Q: What does MOP (Maximum Operating Pressure and/or Motor Overload Protection) mean on an expansion valve?
MOP refers to the maximum value that the suction pressure is allowed to rise to before the expansion valve tends to close off and restrict a further increase in flow. This is accomplished at the point where the gas in the expansion valve’s power element charge has become superheated and can exert only slightly more opening pressure as the temperature rises. The purpose of MOP is to prevent the suction pressure from rising so high that the compressor motor cannot start due to too high of an initial load. An MOP type valve tends to serve the same function as a crankcase pressure regulating (CPR) valve; however, it will not control as precisely as the CPR valve. It usually is not recommended to use both a CPR and an MOP type valve on the same system as there is a possibility that they will ‘fight’ each other as they both try to control.