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In article , Dave Hinz wrote:
On Wed, 26 Apr 2006 21:38:24 +0100, David Billington
wrote:
I can't remember the specific equations now that show when water hammer
will occur but water hammer is caused when the valve controlling flow is
shut down too rapidly causing a pressure pulse in the system. Is there
any way to slow the closure of the control valve.

It can also happen when opening a valve too rapidly. Think pulse-jet,
it's probably the same mechanism. But the fix is to, as someone else
said, puta vertical air-filled column to the pipe in question, to
absorb the impulses and smooth them out so they can damp down to zero.

You can buy fancy spring-loaded space age water hammer arrestors, or you
can make 'em out of 18" of 3/4" pipe and a cap. The latter works great
for my house.

The reason for the fancy commercial ones is two-fold: 1) they will fit in
places where several feet of pipe might be a problem, and 2) the air in
the home brew version will slowly dissolve in the water, and the thing
will stop working. You can always shut off the water & drain the whole
system to get air back in, but why bother?

There are two kinds of water hammer. One is the loud bang you get when a
pressure pulse is created by a rapidly opening or closing valve (closing
is more common). The other is an oscillation in water pressure that can
make the pipes moan or in bad cases go bang bang bang as the water
pressure bops up & down. This requires flowing water. Imagine blowing
over an open soda bottle, only under water. The big spikes can be
handled by pre-compressed piston jobs like the Oatey "Quiet Pipes" sold
at Home Despot & some hardware stores. However, eventually the pistons
can stick, and they won't work well for low pressure fluctuations. They
also make water hammer arrestors with a diaphram that are better for
stuff like that, and they can't stick. Those you'll need to get from a
plumbing supply place, probably special order.

Doug White
(who spent several years beta testing the Oatey devices on a house with
low water pressure)