Relief Valve Sizing - API 520 / 521

The ReliefValveSizing class calculates required orifice area and mass flow capacity for pressure safety valves (PSVs) using API 520 / 521 methods. It supports gas, liquid, and two-phase service.

Class: neqsim.process.util.fire.ReliefValveSizing

Capabilities

Scenario Method Standard
Gas / vapour relief calculateRequiredArea() API 520 Section 5
Liquid relief calculateLiquidReliefArea() API 520 Section 5.8
Two-phase relief calculateTwoPhaseReliefArea() API 520 Appendix D (Leung omega)
Fire heat input calculateAPI521FireHeatInput() API 521 Table 4
Cv rating calculateCv() ISA/IEC
Mass flow capacity calculateMassFlowCapacity() API 520
Blowdown pressure calculateBlowdownPressure() API 520/526

Gas Relief Sizing

The standard gas PSV sizing determines the required orifice area for critical (choked) or subcritical flow.

Java Example

import neqsim.process.util.fire.ReliefValveSizing;
import neqsim.thermo.system.SystemSrkEos;

SystemSrkEos fluid = new SystemSrkEos(273.15 + 60, 100.0);
fluid.addComponent("methane", 0.85);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.setMixingRule("classic");

ReliefValveSizing psv = new ReliefValveSizing(fluid);
psv.setReliefPressure(110.0);              // bara
psv.setBackPressure(1.013);                // bara
psv.setReliefTemperature(273.15 + 60.0);   // K
psv.setReliefMassRate(5000.0);             // kg/hr

double area = psv.calculateRequiredArea();
double capacity = psv.calculateMassFlowCapacity();
System.out.println("Required area: " + area + " m2");
System.out.println("Mass flow capacity: " + capacity + " kg/hr");

Liquid Relief Sizing

For liquid-filled equipment (pumps, blocked-in liquid lines), API 520 Section 5.8 applies.

Java Example

ReliefValveSizing psv = new ReliefValveSizing(liquidFluid);
psv.setReliefPressure(25.0);
psv.setBackPressure(1.013);
psv.setReliefTemperature(273.15 + 40.0);
psv.setReliefMassRate(50000.0);   // kg/hr

ReliefValveSizing.LiquidPSVSizingResult result = psv.calculateLiquidReliefArea();
System.out.println("Required area (liquid): " + result.getRequiredArea() + " m2");
System.out.println("Kd (liquid): " + result.getKd());
System.out.println("Kw (back-pressure): " + result.getKw());
System.out.println("Kc (combination): " + result.getKc());
System.out.println("Kv (viscosity): " + result.getKv());

Key Correction Factors

Factor Symbol Description Default
Discharge coefficient $K_d$ API 520 liquid discharge 0.65
Back-pressure $K_w$ Balanced bellows correction 1.0
Combination $K_c$ Rupture disk + PSV 1.0
Viscosity $K_v$ Re-based viscosity correction 1.0

Two-Phase Relief Sizing

Uses the Leung omega method (API 520 Appendix D) for flashing and non-flashing two-phase mixtures.

Java Example

ReliefValveSizing psv = new ReliefValveSizing(twoPhaseFluid);
psv.setReliefPressure(80.0);
psv.setBackPressure(5.0);
psv.setReliefTemperature(273.15 + 100.0);
psv.setReliefMassRate(30000.0);
psv.setInletVapourMassFraction(0.3);  // quality at inlet

double area = psv.calculateTwoPhaseReliefArea();
System.out.println("Two-phase required area: " + area + " m2");

Method

The Leung omega parameter characterises the compressibility of the two-phase mixture:

\[\omega = \frac{x \cdot v_g + (1-x) \cdot v_l}{v_{mix}} \cdot \frac{c_p \cdot T \cdot P}{h_{fg}^2}\]

The critical pressure ratio and mass flux are then calculated from $\omega$ to obtain the required area.

API 521 Fire Heat Input

Calculates the total heat absorption rate for fire-exposed equipment per API 521 Table 4. Used to determine thermal relief requirements.

Java Example

double wettedArea = 80.0;  // m2
boolean drainage = true;    // adequate drainage and firefighting

double Q = ReliefValveSizing.calculateAPI521FireHeatInput(wettedArea, drainage);
System.out.println("Fire heat input: " + Q + " W");
System.out.println("Fire heat input: " + (Q / 1000.0) + " kW");

Correlations

For adequate drainage and firefighting facilities:

\[Q = 43200 \cdot F \cdot A_{wetted}^{0.82} \quad \text{(W)}\]

where $F$ is the environmental factor (default 1.0) and $A_{wetted}$ is in m$^2$.

For inadequate drainage:

\[Q = 70900 \cdot F \cdot A_{wetted}^{0.82} \quad \text{(W)}\]

Python Example

from neqsim import jneqsim

SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
ReliefValveSizing = jneqsim.process.util.fire.ReliefValveSizing

fluid = SystemSrkEos(273.15 + 60.0, 100.0)
fluid.addComponent("methane", 0.85)
fluid.addComponent("ethane", 0.10)
fluid.addComponent("propane", 0.05)
fluid.setMixingRule("classic")

psv = ReliefValveSizing(fluid)
psv.setReliefPressure(110.0)
psv.setBackPressure(1.013)
psv.setReliefTemperature(273.15 + 60.0)
psv.setReliefMassRate(5000.0)

area = psv.calculateRequiredArea()
print(f"Required orifice area: {area:.6f} m2")

# Fire heat input
Q = ReliefValveSizing.calculateAPI521FireHeatInput(80.0, True)
print(f"Fire heat input: {Q/1000:.0f} kW")