Lawn sprinkler systems and any external hose spigots in your home require an anti-siphon valve to be installed to prevent water pressure from pushing back through your water lines. Backpressure can carry impurities into your piping and contaminate your water. Over time these valves will wear out and will have to be replaced. The valve itself is very simple, as it has a flap that allows water to only flow out, and not in, like a one-way door, and it is generally this flap that wears out. Your cap should be made of PVC and the mainline is metal piping, this is for the purpose of holding proper water pressure and safer conditions in freezing environments.
Step 1 - Shutoff
Before doing anything, be sure to shut off the main water supply to the part of your home where you're installing the anti-siphon valve to prevent harm from water pressure. In many cases, there is not a secondary water cutoff to a spigot, so you will likely have to shut down all your home's water long enough for the installation and replacement. If your water valve is attached to a line with its own shut off then you are safe going to the store while the old valve is removed, if not you will want to take measurements and match these at the store.
Step 2 - Remove the Valve
Once the water is shut off, take a pair of pipe wrenches and remove the central fitting so you can unscrew the anti-siphon valve cap from the main pipeline. You can then take it to the store with you when you get the replacement, as to get a proper fit. This will ensure you purchase the appropriate anti-siphon valve for the job at hand.
Step 3 - Wrap Around Pressurized Seals
Take your plumber's tape and wrap the threads of the male end of your pipes. This is the end with the threads and will be on the spigot end of your operation. Wrap this tape several times around the threads. Once the anti-siphon valve has been installed over this plumbers tape, it will cause a pressurized, watertight seal.
Step 4 - Installing your New Valve
Once you have the hardware needed for the task, simply use your wrench to install the connector to your existing water line. Your anti-siphon valve is a simple screw-on attachment to your existing water connection. Make sure that the fittings are tight and secured to prevent the water pressure from blowing the new cap off once it is installed.
Step 5 - Test for Pressure Leaks
If you follow these instructions and complete your task, simply turn your water back on and check for pressure leaks. If your valve does not leak when you turn your water on, you have installed it successfully.