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Steel pipe sizes around the world are standardized on historic dimensions that are now referred to as Nominal Pipe Sizes (NPS).
In addition to standardizing the nominal outside diameter (OD), a series of standard wall thicknesses was developed years ago and these were labeled as Schedules (SCH). The units for both of NPS and SCH are dimensionless. A larger number indicates a larger OD size or a heavier wall thickness. A typical description might look like 4 NPS SCH 40. In this case this description indicates a nominal OD of 4.500” (114.3 mm) and a nominal wall thickness of 0.237” (6.02 mm). Through 8 NPS, the nominal dimensions for both carbon alloy and stainless alloy pipe are the same. In larger sizes, there are differences in some nominal wall thicknesses.
Historically, the dimensions for smaller diameter pipe were such that the inside diameter (ID) of what are now known as SCH 40 thicknesses were very close to the OD size designator. In the example above, the nominal ID is actually 4.026” for 4 NPS. As it became possible to make lighter (and heavier) wall thicknesses, the schedule of sizes was expanded. The sizes are standardized in ANSI B36.10 for carbon pipe and in ANSI B36.19 for stainless and nickel alloys.
Pipe tolerances will depend on the alloy and the standard to which it was produced. The general requirement standard covering most stainless pipe is ASTM/ASME A/SA 999. The OD under tolerance on all sizes is -0.031”. The over tolerance increases with OD size but for the range of 1-1/2 to 4 NPS the plus tolerance is also 0.031”. This introduces the concept of ovality, where if the pipe exhibited both the maximum and minimum OD permitted, it would be out of round (oval) by 0.062”. An additional ovality allowance is permitted for thin wall pipe which is defined as a pipe where the nominal wall is 3% or less of the OD. In such a case, the OD tolerance range applies to the mean OD and the ovality is permitted to be 1.5% of the OD. (See Inset).
Only the minimum wall thickness is limited, which is -12.5% of the nominal wall. There is no maximum wall thickness limit. For seamless pipe, there is a limit of 10% by weight (mass) over the nominal weight. Note that this limits the average wall thickness of the seamless pipe to being no more than 10% heavy, but does not limit the specific maximum wall thickness at any given point. There is no maximum thickness or weight limit for welded pipe.
Tolerances are also provided for stainless pipe on straightness (1/8” in 10’) and for cut lengths (-0, +1/4”). Notice that there is no requirement or tolerance for the ID.
The tolerances for most nickel alloy welded pipe are identical or very similar to those for stainless. The general requirements for nickel alloy welded pipe are contained in ASTM/ASME B/SB 775. General requirements for seamless nickel alloy pipe are published in ASTM/ASME B/SB 729. Because nickel alloys are more difficult to hot work, tolerances for hot finished pipe are more generous than for stainless steel or welded and cold finished nickel alloy products.
Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes and fittings 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.
The name NPS is based on the earlier Iron Pipe Size (IPS) system. That IPS system was established to designate the pipe size. The size represented the approximate inside diameter of the pipe in inches. An IPS 6" pipe is one whose inside diameter is approximately 6 inches. Users started to call the pipe as 2inch, 4inch, 6inch pipe and so on. To begin, each pipe size was produced to have one thickness, which later was termed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipe was standardized.
As the industrial requirements handling higher pressure fluids, pipes were manufactured with thicker walls, which has become known as an extra strong (XS) or extra heavy (XH). The higher pressure requirements increased further, with thicker wall pipes. Accordingly, pipes were made with double extra strong (XXS) or double extra heavy (XXH) walls, while the standardized outside diameters are unchanged. Note that on this website only terms XS and XXS are used.
Nominal Bore (NB) is the European designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm), in which sizes are measured in millimeters. NB is also frequently used interchangeably with NPS.
Outside diameter (OD) is the outside diameter of the pipe and is fixed for a given size. The pipes are always specified by outside diameter, never by inside diameter.
The pipe schedule sets the pipe Wall Thickness (WT). Obviously increasing the wall thickness of the pipe increases the mechanical strength of the pipe, allowing it to handle higher design press. As the schedule is increased, so does the wall thickness.
Nominal pipe size NPS is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 6 indicates a pipe whose outside diameter is 168.3 mm. The NPS is very loosely related to the inside diameter in inches, and NPS 12 and smaller pipe has outside diameter greater than the size designator. For NPS 14 and larger, the NPS is equal to 14inch. For a given NPS, the outside diameter stays constant and the wall thickness increases with larger schedule number. The inside diameter will depend upon the pipe wall thickness specified by the schedule number.
“S” schedules are specific to stainless steels and schedules without the “s” are intended for carbon steels.