add a vertical piece of dead end pipe or hose to hold air which will
absorb the jolt of water.
see also google group search for water hammer at:
Water hammer is a crazy thing. You can temporarily fix it by adding a length
of vertical capped pipe near the feeder line to the irrigation system. The
problem with this cheap approach is that over time, the air in the capped
length of pipe will actually be absorbed by the water inside the system and
at some point, the length of pipe will actually fill up again with water and
the HAMMER will appear. What you really need is an air bladder version
which has some ambient pressure charge on one side of the bladder. This
absorbs the inrush pressure when quickly switched, but then expands back to
equalize the pressure within a few seconds. The bladder prevents the air
from being absorbed by the water. These charged bladder units run around $50
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