"Grant Erwin" wrote in message
...
I just ran into my old boss. He is now retired, and an avid gardener. He
put in
a 12-zone automated watering system, and there is a problem in it which is
driving him bonkers. I'm posting this because you guys know everything
worth
knowing.

His water system starts at the meter, then it runs to a tee, and off of
one leg
goes his entire outside watering network. The other leg of the tee goes to
his
house. Where the water enters his house, he has a pressure reducing valve.
On
the outside watering leg, there is a backflow prevention valve to ensure
that
even in the event of negative city water pressure his gardening water can
never
wind up in his neighbor's water glass.

At 4AM his watering system turns on. When the first zone valve cuts on,
the
water pipes *in his house* begin hammering loudly. Eventually it dies down
and
then the first zone valve cuts off and the second one cuts on, no more
problems.
But the water hammering wakes him and his family up every morning, and
they
worry about it damaging their house piping because it sounds so violently
loud.

They have had the water department guys out to check his pressure reducing
valve, his backflow prevention valve, and the water pressure everywhere,
and
they say everything checks out OK. He has had two different irrigation
consultants come out and other than suggesting things like reprogramming
to try
a different zone valve to come on first (didn't help) they were similarly
unable
to come at the root cause of the problem, nor could they suggest a
workaround.

I don't really understand the phenomenon of water hammering, but I do
understand
that it is a pressure oscillation which is characteristic of an
underdamped
mechanical system. I suggested that he try adding resistance (some kind of
flow
reducer, maybe a gate valve) or capacitance (one of those bulb thingys)
but he
is on fire to find the actual root cause and solve it, not just find a
workaround. I suggested he try shutting the gate valve, the main water
shutoff
valve to his house, about 95% tonight to see if the added resistance might
do
the trick.

Anyone got any bright ideas? This guy is no dummy; he was an engineering
manager
and has a EE degree from Rensellaer Polytechnic ..

Grant Erwin
Kirkland, Washington

The only sure way to get rid of 'water hammering' is to install a vertical
run of pipe with a cap on the top end. Air trapped in the pipe acts as a
cushion and the hammering problem is resolved. Since the hammering occurs on
his 'house side' then this 'air trap' needs to be added someplace in that
circuit. Most houses less than 20 years old have these air traps already (on
both hot and cold circuits).