On Sat, 16 Jul 2011 20:04:31 +0000, Andy Breen wrote:

On Sat, 16 Jul 2011 20:49:23 +0100, Clive wrote:

In message , Andy Breen
writes
The pet-cock,
Any similarity between this and the modern clack valve?

As I understand it, the pet-cock was for bleeding steam off from the
pump: "by opening (the pet-cock) the steam in the pump was let out, and
the action renewed" (Marshall, again).

I'd not expect a direct similiarity to anything on a "modern" steam
engine, as even the ones that use hot feed and pumps have the pump well
away from the boiler (and seals are much better, anyway)

Thinking some more...

The clack valve is - essentially - a non-return valve to stop boiler steam
getting back into the feed system, nyet? Presumably the pumps used in early
locomotives had non-return valves in them (well, they must have, being
intended to push water one way...), but it's clear from Marshall's comments
("hot water entered from the leaking of the valves, causing them (the pumps)
to be filled with steam") that they didn't work well enough to keep the
pumps at their duty. Given the materials available at the time, it's
easy to see how this happened - and given that until then most applications
requiring continuous working[1] had been the domain of low-pressure engines,
with Trevithick-type[2] high-pressure engines restricted to more intermittent
tasks[3] where the boiler could be topped up after each burst of work
(pressure having dropped then, anyway..), it's easy to see how the problem
hadn't been encountered before. The valves had worked well enough for low-
pressure (and stationary!) boilers, after all (actually, the battering
taken by the machinery in an unsprung locomotive on short - 3'-4' rails -
can't have helped the functioning of the pump valves at all!).

Essentially, if they could have made a really good clack valve, they'd
not have had the problem with the pumps locking and the pet-cock wouldn't
have been needed. As materials available meant they couldn't, the pet-cock
was a real breakthrough in producing a locomotive which could run more than
a very short distance between extended (and madly hazardous!) stops.

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