Fusible plugs

NW Mailing List nw-mailing-list at nwhs.org
Sat Apr 13 23:27:09 EDT 2019

A fun euphemism for someone losing his termper was that "he blew his soft 
plug . . .."

Ed King

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From: NW Mailing List
Sent: Thursday, April 04, 2019 9:00 AM
To: nw-mailing-list
Subject: Fusible plugs

Several people here have commented about fusible plugs. The engines
described which did blow up ALL had the fusible plugs in them. Those
were a part of loco boilers from as early as the 1830s from my
research. I don't know (but strongly suspect), they were a required
part of all boilers but if not, they were at least widely used from
very early on. 1830s references specifically mention them as letting
loose from poor management of water on the crown sheet. Better that
than "boom" I guess.

To reiterate though, Ed King's excellent article which he referenced
earlier "Big Bang, No theory," relates what happens in a fraction of a
second when a boiler goes ka-plooey. And I suspect that for whatever
the reasons, the sudden expansion of water into steam didn't afford
ample opportunity of time for the plug to soften and release. Also
remember that something like a cubic inch of water in a closed
pressure vessel like a boiler will expand something like 1540 times in
an instant, hence the big bang and fact that the containment vessel
(boiler) doesn't have an endless expansion capability. I understand
that regarding steam, the safety factor was something like 4 before it
would let loose, nowhere near what actually they were momentarily
subjected to before the trip to the promised land.

Thanks Ed.

Bob Cohen
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