Question:

Why does COMPRESS apply different values for "C" for internal and external pressure cases when designing a bolted cover per UG-34?

Answer:

Design of bolted covers is performed per paragraph UG-34. This paragraph addresses internal pressure only; ie: P is defined as internal pressure. External pressure is not considered by this paragraph and there are no other paragraphs that would do so.

However, it is not unreasonable to want to design a bolted cover for a vessel that also sees vacuum.

For internal pressure, COMPRESS determines factor C as defined in UG-34. For external pressure, COMPRESS applies the constant factor C = 0.33, which is the most conservative value. This was done due to the lack of Code guidance for this case.