Steel Pipes Size: Analysis Of International Standards

Core parameters of steel pipes size

Definition and measurement method of nominal diameter (NPS/NB)

The nominal diameter of steel pipe (NPS, Nominal Pipe Size, or NB) is one of the important parameters in the pipeline system. It is usually expressed in British (inch) units and provides the most intuitive data for the design and installation of the pipeline system. However, the nominal diameter and the actual outer diameter (OD) are not a simple linear correspondence. This relationship will be different in pipes of different diameters. For example:
The actual outer diameter corresponding to NPS 1/2 is 21.3mm, and the outer diameter corresponding to NPS 12 is 323.9mm.
When measuring the pipeline, the vertical section at one end of the steel pipe should be used as the standard for measurement to ensure the accuracy of the data and avoid errors.

Outer diameter (OD) and inner diameter (ID) calculation formula

The inner diameter (ID) of the steel pipe is calculated by the following formula:
ID=OD−2×WT
WT is the wall thickness, which is an important factor affecting the flow cross-sectional area. For example, the SCH 40 NPS 6 steel pipe has an outer diameter (OD) of 168.3mm and a wall thickness (WT) of 7.11mm. The inner diameter (ID) is calculated to be 154.08mm by the above formula.

Comparison of international standard systems

ASTM/ASME American Standard System

In the American Standard System, the nominal diameter is the basis of the steel pipe size. When the NPS value is greater than or equal to 14, the outer diameter (OD) value of the pipe is the same as the nominal diameter value.
Use “Schedule” (SCH) to define different wall thickness grades. The higher the SCH value, the stronger the pressure bearing capacity. The wall thickness of SCH 160 steel pipe can reach 22.23mm (NPS 12).

ISO/DIN European Standard System

In the European Standard System, the size of steel pipes is usually expressed by nominal diameter (DN). For example, DN15 in the European Standard is NPS 1/2 (OD 21.3mm) in the American Standard, and DN100 corresponds to NPS 4 (OD 114.3mm).
The European Standard has stricter tolerances. The EN 10220 standard requires an outer diameter tolerance of ≤±1%, which is even higher than the ±1.5% of the American Standard ASTM A53.

Wall thickness grade and pressure bearing capacity

Data source: ASTM A106 standard

The influence of material on dimensional tolerance

Data source: EN 10217 standard