Just as manufacturing methods differ, there are also different ways to categorize the size of a pipe. Pipe is identified by three different size categories: nominal pipe size, outside diameter, and inside diameter (see Figure 2-5).

Nominal pipe size (NFS) is used to describe a pipe by name only. In process piping, the term nominal refers to the name of the pipe, much like the name 2 x 4 given to a piece of lumber.

The lumber does not actually measure 2" x 4", nor does a 6" pipe actually measure 6" in diameter. It's just an easy way to identify lumber and pipe.

Outside diameter (OD) and inside diameter (ID), as their names imply, refer to pipe by their actual outside and inside measurements.

Pipe i/g" to 12" has an outside diameter greater than its nominal pipe size, while pipe 14" and above has an outside diameter equal to its nominal pipe size. In process piping, the method of sizing pipe maintains a uniform outside diameter while varying the inside diameter.

This method achieves the desired strength necessary for pipe to perform its intended function while operating under various temperatures and pressures.

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Author said...

As stated in Pipe Drafting and Design by Roy A. Parisher and Robert A. Rhea (ISBN 9 780750 674393), the textbook from which this information was plagiarized, Nominal Pipe Size is abbreviated as NPS, not NFS. Also, the typo in the fifth paragraph should read 1/8”, not i/g”.

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