Relief Valve Sizing Calculator
π‘οΈ Relief Valve Sizing Calculator
API 520/521/526 Based Relief Valve Sizing for Gas, Liquid, Steam & Two-Phase (Preliminary)
Purpose: Preliminary sizing of pressure relief valves in
line with API 520/521/526
Applications: Fire relief, blocked outlet, thermal expansion, control/cooling failure, gas blowdown
Important: Results are for engineering estimates. Final sizing and allowable overpressure/backpressure must be verified by a qualified engineer using the latest API standards and vendor data.
Applications: Fire relief, blocked outlet, thermal expansion, control/cooling failure, gas blowdown
Important: Results are for engineering estimates. Final sizing and allowable overpressure/backpressure must be verified by a qualified engineer using the latest API standards and vendor data.
System Configuration
Operating Conditions
Relief Load
Fluid Properties
Installation Details
Note: Sizing is preliminary. Verify with detailed
API 520/521/526 calculations and vendor curves.
Design Status: Acceptable (Preliminary)
Relief Valve Sizing Results
Relief Pressure (Set + Overpressure):
1100.00 kPa
Critical Pressure:
554.27 kPa
Flow Type:
Critical
Back Pressure Correction (Kb):
1.000
Required Orifice Area:
1661.59 mmΒ²
Effective Discharge Area:
1810.00 mmΒ²
Recommended Orifice Size:
L (1810.00 mmΒ²)
Valve Designation:
L
Performance Verification
Actual Relief Capacity:
10893.17 kg/h
Safety Margin:
8.9 %
Inlet Velocity:
64.34 m/s
Discharge Velocity:
28.60 m/s
Pressure Drop (Inlet):
2.07 kPa
Design Status:
Acceptable (Preliminary)
Export & Reset Functions
API 520 / 526 Standard Orifice Sizes
API 520/526 Standard Orifice Areas:
D: 71.00 mmΒ²
E: 126.00 mmΒ²
F: 198.00 mmΒ²
G: 314.00 mmΒ²
H: 506.00 mmΒ²
J: 804.00 mmΒ²
K: 1267.00 mmΒ²
L: 1810.00 mmΒ² β Recommended
M: 2323.00 mmΒ²
N: 2835.00 mmΒ²
P: 3668.00 mmΒ²
Q: 4645.00 mmΒ²
R: 6452.00 mmΒ²
T: 10323.00 mmΒ²
Technical Notes
Variable Definitions
Pset: Set pressure at PRV inlet (gauge).
Overpressure: Accumulation used to compute relieving pressure (API 520/521 guidance).
Prel,g: Relieving pressure (gauge) = Pset(1 + Overpressure/100).
Prel,abs: Absolute relieving pressure = Prel,g + Patm (Patm = 101.325 kPa).
Pb,abs: Absolute backpressure = Pb,g + Patm.
W: Required relief mass flowrate (kg/h).
Kd: Discharge coefficient (dimensionless).
Kb: Backpressure correction factor (dimensionless; for conventional valves use API charts; here a screening approximation is used).
M: Molecular weight (kg/kmol).
k: Isentropic exponent Cp/Cv (dimensionless).
A: Required orifice area (mmΒ²).
Formulas / Logic
1) Relieving pressure: Prel,g = Pset(1 + OP/100). Absolute pressures are used for gas/steam sizing.
2) Critical pressure (screening): Pcrit,abs = Prel,absΒ·(2/(k+1))k/(kβ1). If Pb,abs < Pcrit,abs, flow is treated as choked (critical).
3) Gas/Vapor/Steam orifice area (API 520-style, critical-flow screening form):
Β Β A = (W / (CΒ·KdΒ·KbΒ·Prel,abs)) Β· β(TΒ·Z/M), where C is computed from k (Z assumed 1.0 in this screening tool).
4) Liquid orifice area (incompressible, screening): A = Q / (KdΒ·β(2ΞP/Ο)), with Q = (W/Ο)/3600 and ΞP = (Prel,abs β Pb,abs).
5) Standard orifice selection: Next-higher API 526 orifice area is selected.
Assumptions / Notes
β’ This calculator is intended for preliminary / screening sizing. Final PSV sizing must follow the latest API 520/521/526 requirements, including viscosity correction (Kv), backpressure correction from API charts, built-up vs superimposed backpressure, inlet/discharge line losses, two-phase methodology (DIERS/HEM or vendor), and certified vendor coefficients.
β’ Two-phase sizing here uses a conservative uplift on required mass flow (screening only). Use API/vendor/DIERS method for real two-phase relief.
β’ Ensure inputs correspond to the relieving conditions (pressure/temperature) and correct required load basis (fire, blocked outlet, etc.).
Pset: Set pressure at PRV inlet (gauge).
Overpressure: Accumulation used to compute relieving pressure (API 520/521 guidance).
Prel,g: Relieving pressure (gauge) = Pset(1 + Overpressure/100).
Prel,abs: Absolute relieving pressure = Prel,g + Patm (Patm = 101.325 kPa).
Pb,abs: Absolute backpressure = Pb,g + Patm.
W: Required relief mass flowrate (kg/h).
Kd: Discharge coefficient (dimensionless).
Kb: Backpressure correction factor (dimensionless; for conventional valves use API charts; here a screening approximation is used).
M: Molecular weight (kg/kmol).
k: Isentropic exponent Cp/Cv (dimensionless).
A: Required orifice area (mmΒ²).
Formulas / Logic
1) Relieving pressure: Prel,g = Pset(1 + OP/100). Absolute pressures are used for gas/steam sizing.
2) Critical pressure (screening): Pcrit,abs = Prel,absΒ·(2/(k+1))k/(kβ1). If Pb,abs < Pcrit,abs, flow is treated as choked (critical).
3) Gas/Vapor/Steam orifice area (API 520-style, critical-flow screening form):
Β Β A = (W / (CΒ·KdΒ·KbΒ·Prel,abs)) Β· β(TΒ·Z/M), where C is computed from k (Z assumed 1.0 in this screening tool).
4) Liquid orifice area (incompressible, screening): A = Q / (KdΒ·β(2ΞP/Ο)), with Q = (W/Ο)/3600 and ΞP = (Prel,abs β Pb,abs).
5) Standard orifice selection: Next-higher API 526 orifice area is selected.
Assumptions / Notes
β’ This calculator is intended for preliminary / screening sizing. Final PSV sizing must follow the latest API 520/521/526 requirements, including viscosity correction (Kv), backpressure correction from API charts, built-up vs superimposed backpressure, inlet/discharge line losses, two-phase methodology (DIERS/HEM or vendor), and certified vendor coefficients.
β’ Two-phase sizing here uses a conservative uplift on required mass flow (screening only). Use API/vendor/DIERS method for real two-phase relief.
β’ Ensure inputs correspond to the relieving conditions (pressure/temperature) and correct required load basis (fire, blocked outlet, etc.).