In article , Robert Swinney says...

The crown sheet forms the top of the furnace in a typical "enclosed furnace"
type of boiler. As the water level becomes low, the crown sheet is
eventually exposed to the intense heat of the furnace, with no cooling water
above it. The crown sheet is exposed to full heat of the furnace with no
cooling water on the other side until it finally burns through. What's the
big deal? Won't the ruptured crown sheet let the water flood into the
furnace and put out the fire? Here, in a nutshell, is what happens: The
water level is already low in the boiler, probably less than 1/2 volume and
it is boiling furiously, in contact with the flues and hot sides of the
furnace. The water temperature is much hotter than its normal,
unpressurized boiling point. As the crown sheet bursts it releases some of
the boiler pressure but does nothing to release the energy contained in the
boiling water - at that temperature. Now with pressure slightly reduced,
the heat energy contained in the water causes more rapid boiling, more
pressure and in a brief instant, Boom! Flash effect. As the opening in the
crown sheet increases, pressure drops slightly, more and more water flashes
into steam, etc. etc. A boiler doesn't burst in the manner of a balloon
filled with air. A boiler explosion is more like the bursting of a bomb -
all the energy contained in the heated water flashes into steam. Generally,
boiler explosions are the result of a weakened crown sheet.

Good description. The feature I heard about that typically accompanies
crown sheet explosions is that the boiler is invariably found to be
completely empty of water. This is not because it was run dry and then
blew up, quite the opposite. All of the water inside will flash to
vapor when it blows up.

The fatal explosion is ohio had been described with photos, and the
crownsheet stay bolts were so badly eroded it would never have passed
any kind of public safety inspection. I think the bolts were supposed
to be 3/4 inch diameter, they were necked down to less than 1/4 inch
in some spots.

Interestingly at the CAMA fair a few weekends ago the had a steam tractor
undergoing restoration. I noticed the inspection port for the crownsheet
stays was opened up, and could not resist peeking inside.

All of the staybolts seemed to be full size for their entire length,
and in very good condition.

Jim


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