PSV sizing is one of the most consequential calculations in process design — get it wrong and you either can’t relieve in time, or you’re specifying a valve that chatters and fails. Here’s the API 520 method done properly.
API 520 vs API 521 — What’s the Difference?
API 520 covers PSV sizing and selection — the calculation to determine required orifice area. API 521 covers pressure relief and depressuring systems — determining the relief load (how much flow must be relieved). In practice, you use API 521 first to calculate the relief mass flow rate, then API 520 to size the valve.
Critical Flow Equation — API 520 Gas/Vapor Service
A = W × √(T × Z / M) / (C × Kd × P₁ × Kb × Kc)
API 520 Equation — Required orifice area for gas/vapor critical flow
Where: A = required effective discharge area (in²), W = required relieving capacity (lb/hr), T = relieving temperature (°R = °F + 460), Z = compressibility factor, M = molecular weight, C = coefficient determined from specific heat ratio k, Kd = effective coefficient of discharge (0.975 for preliminary, use actual from manufacturer), P₁ = upstream relieving pressure (psia), Kb = back pressure correction factor (1.0 for conventional PSV with P₂/P₁ < 0.1), Kc = combination correction factor (1.0 if no rupture disk).
Step 1 — Calculate Coefficient C
The coefficient C accounts for the specific heat ratio k (Cp/Cv) of the gas:
C = 520 × √[ k × (2/(k+1))(k+1)/(k-1) ]
For natural gas with k = 1.31: C = 520 × √[1.31 × (2/2.31)^(2.31/0.31)] = 520 × √[1.31 × 0.5239] = 520 × 0.8292 = 431
Worked Example
Scenario: Gas-fire case on a production separator. Relief mass flow = 50,000 lb/hr, Relieving temperature = 150°F (610°R), Molecular weight = 19, Z = 0.95, Set pressure = 250 psig → P₁ = 265 psia (10% accumulation), k = 1.31, Kd = 0.975.
A = 50,000 × √(610 × 0.95 / 19) / (431 × 0.975 × 265 × 1.0 × 1.0)
A = 50,000 × √(30.5) / (111,389)
A = 50,000 × 5.523 / 111,389 = 2.477 in²
// Select from API 526 orifice table:
Next standard size above 2.477 in² = Orifice L = 2.853 in²
→ Select API 526 Orifice L
API 526 Orifice Designation Reference
| Designation | Area (in²) | Designation | Area (in²) |
|---|---|---|---|
| D | 0.110 | J | 1.287 |
| E | 0.196 | K | 1.838 |
| F | 0.307 | L | 2.853 |
| G | 0.503 | M | 3.600 |
| H | 0.785 | N | 4.340 |
| J | 1.287 | P | 6.380 |
Common Mistakes on EPC Projects
- Using Kd = 1.0 instead of 0.975 for preliminary sizing — undersizes the orifice
- Forgetting back pressure correction (Kb) — critical for balanced bellows PSVs on high back-pressure systems
- Using set pressure instead of relieving pressure — P₁ must include 10% accumulation for fire cases, 3% for process cases
- Not checking subcritical flow — for high back-pressure systems where P₂/P₁ > 0.55, flow may not be critical and a different equation applies
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