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Modern boiler construction methods for the model engineer

Model Engineering;228:4686 & 4688

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Luker08/04/2022 17:07:51
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164 forum posts
122 photos

The recent articles in ME on my methods of boiler construction using 'modern' techniques and materials hinted to the FEA simulations I do on all my boiler designs to check that they are safe. Here's a short video of two very different boilers I have built, one a Martin Evans copper boiler and the other the Wahya. Both simulations evaluated the stresses for the pressure test and the running conditions of the loco when in service. The failure modes for the pressure tests for the two different materials are similar but for running the design considerations are different. I hope you guys find the videos interesting... Something different in ME...

Jon Lawes08/04/2022 17:11:38
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926 forum posts

I love this application of modern techniques to our hobby. Brings it into the current age whilst maintaining the spirit of ME.

I think I'm a data nerd. When I was younger my cars all had as many gauges as there were things to measure...

Dave Smith 1408/04/2022 23:05:41
213 forum posts
43 photos

Luker

Similar to you I used FEA on ME boilers. First photo a section cut through the FEA on a steel version of the Highlander Boiler. I did this as a favour for a member of this forum and wrote a full report which used 3 justifications, first principles hand calculation, a comparison with the Aussie code and the FEA. This was done before not long before I retired and I had my Chief Stress Engineer do the a peer review on the FEA methodology and results as a check. The FEA showed up some issues with design as presented to me and allowed me to easily modify the design to eliminate the problems.

stress 1.jpg

I am also building a Don Young Aspinall. I have modified the boiler to remove the crown girder stays of the original design and replace them with crown rod stays. The boiler design was checked to see if it complied with and then modified in accordance with the Aussie code, how ever I also carried FEA on the boiler as designed and modified to for rod stays. The FEA results were quite interesting. The crown girders are very benign with reasonably low stress. However the some of the crown rod stays when designed in accordance with the Aussie code guidelines were marginal and needed to be increased in diameter to get the levels to a point where I was happy with them. The change to stays has been cleared with our BI's to the Aussie code but they are aware that the FEA has initiated further modification and fully endorse it. Little steps, but I am lucky in that one our BI's is a retied professional engineer like myself. Second picture is from the Aspinall FEA.

von mises.jpg

Luker09/04/2022 05:49:59
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164 forum posts
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Posted by Jon Lawes on 08/04/2022 17:11:38:

I love this application of modern techniques to our hobby. Brings it into the current age whilst maintaining the spirit of ME.

I think I'm a data nerd. When I was younger my cars all had as many gauges as there were things to measure...

Thanks Jon, aren't we all!laugh

Luker09/04/2022 05:55:03
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164 forum posts
122 photos
Posted by Dave Smith 14 on 08/04/2022 23:05:41:

Luker

Similar to you I used FEA on ME boilers. First photo a section...

Hi Dave,

Steel boilers tend to solve nicely in FEA, they don’t have the non-linear material properties typical with copper. Your simulations look good, and it’s good to know that other model engineers are using modern techniques to move the hobby forward!

It’s interesting that you also found the (crown) rod stays in the Aussie code to be marginal. For the Stirling mentioned in the article (my avatar loco) I built the boiler to that code using crown stays and I also found the code to be marginal (in fact there’s a few aspects of that code that can be improved upon). I also had to do a little redesign in this area. The crown stays are stronger and all my later boiler designs use this configuration.

I think the problem with crown stays comes from the old days where the silver solder came loose with subsequent heating, but with welding this is not a problem. I’ve also seem two boilers recently where the cross strut was left out, and the comment was that little plate does nothing. Took a little explaining as to why that little plate is so important for the crown stay!

For the record, I don’t want to create the impression that all boilers need to be verified with FEA. If the design is published, or proven and built to the drawings then no calculations are needed and you have to assume the designer has done his bit. The only reason I have had to use FEA in my designs is because they use materials and manufacturing techniques not commonly used in ME.

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