External P

Module from the Pressure Vessel Engineering Curriculum curriculum.

Module 5 - External Pressure and Stiffeners · Learning Objectives · 1. Apply UG-28 external pressure design procedure using Section II Part D charts · 2. Determine L/Do and Do/t to find Factor A from Figure G · 3. Use material Chart (CS Chart CS-2) to find Factor B, then Pa · 4. Verify Pa >= P_external (full vacuum = 14.7 psi) · 5. Add stiffening rings (UG-29) when cylinder too long for single span · UG-28 Design Procedure · Step 1: Assume t. Step 2: Compute L/Do and Do/t. Step 3: From Figure G, read Factor A. Step 4: From material chart (e.g., CS-2), enter A, read B. Step 5: Compute Pa = 4B / (3 * Do/t). If Pa < P_ext, increase t or add stiffeners. Typical vacuum service: Pa = 15 psi minimum. For shells longer than Lcr = 1.11*Do*(Do/t)^0.5, add intermediate stiffeners per UG-29. · Typical External Pressure Design Cases

Vessel	Do (in)	L (ft)	t (in)	Stiffeners	Notes
Vacuum tower	180	120	1.25	3 rings	Common refinery
Steam-out drum	60	20	0.375	None	L/Do < 1.11
Condenser shell	84	40	0.5	None	Under vacuum end
Nitrogen purge drum	48	12	0.25	None	Short enough
Cold box	72	30	0.625	2 rings	Stainless

Source: Pressure_Vessel_Engineering_Curriculum_v1.xlsx · Sheet: External P

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