In a pressure relief valve nozzle, critical (choked) flow occurs when the downstream backpressure is less than or equal to the critical flow pressure: P_b / P_1 ≤ [2 / (k + 1)]^[k / (k - 1)]. In this regime, gas velocity reaches the speed of sound (Mach 1.0) at the nozzle throat, making mass discharge rate completely independent of downstream backpressure. If backpressure rises above this critical ratio (typically ~50% to 55% of absolute relieving pressure), flow becomes subcritical (subsonic), decreasing mass discharge and requiring a subcritical flow correction factor.

API 520 Part II and ASME Section VIII strictly mandate that the total frictional and fitting pressure drop in the piping between the protected equipment and the PRV inlet nozzle must not exceed 3% of the valve set pressure (gauge). When a PRV pops open, high flow instantaneously induces inlet friction. If the inlet line loss exceeds 3% (which corresponds to the valve closing blowdown differential), the pressure at the valve seat drops below reseat pressure, causing the valve to slam shut. Pressure then rebounds, reopening the valve in a destructive cycle called "chattering" (hundreds of cycles per minute) that destroys seating surfaces and snaps pipe welds.

API Standard 526 establishes standardized nominal effective discharge areas for spring-loaded and pilot-operated relief valves, designated by letters from D (smallest: 0.110 in² / 71 mm²) to T (largest: 26.000 in² / 16,774 mm²). Valve sizing calculations compute the theoretical minimum required orifice area (A_req). The engineer then selects the next larger standard API letter orifice having an area A_API ≥ A_req. The valve manufacturer builds the selected valve with the standardized flange ratings, face-to-face dimensions, and certified nozzle throat.

Conventional spring-loaded PRVs are directly affected by downstream backpressure: built-up or superimposed backpressure acts on the back of the disc, altering set pressure and reducing effective spring lift. Conventional valves are strictly limited to applications where total backpressure does not exceed 10% of set pressure (at 10% accumulation). When backpressure exceeds 10% (and up to 30% to 50%), a balanced bellows PRV must be used, which isolates the upper disc area with a metal bellows. For backpressures exceeding 50%, pilot-operated relief valves (POPRVs) are required.

ASME Section VIII, Division 1 specifies maximum allowable accumulation depending on the relief contingency scenario: (1) Non-fire, single relief valve: 10% overpressure above Maximum Allowable Working Pressure (MAWP) or 3 psi (whichever is greater); (2) Non-fire, multiple relief valves: 16% overpressure (with the secondary valve set up to 105% of MAWP); (3) Fire exposure emergency case: 21% overpressure above MAWP to account for external pool fires or boiling liquid expansion.