I occasionally read about the concern with the miniature steam boilers we use in our live steam hobbies. There is some concern about these devices being potential BOMBS. I purposefully emphasize the word bombs. Scare writers also like to use the word EXPLODE. Again emphasis added. These are nice scary words designed by writers and editors to grab your attention. I used the same scam in the title of this article. There is a modicum of truth to the use of these terms in dealing with heating closed vessels of water. So I will concede that their use is justified but over dramatic.
We live in a dangerous world. But humans don’t really like to live without some danger. We call it thrill, like the thrill of the hunt. I could go way off track on this subject (and I have a little) but suffice to say creating a danger free world by banning dangerous activities is an inglorious goal. Identifying risk is the noble cause. Determining acceptable risk is a judgment of the participants and all others that may be directly affected.
It’s a matter of perspective. Driving a car, flying in an airplane or Alaska crab fishing all look like murder compared to the live steam hobby, where miniature boilers live. I have not heard of a death by a properly designed and maintained miniature steam boiler.
Copper is the preferred construction material in miniature boilers. It conducts heat well and is fairly easy to work. However, because of its cost and ductility it is limited to use in small or miniature hobby boilers or as tubes in larger boilers. There are exceptions of course. Note here that I said tubes. For this article I will be referring only to the miniature copper constructed steam boiler.
Copper boiler failures are not immediate explosions in the sense of a black powder pipe bomb or even a paper firecracker. There is not an extremely rapid (microsecond) build up of extreme pressure that far exceeds (>1000X) the failure point of the containment case. Our fuel doesn’t burn that fast.
What actually happens is a relatively slow build-up of pressure over many minutes of time. In copper there is usually some visible evidence of stretching and deformation because of the ductility of the copper. Finally, the weakest point in the vessel cracks or splits. This metal expansion is not usually a sudden event in normal operation unless we start using an old untested boiler at a higher than normal pressure. This could occur if safety relief valves were not present or functioning and there was a runaway heat source.
In properly designed boilers we purposely design the weakest point to be the pressure relief valve(s). The pressure setting of this intentional weak point is designed well below the material failure point.
When superheated water is suddenly reduced in pressure (because of the split boiler shell or opening of the relief valve), it creates a super hot jet of steam. That is what is seen as and is the explosive force. It is more like a rocket blast than a bomb. There is only a small line between the two definitions. People will still get hurt being in the wrong place, like having their face over a relief valve on a steaming miniature boiler. Is this is an acceptable risk?
I have tested copper pipe and solder joints to the failure point with hydraulic pressure. In every test, the pipe sidewall failed by bulging before splitting. This occurred before the solder joint could fail. The full failure was sudden, like an explosion. That was for both soft and hard solders. The copper is annealed (softened) by the heat of silver (braze) soldering of the pipe. This caused wall failure to occur sooner (at lower pressure) than with lower temperature soft soldering. In no case did the joints fail in either type of soldering. These were properly soldered standard copper plumbing fittings (socket type) with deep overlap joints and seams.
If you have ever replaced frozen copper water pipes, you have seen how copper fails. Sometimes the expanding ice will push a poorly made solder joint apart, but the normal failure is an expanded and split wall of the pipe. The copper pipe doesn’t burst into flying fragments like a hand grenade.
So the truth is copper does not explode, it fails. What explodes is the superheated water into steam and this is quite violent. Semantics yes, but we need to know what the actual risk is. Unless the boiler launches like a rocket (Evil Kenevil’s canyon jump) the risk is severe scalding by the steam. The volume of water in miniature boilers is low so the effective injury danger area is small. Being outside is safer than being enclosed in a small space.
The hydraulic pressure tests we perform are really checking the soundness of the solder joint on new properly designed boilers. On old boilers it is a test of corrosion damage that thins and weakens the copper sidewall. Pressure testing old boilers also checks for work hardening from expansion and contraction that could make the copper or joints crack. The older and more used the boiler, the more critical doing this test becomes. All boilers expand and contract when heating and going cold. We really need to do this testing.
Because of the testing, we can assume the operation pressure of a small copper model boiler is still far from the failure conditions. A lot of small boilers are run at only 30 PSI. Even 100 pounds in a small surface area boiler is not dangerous operation. Well designed small round tube copper boilers using proper and new materials can likely take 10X or more pressure before failure. However it may well be a bit “fatter.” Any old copper boiler that distorts on a 2X operating (relief point) pressure test must be put out of service and replaced.
I remember as a kid, my fat tired bike needed about 25 PSI of air pressure. I was amazed when kids that had the new skinny tired “English” bikes were using over 100 PSI. I thought the tire was going to “explode! It all has to do with surface area and volume.
A 54 square inch boiler (a box 3x3x3) at 100 PSI has a total force potential of 5,400 in/lbs. A boiler twice as big (6x6x6) at the same pressure has 4 times the area and has a force potential of 21,600 in/pounds. Keep the boiler pressure fixed as you take away the steam volume to work (convert more water to steam) and you realize how much more work (Horse Power) the larger boiler can produce just because of its size. This is an extremely simplified explanation, but it is surface area and water volume that determine input firing rate. Then it is the volume of heat (BTUs) you take out while maintaining design pressure that is the power of the boiler.
The safest and easiest to build copper boilers are the exterior round tube designs. Flat copper surfaces should be kept small and heavy walled or well supported. The ends of a boiler tube are about the only flat areas we allow and are convenient for us to make the fire holes and connections. Fittings and rings (fire tubes) help strengthen those areas. It is the desire to produce scale looking copper locomotive boilers with flat sides that cause the most complex designs and problems.
I once designed a large square open top high flow (in and out flow) water tank using stainless steel sheet. It was about five feet square and five feet high. It was a big as we could go in the space available. When we first put water into the tank, the sides wanted to buckle outwards. We had to drain the tank and weld in quite a few stay ties between opposite sides of the tank to prevent failure of the tank. Had the tank been round, the stays would not have been needed, but the volume would have been less. Lesson learned.
Think about large propane tanks in a yard. They are cylinders with round (ball) ends. So are pressure tank cars on train cars. All well designed pressure vessels have domed ends. Look at your certified tank on your air compressor in your shop. That’s because tubes and domes are stronger. If we could build bigger boilers, domed ends should be a consideration.
What is important to remember and practice is our model sized motive boilers are not left alone to run on automatic for long periods of time. We baby these devices. We constantly check that the pressure safeties are working. We carefully blow them down and clean them out. We watch water levels. We use distilled feed water. The live steam hobby is a real hands-on operating experience and we have a lot of time and money invested. The last thing we would do is leave a firing boiler unattended. All of this care and attention mitigates risk.
I have worked on many old large commercial coal fired boilers. I remember the soft plug in the crown of these boilers. It was designed to melt out and blow steam on the coal fire if the crown sheet was not covered by water. I don’t know the melt point or size of the solder plug. It was only intended to extinguish the heat source, not act as a pressure safety. It still seems like a good idea and is used in some model boilers.
If you wonder how this low temperature plug could work, you need to realize that it is possible to boil water in a paper cup over an open fire. When my brother and I were young boys, I bet him that I could boil water in a paper cup. (I think I saw it done on “Watch Mr. Wizard” on TV in the 50’s) We set several paper cups of water on a cooking grate over an open camp fire. The top of one cup burned off down to the water level. The other cup which was full did nothing. In less than ten minutes water was boiling in both cups. He was so mad that I had been correct that he kicked over the cups with his tennis shoe covered foot. His entire foot became one big second degree burn blister. Lesson learned the hard way.
Back to amateur built boilers. Everything I said above concerns properly designed and constructed copper steam boilers. There may be a hundred improperly built boilers in use but without problems. Anyone can purchase a book and attempt to build a model steam boiler. Leave off the relief valve and water level devise and put a shut off valve in the steam line and you could become a statistic. I have never heard about this sort of incident. Hobby boiler makers and users are probably 1 in 10,000 of the world population that can enjoy the luxury of a hobby. So exposure to notice is very low.
Perhaps some people just can’t make a good solder joint. They go get qualified help or give up and buy a certified boiler. That’s good too. I have been in the homebuilt aircraft hobby (EAA) most of my adult life. I have seen some very poor welds in steel aircraft tubing and elsewhere. Thank goodness there is a lot of help available in clubs with a friendly but strict inspection code. That goes for the FAA as well as the club inspections. We all want safe airplanes in the sky! but are any airplanes safe?
Anyone building a steam boiler needs to be and usually is committed to doing the proper job. As I have stated, boilers by their very nature are not safe devices. They contain fire. They are very hot. They contain pressure. They are not to be feared, but respected. They are a simple energy conversion device that humans have been using for hundreds of years. It’s that long history of learning from what has gone wrong that makes us safer today.
For the hobbyist, boiler making is a personal test. Miniature steam boilers are not a matter of survival. We are testing ourselves to learn new skills. To us it is a form of educational entertainment, a history lesson. It is in self pride that we say, “I made this. I know every part and how it goes together and how it works. Every part is properly made for a purpose” I think we also throw in there some thought about safety when we know there is certain risk involved. It is a big part of “properly made.”
And, well! It’s just fun and a thrill!