1. Nominal Pipe Size (NPS)
NPS is the nominal size of the pipe in inches.
- It is an imperial standard designation, not the physical diameter.
- Example: NPS 2″ means a flow capacity of 2″, but the actual OD is 60.3 mm.
2. Diameter Nominal (DN)
DN is the metric equivalent of NPS in millimeters.
- DN 50 is equivalent to NPS 2″; it is only a label, not the actual diameter.
- It facilitates ordering and component comparison in the global market.
3. Outer Diameter (OD) & Inner Diameter (ID)
- OD (Outer Diameter): the outer diameter of the pipe, measured from outside to outside.
- ID (Inner Diameter): the lumen diameter, measured from inside to inside.
- Formula: ID = OD – 2 × wall thickness
Conversion of NPS to DN in ASME B36.10
ASME B36.10 standardizes the OD of pipes based on NPS, then assigns the metric label DN without changing the dimensions. For example:
|
NPS (inch)
|
DN (mm)
|
Actual OD (mm)
|
|
2
|
50
|
60.3
|
|
2 ½
|
65
|
73.0
|
- NPS 2″ ≡ DN 50, but the OD remains 60.3 mm according to ASME B36.10.
- NPS 2 ½″ ≡ DN 65, but the OD remains 73.0 mm according to ASME B36.10.
- DN only facilitates metric referencing; design specifications still refer to OD & schedule.
5. Schedule (SCH) – Wall Thickness Code
Schedule indicates the wall thickness of the pipe for various working pressures.
- Examples: SCH 5, SCH 10, SCH 30, SCH 40, SCH 80, SCH 120, SCH 160.
- The higher the schedule number, the thicker the wall and the higher the pressure rating.
- For example, SCH 40 for NPS 2″: thickness 3.91 mm.
6. Identification STD, XS and XXS
These three terms originate from the old system before the Schedule (SCH) concept was introduced.
- STD Standard Weight: Standard wall thickness, suitable for low–medium pressure.
- XS Extra Strong: Thicker wall than STD, suitable for high pressure.
- XXS Double Extra Strong: Very thick wall, used for extreme pressure.
- For example, NPS 2″ STD ≈ SCH 40.
7. Pipe Length
- Nominal Length: standard HFW/ERW BPI products (6 m or 12 m).
- SRL (Single Random Length)
• Random length between 6.0 m and 6.5 m, with a nominal length of 6 m
• Typically for pipes NPS 4″ and below
- DRL (Double Random Length)
• Random length between 10.7 m and 12.8 m, with an average or nominal length of 12.0 m
• Standard for pipes NPS 6″ and above
8. Weight per Unit Length (kg/m & lb/ft)
Weight per unit length is important for load calculations and material cost estimation. It is commonly expressed in kilograms per meter (kg/m) or pounds per foot (lb/ft).
With an understanding of NPS, DN, OD/ID, schedule, weight and length pipe steel, procurement, engineering, and field teams can ensure that steel pipe specifications meet project requirements. If you currently have steel pipe requirements, please contact our Marketing & Sales team. Bakrie Pipe Industries is ready to serve your steel pipe needs.
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