Sch40 vs. Sch40s Pressure Ratings – Calculation Guide

This article provides a detailed guide to calculating pressure ratings for Schedule 40 (carbon steel) and Schedule 40S (stainless steel) pipes. It covers the core ASME B31.3 formula, step-by-step examples, and the critical effect of temperature on allowable stress.

Sch40 vs. Sch40s Pressure Ratings: The Core Difference

The core difference between the pressure ratings of Sch40 and Sch40s lies in the following: For small-diameter pipes (NPS ≤ 8), the pressure ratings are typically the same. For large-diameter pipes (NPS ≥ 10), the rated pressure of Sch40s (stainless steel) is often lower—not primarily due to wall thickness, but because of the material's strength characteristics at different temperatures. This article will break down the calculation behind this difference.

Understanding the Schedule Number

The Schedule (Sch) number does not directly represent a fixed wall thickness. It is a dimensionless number derived from the formula:


Where P is the design pressure (MPa) and σ is the allowable stress of the material at the design temperature (MPa). For the same nominal pipe size (NPS), different schedules correspond to different wall thicknesses, designed to meet specific pressure requirements.

Schedule 40 (carbon steel) conforms to ASME B36.10, with wall thicknesses varying by diameter (e.g., from 2.0 mm for small diameters to over 9 mm for larger ones). Schedule 40S (stainless steel) follows ASME B36.19, where the "S" denotes stainless steel. For small diameters (NPS ≤ 8), the wall thickness is the same for both; for NPS ≥ 10, Schedule 40 is thicker than Schedule 40S.

Core Pipeline Pressure Rating Formula (ASME B31.3)

The pressure rating is calculated using the formula derived from ASME B31.3:


Where:
● P = Maximum allowable internal pressure (psi or MPa)
● S = Allowable stress of the material at the design temperature (psi or MPa) — this is the key variable
● t = Nominal wall thickness (inches or mm)
● D = Outside diameter of the pipe (inches or mm)

△ Critical Note on Allowable Stress (S):
The value of S is not a constant. It must be obtained from material property tables (e.g., ASME B31.3 Appendix A) and depends on:

1. Material Grade: Carbon steel A106 Gr.B and stainless steel 304/316 have different allowable stress values.
2. Design Temperature: As temperature increases, allowable stress decreases — this is known as temperature derating.

Detailed Calculation Examples:

The following examples show how wall thickness and material choice affect the pressure rating.

Example 1: Small Diameter Pipe (NPS 2″) – Same Wall Thickness

For a 2″ NPS pipe:
● Outside Diameter (D): 2.375 inches
● Wall Thickness (t): Both SCH 40 and SCH 40S have a wall thickness of 0.154 inches (3.91 mm).

Step 1: Determine Allowable Stress (S) at Room Temperature
Carbon Steel (e.g., A106 Gr.B): S ≈ 20,000 psi
Stainless Steel (e.g., 304): S ≈ 16,700 psi

Step 2: Apply the Formula
SCH 40 (Carbon Steel): P = (2 × 20,000 × 0.154) / 2.375 ≈ 2,595 psi
SCH 40S (Stainless Steel): P = (2 × 16,700 × 0.154) / 2.375 ≈ 2,166 psi

Conclusion: At room temperature, carbon steel SCH 40 has a higher pressure rating than stainless steel SCH 40S for the same small diameter, because carbon steel has higher allowable stress.

Example 2: Large Diameter Pipe (NPS 10″) – Different Wall Thicknesses

For a 10″ NPS pipe:
● Outside Diameter (D): 10.75 inches

● Wall Thickness (t): Differs by standard:
SCH 40 (B36.10): 0.365 inches (9.27 mm)
SCH 40S (B36.19): 0.318 inches (8.08 mm)

Step 1: Determine Allowable Stress (S) at Room Temperature (same as Example 1)

Step 2: Apply the Formula
SCH 40 (Carbon Steel): P = (2 × 20,000 × 0.365) / 10.75 ≈ 1,358 psi
SCH 40S (Stainless Steel): P = (2 × 16,700 × 0.318) / 10.75 ≈ 988 psi

Conclusion: For large-diameter pipes, carbon steel SCH 40 again shows a higher pressure rating—due to the combined effect of higher allowable stress and thicker wall thickness.

The Critical Effect of Temperature on Pressure Ratings

Pressure ratings are not valid at elevated temperatures. As temperature increases, allowable stress (S) decreases for all materials. The rate at which pressure ratings change with temperature depends on the material type. Carbon steel and stainless steel exhibit different characteristics, which significantly influence material selection for high-temperature applications.

Carbon Steel (A106 Gr.B): Allowable stress drops faster as temperature rises.
Stainless Steel (304): Allowable stress drops more slowly and remains higher at elevated temperatures.

Engineering Implication: 

While carbon steel has a higher pressure rating at room temperature, at temperatures above ~500°F (260°C), stainless steel may equal or surpass carbon steel in pressure rating. This is why stainless steel is often preferred for high-temperature service, despite its lower rating at ambient conditions.

Important Reminder: 

The calculated values are theoretical estimates. Actual pipe design must also consider:

● Corrosion Allowance: Extra wall thickness for material loss over time.
● Manufacturing Tolerances: Actual wall thickness may vary from nominal.
● Safety Factors: Design codes require additional margins.
● Joint Efficiency: For welded pipes, a joint efficiency factor is required.

Summary and Practical Selection Guidance

Understanding pressure rating calculations helps you make better material and schedule decisions. Here are the key takeaways:

1. SCH 40 and SCH 40S are not interchangeable in pressure rating, especially for NPS ≥ 10 where wall thicknesses differ.
2. Material choice matters: Carbon steel gives higher room-temperature ratings; stainless steel excels in corrosion resistance and high-temperature performance.
3. Always account for temperature: Pressure ratings must be de-rated for elevated operating temperatures.
4. Use the correct design code: Always refer to ASME B31.3 (or your applicable code) and material data sheets for accurate values.

▶ For a quick reference on pressure rating values for various NPS sizes, see our Schedule 40 Pressure Rating Data Table.

▶ To compare SCH 40 and SCH 40S across cost, applications, and other dimensions, visit our SCH 40 vs SCH 40S Selection Guide.