Definition and Details of Pipe Nominal Pipe Size

Nominal Pipe Size

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. The name NPS is based on the earlier "Iron Pipe Size" (IPS) system.

That IPS system was established to designate the pipe size. The size represented the approximate inside diameter of the pipe in inches. An IPS 6" pipe is one whose inside diameter is approximately 6 inches. Users started to call the pipe as 2inch, 4inch, 6inch pipe and so on. To begin, each pipe size was produced to have one thickness, which later was termed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipe was standardized.

As the industrial requirements handling higher pressure fluids, pipes were manufactured with thicker walls, which has become known as an extra strong (XS) or extra heavy (XH). The higher pressure requirements increased further, with thicker wall pipes. Accordingly, pipes were made with double extra strong (XXS) or double extra heavy (XXH) walls, while the standardized outside diameters are unchanged. Note that on this website only terms XS and XXS are used.

Pipe Schedule

So, at the IPS time only three walltickness were in use. In March 1927, the American Standards Association surveyed industry and created a system that designated wall thicknesses based on smaller steps between sizes. The designation known as nominal pipe size replaced iron pipe size, and the term schedule (SCH) was invented to specify the nominal wall thickness of pipe. By adding schedule numbers to the IPS standards, today we know a range of wall thicknesses, namely:

SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS and XXS.

Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 6 indicates a pipe whose outside diameter is 168.3 mm.

The NPS is very loosely related to the inside diameter in inches, and NPS 12 and smaller pipe has outside diameter greater than the size designator. For NPS 14 and larger, the NPS is equal to 14inch.

Steel Pipes

For a given NPS, the outside diameter stays constant and the wall thickness increases with larger schedule number. The inside diameter will depend upon the pipe wall thickness specified by the schedule number.

Summary:
Pipe size is specified with two non-dimensional numbers,

and the relationship between these numbers determine the inside diameter of a pipe.

Stainless Steel Pipe dimensions determined by ASME B36.19 covering the outside diameter and the Schedule wall thickness. Note that stainless wall thicknesses to ASME B36.19 all have an "S" suffix. Sizes without an "S" suffix are to ASME B36.10 which is intended for carbon steel pipes.

The International Standards Organization (ISO) also employs a system with a dimensionless designator.
Diameter nominal (DN) is used in the metric unit system. It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 80 is the equivalent designation of NPS 3. Below a table with equivalents for NPS and DN pipe sizes.

NPS 1/2 3/4 1 1.1/4 1.1/2 2 2.1/2 3 3.1/2 4
DN 15 20 25 32 40 50 65 80 90 100

Note: For NPS ≥ 4, the related DN = 25 multiplied by the NPS number.

Do you now what is "ein zweihunderter Rohr"?. Germans means with that a pipe NPS 8 or DN 200. In this case, the Dutch talking about a "8 duimer". I'm really curious how people in other countries indicates a pipe.

Examples of actual O.D. and I.D.

Actual outside diameters

Actual inside diameters of a 1 inch pipe.

Such as above defined, no inside diameter corresponds to the truth 1" (25,4 mm).
The inside diameter is determined by the wall thickness (WT).

Facts you need to know!

Schedule 40 and 80 approaching the STD and XS and are in many cases the same.
From NPS 12 and above the wall thickness between schedule 40 and STD are different, from NPS 10 and above the wall thickness between schedule 80 and XS are different.

Schedule 10, 40 and 80 are in many cases the same as schedule 10S, 40S and 80S.
But watch out, from NPS 12 - NPS 22 the wall thicknesses in some cases are different. Pipes with suffix "S" have in that range thinner wall ticknesses.

ASME B36.19 does not cover all pipe sizes. Therefore, the dimensional requirements of ASME B36.10 apply to stainless steel pipe of the sizes and schedules not covered by ASME B36.19.

Remark(s) of the Author...

The Story Behind Nominal Pipe Size March 9, 2006

  • A question was put to the PM Engineer (PME) staff (one of SUPPLY HOUSE TIMES sister magazines) asking how nominal pipe size came to be. Here is the answer provided by PME Editorial Director Julius Ballanco.
  • The person directly responsible for the nominal pipe size was a gentleman by the name of Robert Briggs. Briggs was the superintendent of the Pascal Iron Works in Philadelphia. In 1862, he wrote a set of pipe specifications for iron pipe, and passed them around to all of the mills in the area.
  • Realize that in 1862, the United States was engaged in the Civil War. Each pipe mill made its own pipe and fittings to its own specifications. Briggs tried to standardize the sizing, which would also help the war effort. The pipe and fittings would be interchangeable between mills. This was rather novel in 1862.
  • The pipe standards went on to become known as the "Briggs Standards". They eventually became the American Standards, and finally the standards used for modern day pipe.
  • The current ASTM A53 Steel Pipe Standard uses basically the Briggs Standard for pipe sizes 1/2 inch through 4 inch. You will notice that after 4 inches, pipe starts to get closer to the actual dimension used to identify the pipe.
  • So, you are probably asking, where did the sizes come from ?. Well, they were the sizes of the dies used in Pascal Iron Works. Briggs made everyone adjust to him. Hence, the name "nominal" pipe size came about, meaning "close to" or "somewhere in the proximity of" the actual dimension.

I found the story behind Nominal Pipe Size on Supplyhouse Times