Unusual issue with irrigation system water hammer


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Old 03-26-18, 05:07 PM
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Unusual issue with irrigation system water hammer

I have an odd irrigation setup. I installed an in-ground system from my hose bib. I have a well, so GPM and pressure were a concern. To that end, I made sure I had only 2 gear driven heads per zone, for 3 zones. The system works well with no issues, except when I put a screw-on backflow preventer on the sillcock.

This causes MASSIVE water hammer when shutting ANY valve (faucet, toilet or sprinkler system) on either side of the system. Water hammer so bad that it will occur until you open any valve and then gently close it.

I have a theory as to why this is. I suspect, since well systems generally don't exert even pressure until the pump is on, when the water flow is stopped, the pressure in the pipe running from the hose bib to the valve box in the irrigation system is always higher than the other side of the hose bib (This is only about a 6-7 foot run, all pipe used in the irrigation system is 1 inch flex). This causes the back-flow preventer to slam shut, causing a water hammer. When the shockwave reverberates back to the valve, the pressure slams open the breaker, causing a quick drop in pressure, and on and on it goes.

This will force me to not use a backflow preventer, but I'd rather not go without. I know they sell hammer arrestors, I could tee one in on the indoor side of the sillcock, but I'm not sure this would work. I'm hoping it dissipate the pressure from traveling back to the backflow preventer and flinging it open.

Any ideas? Water arrestor? Better backflow preventer? No backflow preventer?
 
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Old 03-26-18, 06:59 PM
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Welcome to the forums.

This causes MASSIVE water hammer when shutting ANY valve (faucet, toilet or sprinkler system) on either side of the system.
You're saying the hammer occurs even when using water inside the house. That would preclude the backflow preventer from being the problem. What pressure is your well running at ?
 
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Old 03-26-18, 07:35 PM
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You would think but removing the backflow preventer resolves the issue. The preventer is not defective, as I've tested two different makes with same results.
 
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Old 03-26-18, 07:48 PM
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If the preventer makes a difference inside the house.... then that would seem to indicate the extra 1" sprinkler piping outside is absorbing some of the impact. That usually indicates higher than normal water pressure.

Not the plumbing pro but I made my own water hammer arrestor by using a T a piece of pipe and a cap. The pipe and cap are pointed up. Since it doesn't have a bladder..... I shut the water off and drain the line when I hear the hammer come back.
 
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Old 03-28-18, 11:07 PM
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Do you have a water softener? I have a 33GPM pump on a large house. They used the same pump to water 2 acres of grass. When my irrigation was on, the pressure inside the house and the pressure on the lines became drastically unbalanced when changing zones.The water inside my house would slosh around everytime my zones changed. Due to having to fill and pressurized the lateral lines. Causing backflow from my softener going in any lines. (even thought I had a backflow preventer)

I had a check valve installed at my softener, and it ended all of my sloshing. However, I am still facing the same issue you are, pressure becoming unbalanced, causing a big pressure drop for a split second as soon as the zones change. Then the pump gets everything stable again. I have pretty much been trying to remedy this problem as well.

I believe our issue is the pump' isn't large enough to fill all of the sprinkler lines while keeping your house pressurized at the same time.

My remedy is to install a larger pump.

Do you also have a pressure regulator for your sprinkler system? That could help the issue. Less PSI more flow. 30-50PSI. I have lots of these around in my system, But I am unsure if they are actual all working.
 
 

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