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Monday, June 25th 2007, 8:07am

Water hammer concern

I'm in the process of installation, and I've discovered that the static pressure just past my double-check valve on the sprinker side is 80 PSI, so I'm concerned about water hammer. When my sprinkler valves shut off, how far back would the shock travel? I assume that it would go at least as far back as the double check valve. I know that this isn't good for my double check valve, but would it contain the surge? Or would it pass the surge through to my interior plumbing?


Supreme Member


Monday, June 25th 2007, 12:34pm

It's not water pressure but water velocity that creates water hammer. What you need to do is size your pipes to avoid water velocities from exceeding 5 feet per second.

Most friction loss table (like this one ) will include not only how much pressure you can expect to loose, but also the expected water speed. For instance, the above referenced chart will tell you that if you are using Sch40 PVC 1" pipe with a GPM of 10, the water will be flowing at 3.24 feet/sec.

Here's a gross summary of what you'll discover:
3/4" Sch40 PVC can handle upto 9GPM
1" Sch40 PVC can handle upto 14GPM
1-1/4" Sch40 PVC can handle upto 24GMP.

Assuming we're talking about a typical residential irrigation system, starting with 80psi likely means you won't have to worry too much about pressure drops due to friction losses. So you should just have to determine what the max flow rate through each pipe in the system is going to be and size the pipe accordingly.

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