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Relief Valve Sizing
API 520 / API 521 pressure relief valve orifice sizing for gas, liquid, and steam service
Fluid Type
Process Conditions
P₁ (relieving) = 179.7 psia | P₂/P₁ = 0.082
Gas Properties
Correction Factors
Gas/steam: 0.975, Liquid: 0.65
Required Orifice
Area (in²)
0.3826
Area (mm²)
246.81
API 526 Standard Orifice
G
Area: 0.503 in² (325 mm²)
Flow regime: Critical (choked)
C coefficient: 356.06
Critical pressure ratio (Pc/P₁): 0.5283
API 526 Orifice Sizes
D
0.110 in²
0.110 in²
E
0.196 in²
0.196 in²
F
0.307 in²
0.307 in²
G
0.503 in²
0.503 in²
H
0.785 in²
0.785 in²
J
1.287 in²
1.287 in²
K
1.838 in²
1.838 in²
L
2.853 in²
2.853 in²
M
3.600 in²
3.600 in²
N
4.340 in²
4.340 in²
P
6.380 in²
6.380 in²
Q
11.05 in²
11.05 in²
R
16.00 in²
16.00 in²
S
26.00 in²
26.00 in²
T
44.50 in²
44.50 in²
Overpressure Guide
10% — Standard (non-fire)
16% — Multiple relief devices
21% — Fire case
3% — ASME VIII accumulation