I'm sure that any pipes that can take an internal pressure of 50-80 psi can
handle a vacuum without imploding. However, that would be a very large
volume of water to evaporate. If you do try this I recommend watching a
vacuum gauge as you pump down as you will get boiling and then freezing and
you will see a plateau at 5-15 torr during boiling and evaporative cooling.
Keep pumping until the pressure falls below maybe 5 torr (where the amount
of ice starts to become large), and then vent the system to air and allow
time for the ice to melt before you do another cycle.

Karl, I know you said that all the special risers and valves you need are
not available in PVC, but how about using PVC pipe for the main runs and
adapting to iron for all those fittings? When they rust out you can replace
them but you won't have to dig up all the long runs of pipe, at least.

-----
Regards,
Carl Ijames

"DoN. Nichols" wrote in message
...
On 2010-04-22, John B Slocomb wrote:

[ ... ]

I am assuming from your posts that the pipes require draining to
prevent freezing but I wonder whether, after draining as best as
possible an air compressor could be connected and "blow down" the pipe
for a few days might not prevent further corrosion.

How about pulling a vacuum on the pipes instead -- assuming that
they can take the compressive forces of pulling a vacuum on the pipe.
This will cause the water to evaporate relatively quickly -- assuming
that you have a way of sealing the other end of the pipe.

Enjoy,
DoN.

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