On Fri, 15 Jul 2011 05:05:40 -0700, Andy Dingley wrote:

On Jul 15, 11:54Â*am, Andy Breen wrote:

Boiler explosion or firebox collapse? Actual (external) explosion of
the boiler barrel seems to have been (commendanbly rare for over a
century - the benefits of elfandsafety gorn madde, of course..).

It actually seems to have had a single cause - the avoidance of
grooving, longitudinally inside the boiler barrel. Much of this was in
turn down to one simple design change - boilers went from being wrapped
with an overlap (then riveted) to being rolled as a butt joint without
any overlap, and the joint strengthened by an external cover plate
instead. Despite now being two joints and actually weaker (in terms of
simple strength) the old design had a problem of being non- circular.
Under pressure, flexing was concentrated in the groove of this joint,
which in turn led to erosion localised here.

Good point - and a major, major factor. Combine that with the gradual
vanishing of Salter Valves over the firebox (sooo easily adjusted with
a bit of wood to get some extra pressure...), more comprehensive
boiler inspection regimes and better quality water supplies[1] (many of
the explosions involved engines with boilers 40-odd years old with boiler
plates worn quite terrifyingly thin), and the change is explained..

[1] Water fom any convenient ditch, stream or mine drain - or even
salt water from the harbour -was commonly used in locomotive boilers
for a lot of the 19th century. Even when that stopped, it would have
left its mark in the boilers..

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