neqsim-depressurization-mdmt

NeqSim Depressurization & MDMT Skill

Transient blowdown / depressurization for inventory release on fire or controlled emergency, plus the minimum design metal temperature (MDMT) check that drives material selection. The two are linked: blowdown end-temperatures (often −80 to −120 °C for hydrocarbon gas) usually drive MDMT, which in turn drives whether LTCS, low-temperature carbon-Mn, 3.5 % Ni or 9 % Ni / 304L is required.

When to Use

• Sizing a blowdown / depressurization valve to reach 50 % pressure in 15 min (API 521 §5.20 fire case) or 7 bar in some operator standards
• Generating P(t), T(t), m(t) curves for the relief / flare load case
• Screening MDMT against end-of-blowdown vessel-wall temperature
• Producing source terms for the flare network (neqsim-relief-flare-network)
• Distinguishing depressurization cases from blocked-in liquid fire rupture cases, where neqsim-trapped-liquid-fire-rupture is the primary workflow

Distinct from neqsim-relief-flare-network (steady-state PSV sizing) and neqsim-dynamic-simulation (continuous-process transients) — this skill is the specific blowdown + MDMT pair.

Standards

• API 521 7th ed. — Pressure-relieving and depressuring systems (§5.20 blowdown)
• API STD 520 — PSV sizing, used for choke check at the BDV
• ASME UCS-66 / UCS-66.1 — MDMT impact-test exemption curves (carbon steel)
• ASME UHA-51 — austenitic stainless steel low-temperature service
• API 579 / FFS-1 §3 — fitness-for-service, MDMT for in-service vessels
• EN 13445-2 Annex B — European MDMT and impact-test approach
• NORSOK L-002 — piping system design (low-temperature operation)

Method 1 — Blowdown Simulation (VU-flash)

import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermo.system.SystemInterface;
import neqsim.process.safety.depressurization.DepressurizationSimulator;

SystemInterface gas = new SystemSrkEos(273.15 + 50.0, 100.0);
gas.addComponent("methane", 0.92);
gas.addComponent("ethane",  0.05);
gas.addComponent("propane", 0.03);
gas.setMixingRule("classic");
gas.setTotalNumberOfMoles(5000.0); // mol — representative of vessel inventory

DepressurizationSimulator sim = new DepressurizationSimulator(gas);
sim.setVesselVolume(50.0);     // m³
sim.setOrificeArea(5.0e-4);    // m² — BDV equivalent area
sim.setBackPressure(1.5);      // bara — flare KO drum
sim.setHeatInput(0.0);         // adiabatic; > 0 for fire case
sim.run(900.0, 1.0);           // 15 min, 1 s timestep

double[] t = sim.timeSeries();
double[] p = sim.pressureSeries();
double[] T = sim.temperatureSeries();
double[] m = sim.massFlowSeries();
double pEnd = p[p.length - 1];
double tEnd = T[T.length - 1];
double t50  = sim.timeToPressure(50.0);  // s, time to 50 bar

The simulator uses the U–V flash (ops.VUflash(V, U)) at every step — internal energy decreases by h_out · ṁ · Δt and volume is held constant by the vessel, so each step is a fully consistent thermodynamic state. Joule-Thomson cooling across the BDV is captured via an isenthalpic flash to the back pressure for the exit-temperature output.

Fire case

sim.setHeatInput(60_000.0); // W — API 521 fire heat input

API 521 fire heat input on uninsulated vessels:

Q = 43.2 · F · A^0.82 [W]

with environment factor F (= 1.0 for un-insulated, 0.30 for fireproof insulation, 0.075 for water-spray) and wetted area A in m². The simulator accepts the value directly so any of the API 521, NFPA 30 or NORSOK correlations can be used upstream.

Method 2 — BDV Sizing Iteration

Typical workflow:

1. Start from a target such as 50 % of design pressure in 15 min (API 521), 7 bar in 15 min, or the relevant company/project criterion from the private basis.
2. Guess BDV Cd · A, run sim.run(...), read sim.timeToPressure(target).
3. Iterate area until target is met without choking the flare header.
4. Verify the minimum T(t) is above the vessel MDMT.

A reference iteration loop is available as DepressurizationSimulator.sizeForTargetPressure(targetBar, targetTimeS).

Method 3 — MDMT Assessment

import neqsim.process.safety.mdmt.MDMTCalculator;

MDMTCalculator mdmt = new MDMTCalculator();
mdmt.setMaterial("SA-516-70N");      // normalised CMn, common for CS vessels
mdmt.setThicknessMM(50.0);
mdmt.setStressRatio(0.35);           // operating / allowable stress ratio
double mdmtC = mdmt.computeUCS66();  // °C — ASME UCS-66 + UCS-66.1 reduction

The calculator implements:

• UCS-66 Curve A / B / C / D lookup vs material specification
• UCS-66.1 stress-ratio reduction (lower stress → lower MDMT)
• API 579 §3 Fitness-for-Service path for in-service vessels with crack reassessment factors
• EN 13445-2 Annex B alternative if requested

Pass / fail check

double bdvEndTemp = sim.minTemperatureC();  // °C from blowdown sim
boolean acceptable = bdvEndTemp >= mdmtC;
if (!acceptable) {
    // Either:  thicker vessel, lower stress ratio, LTCS / 3.5%Ni material,
    //          slower BDV, or accept impact testing per UG-84.
}

Many company practices add a 5-10 °C margin between blowdown end-temperature and MDMT. Record the actual project or operator margin in the private task basis instead of hard-coding it in public guidance.

Method 4 — Source Term to Flare Network

double[] mdot = sim.massFlowSeries();
double[] T    = sim.temperatureSeries();
double[] P    = sim.pressureSeries();
// Pass to ReliefValveSizing peak-load aggregator or to
// FlareStack.estimateRadiationHeatFlux at peak ṁ.
double mdotPeak = sim.peakMassFlow();

This is the standard handoff between the depressurization model and the flare network sizing skill (neqsim-relief-flare-network).

Method 5 — Coupled Multi-Vessel Blowdown to a Shared Header (API 521 §7)

When several vessels blow down simultaneously into one flare/disposal header, the combined load — not any single vessel — sizes the header. MultiVesselBlowdownStudy superimposes each source on a common time grid and checks the header Mach at the peak.

import neqsim.process.safety.depressurization.MultiVesselBlowdownStudy;
import neqsim.process.safety.depressurization.MultiVesselBlowdownStudy.MultiVesselBlowdownResult;

MultiVesselBlowdownResult res = new MultiVesselBlowdownStudy()
    .addSource("V-100", bdvSim100)        // configured DepressurizationSimulator
    .addSource("V-200", bdvSim200)
    .addSourceResult("V-300", precomputed) // or a pre-computed DepressurizationResult
    .setHeader(0.6, 1.5, 288.15, 0.020, 1.30) // D[m], P[bara], T[K], M[kg/mol], gamma
    .setMaxAllowableMach(0.70)             // API 521 §7 / NORSOK P-002
    .run();

double peak   = res.getPeakTotalMassFlowKgPerS();
double tPeak  = res.getPeakTimeS();
double mach   = res.getHeaderMach();
boolean okMach = res.isHeaderMachAcceptable();
String report  = res.summary();

Use addSourceResult(...) with a pre-computed DepressurizationResult to avoid re-running the (slow) VU-flash transient for vessels already simulated.

Method 5b — Governed STID/TR2000 Dynamic Blowdown + Flare Handoff

For agentic engineering studies that start from STID/P&ID drawings, line lists, equipment lists, and TR2000 pipe/valve/material evidence, use the governed data source and runner instead of stitching transient notebooks together by hand.

Key classes:

• LineEquipmentListEvidence — reviewed line-list and equipment-list rows used to build the dynamic model.
• DynamicBlowdownFlareStudyDataSource — source-traceable package with one BlowdownSource per protected equipment item plus header, flare, PSV, fire, topology, and evidence status.
• DynamicBlowdownFlareStudyRunner — runs DepressurizationSimulator, aggregates loads with MultiVesselBlowdownStudy, sizes PSV orifices through ReliefValveSizing, and estimates peak/cumulative flare heat, emissions, radiation distance, and capacity utilization.
• DynamicBlowdownFlareStudyHandoff — versioned JSON package containing dynamic_blowdown_flare_result.v1 and dynamic_blowdown_flare_load_handoff.v1.

LineEquipmentListEvidence lineEq = LineEquipmentListEvidence.builder("line-eq-001")
    .lineListReviewed(true)
    .equipmentListReviewed(true)
    .addEquipment("V-100", "separator", 50.0, 85.0, 70.0, 313.15)
    .addLine("BD-100", "V-100", "FLARE-HDR", 6.0, 0.154, 0.007, 45.0, "DD100", "API 5L X52")
    .build();

DynamicBlowdownFlareStudyDataSource.BlowdownSource source =
    DynamicBlowdownFlareStudyDataSource.BlowdownSource.builder("V-100", gas)
        .equipmentTag("V-100")
        .vesselVolumeM3(50.0)
        .orificeDiameterM(0.035)
        .dischargeCoefficient(0.72)
        .backPressureBara(1.5)
        .api521FireCase(120.0, true, true)
        .psvBasis(85.0, 0.21, false, false)
        .build();

DynamicBlowdownFlareStudyDataSource data = DynamicBlowdownFlareStudyDataSource.builder("BD-FLARE-001")
    .lineEquipmentListEvidence(lineEq)
    .addSource(source)
    .flareHeader(0.6, 1.5, 288.15, 0.020, 1.30)
    .flareGeometry(0.8, 50.0, 0.20)
    .stidDiagramReviewed(true)
    .lineEquipmentListsReviewed(true)
    .vesselInventoryReviewed(true)
    .valveSizingBasisReviewed(true)
    .psvBasisReviewed(true)
    .flareSystemBasisReviewed(true)
    .fireCaseReviewed(true)
    .standardsReviewed(false)
    .build();

DynamicBlowdownFlareStudyHandoff handoff = DynamicBlowdownFlareStudyRunner.builder()
    .timeStepSeconds(1.0)
    .maxTimeSeconds(900.0)
    .build()
    .run(data);

Readiness semantics mirror the pipe-fire runner: missing source fluid, volume, BDV/orifice diameter, discharge coefficient, or flare backpressure blocks the calculation; missing reviewed topology, TR2000, PSV, fire, or flare capacity evidence keeps the result at screening level.

Method 6 — ESD Response-Time Budget (NOG 070 / IEC 61511)

The blowdown / isolation only mitigates the relief load if the ESD valve actually closes in time. EsdResponseTimeSimulator sums the SIF loop contributions and compares against the allowable budget.

import neqsim.process.safety.esd.EsdResponseTimeSimulator;
import neqsim.process.safety.esd.EsdResponseTimeSimulator.EsdResponseTimeResult;

EsdResponseTimeResult esd = new EsdResponseTimeSimulator()
    .setSifTag("SIF-2001 ESDV closure")
    .addDetection("PT-2001 detection", 2.0)        // s
    .addLogic("Logic solver scan + 2oo3 vote", 0.5)
    .addValve("ESDV-2001 close", 1.0, 18.0)        // solenoid delay, valve stroke
    .setAllowableResponseTimeS(45.0)
    .evaluate();

double total  = esd.getTotalResponseTimeS();
double margin = esd.getMarginS();
boolean ok    = esd.isWithinBudget();

This is a budgeting tool — it does not replace certified SIS proof testing or FAT/SAT. Pair with neqsim-process-safety for the SIL determination of the SIF.

Method 7 — Vessel Thermomechanical Safety Models

When a single-temperature lumped model is not enough — gas/liquid temperature bifurcation in a fire, transient PSV sizing conservatism, fast filling, cryogenic boil-off, through-wall thermal lag, or wall rupture — use the dedicated thermomechanical classes. They reproduce the application cases of Andreasen (2026), J. Loss Prev. Process Ind. 103, 106088, and are covered by committed regression tests. See docs/safety/vessel_thermomechanical_safety.md for the full guide.

// Two-temperature (non-equilibrium) fire blowdown — gas superheats, liquid stays cold
import neqsim.process.safety.depressurization.NonEquilibriumBlowdownModel;
import neqsim.process.safety.depressurization.NonEquilibriumBlowdownModel.NemResult;
NonEquilibriumBlowdownModel nem =
    new NonEquilibriumBlowdownModel(fluid, 10.0, 0.025, 0.72, 1.0e5);
nem.setFireExposure(0.9, 1100.0, 30.0, 25.0).setWall(8000.0, 470.0);
nem.setTimeStep(1.0).setMaxTime(600.0).setStopPressure(1.5e5);
NemResult bd = nem.run();
double bifurcationK = bd.maxTemperatureBifurcationK;

// Dynamic PSV sizing — quantify API 521 steady-state oversizing (§4.1)
import neqsim.process.safety.depressurization.DynamicPsvSizingStudy;
DynamicPsvSizingStudy.SizingComparison cmp =
    new DynamicPsvSizingStudy(gas, 1.0, 150000.0, 11.0e5, 0.21, 1.0e5)
        .setBlowdownFraction(0.1).setDischargeCoefficient(0.975).run();
double oversizing = cmp.oversizingRatio; // > 1 => steady-state conservative

// Fast filling of a Type IV hydrogen cylinder — liner temperature limits (§4.2)
import neqsim.process.safety.depressurization.VesselFillingSimulator;
VesselFillingSimulator.VesselFillingResult fill =
    new VesselFillingSimulator(h2, 0.06)
        .setInletConditions(283.15, 360.0, 0.015)
        .setTargetPressure(351.0)
        .setLinerTemperatureLimits(233.15, 338.15)
        .setTimeStep(1.0).setMaxTime(4000.0).run();
boolean linerOk = fill.linerLimitsMet;

// Cryogenic boil-off vs insulation thickness (§4.3)
import neqsim.process.util.heattransfer.BoilOffCalculator;
double boilOff = new BoilOffCalculator()
    .setSurfaceArea(150.0).setOuterFilmCoefficient(10.0)
    .setInsulationConductivity(0.025).setAmbientTemperatureK(288.15)
    .setFluidTemperatureK(253.15).setLatentHeat(320000.0)
    .boilOffRateKgPerH(0.30);

// Fire/blowdown wall rupture vs temperature-derated strength (§4.4)
import neqsim.process.safety.rupture.VesselRuptureAnalyzer;
import neqsim.process.safety.rupture.MaterialStrengthCurve;
MaterialStrengthCurve steel = MaterialStrengthCurve.carbonSteel("CS", 245.0e6, 415.0e6);
VesselRuptureAnalyzer.VesselRuptureResult rup =
    new VesselRuptureAnalyzer(0.5, 0.012, steel).analyze(timeS, pressurePa, metalTempK);
boolean ruptured = rup.ruptured; // bare LPG vessel ruptures in minutes; PFP prevents it

Supporting classes: CompositeWallConduction (1D transient multi-layer wall, Crank-Nicolson; use the static biotNumber(...) helper — lumped is fine for Bi < 0.1), VesselHeatTransferCorrelations (Woodfield filling Nusselt, Rohsenow nucleate boiling), and BlockedOutletOverpressureAnalyzer (blocked-in charging overpressure with relief-demand flag).

Common Pitfalls

• Adiabatic vs fire case — running adiabatic blowdown gives the coldest end-temperature (worst for MDMT). Running fire case gives the highest peak flow (worst for flare network). Both must be checked separately.
• Single component vs multi-component — MDMT is driven by the end-of-blowdown temperature, which depends on JT coefficient and is sensitive to ethane / propane content. Always use a representative composition, not a pure-methane simplification.
• Ignoring liquid level — vessels with liquid have huge thermal mass; the gas phase cools quickly while the liquid holds temperature. The simulator handles two-phase systems automatically.
• Choked vs sub-critical flow — the BDV chokes for most of the blowdown. Make sure the simulator's flow model uses choked-flow correlations until P_vessel / P_back < 1/r_critical.
• Stress ratio = 1 — using 1.0 for stress ratio gives the most conservative MDMT. Operating-pressure stress ratio (0.30–0.40) usually relaxes MDMT by 10–30 °C.

Verification Tests

./mvnw test -Dtest=DepressurizationSimulatorTest,MDMTCalculatorTest,MultiVesselBlowdownStudyTest,EsdResponseTimeSimulatorTest
./mvnw test -Dtest=DynamicPsvSizingStudyTest,VesselFillingSimulatorTest,VesselRuptureAnalyzerTest,BoilOffCalculatorTest

See Also

• neqsim-relief-flare-network — PSV sizing, flare radiation, header back-pressure
• neqsim-trapped-liquid-fire-rupture — blocked-in liquid thermal expansion, PFP demand, and rupture source-term handoff
• neqsim-dynamic-simulation — continuous-process transients with controllers
• neqsim-consequence-analysis — what happens after the released gas ignites
• neqsim-flow-assurance — JT cooling and hydrate formation in blowdown
• neqsim-process-safety — LOPA / SIL for the blowdown SIF (BDV-SIF)