Carbon steel seamless pipe pressure rating

The pressure rating of carbon steel (CS) seamless pipe is not a fixed value, but is determined by three key factors: material, outside diameter, and wall thickness. In simple terms, for a given material and pipe size, the wall thickness directly defines the pressure it can withstand.

1. Pressure Rating and Pipe Schedules

In industries such as petroleum, chemical, and power, pressure ratings are typically indicated by a Schedule (Sch) number, which represents a series of wall thickness values. Common examples include:

Sch10S / Sch10–Thin wall

Sch40 / STD–Standard wall (widely used for general pressure piping)

Sch80 / XS–Extra strong wall (for higher pressure service)

Sch160–Extra thick wall (for high-pressure applications)

Note: For the same Schedule (e.g., Sch40), the actual wall thickness changes with nominal diameter (DN). For example, DN50 Sch40 and DN200 Sch40 pipes have different absolute thicknesses, but both belong to the same strength series.

2. Standards and Materials

Pressure ratings and pipe dimensions are governed by international and national standards:

ASME/ANSI B36.10M–Seamless and welded steel pipes (U.S. standard, widely adopted)

GB/T 8163–Seamless pipes for fluid transport (China, medium & low pressure)

GB/T 5310–Seamless pipes for high-pressure boilers (China, high temp & pressure)

Common carbon steel grades include:

ASTM A106 Gr. B–High-temperature seamless carbon steel pipe, widely used

ASTM A53 Gr. B–Seamless/welded pipe, general use, slightly lower performance

Chinese #20 steel–Equivalent to ASTM A106 Gr. B, commonly used under GB/T 8163

3. How Pressure Rating is Calculated

The maximum allowable working pressure can be estimated using the internal pressure formula from ASME B31.3 or ISO 15649:

P=2×S×t×E/D

Where:

P = Maximum allowable pressure (MPa or psi)

S = Allowable stress of the material (e.g., A106 Gr.B≈138 MPa at room temperature)

t = Minimum wall thickness (mm/in)

D = Outside diameter (mm/in)

E = Quality factor (1.0 for seamless pipe)

4. Pipe Selection Process

Define design parameters–Required pressure and temperature.

Choose material–Based on fluid type, temperature, and pressure (accounting for stress reduction at high temperature).

Determine diameter–Calculate DN and OD according to flow rate and velocity.

Wall thickness selection–Calculation method: Use the formula above to find minimum required thickness.

Schedule table method: Compare with ASME B36.10M thickness tables and select a Schedule equal to or greater than the calculated requirement.

Add allowances–Include corrosion allowance (CA) and safety factors for long-term reliability.

5. Key Takeaway

The pressure rating of carbon steel seamless pipe depends on its material, diameter, and wall thickness. Proper selection requires balancing safety, efficiency, and cost:

Too low→Risk of leakage or rupture.

Too high→Unnecessary cost and maintenance burden.

By following design standards and considering real operating conditions, engineers can select the most suitable seamless pipe to ensure safe and reliable operation.