Introduction

In the field of industrial piping, the Sch (Schedule Number) grade is the engineer's “universal code”, which defines the pipe's pressure capacity and safety boundaries with a series of numbers. However, when faced with large seamless stainless steel pipes, the difference in wall thickness between Sch40/40s and Sch80/80s is often confusing - why is the thickness difference more pronounced the larger the pipe diameter? This article will unravel the nature of this issue by using engineering mechanics as an anchor point and combining industry standards with design logic.

 

I. The nature of the Sch grade: not the thickness, but the “pressure formula”

 

Sch does not directly correspond to the absolute value of the pipe wall thickness, but rather to a set of mathematical formulas based on the balance between pipe diameter and pressure. According to ASME B36.19, the value of Sch grade (e.g., 40, 80) is closely related to the design pressure and allowable stress of the pipe.The design formulas：

where P is the design pressure, D is the pipe diameter, S is the allowable material stress, E is the weld coefficient, and A is the corrosion allowance. For large pipe sizes, as the pipe diameter (D) increases exponentially, the wall thickness requirement in Eq. rises nonlinearly, resulting in a significant amplification of the thickness difference between Sch40 and Sch80.

 

 

II. Mechanical Dilemma of Large Size Pipes and Response of Sch Grade

1. Circumferential stress: the larger the pipe diameter, the more “dangerous” the pressure.

When large pipes are used for the transportation of media, the cyclic stresses generated by the internal pressure are directly proportional to the pipe diameter. When the pipe diameter is doubled, the value of the stress at the same pressure is also doubled. sch80 ensures safety by reducing the stress level through a higher wall thickness (t), while sch40 sacrifices part of its pressure capacity to balance the costs.

 

2. Risk of destabilization: the “collapse crisis” of thin-walled pipes

For pipes with a diameter of more than 300 mm, the thin-walled structure of the Sch40 can buckle under vacuum or external pressure due to insufficient stiffness, a risk that is avoided by the thicker design of the Sch80, which effectively increases the moment of inertia of the cross-section.

 

Sch	TK（mm）	Pressure（MPa）
40s	9.53	2.1
80s	17.12	4.8
（based on ASTM A312 316Lstainless steel，temperature≤100℃）

                                                     （data form DN600）

                 

  

III. 40s and 80s: stainless steel “special rules”

The “s” in the Sch suffix (e.g. 40s/80s) is reserved for stainless steel tubing, which is designed to differ slightly from carbon steel tubing (Sch40/80) in ASME B36.19:

 

Wall Thickness Optimization: Stainless steel allows a slight reduction in wall thickness due to its high strength (40s wall thickness ≤ 40 for the same Sch grade).

Corrosion allowance: For the corrosion resistance of stainless steel, the corrosion allowance (A) in the formula can be appropriately reduced to further optimize the cost.

 

IV. Selection Logic: Application Scenarios for Sch40/40s and Sch80/80s

1. Sch40/40s: The choice between economy and light weight

Scenarios: low-pressure water supply, ventilation systems, non-critical process piping.

Advantages: light weight (reduces support structure costs), low material loss (suitable for large-scale procurement).

2. Sch80/80s: synonymous with high pressure and safety

Applicable scenarios: petrochemical high-pressure transportation, steam pipelines, deep-sea oil and gas platforms.

Advantages: strong impact resistance, excellent fatigue resistance (life expectancy under cyclic pressure increased by more than 30%).

 

V. Manufacturing process: the technological threshold behind the thickness difference

The manufacture of large size Sch80/80s seamless pipe needs to break through multiple bottlenecks:

 

Hot rolling accuracy: wall thickness tolerance needs to be controlled within ± 10% to avoid localized weak points.

Solution treatment: Uniform heating requirements for thick-walled stainless steel tubes are extremely high to prevent intergranular corrosion.

Non-destructive testing: ultrasonic flaw detection (UT) needs to cover 100% of the tube body to ensure that no delamination defects.

 

VI. Future Trends: Intelligent Selection and Customized Services

With Industry 4.0, digital selection tools (e.g. wall thickness-pressure simulation software) and customized Sch grades (e.g. Sch60) are becoming an industry trend. Companies can dynamically match customer needs to provide solutions that balance safety and economy.

 

VII. Conclusion: Engineering ingenuity behind thickness differences

The difference in wall thickness between Sch40/40s and Sch80/80s is by no means a simple numbers game, but a product of the deep integration of materials science, mechanical calculations and industrial scenarios. The choice of grade is essentially a search for the optimal solution between safety, cost and efficiency. As a technical service provider in the field of piping, we are committed to using professional data and scenario-based solutions to help you accurately match your needs and master the art of every inch of thickness.

 

Consult our engineers now for customized piping selection solutions!

 

 

                                                                                                         --PS:above Wall thickness data quoted from “ASTM B36.19”.