This is an anti-siphon valve for a swimming pool. The purpose for the anti-siphon valve is to prevent pool water from returning into the tap water. This is also the area where you would shut off and turn on the water to the pool. Notice the wing valves. These valves are the controls for the anti-siphon valve. If the wing valve is in line with the plumbing then the line is open. If the wing valve is dissecting the plumbing meaning cutting it in half then the valve is closed. A common problem with anti-siphon valves is when ever there is a frost the water inside can freeze and bust the cap. If the cap is busted then it causes the anti-siphon valve to leak. This is what has occurred. in the photo above.
The picture above shows what a brand new anti-siphon valve should look like. In order to fix the leaking cap the entire anti-siphon valve would need to be replaced because it is to difficult and expensive to find the insides.
Solution to this problem: When there is going to be a hard freeze be sure to either open the water valve just slightly over night buy keeping the float in your pool slightly depressed over night so that a little bit of water is flowing and preventing ice from building up. Or you can try wrapping the valve and plumbing with towels to help insulate it. Personally I would do both. You don't have make the pool auto fill valve run at full blast and over fill the pool, just enough so that water is moving inside the plumbing. This should do the trick on the rare super cold nights here in Phoenix, AZ.
