Nominal Pipe Size is a North American set of standard sizes for pipes used for high
or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers:
a nominal pipe size for diameter based on inches, and a schedule for wall thickness.
NPS is often incorrectly called National Pipe Size, due to confusion with national pipe
thread. The European designation equivalent to NPS is DN, in which sizes are measured
in millimetres. The term NB is also frequently used interchangeably with NPS.
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, extra-strong, and double extra-strong, based on the iron pipe size
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, 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. 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, 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.
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. Application
Based on the NPS and schedule of a pipe, the pipe outside diameter 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. 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 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. Some specifications use pipe schedules called
standard wall, extra strong, and double extra strong, although these actually belong to
an older system called iron pipe size. 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. However stainless steel pipe can also be available in other
schedules. Both polyvinyl chloride pipe and chlorinated
polyvinyl chloride pipe are made in NPS sizes. NPS tables for selected sizes
NPS ⅛ to NPS 3½ Tolerance: The tolerance on pipe OD is +1/64inch,
-1/32inch. NPS 4 to NPS 9
NPS 10 to NPS 24 Additional sizes
See also
Notes
References ^ ASME B36.10M-2004, p. 1
^ a b c d e f NPS - "Nominal Pipe Size" and DN - "Diamètre Nominal", EngineeringToolbox.com
^ a b c ASME B36.10M-2004, p. iv ^ ASME Standard 36.19M-2004, p. iv
^ a b c d e f g h Table of Pipe Schedules from EngineersEdge.com.
^ ASTM A999 Bibliography
Nayyar, P.E., Mohinder L.. "A1". In Mohinder L. Nayyar, P.E. Piping Handbook. 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
Notes on Pipe—PVC Pipe weights and max PSI Quick calculator to determine standard pipe
dimensions For Carbon Steel and Stainless Steel pipes as per ANSI.
[1] - Online Pipe Schedule Calculator Nominal Pipe Size NPS, Nominal Bore NB, Outside
Diameter OD Montage
Piping Tools