Design pressure must be greater than or equal to which value?

• A. Greater than or equal to maximum operating pressure
• B. Less than maximum operating pressure
• C. Equal to minimum operating pressure
• D. Greater than minimum operating pressure

Answer: (A)

Design pressure is the pressure the vessel is built to contain safely, so it must be at least as high as the highest pressure the vessel will experience in service. That highest pressure is the maximum operating pressure (often called the maximum allowable working pressure, MAWP). By sizing the design pressure to be equal to or greater than the MAWP, you ensure the vessel remains within its safe strength limits under worst-case conditions. If the design pressure were lower than the maximum operating pressure, the vessel could be overstressed during operation, leading to failure. The minimum operating pressure isn’t the controlling factor for strength or safety, since the concern is with handling peak loads, not the low end of the pressure range.