Hello and welcome Norman, to the best DIY'r site on the web.
Repair of a frozen valve is not possible, there is a good change that the pressure of the ice has deformed the valve, replace it, and next winter ensure it is drained completely so as not to cause the issue again.
Jack - Thanks for the response. Instructions indicate after initial installation a discharge from the relief valve may occur due to inadequate initial flushing. how long should this discharge last? is there an adjustment? It says clean check valve if flushing doesn't clear. how is that done?
If this is a true Back Flow Prevention (BPD) device, and not a check valve, then local and/or Federal codes will dictate that it must be replaced if damaged at all. BPD's must be able to pass an annual/periodic inspection and one that has been cracked and repaired most likely will not pass the visual nor the dynamic testing.
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