A problem with most anti-siphon plastic irrigation valve installations on PVC risers is that if the valve fails, one is usually forced to cut off pieces of the plastic pipes to cement new fittings on so that the new valve can be installed, or the valve body must be sawed in half to remove it from the pipe, destroying any hope of repairing the valve. If the original installation was neatly done with all valves at the same height, the repair job looks bad. Moreover, if the valve was originally installed close to ground level, it may end up below ground after the pipes have been shortened. Here's a way to keep your installations looking neat and easily repairable.
Originally, I considered using PVC union fittings, but they are quite expensive and bulky. I finally settled on an inexpensive solution using expendable parts that can be unscrewed from the valve body and risers.
The first photo in this series (above) shows the original backyard valve installation, with some earth dug away to expose the pipes. The valve on the left has been replaced at least once already, requiring the riser to be cut and material to be spliced on to bring the valve back up to its original height.
Instead of cementing the slip x male pipe thread (MPT) PVC fitting directly to the PVC riser, instead we construct short adapter sections consisting of a slip x female pipe thread (FPT) fitting, short piece of PVC pipe, and slip x MPT fitting. We need two of these adapter sets per anti-siphon valve. Cut the pieces of PVC pipe so that about 1/2 inch of pipe will be exposed between the two fittings once they've been cemented on. Since the slip part of PVC fittings varies between manufacturers, you'll need to measure the depth of the fittings with a small ruler or depth gauge; add the two measurements and add 1/2 inch to determine the length of the PVC pipe pieces. A miter box and miter handsaw or an electric miter saw are very useful for getting uniform, square cuts.
As you can see in the photo, I'm using a scrap piece of 1x2 lumber as a stop so that I get absolutely uniform lengths of PVC pipe. You can use the same trick with a miter box, using a C-clamp to hold the stop in place.