Pressure vessels are designed for positive internal pressure (bulging outward). Vacuum vessels (or full vacuum rating) are designed for external pressure (collapse inward). A vessel designed for atmospheric pressure cannot handle a vacuum because the external atmospheric pressure creates a buckling stress (hoop compression) that is very different from tensile hoop stress.
What are the primary loads considered under ASME Section VIII, Div. 1 (UG-22)? static equipment interview questions
If it fails Level 1, perform more detailed stress calculations or Finite Element Analysis (FEA) to determine a safe derating of pressure (MAWP) or schedule a repair/patch plate. Pressure vessels are designed for positive internal pressure