Pressure Safety Valve (PSV) Dynamic Sizing Example

Overview

This example demonstrates how to perform a dynamic safety calculation for sizing a pressure safety valve (PSV) using NeqSim’s transient simulation capabilities. The scenario simulates a blocked outlet condition where a pressure control valve suddenly closes, causing pressure to rise in a separator until the PSV opens to prevent overpressure.

Process Description

Equipment Configuration

  1. Separator: High-pressure separator receiving gas feed at 50 bara
  2. Splitter: Splits the gas outlet into two streams
    • Stream 1 (99.9%): Goes to the pressure control valve (PCV) for normal operation
    • Stream 2 (0.1%): Goes to the pressure safety valve (PSV) for overpressure protection
  3. Pressure Control Valve (PCV-001): Controls normal outlet pressure (5 bara)
  4. Pressure Safety Valve (PSV-001): Protects against overpressure
    • Set pressure: 55 bara
    • Full open pressure: 60.5 bara (110% of set pressure)

Safety Scenario

The simulation models a sudden blocked outlet scenario:

Implementation

Key Code Sections

1. System Setup

// Create gas system
SystemInterface feedFluid = new SystemSrkEos(273.15 + 40.0, 50.0);
feedFluid.addComponent("nitrogen", 1.0);
feedFluid.addComponent("methane", 85.0);
// ... additional components

// Create separator
Separator separator = new Separator("HP Separator", feedStream);
separator.setCalculateSteadyState(false); // Enable dynamic mode

2. Splitter Configuration

// Split gas to PCV and PSV
Splitter gasSplitter = new Splitter("Gas Splitter", separator.getGasOutStream(), 2);
gasSplitter.setSplitFactors(new double[] {0.999, 0.001});
gasSplitter.setCalculateSteadyState(false);

3. PSV Automatic Opening

The SafetyValve class now automatically controls its opening based on inlet pressure during dynamic simulations. When runTransient() is called, the valve:

// PSV configured with set and full open pressures
SafetyValve pressureSafetyValve = new SafetyValve("PSV-001", stream);
pressureSafetyValve.setPressureSpec(55.0);  // Set pressure
pressureSafetyValve.setFullOpenPressure(60.5);  // Full open pressure
pressureSafetyValve.setCalculateSteadyState(false);  // Enable dynamic mode

// PSV automatically calculates opening in runTransient()
// No manual opening calculation needed!

4. Transient Simulation Loop

double dt = 0.5; // Time step in seconds
for (int i = 0; i < numSteps; i++) {
    currentTime = i * dt;
    
    // Simulate blocked outlet at t=50s
    if (currentTime >= 50.0 && currentTime < 51.0) {
        pressureControlValve.setPercentValveOpening(1.0);
    }
    
    // Run transient calculations
    // PSV automatically adjusts its opening based on inlet pressure
    separator.runTransient(dt, id);
    gasSplitter.runTransient(dt, id);
    pressureControlValve.runTransient(dt, id);
    pressureSafetyValve.runTransient(dt, id);  // Automatic PSV control
}

Results

Typical Simulation Results

Key Observations

  1. Pressure Response: When the PCV closes at t=50s, the separator pressure begins to rise steadily
  2. PSV Activation: PSV starts opening at ~140s when pressure reaches 55 bara
  3. Pressure Control: PSV successfully limits maximum pressure to 58.69 bara (6.7% above set pressure)
  4. Flow Distribution: PSV relieves approximately 86% of the feed flow rate at maximum opening

PSV Sizing Validation

The test validates several critical aspects:

  1. ✓ PSV remains closed during normal operation
  2. ✓ PSV opens at the set pressure
  3. ✓ Maximum pressure stays within acceptable limits (< 130% of full open pressure)
  4. ✓ PSV relief capacity exceeds minimum requirements (> 80% of feed rate)
  5. ✓ Pressure rise occurs after PCV blockage
  6. ✓ Overall pressure limited to within 35% of set pressure

Usage

Running the Example

The example is implemented as a JUnit test:

mvnw test -Dtest=SafetyValveDynamicSizingTest

Customization

You can modify the following parameters to study different scenarios:

Best Practices for PSV Sizing

  1. Set Pressure: Typically 10% above maximum allowable working pressure (MAWP)
  2. Accumulation: Full opening at 110% of set pressure (10% accumulation)
  3. Relief Capacity: PSV must handle the maximum credible flow rate
  4. Dynamic Simulation: Validates PSV response and pressure dynamics
  5. Time Step: Use 0.5-1.0 second time steps for valve dynamics
  6. Validation: Compare with API 520/521 or equivalent standards

References