Vessel Depressurization and Blowdown

Comprehensive modeling of pressure vessel filling, depressurization, and blowdown scenarios.

Table of Contents

• Overview
• Calculation Types
• VesselDepressurization Class
  • Basic Usage
  • Configuration Parameters
  • Transient Simulation
• Heat Transfer Models
• Fire Scenarios
• Results and Reporting
• API Reference
• Examples
• Python Examples

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Overview

The VesselDepressurization class models dynamic filling and depressurization of pressure vessels with support for:

1. Multiple Thermodynamic Processes: Isothermal, isenthalpic, isentropic, isenergetic, and energy balance
2. Heat Transfer: Vessel wall thermal modeling, fire exposure
3. Multi-component Mixtures: Full thermodynamic calculations using NeqSim’s equation of state
4. Orifice Flow: Critical and subcritical discharge through orifices

Location: neqsim.process.equipment.tank

Reference: Andreasen, A. (2021). “HydDown: A Python package for calculation of hydrogen (or other gas) pressure vessel filling and discharge.” Journal of Open Source Software, 6(66), 3695.

Physical Model

                    ┌─────────────────────────────────┐
                    │                                 │
                    │   ╔═══════════════════════╗     │
                    │   ║                       ║     │
                    │   ║    VESSEL CONTENTS    ║     │
     Fire ──────────┤   ║    P, T, m, U         ║     ├────── Fire
     (optional)     │   ║                       ║     │     (optional)
                    │   ╠═══════════════════════╣     │
                    │   ║  Vessel Wall (thermal)║     │
                    │   ╚═══════════════════════╝     │
                    │               │                 │
                    └───────────────┼─────────────────┘
                                    │
                              Orifice │ (d_orifice)
                                    ▼
                               Discharge
                            (critical/subcritical)

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Calculation Types

Available Thermodynamic Models

Type	Description	Conservation	Use Case
ISOTHERMAL	Constant temperature	T = const	Fast estimates, isothermal expansion
ISENTHALPIC	Constant enthalpy	H = const	Adiabatic, no PV work (J-T effect)
ISENTROPIC	Constant entropy	S = const	Adiabatic with PV work
ISENERGETIC	Constant internal energy	U = const	Closed adiabatic vessel
ENERGY_BALANCE	Full heat transfer	Energy balance	Most accurate, fire scenarios

Selection Guide

                        Need accuracy?
                             │
              ┌──────────────┴──────────────┐
              │                             │
              ▼                             ▼
           No/Quick                    Yes/Safety
              │                             │
              ▼                             │
        ISOTHERMAL                          │
        (simplest)                          │
                                            │
                              Heat transfer involved?
                                      │
                         ┌────────────┴────────────┐
                         │                         │
                         ▼                         ▼
                        No                        Yes
                         │                         │
                         ▼                         ▼
                   ISENTROPIC              ENERGY_BALANCE
                   or ISENERGETIC          (with fire/walls)

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VesselDepressurization Class

Basic Usage

import neqsim.process.equipment.tank.VesselDepressurization;
import neqsim.process.equipment.stream.Stream;
import neqsim.thermo.system.SystemSrkEos;

// Create high-pressure gas
SystemSrkEos gas = new SystemSrkEos(298.0, 200.0);  // 200 bara
gas.addComponent("hydrogen", 1.0);
gas.setMixingRule("classic");

// Create inlet stream (closed vessel = 0 flow)
Stream feed = new Stream("feed", gas);
feed.setFlowRate(0.0, "kg/hr");
feed.run();

// Create vessel
VesselDepressurization vessel = new VesselDepressurization("HP Vessel", feed);
vessel.setVolume(0.1);  // 100 liters = 0.1 m³
vessel.setCalculationType(VesselDepressurization.CalculationType.ENERGY_BALANCE);

// Configure discharge
vessel.setOrificeDiameter(0.005);  // 5 mm orifice
vessel.setDischargeCoefficient(0.84);
vessel.setBackPressure(1.0);  // 1 bara ambient

// Initial state
vessel.run();
System.out.println("Initial pressure: " + vessel.getPressure() + " bara");
System.out.println("Initial temperature: " + vessel.getTemperature() + " K");

Configuration Parameters

Vessel Geometry

vessel.setVolume(0.5);           // Internal volume (m³)
vessel.setVesselDiameter(0.5);   // Internal diameter (m)
vessel.setVesselLength(2.5);     // Cylinder length (m)

Discharge Configuration

vessel.setOrificeDiameter(0.010);   // Orifice diameter (m)
vessel.setDischargeCoefficient(0.84); // Cd (typical 0.8-0.9)
vessel.setBackPressure(1.0);        // Downstream pressure (bara)

Wall Thermal Properties

// Set wall thermal properties for heat transfer
vessel.setVesselProperties(
    0.015,    // Wall thickness (m)
    7800.0,   // Wall density (kg/m³) - steel
    500.0,    // Wall specific heat (J/kg·K)
    45.0      // Wall thermal conductivity (W/m·K)
);

Heat Transfer Type

// Available heat transfer modes
vessel.setHeatTransferType(HeatTransferType.ADIABATIC);    // No heat transfer
vessel.setHeatTransferType(HeatTransferType.NATURAL_CONVECTION); // Natural convection
vessel.setHeatTransferType(HeatTransferType.FORCED_CONVECTION);  // Forced convection
vessel.setHeatTransferType(HeatTransferType.FIRE);          // Fire exposure

Transient Simulation

import java.util.UUID;

// Run transient depressurization
UUID runId = UUID.randomUUID();
double dt = 0.5;  // Time step (seconds)
double totalTime = 300.0;  // Total simulation time (seconds)

// Storage for results
List<Double> times = new ArrayList<>();
List<Double> pressures = new ArrayList<>();
List<Double> temperatures = new ArrayList<>();

for (double t = 0; t <= totalTime; t += dt) {
    vessel.runTransient(dt, runId);
    
    times.add(t);
    pressures.add(vessel.getPressure());
    temperatures.add(vessel.getTemperature());
    
    // Stop if pressure reaches back pressure
    if (vessel.getPressure() <= vessel.getBackPressure() * 1.01) {
        System.out.println("Depressurization complete at t = " + t + " s");
        break;
    }
}

---

Heat Transfer Models

Natural Convection

Internal and external natural convection correlations:

vessel.setHeatTransferType(HeatTransferType.NATURAL_CONVECTION);
vessel.setAmbientTemperature(288.15);  // 15°C ambient

Forced Convection

For wind-exposed vessels:

vessel.setHeatTransferType(HeatTransferType.FORCED_CONVECTION);
vessel.setExternalHeatTransferCoefficient(25.0);  // W/m²·K
vessel.setAmbientTemperature(288.15);

Fire Scenarios

The TransientWallHeatTransfer class provides detailed fire modeling:

import neqsim.process.util.fire.TransientWallHeatTransfer;

// Configure fire exposure
vessel.setHeatTransferType(HeatTransferType.FIRE);
vessel.setFireHeatFlux(50000.0);  // 50 kW/m² (pool fire)
vessel.setFireCoverage(0.5);     // 50% of vessel exposed

Wall Temperature Tracking

// Get wall temperatures during simulation
double innerWallTemp = vessel.getInnerWallTemperature();
double outerWallTemp = vessel.getOuterWallTemperature();

System.out.println("Inner wall: " + (innerWallTemp - 273.15) + " °C");
System.out.println("Outer wall: " + (outerWallTemp - 273.15) + " °C");

---

Fire Scenarios

Pool Fire (50 kW/m²)

vessel.setHeatTransferType(HeatTransferType.FIRE);
vessel.setFireHeatFlux(50000.0);  // Pool fire: 50-150 kW/m²
vessel.setFireCoverage(0.4);

Jet Fire (150-300 kW/m²)

vessel.setHeatTransferType(HeatTransferType.FIRE);
vessel.setFireHeatFlux(200000.0);  // Jet fire: 150-300 kW/m²
vessel.setFireCoverage(0.3);       // Localized exposure

API 521 Fire Case

Following API 521 guidance for fire case relief:

// API 521 recommends 45-75 kW/m² for pressure relief sizing
vessel.setHeatTransferType(HeatTransferType.FIRE);
vessel.setFireHeatFlux(75000.0);   // Conservative API 521 value
vessel.setFireCoverage(1.0);       // Full exposure (conservative)

// Run until relief device opens
double reliefPressure = 1.1 * designPressure;
while (vessel.getPressure() < reliefPressure) {
    vessel.runTransient(0.1, runId);
}
double timeToRelief = currentTime;
System.out.println("Time to relief opening: " + timeToRelief + " s");

---

Results and Reporting

Transient Results

// Get current state
double pressure = vessel.getPressure();           // bara
double temperature = vessel.getTemperature();     // K
double mass = vessel.getMass();                   // kg
double internalEnergy = vessel.getInternalEnergy(); // J
double enthalpy = vessel.getEnthalpy();           // J
double entropy = vessel.getEntropy();             // J/K

// Get discharge properties
double massFlowRate = vessel.getDischargeFlowRate();  // kg/s
double velocity = vessel.getDischargeVelocity();      // m/s
boolean isCritical = vessel.isCriticalFlow();         // choked flow?

Export to JSON/CSV

// Export results
vessel.exportResultsToJSON("blowdown_results.json");
vessel.exportResultsToCSV("blowdown_timeseries.csv");

Result Structure

{
  "vesselName": "HP Vessel",
  "initialConditions": {
    "pressure_bara": 200.0,
    "temperature_K": 298.0,
    "mass_kg": 12.5,
    "volume_m3": 0.1
  },
  "finalConditions": {
    "pressure_bara": 1.0,
    "temperature_K": 178.5,
    "mass_kg": 0.08,
    "blowdownTime_s": 245.0
  },
  "peakValues": {
    "massFlowRate_kg_s": 2.3,
    "cooldownRate_K_s": 1.5,
    "minTemperature_K": 165.2
  }
}

---

API Reference

Constructor

Constructor	Description
VesselDepressurization(String name, StreamInterface feed)	Create vessel with feed stream

Configuration Methods

Method	Description
setVolume(double)	Set vessel volume (m³)
setCalculationType(CalculationType)	Set thermodynamic model
setHeatTransferType(HeatTransferType)	Set heat transfer mode
setOrificeDiameter(double)	Set discharge orifice (m)
setDischargeCoefficient(double)	Set orifice Cd (0.8-0.9 typical)
setBackPressure(double)	Set downstream pressure (bara)
setVesselProperties(thickness, density, cp, k)	Set wall thermal properties
setFireHeatFlux(double)	Set fire heat input (W/m²)
setAmbientTemperature(double)	Set ambient temperature (K)

Execution Methods

Method	Description
run()	Initialize steady state
runTransient(double dt, UUID id)	Advance by time step dt

Result Methods

Method	Description
getPressure()	Current pressure (bara)
getTemperature()	Current temperature (K)
getMass()	Current mass (kg)
getDischargeFlowRate()	Mass flow rate (kg/s)
isCriticalFlow()	Check if flow is choked
getInnerWallTemperature()	Inner wall temp (K)
getOuterWallTemperature()	Outer wall temp (K)

Enumerations

public enum CalculationType {
    ISOTHERMAL,
    ISENTHALPIC,
    ISENTROPIC,
    ISENERGETIC,
    ENERGY_BALANCE
}

public enum HeatTransferType {
    ADIABATIC,
    NATURAL_CONVECTION,
    FORCED_CONVECTION,
    FIRE
}

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Examples

Complete Blowdown Simulation

import neqsim.process.equipment.tank.VesselDepressurization;
import neqsim.process.equipment.tank.VesselDepressurization.CalculationType;
import neqsim.process.equipment.tank.VesselDepressurization.HeatTransferType;
import neqsim.process.equipment.stream.Stream;
import neqsim.thermo.system.SystemSrkEos;
import java.util.UUID;

public class BlowdownExample {
    public static void main(String[] args) {
        // Natural gas at high pressure
        SystemSrkEos gas = new SystemSrkEos(300.0, 100.0);
        gas.addComponent("methane", 0.90);
        gas.addComponent("ethane", 0.05);
        gas.addComponent("propane", 0.03);
        gas.addComponent("CO2", 0.02);
        gas.setMixingRule("classic");
        
        Stream feed = new Stream("feed", gas);
        feed.setFlowRate(0.0, "kg/hr");
        feed.run();
        
        // Configure vessel
        VesselDepressurization vessel = new VesselDepressurization("Scrubber", feed);
        vessel.setVolume(5.0);  // 5 m³
        vessel.setCalculationType(CalculationType.ENERGY_BALANCE);
        vessel.setHeatTransferType(HeatTransferType.NATURAL_CONVECTION);
        
        // Vessel wall properties (carbon steel)
        vessel.setVesselProperties(0.025, 7850.0, 490.0, 43.0);
        vessel.setAmbientTemperature(288.15);
        
        // Discharge through 2" orifice
        vessel.setOrificeDiameter(0.0508);
        vessel.setDischargeCoefficient(0.85);
        vessel.setBackPressure(1.0);
        
        // Initialize
        vessel.run();
        System.out.println("Initial: P=" + vessel.getPressure() + " bara, " +
                          "T=" + (vessel.getTemperature()-273.15) + " °C");
        
        // Run transient
        UUID id = UUID.randomUUID();
        double dt = 1.0;
        double t = 0;
        double minTemp = Double.MAX_VALUE;
        
        while (vessel.getPressure() > 2.0) {  // Stop at 2 bara
            vessel.runTransient(dt, id);
            t += dt;
            
            minTemp = Math.min(minTemp, vessel.getTemperature());
            
            if (t % 30 < dt) {  // Print every 30 seconds
                System.out.printf("t=%5.0fs: P=%6.2f bara, T=%6.1f °C, mdot=%.3f kg/s%n",
                    t, vessel.getPressure(), vessel.getTemperature()-273.15,
                    vessel.getDischargeFlowRate());
            }
        }
        
        System.out.println("\nBlowdown complete:");
        System.out.println("  Total time: " + t + " s");
        System.out.println("  Minimum temperature: " + (minTemp-273.15) + " °C");
    }
}

Fire Case Analysis

// Same setup as above, then:
vessel.setCalculationType(CalculationType.ENERGY_BALANCE);
vessel.setHeatTransferType(HeatTransferType.FIRE);
vessel.setFireHeatFlux(75000.0);  // 75 kW/m² fire
vessel.setFireCoverage(0.5);      // Half vessel exposed

// Track maximum pressure and temperature
double maxPressure = vessel.getPressure();
double maxWallTemp = vessel.getOuterWallTemperature();

while (t < 1800) {  // 30 minutes
    vessel.runTransient(dt, id);
    t += dt;
    
    maxPressure = Math.max(maxPressure, vessel.getPressure());
    maxWallTemp = Math.max(maxWallTemp, vessel.getOuterWallTemperature());
    
    // Check for relief device opening
    if (vessel.getPressure() >= reliefSetPressure) {
        System.out.println("Relief device opens at t = " + t + " s");
        break;
    }
    
    // Check wall temperature limit (API 521)
    if (maxWallTemp > 593 + 273.15) {  // 593°C steel limit
        System.out.println("WARNING: Wall temperature exceeds material limit!");
    }
}

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Python Examples

Basic Depressurization (Python)

from neqsim.process.equipment.tank import VesselDepressurization
from neqsim.process.equipment.stream import Stream
from neqsim.thermo.system import SystemSrkEos
from java.util import UUID

# Create gas
gas = SystemSrkEos(300.0, 150.0)  # 150 bara
gas.addComponent("nitrogen", 0.95)
gas.addComponent("oxygen", 0.05)
gas.setMixingRule("classic")

feed = Stream("feed", gas)
feed.setFlowRate(0.0, "kg/hr")
feed.run()

# Configure vessel
vessel = VesselDepressurization("N2 Buffer", feed)
vessel.setVolume(1.0)
vessel.setCalculationType(VesselDepressurization.CalculationType.ISENTROPIC)
vessel.setOrificeDiameter(0.010)
vessel.setBackPressure(1.0)

vessel.run()

# Run simulation
run_id = UUID.randomUUID()
results = {'time': [], 'pressure': [], 'temperature': []}

t = 0
dt = 0.5
while vessel.getPressure() > 2.0:
    vessel.runTransient(dt, run_id)
    t += dt
    
    results['time'].append(t)
    results['pressure'].append(vessel.getPressure())
    results['temperature'].append(vessel.getTemperature() - 273.15)

# Plot results
import matplotlib.pyplot as plt

fig, (ax1, ax2) = plt.subplots(2, 1, figsize=(10, 8), sharex=True)

ax1.plot(results['time'], results['pressure'], 'b-')
ax1.set_ylabel('Pressure (bara)')
ax1.grid(True)

ax2.plot(results['time'], results['temperature'], 'r-')
ax2.set_xlabel('Time (s)')
ax2.set_ylabel('Temperature (°C)')
ax2.grid(True)

plt.tight_layout()
plt.savefig('blowdown_results.png', dpi=150)

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Related Documentation

• QRA Integration Guide - Safety analysis integration
• Tank Equipment - General tank modeling
• Safety Simulation - Safety system modeling

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Package Location: neqsim.process.equipment.tank