A typical wall thickness tolerance is 12.5%. It is worth bearing in mind that wall thicknesses come within a specified tolerance, depending onВ the engineering standard used. The pipe ID is givenД«y the pipe NPS minus double the pipe wall thickness (which can be obtained from the pipe schedule).ДЇor example, for a 12 NPS (DN 300 mm) pipe, schedule 40, the OD and wall thickness are respectivelyВ 12.75 inches (324 mm) and 0.406 inches (10.4 mm). The ID of the pipe can be easily calculated, as long as the pipe NPS and schedule are known. As the pipe schedule changes, the ID of the pipe changes. As discussed above, for a given Nominal Pipe Size (NPS), the pipe Outside Diameter (OD) remains constant. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.Д¬alculation of Pipe Internal Diameter (ID)ДЇor process engineers, the most important information is the pipe Internal Diameter (ID), as this is used in.For a given schedule number, the OD increases with NPS while the wall thickness either stays constant or increases.For a given NPS, the OD stays fixed and the wall thickness increases with increase in schedule number.In other words, an NPS 14 pipe is actually 14 inches (360В mm) OD.ДЇew general rules regarding nominal pipe size (NPS) and schedule, For NPS 14 and above вЂ“В The NPS and OD values are equal.To find the actual OD for each NPS value, refer to the reference tables which are based on ASME standards B36.10M and B36.19M. For example, the OD of an NPS 12 pipe is actually 12.75 inches (324В mm). For NPS в…› to 12 вЂ“ The NPS and OD values are different.However the Nominal Pipe Size (NPS) and Outside Diameter (OD) values are not always equal, which can create confusion. Nominal Pipe Size and Schedule collectivelyВ sets the pipe outside diameter (OD) and the wall thickness (and hence the internal diameter (ID) is automatically fixed).В For example, NPS 14 Sch 40 has an OD of 14 inches (360В mm) and a wall thickness of 0.437 inches (11.1В mm). Schedule Double Extra Strong (XXS) wall is thicker than schedule 160 from NPS 1/8В in to NPS 6В in inclusive, and schedule 160 is thicker than XXS wall for NPS 8В in and larger.

Schedule Extra Strong (XS) is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive.

Schedule Standard (STD) is identical to SCH 40S, and 40S is identical to 40 for NPS 1/8 to NPS 10, inclusive.

Due to their thin walls, the smaller вЂњSвЂќ sizes can not be threaded together according to ASME code, but must beВ fusion welded.

However stainless steel pipe can also be available in other schedules. Stainless steel pipes are most often available in standard weight sizes (noted by the SВ designation for example, Sch 10S). Initially thinner pipe schedules 5S and 10S had been created and afterwards other вЂњSвЂќ sizes followed later. Stainless steel pipes, due to less risk of failure due to non-corrosive property, permitted the use of pipes with thinner wall thickness. Today we have a range of wall thicknesses, namely: There have been many revisions and additions to the tables of pipe sizes based on industry use and on standards fromВ API,В ASTM, and others. However with the advancement of industrial age and use of pipes in varying pressure and temperature conditions, these three sizes did not fit all applications.

Initially there are only three pipe wall thicknesses in use вЂ“ For NPS 5 and larger, the Nominal Diameter (DN) is equal to the NPS multiplied byВ 25.

The terms Nominal Bore (NB)В andВ Nominal Diameter (DN) are also frequently used interchangeably with Nominal Pipe Size (NPS).В Nominal Bore (NB) is European Designation equivalent to NPS. Increasing the wall thickness of the pipe increases the mechanical strength of the pipe, allowing it to handle higher design pressures. Pipe schedule sets the pipe wall thickness. For example, when we say pipe size is 2 NPS, it refers to all the pipes having 2.375-inch (or 60.3В mm) as outside diameter irrespective of wall thickness and thus the inside diameter.В Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule (SCH). Nominal pipe size refers to only the outside diameter (OD) of a pipe making it somewhat vague. Nominal Pipe SizeВ (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.

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