Nominal diameter意思

"Nominal diameter" is a term used in engineering and manufacturing, particularly in the context of pipes, tubes, and fittings. It refers to the approximate diameter of a pipe or tube as stated by the manufacturer or in a catalog. The nominal diameter is not the actual outside diameter (O.D.) of the pipe, but rather a name or designation for a pipe size that is used to group pipes into categories based on their intended use and pressure rating.

For example, in the United States, pipe sizes are often given in nominal diameters such as "2 inch" or "4 inch," even though the actual outside diameter of a 2-inch pipe is not exactly 2 inches. The actual O.D. of a 2-inch pipe might be something like 2.375 inches, while the inside diameter (I.D.) would be smaller still due to the thickness of the pipe wall.

The use of nominal diameters simplifies the process of specifying, ordering, and identifying pipes and fittings. It allows for standardization across different manufacturers and ensures that components such as elbows, tees, and valves can be interchangeably used with pipes of the same nominal diameter, despite slight variations in actual dimensions.

It's important to note that the term "nominal diameter" can also be used in other contexts, such as with bolts and screws, where it refers to the major diameter of the thread, which is the diameter of the imaginary smooth cylinder on which the threads are cut. In this context, the nominal diameter is the size of the screw or bolt, and it does not necessarily match the actual thread diameter exactly.