NeqSim Controllability & Operability Skill

The "can this plant be operated?" half of process design — operating envelopes, turndown, startup/shutdown sequencing, control valve sizing, and steady-state operability checks. Pairs with neqsim-dynamic-simulation which solves the transient response.

When to Use

• Mapping operating envelope (P, T, flow, composition window where the plant runs)
• Turndown analysis (10–100% capacity)
• Control valve sizing per ISA-75 / IEC 60534
• Startup / shutdown sequence definition
• Recycle stability / loop interaction screening
• Identifying hidden constraints (compressor surge, tray weeping, HX pinch)
• Evaluating P&ID-derived valve changes such as closing an isolation valve, partly closing a control valve, opening a bypass, or changing controller mode

For P&ID-driven operational changes, load neqsim-pid-process-operations first to classify symbols, define the topology, bind plant tags, and turn each action into a NeqSim model delta.

For MCP or Java workflows that already have a ProcessSystem, use neqsim.process.operations.envelope.OperationalEnvelopeEvaluator or MCP runOperationalStudy with action="evaluateOperatingEnvelope". The evaluator does not duplicate equipment physics; it ranks margins from EquipmentCapacityStrategyRegistry, uses optional tagreader field data through OperationalTagMap, and can turn margin history into simple time-to-limit screening.

Standards: ISA-75.01 / IEC 60534-2-1, API 685 (control valves), NORSOK P-002 (operability).

Pattern 1 — Operating Envelope Map

Sweep two key handles (e.g. flow and composition) and check that every constraint is satisfied in the operating window:

double[] flows = linspace(0.1, 1.0, 10);   // 10–100% turndown
double[] gors  = linspace(50.0, 800.0, 8); // GOR variation

for (double f : flows) {
    for (double g : gors) {
        SystemInterface fluid = makeFluid(g);
        feed.setFlowRate(f * design_kgh, "kg/hr");
        process.run();
        envelope.recordPoint(f, g, checkConstraints(process));
    }
}

Constraints to check on every point:

Pattern 2 — Control Valve Sizing (ISA-75)

import neqsim.process.equipment.valve.ThrottlingValve;

ThrottlingValve cv = new ThrottlingValve("LV-1101", inletStream);
cv.setOutletPressure(P_downstream_bara);
cv.run();

double Cv_required = cv.getCv();          // ISA-75 sizing
double opening     = cv.getPercentValveOpening();
double dpRatio     = cv.getDeltaPressure() / cv.getInletPressure();
boolean choked     = dpRatio > cv.getFL() * cv.getFL() * (1 - cv.getFf() * cv.getXt());

Sizing rules:

• Pick valve such that normal flow is at 60–70% open
• Minimum flow must be at > 20% open (rangeability check)
• Maximum flow must be at < 90% open (margin)
• For choked flow: use Fl × ΔP_choke instead of actual ΔP

Rangeability:

If turndown ratio > rangeability, you need a split-range or two valves in parallel.

Seat Leakage Tightness (screening)

To rank candidate valve tightness requirements (isolation/ESDV block valves vs throttling control valves), use the ValveSeatLeakageClass enum (neqsim.process.equipment.valve). It covers EN 12266-1 Rates A–G (isolation valves) and ANSI/FCI 70-2 Classes II–VI (control valves); Class IV (0.01% of rated capacity) is the typical control-valve default, Class VI is the soft-seat essentially-zero-leakage class:

import neqsim.process.equipment.valve.ValveSeatLeakageClass;

ValveSeatLeakageClass ctrl = ValveSeatLeakageClass.FCI_70_2_CLASS_IV;
boolean tight = ctrl.isEssentiallyZeroLeakage();   // false for Class IV, true for Class VI

// ANSI/FCI 70-2 Class V closed-form liquid leak-rate screening estimate (test medium water)
double leakRateMlPerMin =
    ValveSeatLeakageClass.estimateFciClassVLeakRate(50.0e5 /* Pa */, 0.10 /* m port */);

This is screening-level only and not a substitute for a manufacturer seat-leakage test certificate. See docs/process/ValveMechanicalDesign.md for the full reference.

Pattern 3 — Recycle Stability

A process with strong recycles can have multiple steady states or be unstable:

ProcessSystem.RecycleConvergenceMonitor mon = process.getRecycleMonitor();
process.run();
double[] residual = mon.getMassResidualHistory();
// Plot residual vs iteration — should be monotone decreasing.
// If oscillating: recycle interactions; if diverging: bad initial guess or fundamental instability

Stabilization tactics:

• Increase damping factor on recycle stream (0.3–0.7)
• Set initial guess from previous-load solution (warm start)
• Decompose with ProcessModel so each area converges first
• Add an explicit pressure setpoint instead of free-floating P

Pattern 4 — Startup / Shutdown Sequence

Document each major mode as a state machine:

Use neqsim-dynamic-simulation to validate the transient.

Pattern 5 — Loop Tuning Sanity

For each PID:

• Gain (Kp): 0.5 × Ziegler–Nichols ultimate; tighten only if oscillation
• Integral (Ti): 1–3 × dead time
• Derivative (Td): usually off; only for slow exotherms / temperature

Common bad loops:

• Level on a vessel with rapid throughput change → use averaging level (P-only, low gain)
• Composition (e.g. column reflux) → cascade onto temperature
• Surge control on compressor → fast PI + proportional kicker

Pattern 6 — Operability Scorecard for the Report

Operability Scorecard
=====================
Turndown        : 25–110% of design     PASS (target ≥ 25%)
Compressor      : surge margin 18% @ low end   PASS (≥ 10%)
Column          : flooding margin 22% @ high end  PASS (≥ 15%)
Recycle stab.   : converges in 8 iters @ all loads  PASS
Control valves  : 4 valves outside 20–80% range — RESIZE
Startup time    : 6.5 hr (cold→full)    PASS (target ≤ 8 hr)
SD time         : 12 min API 521 BD     PASS (target ≤ 15 min)

Common Mistakes

Validation Checklist

• Operating envelope plotted with turndown range and at least 2 disturbance axes
• All control valves: 20% < normal opening < 80%
• Compressor surge margin ≥ 10% at lowest expected flow
• All recycles: convergence in ≤ 20 iterations across the envelope
• Startup sequence documented as state machine with controller modes
• Emergency BD validated dynamically per API 521 (≤ 15 min to 50% MAWP)
• Operability scorecard included in the report (results.json → operability section)

Related Skills

• neqsim-dynamic-simulation — solve the transient
• neqsim-process-safety — link operability gaps to safety scenarios
• neqsim-relief-flare-network — startup/SD blowdown loads
• neqsim-platform-modeling — multi-area recycle decomposition
• neqsim-control-authority-screening (community) — the operational counterpart: whether an installed loop has already run out of authority in the historian, as opposed to whether it was sized with enough