Pressure Relief Valve Sizing and Set Pressure Determination

Pressure relief valves (PRVs) are critical safety devices that protect process equipment, vessels, and piping from overpressure. Proper PRV sizing ensures the device can relieve the required flow at the set pressure without exceeding the maximum allowable accumulated pressure. Incorrect PRV sizing—either undersized (insufficient relief capacity) or oversized (chattering and seat damage)—compromises the protection function and may cause equipment damage or safety incidents.

ASME Section VIII Requirements

ASME Boiler and Pressure Vessel Code Section VIII Division 1 governs PRV requirements for unfired pressure vessels. The code requires that every pressure vessel have at least one pressure relief device set at or below the maximum allowable working pressure (MAWP). For fire case scenarios, the total PRV capacity must prevent pressure from exceeding 121% of MAWP. For non-fire scenarios, accumulated pressure must not exceed 110% of MAWP (with a single PRV) or 116% (with multiple PRVs). API 520 and API 521 provide supplemental guidance on PRV sizing and selection for petroleum and chemical process plants.

• Set pressure: PRV opening pressure, not to exceed MAWP

• Accumulation: pressure increase above MAWP during relieving, 10% max (non-fire)

• Overpressure: difference between relieving pressure and set pressure

• Blowdown: pressure drop from set pressure to PRV reseating pressure

• Back pressure: pressure at PRV outlet, affects conventional PRV capacity

Sizing for Gas and Vapor Service

PRV sizing for compressible gas or vapor service uses the ideal gas equation modified for real gas behavior (using compressibility factor Z), molecular weight, and specific heat ratio (k). The required orifice area is calculated to pass the design flow at relieving conditions (set pressure plus overpressure allowance). Standard orifice designations (D through T) per API 526 define available PRV sizes. The selected PRV must have a certified capacity equal to or greater than the required relieving capacity under the governing overpressure scenario.

Back Pressure Considerations

Back pressure at the PRV outlet reduces the effective differential pressure and can affect capacity and set pressure. Conventional spring-loaded PRVs are affected by built-up back pressure from the relief header system and superimposed back pressure from the flare or vent header. When back pressure exceeds 10% of set pressure, a balanced-bellows PRV (which compensates for back pressure effects on the spring) or pilot-operated PRV should be specified. Pilot-operated PRVs maintain their set pressure regardless of back pressure, making them suitable for high back pressure applications.