You want to clear water from the system and the backflow valve. It looks like the hose bib is a tee off the irrigation feed with its own valve.
If there is no inside shut-off valve dedicated to what comes out of the wall, follow these instructions:
- Close valve 3 (turn perpendicular to the pipe).
- Leave valve 1 open (parallel with the pipe).
- Attach the compressor to port 4. Turn the port 4 screw 1/4 turn so it's parallel with that stem.
- Assuming you have an irrigation controller, cycle it through each zone for several minutes in each zone. A "test" cycle may be long enough. Water will spray from the irrigation heads, then it will turn to mist as the water is cleared out of the zone. You want to pressurize each zone until there's nothing but air coming out of the last head in the zone.
- When all of the zones are done, open port 2 (turn the screw 1/4 turn so it's parallel with that stem), and close valve 1. Give it a few seconds to empty any water in that stem.
- Remove the compressor from port 4.
- Open valve 1. Any pressure trapped in the system will escape through the open ports.
- Close the two ports by turning the port 2 and port 4 screws perpendicular to their stems.
- Leave valve 1 open and valve 3 closed (that will protect the backflow valve, which is expensive to replace).
If there is a dedicated inside shut-off valve, follow these instructions, instead:
- Close the inside valve.
- Leave valve 3 open and open the port 4 valve. Open the valve on the hose bib. Close valve 1.
- Attach the compressor to port 4. Blow any water in the feed pipe out through the hose bib.
- Leave the hose bib valve open and complete steps 1 through 8 above (including closing valve 3 and reopening valve 1 in steps 1 and 2).
- Modified step 9: leave both valves 1 and 3 open.
BTW, you want a compressor with a lot of CFM or a large tank, but it doesn't need to be high pressure. The compressed air will drain pretty fast when you're blowing it out of open ports and lots of sprinkler heads.