In relation to operating pressure, design pressure must be what?

• A. Equal to or greater than maximum operating pressure
• B. Equal to maximum operating pressure
• C. Twice the operating pressure
• D. Not related to operating pressure

Answer: (A)

The key idea is that design pressure is the maximum pressure the vessel or piping is built to safely contain. It must be at least as high as the highest pressure the system will experience in operation, including possible pressure surges, so there’s a margin to prevent overpressure in normal service and during transient events. If the design pressure were set only to match the operating pressure without any cushion, a surge or slight measurement error could push the actual pressure above what the structure can safely handle. That’s why the requirement is that design pressure is equal to or greater than the maximum operating pressure. The other not-fitting options would either provide no safety margin, set an arbitrary excess, or ignore the relationship between the operating conditions and the vessel’s strength.