Boiler Material

Copper
chet86 asks:


 * How much stronger is Type K over type L copper tubing? Only 0.009" difference in the wall.

From gwrdriver:


 * Let's assume for the moment you are looking for a boiler barrel (The barrel is the long'ish, tubular, front part of the boiler). If you run it through the applicable formlae for boiler barrels you will find that Type M is actually sufficiently strong for 100psi steam. But because there are always other forces acting on the boiler, both during and after construction, as Bill has just mentioned, I don't use Type M. All three types meet the basic strength requirements, but the extra rigidity and weight of Types K and L allow me to handle the material a little bit more like a solid and machine and work it with less concern for deformation. My personal preference is Type L for barrels and Type K for flues. I can usually get Type L locally but the Type K for flues almost always has to be ordered.

Steel
Bill Shields writes:


 * Pipe and HRS plate are NOT the material to build a boiler, unless the pipe is something like oil well casing which has the same specification / inspection requirements as boiler tubing (I worked for an oil company for 10 years and know this from experience). You need to get certified material that is OK for Pressure Vessel application.  A36 is a grade that comes to mind -> here are others, but my head is in European / DIN grades and for the life of me cannot remember the ANSI equivalents at this time.

Marty Knox writes:


 * I prefer to use rod stays (as opposed to girders). It is easier to calculate the stresses, they don't put a bending stress on the side stays, and they allow better circulation.


 * Seamless pipe, SA106B, would be my first choice for the shell. Flat plates should be Pressure Vessel Quality, SA516-Gr70 is the most common. Couplings and half couplings should be 3000# forged steel, ASTM A105. Stays and mudrings can be made from SA36.


 * 10" pipe is 10 3/4" OD. In a boiler this size (10-inch diameter by 41-inch length) I usually use 5/16" plate.


 * I use only PVQ plate. Sa285 GrC is getting harder to find these days. SA516 Gr70 is much easier to get. Boiler plate used be sold as firebox quality or flange quality. Those terms fell into disuse in the early 70's, about the time I started building boilers. The current term used is PVQ, for Pressure Vessel Quality.



Picking Marty's Brain
Marty Knox shares some views on boiler material and construction:


 * 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 welder 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!


 * 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 traceability. 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.

What not to use
Harold V writes:


 * It's not a good idea to use brass in a boiler, as it can suffer de-zincification (and failure). I expect the pieces you have are made of bronze, not brass. Bronze does not suffer the same fate.