neqsim-depressurization-mdmt

name	neqsim-depressurization-mdmt
version	1.0.0
description	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.
last_verified	2026-04-26
requires	{"java_packages":["neqsim.process.safety.depressurization","neqsim.process.safety.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).

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

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)