On Sat, 16 Jul 2011 02:33:21 -0700, Andy Dingley wrote:

On Jul 16, 10:24Â*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).


but, as stated, Trevithick used much higher pressures - not least because
his engines did most of their work via steam pressure, not vacuum)

Filling locomotive boilers against ~50psi pressures in the earlies was
a problem. Trevithick seems to have tried a pump on the Pen-y-Darren machine,
but makes much of not needing to use it (there were real issues with
pump-feed until Stephenson's invention of the pet-cock to prevent locking,
somewhere about 1820s - it may be Trevithick didn't want to risk working the
pump..).
The Middleton machines were filled before the start of a run, then blown down
at the end of it and refilled. The refil /may/ have been with cold water at
first, but the time needed to raise steam again (don't even think of the
thermal stresses on a cast boiler being refilled with cold water!) meant
that they shifted to boiling-water feed from lineside 'kettles' very soon.
These 'kettles' seem to have been a common factor across most early locomotive
applications, although Chapman and Buddle did use feed water heaters on the
engine (as well as or instead?).
Even after the pet-cock came in and allowed the boiler to be refilled while
the engine was working, hot feed from a kettle to the feed-tank seems to
have been the norm until the bigger and more efficient boilers introduced
by Hackworth (1827 on Royal George) and Stephenson (1828-1829) came in.

There's a thought on operation: fill the boiler, wait until you have pressure.
Now tie the safety valve hard down, get moving. At the end of it, release the
valve, blow down the boiler and gravity-feed with boiling water. All with a
cast boiler. The Middleton machines worked like that for over 20 years..

--
From the Model M of Andy Breen, speaking only for himself