Pipe wall thickness
What is pipe? Pipe is a round tubular equipment to distribute fluids ( liquid, gases or slurries ) that designated by a nominal pipe size (NPS or DN) that represents capacity and wall thickness that represents strength. When we talk about pipe, the important dimensions for a pipe is the outer diameter (OD) and pipe wall thickness. To make it simple, we use pipe SCHEDULE to represents pipe wall thickness. Pipe inside diameter is calculated by outside diameter (OD) and wall thickness (WT). ASME B36.10 and ASME B36.19 (respectively, carbon and alloy pipes, and stainless steel pipes) specify the standard combinations of pipe Nominal Pipe Size and Wall Thickness (schedule).
Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes. “Nominal” refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number. For example – “2-inch nominal steel pipe” or NPS 2, consists of many varieties of steel pipe with the only criterion being a 2.3/8″ or 2.375-inch or 60.3 mm as the outside diameter.
Actual outside diameter versus NPS
NPS 1 actual O.D. = 1.5/16″ or 1.315 in (33.4 mm)
NPS 2 actual O.D. = 2.3/8″ or 2.375 in (60.3 mm)
NPS 3 actual O.D. = 3½” or 3.500 in (88.9 mm)
NPS 4 actual O.D. = 4½” or 4.500 in (114.3 mm)
NPS 10 actual O.D. = 10¾” or 10.750 in (273.05 mm)
NPS 12 actual O.D. = 12¾” or 12.750 in (323.9 mm)
NPS 14 actual O.D. = 14″
NPS 16 actual O.D. = 16″
NPS 18 actual O.D. = 18″
NPS 20 actual O.D. = 20″
NPS 22 actual O.D. = 22″
NPS 24 actual O.D. = 24″
Did you see the pattern or difference between NPS and Outside diameter? Here the clue for take a way :
- Start from NPS 14 to NPS 36, the NPS equal with outside diameter.
- For NPS 10 and 12, the outside diameter is NPS + 0.750 in
- For NPS 3 and 4, the outside diameter is NPS + 0.500 in
The wall thickness is expressed in “schedules“. The pipe schedule is abbreviated as SCH. For a given size and schedule the thickness of the pipe is fixed and defined in the applicable ASME standard. Please see ASME B36.10 for carbon steel & alloy pipes ( welded and seamless ) and also ASME B36.19 for stainless steel pipes.
The outside diameter for all NPS remains constant. Therefore any variation schedule that represent wall thickness, affects only the inside diameter. Increase in the schedule number means the wall thickness increases, and the actual bore is reduced. The most popular is pipe schedule 40, 80 and 160. We also familiar with schedule STD (standard), XS (Extra strong) and XXS (Extra Extra Strong). Here the clue :
- STD is identical to SCH 40 for NPS 1/8 to NPS 10, inclusive.
- XS is identical to SCH 80 for NPS 1/8 to NPS 8, inclusive.
- XXS wall is thicker than SCH 160 from NPS 1/8″ to NPS 6″ inclusive,
- SCH 160 is thicker than XXS wall for NPS 8″ and larger.
Schedule 20, 40, 60, 80 pipe wall thickness in mm
In general, the Pipe schedule chart is expressed in inches. if you want to change it in mm units, then the value is multiplied by 25.4. For example, the thickness of the NPS 16 sch 30 pipe is 0.375 in, so we can convert the value to 0.375 x 25.4 = 9.525 mm
The details of pipe dimensions including the outside diameter, pipe wall thickness for any pipe schedule can be seen in this attached documents:
related article:
ASME B31.3
ASME B31.4
ASME B31.8
disclaimer: Please always follow the updated ASME documents for your work