neqsim-dynamic-simulation

name	neqsim-dynamic-simulation
description	Dynamic simulation guidance for NeqSim. USE WHEN: running transient simulations, modeling startup/shutdown, tuning PID controllers, analyzing pressure/level dynamics, performing blowdown/depressurization, or setting up measurement devices and control loops. Covers runTransient, DynamicProcessHelper, controller tuning, and dynamic equipment configuration.
last_verified	2026-08-27

Dynamic Simulation Guidance

Guide for transient/dynamic process simulation in NeqSim.

When to Use Dynamic Simulation

• Startup and shutdown sequences
• Controller tuning and loop analysis
• Pressure relief / blowdown scenarios
• Level and pressure dynamics
• Compressor surge analysis
• Pipeline transients (slug flow)
• Emergency depressurization (EDP/ESD)
• P&ID-derived valve actions where pressure, level, controller response, or inventory release changes with time

For valve-action studies that start from P&ID symbols and plant data, also load neqsim-pid-process-operations to define the process graph, valve semantics, historian tag mapping, and event schedule before running runTransient.

Dynamic Simulation Architecture

NeqSim dynamic simulation advances a ProcessSystem with runTransient() (using the configured timestep) or runTransient(double dt, UUID id) (using an explicit timestep and calculation identifier). Pass a finite, positive timestep. Process and model entry points reject zero, negative, NaN, and infinite values before any area or equipment state changes. Adaptive stepping rejects invalid requests rather than clamping them.

Each timestep:

1. Flowsheet-wide settings are applied. Each setter unit receives exactly one transient call, applying its specification and advancing its own clock once.
2. Simulation time advances, then due events and field inputs are applied.
3. Equipment updates, including its thermodynamic calculations, run in insertion order or dependency-aware graph levels when parallel transient execution is enabled. Setter units are excluded from these explicit, semi-implicit, and parallel equipment passes. Steady-state algebraic equipment still evaluates on every requested pass, but its local clock advances once per non-null timestep calculation identifier. Reusing an identifier may refine a recycle without advancing physical time; use a new identifier for the next physical timestep.
4. Standalone controllers run; controllers actually executed inside equipment retain their equipment-specific timing for compatibility. Execution is coalesced by object identity and the timestep calculation identifier. Merely attaching a controller to equipment that does not execute it must not suppress the standalone update; repeated standalone registration runs once per step. ControllerDeviceBaseClass also makes repeated calls with one identifier idempotent, including semi-implicit equipment passes. Custom controllers integrated by equipment must implement the same identifier contract through hasRunTransient(UUID).
5. Measurement devices are sampled, alarms are evaluated, and history is stored.

A non-null UUID therefore identifies one physical timestep, not an entire outer time-marching simulation. Refinements of that physical step reuse the same physical-step UUID; the next accepted physical step must use a different UUID. For ordinary loops runTransient() is safe because it creates a fresh UUID each call. Deterministic safety/OTS workflows can use TransientStepIdentifier.deterministicPhysicalStep(scope, stepIndex).

ProcessSystem.runTransientAdaptive(...) is not yet a transactional adaptive integrator. The current implementation validates the request, advances trial state directly, estimates a temperature-change heuristic, and chooses a subsequent timestep. It does not yet provide rejected-step process/event/control rollback or a full-step versus two-half-step error estimate. Do not use it as evidence of qualified stiff/adaptive professional dynamics until the #2911 transactional-step work is complete.

Basic Dynamic Setup

import neqsim.process.processmodel.ProcessSystem;
import neqsim.process.equipment.stream.Stream;
import neqsim.process.equipment.separator.Separator;
import neqsim.process.equipment.valve.ThrottlingValve;
import neqsim.process.controllerdevice.ControllerDeviceInterface;
import neqsim.process.controllerdevice.ControllerDeviceBaseClass;
import neqsim.process.measurementdevice.LevelTransmitter;
import neqsim.process.measurementdevice.PressureTransmitter;

// Build steady-state process first
SystemInterface fluid = new SystemSrkEos(273.15 + 25.0, 50.0);
fluid.addComponent("methane", 0.80);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.addComponent("n-pentane", 0.05);
fluid.setMixingRule("classic");

Stream feed = new Stream("feed", fluid);
feed.setFlowRate(10000.0, "kg/hr");

Separator sep = new Separator("HP Sep", feed);
sep.setInternalDiameter(2.0);  // m — for dynamic simulation, set directly for level dynamics
sep.setSeparatorLength(6.0);   // m — for design purposes, use SeparatorMechanicalDesign instead

ThrottlingValve gasValve = new ThrottlingValve("gas valve", sep.getGasOutStream());
gasValve.setOutletPressure(20.0, "bara");

ThrottlingValve liqValve = new ThrottlingValve("liq valve", sep.getLiquidOutStream());
liqValve.setOutletPressure(10.0, "bara");

ProcessSystem process = new ProcessSystem();
process.add(feed);
process.add(sep);
process.add(gasValve);
process.add(liqValve);

// Run steady state first
process.run();

Adding Measurement Devices

// Pressure transmitter
PressureTransmitter PT100 = new PressureTransmitter("PT-100", sep);
PT100.setUnit("bara");
PT100.setMaximumValue(100.0);
PT100.setMinimumValue(0.0);
process.add(PT100);

// Level transmitter
LevelTransmitter LT100 = new LevelTransmitter("LT-100", sep);
LT100.setUnit("m");
process.add(LT100);

// Temperature transmitter
TemperatureTransmitter TT100 = new TemperatureTransmitter("TT-100", sep);
TT100.setUnit("C");
process.add(TT100);

// Flow transmitter
VolumeFlowTransmitter FT100 = new VolumeFlowTransmitter("FT-100", feed);
FT100.setUnit("kg/hr");
process.add(FT100);

Controller Configuration

PID Controller

// Level controller on liquid OUTLET valve — DIRECT acting (reverseActing = false):
// level up (error > 0) -> output up -> outlet valve opens -> level falls. Setting
// reverseActing(true) here inverts the sign and makes the loop unstable (runaway).
ControllerDeviceInterface LC100 = new ControllerDeviceBaseClass();
LC100.setControllerSetPoint(1.0);          // Target level = 1.0 m
LC100.setTransmitter(LT100);              // Controlled variable
LC100.setReverseActing(false);            // liquid-outlet level valve is direct acting
LC100.setControllerParameters(0.5, 100.0, 10.0); // Kp, Ti (s), Td (s)

// Attach controller to valve
liqValve.addController("LC-100", LC100);

// Pressure controller on gas valve
ControllerDeviceInterface PC100 = new ControllerDeviceBaseClass();
PC100.setControllerSetPoint(50.0);         // Target pressure = 50 bara
PC100.setTransmitter(PT100);
PC100.setReverseActing(false);            // Pressure up -> valve opens more
PC100.setControllerParameters(1.0, 50.0, 0.0);

gasValve.addController("PC-100", PC100);

Controller Tuning Guidelines

Loop Type	Typical Kp	Typical Ti (s)	Typical Td (s)
Level (averaging)	0.5-2.0	60-300	0
Level (tight)	2.0-5.0	30-60	0-10
Pressure (gas)	0.5-2.0	20-100	0-5
Flow	0.3-1.0	5-30	0
Temperature	0.5-2.0	60-600	10-60

Controller Deadband (SP-PV)

ControllerDeviceBaseClass supports a native SP-PV deadband via setDeadBand(double) / getDeadBand(). While the absolute control error stays inside the band the controller output is frozen (holds the last valve position) and the integral term does not accumulate; default 0 disables it. The deadband is in the controller error unit (percent in the default percent mode, else the configured engineering unit). This is the standard DCS averaging- level deadband used to stop valve cycling.

levelController.setDeadBand(0.5);   // hold the valve while |PV - SP| <= 0.5 %

Beware the deadband limit cycle. On an integrating (level) process a deadband delays correction until the level reaches the band edge; the delayed correction then overshoots and the cycle repeats, giving a square-wave valve trace. Removing (or shrinking) the deadband is the usual fix. If the installed pip neqsim predates setDeadBand, emulate it by toggling controller mode each step: setMode(ControllerMode.MANUAL) while |PV%-SP%| <= deadband (holds output) and setMode(ControllerMode.AUTO) otherwise (bumpless resume) - this is numerically identical to the native deadband.

Dynamic level-loop recipe (get the sequence right)

A dynamic separator level loop only responds if the vessel is switched out of steady-state mode and the liquid outlet valve is direct acting. The exact, easy-to-get-wrong sequence is:

// 1. Build and solve the steady state first (sets inventory, flows, holdup).
process.run();

// 2. Switch the vessel (and its outlet valve) to dynamic mode. If this is left
//    on, the separator recomputes steady state every step and the level is
//    pinned at its default (0.5 fraction) no matter what the controller does.
sep.setCalculateSteadyState(false);
liqValve.setCalculateSteadyState(false);

// 3. Set the physical geometry and the starting liquid level (as a 0..1 fraction
//    of the vessel). Do this AFTER run() so it is not overwritten by steady state.
sep.setInternalDiameter(2.0);   // m — drives holdup volume / level dynamics
sep.setSeparatorLength(6.0);    // m
sep.setLiquidLevel(0.30);       // start at 30 %

// 4. Direct-acting level controller on the liquid OUTLET valve (see above).
LevelTransmitter LT100 = new LevelTransmitter("LT-100", sep);
LT100.setUnit("m");
ControllerDeviceInterface LC100 = new ControllerDeviceBaseClass();
LC100.setTransmitter(LT100);
LC100.setControllerSetPoint(0.30 * sep.getInternalDiameter()); // SP in the LT unit
LC100.setReverseActing(false);                 // liquid-outlet level valve = direct acting
LC100.setControllerParameters(1.0, 300.0, 0.0); // averaging level: loose Kp, long Ti, no Td
liqValve.addController("LC-100", LC100);

// 5. Advance the transient with a fresh physical-step identity each step.
for (long step = 0; step < 600; step++) {
  java.util.UUID physicalStepId =
      neqsim.process.dynamics.TransientStepIdentifier.deterministicPhysicalStep("level-loop", step);
  process.runTransient(1.0, physicalStepId);   // dt = 1 s
}

Gotchas:

• Level pinned at 0.5 — the vessel is still in steady-state mode; call setCalculateSteadyState(false) on the separator (and its outlet valve).
• Level runs away — the liquid-outlet level controller is reverse acting; it must be setReverseActing(false) (direct acting). A gas-outlet pressure valve is also direct acting (false); reverse acting is for cases where more output reduces the measured value (e.g. a controller manipulating an inlet/feed valve).
• Set setLiquidLevel after run() — a steady-state solve resets the level, so set the starting level and geometry after the first run().
• One UUID per physical step — do not create one UUID before the outer time loop and reuse it. Built-in controllers/equipment intentionally treat repeated calls with one UUID as refinements of the same physical timestep.
• For an averaging level loop, use a loose Kp and long Ti (see the tuning table) and consider an SP-PV deadband only with care (see the limit-cycle note above).

After the run, use ControllerPerformanceMetrics.fromEventLog(LC100.getEventLog()) (or LC100.getPerformanceMetrics()) to score the tuning (IAE/ISE/ITAE, PV variability, valve travel and reversals, settling time) — see the KPI section below.

Loop-tuning KPIs (ControllerPerformanceMetrics)

ControllerPerformanceMetrics (neqsim.process.controllerdevice.ControllerPerformanceMetrics) computes the standard loop-tuning KPIs from a controller event log (or from raw time / PV / SP / output arrays) so tuning studies report consistent numbers without re-implementing the definitions. It is the preferred way to compare two PID tunings on the same disturbance.

Metrics: getIntegralAbsoluteError() (IAE), getIntegralSquaredError() (ISE), getIntegralTimeAbsoluteError() (ITAE, time referenced to the first sample), getProcessValueStandardDeviation() (PV variability), getPeakAbsoluteError(), getControllerOutputTravel() (total valve travel), getControllerOutputReversals() (valve direction reversals), and getSettlingTime() (time of the last sample outside the settling band, default 2 % of max(|SP|, 1)).

// After a runTransient loop with a logging controller:
ControllerPerformanceMetrics kpi = LC100.getPerformanceMetrics();          // from getEventLog()
// or, explicitly / with a custom settling band:
ControllerPerformanceMetrics kpi2 =
    ControllerPerformanceMetrics.fromEventLog(LC100.getEventLog(), 0.05);   // 5 % band

double iae = kpi.getIntegralAbsoluteError();
double valveTravel = kpi.getControllerOutputTravel();
int reversals = kpi.getControllerOutputReversals();
double settlingTime = kpi.getSettlingTime();
logger.info("IAE={} travel={} reversals={} settle={} s", iae, valveTravel, reversals, settlingTime);

// Or build directly from arrays (e.g. PV/OP pulled from a historian):
ControllerPerformanceMetrics kpi3 =
    ControllerPerformanceMetrics.fromArrays(time, pv, sp, op);

• Integral criteria use trapezoidal integration over the sample intervals, so irregular time steps are handled correctly.
• A lower IAE/ISE/ITAE means tighter regulation; lower valve travel and reversals means gentler actuator duty. Tuning trade-offs usually pit the two against each other (tighter control costs more valve movement).
• resetEventLog() on the controller before the disturbance so the KPIs cover only the window of interest.

Anti-Surge Control (dynamic)

AntiSurgeController (neqsim.process.controllerdevice.AntiSurgeController) is a purpose-built reverse-acting PI controller that reads the compressor getDistanceToSurge() and drives a recycle (spill-back) ThrottlingValve open when the margin falls below the set point, then closes it again on recovery.

import neqsim.process.controllerdevice.AntiSurgeController;

// distance to surge ~ (operating flow / surge flow - 1); only meaningful once a
// compressor chart with an active surge curve exists.
AntiSurgeController asc = new AntiSurgeController("anti-surge", compressor, recycleValve);
asc.setSurgeMarginSetPoint(0.10);   // protect a 10% distance-to-surge margin
asc.setProportionalGain(400.0);     // percent opening per unit margin error
asc.setIntegralTime(20.0);          // s
asc.setOpeningRange(0.0, 100.0);    // valve opening clamp (%) with anti-windup
asc.setActive(true);
recycleValve.addController("anti-surge", asc);

Control law each transient step: error = setPoint - distanceToSurge, integral += Kp/Ti * error * dt, opening = clamp(Kp*error + integral) with anti-windup; the controller applies the opening directly to the recycle valve.

Reproducible benchmark. AntiSurgeDynamicBenchmark (neqsim.process.util.scenario.AntiSurgeDynamicBenchmark) drives the real controller against a transparent first-order gas-path surrogate m_{k+1} = m_k - d*dt + a*(u/100)*dt (m = distance to surge, d = disturbance rate, a = recycle authority, u = valve opening %). It is deterministic and always converges, so it is the preferred way to verify or tune the control law:

import neqsim.process.util.scenario.AntiSurgeDynamicBenchmark;

AntiSurgeDynamicBenchmark bench = new AntiSurgeDynamicBenchmark();
bench.setInitialMargin(0.30);
bench.setDisturbanceRate(0.020);   // flow loss erodes the margin (/s)
bench.setRecycleAuthority(0.060);  // fully open recycle restores margin (/s)
bench.setTimeStep(1.0);
bench.setNumberOfSteps(120);
bench.getController().setSurgeMarginSetPoint(0.10);
bench.run(false);                   // open loop  -> surges (margin < 0)
bench.run(true);                    // closed loop -> margin held at set point
boolean safe = bench.isSurgeAvoided();

Critical gotchas when wiring a full dynamic recycle flowsheet:

• A fixed-factor Splitter (setSplitFactors([0.97, 0.03])) pins the recycle fraction in dynamic mode, so the anti-surge valve has no authority over the actual recycle flow — the controller can hit 100% with no effect. Let the recycle flow be set by the valve (setCv/resistance), or use the steady-state anti-surge Calculator pattern instead.
• Once the operating point crosses left of the surge line, getDistanceToSurge() clamps at -1.0 and the steady solver cannot climb back out; a flowsheet driven into deep surge will not self-heal even after the inlet is reopened. Apply gradual/ramped disturbances and keep the machine off deep surge.
• Aggressive proportional gain can slam the recycle valve to its minimum opening, starve a stream, and trigger an SRK flash NaN (PhaseSrkEos:molarVolume ... NaN). Keep gains moderate and the valve off hard minimum.
• To demonstrate or tune the control law cleanly, prefer AntiSurgeDynamicBenchmark (or a transparent gas-path surrogate) over a full recycle flowsheet that can stick in deep surge.

For coordinated compressor-train studies, use CompressorAntiSurgeApplication (neqsim.process.equipment.compressor) as the supervisory scan layer. Each StageApplication can bind directly to real NeqSim topology objects with bindTopology(process, compressor, hotRecycleValve, coldRecycleValve, recycleCooler, suctionMixer, hotRecycle, coldRecycle). A scan then writes hot/cold recycle valve openings and optional compressor speed runback to the real units, and runDynamicStep(scanInput, dt) advances the bound ProcessSystem with runTransient().

Use this application layer when the study needs stage coordination, startup/shutdown or trip states, hot/cold recycle split, operator diagnostics, commissioning checks, or speed runback in one executable dynamic model. Keep Recycle blocks algebraic unless they have explicit transient inventory support; valve, compressor, cooler, mixer, and volume-capable equipment should carry the dynamic response. The application layer reports NOT_CERTIFIED_FOR_PROTECTION and is for simulation/advisory studies, not a certified machinery-protection package.

For production-readiness evidence, pass the actual compressor cases and transient response limits to CompressorProtectionQualificationCalculation; it checks map margins, extrapolation, driver/start-up/rundown, response time, rotor separation, settle-out and vendor acceptance without certifying the protection system. For piping, pass ordered TwoFluidPipe, water-hammer, or externally governed solver samples to TransientPipingQualificationCalculation. That module checks acoustic resolution and line-pack balance before pressure, slug, velocity and stress limits, and deliberately does not treat a quasi-steady time series as distributed-transient evidence. Production mode requires the controlled context attribute distributedTransientModel=approved.

TwoFluidPipe Phase Appearance and Disappearance

When TwoFluidPipe.setIncludeMassTransfer(true) is enabled, flash-driven transfer is phase resolved. Condensation must use equilibrium hydrocarbon-liquid and aqueous-liquid mass contributions; never use the current cell water cut to identify a phase that is not yet present. For evaporation, withdraw from the actual oil and water conservative inventories and bound each withdrawal by phase mass / relaxation time. An absent phase must have exactly zero evaporation source.

Transferred momentum follows donor velocity. During condensation, gas loses mass and momentum at gas velocity and the receiving oil/water phases gain that momentum. During evaporation, each liquid loses momentum at its own velocity and gas receives their sum. Validate both invariants:

gas mass source + oil mass source + water mass source = 0
gas momentum source + oil momentum source + water momentum source = 0

For a phase-transition regression, cross a real SRK/CPA dew point in both directions without finite oil or water seeding. Check gas, oil, water, liquid, and total closure with TwoFluidMassBalanceReport; sweep nearby temperatures, refine time step and mesh, repeat the run, and compare rigorous flash with FlashTable. The flash table must retain the oil/aqueous liquid mass split. Record EOS, mixing rule, composition, absolute pressure, temperature, mass-transfer relaxation time, and units. The current hydrodynamic state transports bulk phase inventories, not a full component-composition vector per cell, and does not establish equivalence with any commercial transient multiphase simulator.

TwoFluidPipe Coupled Pressure-Momentum Gate

For a liquid-rich pressure outlet that physically permits phase fallback, the coupled path requires all four options. Keep the nonlinear controls explicit in reproducible studies:

pipe.setEnableInterfacialPressure(true);
pipe.setImplicitInterfacialPressureCoupling(true);
pipe.setEnableCoupledPressureMomentum(true);
pipe.setAllowOutletPhaseBackflow(true);
pipe.setCoupledPressureMomentumMaximumIterations(24); // default
pipe.setCoupledPressureMomentumRelativeVolumeTolerance(1.0e-7); // default

The previous budget of 12 stopped the public Tengesdal progress case near a 6e-7 relative cell-volume residual, above the 1e-7 gate. The current 16-section Test 3 probe completes 50/50 calls of 0.1 s and a 24-section refinement completes 100/100 calls of 0.05 s. Neither rejects a nonlinear substep, and phase and total discrete mass residuals remain below 1e-9.

A coupled call that exhausts its adaptive retries throws with accepted/requested elapsed time, residual/tolerance, iterations/cap, and limiter state. Do not catch that exception and advance the flowsheet clock. Successful completion still requires the sticky diagnostics to be inspected after the full window:

if (pipe.isTransientOutletBackflowClamped()
    || pipe.isTransientCoupledPressureMomentumFailureDetected()
    || pipe.isTransientCoupledPressureMomentumCorrectionLimited()
    || pipe.getTransientCoupledPressureMomentumRejectedSubsteps() > 0) {
  throw new IllegalStateException("TwoFluidPipe transient is not qualified");
}

These flags reset on the next steady run(). A pressure-limiter event is not itself a rejected substep because the volume residual can converge while the bounded correction is active, but it is mandatory qualification evidence. Use isCoupledPressureMomentumPressureCorrectionLimited() only for the latest correction; use the isTransient... form for the complete window. The current Tengesdal progress run still activates that flag and spans -18.55 to 6.88 kg/s liquid outlet versus the stored 0.375 to 4.03 kg/s comparison. It is therefore numerical-progress evidence only, not sustained-severe-slugging or commercial-parity evidence. Never tune a public closure to the commercial trace; validate the next boundary-coupling increment against the public Tengesdal experiment, conservation, nearby points, and mesh/time-step refinement.

Running Dynamic Simulation

// Timestep loop
double dt = 1.0;  // seconds
int nSteps = 3600; // 1 hour

// Storage for time history
double[] time = new double[nSteps];
double[] pressure = new double[nSteps];
double[] level = new double[nSteps];

for (int i = 0; i < nSteps; i++) {
    time[i] = i * dt;

    // Introduce disturbance at t = 300 s
    if (i == 300) {
        feed.setFlowRate(15000.0, "kg/hr");  // Step change +50%
    }

    process.runTransient(dt);

    pressure[i] = PT100.getMeasuredValue();
    level[i] = LT100.getMeasuredValue();
}

For large flowsheets with independent branches, enable process.setParallelTransientEnabled(true) and set the maximum worker count with process.setTransientThreadPoolSize(n). The per-process worker pool is created lazily and reused across timesteps; changing the worker count or disabling the option retires it. Execution follows cached process-graph levels, so upstream groups complete before downstream equipment is submitted while independent groups within a level remain parallel. A worker exception propagates to the caller with its runtime type and message, cancels later queued groups, prevents downstream dependency levels from being submitted, and skips controller, measurement/alarm/history, timestep-counter, and calculation-identifier commit phases. The process clock, due-event effects, and equipment state already mutated by a same-level sibling or earlier unit are not rolled back; treat that timestep as incomplete. If the caller is interrupted while waiting, NeqSim restores the interrupt status, cancels queued work without interrupting equipment already updating state, does not submit downstream levels, and likewise aborts the remaining timestep phases. Graph ordering does not define transient recycle convergence or transactional rollback, so keep parallel execution off for recycle loops and other implicit couplings until their transient contract is explicitly supported.

Python Dynamic Simulation

from neqsim import jneqsim
import numpy as np
import matplotlib.pyplot as plt

# Build process (same pattern as Java)
# ... create fluid, equipment, controllers ...

process.run()  # Steady state

dt = 1.0
n_steps = 3600
times = np.zeros(n_steps)
pressures = np.zeros(n_steps)
levels = np.zeros(n_steps)

for i in range(n_steps):
    times[i] = i * dt
    if i == 300:
        feed.setFlowRate(15000.0, "kg/hr")

    process.runTransient(dt)
    pressures[i] = PT100.getMeasuredValue()
    levels[i] = LT100.getMeasuredValue()

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

ax1.plot(times / 60, pressures)
ax1.set_ylabel("Pressure (bara)")
ax1.set_xlabel("Time (min)")
ax1.grid(True)

ax2.plot(times / 60, levels)
ax2.set_ylabel("Level (m)")
ax2.set_xlabel("Time (min)")
ax2.grid(True)
plt.tight_layout()

P&ID Valve-Action Dynamic Studies

Use this pattern when evaluating actions such as closing an outlet valve, opening a bypass, tripping a shutdown valve, or opening a drain/vent:

1. Run and validate the steady-state base case.
2. Define an event schedule with action type, affected valve, start time, and ramp duration.
3. Map each P&ID valve to the correct NeqSim role: control valve, boundary switch, check-valve direction constraint, or blowdown/flare path.
4. Run process.runTransient(dt) for controller and inventory dynamics, or use neqsim-depressurization-mdmt for dedicated blowdown/MDMT cases.
5. Save time series for pressure, level, temperature, valve position, flow, and any flare or vent stream.

Minimum result keys: max_pressure_bara, max_level_m, min_temperature_C, peak_flare_flow_kg_s, time_to_alarm_s, and time_to_new_steady_state_s.

Depressurization / Blowdown

// For vessel depressurization, use the safety/depressuring agent
// Key pattern: open a blowdown valve at t=0 and track P, T vs time

ThrottlingValve bdv = new ThrottlingValve("BDV", sep.getGasOutStream());
bdv.setOutletPressure(1.0, "bara");  // Vent to atmosphere
bdv.setCv(500.0);  // Valve Cv

// Controller: fully open at t=0
// Or use step change in valve opening

Transfer Function Blocks

For more advanced control logic:

import neqsim.process.controllerdevice.TransferFunctionBlock;

TransferFunctionBlock leadLag = new TransferFunctionBlock();
// Configure lead-lag, deadtime, filters as needed

Common Pitfalls

1. Always run steady state first: Call process.run() before runTransient()
2. Timestep size: Start with 1.0 s, reduce if oscillating (0.1-0.5 s)
3. Liquid-outlet level-controller direction: use setReverseActing(false) so level up drives the outlet valve further open. Reverse acting on this configuration drives the loop the wrong way.
4. Controller windup: Large setpoint changes can cause integral windup
5. Separator dimensions: Must set setInternalDiameter() and setSeparatorLength() for meaningful level dynamics. For dynamic simulation, set directly on the separator; for design purposes, configure via SeparatorMechanicalDesign (see neqsim-api-patterns skill)
6. Measurement range: Set min/max on transmitters to match process range
7. Enable dynamic (inventory) mode for level loops: after process.run() (steady), call setCalculateSteadyState(false) on the separator AND every valve, then separator.setLiquidLevel(startFraction), before runTransient. If steady-state mode is left on, the separator liquid level stays pinned at its default (0.5) and the level controller never acts. The valve Cv is auto-derived from the steady solve. A liquid-outlet level valve is setReverseActing(false) (level up -> valve opens); put a pressure controller on the gas-outlet valve so the vessel pressure is held and the level loop is isolated.
8. Physical-step UUIDs: never reuse one non-null UUID across multiple outer timesteps. Reuse is reserved for refinement/evaluation of one physical step.

Pluggable Integrator Strategies

ProcessSystem accepts a pluggable IntegratorStrategy, but this hook does not by itself make the complete flowsheet a stiff vector ODE/DAE system. Implementations live in neqsim.process.dynamics:

Strategy	Class	Notes
Explicit Euler	ExplicitEulerIntegrator	Default; fast, conditionally stable
BDF-1 (Implicit Euler)	BDFIntegrator	Experimental local strategy. Newton + FD Jacobian; can fall back to explicit Euler when Newton diverges. Inspect lastStepFellBack() and do not treat fallback runs as qualified stiff/DAE evidence.

import neqsim.process.dynamics.BDFIntegrator;
import neqsim.process.dynamics.ExplicitEulerIntegrator;
import neqsim.process.dynamics.IntegratorStrategy;

BDFIntegrator bdf = new BDFIntegrator();
process.setIntegratorStrategy(bdf);
// process.setIntegratorStrategy(new ExplicitEulerIntegrator()); // explicit default
// process.setIntegratorStrategy(null);  // reset to default ExplicitEulerIntegrator
IntegratorStrategy current = process.getIntegratorStrategy();

For multi-area plants the strategy is propagated to every child area: plant.setIntegratorStrategy(new BDFIntegrator()).

A professional stiff-solver acceptance path must fail loudly or expose explicit fallback diagnostics; silent fallback is not an acceptable qualification result.

Event Scheduling (ESD, IOA, setpoint changes)

Time-stamped events (ESD trips, valve closures, setpoint ramps) are managed by EventScheduler in neqsim.process.dynamics. ProcessSystem.runTransient(dt, id) advances the process clock, then fires events with time <= currentTime before equipment runs.

import neqsim.process.dynamics.EventScheduler;

EventScheduler events = new EventScheduler();
events.scheduleEvent(120.0, "ESD trip", new Runnable() {
  public void run() { esdValve.setPercentOpen(0.0); }
});
events.scheduleEvent(300.0, "Setpoint ramp", new Runnable() {
  public void run() { pressureController.setControllerSetPoint(45.0); }
});
process.setEventScheduler(events);

for (int i = 0; i < nSteps; i++) {
  process.runTransient(dt);   // fresh physical-step ID; due events fire automatically
}

int fired = events.getFiredEvents().size();
int pending = events.getPendingEvents().size();

For event-aware/adaptive work, inspect event state without mutating it and checkpoint scheduler bookkeeping explicitly:

EventScheduler.Snapshot eventState = events.snapshot();
double nextEventTime = events.getNextEventTime();
java.util.List<EventScheduler.ScheduledEvent> due = events.getDueEvents(process.getTime());

// If a trial is rejected, restore pending/fired membership.
events.restore(eventState);

restore(...) restores only scheduler pending/fired membership. It cannot undo an already executed Runnable side effect. A rejected trial must defer external actions until acceptance or restore every object those actions mutate as part of the same transaction. This is required before safety/OTS event replay can be called transactional.

For multi-area plants install the scheduler once on the ProcessModel; it is propagated to every child area, and plant.runTransient(dt, id) advances all areas:

plant.setEventScheduler(events);
plant.runTransient(dt, java.util.UUID.randomUUID());

Note: EventScheduler is declared transient on ProcessSystem because event Runnable payloads (lambdas, anonymous classes) are usually not serializable. Re-install the scheduler after deserialising a saved process. EventScheduler.Snapshot itself can be serialized when its event actions are serializable, which is useful for explicit checkpoint/restart coordination.

New Measurement Devices (v3.11)

Three new measurement devices in neqsim.process.measurementdevice complement the existing PT/TT/LT/FT family:

Class	Reads	Unit
DifferentialPressureTransmitter(name, high, low)	pHigh - pLow across two streams	bar
CompositionAnalyzer(name, stream, component, phase)	Mole fraction; phase OVERALL / GAS / LIQUID	mole/mole
FlowRatioMeter(name, num, den, basis)	Flow ratio; basis MASS / MOLE / VOLUME	dimensionless

import neqsim.process.measurementdevice.DifferentialPressureTransmitter;
import neqsim.process.measurementdevice.CompositionAnalyzer;
import neqsim.process.measurementdevice.FlowRatioMeter;

DifferentialPressureTransmitter dpdt = new DifferentialPressureTransmitter("dPT-1", upstream, downstream);
CompositionAnalyzer ax = new CompositionAnalyzer("AX-1", sweetGas, "methane",
    CompositionAnalyzer.AnalyzerPhase.GAS);
FlowRatioMeter rxn = new FlowRatioMeter("FR-1", recycleStream, feedStream,
    FlowRatioMeter.FlowBasis.MASS);