What is SCH (Schedule Number)?

SCH stands for “Schedule,” which is a numbering system used in the American Standard Pipe System to indicate wall thickness. It is used in conjunction with nominal diameter (NPS) to provide standardized wall thickness options for pipes of different sizes, facilitating design, manufacturing, and selection.

 

SCH does not directly indicate wall thickness but is a grading system that corresponds to specific wall thicknesses through standardized tables (e.g., ASME B36.10M, B36.19M).

 

In the early stages of standard development, an approximate formula was proposed to describe the relationship between SCH, pressure, and material strength:
SCH ≈ 1000 × P / S
Where:
P — Design pressure (psi)
S — Allowable stress of the material (psi)

 

Although this formula reflects the relationship between wall thickness design and usage conditions, in actual selection, the corresponding wall thickness values must still be referenced from standard tables.

 

Origin and Related Standards of SCH (Schedule Number)

The SCH system was originally established by the American National Standards Institute (ANSI) and later adopted by the American Society of Mechanical Engineers (ASME), incorporated into the B36 series of standards, to indicate the relationship between pipe wall thickness and pipe diameter.

 

Currently, the commonly used standards include:

ASME B36.10M:
Applicable to carbon steel and alloy steel pipes, covering SCH 10, 20, 40, 80, 160, etc.;

ASME B36.19M:
Applicable to stainless steel pipes, including lightweight series such as SCH 5S, 10S, 40S, etc.

 

The introduction of SCH numbers resolved the issue of inconsistent wall thickness representation across different nominal diameters, thereby standardizing pipeline design.

 

How is SCH (schedule number) represented?

In American standards, pipelines are typically denoted using the format “NPS + SCH,” such as NPS 2" SCH 40, indicating a pipeline with a nominal diameter of 2 inches and wall thickness conforming to the SCH 40 standard.

NPS: Nominal pipe size, measured in inches, which is not the actual outer diameter but an industry-standard dimensional identifier. For example, the actual outer diameter of NPS 2" is approximately 60.3 millimeters.

SCH: Wall thickness grade, where higher numbers indicate thicker walls, resulting in greater pipe strength and pressure resistance.

Using NPS 2" as an example, the wall thicknesses for different SCH numbers are as follows (units: mm):

SCH 10: 2.77 mm
SCH 40: 3.91 mm
SCH 80: 5.54 mm

 
【Important Note】
— SCH is merely a designation, not a direct measurement of wall thickness;
— Pipes with the same SCH designation but different NPS sizes have varying wall thicknesses;
— The higher the SCH rating, the thicker the pipe wall and the higher the applicable pressure rating.