v1.0.0 to v1.0.0

---

name: neqsim-controllability-operability

version: "1.0.0"

description: "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."

last_verified: "2026-04-26"

requires:

java_packages: [neqsim.process.equipment.valve, neqsim.process.controllerdevice]

---

# 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)

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.

## 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