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DELOACH BLOG

Understanding Pipe Sizing

Posted by Matthew C. Mossman P.E. on Nov 5, 2021 1:00:00 PM

One issue that I run into relatively often with new technicians, or with some non-technical project managers is confusion over pipe sizing. 

A typical example looks like this: I ask a new technician-in-training to get me a count of the 1-inch pipe that we have in storage. I take the info, and then later find out that what the trainee inventoried was the 3/4-inch pipe. The reason for this confusion lies in the way that pipe sizes are named. The 1-inch, 3-inch, 6-inch, etc. pipe designations are closer to names than sizes. This is because pipe sizing goes by a nominal size standard which is somewhat non-intuitive.

For many people, if they are asked to locate a pipe of a given size, 3-1/2” for example, they will take a tape measure and instinctually measure the outside diameter of the pipe, which will lead them to an incorrect identification. This has to do with the sizing conventions for pipes. Pipes are sized using a nominal pipe size (NPS) designation, and a pipe schedule (SCH) to fully define the size. The nominal size refers only to the approximate inside diameter, and the schedule refers to the wall thickness of the pipe. Because of this, the inner and outer dimensions of a pipe do not directly align with the “name” of the pipe size

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Topics: PVC, Pipe Size, Schedule 40, Schedule 80

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