Question 184Steam Plants - Assistant Engineer

The correct answer is D) diameter of the inlet opening. The term "nominal size" in the context of safety valves refers to the diameter of the inlet opening, which must be between 1 1/2 and 4 inches as per the regulations for propulsion boilers and superheaters. This is the critical dimension that determines the flow capacity and performance of the safety valve. The other options are incorrect because they do not correspond to the definition of "nominal size" as it applies to safety valves. The free spring length, diameter of the feather, and diameter of the huddling chamber are not the primary factors that define the nominal size of the safety valve.