Relief valve set pressure must be equal to or less than which parameter?

• A. Greater than MAWP
• B. Irrelevant to MAWP
• C. Equal to MAWP plus 10 psi
• D. Equal to or less than MAWP

Answer: (D)

Relief devices are there to prevent overpressure beyond the vessel’s design limits. The relief valve’s set pressure—the pressure at which it starts to lift—must be at or below the maximum allowable working pressure (MAWP) of the vessel. If the set pressure were higher than MAWP, the system could reach or exceed the vessel’s design limit before relief occurs, risking failure. MAWP defines the allowable pressure the vessel can operate at for the specified temperature, so tying the relief set point to MAWP ensures protection. In practice, setting the valve at or just below MAWP provides a margin for measurement tolerances and pressure losses in the relief line, but never above MAWP.