WARNING! Wear ANSI-approved safety eye protection! Extreme care must always be taken when blowing out an irrigation system with compressed air. Compressed air can cause serious injury, including serious eye injury, from flying debris. Always wear ANSI approved safety eye protection and do not stand over any irrigation components (pipes, sprinklers, and valves) during air blow out. Serious personal injury may result if you do not proceed as recommended! It is best for a qualified licensed contractor to perform this type of winterization method.
The blow out method utilizes an air compressor with a Cubic Foot per Minute (CFM) rating of 80-100 for any mainline of 2" or less. These types of compressors can be rented at your local equipment rental yard. Caution: a small shop compressor (1-3 HP) will not have enough "free" air to properly winterize the system. Do not attempt to fully charge the holding tank then release the highly-pressurized air flow into the mainline to compensate for the compressor's lack of CFM. The compressor is attached to the mainline via a quick coupler, hose bib or other type connection, which is located after the backflow device. Compressed air should not be blown through any backflow device. To start the blow out, shut off the irrigation water supply and, with the compressor valve in the closed position, attach the air compressor hose to the fitting. Activate the station on the controller that is the zone or sprinklers highest in elevation and the furthest from the compressor. Close the backflow isolation valves. Then slowly open the valve on the compressor; this should gradually introduce air into the irrigation system. The blow out pressure should remain below the maximum operating pressure specification of the lowest pressure rated component on that zone and should NEVER exceed 80 PSI.
Each station/zone should be activated starting from the furthest station/zone from the compressor, slowly working your way to the closest station/zone to the compressor. Each station/zone should be activated until no water can be seen exiting the heads; this should take approximately two minutes or more per station/zone. It is better to use two or three short cycles per station/zone than to have one long cycle. Once the station/zone is dry, you should not continue to blow air through the pipe. Compressed air moving through dry pipes can cause friction, which will create heat and could cause damage. Never run the compressor without at least one irrigation control valve open.