Determining the correct wall thickness for steel pipe is one of the most fundamental engineering tasks in piping design. An undersized wall risks catastrophic failure under pressure, while an oversized wall wastes material and drives up costs. The challenge lies in balancing safety, economy, and code compliance.

CoreMetal Steel supplies steel pipe in all wall thicknesses from schedule 5 through XXS, with precision manufacturing to exact specifications.

Fundamental Pressure-Thickness Relationships

Barlow’s Formula

The most basic wall thickness calculation: t = P x D / (2 x S), where t is wall thickness, P is internal design pressure, D is outside diameter, and S is allowable stress. This assumes thin-wall theory (D/t greater than 20) and provides a conservative estimate for preliminary sizing.

ASME B31.3 Process Piping Method

Pressure Design Thickness

The governing equation: t = P x D / (2 x (S x E + P x Y))

Where E is the quality factor (1.0 for seamless, 0.85 for ERW) and Y is the coefficient from Table 304.1.1 (0.4 for ferritic steel below 482C).

Minimum Required vs. Ordered Thickness

The actual ordered thickness must account for corrosion allowance (typically 1.5-3.0 mm), mill tolerance (12.5% for seamless pipe per ASTM A106), and thread depth for threaded connections.

Allowable Stress Values (Common Materials)

• A106 Grade B (seamless carbon steel): 138 MPa at ambient
• A312 TP304 (seamless austenitic SS): 138 MPa at ambient
• A312 TP316 (seamless austenitic SS): 138 MPa at ambient
• A335 P91 (9Cr-1Mo-V): 165 MPa at ambient

External Pressure Design

For vacuum service or external pressure applications, wall thickness is governed by buckling. ASME Section VIII Division 1 method uses material charts to calculate MAWP for external pressure.

Combined Loading Considerations

• Internal pressure plus weight creates combined hoop and longitudinal bending stress
• Thermal stress adds to pressure stress and must not exceed displacement stress range
• Equipment nozzle loads, wind, and seismic forces add additional stress components

Practical Calculation Example

Design: 5.0 MPa internal pressure, 300C, A106 Grade B, 3.0 mm corrosion allowance, DN 200 (8-inch pipe).

At 300C: S = 117 MPa, E = 1.0, Y = 0.4, D = 219.1 mm.

Pressure design thickness: t = 5.0 x 219.1 / (2 x (117 x 1.0 + 5.0 x 0.4)) = 4.60 mm

Minimum with corrosion: 4.60 + 3.0 = 7.60 mm

Ordered with 12.5% tolerance: 7.60 / 0.875 = 8.69 mm

Schedule 40 (8.18 mm) insufficient. Schedule 60 (10.31 mm) adequate.

Mill Tolerance Standards

• ASTM A106 (seamless): Plus/minus 12.5%
• ASTM A312 (welded SS): Plus/minus 10% or 0.3 mm
• API 5L (line pipe): -12.5% only
• EN 10216 (European seamless): Plus/minus 10% or 0.5 mm

Why Choose CoreMetal Steel

CoreMetal Steel supplies full range of schedules from 5S through XXS, tight tolerance manufacturing, carbon/alloy/stainless steel grades, complete material test reports with actual wall thickness measurements, and custom wall thickness for special applications.

Contact CoreMetal Steel today for pipe wall thickness consultation and supply.