Question
Hi Tony,

I apologize if this question is long. I just wanted to give as much detail as possible about my sprinkler issue. I have a sprinkler system with six zones. The control valves for zones 1,2 & 3 are connected in series, and those for zones 4, 5, 6 are connected in series. 1 & 2 cover the front lawn and 3 is a drip system. Zones 4, 5, 6 and six cover the back yard and every time any these three zone valves open I experience water-hammering for about 10 seconds... as is well once a steady flow is reached. These are a couple of things I observed:

1) I've removed the problematic zones from the automatic timer - no automatic watering. Twice, I tried to manually open the zones, about four days apart, each time. The first time I open ANY zone, there is no hammering. But after the first time, I see hammering every time I open any of the zones.
2) When I manually ran the sprinkler from the automatic timer - using the "Manual" button, I hear a buzzing noise when the solenoids for zones 4, 5, 6 are active.
3) When the hammering happens, my vacuum breaker is "pumping" - i.e., the diaphragm in the breaker moves up and down, in sync with the hammering.
4) When the sprinklers are not running the vacuum breaker is sealed shut.

This is what I infer so far based on the above observations.

a) Since zones 1, 2 & 3 run fine, it cannot be a water pressure issue or a vacuum breaker issue.
b) Since the issue happens even when I manually open the control valve, it probably is not a bad solenoid.
c) Since the first zone to open after a long period of time doesn't exhibit hammering, I probably is not a issue with the water draining out of the irrigation pipe.

Couple of side notes:
i) I have a walkout basement => zones 4, 5 & 6 are about a floor below their controller valves.
ii) This started happening only this spring - everything was fine last year.

Do you think you might be able to help me out or recommend that I call in a pro? Again, apologies for the lengthy description.

Thanks in advance for your advice.
Ravi

Answer
Hi Ravi

Water hammer is the resultant "shock wave" in a pipe system that occurs when water is forced to change direction quickly.  An issue which can compound water hammer is downhill slopes which increases the velocity and dynamic pressure of the water.  This increased velocity may actually create a vacuum in the pipe and be problematic for check valves and vacuum breakers trying to stay open.  When the lateral lines are empty, the sprinklers act as air relief valves and you don't experience the hammer problem, however when they are partially filled, the rapid introduction of water back into the lines bounces off the residual water left in the lateral and creates your hammer effect.  

I think you have to slow the water down as it is introduced to the lateral lines.  Pressure reducers or regulators are commonly used to control the pressure levels and conversely, flow regulators (or flow control valves) also help to reduce the flow which ultimately are the source of the problem.

Good luck

Tony