SCH 40 vs SCH 40S Pipes

SCH 40 vs SCH 40S Pipes:

The core difference between SCH 40 and SCH 40S lies in their respective standard systems and applicable pipe materials:

SCH 40 Pipe follows the ASME B16.10M standard and is a general wall thickness series suitable for carbon steel and alloy steel pipes. SCH 40 is designed based on the mechanical properties of carbon steel and is commonly used in general industrial piping systems.

SCH 40S Pipe follows the ASME B16.19M standard and is a wall thickness series specifically designed for stainless steel pipes, where "S" stands for "stainless." SCH 40S is mainly used in applications requiring high corrosion resistance and strength. It is typically made of 304 or 316 stainless steel and has good toughness and weldability.

Detailed Comparison of SCH 40 and SCH 40S:

1. Different Origins and Standards

SCH 40 and SCH 40S follow different international standards.

SCH 40 follows the ASME B16.10M standard, which specifies a general wall thickness series for carbon steel and alloy steel pipes. ASME B36.10 defines the outer diameter. Regardless of whether the pipe is SCH 40, SCH 80, or SCH 160, as long as the nominal size (NPS) is the same, its outer diameter remains constant.

SCH 40S follows ASME B16.19M, a standard specifically designed for stainless steel pipes. Its wall thickness calculations take into account the higher allowable stress values of stainless steel. B36.19 defines specific wall thicknesses without redefining the outer diameter; it fully adopts the outer diameter system of B36.10. Its primary function is to specify the wall thickness series specific to stainless steel pipes, i.e., Schedule numbers with the suffix "S".

2. Differences in Wall Thickness

For most common small-diameter steel pipes (e.g., from 1/8 inch to 8 inches), the wall thickness of SCH 40 and Sch40s is exactly the same. This is why they are often used interchangeably.

However, for large-diameter pipes (typically NPS ≥ 10 inches), the wall thickness in the B36.19 standard is thinner than that in B36.10. Because stainless steel has different mechanical properties and corrosion resistance than carbon steel, the B36.19 standard optimizes the wall thickness of large-diameter stainless steel pipes, making them typically thinner than B36.10 Sch40 carbon steel pipes of the same size.

This is because stainless steel has a higher allowable stress (the maximum stress it can withstand) than carbon steel. Therefore, stainless steel pipes can use thinner walls to save materials and reduce costs while meeting the same pressure requirements. At the same time, the reduced weight of the steel pipe facilitates installation and support. Furthermore, corrosion resistance is a primary function of stainless steel, and in many operating conditions, the thicker walls required to resist corrosion, as seen in carbon steel, are not necessary.

3. Cost Difference

The initial cost of stainless steel pipe (SCH40S) is significantly higher than that of carbon steel pipe (SCH40), typically by 3-5 times or even more. However, in corrosive environments, carbon steel pipes require frequent maintenance and replacement, potentially resulting in higher total lifecycle costs.

4. Application Scope

The choice between SCH 40S and SCH 40 depends primarily on the application environment. For fields requiring high hygiene and corrosion resistance, such as food processing, chemical engineering, and marine engineering, SCH 40S is the ideal choice; while for general plumbing, building structures, non-corrosive or weakly corrosive environments, and low-pressure scenarios, SCH 40 carbon steel pipes are sufficient. Budget is also a crucial factor; while SCH 40S offers superior performance, it is more expensive.

Typical Applications:

SCH 40:
Low-pressure fluid transport: Cold/hot water supply in buildings, fire sprinkler systems, drainage systems, low-pressure compressed air piping.
HVAC: Ventilation ducts, air conditioning condensate drain pipes.
Structural applications: Scaffolding, guardrails, equipment supports.
Municipal engineering: Low-pressure water supply.

SCH 40S: 

Corrosive environments: Process piping for conveying chemical media, acids, alkalis, and salt solutions.

High cleanliness requirements: Process piping in the food, beverage, pharmaceutical, and biotechnology industries.
High hygiene requirements: Piping for drinking water systems, hospitals, and laboratories.
Appearance and durability: Outdoor or building interior piping requiring aesthetic appeal or long-term durability.

Meaning of the suffix "S":

The suffix "S" explicitly indicates that the piping should be manufactured and supplied according to the stainless steel pipe standard B36.19. It is an important identifier in orders and design documents, ensuring that the supplier provides piping that conforms to standard stainless steel dimensions.

Why has the B36.19 standard?

Due to the economics and material properties of stainless steel pipes, a separate standard was created for them:

1. Cost considerations: Stainless steel is much more expensive than carbon steel. B36.19 significantly saves material costs by providing thinner wall thickness options for large-diameter pipes while still meeting pressure requirements.
2. Corrosion Resistance is a Primary Function: The core value of stainless steel lies in its corrosion resistance. In many applications, pipe failure is caused by corrosion, not pressure. Therefore, it doesn't require the excessive wall thickness "corrosion allowance" required for corrosion resistance, as is the case with carbon steel.
3. Strength Characteristics: Stainless steel has different mechanical properties than carbon steel, and the wall thickness series of B36.19 is optimized based on the strength characteristics of stainless steel.

Are SCH 40S and SCH 40 pipes interchangeable?

Yes, but not recommended. For large-diameter pipes, the wall thicknesses specified in ASME B36.10 (SCH40) and ASME B36.19 (SCH40S) standards are different. If you need a 10-inch stainless steel pipe but mistakenly order "SCH40 stainless steel pipe," you might receive a pipe with a wall thickness of 9.27mm instead of the 8.08mm you intended.

This will result in material waste and increased costs, paying more for an unnecessary, thicker pipe wall. Furthermore, thicker walls mean different weights, pressure-bearing capacities, and stress characteristics, which may affect support design and fluid performance.