On 19/07/2011 11:49, Andy Breen wrote:
On Tue, 19 Jul 2011 02:21:32 -0700, Andy Dingley wrote:

On Jul 18, 8:07 am, wrote:

There is a problem with all boiler feed water pumps and that is
cavitation.

Utter ********. Cavitation is a problem of _low_ pressure in pumps,
sufficient to form bubbles (i.e. "cavities"). The one thing we know
about a boiler feedwater pump of any form is that it has to work at
around boiler pressure. Whilst the Giffard injector stops working with
hot water, heat isn't a big problem for pumps.

Um.. Cavitation occurs when water boils against a surface, and the collapsing
bubbles can lead to pitting and damage to that surface. The cause is localised
boiling as a result of low pressures in those regions.
In marine propellors, of course, it occurs at ambient temperatures and
very low pressures, but there's no reason at all why it couldn't occur
in a pump if the feed temperature were hot enough and the small drop
in pressure produced at some point in the pump were large enough for
the water to begin to boil.

Cavitation occurs at any place within a pump where the liquid pressure
falls below the vapour pressure of the liquid being pumped. This could
be at a surface, but doesn't have to be.

It typically occurs where the fluid enters the pump, where it has been
accelerated, but before energy has been imparted to the liquid by the
pump mechanism. According to Bernoulli's law, as the liquid is
accelerated, its pressure falls.

It's a particular problem where the liquid is close to its boiling
point. As the problem occurs at the entry to the pump, the pressure
that the pump is delivering to is irrelevant, so it can occur with a
boiler feed pump working with condensate.

If you were pumping condensate that was not sub-cooled (i.e. still at
its boiling point and condensation pressure at the pump inlet), then
cavitation is very likely.

If you ever look at an oil refinery or petrochemical plant, you will
notice that the distillation columns do not sit on the floor, but are
raised several metres above ground level on "skirts". This is so there
is a hydrostatic head above the bottom product pumps, so that cavitation
doesn't occur.

--
Jeremy Double {real address, include nospam}
Rail and transport photos at
http://www.flickr.com/photos/jmdoubl...7603834894248/