yes, but that doesn't mean his equations are good.
Anyway...I'm sure you're bored of me by now!
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Post by Jim on Jul 22, 2014 12:53:30 GMT
Anyway...I'm sure you're bored of me by now! Not at all Ed. It is interesting to hear and learn about the different way things are done in other places. By comparison we are very fortunate in having a very clear cut set of guidelines that removes the sort of quandaries that you and others are faced with. I can appreciate your predicament. Jim |
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fang
Seasoned Member
Posts: 100
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Post by fang on Jul 25, 2014 16:18:17 GMT
Totally agree with ejparrott. Without a written set of calcs, etc to work to, how does anyone know exactly what anybody is working too?? All I'm looking for is guidelines to design a boiler too, not ultra strict regulations which will hinder us all, but as stated with number of years people have been building model boilers there will be right ways and wrong ways to design and build them, and yes the club boiler inspectors can help and advise, but there is nothing written that ties up the person sat at home, the green book and the person inspecting the boiler.
Martin Evans "Model locomotive boilers" book, page 30 there is a table given "The following table therefore gives suitable thicknesses of seamless tubes and flues for boilers working at 80 to 120 lb. per sq. in." The table then gives tube diameters and thicknesses for copper, mild steel and stainless steel.
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jul 25, 2014 17:04:34 GMT
fang,
you have taken the above table on p.30 out of context as it refers (or is supposed to refer) only to flue tubes (going up only to 1 1/2" dia) NOT barrel material or firebox plates for steel.
cheers,
julian
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