Choke Collapse PSD Protection Scenario

Overview

This document describes a critical safety scenario where an inlet choke valve (throttle valve) suddenly fails open to 100%, causing rapid pressure rise in downstream equipment. A Process Shutdown (PSD) valve monitors the pressure and automatically closes when a High-High (HIHI) alarm is triggered, protecting the system from overpressure.

Scenario Description

Normal Operation

• High pressure feed: 100 bara at inlet
• Choke valve: Operates at 30% opening to control pressure
• Target downstream pressure: ~50 bara
• PSD valve: 100% open (normally passing flow)
• HIHI alarm setpoint: 55 bara

Failure Event

1. t = 0.0 s: Choke valve fails open to 100%
2. t = 0-3 s: Pressure rises rapidly from 50 to 56 bara
3. t = 2.5 s: Pressure reaches 55 bara, triggering HI alarm
4. t = 3.0 s: Pressure exceeds HIHI setpoint, PSD valve trips and closes
5. t > 3.0 s: PSD valve fully closed, system isolated and protected

Protection Response

• Response time: 3 seconds from failure to PSD closure
• Maximum pressure: 56 bara (contained below safe limits)
• PSD closure time: 2 seconds (fast-acting)
• Trip latching: PSD remains closed until manual reset

Implementation

System Configuration

// High pressure feed at 100 bara
SystemInterface feedGas = new SystemSrkEos(273.15 + 40.0, 100.0);
feedGas.addComponent("methane", 85.0);
feedGas.addComponent("ethane", 8.0);
// ... add other components
feedGas.setMixingRule("classic");

Stream feedStream = new Stream("High Pressure Feed", feedGas);
feedStream.setFlowRate(5000.0, "kg/hr");
feedStream.setPressure(100.0, "bara");

// Choke valve - normally at 30% opening
ThrottlingValve chokeValve = new ThrottlingValve("Inlet Choke Valve", feedStream);
chokeValve.setPercentValveOpening(30.0);
chokeValve.setOutletPressure(50.0);

Stream chokeOutlet = new Stream("Choke Outlet", chokeValve.getOutletStream());

// PSD valve for protection
PSDValve psdValve = new PSDValve("PSD Inlet Protection", chokeOutlet);
psdValve.setPercentValveOpening(100.0);
psdValve.setClosureTime(2.0); // Fast closure

// Pressure transmitter with HIHI alarm
PressureTransmitter pressureTransmitter = new PressureTransmitter(
    "Separator Inlet PT", chokeOutlet);

AlarmConfig alarmConfig = AlarmConfig.builder()
    .highHighLimit(55.0)
    .highLimit(52.0)
    .deadband(0.5)
    .delay(1.0)
    .unit("bara")
    .build();
pressureTransmitter.setAlarmConfig(alarmConfig);

// Link PSD valve to pressure transmitter
psdValve.linkToPressureTransmitter(pressureTransmitter);

Simulating the Failure

// Run initial steady state
feedStream.run();
chokeValve.run();
psdValve.run();

// FAILURE EVENT: Choke valve fails open
chokeValve.setPercentValveOpening(100.0);

// Simulate dynamic response
double timeStep = 0.5; // 0.5 second time steps
for (double time = 0.0; time <= simulationTime; time += timeStep) {
    // Run process
    feedStream.run();
    chokeValve.run();
    chokeOutlet.run();
    
    // Evaluate alarm
    pressureTransmitter.evaluateAlarm(
        chokeOutlet.getPressure("bara"), timeStep, time);
    
    // Run PSD transient behavior
    psdValve.runTransient(timeStep, UUID.randomUUID());
    
    // Check if PSD tripped
    if (psdValve.hasTripped()) {
        System.out.println("PSD valve tripped at " + time + " s");
        break;
    }
}

Test Results

Choke Collapse Test

===== CHOKE COLLAPSE SCENARIO =====
Initial Configuration:
  Feed pressure: 100.0 bara
  Choke opening: 30.0% (normal operation)
  PSD opening: 100.0% (normal operation)
  PSD HIHI setpoint: 55.0 bara

Time (s) | Choke Opening | Pressure (bara) | Alarm State | PSD Opening | PSD Tripped
---------|---------------|-----------------|-------------|-------------|------------
    0.0  |     100.0%    |      50.00      |   NONE      |    100.0%   |    NO
    0.5  |     100.0%    |      51.00      |   NONE      |    100.0%   |    NO
    1.0  |     100.0%    |      52.00      |   NONE      |    100.0%   |    NO
    1.5  |     100.0%    |      53.00      |   HI        |    100.0%   |    NO
    2.0  |     100.0%    |      54.00      |   HI        |    100.0%   |    NO
    2.5  |     100.0%    |      55.00      |   HI        |    100.0%   |    NO
    3.0  |     100.0%    |      56.00      |   HIHI      |      0.0%   |    YES

Results:

• ✓ Choke failed open at t = 0.0 s
• ✓ PSD valve tripped at t = 3.0 s
• ✓ Maximum pressure: 56.00 bara (safely contained)
• ✓ System protected from overpressure

Recovery Test

===== CHOKE REPAIR AND PSD RESET TEST =====
Step 1: Simulating choke collapse...
  PSD tripped at 56.0 bara

Step 2: Repairing choke valve (returning to 30% opening)...
  Choke repaired and pressure returned to 50 bara

Step 3: Attempting to open PSD valve while still tripped...
  ✓ PSD correctly prevents opening while tripped

Step 4: Resetting PSD valve...
  PSD valve reset complete

Step 5: Opening PSD valve to resume operation...
  ✓ PSD successfully opened to 100.0%

===== RESET TEST SUMMARY =====
✓ Choke collapse triggered PSD trip
✓ Choke repaired (returned to 30% opening)
✓ PSD prevented opening while tripped
✓ PSD reset successful
✓ System ready to resume normal operation

Key Features Demonstrated

1. Rapid Failure Detection

• Pressure rise detected within 1.5 seconds
• HI alarm activated at 53 bara
• HIHI alarm activated at 56 bara

2. Automatic Protection

• PSD valve automatically trips on HIHI alarm
• Fast closure (2 seconds) minimizes pressure overshoot
• No manual intervention required for trip

3. Trip Latching

• PSD valve remains closed after trip
• Cannot be opened while in tripped state
• Requires manual reset before operation can resume

4. Recovery Procedure

1. Repair/replace failed choke valve
2. Verify pressure has returned to safe levels
3. Reset PSD valve to clear trip state
4. Manually open PSD valve to resume operation

Safety Analysis

Layers of Protection

Layer	Device	Setpoint	Action	Response Time
1	HI Alarm	52 bara	Operator notification	Immediate
2	HIHI Alarm	55 bara	Triggers PSD closure	1 second delay
3	PSD Valve	On HIHI	Closes to 0%	2 seconds

Effectiveness

• Pressure containment: Maximum 56 bara vs. 100 bara feed pressure
• Response time: 3 seconds from failure to isolation
• Protection margin: 11% overshoot above HIHI setpoint
• Recovery capability: System can be safely restarted after repair

Best Practices

1. Alarm Configuration

AlarmConfig alarmConfig = AlarmConfig.builder()
    .highHighLimit(55.0)      // Trip setpoint
    .highLimit(52.0)          // Early warning
    .deadband(0.5)            // Prevent chattering
    .delay(1.0)               // Confirmation time
    .unit("bara")
    .build();

2. PSD Valve Settings

• Closure time: Fast enough to prevent overpressure (2-3 seconds typical)
• Trip enabled: Always enabled during normal operation
• Reset requirement: Manual reset prevents inadvertent restart

3. Testing

• Test PSD trip function regularly
• Simulate failure scenarios in training mode
• Verify alarm setpoints match design basis

Related Safety Devices

This scenario complements other safety devices in NeqSim:

Comparison Matrix

Feature	PSD Valve	Safety Valve (PSV)	Rupture Disk
Activation	HIHI alarm	Set pressure	Burst pressure
Response	Fast closure	Opens to relieve	Bursts open
Reset	Manual	Self-resetting	Requires replacement
Use Case	Process shutdown	Overpressure relief	Last-resort protection
Typical Setpoint	55 bara (HIHI)	55 bara (set)	65 bara (burst)

Integrated Protection Strategy

For complete system protection, use all three in series:

1. PSD Valve (Primary): Isolates on abnormal process conditions
2. PSV (Secondary): Relieves pressure if PSD fails
3. Rupture Disk (Ultimate): Prevents catastrophic failure

Example Application

Production Separator Protection

// High pressure feed from wellhead
Stream wellheadStream = new Stream("Wellhead", highPressureGas);
wellheadStream.setPressure(100.0, "bara");

// Choke valve for flow control
ThrottlingValve chokeValve = new ThrottlingValve("Production Choke", wellheadStream);
chokeValve.setPercentValveOpening(30.0);

// PSD valve for emergency isolation
PSDValve psdValve = new PSDValve("ESD Inlet", chokeValve.getOutletStream());
psdValve.linkToPressureTransmitter(pressureTransmitter);

// Production separator
Separator separator = new Separator("Production Sep", psdValve.getOutletStream());
separator.setInternalDiameter(1.5);
separator.setSeparatorLength(4.0);

// PSV for overpressure relief
SafetyValve psv = new SafetyValve("PSV-101", separator.getGasOutStream());
psv.setSetPressure(55.0, "bara");

// Rupture disk as last resort
RuptureDisk ruptureDisk = new RuptureDisk("RD-101", separator.getGasOutStream());
ruptureDisk.setBurstPressure(65.0, "bara");

References

• PSD Valve HIHI Trip Documentation
• Safety Valve (PSV) Documentation
• Rupture Disk Documentation
• Process Alarm System

Test Class

Complete test implementation: neqsim.process.equipment.valve.ChokeCollapsePSDProtectionTest

Run tests:

mvn test -Dtest=ChokeCollapsePSDProtectionTest