PSV Dynamic Sizing Example

This example demonstrates how to perform a dynamic safety calculation for a pressure safety valve (PSV) sizing using NeqSim’s transient simulation capabilities.

Scenario

A high-pressure separator operates at ~50 bara with gas output flowing through a splitter:

Dynamic Event Sequence

  1. Normal operation (t=0-50s): Separator operates normally, PCV at 50% opening, PSV closed
  2. Blocked outlet (t=50s): PCV suddenly closes to 1% (simulating blocked outlet)
  3. Pressure rise (t=50-130s): Separator pressure increases from ~32 bara to 55 bara
  4. PSV opens (t=130s): PSV starts opening when pressure exceeds set pressure (55 bara)
  5. Relief phase (t=130-200s): PSV relieves gas to control pressure at ~58.7 bara
  6. Recovery (t=200s): PCV reopens to 50%, allowing normal flow path
  7. Pressure drops (t=200-260s): System pressure decreases as both valves relieve
  8. PSV closes (t=260s): PSV reseats when pressure drops to blowdown pressure (~51.15 bara)

PSV Hysteresis Behavior

The PSV implements realistic hysteresis (blowdown) behavior to prevent valve chattering:

Key Point: Once the PSV opens, it does NOT close immediately when pressure drops below the set pressure. It stays open until pressure drops to the blowdown/reseat pressure. This prevents rapid cycling (chattering) that could damage the valve.

Typical Blowdown Values

Code Structure

// Setup equipment
Separator separator = new Separator("HP Separator", feedStream);
Splitter gasSplitter = new Splitter("Gas Splitter", separator.getGasOutStream(), 2);
ThrottlingValve pressureControlValve = new ThrottlingValve("PCV-001", gasSplitter.getSplitStream(0));
SafetyValve pressureSafetyValve = new SafetyValve("PSV-001", gasSplitter.getSplitStream(1));

// Configure PSV with automatic opening
pressureSafetyValve.setPressureSpec(55.0);  // Set pressure (bara)
pressureSafetyValve.setFullOpenPressure(60.5);  // Full open at 110% of set
// Blowdown is automatically set to 7% (reseat at 51.15 bara)

// Alternative: Explicitly set blowdown percentage
pressureSafetyValve.setBlowdown(10.0);  // 10% blowdown for liquid service

// Dynamic simulation loop
for (int i = 0; i < numSteps; i++) {
    currentTime = i * dt;
    
    // Simulate events (blockage, recovery, etc.)
    if (currentTime >= 50.0 && currentTime < 51.0) {
        pressureControlValve.setPercentValveOpening(1.0);  // Block outlet
    }
    if (currentTime >= 200.0 && currentTime < 201.0) {
        pressureControlValve.setPercentValveOpening(50.0);  // Recover
    }
    
    // Run transient calculations
    // PSV opening is calculated automatically based on inlet pressure
    separator.runTransient(dt, id);
    gasSplitter.runTransient(dt, id);
    pressureControlValve.runTransient(dt, id);
    pressureSafetyValve.runTransient(dt, id);  // Automatic PSV control with hysteresis
}

Automatic PSV Control

The SafetyValve.runTransient() method automatically:

  1. Monitors inlet pressure
  2. Calculates valve opening percentage based on:
    • Set pressure (starts opening)
    • Full open pressure (100% open)
    • Current valve state (open/closed)
  3. Implements hysteresis:
    • When closed: Opens when P ≥ P_set
    • When open: Closes when P ≤ P_blowdown (reseat pressure)
  4. Prevents chattering through state tracking

Results

From the test simulation with 5000 kg/hr feed:

Parameter Value
Feed flow rate 5000 kg/hr
PSV set pressure 55.0 bara
PSV full open pressure 60.5 bara
PSV blowdown pressure 51.15 bara (7% blowdown)
Maximum separator pressure 58.69 bara
Maximum PSV relief flow 6086 kg/hr
PSV opening at max pressure 67.1%

Key Observations

  1. PSV prevents catastrophic overpressure: Maximum pressure (58.69 bara) is well below the full open pressure (60.5 bara), demonstrating effective pressure control.

  2. Adequate relief capacity: PSV relieves 6086 kg/hr, which exceeds the feed rate (5000 kg/hr), ensuring the valve can handle the relief scenario.

  3. Hysteresis prevents chattering:
    • PSV opens at 55.0 bara
    • PSV stays open even when pressure drops to 52-54 bara
    • PSV only closes when pressure reaches 50.95 bara (below the 51.15 bara blowdown)
  4. Smooth pressure control: The automatic PSV control provides smooth pressure regulation during both pressure buildup and recovery phases.

Best Practices

  1. Always use dynamic mode: Set setCalculateSteadyState(false) for all equipment in transient simulations

  2. Size PSV conservatively: Ensure PSV can handle at least 100% of the feed flow rate

  3. Set appropriate blowdown: Use 7-10% for gas, 10-20% for liquid service to prevent chattering

  4. Use unique UUID: Create one UUID per simulation run to track transient state correctly

  5. Choose appropriate time step: 0.5 seconds provides good resolution for PSV dynamics

  6. Monitor key parameters: Track separator pressure, valve openings, and flow rates throughout the simulation

See Also