On Jul 16, 10:24*am, Guy Gorton wrote:

Then again, early stationary engines normally ran at a maximum
of about 3 or 4 psi.

That seems very low. *

No, this was the sort of pressure used. Early boiler feedwater wasn't
pumped in, it was fed by a gravity cistern up in the rafters of the
engine shed (it was raised here by the pumping engine itself).

The distinction in strength between Newcome or Watt's boilers and the
first high-pressure boilers (such as Trevithick''s "strong steam" of
25psi) is that the newer boilers also had a margin of safety over-
design to them. Trevithick knew that safety valves were unreliable and
often adjusted by enginemen (this wasn't even seen as a bad practice
at the time), so he not only designed for 25psi, but he designed to
not fail at 25psi, and to survive overpressure too.

The first haystack or balloon boilers used by Newcomen and Watt
weren't even iron, they were copper brewer's mash tubs with lead lids
fitted. These had a fairly benign failure on over-pressure, as their
seams would leak or split without causing the whole pressure vessel to
explode. As a result, fear of boiler explosions was much less in the
early days, despite them being more common, as the injuries would only
be scalding (possibly fatal) to a few enginemen, rather than the
expensive destruction of the engine house. The real concerns over
boiler explosions didn't begin until pressures had raised and engines
were working as expansion engines.