Nominal Pipe Size (NPS) 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 (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. The term NB (nominal bore) is also frequently used interchangeably with NPS.
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.
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 astandard 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.
Tolerance: The tolerance on pipe OD is +1/64 (.0156)inch, -1/32 (.0312)inch.
source from: wikipedia.org
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