Understanding Schedule and Wall Thickness in Stainless Steel Forged Flanges

Difference Between Schedule and Wall Thickness in Stainless Steel Forged Flanges

The difference between schedule and wall thickness in stainless steel forged flanges lies in their definitions, usage, and how they relate to pipe and flange dimensions. Below is a detailed, scientific, and data-supported explanation, including comparisons and tables to clarify these concepts.

1. Definition of Schedule

• Schedule (Sch) is a dimensionless number representing a series of wall thicknesses for pipes or flanges. It is derived from the ratio of the pipe’s design pressure (P) to the allowable stress (σ) of the material at the design temperature, multiplied by 1000:
  Schedule = (P / σ) × 1000
• It is a standardized system introduced by ANSI (American National Standards Institute) to categorize pipes and fittings by their wall thickness relative to their nominal pipe size (NPS).
• Schedule numbers do not represent a specific wall thickness but rather a series of thicknesses that vary with the nominal pipe size. For example, Sch 10, Sch 40, Sch 80, etc., are common schedules, with higher numbers indicating thicker walls.
• For stainless steel pipes, schedules often have an “S” suffix (e.g., 10S, 40S, 80S) to differentiate from carbon steel schedules.
• The schedule ensures that pipes and flanges of the same nominal size but different pressure ratings can be matched correctly.

2. Definition of Wall Thickness

• Wall thickness is the actual physical thickness of the pipe or flange material at a given point, usually measured in millimeters or inches.
• For forged stainless steel flanges, the wall thickness is typically measured at the flange hub, where the flange connects to the pipe. This thickness must match the wall thickness of the corresponding pipe to ensure a secure and proper fit.
• Wall thickness varies depending on the nominal pipe size, schedule, and pressure class of the flange.

Key Differences Between Schedule and Wall Thickness

Aspect	Schedule (Sch)	Wall Thickness
Definition	A dimensionless number representing a series of wall thicknesses based on design pressure and allowable stress	The actual physical thickness of the pipe or flange wall
Purpose	Standardizes pipe/flange thicknesses for different pressure ratings and sizes	Ensures mechanical strength and fit with pipe/flange
Measurement Unit	None (dimensionless)	Millimeters (mm) or inches (in)
Relation to Pipe Size	Schedule number applies to nominal pipe size series, wall thickness varies with NPS and schedule	Direct measurement, varies with schedule and pipe size
Usage	Used to specify pipe/flange thickness series during procurement and design	Used during manufacturing and quality control
Examples	Sch 10S, Sch 40, Sch 80S	2.9 mm wall thickness for Sch 10S at certain NPS

Relationship Between Schedule and Wall Thickness in Stainless Steel Forged Flanges

The schedule number determines the wall thickness for a given nominal pipe size. For example, a 2-inch pipe with Sch 10S will have a thinner wall than the same pipe with Sch 40S.

In forged stainless steel flanges, the flange thickness (especially at the hub) must match the wall thickness of the pipe schedule it is designed to fit. This ensures no mismatch in internal diameters, avoiding flow resistance or pressure drops.

The flange thickness also depends on the pressure class (e.g., 150#, 300#, 600#), with higher classes requiring thicker flanges.

Scientific and Engineering Data Comparison

ANSI B36.10 and B36.19 Pipe Schedule Wall Thickness Data (inches)

NPS (in)	Outside Diameter (OD)	Sch 10S Wall Thickness	Sch 40S Wall Thickness	Sch 80S Wall Thickness
1/2	0.840	0.065	0.083	0.109
1	1.315	0.065	0.133	0.179
2	2.375	0.083	0.154	0.218
4	4.500	0.109	0.237	0.337
6	6.625	0.147	0.280	0.432

Note: “S” suffix indicates stainless steel schedules per ANSI B36.19.

Stainless Steel Flange Thickness vs. Nominal Pipe Size and Schedule

NPS (in)	Flange Thickness (in) for Class 150	Flange Thickness (in) for Class 300	Typical Pipe Wall Thickness (Sch 40S)
1/2	0.44	0.56	0.083
2	0.62	0.81	0.154
6	0.88	1.12	0.280
12	1.12	1.50	0.406

Detailed Explanation of the Concepts

Schedule as a Design Parameter

Schedule is derived from Barlow’s formula, which relates pipe wall thickness to pressure and material strength:

t = (P × D) / (2 × S × E + P × Y)

• t = wall thickness
• P = design pressure
• D = pipe diameter
• S = allowable stress
• E = weld joint efficiency
• Y = coefficient

The schedule simplifies this into a standardized series for easy specification and procurement.

Wall Thickness as a Manufacturing Parameter

• Wall thickness is a direct measurement ensuring the pipe or flange can withstand the specified pressure and mechanical stresses.
• In forged flanges, the thickness at the hub is critical for welding and mechanical strength.
• Mismatched wall thickness between pipe and flange can cause mechanical failure or leaks.

Summary Table: Schedule vs. Wall Thickness in Stainless Steel Forged Flanges

Feature	Schedule (Sch)	Wall Thickness
Nature	Dimensionless rating number	Physical measurement
Basis	Calculated from design pressure and material allowable stress	Actual thickness of flange or pipe wall
Application	Defines series of wall thicknesses for pipes and flanges	Ensures mechanical integrity and fit
Relation to Nominal Size	Same NPS can have multiple schedules	Thickness varies with schedule and NPS
Standards	ANSI B36.10 (carbon steel), ANSI B36.19 (stainless steel)	Measured as per ASME B16.5 for flanges
Importance in Flanges	Flange schedule must match pipe schedule for proper fit	Thickness must match pipe thickness at hub

Conclusion

Schedule is a standardized classification system representing a series of wall thicknesses for pipes and flanges, based on design pressure and material strength.

Wall thickness is the actual physical thickness of the pipe or flange, critical for mechanical strength and proper fit.

In stainless steel forged flanges, the flange wall thickness (especially at the hub) must match the pipe schedule wall thickness to ensure compatibility, structural integrity, and avoid flow disturbances.

Understanding both concepts and their relationship is essential for selecting the right flange and pipe combination for high-pressure and corrosion-resistant applications.

If you want, I can provide detailed tables of pipe sizes, schedules, and corresponding flange thicknesses for specific stainless steel grades or pressure classes. Would you like that?