Nominal Pipe Size
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Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.^[1] Pipe size is specified with two non-dimensional numbers: a Nominal Pipe Size (NPS) based on inches, and a Schedule (Sch). NPS is often incorrectly called National Pipe Size, due to confusion with National Pipe Thread. For other pipe size standards, see Pipe (material) - Sizes. The European designation equivalent to NPS is DN (Diametre Nominel), which is based on millimetres.^[2]

History

In March 1927, the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: Standard weight (STD), Extra-Strong (XS), and Double Extra-Strong (XXS), based on the Iron pipe size (IPS) system of the day. However these three sizes did not fit all applications. The committee surveyed industry and created a system of Schedule Numbers that designated wall thicknesses based on smaller steps between sizes,^[3] although IPS and NPS numbers remain equivalent.

The original intent was that each Schedule would relate to a given pressure rating, however the numbers deviated so far from wall thicknesses in common use that this original intent could not be accomplished.^[3][4] Also, in 1939, it was hoped that the designations of STD, XS, and XXS would be phased out by Schedule numbers, however those original terms are still in common use today (although sometimes referred to as Standard, Extra-Heavy, and Double Extra-Heavy, respectively). Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others.^[3]

Stainless steel pipes, which were coming into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. By 1949 thinner Schedules 5S and 10S, which were based on the pressure requirements modified to the nearest BWG number, had been created, and other "S" sizes followed later. Due to their thin walls, the smaller "S" sizes can not be threaded together according to ASME code, but must be fusion welded.^[5]

Application

Based on the NPS and Schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and 36.19M. For example, NPS 14 Sch 40 has an OD of 14 inches and a wall thickness of 0.437 inches. However the NPS and OD values are not always equal, which can create confusion.

• For NPS ⅛ to 12 inches, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size is always the actual OD.)
• For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD.

The reason for the discrepancy for NPS ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.

For a given NPS, the OD stays fixed and the wall thickness increases with Schedule. For a given Schedule, the OD increases with NPS while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant Schedule.^[4]

The "S" designation, for example "NPS Sch 10S", most often indicates stainless steel pipes. However some stainless steel pipes are available in steel designations, so strictly speaking the "S" designation only differentiates B36.19M pipe from B36.10M pipe.^[1]

Both polyvinyl chloride pipe (PVC) and chlorinated polyvinyl chloride pipe (CPVC) are made in NPS sizes.

NPS tables for selected sizes

NPS ⅛ to NPS 3½

NPS 4 to NPS 9

NPS 10 to NPS 24

Note: --- = size not designated.

See also

• British standard pipe sizes
• British standard pipe thread sizes

Sources

• Oberg, Erik; Franklin D. Jones, Holbrook L. Horton, and Henry H. Ryffel (2000). in ed. Christopher J. McCauley, Riccardo Heald, and Muhammed Iqbal Hussain: Machinery's Handbook, 26th Edition, New York: Industrial Press Inc.. ISBN 0-8311-2635-3. 
• Nayyar, P.E., Mohinder L. (2000). "A1", in Mohinder L. Nayyar, P.E.: Piping Handbook, 7th, New York: McGraw-Hill. ISBN 0-07-047106-1. 
• Engineering Department (1988). Flow of Fluids Through Valves, Fittings, and Pipe (Technical Paper No. 410), 25th Printing—1991, Joliet, IL: Crane Co.. 
• ASME B36.10M - Welded and Seamless Wrought Steel Pipe. 
• ANSI/ASME Standard 36.19M - Stainless Steel Pipe. 
• ANSI/ASTM Reference R0036 - Stainless Steel Pipe. 
• Table of Pipe Schedules from EngineersEdge.com.

References

1. ^ ^{a b} ASME B36.10M-2004, p. 1
2. ^ NPS - "Nominal Pipe Size" and DN - "Diametre Nominal", EngineeringToolbox.com
3. ^ ^{a b c} ASME B36.10M-2004, p. iv
4. ^ ^{a b} Contrary to the McGraw Hill Piping Handbook, which says that the schedule number can be converted to pressure by dividing the schedule by 1000 and multiplying by the allowable stress of the material. [Nayyar and Mohinder, 2000.]
5. ^ ASME Standard 36.19M-2004, p. iv