Neqsim Self Heating Ignition

NeqSim Self-Heating and Spontaneous Ignition Skill

Screening for low-temperature self-heating leading to spontaneous ignition — the mechanism behind lagging fires, where a combustible liquid soaks into porous thermal insulation and ignites with no external ignition source.

When this skill applies

Reach for this skill when you see any of these signals:

• A fire or smoke event on insulated pipework, vessels or valves with no identified ignition source.
• A leak of a combustible liquid — glycol (MEG/DEG/TEG), heat-transfer oil, lubricating oil, hydraulic fluid, amine, diesel — onto or into porous insulation (mineral wool, glass wool, calcium silicate, perlite).
• Questions phrased as "can this liquid self-ignite?", "is this insulation safe?", "how thick can the lagging be?", "why did it catch fire hours after the leak stopped?"
• A request to interpret oven / basket / hot-storage test results, or to extrapolate small-scale test data to plant scale.

The one thing to get right

Self-heating is a size-dependent problem, not a temperature-only problem.

Heat is generated throughout the volume (scales with $r^3$) but lost through the surface (scales with $r^2$). So the same material at the same temperature can be perfectly stable as a thin film and ignite as a thick layer.

This is why the lumped adiabatic screening in neqsim.process.safety.reaction.RunawayReactionAnalyzer cannot answer this question — it assumes a well-stirred mass with no spatial conduction, so it has no concept of a critical thickness or a critical surface temperature. Do not substitute it. Use neqsim.process.safety.selfheating.

Equally, do not use a Gibbs/equilibrium calculation to decide whether something self-ignites. GibbsReactor will report that any hydrocarbon is fully oxidised to CO₂ and H₂O at 20 °C, because that is thermodynamically true and kinetically irrelevant. Equilibrium gives you the fuel ($\Delta H$); only kinetics give you the hazard.

Model selection

Governing relations

Frank-Kamenetskii criticality parameter:

$$ \delta = \frac{E,P,r^{2}}{\lambda,R,T_a^{2}}\exp!\left(-\frac{E}{R,T_a}\right) $$

with $P = A,Q,\rho$ the volumetric heat-release pre-exponential factor [W/m³], $E$ the activation energy [J/mol], $r$ the characteristic half-dimension [m], $\lambda$ the effective conductivity of the wetted medium [W/(m·K)] and $T_a$ the boundary temperature [K]. A steady state exists only while $\delta \le \delta_{crit}$, where $\delta_{crit}$ depends only on shape: slab 0.878, infinite cylinder 2.00, sphere 3.32, cube 2.52, equicylinder 2.76.

Semenov criticality parameter, $\psi_{crit} = 1/e$:

$$ \psi = \frac{E,V,P}{h,S,R,T_a^{2}}\exp!\left(-\frac{E}{R,T_a}\right) $$

Standard workflow

1. Get the kinetics — they are measured, never calculated

$E$ and $P$ cannot come from thermodynamics. Sources, in order of preference:

1. Basket / hot-storage test on the actual soaked material → BasketTestRegression.
2. Published data for the same liquid/substrate pair.
3. A screening estimate, clearly flagged as an assumption in results.json.

BasketTestRegression fit = new BasketTestRegression();
fit.setEffectiveThermalConductivity(0.09);       // wetted insulation, not dry
fit.addPoint(SelfHeatingGeometry.CUBE, 25.0, "mm", 168.0, "C");
fit.addPoint(SelfHeatingGeometry.CUBE, 50.0, "mm", 149.0, "C");
fit.addPoint(SelfHeatingGeometry.CUBE, 100.0, "mm", 132.0, "C");
BasketTestRegressionResult k = fit.regress();
// k.getActivationEnergyJPerMol(), k.getVolumetricPreFactorWPerM3(), k.getRSquared()

Require at least three basket sizes and check getRSquared() before trusting the fit.

2. Screen the plant-scale geometry

PorousMediaSelfHeatingResult r = k
    .createAnalyzer(SelfHeatingGeometry.SLAB, 50.0, "mm", 180.0, "C")
    .analyze();

r.getVerdict();                 // SUBCRITICAL / MARGINAL / SELF_IGNITION
r.getCriticalTemperatureK();    // hottest safe surface for this thickness
r.getCriticalDimensionM();      // thickest safe layer at this temperature

For lagging on a hot line, use the convenience configuration, which applies a conservative bounding assumption (slab half-dimension = full insulation thickness, boundary temperature = pipe wall temperature) and records that assumption in getWarnings():

new PorousMediaSelfHeatingAnalyzer()
    .setEffectiveThermalConductivity(0.09)
    .setActivationEnergy(110.0, "kJ/mol")
    .setVolumetricHeatReleasePreFactor(5.0e13)
    .forPipeInsulation(50.0, "mm", 180.0, "C")
    .analyze();

3. If supercritical, get the induction time

SelfHeatingInductionResult t = new SelfHeatingInductionSolver()
    .setGeometry(SelfHeatingGeometry.SLAB)
    .setCharacteristicDimension(50.0, "mm")
    .setEffectiveThermalConductivity(0.09)
    .setBulkProperties(150.0, 1200.0)      // bulk density, specific heat
    .setActivationEnergy(110.0, "kJ/mol")
    .setVolumetricHeatReleasePreFactor(5.0e13)
    .setBoundaryTemperature(180.0, "C")
    .setMaxTime(30.0, "day")
    .solve();

t.isIgnited();
t.getInductionTimeHours();     // typically hours to days — not seconds

4. Hand off the consequence

A predicted ignition is a source term, not the end of the study. Continue to neqsim-consequence-analysis (pool/jet fire radiation) and neqsim-relief-flare-network if the fire exposes pressurised equipment.

Input guidance and common mistakes

Interpreting the verdict

• SUBCRITICAL — a stable steady state exists. Still report the margin; the steady self-heating excess is only a few kelvin, so there is no temperature warning before runaway.
• MARGINAL — within 30 % of criticality. Small changes in liquid loading, thickness or surface temperature tip it over. Recommend confirmatory testing.
• SELF_IGNITION — no steady state exists. Report the induction time and treat as a credible fire scenario.

Reporting

Populate results.json with:

• key_results: delta, delta_crit, critical_temperature_C, critical_thickness_mm, induction_time_hours, verdict
• assumptions: kinetic-parameter provenance (measured vs assumed), effective conductivity basis, and the pipe-insulation bounding assumption if used
• standards_applied: EN 15188, ASTM E2021 for the test method
• Carry every entry of getWarnings() into the assumptions/gaps register

Mitigation hierarchy (for recommendations)

1. Eliminate absorbency — closed-cell or non-absorbing insulation, so there is no porous matrix to hold the liquid and trap the heat. This removes the mechanism rather than managing it, and is normally the correct recommendation.
2. Prevent wetting — eliminate the leak; fit liquid-tight cladding and drainage that sheds leakage clear of the lagging.
3. Reduce thickness below the critical dimension, or lower the surface temperature below the critical temperature.
4. Detect and inspect — remove and replace contaminated insulation promptly; note that detection is weak as a sole barrier because the pre-runaway temperature signature is only a few kelvin.

Related skills

• neqsim-process-safety — HAZOP guideword Other than → self-ignition, LOPA, risk matrix
• neqsim-consequence-analysis — fire radiation once ignition is predicted
• neqsim-reaction-engineering — KineticReaction if a full reactor model of the oxidation is needed
• neqsim-hazid-fmea-eta-fta — placing the scenario in an event tree
• neqsim-standards-lookup — EN 15188, ASTM E2021, insulation standards