Water hammer when lawn valve turns on

When my automatic lawn sprinkler valve turns "on" I hear water hammer. What causes water hammer when the valve turns "on"? How do I stop it?
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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: http://groups.google.com/groups?sourceid=navclient&ie=UTF-8&rls=GGLG,GGLG:2006-18,GGLG:en&q=water%20hammer&sa=N&tab=wg
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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 or so.

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