Pressure Vessel Design

Please stop SHOUTING. Other readers here are trying to get some sleep!

Is there not a more experienced individual at the facility that might become a Mentor?

Because the external pressure is often atmospheric, and inside the vessel it is partial vacuum. Of course, that is not always the case, as in, say, submarine design.

"Full Vacuum design pressure means the vessel is rated to be used at any vacuum level that can be achieved under standard atmospheric pressure conditions. It is usually associated with a temperature range as well. Note that this assumes all materials that contact the vessel are compatible with it and do not cause unacceptable levels of physical or chemical corrosion and abrasion."

https://www.quora.com/What-is-the-meaning-of-design-pressure-5-FV

The use of internal/external indicates the direction of the pressure acting on he vessel wall

With a vacuum in the vessel and atmospheric pressure on the outside the direction of the forces on the vessel wall are from the exterior of the vessel towards the interior.

Hence the force on the vessel trying to crush it is external to the vessel.

Internal Design Pressure (normally just called Design Pressure) is the opposite with a high pressure inside the vessel and atmospheric pressure externally the forces trying to destroy the vessel are internal to the vessel acting outwards.

Here's a reason - click the link - then click on 'start' - then jump to 45 second to see the result ...(or from the beginning if you want the story)

Vacuum test