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) for diameter based on inches, and a schedule (Sched. or Sch.) for wall thickness. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). The European designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm), in which sizes are measured in millimetres.[2] The term NB (nominal bore) is also frequently used interchangeably with NPS.

History[edit]

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 Nayyar and Mohinder, 2000. 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.[4]

Application[edit]

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 indicates actual dimensions, not nominal.)
  • 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.[note 1]

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. XXS wall is thicker than schedule 160 from NPS 1/8" to NPS 6" inclusive, and schedule 160 is thicker than XXS wall for NPS 8" and larger.

Blockage or Ball Test: When a pipe is welded or bent the most common method to inspect blockages, misalignment, ovality, and weld bead dimensional conformity is to pass a round ball through the pipe coil or circuit. If the inner pipe dimension is to be measured then you should subtract the weld bead, if welding is applicable. Typically, the clearance tolerance for the ball must not exceed 1mm. Allowable ovality of any pipe is measured on the inside dimension of the pipe, normally 5% to 10% ovality can be accepted. If no other test is conducted to verify ovality, or blockages, this test must be seen as a standard requirement. A flow test can not be used in lieu of a Blockage or Ball Test. See pipe dimensional table, Specification ASME B36.10M or B36.19M for pipe dimensions per schedule.

Stainless steel pipe is most often available in standard weight sizes (noted by the "S" designation, for example "NPS Sch 10S"). However stainless steel pipe can also be available in other schedules.

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

NPS tables for selected sizes[edit]

NPS ⅛ to NPS 3½[edit]

NPS[5] DN
[2]
OD
[in (mm)]
Wall thickness
[in (mm)]
SCH 5s SCH 10s/10 SCH 30 SCH 40s/40
/STD
SCH 80s/80
/XS
SCH 120 SCH 160 XXS
6 0.405 (10.29) 0.035 (0.889) 0.049 (1.245) 0.057 (1.448) 0.068 (1.727) 0.095 (2.413)
¼ 8 0.540 (13.72) 0.049 (1.245) 0.065 (1.651) 0.073 (1.854) 0.088 (2.235) 0.119 (3.023)
10 0.675 (17.15) 0.049 (1.245) 0.065 (1.651) 0.073 (1.854) 0.091 (2.311) 0.126 (3.200)
½ 15 0.840 (21.34) 0.065 (1.651) 0.083 (2.108) 0.095 (2.413) 0.109 (2.769) 0.147 (3.734) 0.188 (4.775) 0.294 (7.468)
¾ 20 1.050 (26.67) 0.065 (1.651) 0.083 (2.108) 0.095 (2.413) 0.113 (2.870) 0.154 (3.912) 0.219 (5.563) 0.308 (7.823)
1 25 1.315 (33.40) 0.065 (1.651) 0.109 (2.769) 0.114 (2.896) 0.133 (3.378) 0.179 (4.547) 0.250 (6.350) 0.358 (9.093)
32 1.660 (42.16) 0.065 (1.651) 0.109 (2.769) 0.117 (2.972) 0.140 (3.556) 0.191 (4.851) 0.250 (6.350) 0.382 (9.703)
40 1.900 (48.26) 0.065 (1.651) 0.109 (2.769) 0.125 (3.175) 0.145 (3.683) 0.200 (5.080) 0.281 (7.137) 0.400 (10.160)
2 50 2.375 (60.33) 0.065 (1.651) 0.109 (2.769) 0.125 (3.175) 0.154 (3.912) 0.218 (5.537) 0.250 (6.350) 0.343 (8.712) 0.436 (11.074)
65 2.875 (73.03) 0.083 (2.108) 0.120 (3.048) 0.188 (4.775) 0.203 (5.156) 0.276 (7.010) 0.300 (7.620) 0.375 (9.525) 0.552 (14.021)
3 80 3.500 (88.90) 0.083 (2.108) 0.120 (3.048) 0.188 (4.775) 0.216 (5.486) 0.300 (7.620) 0.350 (8.890) 0.438 (11.125) 0.600 (15.240)
90 4.000 (101.60) 0.083 (2.108) 0.120 (3.048) 0.188 (4.775) 0.226 (5.740) 0.318 (8.077) 0.636 (16.154)

Tolerance: The tolerance on pipe OD is +1/64 (.0156)inch, -1/32 (.0312)inch.[6]

NPS 4 to NPS 9[edit]

NPS[5] DN
[2]
OD
[in (mm)]
Wall thickness
[in (mm)]
SCH 5 SCH 10s/10 SCH 20 SCH 30 SCH 40s/40
/STD
SCH 60 SCH 80s/80
/XS
SCH 100 SCH 120 SCH 140 SCH 160 XXS[5]
4 100 4.500 (114.30) 0.083 (2.108) 0.120 (3.048) 0.188 (4.775) 0.237 (6.020) 0.281 (7.137) 0.337 (8.560) 0.437 (11.100) 0.531 (13.487) 0.674 (17.120)
115 5.000 (127.00) 0.247 (6.274) 0.355 (9.017) 0.710 (18.034)
5 125 5.563 (141.30) 0.109 (2.769) 0.134 (3.404) 0.258 (6.553) 0.375 (9.525) 0.500 (12.700) 0.625 (15.875) 0.750 (19.050)
6 150 6.625 (168.28) 0.109 (2.769) 0.134 (3.404) 0.280 (7.112) 0.432 (10.973) 0.562 (14.275) 0.719 (18.263) 0.864 (21.946)
7[5] 7.625 (193.68) 0.301 (7.645) 0.500 (12.700) 0.875 (22.225)
8 200 8.625 (219.08) 0.109 (2.769) 0.148 (3.759) 0.250 (6.350) 0.277 (7.036) 0.322 (8.179) 0.406 (10.312) 0.500 (12.700) 0.593 (15.062) 0.719 (18.263) 0.812 (20.625) 0.906 (23.012) 0.875 (22.225)
9[5] 9.625 (244.48) 0.342 (8.687) 0.500 (12.700)

1 -25mm

NPS 10 to NPS 24[edit]

NPS[5] DN
[2]
OD
[in (mm)]
Wall thickness
[in (mm)]
SCH 5s SCH 5 SCH 10s SCH 10 SCH 20 SCH 30 SCH STD/40S
10 250 10.75 (273.05) 0.134 (3.404) 0.134 (3.404) 0.165 (4.191) 0.165 (4.191) 0.250 (6.350) 0.307 (7.798) 0.365 (9.271)
12 300 12.75 (323.85) 0.156 (3.962) 0.165 (4.191) 0.180 (4.572) 0.180 (4.572) 0.250 (6.350) 0.330 (8.382) 0.375 (9.525)
14 350 14.00 (355.60) 0.156 (3.962) 0.156 (3.962) 0.188 (4.775) 0.250 (6.350) 0.312 (7.925) 0.375 (9.525) 0.375 (9.525)
16 400 16.00 (406.40) 0.165 (4.191) 0.165 (4.191) 0.188 (4.775) 0.250 (6.350) 0.312 (7.925) 0.375 (9.525) 0.375 (9.525)
18 450 18.00 (457.20) 0.165 (4.191) 0.165 (4.191) 0.188 (4.775) 0.250 (6.350) 0.312 (7.925) 0.437 (11.100) 0.375 (9.525)
20 500 20.00 (508.00) 0.188 (4.775) 0.188 (4.775) 0.218 (5.537) 0.250 (6.350) 0.375 (9.525) 0.500 (12.700) 0.375 (9.525)
22 550 22.00 (558.80) 0.188 (4.775) 0.188 (4.775) 0.218 (5.537) 0.250 (6.350) 0.375 (9.525) 0.500 (12.700) 0.375 (9.525)
24 600 24.00 (609.60) 0.218 (5.537) 0.218 (5.537) 0.250 (6.350) 0.250 (6.350) 0.375 (9.525) 0.562 (14.275) 0.375 (9.525)
NPS[5] Wall thickness
[in (mm)]
SCH 40 SCH 60 SCH 80s/XS SCH 80 SCH 100 SCH 120 SCH 140 SCH 160
10 0.365 (9.271) 0.500 (12.700) 0.500 (12.700) 0.593 (15.062) 0.718 (18.237) 0.843 (21.412) 1.000 (25.400) 1.125 (28.575)
12 0.406 (10.312) 0.562 (14.275) 0.500 (12.700) 0.687 (17.450) 0.843 (21.412) 1.000 (25.400) 1.125 (28.575) 1.312 (33.325)
14 0.437 (11.100) 0.593 (15.062) 0.500 (12.700) 0.750 (19.050) 0.937 (23.800) 1.093 (27.762) 1.250 (31.750) 1.406 (35.712)
16 0.500 (12.700) 0.656 (16.662) 0.500 (12.700) 0.843 (21.412) 1.031 (26.187) 1.218 (30.937) 1.437 (36.500) 1.593 (40.462)
18 0.562 (14.275) 0.750 (19.050) 0.500 (12.700) 0.937 (23.800) 1.156 (29.362) 1.375 (34.925) 1.562 (39.675) 1.781 (45.237)
20 0.593 (15.062) 0.812 (20.625) 0.500 (12.700) 1.031 (26.187) 1.280 (32.512) 1.500 (38.100) 1.750 (44.450) 1.968 (49.987)
22 0.875 (22.225) 0.500 (12.700) 1.125 (28.575) 1.375 (34.925) 1.625 (41.275) 1.875 (47.625) 2.125 (53.975)
24 0.687 (17.450) 0.968 (24.587) 0.500 (12.700) 1.218 (30.937) 1.531 (38.887) 1.812 (46.025) 2.062 (52.375) 2.343 (59.512)

Additional sizes (NPS)[edit]

NPS[5] DN
[2]
OD
[in (mm)]
Wall thickness
[in (mm)]
SCH 5s SCH 10s SCH 10 SCH 20 SCH 30 SCH 40s/STD SCH 40
26 650 26.000 (660.400) 0.312 (7.925) 0.500 (12.700) 0.375 (9.525)
28 700 28.000 (711.200)  ? 0.312 (7.925) 0.500 (12.700) 0.625 (15.875) 0.375 (9.525)
30 750 30.000 (762.000) 0.250 (6.350) 0.312 (7.925) 0.312 (7.925) 0.500 (12.700) 0.625 (15.875) 0.375 (9.525)
32 800 32.000 (812.800) 0.312 (7.925) 0.500 (12.700) 0.625 (15.875) 0.375 (9.525) 0.688 (17.475)
34 850 34.000 (863.600) 0.312 (7.925) 0.500 (12.700) 0.625 (15.875) 0.375 (9.525) 0.688 (17.475)
36 900 36.000 (914.400) 0.312 (7.925) 0.500 (12.700) 0.375 (9.525)
NPS DN
[2]
OD
[in (mm)]
Wall thickness
[in (mm)]
SCH STD SCH XS
40 1000 40.000 (1,016.000) 0.375 (9.525) 0.500 (12.700)
42 1050 42.000 (1,066.800) 0.375 (9.525) 0.500 (12.700)
44 1100 44.000 (1,117.600) 0.375 (9.525) 0.500 (12.700)
46 1150 46.000 (1,168.400) 0.375 (9.525) 0.500 (12.700)
48 1200 48.000 (1,219.200) 0.375 (9.525) 0.500 (12.700)
52 1300 52.000 (1,320.800) ? ?
56 1400 56.000 (1,422.400) ? ?
60 1500 60.000 (1,524.000) ? ?
64 1600 64.000 (1,625.600) ? ?
68 1700 68.000 (1,727.200) ? ?
72 1800 72.000 (1,828.800) ? ?
76 1900 76.000 (1,930.400) ? ?
80 2000 80.000 (2,032.000) ? ?
88 2200 88.000 (2,235.200) ? ?

See also[edit]

Notes[edit]

  1. ^ 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.]

References[edit]

  1. ^ ASME B36.10M-2004, p. 1
  2. ^ a b c d e f NPS - "Nominal Pipe Size" and DN - "Diamètre Nominal", EngineeringToolbox.com
  3. ^ ASME B36.10M-2004, p. iv
  4. ^ ASME Standard 36.19M-2004, p. iv
  5. ^ a b c d e f g h Table of Pipe Schedules from EngineersEdge.com.
  6. ^ ASTM A999

Bibliography[edit]

  • 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. 
  • ASME B36.10M - Welded and Seamless Wrought Steel Pipe. 
  • ANSI/ASME Standard 36.19M - Stainless Steel Pipe. 

External links[edit]