Boiler tube material choices?

[Steam Captain]

Hello,

Since firing wood releases sulfur compounds and sulfur is making copper and many sorts of steel brittle, which materials do the wood burning steamers have for their steam boilers?

friendly hello,

Ramón

[fredrosse]

Steel boiler tubes have been used for more than a century with high sulfur coals as well as wood fired equipment, and I have never herd of sulfur difficulties here. Steam boilers operate with the boiler metal temperatures only a little higher temperature than the steam, thus maybe 200C - 250C. ASTM A-178 ERW boiler tubes are what we should use, not tubes made from common water pipe.

For sulfur to begin affecting the metalurgical properties of steel, it would require steel temperatures several hundred C above these temperatures, so not an issue with our hobby.

As to copper tubes, they would generally not be recommended for new boiler construction if possible, although many many steamboats still use copper boilers, as well as the model railroad community. Again I have never herd of difficulties of sulfur affecting metalurgical properties of copper at the expected temperatures.

I have herd of some difficulties with some of the silver solders used to construct copper boilers, but my knowledge of this facet is nil.

[fredrosse]

Sorry, computer acting up,

[fredrosse]

Same problem

[barts]

I'll echo what Fred explained: both copper and steel are in common use for tubes in small steamboat boilers, and have a long successful history in such service, including use with high sulfur fuels. As Fred alluded to, Sil-Fos silver solder (a silver-phosphorous alloy) is not recommended for use in sulfur-bearing atmospheres. Such solder can only be used for all-copper boilers; boilers using silver-soldered copper tubes w/ steel drums (such as the boiler in my own boat) use a high-silver content alloy which doesn't contain phosphorous and is not subject to such corrosive attack.

- Bart

[Steam Captain]

Thanks for these crucial informations - especially for the sil fos soldering issue, since my first boiler ( scotch type model size - 30 cm long 25 cm dia drum) was soldered with sil fos and I considered using it in my recent, real size boiler project. This sounds all good, since my preferred choice of material for my 10 at monotube "beer keg" would be copper.
Thanks for the high quality answers!

Ramón

[PeteThePen1]

Hi

Just a thought from a paper pusher (not an engineer!) but it may be worth thinking about. The SBA will not allow copper in boilers where it is possible for the fire to impinge directly onto the tube, and as a rule of thumb limit maximum boiler pressure with copper tubes to 100 psi. I have also noticed that various monotube coil boilers in the SBA tend to be of stainless steel. I have not done a review of the SBA register for you, but it might be worth searching to get a feel for what is currently in service.

Hopefully, our metal and engineering experts will jump in here and give us chapter and verse.

Regards


Pete

[barts]

Hi

Just a thought from a paper pusher (not an engineer!) but it may be worth thinking about. The SBA will not allow copper in boilers where it is possible for the fire to impinge directly onto the tube, and as a rule of thumb limit maximum boiler pressure with copper tubes to 100 psi. I have also noticed that various monotube coil boilers in the SBA tend to be of stainless steel. I have not done a review of the SBA register for you, but it might be worth searching to get a feel for what is currently in service.

Pete

The ASME boiler code limits copper tube to 400F service using brazed joints; this is approx 2x the 100 psig limit you mention above
for saturated steam so long as the wall stresses are acceptable. For high output monotubes, 400 F is far too low; I've seen small monotubes (built from 1/8" pipe) at 800F and 800 psi.

In my experience with copper watertube boilers, the most important thing to avoid is brazed joints that are exposed to radiant heat that can be easily uncovered in the case of low water. For example, a solid copper silver-brazed Yarrow boiler is a tube failure waiting to happen, since the tubes entering the top drum are directly exposed to radiant heat and are only covered by a small amount of water. Several of these boilers were built in the USA; as far as I know every one has had at least one tube joint fail due to momentary low water conditions.

A common design pattern is copper coils brazed into steel nipples welded into a steel drum; various designs along these lines have been built and delivered good service so long as water levels are maintained. The common use of 1/2" copper tubes means that in the case of a tube or joint failure the steam is released at a limited rate.

- Bart