Emergency depressurization (blowdown) per API 521 §5.20 and minimum design metal temperature (MDMT) assessment per ASME UCS-66 / API 579 / EN 13445 — VU-flash transient inventory model, time-to-target-pressure, low-temperature embrittlement screening, and integration with PSV/flare loads. USE WHEN: a task requires sizing a blowdown valve, generating a P-vs-time curve for a vessel under fire / depressurization, checking MDMT against blowdown end-temperature, providing source terms for relief and flare networks, or distinguishing blowdown from trapped-liquid fire rupture screening. Anchors on neqsim.process.safety.depressurization.DepressurizationSimulator and neqsim.process.safety.mdmt.MDMTCalculator.
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Emergency depressurization (blowdown) per API 521 §5.20 and minimum design metal temperature (MDMT) assessment per ASME UCS-66 / API 579 / EN 13445 — VU-flash transient inventory model, time-to-target-pressure, low-temperature embrittlement screening, and integration with PSV/flare loads. USE WHEN: a task requires sizing a blowdown valve, generating a P-vs-time curve for a vessel under fire / depressurization, checking MDMT against blowdown end-temperature, providing source terms for relief and flare networks, or distinguishing blowdown from trapped-liquid fire rupture screening. Anchors on neqsim.process.safety.depressurization.DepressurizationSimulator and neqsim.process.safety.mdmt.MDMTCalculator.
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 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;
SystemInterfacegas=newSystemSrkEos(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 inventoryDepressurizationSimulatorsim=newDepressurizationSimulator(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 timestepdouble[] t = sim.timeSeries();
double[] p = sim.pressureSeries();
double[] T = sim.temperatureSeries();
double[] m = sim.massFlowSeries();
doublepEnd= p[p.length - 1];
doubletEnd= T[T.length - 1];
doublet50= 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:
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.
Guess BDV Cd · A, run sim.run(...), read sim.timeToPressure(target).
Iterate area until target is met without choking the flare header.
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;
MDMTCalculatormdmt=newMDMTCalculator();
mdmt.setMaterial("SA-516-70N"); // normalised CMn, common for CS vessels
mdmt.setThicknessMM(50.0);
mdmt.setStressRatio(0.35); // operating / allowable stress ratiodoublemdmtC= mdmt.computeUCS66(); // °C — ASME UCS-66 + UCS-66.1 reduction
The calculator implements:
UCS-66 Curve A / B / C / D lookup vs material specification
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
doublebdvEndTemp= sim.minTemperatureC(); // °C from blowdown simbooleanacceptable= 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.doublemdotPeak= 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.
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.
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.
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 coldimport neqsim.process.safety.depressurization.NonEquilibriumBlowdownModel;
import neqsim.process.safety.depressurization.NonEquilibriumBlowdownModel.NemResult;
NonEquilibriumBlowdownModelnem=newNonEquilibriumBlowdownModel(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);
NemResultbd= nem.run();
doublebifurcationK= bd.maxTemperatureBifurcationK;
// Dynamic PSV sizing — quantify API 521 steady-state oversizing (§4.1)import neqsim.process.safety.depressurization.DynamicPsvSizingStudy;
DynamicPsvSizingStudy.SizingComparisoncmp=newDynamicPsvSizingStudy(gas, 1.0, 150000.0, 11.0e5, 0.21, 1.0e5)
.setBlowdownFraction(0.1).setDischargeCoefficient(0.975).run();
doubleoversizing= 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.VesselFillingResultfill=newVesselFillingSimulator(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();
booleanlinerOk= fill.linerLimitsMet;
// Cryogenic boil-off vs insulation thickness (§4.3)import neqsim.process.util.heattransfer.BoilOffCalculator;
doubleboilOff=newBoilOffCalculator()
.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;
MaterialStrengthCurvesteel= MaterialStrengthCurve.carbonSteel("CS", 245.0e6, 415.0e6);
VesselRuptureAnalyzer.VesselRuptureResultrup=newVesselRuptureAnalyzer(0.5, 0.012, steel).analyze(timeS, pressurePa, metalTempK);
booleanruptured= 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
