How do I prevent the drain valve from being damaged by frost.

It opens at a pressure of < 0.2 bar. It has a 3/4" male thread and is screwed into T coupling 25 mm x 3/4" female thread within the pipe run or into coupler 25 mm x 3/4" female thread at the end of the pipe.  Each separate sprinkler section must be fitted with at least one drain valve. Coarse gravel packing covering an area of at least 30x30x30 cm must be provided underneath the drain valve. On slopes where the pipe run goes up or down, a drain valve must be fitted every 2 metres of difference in height.   Drain Valve (2760)