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 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 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 from a pipe do not directly align with the “name” of the pipe size

PVC-SIZE-DIMENSION (2)In addition, different types of pipes have different standard wall thicknesses and overall dimensions based on material of construction and thickness schedule. For example, see the diagram above. This table represents a small selection of PVC nominal sizes in two different schedules, schedule 40 and schedule 80. Schedule 40 is often referred to as standard wall and 80 as heavy wall pipe. You will see that the table refers to average ID and wall thickness. In this case, the average ID language is a bit of a trap because the inside diameter of PVC pipe is not fixed within a given nominal size. For the two schedules to be compatible with each other for reasons such as making transitions from a heavy schedule to a standard schedule, the outside diameter of the pipe is a fixed quantity. Changes in schedule result in variation in the ID that will vary around the nominal size designation.

You can see that for 1-inch pipe, schedule 40 and schedule 80 pipe have the same outside diameter of 1-5/16” while schedule 40 has a 1/8” wall thickness and schedule 80 has a 3/16” wall thickness. This can be very confusing to people just becoming acquainted to working with pipe, especially because the sizes and the number of standard thickness schedules changes between pipe materials. However, once a person understands that pipe sizes should be treated more like names than measurements, it gets a little more straightforward.

I recommend that all my new technicians and junior engineers get comfortable with using reference tables in the beginning. Pipe size tables are readily available on the internet or in traditional engineering print references. I would recommend sticking to engineering specific online resources such as “Engineering Toolbox” or “Engineers Edge” to make sure the information is accurate.

Selecting the correct pipe size, material, and schedule is critical to ensuring the performance of fluid handling systems and is a topic much larger than a single blog post. For those just getting involved in fluid handling, or for non-technical people looking to better understand a plumbing project, understanding nominal pipe sizing, thickness schedules, and dimensional charts can help to remove some of the confusion. I recommend getting acquainted with the tables, and after time you will be able to recognize the size and schedule by sight most times, and then maybe you can give your own “new guys” the same learning experience.  The professionals at DeLoach Industries are here to help. If you have any further questions or have an idea for a blog post, or need more information on a specific subject or water treatment project. Please feel free to contact us at 941-371-4995.

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

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