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 (Sched. or Sch.). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). For other pipe size standards, see pipe (material) - sizes. The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter), in which sizes are measured in millimetres.[2] The term NB (nominal bore) is also frequently used interchangeably with NPS.

[edit] 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 the 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 (XH), and double extra-heavy (XXH), 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]

[edit] 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 B36.19M. 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]

Some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS), although these actually belong to an older system called iron pipe size (IPS). The IPS number is the same as the NPS number. STD is identical to SCH 40S, and 40S is identical to 40 for NPS 1/8 to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive. Different definitions exist for XXS, but it is generally thicker than schedule 160.

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.

[edit] NPS tables for selected sizes

[edit] NPS ⅛ to NPS 3½

NPS[6] DN
mm		 OD
inches (millimeters)		 Wall Thickness
inches (millimeters)
SCH 5 SCH 10 SCH 30 SCH 40 SCH 80 SCH 120 XXS
⅛ 6 0.405 in (10.29 mm) 0.035 in (0.889 mm) 0.049 in (1.245 mm) 0.057 in (1.448 mm) 0.068 in (1.727 mm) 0.095 in (2.413 mm) --- ---
¼ 8 0.540 in (13.72 mm) 0.049 in (1.245 mm) 0.065 in (1.651 mm) 0.073 in (1.854 mm) 0.088 in (2.235 mm) 0.119 in (3.023 mm) --- ---
⅜ 10 0.675 in (17.15 mm) 0.049 in (1.245 mm) 0.065 in (1.651 mm) 0.073 in (1.854 mm) 0.091 in (2.311 mm) 0.126 in (3.200 mm) --- ---
½ 15 0.840 in (21.34 mm) 0.065 in (1.651 mm) 0.083 in (2.108 mm) 0.095 in (2.413 mm) 0.109 in (2.769 mm) 0.147 in (3.734 mm) 0.170 in (4.318 mm) 0.294 in (7.468 mm)
¾ 20 1.050 in (26.67 mm) 0.065 in (1.651 mm) 0.083 in (2.108 mm) 0.095 in (2.413 mm) 0.113 in (2.870 mm) 0.154 in (3.912 mm) --- 0.308 in (7.823 mm)
1 25 1.315 in (33.40 mm) 0.065 in (1.651 mm) 0.109 in (2.769 mm) 0.114 in (2.896 mm) 0.133 in (3.378 mm) 0.179 in (4.547 mm) --- 0.358 in (9.093 mm)
1¼ 32 1.660 in (42.16 mm) 0.065 in (1.651 mm) 0.109 in (2.769 mm) 0.117 in (2.972 mm) 0.140 in (3.556 mm) 0.191 in (4.851 mm) --- 0.382 in (9.703 mm)
1½ 40 1.900 in (48.26 mm) 0.065 in (1.651 mm) 0.109 in (2.769 mm) 0.125 in (3.175 mm) 0.145 in (3.683 mm) 0.200 in (5.080 mm) --- 0.400 in (10.160 mm)
2 50 2.375 in (60.33 mm) 0.065 in (1.651 mm) 0.109 in (2.769 mm) 0.125 in (3.175 mm) 0.154 in (3.912 mm) 0.218 in (5.537 mm) 0.250 in (6.350 mm) 0.436 in (11.074 mm)
2½ 65 2.875 in (73.02 mm) 0.083 in (2.108 mm) 0.120 in (3.048 mm) 0.188 in (4.775 mm) 0.203 in (5.156 mm) 0.276 in (7.010 mm) 0.300 in (7.620 mm) 0.552 in (14.021 mm)
3 80 3.500 in (88.90 mm) 0.083 in (2.108 mm) 0.120 in (3.048 mm) 0.188 in (4.775 mm) 0.216 in (5.486 mm) 0.300 in (7.620 mm) 0.350 in (8.890 mm) 0.600 in (15.240 mm)
3½ 90 4.000 in (101.60 mm) 0.083 in (2.108 mm) 0.120 in (3.048 mm) 0.188 in (4.775 mm) 0.226 in (5.740 mm) 0.318 in (8.077 mm) --- 0.636 in (16.154 mm)

[edit] NPS 4 to NPS 9

NPS[6] DN
mm		 OD
inches (millimeters)		 Wall Thickness
inches (millimeters)
SCH 5 SCH 10 SCH 20 SCH 30 SCH 40/STD SCH 60 SCH 80 SCH 100 SCH 120 SCH 140 SCH 160
4 100 4.500 in (114.30 mm) 0.083 in (2.108 mm) 0.120 in (3.048 mm) --- 0.188 in (4.775 mm) 0.237 in (6.020 mm) 0.281 in (7.137 mm) 0.337 in (8.560 mm) --- 0.437 in (11.100 mm) --- 0.531 in (13.487 mm)
4½ 115 5.000 in (127.00 mm) --- --- --- --- 0.247 in (6.274 mm) --- 0.355 in (9.017 mm) --- --- --- ---
5 125 5.563 in (141.30 mm) 0.109 in (2.769 mm) 0.134 in (3.404 mm) --- --- 0.258 in (6.553 mm) --- 0.375 in (9.525 mm) --- 0.500 in (12.700 mm) --- 0.625 in (15.875 mm)
6 150 6.625 in (168.27 mm) 0.109 in (2.769 mm) 0.134 in (3.404 mm) --- --- 0.280 in (7.112 mm) --- 0.432 in (10.973 mm) --- 0.562 in (14.275 mm) --- 0.719 in (18.263 mm)
8 200 8.625 in (219.08 mm) 0.109 in (2.769 mm) 0.148 in (3.759 mm) 0.250 in (6.350 mm) 0.277 in (7.036 mm) 0.322 in (8.179 mm) 0.406 in (10.312 mm) 0.500 in (12.700 mm) 0.593 in (15.062 mm) 0.718 in (18.237 mm) 0.812 in (20.625 mm) 0.906 in (23.012 mm)

[edit] NPS 10 to NPS 24

NPS[6] DN
mm		 OD
inches (millimeters)		 Wall Thickness
inches (millimeters)
SCH 5s SCH 5 SCH 10s SCH 10 SCH 20 SCH 30
10 250 10.75 in (273.05 mm) 0.134 in (3.404 mm) 0.134 in (3.404 mm) 0.165 in (4.191 mm) 0.165 in (4.191 mm) 0.250 in (6.350 mm) 0.307 in (7.798 mm)
12 300 12.75 in (323.85 mm) 0.156 in (3.962 mm) 0.165 in (4.191 mm) 0.180 in (4.572 mm) 0.180 in (4.572 mm) 0.250 in (6.350 mm) 0.330 in (8.382 mm)
14 350 14.00 in (355.60 mm) 0.156 in (3.962 mm) 0.156 in (3.962 mm) 0.188 in (4.775 mm) 0.250 in (6.350 mm) 0.312 in (7.925 mm) 0.375 in (9.525 mm)
16 400 16.00 in (406.40 mm) 0.165 in (4.191 mm) 0.165 in (4.191 mm) 0.188 in (4.775 mm) 0.250 in (6.350 mm) 0.312 in (7.925 mm) 0.375 in (9.525 mm)
18 450 18.00 in (457.20 mm) 0.165 in (4.191 mm) 0.165 in (4.191 mm) 0.188 in (4.775 mm) 0.250 in (6.350 mm) 0.312 in (7.925 mm) 0.437 in (11.100 mm)
20 500 20.0 in (508.00 mm) 0.188 in (4.775 mm) 0.188 in (4.775 mm) 0.218 in (5.537 mm) 0.250 in (6.350 mm) 0.375 in (9.525 mm) 0.500 in (12.700 mm)
24 600 24.00 in (609.60 mm) 0.218 in (5.537 mm) 0.218 in (5.537 mm) 0.250 in (6.350 mm) 0.250 in (6.350 mm) 0.375 in (9.525 mm) 0.562 in (14.275 mm)


NPS[6] Wall Thickness
inches (millimeters)
SCH 40s SCH 40 SCH 60 SCH 80s SCH 80 SCH 100 SCH 120 SCH 140 SCH 160
10 0.365 in (9.271 mm) 0.365 in (9.271 mm) 0.500 in (12.700 mm) 0.500 in (12.700 mm) 0.593 in (15.062 mm) 0.718 in (18.237 mm) 0.843 in (21.412 mm) 1.000 in (25.400 mm) 1.125 in (28.575 mm)
12 0.375 in (9.525 mm) 0.406 in (10.312 mm) 0.500 in (12.700 mm) 0.500 in (12.700 mm) 0.687 in (17.450 mm) 0.843 in (21.412 mm) 1.000 in (25.400 mm) 1.125 in (28.575 mm) 1.312 in (33.325 mm)
14 0.375 in (9.525 mm) 0.437 in (11.100 mm) 0.593 in (15.062 mm) 0.500 in (12.700 mm) 0.750 in (19.050 mm) 0.937 in (23.800 mm) 1.093 in (27.762 mm) 1.250 in (31.750 mm) 1.406 in (35.712 mm)
16 0.375 in (9.525 mm) 0.500 in (12.700 mm) 0.656 in (16.662 mm) 0.500 in (12.700 mm) 0.843 in (21.412 mm) 1.031 in (26.187 mm) 1.218 in (30.937 mm) 1.437 in (36.500 mm) 1.593 in (40.462 mm)
18 0.375 in (9.525 mm) 0.562 in (14.275 mm) 0.750 in (19.050 mm) 0.500 in (12.700 mm) 0.937 in (23.800 mm) 1.156 in (29.362 mm) 1.375 in (34.925 mm) 1.562 in (39.675 mm) 1.781 in (45.237 mm)
20 0.375 in (9.525 mm) 0.593 in (15.062 mm) 0.812 in (20.625 mm) 0.500 in (12.700 mm) 1.031 in (26.187 mm) 1.280 in (32.512 mm) 1.500 in (38.100 mm) 1.750 in (44.450 mm) 1.968 in (49.987 mm)
24 0.375 in (9.525 mm) 0.687 in (17.450 mm) 0.968 in (24.587 mm) 0.500 in (12.700 mm) 1.218 in (30.937 mm) 1.531 in (38.887 mm) 1.812 in (46.025 mm) 2.062 in (52.375 mm) 2.343 in (59.512 mm)

Note: --- = size not designated.

[edit] See also

[edit] Sources

  • Oberg, Erik; Franklin D. Jones, Holbrook L. Horton, and Henry H. Ryffel (2000). ed. Christopher J. McCauley, Riccardo Heald, and Muhammed Iqbal Hussain. ed. Machinery's Handbook (26th Edition ed.). 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 ed.). 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 ed.). 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. 

[edit] References

  1. ^ a b ASME B36.10M-2004, p. 1
  2. ^ NPS - "Nominal Pipe Size" and DN - "Diamètre Nominale", EngineeringToolbox.com
  3. ^ a b c ASME B36.10M-2004, p. iv
  4. ^ a b This is 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
  6. ^ a b c d Table of Pipe Schedules from EngineersEdge.com. Table shows additional schedules, sizes and sizes greater than 24".
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