How to fully close large (municipal size) gate valves that leak?

[Dajguy]

The short:
Any suggestions for how to fully close a 6" gate valve that presently lets a lot of water through despite turning it closed with a lot of torque?

The long:
For better or worse, our private property has what is effectively a municipal style 6" PVC water system on it that feeds a number of buildings here. There is a 53,000 gallon storage tank on the property for storage and even several fire hydrants. From electronically monitoring the water level in that tank on a per-minute basis, I know that I am losing about a gallon a minute (~ 1,500 GPD) of water somewhere (of a combination of several places) on the property. That is about 1/2 of our total usage so the leak(s) is/are material to us and we want to locate it/them.

There are several gate valves on that 6" line that break up the water system into zones. I want to systematically close gate valves on the various zones to see if closing off a particular zone reduces the baseline consumption that is due to leaks. In trying to close one zone down today I discovered that there is still a surprisingly large amount of water going past a closed gate valve. As I was closing it, I would stop and go one turn open again and then go back to closing, and I did this several times in the closing process as I closed it down in an attempt to get it to complexly close but several gallons per minute are still getting through. Do you have any suggestions of how to get them fully closed without digging them out and rebuilding them? We also used a cheater bar on the closing tool (as seen in the picture) to the point that I felt very uncomfortable putting more torque on it. The helped close it a little more, but didn't stop the flow. Speaking of that, how much torque can you put on this sized valve before you risk breaking something?

Thanks for any advice that you can offer. It may be that rebuilding is the only option. If so, can that typically be done without removing the gate valve housing from the pipe?

David

Thanks for your thoughts.

 

[John Gayewski]

I think i would keep exercising it down and up and let it run as it's down. You need to blow the crap out of the seat. When you close the valve and let it run out the other side your increasing the velocity through the valve and over the seat, which hopefully acts like a pressure washer cleaning the seat. I don't think I'd keep going with the cheater bar. If you keep giving the cleaning treatment and it doesn't stop its replace or rebuild time.

 

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[Jadnashua]

There is very likely some sand, silt, or similar crud preventing the valve gate from fully seating. We ran into that on our codo's pump station. The valves weren't exercised on a regular basis. The silt had hardened into almost rock hard layer. We had to cut them out (CI) and replace. We had hoped that flow tests where the fire hydrants were opened at least once a year would flush out anythig, but that was not enough.