ASME B31.3 Steel Pipe Pressure and Wall Thickness Calculation

Relationship between Steel Pipe Wall Thickness and Pressure: 

The wall thickness of a steel pipe is directly related to its pressure-bearing capacity. Generally, the greater the wall thickness, the greater the pressure the pipe can withstand. However, this relationship is not a simple direct proportion; it requires precise calculation using standard formulas. This article will introduce the most commonly used wall thickness calculation formulas in the American and Chinese standard systems, as well as the key parameters to consider during the calculation process.

American Standard System: ASME B31.3 Wall Thickness Calculation Formulas

In the American standard system, ASME B31.3, "Process Piping Specification," is the most widely used piping design standard. The formula for calculating the wall thickness of steel pipes is as follows:

Minimum wall thickness required for straight pipes:

Where:
t = Minimum required wall thickness (unit: inches or millimeters)
P = Design pressure (unit: psi or MPa)
D = Outer diameter of steel pipe (unit: inches or millimeters)
S = Allowable stress of material at the calculation temperature (unit: psi or MPa)
E = Weld coefficient (1.0 for seamless pipes, 0.85 for welded pipes, or according to standard values)
Y = Temperature coefficient (0.4 for carbon steel below 482℃)

Simple empirical formula (quick estimation in low-pressure applications):

For low-pressure, normal-temperature conditions, a simplified formula is commonly used in engineering for quick estimation:
This formula essentially takes E = 1.0 and ignores P × Y in the denominator. The approximate simplification of the terms is applicable to preliminary estimations when the design pressure is low, the wall thickness is thin, and the weld coefficient is 1.0.

Chinese Standard System: GB/T 20801 Wall Thickness Calculation Formula

The calculation formula in the national standard system GB/T 20801 "Pressure Piping Specification - Industrial Piping" is basically consistent with ASME B31.3:

Straight pipe wall thickness calculation:

Where:
[σ]t = Allowable stress of the material at the design temperature (unit: MPa)
φ = Weld coefficient (1.0 for seamless pipes)
Y = Temperature correction factor (0.4 for carbon steel)

Key Parameters in the Calculation Process:

1. Selection of Allowable Stress 

Allowable stress is the maximum stress value that a material can withstand under safe operating conditions, and it is the most crucial parameter in wall thickness calculation. Allowable stress is obtained by dividing the material's tensile strength and yield strength by the corresponding safety factor. Specific values can be found in Appendix A of ASME B31.3 or the allowable stress table in GB/T 20801.

Allowable stress is directly related to temperature. As temperature increases, the allowable stress of a material decreases. For example, the allowable stress of A106 Gr.B carbon steel pipe is approximately 138 MPa at room temperature, but drops to approximately 70 MPa at 400°C. Therefore, under high-temperature conditions, even with constant pressure, the required wall thickness will increase significantly.

2. Selection of Weld Coefficient
● Seamless Steel Pipe: Since there is no weld seam, there is no potential weak point; the weld coefficient is taken as 1.0.
● Electric Resistance Welded Pipe (ERW): The weld coefficient is usually taken as 0.85.
● Straight Seam Submerged Arc Welded Pipe (LSAW): Requires non-destructive testing according to standards; the weld coefficient is taken as 0.85-1.0.

3. Meaning of Safety Factor
In the formula, the safety factor is actually implicitly included in the value of the allowable stress (S).

Taking ASME B31.3 as an example: 

The allowable stress is taken as the smaller of 1/3 of the tensile strength (approximately 0.333 times) and 2/3 of the yield strength (approximately 0.67 times);

This is equivalent to reserving approximately 3 times the safety margin based on the material's ultimate strength.

Therefore, it is not necessary to multiply by an additional safety factor in the wall thickness formula. The "safety factor" mentioned in the original text is already reflected in the allowable stress value under both American and Chinese standards, and does not require a separate F coefficient in the formula.

4. Corrosion Allowance
In addition to the pressure-bearing wall thickness calculated by the formula, an additional corrosion allowance is needed to compensate for wall thickness reduction caused by media corrosion during the pipeline's service life. Generally, 1.5mm is used; for corrosive media, 2.0mm-3.0mm is used; and for non-corrosive media, 0mm can be used.

Differences in Application and Calculation for Different Steel Pipe Types:

1. Classification by Material

● Carbon steel pipe (A106 Gr.B, 20#): Suitable for -29℃ to 425℃, allowable stress decreases significantly with increasing temperature.
● Alloy steel pipe (A335 P11, 15CrMo): Suitable for 425℃-550℃, high-temperature strength is superior to carbon steel.
● Stainless steel pipe (304, 316): Suitable for -196℃ to 750℃, no low-temperature brittleness, higher allowable stress.
● Low-temperature carbon steel pipe (A333 Gr.6): Suitable for -46℃ to 343℃, excellent low-temperature toughness.

2. Classification by Application

● Oil/gas pipelines: Usually calculated according to ASME B31.4 or B31.8, allowing a design factor of 0.72 times the yield strength.
● Chemical/process pipelines: Calculated according to ASME B31.3, with a higher safety factor.
● Power plant boiler tubes: According to ASME B31.1. Calculations, High-Temperature Creep is a Key Consideration
● Water Supply Pipelines: According to standards such as AWWA C200, wall thickness design mainly considers internal pressure and soil load.

For Example:

A water supply pipeline at ambient temperature has a design pressure P = 1.6MPa and uses 20# seamless steel pipe with an outer diameter D = 219mm. Calculate the minimum required wall thickness.

Given: Allowable stress of 20# steel at ambient temperature S = 130MPa, weld coefficient of seamless pipe φ = 1.0, Y = 0.4.

Calculation:
t = (1.6 × 219) / (2 × 130 × 1.0 + 2 × 1.6 × 0.4)
= 350.4 / (260 + 1.28)
= 350.4 / 261.28
≈ 1.34mm

Adding a corrosion allowance of 1.5mm, the design wall thickness = 1.34 + 1.5 = 2.84mm

Rounding up to the standard wall thickness grade, Sch20 or Sch40 wall thickness series steel pipes can be selected (actual wall thickness approximately 6.35mm).

Conclusion: 

The calculation of steel pipe wall thickness is not simply "the greater the pressure, the greater the wall thickness," but rather a comprehensive consideration of multiple factors such as pressure, pipe diameter, allowable stress, weld coefficient, temperature correction, and corrosion allowance through a standard formula. Whether it's the American standard or the Chinese standard, the core formula is consistent. In practical engineering projects, it is recommended that professional engineers perform calculations according to relevant standards, taking into account economic efficiency, construction feasibility, and long-term operational safety.

Read more: Seamless Steel Pipe Sizes and Weights