Safety and Relief Valve Sizing: API 520 Method and Selection
Quick Answer
- A safety (relief) valve auto-opens at set pressure to protect against overpressure; sizing follows API 520 and ISO 4126.
- Set pressure = MAWP (maximum allowable working pressure); relieving pressure = set + allowable overpressure (10% standard, 21% fire).
- Size by the governing scenario (fire, blocked outlet, thermal expansion, tube rupture, control-valve failure) — pick the largest required orifice.
- Select the orifice letter per API 526 (D through T); verify inlet pressure drop < 3% of set pressure.
- Type: conventional spring (air/steam), balanced bellows (backpressure/toxic), pilot-operated (high set / tight shutoff), modulating (liquid).
Why Sizing Matters
An undersized PSV cannot relieve the load and equipment can rupture. An oversized valve chatters and damages its seat. Sizing is code-mandated (ASME VIII, PED, ISO 4126).
Overpressure Scenarios (API 521)
| Scenario | Relief load basis |
|---|---|
| Fire case | API 521 heat input: wetted area × environmental factor F (bare 1.0, deluge 0.6, insulated per formula) |
| Blocked outlet | Maximum normal flow |
| Pump dead-headed | Pump shutoff pressure |
| Thermal expansion | Trapped liquid expansion |
| Tube rupture | Flow from high-pressure side |
| Control valve failure | Depends on fail-open or fail-close position |
| Cooling water failure | Maximum vapor load to condenser |
The Sizing Workflow
- List all credible overpressure scenarios.
- Calculate the required relieving rate W for each (gas in lb/hr, liquid in GPM).
- Determine the governing scenario (largest required area).
- Apply the appropriate API 520 formula (gas/vapor, liquid, steam, or two-phase).
- Select the smallest standard API 526 orifice whose effective area is ≥ the calculated area.
- Verify inlet pressure drop < 3% of set pressure and that backpressure is within the valve type limit.
Gas/Vapor Sizing (API 520)
The effective discharge area (in²) is:
A = W / (C · Kd · P1 · Kb · Kc) · √(T · Z / M)
- W = required relieving rate
- C = gas constant coefficient (function of k = Cp/Cv)
- Kd = discharge coefficient (~0.975 for API-certified valves)
- P1 = relieving pressure absolute = set pressure × 1.10 + atmospheric
- Kb = backpressure correction factor (1.0 for conventional)
- Kc = combination factor (1.0 alone, 0.9 with rupture disc)
- T = relieving temperature, Z = compressibility, M = molecular weight
Steam uses the Napier formula; liquid uses a viscosity-corrected form (Kv, Kw); two-phase service sums the vapor-phase and liquid-phase areas.
Type Selection
| Application | Recommended valve type |
|---|---|
| Gas / steam relief | Full-lift spring-loaded |
| Liquid relief | Modulating or full-lift |
| Toxic / corrosive media | Balanced bellows |
| High backpressure | Balanced bellows or pilot-operated |
| High temperature (>235 °C) | Finned safety valve |
| Critical equipment | Dual valves with interlock |
| Polymerizing / solidifying fluid | Rupture disc mandatory |
Installation Rules
- Install vertically; route discharge to a safe location (flare header, atmospheric vent, or blowdown drum).
- Open bonnet for steam/air/non-toxic venting; closed bonnet when atmospheric discharge is unacceptable.
- Inlet piping pressure drop must not exceed 3% of set pressure — a commonly violated rule that causes chatter and premature wear.
- Full-nozzle construction for corrosive or high-pressure process; semi-nozzle for clean, moderate service.
Set Pressure Reference (per design pressure)
| Condition | Maximum set / relieving pressure |
|---|---|
| Primary (main) valve | 100% set / 110% relieving (non-fire) |
| Fire-case auxiliary | 105% set |
| Non-fire auxiliary | 105% set |
| Non-fire primary | 116% relieving |
| Fire-case primary | 121% relieving |
| Reseating (blowdown) | 93–97% of set |
Standards
API 520 (sizing), API 526 (orifice standardization), API 521 (scenarios), ASME VIII (construction), ISO 4126, PED.
Use ValveSpecs Pro to check pressure class and certifications. For high-temperature duty see high-temperature valve selection; for standards see API standards explained.
FAQ
How do I size a safety valve?
Follow API 520: identify all credible overpressure scenarios (API 521), calculate the relieving rate for each, determine the governing case, apply the gas/liquid/steam formula, and select the smallest API 526 orifice whose area is at least the calculated area. Always verify the inlet pressure drop is below 3% of set pressure.
What is the difference between set pressure and relieving pressure?
Set pressure is the pressure at which the valve begins to open. Relieving pressure is set pressure plus the allowable overpressure (typically 10% for non-fire, up to 21% for fire case). Sizing uses the relieving pressure as P1.
When should I use a balanced bellows instead of a conventional valve?
Use a balanced bellows (or pilot-operated) valve when built-up backpressure is high or variable, or when the fluid is toxic or corrosive and the spring must be isolated from the process.
When is a rupture disc used instead of a relief valve?
When the fluid can polymerize, crystallize, or foul the valve (e.g., sticky polymers), or when maximum effective area is needed. Rupture discs are also combined upstream of a PSV, in which case Kc = 0.9.
Frequently Asked Questions
How do I size a safety valve?
Follow API 520: identify all credible overpressure scenarios per API 521, calculate the relieving rate for each, determine the governing case, apply the gas, liquid, or steam formula, and select the smallest API 526 orifice whose area meets or exceeds the calculated area. Verify the inlet pressure drop stays below 3% of set pressure.
What is the difference between set pressure and relieving pressure?
Set pressure is the pressure at which the valve begins to open. Relieving pressure is set pressure plus the allowable overpressure, typically 10% for non-fire service and up to 21% for fire case. Sizing uses the relieving pressure as P1.
When should I use a balanced bellows instead of a conventional valve?
Use a balanced bellows or pilot-operated valve when built-up backpressure is high or variable, or when the fluid is toxic or corrosive and the spring must be isolated from the process.
When is a rupture disc used instead of a relief valve?
When the fluid can polymerize, crystallize, or foul the valve, or when maximum effective area is needed. Rupture discs are also combined upstream of a PSV, in which case the combination factor Kc is 0.9.