The key dimensional parameters of steel pipes explained: “NPS”, “Schedule”, “Length” .
PIPE DIAMETER AND WALL THICKNESS
The two key steel pipe sizes are the nominal pipe size (NPS) and wall thickness (WT):
- NPS means “nominal pipe size”, which is the internal flow capacity of the pipe (pipe bore capacity). The comparable European term for “NPS” is “DN” (i.e. “Diamètre nominal” or “Nominal diameter” and “Durchmesser Nach” in German – source: Wikipedia)
- WT means “wall thickness”, i.e. the thickness of the pipe wall expressed in inches or millimeters. The higher the thickness of a pipe at a given NPS, the stronger the resistance of the pipe to the pressure of the fluid and its possible corrosion. The pipe thickness is called also “pipe schedule” (abbreviated, “SCH.”). For a given NPS and schedule, the thickness of the pipe is fixed and defined in the applicable ASME standard B36.10 for carbon/alloy steel and ASME B36.19 for stainless and nickel alloy pipes)
A similar concept exists also for European pipes, whose dimensions are covered by the EN 10220 (carbon/alloy steel) and EN 1127 (stainless/duplex/nickel alloys) specifications.
NPS vs. IPS Pipe System
The term “NPS” replaced the previously used “Iron Pipe Size” (IPS) system, which was similarly used to designate the sizes of steel pipes. The term IPS referred to the approximate inside diameter of a pipe, expressed in inches (so a 4″ inches IPS pipe had an inside diameter of approximately 4 inches). Under the IPS system, pipes were manufactured with one wall thickness only (which was called “standard”, or “STD”): the outside diameter of the pipe resulted from the sum of the inside diameter and the wall thickness. With time, the petrochemical industry led to the introduction of additional wall thicknesses, like the “extra-strong” (XS or XH, i.e. “extra-heavy”), and the double-extra strong (XXS or XXH, i.e. double extra heavy).
Under the IPS system, and until the year 1927, manufacturers produced pipes in three thicknesses only. In the Thirties, ASME introduced a new system were pipes sizes were designated as we know them today (NPS). The NPS system showed some of the typical thicknesses existing under the IPS (STD, XS/XH, XXS/XXH) and introduced new ones (Sch. 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160).
“NPS xx” designates a pipe size, but not an exact inside diameter of the pipe in inches: pipes with NPS under 12, have an outside diameter which is larger than the size designator (for example a pipe NPS 4 has an actual OD of 4 ½”, i.e. 114.3 mm); for pipes above 14 inches, the outside diameter and the NPS match (for example, a pipe NPS 14 has an actual OD of 14″, i.e. 355.6 mm).
Pipes of a certain NPS have a constant outside diameter, but different inside diameters depending on the wall thickness (SCH): an NPS 6 pipe Sch. STD has the same OD of an NPS 6 pipe Sch. XXS, but a larger inside diameter (as the wall is smaller).
The terms used to designate the length of steel pipes are:
- “SRL” (“single random length”): means that the pipe has any random size between 5-7 meters; generally, pipes below 2 inches in diameter are manufactured with SRL, i.e. shorter (or half measures) of larger bore pipes
- “DRL” (double random length): meaning that the pipe has any random size between 11-13 meters. Pipes above 2 inches in diameter are available in DRL size
- Cut Lengths: pipes are cut according to project specifications. Custom sizes are used to save welding costs at the installation site.
The term “random” refers to the fact that the pipe mill can control that the pipe length is between a min-max value, but cannot control the exact length of every single pipe (which will be variable, within the given range).
A double random length pipe has an expected length twice the length of an SRL pipe.
PIPE END TYPES
The term “pipe end” refers to how the pipe is finished at its extremities.
The common pipe end types are:
- Plain ends (PE): plain ends are generally used for smaller diameters and require slip-on flanges and socket weld fittings. Plain ends are also common for stainless, duplex and nickel-alloy pipes
- Beveled ends (BE): this is the most common pipe end type (beveled end pipes are joined by welding).
- Threaded ends (TE): threaded ends (which are generally NPT as per ASME B1.20.1 for petrochemical pipes) require threaded fittings and flanges and are used for smaller size pipelines or gas lines
- Threaded and coupled ends (T&C), generally used for gas distribution
- Grooved ends (example Victaulic pipes): these are pipes that allow a quick connection, used for non-critical applications