Consequence Modeling of BLEVE Events Using PHAST - Chemical Engineering Knowledge Platform

Industry Applications

Refineries, LNG terminals, chemical manufacturing, rail/road transport depots

Key Standards

API RP 752, NFPA 58, CCPS Guidelines, EN 14015

Typical Scale

BLEVE overpressure > 50 kPa at 100 m; fireball diameter 30–120 m for 20–100 tonne inventories

⚠️ Why It Matters

1

Inadequate vessel insulation or fire exposure

↓

2

Rapid wall temperature rise beyond yield strength

↓

3

Loss of structural integrity under pressure

↓

4

Uncontrolled vapor release with flash fraction > 20%

↓

5

Thermal radiation hazard exceeding 15 kW/m² at 100 m

↓

6

Fatalities, facility destruction, and domino effect ignition

📘 Definition

Boiling Liquid Expanding Vapor Explosion (BLEVE) is a catastrophic failure mode in pressurized vessels containing liquefied gases above their atmospheric boiling point, triggered by loss of containment integrity under thermal stress—resulting in rapid phase transition, supersonic vapor expansion, and potential fireball formation if flammable. It is governed by thermodynamic instability, vessel wall failure mechanics, and two-phase flow dynamics during rupture.

🎨 Concept Diagram

AI-generated illustration for visual understanding

💡 Engineering Insight

BLEVE modeling is not about predicting *if* a vessel will fail—it’s about quantifying *how much energy releases, how fast, and where it goes*. PHAST’s value lies not in its default correlations, but in how rigorously you constrain inputs: flash fraction errors of ±0.05 shift thermal exclusion zones by ±35 m; wall temperature uncertainty of ±50 °C changes failure time by >3 minutes—enough to activate mitigation systems. Always anchor PHAST inputs to measured or validated thermodynamic states—not textbook averages.

📖 Detailed Explanation

A BLEVE begins when external heating (e.g., pool fire) raises the vessel wall temperature beyond the point where yield strength drops below internal pressure. Unlike mechanical rupture, BLEVE is thermally driven: the liquid remains subcooled until containment fails, then flashes explosively due to stored enthalpy. PHAST models this using simplified thermodynamic equilibrium assumptions—primarily the ‘TNT equivalence’ method for blast and the ‘Hansel’ or ‘Mudan’ correlations for fireball growth.

PHAST’s BLEVE module relies on five core inputs: inventory mass, liquid temperature, saturation pressure, ambient conditions, and vessel geometry. The software calculates flash fraction using the homogeneous equilibrium model (HEM), assuming instantaneous mixing and equilibrium between vapor and liquid phases post-rupture. For accuracy, users must replace PHAST’s default HEM flash fraction with values derived from rigorous PVT data—especially for non-ideal mixtures like propylene-propane blends or refrigerated ammonia.

Advanced modeling includes coupling PHAST with CFD tools (e.g., FLACS) for asymmetric fireball development, or integrating time-resolved wall heating from ANSYS Mechanical to drive dynamic failure timing. Recent CCPS guidance (2022) emphasizes ‘failure-time-aware’ modeling: instead of assuming instantaneous rupture, simulate heat-up curves to determine *when* failure occurs—enabling realistic assessment of whether deluge systems can prevent BLEVE altogether. This shifts the analysis from consequence estimation to prevention verification.

🔄 Engineering Workflow

Step 1

Step 1: Identify hazardous inventory (mass, composition, phase state, operating T/P)

→

Step 2

Step 2: Determine credible failure scenario (fire exposure duration, vessel geometry, material grade)

→

Step 3

Step 3: Calculate flash fraction using REFPROP or NIST WebBook thermodynamic data

→

Step 4

Step 4: Configure PHAST BLEVE module: select fireball/blast model, define terrain, humidity, wind

→

Step 5

Step 5: Run consequence simulations for overpressure, thermal radiation, and toxic dispersion (if applicable)

→

Step 6

Step 6: Generate exclusion zones, verify against API RP 752 siting criteria and NFPA 58 separation distances

→

Step 7

Step 7: Integrate results into QRA and safeguard verification (e.g., deluge system adequacy, blast walls)

📋 Decision Guide

Rock/Field Condition	Recommended Design Action
Liquefied petroleum gas (LPG) storage > 10 tonnes, no water spray protection	Model full BLEVE with fireball using PHAST’s 'Fireball' module; apply 100% flash fraction default unless PVT data available
Ammonia tank exposed to pool fire > 10 min duration	Use PHAST ‘Thermal Failure’ submodel with time-dependent wall heating; assign 0.15 flash fraction based on NH₃ saturation data at 120 °C
Chlorine rail tank with insulated jacket and emergency depressurization system	Model sequential failure: first depressurization (reducing flash fraction), then BLEVE only if jacket fails; use PHAST ‘Depressurization + BLEVE’ workflow

📊 Key Properties & Parameters

Flash Fraction

0.1–0.4 (dimensionless)

Mass fraction of liquid that instantly vaporizes upon pressure release, determined by thermodynamic state relative to saturation curve.

⚡ Engineering Impact:

Directly scales blast overpressure magnitude and fireball diameter; values >0.25 dominate PHAST consequence output.

Vessel Wall Temperature

300–750 °C for carbon steel vessels

Metal temperature at time of failure, critical for determining residual strength and failure mode (ductile vs. brittle).

⚡ Engineering Impact:

Controls failure timing and rupture geometry—higher temperatures reduce tensile strength by up to 80%, accelerating fragmentation.

Saturation Pressure

0.7–2.5 MPa for LPG at 40–60 °C

Vapor pressure of the stored liquid at its bulk temperature, defining the driving force for expansion upon rupture.

⚡ Engineering Impact:

Higher saturation pressure increases initial expansion velocity and shock front intensity—dominant input for PHAST’s TNT-equivalent scaling.

Vapor Expansion Ratio

150–400 (dimensionless) for propane, butane, and ammonia

Ratio of saturated vapor specific volume to liquid specific volume at storage conditions.

⚡ Engineering Impact:

Determines volumetric expansion energy; ratios >200 produce supersonic expansion fronts detectable by PHAST’s shock wave model.

Fireball Duration

1.5–12 s for hydrocarbon BLEVEs (depends on mass and flame speed)

Time interval during which the ignited vapor cloud sustains self-sustaining combustion and radiates thermal energy.

⚡ Engineering Impact:

Drives thermal dose calculation in PHAST; durations >5 s exceed 3rd-degree burn threshold at distances >150 m.

📐 Key Formulas

Flash Fraction (HEM)

FF = (h - h_f) / (h_fg)

Mass fraction vaporized upon sudden pressure release, calculated from specific enthalpy balance.

Variables:

Symbol	Name	Unit	Description
FF	Flash Fraction		Mass fraction vaporized upon sudden pressure release
h	Specific Enthalpy of System	kJ/kg	Specific enthalpy of the fluid before pressure release
h_f	Specific Enthalpy of Saturated Liquid	kJ/kg	Specific enthalpy of saturated liquid at final pressure
h_fg	Latent Heat of Vaporization	kJ/kg	Difference between specific enthalpy of saturated vapor and saturated liquid at final pressure

Typical Ranges:

Propane at 40 °C

0.22–0.30

Butane at 20 °C

0.12–0.18

Ammonia at 50 °C

0.14–0.21

⚠️ Use REFPROP or NIST data; avoid generic HEM defaults if T > 0.7·T_c

Fireball Diameter (Mudan Correlation)

D_fb = 2.7 × M^(1/3)

Empirical fireball diameter (m) as function of flammable mass (kg).

Variables:

Symbol	Name	Unit	Description
D_fb	Fireball Diameter	m	Empirical fireball diameter
M	Flammable Mass	kg	Mass of flammable material

Typical Ranges:

LPG BLEVE (10–100 t)

32–115 m

Chlorine (non-flammable)

N/A — no fireball

⚠️ Valid only for hydrocarbons; do not apply to toxic-only releases

Overpressure (TNT Equivalence)

ΔP = 0.11 × (W_TNT / R)^1.33

Peak side-on overpressure (bar) at distance R (m) from equivalent TNT mass W_TNT (kg).

Variables:

Symbol	Name	Unit	Description
ΔP	Peak side-on overpressure	bar	Overpressure at distance R from the TNT equivalent explosion
W_TNT	Equivalent TNT mass	kg	Mass of TNT that produces equivalent blast effect
R	Distance from explosion	m	Radial distance from the center of the explosion to the point of interest

Typical Ranges:

40 t propane BLEVE → 1,200 kg TNT eq.

0.05–0.3 bar at 100 m

⚠️ Use only for R > 5× D_fb; invalid inside fireball zone

🏭 Engineering Example

BASF Ludwigshafen Site – Propane Storage Yard

N/A (vessel material: ASTM A516 Gr. 70 carbon steel)

Inventory Mass

42,000 kg

Liquid Temperature

45 °C

Saturation Pressure

1.62 MPa

Vessel Wall Temp at Failure

612 °C

Fireball Diameter (PHAST output)

89 m

Flash Fraction (REFPROP-derived)

0.28

🏗️ Applications

Process safety barrier design
Land-use planning for tank farms
Emergency response planning
QRA for facility siting

📋 Real Project Case

Ammonia Refrigeration System HAZOP & LOPA Integration at Midwest Food Processing Plant

Retrofit of legacy ammonia chiller system serving 300k sq ft food processing facility

Challenge: Unplanned releases during maintenance due to undocumented isolation points and missing P&IDs

Read full case study →

Ethylene Oxide Sterilization Facility QRA and Bow-Tie Implementation Grain Elevator Dust Explosion Mitigation Using ASTM E1226-Based Risk Model Offshore LNG Transfer System Fault Tree Analysis and SIS Architecture Optimization

🎨 Technical Diagrams

📚 References

[1]

Guidelines for Hazard Evaluation Procedures — CCPS (Center for Chemical Process Safety)

[2]

API RP 752 – Management of Hazards Associated with Location of Process Plant Buildings — American Petroleum Institute

[3]

NFPA 58 – Liquefied Petroleum Gas (LPG) Code — National Fire Protection Association

[4]

PHAST User Manual v6.9 — DNV GL

🔗 Related Knowledge

Escalation Vector Analysis for Fire-Induced Domino Effects