"Eric R Snow" wrote in message
...
On 26 Apr 2006 20:48:00 GMT, 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.
When installing new shower valves and new washing machine valves in
my house I made up my own water hammer prevention devices. I just used
lengths of copper pipe and caps like everyone else does. I told a
plumber friend about this cheap fix compared to the ones at the store
and he said that the ones I made will eventually become waterlogged
and the only fix would be to drain them. In his years being a plumber
he had seen this many times. However, the ones I made stopped the
water hammer and lasted at least 6 years before I moved. And the ones
in my shop have been working for 7 or 8 years so I'm not sure it
always happens.
ERS

Maybe not...

On the cold side, as the cold water sits in the pipe and warms up, air will
come out of it and some will be caught in the anti-hammer pipe and keep it
nice and full. On the other hand, the hot water will probably have little
bubbles of air in it from the water heater which will likewise fill the
anti-hammer pipe. This, of course, assumes that water flows past the
anti-hammer pipe. So, for best results, tee the anti-hammer pipe off the
line before the last valve... (Things get a little more complicated on a
vertical pipe where the flow is downward...)

Jerry