Standard Pipe Sizes

Process Facility

The pipe size is normally specified with two numbers i.e. the nominal pipe diameter (or bore) and the pipe schedule. This then sets the outside pipe diameter, the wall thickness and thus the internal diameter. With the exception of specialist applications, commercially available pipes come in standard sizes. Pipes manufactured in standard sizes are available in a variety of materials such as carbon steel, steel alloys, in addition to some plastics.

Pipe Diameters

The common pipe size standards include: the American (ANSI/ASME/API), which is in imperial units, and the European (DIN) system which uses metric units. In the American system, the pipe diameter is referred to as “Nominal Pipe Size” (NPS) or “Nominal Bore” (NB). On the other hand, in the European system, it is known as the “Nominal Diameter” (DN).

The most common standard diameters include: 0.5 inch (15 mm), 0.75 inch (20 mm), 1 inch (25 mm), 1.5 inch (40 mm), 2 inch (50 mm), 3 inch (80 mm), 4 inch (100 mm), 6 inch (150 mm), 8 inch (200 mm), 10 inch (250 mm), 12 inch (300 mm), 14 inch (350 mm), 16 inch (400 mm), 18 inch (450 mm), 20 inch (500 mm), 22 inch (550 mm) and 24 inch (600 mm). Additional sizes up to 36 inch are also available. Note that some intermediate standard pipe sizes are also available commercially like 5 inch (125 mm), but these are less common.

For pipe sizes with nominal bore (NB) of 14 inch (DN 350 mm) and above, the nominal bore is the same as the Outside Diameter (OD). For pipe sizes with NB of 12 inch (DN 300 mm) and below, the nominal bore and outside diameter are different. For instance, a pipe with a 12 inch NB (DN 300 mm) has an OD of 12.75 inches, or 324 mm.

Pipe Schedule

The pipe schedule sets the pipe wall thickness. Obviously increasing the wall thickness of the pipe enhances the mechanical strength of the pipe, allowing it to handle higher design pressures.

The following pipe schedules are available (in the order of increasing wall thickness): 5S, 10S, 10, 20, 30, 40S, STD, 40, XS (Extra Strong), 60, 80, 100, 120, 140, XXS (Double Extra Strong) and 160. Thus for pipe of 12 inch NB (DN 300 mm) with a pipe schedule 5S, it has wall thickness is 0.156 inch (4 mm). It is obvious that as the schedule is increased, so does the wall thickness. At a pipe schedule of 160, a 12 inch NB pipe has a wall thickness of 1.312 inches (33.3 mm). Note that not all pipe schedules are available for all pipe sizes.

Pipe Internal Diameter (ID) Calculation

Pipe Internal Diameter (ID) is an important parameter required for line sizing calculations during process engineering design. As aforementioned, for given Nominal Bore or Diameter, the pipe Outside Diameter (OD) remains constant; hence as the pipe schedule changes, the internal diameter of the pipe changes.

The Internal Diameter (ID) can be calculated provided the pipe NB/DN and schedule are known. The pipe ID is given by the pipe NB minus double the pipe wall thickness (obtained from the pipe schedule).

Example:

For a 12 NB (DN 300 mm), schedule 40, the OD and wall thickness are 12.75 inches (324 mm) and 0.406 inches (10.4 mm) respectively. Hence:

Pipe ID = 12.75 inches – 2 x 0.406 inches = 11.94 inches, alternatively

Pipe ID = 324 mm – 2 x 10.4 = 303.2 mm

It is important to keep in mind that wall thickness is provided within a specified tolerance, depending on the engineering standard employed. A typical wall thickness tolerance is 12.5%. This implies that the actual internal pipe diameter may vary slightly from the value calculated above.