At 04:57 PM 8/20/01 EDT, you wrote:
>Harry,
>What would you predict the results would be if the boiler was very hot,
>almost dry, and water was introduced? This was the apparent reason for the
>traction engine failure recently resulting in loss of life. Obviously size
is
>of great importance here.
>Walt & Lunk
Yes size is of importance, but with regard to the Ohio case despite
official reports there is still a great deal of speculation and
misinformation about what actually happened there, and there are well
respected authorities on boiler design and behavior as well as historical
empirical data which contends that for reasons having to do with the laws
of thermodynamics overpressuring due to flash steam could not have been
sufficient cause as has been widely reported. Convenient, but not
conclusive. I don't speculate one way or another because I don't know
enough about any of the circumstances to say but it is safe to say that
like so many other mishaps a number of factors contributed to the accident
which individually were unlikely to have caused it.
As we all should know by now the amount of heat needed to turn water
to steam rises as the pressure rises. At one atmosphere the boiling point
of water is 212 deg F. At 60 psi it's 307 deg F. (153 C.) In an
overheated or stuck valve condition in one of our boilers the pressure will
reach a point where the burner in incapable of adding enough heat to the
system to turn any more water to steam. Thus it has reached thermal
equilibrium, in my guess below the critical point of the boiler, and will
sit there and sizzle, as I mentioned in my earlier post, but I don't know
where that equilibrium point is.
To answer your question, I have no prediction because I don't know. My
guess would be that there would be a momentary increase in pressure,
probably large on a % of WP (working pressure) basis, but essentially then,
nothing. However while the boiler was being run dry the shell was busy
taking on a great deal of additional heat which when presented with a
little more water could indeed flash into enough pressure to be
destructive. I guess we're just going to have to try it and see.
I suppose I should add this qualification, which is that all of this
assumes that fittings, closures, gauges (if included), etc., are not the
source of a failure. Naturally our fittings, such as water gauges, aren't
intended to withstand these conditions and so could not be considered a
'failure". Bushings on the other hand must be capable of withstanding as
much if not more pressure than the basic boiler shell.
Cheers,
Harry
