Which statement is true regarding relief valve setting in relation to MAWP?

Which statement is true regarding relief valve setting in relation to MAWP?

• A. Relief valve set pressure must be equal to or less than MAWP
• B. Relief valve set pressure should be twice MAWP
• C. Relief valve set pressure is independent of MAWP
• D. Relief valve set pressure must be less than half MAWP

Answer: (A)

The main idea is that safety devices protect against overpressure by opening before the vessel is pressed beyond what it is allowed to handle. MAWP is the maximum pressure the vessel is permitted to operate at a given temperature. The relief valve’s setting is the pressure at which the valve starts to open to relieve pressure. To ensure the vessel never exceeds its allowed limit, this set pressure must not be higher than MAWP; it should be equal to or below MAWP so relief begins before the design limit is reached. In practice, engineers may set it at or just below MAWP to account for measurement tolerances, temperature effects, and the pressure rise that can occur as relief begins to act. The other options would either risk allowing overpressure, be irrelevant to how relief devices protect the vessel, or be unnecessarily conservative.

The main idea is that safety devices protect against overpressure by opening before the vessel is pressed beyond what it is allowed to handle. MAWP is the maximum pressure the vessel is permitted to operate at a given temperature. The relief valve’s setting is the pressure at which the valve starts to open to relieve pressure. To ensure the vessel never exceeds its allowed limit, this set pressure must not be higher than MAWP; it should be equal to or below MAWP so relief begins before the design limit is reached. In practice, engineers may set it at or just below MAWP to account for measurement tolerances, temperature effects, and the pressure rise that can occur as relief begins to act. The other options would either risk allowing overpressure, be irrelevant to how relief devices protect the vessel, or be unnecessarily conservative.