The best way to avoid valves from freezing is to apply a direct source of consistent heat. When pipes and valves freeze up, it can cause some down time along with additional costs for damaged valves.
Regardless of what is flowing through the pipelines, avoiding damage means to avoid freezes, especially during the times that the pipelines are not flowing. To prevent valves from freezing I recommend heating them. A safe electrical heating source is the best way to go. You need to be able to control the temperature safely and evenly to avoid damaging the pipes or the contents inside. Finding a solution with an even distribution of heat is also important to avoid hot and cold spots.
It is also easier to find a method of freeze protection vs. finding a solution to thaw a valve that is already frozen. It takes a lot more energy to thaw a solid, than it does to warm it enough to avoid freezing. So I also recommend finding a solution and getting it installed before the cold season begins. So when searching for a solution, look for a cost effective solution that can be easily maintained and installed without the costs of additional labor.