How is allowable external pressure for thin-wall pipe subjected to vacuum determined?

• A. Using internal pressure charts
• B. Using external pressure design charts
• C. Wall-thickness alone governs
• D. Operator preference determines

Answer: (B)

When a thin-wall pipe is subjected to vacuum, the governing failure mode is buckling under external pressure, not the material’s strength under tension. That’s why allowable external pressure is determined using external pressure design charts. These charts incorporate how geometry (outside diameter, wall thickness, length) and boundary conditions (end closures, support) interact with material properties to set a safe limit for the external pressure the shell can withstand before instability occurs. The actual vacuum pressure is compared to the chart’s allowable external pressure to ensure safety. Relying on wall thickness alone ignores how diameter, length, and ends influence buckling resistance, and internal-pressure charts apply to different loading scenarios. Operator preference has no role in this design; the charts provide the code-approved basis for the allowable external pressure.