My 1st Boiler Design and Build

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Jan 4, 2010
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Hi All,
I have been reading the various very interesting threads on this forum and thought I might add what I have been doing...

I have always wanted to build a live steam locomotive and during the final year of my engineering degree, a couple of years ago, I thought I might be able to come up with a boiler design, as the first step and the topic for my final year project.

I took this idea to one of my lecturers and he looked at it and said; "Steve I don't think you should design this.... You should design and build it!"

Without having a specific loco in mind I started with some general external dimensions for a 4-6-0 type 5" gauge loco boiler. From there I conducted some research of current design practices and an extensive heat transfer analysis of various fuels and boiler configurations.

One of the objectives of my project was to come up with a design that was efficient as possible based on the heat transfer 'theory'. As I found out, this requires a number of assumptions and is not straightforward. An excellent book on the subject; “Steam, its Generation and Use”, Babcock and Wilcox, 1978 states on calculating furnace temperatures;

“An analytical solution of the problem of heat transfer in the furnace of a steam generating unit is extremely complex. It is not possible to calculate furnace outlet temperatures by theoretical methods alone.” It goes on to list factors to be considered such as furnace geometry, fuel variation, surface variation and load as some of the factors that influence temperature. Also; “Temperature varies throughout the furnace. Fuel and air enter at relatively low temperatures, reach a high temperature during combustion, and cool again as the products of combustion give up heat to the furnace enclosure. All temperatures change with load, excess air, burner adjustment and other operating conditions.

Based on a theoretical load, the completed design would need to produce 10.5kg of steam per hour, at 700 kPa which would require 7.7 kW of energy input. In order to do this efficiently a Firebox with as large a surface area as possible was incorporated. 18 Firetubes were included of 11.1mm ID (Inside Diameter). This was slightly larger than the optimum diameter for heat transfer in order to allow for soot build up and cleaning.

Analysis of the draft design was based on an optimum theoretical efficiency of 82%. This equates to the burning of 1.027kg of Bituminous Coal per hour and an energy input rate of 9.38 kW. Based on these figures the calculated theoretical efficiency was found to be 91.5%. With the aid of these results a detailed design was prepared in CAD. The draft design was developed IAW the AMSBC (Australian Miniature Boiler Safety Committee) Code for copper boilers and in consultation with a certified boiler inspector.

Theoretical and Computational Stress Analysis of the boiler design were calculated and amendments to the design were made as required. The Maximum Shear Stress (Tresca Stress) was found to be 15.87 MPa and the Factors of Safety were calculated for the Boiler and found to be 2.65 against Yielding and 7.1 against Ultimate failure.

With the design approved material and tools were purchased and construction began with the Waterjet cutting of all copper plate and Laser cutting of 16mm mild steel “Formers”. All of the boiler plate was formed to shape and the Barrel rolled to the correct diameter. The Butt Strap was Silver Brazed to the Barrel. The Throatplate was fitted with a combination of Bronze and Silver Brazing. All the Boiler Bushes and the components for the Boiler and Regulator were machined to size. The Inner Assembly was completed, passed inspection and fitted into the Barrel.

So over 12 months I embarked on a very steep learning curve that started out as a blank sheet of paper and ended up with a successful hydro test...! As you can see the boiler I ended up with, is not to dissimilar to many standard designs. For the construction the book; "Model Locomotive Boilermaking" by Alec Farmer was always close at hand in the shed and highly recommended.

I had never built anything like this before and it was a real challenge and a very valuable learning experience. For those considering it, have a go, working with copper is great fun.

Here are some images....

This is the final design, completed with Solidworks 2006


Here are the boiler components being profile cut on a Waterjet machine


Forming the firebox tubelplate. This required annealing 5-6 times from memory.


Some of the completed boiler components


More pics to come...

Looks like a very nice boiler. :bow:

Welcome to our forum. wEc1

I see you are wearing "stokers overalls," although you look to young to have served when the RAN had steam engines and proper boilers. ;D

Now you've made me curious, how about posting a little about yourself in the welcome thread. ;)

Best Regards

Welcome aboard. wEc1

Where did you earn your degree?

Please keep the build photos coming. th_wwp

Thanks for sharing.

Glad to have you. Keep those build photos coming, please.
Welcome to the forum, I will be very interested to see your test results when you fire your boiler. I think that your efficiency ratings are very optimistic, if they could have obtained those they would still be running steam.
"Analysis of the draft design was based on an optimum theoretical efficiency of 82%. This equates to the burning of 1.027kg of Bituminous Coal per hour and an energy input rate of 9.38 kW. Based on these figures the calculated theoretical efficiency was found to be 91.5%. "
thanks for the positive replies,

well spotted with the overalls, indeed they are an old pair of my stoker's kit and I am still a member of the RAN. Are you an old stoker yourself? I like your profile pic of DDG 41, a great ship. Here is a pic of me at sea in BRISBANE as a Leading Hand in 2001. The DDGs had a fascinating steam plant and to hear them in full flight was a treat. Can you identify the location of this pic?


I went to RMIT in Melbourne, Australia.

you are quite correct on my optimistic efficiency figure for a firetube boiler! However in order for me to start the project of designing an efficient boiler, I had too many unknowns and had to make some assumptions and then work back from there. The 82% is based on 'theoretical' optimal efficiency given for a firetube boiler by Malek in “Power Boiler Design, Inspection and Repair”, McGraw Hill, 2005. As we know in reality the losses in our boilers are large and very hard to measure and the actual efficiencies are far less than 82%. See the table below for various types of boilers.

Boiler Efficiencies of Various Design

Watertube 85%
Packaged Watertube 88%
Firetube 82%
Firetube 4 pass 88%
Packaged Firetube 87%
Tubeless vert 81%
Electric 83%

Referring to the table above, it can be seen with this being a single pass Firetube boiler the optimum efficiency that can be hoped for is 82%.

Mohammed Malek in his book, explains Boiler Efficiency:

“The actual loss in the system is not simple to calculate. To calculate the exact efficiency, all the heat losses should be calculated. The total heat loss is the sum of heat losses due to steam leaks, radiation, blowdown, heating of combustion air, condensate, makeup water (not relevant here as not a closed system, i.e. no condenser, feedwater supplied instead.) and stack. The biggest energy loss is in a boiler goes up the chimney. The stack gas heat loss includes heat loss due to dry gas, moisture in fuel, and water from the combustion of hydrogen.”

I too, will be most keen to see how the boiler performs in reality, I am part way through building a 'Springbok' to Martin Evan's design to go under my boiler, but I have a way to go yet...

Welcome Dusti, I am looking forward to seeing more pics of your boiler.... very professional approach to a very professional looking boiler.... lets see more of it.


Welcome Dusti

Thats looks a great boiler, interesting that you designed it up from first principles, I've always had an yearning to build a model of a tram engine that was used to move goods around Crewe Loco works, the design would require a navel boiler (round fire box) I've skeamed a design out with the help of Martin Evans' "Model Locomotive Boilers their design and construction" for model boilers he quotes very low efficiencies, one way to spark a lively debate on model locos is to bring up the ? of efficiencies, and to heat up the debate (pun intended) just throw supperheating into the ring.

Springbok is a very powerful Engine I've seen one running on our club track, but they take quite some time to build, I've been building my Loco now for about 3 years, but I havn't worked on it continuously, I like to take a break from it, from time to time, and build smaller stationery engines, something you can complete in a couple of months.

Look forward to seeing more pics and progress reports on the loco build.


Thanks Stew,
you are right. I certainly don't want to start a major debate, I do not claim to be an expert, this is just what I came up with based on a number of initial assumptions. Also most of the modern literature understandably concerns boilers used in power generation. Not many people designing steam locomotive boilers these days!
I have been admiring the beautiful work in your thread Stew and it inspired me to get my pics on the forum. Funny you should mention smaller engines, I was looking at videos of the Stuart Victoria Twin

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on YouTube, a beautiful engine and am thinking about one of those as a side project.... not that I need another.....
Here are some images of some of the calculated stresses in boiler during operation. The images were created using the “COSMOSWorks” add-in for Solidworks. Represented is the stress distribution throughout the boiler shell at the operating pressure of 700 kPa. The Boiler Barrel CAD file model was restrained in the analysis, as it would be in the assembly. The Yield Strength for the copper in the simulation was entered as 42 MPa. It can be seen from the analysis that there will be some localised plastic deformation, notably at where points where stresses will be concentrated around the stays. Any localised yielding of the copper that was to occur, will be during the hydrostatic testing which will be at twice the operating pressure.


I looked at various methods of providing the correct amount of support to the backhead and smokebox tubeplate. Longintudinal stays really cluttered up things and I wasn't keen on them.


So IAW with the AMBSC code I went with Girder Stays which I think were a better option.


Here is a computational representation of the backhead with the girder stay in place. More room!


Thats a lovely bit of modeling Dusti

I was talking to our boiler inspector and he was telling a horror story of a guy who tried pressure testing a boiler with a stuck pressure gauge he just kept pumping only stopped when he noticed the copper arround the stays was begining to look like checker plate:- just how your model shows.

Stuart victoria does build up to a very nice engine but personaly I like engines made from bar stack not casting as the quality of some casting you get with kits are very poor.

Hi Dusti,
I hadn't realized that they were rating boiler at that efficiency. I haven't done any work with full size ones since the 80s, and if I remember correct we were rating them at about 20% below that. I looked up the book you mentioned :power Boiler Design, Inspection and Repair" at my local library system, they have two copies both reference only at the main library which is downtown. I will have to keep a look out for a used one. Please keep us informed on how this goes.

No2 Engine room and you've got your hand on the port ahead throttle. I was responsible for the aft unit when she was being built by Defoe in Bay City Michigan 1967. At the time I was an ERA1.

Of all the ships I served in, Brisbane was my favourite, as you can see by my avatar. ;D

Later I went back to her as Senior.

The other bike shop.


Best Regards
a heartbreaking and expensive lesson in that story! Certainly when it came to applying pressure to this boiler for the first time, as part of the hydro testing, we took it very carefully and slowly. Just to be sure everything was as it should be. If nothing else, as you know, these boilers are a big investment in materials alone........ not to mention time!

the topic of boiler efficiency is a complex one. I have been having a read recently of 'The Red Devil and Other Tales from the Age of Steam' by David Wardale. A most interesting book for those interested in steam locomotive development in more recent times. In it he discusses the South African Railways 25NC class and it's boiler performance characteristics. He presents a graph where the maximum boiler efficiency is given as 37%, at the Grate Limit, the Grate Limit being the point at which admission of more fuel/air produced no more steam. I have a lot more to learn on this subject......

great clipping and you are dead right on the photo location, No 2 engine room. I am sure you had a great experience during the ship build.... They (steam powered destroyers) have been gone nearly 10 years now Bob, but as you would imagine they are still talked about regularly and fondly by everyone that served in them. I remember the brief time I spent in BRISBANE was like going back in time..... Brilliant.

Here are some more pics;

Backhead Stress Distribution.


Boiler Displacement During Operation


The above image is included as a graphical representation of the displacement measured in millimetres of the Boiler Barrel. It must be noted, that the analysis does not consider any thermal expansion, only that caused by internal pressure although the material properties are representative of the operating temperature.

It can be seen that the maximum displacement is 0.01172mm, less than half of 1/1000th of an inch. What is clearly evident from this analysis is that the average displacement of the flat stayed area is approximately 4 times less than the red areas on the curved surface of the barrel. This would suggest that the copper in the flat stayed area is well reinforced and this could possibly be reduced either by reducing stay diameter or increasing the stay pitch.

The stay diameters used in this area are approximately 15% larger than the AMBSC Code requires and the pitch used was slightly smaller. Considering there are already substantial factors of safety built into this Code, this area of the boiler constructed accurately, should be very strong.

Annealing the Throatplate


Bending up the firebox wrapper. One of the terrific things I found I like about boilermaking is all you really need is some very basic hand-tools. Or in this case some 3/4" stock, a vice and a piece of wood!


Some finished boilerplates.....

Hi Dusti,
I admire the work you have done, both in the design and construction of your boiler, however, I would like to put a couple of thoughts up for your consideration.
The AMBSC code allows a maximum allowable stress of 26MPa for annealed copper, in which state most copper will be if it has been silver soldered, while you have apparently used 42 MPa in your Cosmos analysis. As I didn't get very far with Cosmos before I retired, and I don't have access to it now, may I ask if you would perhaps re-run your analysis using the lower value, just to see what difference it makes, please? Also, I suspect it would be more meaningful if you perhaps drew a complete boiler (ie, including tubeplates, tubes, and stays), then sectioned it along its vertical centereline, before "applying" the internal pressure. As drawn, the "restraint" you have applied to the barrel is a little suggestive of all edges of the drawn barrel being totally fixed. Copper being quite plastic, this assumption may be a little unrealistic. Please do not think I am being obstructive. I strongly believe in using FEA for boiler design/analysis, I just do not yet have the experience I would like to have in using it meaningfully, and wish to learn more.
From your photos, it is unclear to me if there is an internal radius to your flanges. Is there in fact a radius there, and if so, what is that radius please?
Best regards,
Ian Kirby.
Wollongong NSW
(Recently retired AMBSC Chairman)
appreciate the input, I would not have started the thread if I wasn't willing to get some feedback. I would have liked to have had a chat to you during my project!

I don't have access to my notes at present Ian, but will attempt to answer your questions as best I can from memory. (I did all this work over 3 years ago now..) The COSMOS add-in for Solidworks that I used, was limited in its application. It could analyse individual parts, but not assemblies.

I agree the analysis is not as realistic as it could be. You have rightly pointed out that an analysis of the complete assembly would have been better, however my version of the Software didn't have this capability. All it could do was analyse individual parts, so I set up a simulation to model the boiler shell, restrained as closely as I could as if it was part of a complete boiler. With the old computer I had the simulation would still be processing now if I had manage to model a complete assembly!

Why I used the value of 42 MPa in the analysis for the copper I cannot clearly recall. I believe from memory, it was the Yield stress of the material I used at boiler operating temperature. But I will clarify this when I check my records (including the flange radii).

I agree it would be worth re-running the simulation at 26 MPa and I will look into that.

This is a really nice build. Enjoying the pix!


G'day all,
have been away recently, hence no recent additions to post. Was lucky enough to get to the annual steam rally at the Guildford MES in the UK, which was terrific. Here are some more images of boiler construction.

This is some trial fitting of boiler tubes into the firebox tubeplate.


Beautiful copper swarf


Girder stay bronze brazed into smokebox tubeplate


Here is the boiler former and plate prior to work. The former was made from pieces of pine timber, glued together and then turned on a wood lathe. It worked very well.


The former and boiler barrel plate set up prior to bending.


This is the fun part. The method looks pretty agricultural and I guess it is, but with a soft hammer, plenty of annealing and persistence I was happy with the final result.


Squaring up the barrel with a couple of long clamps.


The finished barrel in the pickle tub.


Trial assembly on the office floor....


Trial assembly from the backhead.


Fantastic work Dusti :bow: You should be very pleased with it. :D

Best Regards
Dusti, Thatis sure a fine start on that boiler,really looks good, thanks for the pictures. larry
Man that looks great. Really impressive...

I'm a SolidWorks guy and love modeling parts prior to making them. Extremely impressive.