Question
We hear a loud bang (a single one, no continuous bumps) ONLY when the automatic sprinkler system turns on and again when it turns off. The "certified" plumber installed a water hammer arrester next to (after) the shut-off valve of the sprinkler system pipe. He also checked that the Pressure Vacuum Breaker was working well. All this did not help at all.

Can you guys help me suggesting some solutions?

Other facts.

This is a 1997 built hose that we bought 6 months ago, but the problem just started recently (1 month ago)
Some sprinkler heads are not working properly and one of them is missing, which makes water just spill in the ground (we of course want to fix this, but we first want to solve the water hammer problem)

thanks a lot for your help

Answer
Hey Rodrigo, the reason a system develops a water hammer effect is because the water rushes through the pipes and then as the valve shuts off it creates the hammer effect. The more water that runs through the pipe, the worse the effect is. First thing I would do is replace the head that is broken off in the ground where you are seeing the water spill in the ground. This is causing alot more water to be used then if you had a head installed, thus more water is moving through the pipe then what it used to be when the head was installed. Also you might want to think about installing a pressure regulator on the system either before or after your Vacuum Breaker. What this will do is decrease the volume (speed) that the water is flowing through the pipes, thus cutting down on the amount of water flowing through the pipes and eliminating the water hammer. The only thing you will have to watch, is how the system was designed to begin with, and if you do cut down on the pressure, that the existing system is designed to work on less pressure then what you crrently have. Good luck and have a great day.