Boiler Design – issue 4765

[JasonBModerator @jasonb]

It has worked for many years and I would not be too worried about sitting astride a copper or steel boiler that a UK inspector has passed. I said they tend to be people with experience in making and running model boilers and that comes before they become an inspector. Many will also have been or still are involved in engineering as their day job. So will likely have experience before they even become a “new” inspector.

As I said the UK code does not have much in the way of technical guidance, it says suitable material but as far as I am aware does not state what is suitable. Again knowing what is generally accepted comes with experience or asking if you don’t have that experience hence my comment that the boiler inspector should be consulted early on and they will guide you through what they require. Can’t say where I picked it up from but must have taken it in some time in the last 43years which is how long ago I made my first steam engine and boiler

[lezsmithParticipant @lezsmith]

Hi Jason,
Thank you for the update, I agree the best source of knowledge at this time are the dedicated model boiler inspectors.

However, I do worry without appropriate documented codes or authoritative design documents, I’m not talking about legislation, as the Federation of model engineering societies state, “At the present time there are no nationally agreed boiler design formula for the hobby of Model Engineering.”

Changing clubs or moving to a different part of the country where different boiler inspectors work, could lead to problems. We need a nationally agreed standard that all designs must comply with.

Yes, we do have legislation in the UK, but it is so convoluted and lacking in detail it is practically worthless, I’m referring to the “PSSR 2000” Joining the EU and then leaving the EU did not help, however the PSSR is recognized as the current legislation in force by the UK.

It is commendable that the Federation of Model Engineering Societies (https://fmes.org.uk) took the initiative to define Boiler test codes and Design codes to try to fill the lack of information to help the model engineering enthusiasts. Also interesting there has been discussion about using the AMBSC codes in the UK.

During my research I have uncovered some interesting facts about the ‘6-10’ safety factor in the UTS method, that shines a light on why it is included in the UTS formula. However, this post is getting a bit long so I will post the details in a separate post.

[JasonBModerator @jasonb]

Changing clubs or moving to a different part of the country where different boiler inspectors work, could lead to problems. We need a nationally agreed standard that all designs must comply with.

 

This would be quite problematic and could well end the hobby as we know it. Many locos and traction engines are running to legacy designs and some to updated legacy designs. If all these boilers are then to be tested to a new revised code then many may not comply so you have many club members unable to run their models which will have become almost worthless.

Also why the “We” you are not in the UK so will have different rules to follow if any. Or do you want an international code that may include Duplex like the Aussie code does and stainless like Europe and SA allow at club level testing?

[lezsmithParticipant @lezsmith]

As an Engineer and probably many of the members of this forum included I do not like formulas that include hard values with no explanation as to what they represent and why the value is as stated.

First one has to consider when the formula was written and the circumstances of that time. Martin Evans and K.N.Harris were clearly two of the pioneers of the model engineering hobby, books such as Manual of model steam locomotive construction (1960) and Model boilers & boiler making by K.N.Harris (1967) are still referenced today.

The famous UTS formula P = D x F x WP / TS x T x 2
Where:
P = plate thickness in inches
D = boiler diameter in inches
F = the safety factor. (this is between 6 and 10 but usually a factor of 8 is used)
WP = the working pressure in lb/in²
TS = the tensile strength of copper (a suggested figure is 25,000 lb/in²)
T = An allowance of 0.8 for pressures between 60-100 lb/in² and 0.7 for pressures from 110 to 150 lb/in²)

Included ‘F’ a safety factor, but we have a safety factor of 2xWP so why do we need another safety factor of ‘8’?

Back to the 60’s and early 70’s when Evens and Harris were experimenting with miniature boilers and writing their books, the available material at that time was still limited, copper piping was just starting to replace lead piping and there was a shortage of copper, also the machinery did not have the accuracy we expect from modern machinery and CNC was still being developed by MIT.

So, the UTS formula had to make an allowance for inconsistent plate thickness, cylindrical tubes not being true cylinders, handmade endplates with varying gap tolerances and plate thickness, quality and availability of silver solder, etc. Therefore, some compensation factor had to be built into the formula to account for these variables.

It is unfortunate that these compensation factors were lumped together and called ‘safety factors’, I now understand why MEinThailand objected to calling the ‘F’ variable a safety factor, in reality it is a compensating factor for design issues in the materials and construction that were unavoidable at the time the formula was first published.

So where does that leave the YP method?
First, we must consider do the same limitations and factors still apply to modern model boiler construction. I personally believe some do and some do not.

Most modern workshop equipment has accuracies and repeatability significantly better than was available in the late 60’s early 70’s, with CNC retrofitting of manual equipment the accuracy and repeatability is constantly increasing. However some things like manually working copper to form an end plate has not changed much in the last 65 years.

But what about moving forward, tig welding was not considered to be something a hobby workshop would have 60 years ago, what about 3D metal printing and laser cutting, tools and technologies that were only talked about in academic circles are becoming available to the hobby workshop.

Some have mentioned FEA is the way forward, the reality is most FEA models are still very simplistic, yes there are better models and more advanced analytics, but these are generally out of the reach of the hobby engineer some costing thousands of dollars. I hope professor Beach or one of the other professors at MIT can help move that forward.

Should we cling to a formula that was designed to give a reasonable assurance that a copper boiler built with all the limitations and artifacts of the 60’s /70’s would be ok, or do we move the work practices and formulas forward as the technology advances and materials are produced with higher quality and tighter quality controls.

I will leave this question with you whilst you think about the future of our hobby.

[lezsmithParticipant @lezsmith]

Hi Jason,
Firstly, thank you for responding.

I understand your concern however I believe safety always comes first, therefore if the current boilers are as many have said in the preceding posts “built to a standard that is proven to be safe” there should be no reason to doubt they would pass any reasonable test.

I also believe any reasonable standard would allow grandfathering of existing certified boilers, with the caveat boilers just like some other potentially dangerous items should have an annual or biannual test to ensure continued safety.

Whilst it is true I now reside in Thailand, being born and raised in Yorkshire but spent most of my working life in the USA (although much of that I was traveling for extended periods of time in SE Asia and Europe) I still consider myself to be British and proud of it.

Regarding a new standard, yes it would like to see the UK be a leader to define a new international standard it would have been easier for me to work with model engineers in the USA given my federal and academic connections, I also have a large number of relatives in Australia, I chose the UK and MEW to start the ball rolling, given all the negative feedback perhaps that was a mistake.

[Nigel BennettParticipant @nigelbennett69913]

I was only looking at the KN Harris book the other day and was quite frankly horrified how out-of-date it is. It recommends phosphorus-bearing silver solders (pp80-81) and a butt-jointed barrel (p67). Using “Good quality solid drawn brass tubes” in flues (p26) is not something I’d be prepared to accept, either.

[lezsmithParticipant @lezsmith]

Just for completeness my research has taken me down many rat holes, however a few nuggets were found along the way.

One of the Best resources I found for Model builders in the UK is:
https://fmes.org.uk

Here you will find the best and possibly the most current information available on the subject of model boiler making and testing. The boiler testing FAQ volume 2 answers many of the questions regarding the current UK standards.

https://fmes.org.uk/wp-content/uploads/2021/04/BTC2018V2-FAQ-February-2019.pdf

Additionally:
https://fmes.org.uk/wp-content/uploads/2022/01/Notes-on-copper-specification-November-2021.pdf

and

https://fmes.org.uk/wp-content/uploads/2025/04/Boiler-design-calculations.pdf

Are a great place to find relevant information on material selection and boiler design.

For those of us that are relatively new to model boiler building or just want to a place to start I recommend:

A guide to boiler and other pressure vessel construction, testing, certification and operation

 

[noel shelleyParticipant @noelshelley55608]

As this thread has now moved from YP/UTS to the future of the hobby ! That the 2 methods of calculating the minimum shell thickness have been shown to for practical purposes give similar results and that there seem to be few if any recorded incidents where the 2 methods of calculation would have made any difference may be it is better to leave sleeping dog lie ?

That modern methods are used and yet buildings and bridges fail catastrophically rather than be well  designed with a large and more than adequate allowance for any variables, of course money comes into this and sometimes dishonesty.

As the owner and operator of 2 small boilers both about 50 years old, very well made all be it ” back in the dark ages ” that currently are certified. I would still operate them even IF the regulations were changed. I have little doubt that many others would do the same so I fail to see HOW any change would increase safety. As for the future of the hobby it would have a serious impact, many clubs have tracks that may become so little used that their viability and that of the club called into question.  The making of a copper boiler involves many skills that need to be carried forward for the benefit of our young ( they will not ALL become computer programmers ), not ham strung by yet more legislation that will achieve little. If one were to add plate certification and possibly a composition analysis then the costs involved may be the straw that broke the camels back, copper is already very costly, stainless not permitted ( in the UK )and steel effectively only if commercially built.

To those who say I am prepared to operate an unsafe boiler, my answer is that I have both the knowledge and equipment to carry out tests to prove them.

To the two authors of the interesting original article I would say thank you, but having had your say, please leave well alone ! Noel.

[Author]

Posts