Sch40 vs. Sch40s Pressure Ratings

Sch40 vs. Sch40s Pressure Ratings:

The core difference between the pressure ratings of Sch40 and Sch40s lies in the following: For small-diameter pipes, their pressure ratings are usually the same; for large-diameter pipes, the rated pressure of Sch40s (stainless steel) may be lower, but this is not due to its wall thickness, but rather to the material's strength characteristics at different temperatures.

Understanding the Sch Wall Thickness Series: 

First, we need to understand the Sch wall thickness series. Sch, the pipe designation number, does not directly represent the wall thickness, but rather a numerical value indicating a series of wall thicknesses. This value is calculated based on the ratio of the design pressure to the allowable stress of the material at the design temperature. Specifically, Sch = (P/σ) × 1000, where P is the design pressure (MPa), and σ is the allowable stress of the material at the design temperature (MPa). This means that for the same pipe diameter, the wall thickness will differ across different pipe designations.

Sch40 Pressure Rating and Wall Thickness: 

Sch40 represents a specific pressure rating and corresponding wall thickness. It is typically made of carbon steel material and conforms to ASME B36.10. Generally, Sch40 corresponds to a minimum working pressure of ≥40 MPa, with a wall thickness range of 2.0–4.0 mm. This standard primarily focuses on the safety performance of the pipe under design pressure and temperature. In practical engineering, we need to select the appropriate pressure rating and wall thickness based on specific circumstances to ensure the safe operation of pressure pipelines.

Sch40s Pressure Rating: 

Sch40s is commonly used for stainless steel pipes, where "s" represents stainless steel, conforming to ASME B36.19. Although the pipe designation for Sch40s is the same as Sch40 (40), the difference in material properties between stainless steel and carbon steel means that, at the same pressure rating, the wall thickness of stainless steel may be less than that of carbon steel. Therefore, we need to pay special attention to these material differences when selecting and using these pipes.

When NPS ≤ 8", both have the same wall thickness. When NPS > 8", Sch40s has a thinner wall.

High Temperature Resistance:
Sch40 (using carbon steel A106 B as an example) experiences a faster decrease in allowable stress with increasing temperature.
Sch40s (using stainless steel 304 as an example) experiences a slower decrease in allowable stress with increasing temperature, and maintains this better at specific temperatures.

Impact on Pressure Rating: At room temperature, carbon steel has higher pressure resistance; at high temperatures (e.g., >500°F/260°C), stainless steel may surpass it.

Applications and Selection: 

In industrial applications, whether using Sch40 or Sch40s, the appropriate wall thickness and pressure rating must be selected based on factors such as actual operating pressure, temperature, and fluid properties. Excessively thin walls may cause deformation or rupture of the pipeline under high pressure, while excessively thick walls may increase manufacturing costs and installation difficulty. Therefore, proper selection and matching are crucial to ensuring the safe and efficient operation of the piping system.

Calculation of Pipeline Pressure Rating:

The pressure rating of a pipeline is not a fixed value, but is determined by the following formula and standard:
P = (2 × S × t) / D
Where, S: Allowable stress of the material (this is a key variable, from ASME standard); t: Wall thickness; D: Outer diameter.

Example of pressure rating comparison:

1. Small diameter pipes (e.g., NPS 2")

Same wall thickness: For a 2" pipe, the wall thickness of both Sch40 and Sch40s is 3.91mm.
At room temperature: Due to the higher allowable stress (S) of carbon steel (Sch40), its calculated pressure rating will be higher than that of stainless steel (Sch40s).

Example calculation formula:
Sch40 carbon steel: P = (2 × 20,000 psi × 0.154 in) / 2.375 in ≈ 2,595 psi
Sch40s stainless steel: P = (2 × 16,700 psi × 0.154 in) / 2.375 in ≈ 2,166 psi

In this case, the pressure rating of Sch40 is significantly higher than that of Sch40s.

2. Large-diameter pipes (e.g., NPS 10)

Different wall thicknesses:
Sch40 (B36.10): Wall thickness 0.365 in (9.27 mm)
Sch40s (B36.19): Wall thickness 0.318 in (8.08 mm)

Example calculation formula:
Sch40 carbon steel: P = (2 × 20,000 psi × 0.365 in) / 10.75 in ≈ 1,358 psi
Sch40s stainless steel: P = (2 × 16,700 psi × 0.318 in) / 10.75 in ≈ 988 psi

Due to the lower allowable stress and thinner wall thickness of stainless steel, the pressure rating of Sch40 is significantly higher in this case. Sch40s.

Conclusion: 

Sch40 and Sch40s are two different wall thickness designations, representing the wall thickness standards for carbon steel pipes and stainless steel pipes at certain pressure ratings, respectively. Understanding and correctly applying these standards is crucial for ensuring the safe operation of pipeline systems and improving industrial production efficiency. In practical applications, we need to make reasonable selections and combinations based on specific circumstances and requirements.

Read more: Schedule 40 vs Schedule 80 Steel Pipe