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I have a 3/4 inch inline automatic sprinkler valve that will not shut off, whether it is turned on electrically or by unscrewing the solenoid. It drives a drip line with about 15 heads, so the flow rate is rather low. If I close the main shutoff valve to the system and reopen it or open a drain valve downstream so the flow increases dramatically it closes fine, so it appears to depend on the downstream pressure being high. Can anyone help with the cause and fix? Normally I would just replace the valve as they are not expensive enough to fight with, but I am concerned about getting a new one in. I have unions both sides of the valve, but the pipes seem fixed in the ground. I would have to find one the same length and get the unions to mate without pulling or pushing the pipe.

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Opening a drain valve downstream will not raise downstream pressure, it will increase flow and lower total system pressure.

If there is any flow the valve should close, as pressure above the diaphragm equalizes with supply pressure upon closing of the solenoid pilot port.

Get the brand and model # of the valve and order a rebuild kit for it as your problem is very likely to be mechanical. Turn off the supply and remove the valve bonnet, inspect and clean internal components (flush all ports to remove debris, check the diaphragm and support rings for damage or cracks, inspect the sealing surfaces and seats). Replace the washers and diaphragm with new ones from the rebuild kit.

  • Opening a valve downstream will lower the downstream pressure and increase the delta pressure across the valve. I don't know if that points to the problem, but it seemed it might be helpful data. I agree the problem is mechanical, but there are many mechanical components. I have not succeeded in reading a model number (else I would have specified it)-the valve is molded black plastic. I'll try harder tomorrow. I bought a new solenoid for another valve in the bank (it solved that problem-open circuit) and tried it here first-maybe a leak there. No help. – Ross Millikan Mar 20 '15 at 4:37
  • I had a similar issue and had to tighten the solenoid a little more than I expected, This does not sound like its your issue, but I think its worth pointing out. – Hightower Mar 20 '15 at 6:36
  • @Hightower: trying the replacement solenoid convinced me that the solenoid is sealing fine. The fact that the valve stays closed when the pressure is applied supports that as well. The valve only opens on command. – Ross Millikan Mar 20 '15 at 15:22
  • Your original post stated: "If I... open a drain valve downstream... it closes fine, so it appears to depend on the downstream pressure being high." That statement is contradictory to your comment above and to the laws of fluid dynamics. My suspicion is that your problem is due to failure of pressure to equalize above and below the diaphragm. Flush/clean the valve and replace diaphragm and seals or bite the bullet and hack out the valve and install a new one. – Jimmy Fix-it Mar 20 '15 at 17:26
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    If you can find the make and model number, look up the minimum flow rating for operation. Some of these valves don't work well after drip conversion as they don't have enough water flow to actuate the diaphragm effectively. Which is what happens when you open the downstream valve. – Fiasco Labs Apr 19 '15 at 4:38

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