stig,

If you have drains at the low spots congratulations on good maintenance planning You only have to worry about low spots that have lines full of water. Also people used compressed air to irrigation clear lines for winter. There are YouTube videos showing how to do this. The hard part is the connections for the compressor and how big of one to use. In your case low pressure or none at all would work. No more than about 5 psi to push water out the drains. GL JMHO Aloha

For what it's worth, I use polyethyene instead of PVC for my irrigation system. In the fall I open everything up, and let it drain.

PE tolerates short sections of full pipe. The ice plug just expands into the air space on either end. It also really reduces the number of fittings. A 300 foot supply line with two hosebibs on it (one for the projected greenhouse, one for a demo garden) took me about a half a day to put in. Dig a 2" deep x 2" wide groove in the sod with a grubhoe, lay the pipe, pinning it down with geotex staples, and cover it over, leaving the top showing now and then so I can find and follow it later. (This is a farm, not a show place.)

I learned early on to make sure my connections, which are more rigid, a few inches above the lie of the pipe. They don't like to freeze. Nor do taps.

In the spring I usually have a small amount of breakage. It takes me most of a day to get the system running and fix things. But i have 1500 feet of mainline, 10,000 feet of laterals, 9 hose bibs, and I buy drippers in boxes of a thousand.