Carbon steel pipe pressure resistance

The pressure resistance of carbon steel pipe (CS pipe) is not a single fixed value; instead, it depends on several key factors, including material grade, wall thickness, outer diameter, manufacturing method, and operating temperature. Below is a systematic overview of the main considerations.

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Key Factors Influencing Pressure Resistance of Carbon Steel Pipe

- Material Grade

Ordinary carbon steel (e.g., Q235) has a tensile strength of around 375–500 MPa.

High-strength pipeline steels (e.g., API 5L X60) can exceed 517 MPa.

In general, higher material strength results in greater pressure resistance.

- Wall Thickness and Outer Diameter

Barlow’s Formula:

P=2⋅S⋅t/D

where P = pressure, S = tensile strength, t = wall thickness, D = outside diameter.

Example: A DN100 SCH40 pipe (6 mm wall) withstands ~5.6 MPa, while SCH80 (8 mm wall) increases to ~7.4 MPa.

- Manufacturing Process

Seamless pipe (ASTM A106) generally withstands higher pressures than welded pipe (ASTM A53) due to the absence of weld seams.

- Operating Temperature

Strength decreases with rising temperature.

For ASTM A106 Grade B: allowable stress is ~138 MPa at room temperature but drops to ~100 MPa at 400°C.

Pressure Rating Methods of Carbon Steel Pipe

- Nominal Pressure (PN)

A standardized classification used for flanges, valves, and fittings.

Examples: PN6, PN10, PN16, PN25, PN40, PN63, PN100.

PN16 indicates a max working pressure of 16 bar at 20°C, with reduced values at higher temperatures.

Standards:

ANSI/ASME (B16.5): Classes 150, 300, 600, 900, 1500, 2500.

EN 1092-1: PN2.5 up to PN250.

JIS: 5K, 10K, 16K, 20K, 30K, 40K.

- Wall Thickness Series (Schedule, Sch)

A common way to indicate pipe strength.

The higher the schedule number, the thicker the wall and the greater the pressure capacity.

Typical ranges:

Sch5S / Sch10S: Thin-wall

Sch40 (STD): Standard weight, widely used

Sch80 (XS): Extra strong

Sch120 / Sch160 / XXS: Heavy-duty, high-pressure service

Standard Specifications and Reference Values of Carbon Steel Pipe

ASTM A106 Seamless Pipe: Grade B can handle ~15.8 MPa at RT (DN150, 12 mm wall).

API 5L Line Pipe (X42): Typically designed for 10–12 MPa (SF = 0.72).

GB/T 8163 #20 Steel Pipe: At 20°C, 6.4 MPa for DN80, 5 mm wall.

Application Standards and Safety Factors of Carbon Steel Pipe

ASME B31.1 – Power piping (boilers, power plants).

ASME B31.3 – Process piping (oil, gas, chemicals).

Safety factors typically range 1.5–4× design pressure, with higher values for hazardous fluids.

High temperatures (>200°C) reduce pressure capacity by 20–30%.

Corrosive media (e.g., H₂S, CO₂) may require sulfur-resistant steel or a pressure reduction of at least 30%.

Practical Selection Guidelines of Carbon Steel Pipe

Define Design Conditions: Pressure and temperature.

Choose Material: Match grade to fluid properties.

Determine Pipe Size: Based on flow requirements.

Select Wall Thickness:

Calculate using Barlow’s formula.

Cross-check with ASME B36.10 tables (e.g., Sch40, Sch80).

Verify Ratings: Ensure chosen pipe exceeds design requirements with a proper safety margin.

In short: The pressure resistance of carbon steel pipe is determined by material, dimensions, manufacturing process, and service conditions. Engineers must evaluate these factors together, using standards and safety codes, to ensure reliable and safe pipeline operation.