The Steamboating Forum

I don't know that the steel deteriorates from a metallurgical point of view but it does rust and erode especially when it is being fed dubious water in a non-condensing application.

That boiler that blew at Medina was down to 1/8" thick around the crown sheet stay bolts. It wasn't so much that the crown sheet ran dry as that it shouldn't have been steamed at all. The investigation found that the operator had recently bought the tractor and had been told (in writing) that the boiler should not be steamed up and should be either replaced or at least have extensive repairs. The new owner was told by knowledgeable people that he should not be steaming it and refused the advice. The post mortem photographs are frightening.

I suppose from a crude Darwinian point of view it is slightly comforting that the only fatalities were family members.

I don't know if the traction engine people have instituted an inspection requirement yet. These matters are dealt with on a state by state level in the U.S.

One of the direct results of that disaster was our safety inspection that has been setup for the members of our Northwest Steam Society. We do not allow a boiler that has not passed our annual combination of hydro test and inspection to be steamed up at any of our events. We have a committee of experienced and knowledgeable people who observe the tests. Our safety manual is freely downloadable from our web site.
http://www.northweststeamsociety.org/safety

A boiler inspector has a pretty tough job I'd say. They are required to visually examine a boiler/system and oversee hydro tests and then sign off on it. There was a boiler explosion (video) around 1970 in a college in Canada that let go right after being inspected. A crack started right next to one of the welds in the A53 shell internally caused by corrosion and was not apparent during the inspection.

Over here at Greenfield village they had an old VFT shop boiler that had been approved every year, and one year the inspector realized that the whole top end of the boiler was cast iron and wouldn't pass it that year. Hasn't been fired since.




-Ron

Very interesting video.

What is not apparent to me is why a crack of this size wasn't found during a hydro test. Even if the test was at the operating pressure rather than an elevated pressure it would seem that the pressure drop would have signaled that there was something amiss. Pressure drop during hydro is one of our flags for further inspection.

Also, for an instructional facility it is curious that there didn't seem to be any system of boiler water testing. Such a test would have surely showed the high oxygen situation. Much left out of that video.

Which brings home the old adage that an accident does not happen as a result of a single failure. Hard lessons. Selah. Pause and consider.

What is not apparent to me is why a crack of this size wasn't found during a hydro test.

If I remember well my mechanical engineering courses back in College, temperature dramatically changes the level of stress. When you run a hydro-test at room temperature (something between 0 °C and 40 °C, for example), you don't get the same level of stress than that of a "running" boiler. Thermal stress add to pressure stress, this makes quite a difference.

Hmm. The difference between room temperature and the temperature of my boiler at the point where the safety valve opens seems pretty small compared to the gap between room temperature and the point where the tensile strength of steel is very much affected.

What you suggest may be a factor but I strongly suspect that contempt arising from familiarity was a much more important issue. I know where every leak in my system comes from. If it is a valve packing I don't worry too much but steam coming out from under the insulation and lagging gets my attention right now!

This is very much the same situation that led to the Challenger disaster only writ much smaller. "It didn't blow up before so why should it do it now?" Any system with large stored energy has to be watched carefully whether it be chemical, thermodynamic or inertial.

I have a sign in my shop that provokes thought and comment. It reads, "He was a good man but he didn't understand flywheels."

Enough preaching. Sorry.

Ok This piqued my interest so I went on a Gookle search. It appears structural steel is strongest at about 440 degrees? But ouch on those superheated surfaces, areas where fire impinges and no water on the other side.

Stainless drops at any temp above ambient.



-Ron

Very interesting graph.

I wonder what they mean by structural steel. Not that it matters as when I do any sort of calculations I use numbers down around 20,000 p.s.i. and crank in safety margins of 5 or 10. Not being an engineer means taking no chances. You could probably use my little shop VFT for a scuba tank.

Yes it is interesting and a bit surprising. I knew stainless was good for higher temperatures, but mild steel actually gaining strength above ambient?? Boiler components are likely classified with structural steels, I-beams, channels, square and round tube, 1014 etc.

-Ron