Richard
Any demand of water flow through a back flow preventer can cause debris to be carried into the valve body. No matter if you are flushing a system to remove air or if you have an expansion tank fail and the relief valve trips. What causes the double check to float is when there is a pressure fluctuation on the incoming supply. This will allow the checks break contact just long enough for debris to get wedged in the narrow sealing surface and the valve leaks through the vent. The longevity of a back flow preventer will always depend upon the water quality and weather or not you have problems with water hammering. The checks can be removed and cleaned most of the time but, it takes time to take the valve apart, clean it and re-install it. For most it is less time consuming to just replace the leaking valve.
Richard
Any demand of water flow through a back flow preventer can cause debris to be carried into the valve body. No matter if you are flushing a system to remove air or if you have an expansion tank fail and the relief valve trips. What causes the double check to float is when there is a pressure fluctuation on the incoming supply. This will allow the checks break contact just long enough for debris to get wedged in the narrow sealing surface and the valve leaks through the vent. The longevity of a back flow preventer will always depend upon the water quality and weather or not you have problems with water hammering. The checks can be removed and cleaned most of the time but, it takes time to take the valve apart, clean it and re-install it. For most it is less time consuming to just replace the leaking valve.