Pressure Relief Valve Sizing and Selection: API 520 & API 521 Guide

Why PRV Sizing Matters

A pressure relief valve (PRV) — also called a pressure safety valve (PSV) or safety relief valve (SRV) — is a spring-loaded device that opens automatically when upstream pressure exceeds the set pressure, venting fluid to a flare, blowdown drum, or atmosphere to protect vessels, heat exchangers, and piping systems from structural failure. Undersizing results in inadequate relieving capacity: the vessel continues to pressurise beyond the Maximum Allowable Accumulated Pressure (MAAP), risking catastrophic rupture. Oversizing causes valve chatter — the disc repeatedly lifts and reseats rapidly — causing seat damage and premature failure. API Standard 520 Part I (Sizing) and API Standard 521 (Pressure-Relieving and Depressuring Systems) are the globally recognised standards for PRV sizing and system design in oil, gas, petrochemical, and refining plants.

Key Definitions and Pressure Levels

PRV Types and When to Use Each

• Conventional Spring-Loaded PRV: Opening pressure is directly affected by back pressure on the outlet; suitable when superimposed back pressure is constant and ≤ 10% of set pressure; lowest cost and most common type
• Balanced Bellows PRV: A bellows element cancels the effect of variable back pressure on the disc — suitable when built-up back pressure varies or exceeds 10% of set pressure; used on liquid service and in flare-connected systems
• Pilot-Operated PRV (POPRV): Main valve held closed by process pressure acting on a larger top-seat area via pilot; opens sharply at set pressure with minimal simmer loss; suitable for high back-pressure service, large-bore valves, and gas service requiring tight shutoff
• Rupture Disc + PRV in Series: Rupture disc upstream of PRV eliminates process fluid contact with PRV springs and internals — used for toxic, corrosive, or highly viscous services where PRV simmer losses or spring contamination are unacceptable; API 520 requires the space between disc and PRV to be monitored with a pressure gauge
• Thermal Relief Valve: Very small valve to relieve trapped liquid thermal expansion in blocked-in piping — not sized for full process relief; typically 3/4" x 1" screwed body

API 520 Sizing: Vapour and Gas Service

For gas or vapour service, the required effective discharge area A (in sq in) is calculated using the compressible flow equation from API 520 Part I, Section 5. The key variables are: W (required relieving capacity in lb/hr); C (gas constant based on ratio of specific heats k = Cp/Cv, approximately 315-356 for most hydrocarbons); Kd (effective coefficient of discharge, 0.975 for vapour); P1 (absolute relieving pressure in psia = set pressure + overpressure allowance + 14.7); Kb (back pressure correction factor, 1.0 for low back pressure); Kc (combination factor, 0.9 with rupture disc upstream, 1.0 without); T (relieving temperature in degrees Rankine = F + 460); Z (compressibility factor); and M (molecular weight of the gas or vapour).

API 521: Credible Relief Scenarios

1. 1Fire Case: Heat input from pool fire per API 521 Section 5.15 — F x A^0.82 formula for wetted surface area; typically produces the largest vapour generation rate for liquid-containing vessels
2. 2Blocked Outlet: Full pump or compressor discharge flow with no outlet flow — maximum inlet flow at relief conditions
3. 3Control Valve Failure: Failure of FCV to full-open position — excess flow above design rate enters the system
4. 4Reflux Failure: Loss of cooling to distillation column — vapour overhead rate determined by reboiler duty divided by latent heat
5. 5Heat Exchanger Tube Rupture: High-pressure shell fluid leaks into low-pressure tube side — HP fluid flash volume at LP system MAWP
6. 6Utility Failure (Power/Cooling Water): Loss of heat removal driving temperature and pressure rise
7. 7Thermal Expansion: Trapped liquid in blocked-in piping with heat input — small thermal relief required
8. 8Runaway Reaction: For reactive chemistry, worst-case uncontrolled exotherm — requires DIERS methodology per API 521

API Standard Orifice Designations

Installation and Inspection Requirements

• PRVs must be installed in the upright (vertical) position with the spindle vertical — horizontal installation is permitted only for specific balanced or pilot-operated designs confirmed by the manufacturer
• Isolation block valve: A full-bore gate or ball valve may be installed below the PRV to allow in-situ removal for testing — must be car-sealed or locked open with interlocking to a spare PRV on a switching manifold (API 576)
• PRV inlet pressure drop at full flow must not exceed 3% of set pressure (API 520) — excessive inlet pressure drop causes instability and chatter
• PRV discharge line must be designed for maximum relieving load — back pressure at outlet must remain within the type-specific limits
• API 576 inspection frequency: PRVs in clean, non-corrosive service — test every 5 years minimum; PRVs in sour or corrosive service — test every 2 years
• Every PRV must have a nameplate per ASME Section VIII and API 526 showing set pressure, cold differential test pressure, orifice designation, fluid, temperature, and capacity at rated conditions