Boiler Design and Material Selection

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Marty_Knox
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Boiler Design and Material Selection

Postby Marty_Knox » Wed Oct 07, 2009 9:13 pm

After reading and re-reading the threads on CBQ's Boiler and the proposed boiler code I thought I would make some comments on the design of boilers and the desirable material properties. My experience with boilers started with building my first locomotive. Like many live steamers initially I was more interested in the machining side of the things. I had that locomotive running on air and needed a boiler. I wrote to a gentleman who advertised in Live Steam magazine and he quoted $600 to build my boiler. Well, $600 was an astronomical sum to a college student in the mid-70s so I figured I would build my own. I ended up building one in steel with copper tubes; I paid the best weldor I knew $125 to weld it up for me. That experience grew into a life-long interest and career. My comments are based on 30 years of building, inspecting, operating, and maintaining boilers.
The first quality of a boiler material that comes to mind is ductility - you want a material that will withstand many,many heating and cooling cycles, with out fatiguing and cracking. Strength is almost secondary - the strength of a boiler is in its structure. Up to about 5 1/2 - 6" diameter and 100 PSI copper has many advantages,above that,low carbon steel is the material of choice. I worked as a boilermaker for DuPont. They have a whole department that works on material specifications. They never hesitate to buy the best material for the application. We worked with a wide variety of materials - Inconel, Hastelloy, and Carpenter 20 to name a few - yet all our boilers were low carbon steel!

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Fender
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Postby Fender » Wed Oct 07, 2009 10:15 pm

Marty,
I'm glad you started this thread, as it is important for live steamers to understand why certain materials are better suited than others for boiler construction. Perhaps you can explain, for instance, what the difference is between A36 or 1018 steel (both low carbon steels), and one of the boiler plate specifications such as A 516.
Thanks,
Dan Watson

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Marty_Knox
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Postby Marty_Knox » Thu Oct 08, 2009 6:15 am

Boiler plate and even the terms used to describe have evolved over the years. Back when I was first learning about boilers you still heard terms like 'Firebox Quality' and 'Flange Quality'. Today the term used is 'PVQ' - Pressure Vessel Quality. The specifications to look for are SA285 Grade C and SA 516-70. For bars and rods the specification is A-36. These are the ASTM (American Society for Testing and Materials) classes. A-36 also can apply to plate. In fact, SA 516-70 meets the A-36 spec, since it is so broad. The difference is the testing and quality control. The 516 has to meet much tighter specs of tensile strength and chemical analysis. When we ordered the plate for the floor of 464's new tender I spec'ed A-36 - the supplier sent us 516!
How do you know what plate you are getting? You should ask for the MTR's - Mill Test Reports, also known as certification papers. These will have the heat numbers which identifies that particular plate along with the tensile strength, yield strength, and chemical analysis. It is common to have multiple heat numbers listed on the same MTR, but you should check the heat number stamped into your plate and make sure it is listed. And yes, the heat number should be stamped into the plate even if you are not buying the whole plate. This is one of the requirements if you are building to the ASME(American Society of Mechanical Engineers) code - the heat number has to be stamped into each part. This is to provide traceablity. If there ever turns out to be a problem you can go back to the manufacturer and prove that it is a plate that he made. For hobby work this isn't as important as in ASME work.

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Fred_V
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Postby Fred_V » Thu Oct 08, 2009 7:26 am

what are your thoughts on building a boiler with no stays in the firebox side sheets?
Fred V
Pensacola, Fl.

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Chris Hollands
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Boiler Standards

Postby Chris Hollands » Thu Oct 08, 2009 9:01 am

If people want a very good boiler standard for minature boilers then get your self a copy of the Australian miniture boiler code Im sure you can buy it from various sorces (Australian model engineer magazine would be one way or Phil could let us know where)this explains and shows how things have to be done and calulations required this code would pass any test , it is not perfect and some people would not like certain things Im sure Like I did with only being able to build to 50litres max and 100psi but it is safe and for the average person in any country it would be of great help .
This is the national standard in Australia and I think now New Zealand now.

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Marty_Knox
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Postby Marty_Knox » Thu Oct 08, 2009 9:36 am

Fred, I'm not here to rake anyone over the coals, but to describe what I have found to be the best practices.If you're talking about CBQ's boiler, from the pictures I saw it looks like his boiler has dry sides like a Briggs boiler. If that is the case then the sides don't need any stays.
I tend to be very conservative in staying a boiler. I'm used to full size locomotive practice, where you figure of load of 7500 psi maximum on the staybolts. To plug in some dimensions, with 1/4" plate I will use 3/8" diameter stays on 2" centers. In 5/16" plate I use 1/2" diameter stays on 3" centers.

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cbrew
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Postby cbrew » Thu Oct 08, 2009 9:53 am

Marty,
First off, thanks for starting this thread.
I have a question,
what are your thoughts on threaded stays that are seal welded?
I can how this can help with assembly of the fire box area.
Thanks
Chris
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SteveR
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Postby SteveR » Thu Oct 08, 2009 12:38 pm

Marty, If my boiler operating pressure is 100psi, why would I figure a load of 7500psi on the staybolts? Both numbers are per square inch.
Thanks,
Steve
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Fred_V
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Postby Fred_V » Thu Oct 08, 2009 12:43 pm

Marty_Knox wrote:Fred, I'm not here to rake anyone over the coals, but to describe what I have found to be the best practices.If you're talking about CBQ's boiler, from the pictures I saw it looks like his boiler has dry sides like a Briggs boiler. If that is the case then the sides don't need any stays.
I tend to be very conservative in staying a boiler. I'm used to full size locomotive practice, where you figure of load of 7500 psi maximum on the staybolts. To plug in some dimensions, with 1/4" plate I will use 3/8" diameter stays on 2" centers. In 5/16" plate I use 1/2" diameter stays on 3" centers.


i'm not raking anyone either but i am very very concerned. it isn't a Briggs design it is a conventional water leg boiler with over 12 gallons of water in it at 350 deg. F. We all know what happens to that water when a seam opens up.
Fred V
Pensacola, Fl.

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Bill Shields
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stay calculations

Postby Bill Shields » Thu Oct 08, 2009 2:40 pm

General rule with boilers:

you operate at 100, but the safeties are set at what - 125? OK that's the operating pressure you design to.

4X operating for safety margin - minimum. For stays, I go 6X, which allows plenty of room for corrosion.

what does it actually cost to use a slightly larger stay or put in a few more? Is there a competition to see who can build a boiler with the fewest stays and not kill himself?

I would hate to be the runnner-up in that contest.

Not to speak for Marty, but threaded stays are OK from a mechanical standpoint, the problem is that often you are going into the firebox or outer shell at an angle, and getting the taps going at exactly the correct angle to get the stay in can be a bear. OK, if you have a STAYBOLT TAP, it's easy, but try to find one these days.

If you use threaded stays, you have to calculate to the root diameter of the thread and allow for 'notch sensitivity' - cracks will start at the root of the thread if you aren't careful.

I find it easier to just drill, chamfer, center punch to hold and weld it up. It's a lot easier than tapping for stays.

99.9999% of the time, I put the stays in after the firebox is fixed in place by the mud ring, so I don't depend on them to hold the firebox in place anyway.

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Marty_Knox
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Postby Marty_Knox » Thu Oct 08, 2009 4:51 pm

I'd like to answer Steve first. What we are talking about is an area of many square inches supported by far less than 1 square inch. To use a specific example with 3/8" diameter stays on 2" centers you have 4 square inches supported by .1105 square inches. At 100 psi you have a load of 400 lbs. divided by .1105 for an actual load on the stay of 3620 psi. Another case would be 1/2" diameter stays on 3" centers, at 100 psi you have 900 lbs. divided by .1964 for an actual load of 4582 psi.
As for Chris's question - yes, I have thought about it, and have actually done it on a full size locomotive. I came up with a different way to solve the problem. I now weld the stays into the inside sheets before I assemble the firebox.
The Australian Model Boiler Safety Code is an excellent document, and I have copies of section 1 (copper boilers) and section 2 (steel boilers). I would refer to it more if I could think metrically, but I still think in inches and feet.

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Re: stay calculations

Postby Curtis_F » Thu Oct 08, 2009 5:08 pm

Marty,

Thank you for what is shaping up to be an interesting thread.


------
Bill Shields wrote:4X operating for safety margin - minimum. For stays, I go 6X, which allows plenty of room for corrosion.

Just as a side note, the 19" gauge locomotives built by Louis MacDermott for the Pan Pacific International Exposition in 1915, have been found to have been built to a safty factor of 9!

Which means at almost 100 years old they're still capibile of running at a full 200 psi boiler pressure.


Cheers,

Curtis F.
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