# Neqsim Controllability Operability > Process controllability and operability — operating envelope mapping, turndown analysis, startup/shutdown sequencing, control valve sizing per ISA-75 (Fl, Fp, choked flow), rangeability, hunting/loop tuning, recycle stability. USE WHEN: a task involves operability assessment, turndown studies, control valve sizing, startup/shutdown procedures, or 'will this design operate?' questions. Complements neqsim-dynamic-simulation (transient solver) with steady-state operability framing. - Skill: `equinor/neqsim-controllability-operability` (Agent Skill) - Install (CLI): `npx skillmds@latest add equinor/neqsim-controllability-operability` - Raw SKILL.md: https://api.skillmd.com/api/skills/equinor/neqsim-controllability-operability/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Productivity - Author: equinor (https://skillmd.com/u/equinor) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/equinor/neqsim-controllability-operability --- # 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`](../neqsim-dynamic-simulation/SKILL.md) 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: ```java 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: | Constraint | Check | Source | | ----------------------- | ---------------------------------------------------- | --------------------- | | Compressor surge | Q_actual > Q_surge × 1.10 | Vendor curve | | Compressor stonewall | Q_actual < Q_choke × 0.95 | Vendor curve | | Separator vapor velocity | v < K × √((ρL−ρV)/ρV) | NORSOK P-100 (K=0.107) | | Separator residence time | t_res ≥ 3 min liquid, ≥ 30 s gas | API 12J | | HX ΔTmin | ΔT > 5 °C everywhere | Pinch design | | Column flooding | C_factor < 0.85 × C_flood | Souders–Brown | | Column weeping | C_factor > 0.55 × C_flood | Tray vendor | | Pump NPSHa | NPSHa > NPSHr + 1 m margin | API 610 | | Control valve opening | 20% < travel < 80% | ISA-75 | ## Pattern 2 — Control Valve Sizing (ISA-75) ```java 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:** | Trim type | Rangeability | | ---------------- | ------------ | | Linear | 30:1 | | Equal percentage | 50:1 | | Quick opening | 10:1 | 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: ```java 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](../../../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: ```java 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: | State | P / T / flow | Active controllers | Active SIFs | | -------------- | ------------------------------------------- | ----------------------------------- | ------------- | | Cold shutdown | atmospheric, ambient | none | all in OOS | | Pressurization | ramp to design × 0.3 in 30 min | PIC slow ramp | HIPPS armed | | Cold circulation | full rate, ambient (recycle to flare) | LIC, FIC | all armed | | Heat-up | ramp 50 °C/h to design T | TIC, LIC, FIC | all armed | | Normal | design point | all loops in AUTO | all armed | | Emergency BD | depressurize per API 521 in 15 min | controllers tripped | BDV open | Use [`neqsim-dynamic-simulation`](../neqsim-dynamic-simulation/SKILL.md) 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 | Mistake | Fix | | --------------------------------------------------------- | ------------------------------------------------------------------ | | Sizing CV with normal ΔP and forgetting choked flow | Always check σ = ΔP / Fl² × ΔP_max; if exceeded use choked formula | | One CV across full 10–110% range | Rangeability > 30:1 needs split-range or two valves | | Designing only at 100% flow | Min/max constraints often govern (surge, weeping, NPSH) | | Ignoring recycle interactions until commissioning | Decompose with `ProcessModel`, verify convergence at each load | | Aggressive Kp on level control | Causes throughput oscillation downstream; use P-only averaging | | No startup BD path | Cold flare/atmospheric vent path required for trip-from-pressurized | | Using "design rate" pump curve | Pumps often run at 60–80% Qbep at normal — pick BEP at normal | ## 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`](../neqsim-dynamic-simulation/SKILL.md) — solve the transient - [`neqsim-process-safety`](../neqsim-process-safety/SKILL.md) — link operability gaps to safety scenarios - [`neqsim-relief-flare-network`](../neqsim-relief-flare-network/SKILL.md) — startup/SD blowdown loads - [`neqsim-platform-modeling`](../neqsim-platform-modeling/SKILL.md) — multi-area recycle decomposition