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Banging is usually an oscillation. Valve opens, water rushes through, and the pressure upstream of the valve drops to a point where the valve no longer stays fully opened, and a nearly-closed zone valve brings the supply pressure back up again. This repeats and you get the banging.
Make certain that every shutoff valve in the pathway to your sprinklers is fully opened.
Are your zone valves flow-control types? Throttling down the flow controls will make a difference here.
How to get less flow? install smaller GPM nozzle?Banging pipes can eventually break. What you should do is to experiment with one or more zones, and see if you can get less flow, and maybe stop the banging.
All this is an indication of an undersized supply line, in which pressure drops when a zone is opened.
Very simply put, sprinkler systems should not be operating with banging plumbing. If you are betting the ranch that there are no partially-closed valves in the supply chain, and you have no flow-controls on the zone valves (very bad choice on the part of the installer, by the way) then all there is left to play around with is the flow in the zones that are doing the banging. You are trying to find a point of lower flow rates where the banging ceases.
I agree with everything boots says as possible reasons
My only input is to think about is. Do you have a Pressure Reducing Valve? If you do it's possible it has failed and preventing the full amount of flow available being allowed into the sprinkler system . It's also possible if there is a PRV on the cities side and it has failed ( I've dealt with 3 failed PRV's this past week and half so they are fresh in my mind)
Can you cut your main line on the irrigation side and get a flow rate test?
Does every zone cause the banging? I'm highly leaning towards flow restriction either from a PRV or a shut off not fully open.. Make sure your water meter is fully on too
I also recommend to get a dynamic pressure reading when the sprinkler zones are running
This post has been edited 1 times, last edit by "brisk" (Jul 13th 2011, 7:16am)