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TOP EVENTS CONSEQUENCE ANALYSIS MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmailPowerPoint Presentation

TOP EVENTS CONSEQUENCE ANALYSIS MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

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TOP EVENTS CONSEQUENCE ANALYSIS MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

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TOP EVENTS CONSEQUENCE ANALYSIS MODELS Antony Thanos Ph.D. Chem. Eng. antony.thanos@gmail

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This project is funded by the European Union

Projekat finansira Evropska Unija

TOP EVENTSCONSEQUENCE ANALYSIS MODELS Antony ThanosPh.D. Chem. Eng.antony.thanos@gmail.com

- Consequence analysis framework

Release

scenarios

Accident

type

Hazard Identification

Event

trees

Dispersion models

Release models

Consequence

results

Release

quantification

Fire, Explosion Models

Domino effects

Limits of

consequence analysis

- Pool fire
- Ignition of flammable liquid phase

Main consequence

Thermal radiation

Liquid fuel tank fire

- Pool fire characteristics
- Pool dimensions (diameter, depth)
- Confined pool (liquid fuels tank/bund fire) :
- Tank fire pool : diameter equal to tank diameter dimension
- bunds : pool diameter estimated by equivalent diameter of bund

- Confined pool (liquid fuels tank/bund fire) :

- Pool dimensions (diameter, depth)

- Pool fire characteristics (cont.)
- Pool dimensions (diameter, depth) (cont.)
- Unconfined pool (LPG pool from LPG tank failure –no dike present):
- Theoretically maximum pool diameter is set by balance of release feeding the pool and combustion rate from pool

- Unconfined pool (LPG pool from LPG tank failure –no dike present):

- Pool dimensions (diameter, depth) (cont.)

Release

to pool

Combustion

rate

- Pool fire characteristics (cont.)
- Pool dimensions (diameter, depth) (cont.)
- Unconfined pool :
Min : release rate (kg/sec)

Mcomb : combustion rate (kg/sec)

mcomb : specific combustion rate (kg/m2.sec)

- In real life, pool is restricted by ground characteristics. Typical values for assumed depth: 0.5-2 cm (depending on ground type, higher values reported for sandy soils)

- Unconfined pool :

- Pool dimensions (diameter, depth) (cont.)

- Pool fire characteristics (cont.)
- Flame height, inclination (angle of flame from vertical due to wind)
- Long duration (hours to days)
- Combustion rate

- Pool fire models
- Combustion rate per pool surface on empirical equations (Burges, Mudan etc.)
- Example :

- Combustion rate per pool surface on empirical equations (Burges, Mudan etc.)

- Pool fire models (cont.)
- Combustion rate for liquids not exceeding 0.1 kg/m2.sec. Upper range for low boiling point hydrocarbons
- Flame dimension from empirical equations (Thomas, Pritchard etc.)
- Example, Thomas correlation :
- Big pools : Hf/Dp in the range of 1-2

- Pool fire models (cont.)
- Point source model (cont.)
- No flame shape taken into account
- A fraction of combustion energy is considered to be transmitted by ideal point in pool center

- Point source model (cont.)

Thermal radiation

transmitted

semi spherically

- Pool fire models (cont.)
- Point source model (cont.)
- Increased inaccuracies near pool end (important for Domino effects)

- Point source model (cont.)

Pool diameter

Flame height

Pool depth

- Pool fire models (cont.)
- Solid flame radiation model, radiation emitted via flame surface

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Calculation based on :
- flame shape (usually considered cylinder -tilted or not-),
- distance from flame (View Factor),
- emissive power (thermal radiation flux at flame surface)

- Calculation based on :

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Calculation equation :

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- View Factor : function of distance of receptor from flame and flame dimensions. Different equation for different flame shapes
- Transmissivity coefficient : Absorbance of thermal radiation by atmosphere components - e.g. humidity, CO2 –
- Correlation with relative humidity (R.H.) level and distance to “receptor”)
- High R.H, low transmissivity coefficient
- More important for far-field effects (due to increased distance)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power :
- Depending on pool size, substance
- For big pools, soot formation (20 kW/m2), masking of flame, significant reduction of average flame emissive power

- Emissive power :

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power : (cont.)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power : (cont.)
- Experimental Gasoline pool examples :
Dp=1 m, E=120 kW/m2

Dp=50 m, E= 20 kW/m2

- Medium to low emissive power for big pools (thermal radiation flux, up to 60 kW/m2 for liquid fuels)
- LPGs, LNG, provide higher emissive power (up to 150-270 kW/m2 for LPG, 250 kW/m2 for LNG)

- Experimental Gasoline pool examples :

- Emissive power : (cont.)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power : (cont.)
- One example of correlations available for max emissive power :

- Emissive power : (cont.)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power : (cont.)
- Final emissive power must take into account smoke production. Example correlations :
s, smoke coverage of surface

Dp, pool diameter (m)

Esmoke, emissive power of smoke (kW/m2)

- Final emissive power must take into account smoke production. Example correlations :

- Emissive power : (cont.)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- Solid flame radiation model (cont.)
- Emissive power : (cont.)
- Please be careful !!!!
- Make sure radiation fraction used is in-line with experimental data if available
- Evaluate calculation results for emissive power with experimental results, if available

- Please be careful !!!!

- Emissive power : (cont.)

- Solid flame radiation model (cont.)

- Pool fire models (cont.)
- UK HSE suggestions for LPGs :
- Emissive power : 200 kW/m2 over half flame height

- Some conservative assumptions
- For unconfined LPG cases, for theoretical pool calculation :
- butane fire instead of similar propane release (lower boiling rate, higher pool diameter
- low ambient temperature examined (as above)

- Low relative humidity examined (high transmissivity coefficient)

- For unconfined LPG cases, for theoretical pool calculation :

- UK HSE suggestions for LPGs :

- Pool fire models (cont.)
- Example results for propane pool fire Dp=10 m, wind speed 5 m/sec T=25 °C (confined fire, Aloha),
- flame height Hf : 21 m
- combustion rate M : 400 kg/min

- Example results for propane pool fire Dp=10 m, wind speed 5 m/sec T=25 °C (confined fire, Aloha),

- Pool fire models (cont.)
- Example results for Methanol tank, Dtank=20 m, H tank=20 m, T= 25 C°, atmospheric conditions D5, 2 in hole on tank shell at ground level (burning unconfined pool, Aloha)
- pool diameter Dp = 27 m
- flame length : 11 m

- Example results for Methanol tank, Dtank=20 m, H tank=20 m, T= 25 C°, atmospheric conditions D5, 2 in hole on tank shell at ground level (burning unconfined pool, Aloha)

- Fireball, BLEVE (Boiling Liquid Expanding Vapour Explosion)
- Rapid release and ignition of a flammable under pressure at temperature higher than its normal boiling point

Main consequence

Thermal radiation

Secondary consequences:

- Fragments (missiles)
- Overpressure

LPG BLEVE (Crescent City)

- Fireball/BLEVE characteristics
- Very rapid phenomenon (expanding velocity 10 m/sec)
- Limited duration (up to appr. 30 sec, even for very large tanks)
- Significant extent of fireball radius (in the order of 300 m for very big tanks, ≈ 4000 m3)
- Very high emissive power (in the order or 200-350 kW/m2)
- No precise capability for prediction of when it will happen (usual initial step for tanks exposed to heat -pool fire, jet flame-, opening of PSVs)

- Fireball/BLEVE characteristics and models (cont.)
- Radius and duration from correlations with tank content, example(AIChE CCPS) :
- t, duration (sec)
- m, mass (tn)
- No significant deviations for various correlations available, example results for full propane tank BLEVE (100 m3)

- Radius and duration from correlations with tank content, example(AIChE CCPS) :

- Fireball/BLEVE characteristics and models (cont.)
- Radius and duration from correlations with tank content, example(AIChE CCPS) :
- t, duration (sec)
- m, mass (kg)

- Radius and duration from correlations with tank content, example(AIChE CCPS) :

- Fireball/BLEVE characteristics and models (cont.)
- Mass in fireball calculations :
- Typically whole tank content (worst case approach.)
- Netherlands (BEVI method) : gas phase + 3 x flash fraction of liquid phase at failure pressure.
- For typical failure pressure in LPGs with hot BLEVEs, results to whole tank content
- For propane at usual atmospheric conditions, results to whole tank content

- Mass in fireball calculations :

- Fireball/BLEVE characteristics and models (cont.)
- Solid flame model
- radiation emitted via fireball surface,
- Usually fireball considered as sphere touching ground (conservative approach, adopted by UK HSE)

- Solid flame model

Evolution of fireball/BLEVE

- Fireball/BLEVE characteristics and models (cont.)
- Solid flame radiation model (cont.)
- Calculation based on :
- sphere shape at contact with ground,
- distance from fireball (sphere View Factor),
- fireball emissive power (thermal radiation flux at fireball surface)

- Calculation based on :

- Solid flame radiation model (cont.)

- Fireball/BLEVE characteristics and models(cont.)
- Solid flame radiation model (cont.)
- View Factor : function of distance of receptor from flame and fireball radius
- Transmissivity coefficient : as in pool fire case

- Solid flame radiation model (cont.)

- Fireball/BLEVE characteristics and models (cont.)
- Emissive power in fireball calculations :
- Correlations are available for emissive power calculation based on :
- vapour pressure at failure conditions (AIChE CCPS)
Pv, vapour pressure at failure (MPa)

- and/or mass involved, duration, size of fireball

- vapour pressure at failure conditions (AIChE CCPS)
- Experimental data provide values up to 350 kW/m2
- UK, HSE suggestion >270 kW/m2

- Correlations are available for emissive power calculation based on :

- Emissive power in fireball calculations :

- Fireball/BLEVE models (cont.)
- Example results (full 100 m3 propane tank BLEVE, Aloha)
- But, duration is only 13 sec. For limit values set in TDU (not in kW/m2), the relevant thermal radiation flux limit must be calculated

- Example results (full 100 m3 propane tank BLEVE, Aloha)

- Fireball/BLEVE models (cont.)
- Example results (full 100 m3 propane tank BLEVE, Aloha) (cont.)
- For t=13 sec,
- 1500 TDU corr. to 35 kW/m2
- 450 TDU corr. to 14 kW/m2
- 170 TDU corr. to 6.9 kW/m2

- For t=13 sec,

- Example results (full 100 m3 propane tank BLEVE, Aloha) (cont.)

- Jet flame
- Ignition of gas or two-phase release from pressure vessel

Main consequence

Thermal radiation

Propane jet flame test

- Jet flame characteristics
- Results as outcome of gas or two phase releases of flammable substances
- Cone shape
- Long duration (minutes to hours, depends on source isolation)
- Very high emissive power (in the order or 200 kW/m2)
- Soot expected, but not affecting radiation levels

- Jet flame models
- Combustion rate determined by release rate
- Dimensions from empirical equations. Example of simplified Mudan-Cross equation
L= jet flame length

d= release point diameter

Ct= fuel content permole in stoichiometric mix

of fuel/air

ΜWa= air molecular weight

MWf= fuel molecular weight

- Jet flame models (cont.)
- Dimensions from empirical equations. Example of simplified Considine-Grint equation for LPGs
L= jet flame length

M= release rate (kg/sec)

W= jet radius at flame tip (m)

- Dimensions from empirical equations. Example of simplified Considine-Grint equation for LPGs

- Jet flame models (cont.)
- Point source models
- Single point : all energy is released from flame “center”. Similar to relevant point source model for pool fires
- Multipoint source : several point along jet trajectory taken into account

- Point source models

- Jet flame models (cont.)
- Solid flame radiation model
- Radiation emitted via flame surface
- Calculation based on :
- shape (cylinder, tilted or not)
- distance (View Factor)
- emissive power

- Solid flame radiation model

- Jet flame models (cont.) (cont.)
- Solid flame radiation model (cont.)
- Calculation equation :

- Solid flame radiation model (cont.)

- Jet flame models (cont.)
- Solid flame radiation model (cont.)
- View Factor : function of distance of receptor from flame and flame dimensions for shape assumed
- Transmissivity coefficient : as in pool fires, fireball/BLEVE
- Emissive power : Estimated by flame dimension (surface) and energy released
E= Emissive power (kW/m2)

M= release rate (kg/s)

ΔΗc= combustion energy (kJ/s)

A= jet surface area, m2

Fs= fraction of combustion energy radiated

uj= expanding jet velocity (m/sec)

- Solid flame radiation model (cont.)

- Jet flame models conservative approaches
- Examination of horizontal jet
- Produce more extended thermal radiation zones
- Have direct effect via impingement in near by equipment

- Wind speed (for models taking into account flame distortion due to wind) :
- Vertical jets : High wind speed (UK HSE suggestion 15 m/sec)
- Horizontal jets : Low wind speed (UK HSE suggestion 2 m/sec)

- Examination of horizontal jet

- Jet flame models example results (cont.)
- Example results, 2 in hole in top of propane tank/gas phase, vertical jet (Aloha)

- END OF PART A

- Vapour cloud dispersion (cont.)
- Extent of cloud : dimensions, downwind/crosswind till specific endpoints (concentration)

- Vapour cloud dispersion (cont.)
- Endpoints :
- Toxics : several toxicity endpoints (e.g. IDLH, LC50)
- Flammables : LFL, ½ LFL
- Deaths expected within cloud limits where ignition is possible (Flash fire) due to thermal radiation and clothes ignition
- Reporting of LFL, ½ LFL is for theoretical extend of cloud, as no ignition is assumed on cloud path
- Very extended clouds expected for LPGs, especially in catastrophic failure cases (in the order of 500-1500 m)

- Endpoints :

- Vapour cloud dispersion (cont.)
- Endpoints : (cont.)
- Flammables : (cont.)
- Usually ignition sources outside establishment premises limit actual cloud
- Protection zones not justified to take into account flammable dispersion till LFL, ½ LFL

- Flammables : (cont.)

- Endpoints : (cont.)

- Vapour cloud dispersion (cont.)
- Example results for LPG dispersion (SLAB) at ground level centerline

- Vapour cloud dispersion (cont.)
- Example results for LPG dispersion (SLAB) (cont.) at ground level

- Vapour cloud (gas) dispersion (cont.)
- Release conditions affecting dispersion :
- substance properties (Boiling Point etc.)
- pressure, temperature at containment
- release rate and area
- release point height
- release direction (upwards –PSV-, horizontal)

- Release conditions affecting dispersion :

- Vapour cloud (gas) dispersion (cont.)
- Meteorological conditions affecting dispersion :
- atmospheric stability class (A-F),
- wind speed,
- air temperature,
- humidity (for some substances reacting with water as for example HF or other polar substances : SO2, NH3 etc.)
- Type of area : rural/industrial/urban, roughness factor

- Meteorological conditions affecting dispersion :

- Vapour cloud (gas) dispersion (cont.)
- Atmospheric stability :
- Expression of turbulent mixing in atmosphere. Related with atmospheric vertical temperature gradient (dT/dz)

- Atmospheric stability :

- Vapour cloud (gas) dispersion (cont.)
- Atmospheric stability : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Atmospheric stability : (cont.)
- Usually attributed to standardized class A-F (Pasquill)
- A : unstable, in combination with high winds favors dispersion
- D : neutral
- F : stable, minimum mixing in atmosphere

- Other parameter to attribute atmospheric stability, Monin-Obukhov length (positive for stable conditions, negative for unstable conditions)

- Usually attributed to standardized class A-F (Pasquill)

- Atmospheric stability : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Atmospheric stability : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Atmospheric stability : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Wind speed :
- Wind speed referred in meteorological data usually refer to measurement at 9-10 m height
- Boundary layer effect (variation of speed with height)

- Wind speed :

- Vapour cloud (gas) dispersion (cont.)
- Wind speed : (cont.)
- Simplified function :
p, function of :

- stability class
- surface roughness

- Simplified function :

- Wind speed : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Wind speed : (cont.)
- Variation with stability class for rural environment :

- Wind speed : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Meteorological conditions :
- Typical set under interest in Safety Reports :
- D5 : stability class D, uref=5 m/sec (unstable conditions)
- F2 : stability class D, uref=2 m/sec (stable conditions). Worst case for extent of vapour cloud, especially in heavy gas dispersion

- Typical set under interest in Safety Reports :

- Meteorological conditions :

- Vapour cloud (gas) dispersion (cont.)
- Type of surroundings : rural/industrial/urban
- Refers to variation of height in elements of surrounding
- Usually attributed via “roughness factor”

- Type of surroundings : rural/industrial/urban

- Vapour cloud (gas) dispersion (cont.)
- Type of surroundings (cont.)
- Conservative approach : open country (rural)

- Averaging time :
- Variation in time, due to turbulence, of wind characteristics :
- speed
- direction

- Variation in time, due to turbulence, of wind characteristics :

- Type of surroundings (cont.)

y

not exposed

x

T=0

average wind

direction

y

exposed

x

T=t

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- For continuous releases, concentration at constant location (x, y, z) is not constant

- Averaging time : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- increase of averaging time :
- plume boundaries widen
- concentration distribution flattens

- increase of averaging time :

- Averaging time : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- Very important to use averaging time in models, suitable to exposure time under interest
- Models may use parameters for certain averaging time, which might not be suitable for application in Safety Report. PLEASE ALWAYS CHECK !!!
- Gaussian models use implicit 10-min averaging time but…

- Averaging time : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- Toxics exposure usually under interest for period of 30 min (due to LC50 30 min endpoints etc.)
- Aloha-DEGADIS (Heavy Gas Dispersion) uses 5 min for toxics
- Ignition of flammable cloud is related with very low exposure time (time just for ignition to happen).
- Aloha-DEGADIS uses 10 sec for flammables (e.g. LPGs, no matter if toxic effect is examined)

- Averaging time : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- Example results for propane release from liquid phase piping (SLAB)

- Averaging time : (cont.)

Averaging time = 1 sec

Averaging time = 30 min

- Vapour cloud (gas) dispersion (cont.)
- Averaging time : (cont.)
- Reason for reporting both LFL, ½ LFL in flammable dispersion
- ½ LFL reporting contributes to uncertainty of averaging time (conservative approach)

- Reason for reporting both LFL, ½ LFL in flammable dispersion

- Averaging time : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) :
- Release of gas with density equal or higher than air

- Passive (neutral) dispersion (Gauss) :

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) :
- Basic characteristics:
- Maximum concentration at centreline
- Concentration reducing with increasing distance from source
- If release at ground level, maximum concentration at ground level

- Basic characteristics:

- Passive (neutral) dispersion (Gauss) :

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) :
- Basic equation for point source continuous release

- Passive (neutral) dispersion (Gauss) :

u, wind speed at z (m/sec)

M, release rate flow (kg/sec)

Hd, active release height (m)

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) : (cont.)
- σy, σz :
- functions of stability class with x and roughness factor
- usually given for 10-min averaging time
- proper correction of σy based on necessary averaging time is required

- σy, σz :

- Passive (neutral) dispersion (Gauss) : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) : (cont.)
- Example results for dispersion for NH3 release by 2 in hole in 6 bar gas vessel, D5 (Aloha)

- Passive (neutral) dispersion (Gauss) : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) : (cont.)
- Equations provided for point stationary source (no momentum)
- But jets of releases have significant momentum due to high velocity… modifications needed in model to take into account release momentum

- Passive (neutral) dispersion (Gauss) : (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Passive (neutral) dispersion (Gauss) : (cont.)
- For jets of releases modifications are needed, typical example : plume rise parameter to modify the release source point at downstream location

- Passive (neutral) dispersion (Gauss) : (cont.)

Modified

Original

- Vapour cloud (gas) dispersion (cont.)
- Flue gases dispersion
- Example : pool fire combustion products, e.g., SO2
- Special characteristics :
- Large area of source (e.g. tank area, bund area), (not point source). Modifications are available to models or sources are treated as point ones
- High temperature of flue gases
- Relevant plume rise equations provided for stacks (Briggs, Holland equations), provide unrealistic plume rise height

- Conservative approach, no plume rise, dispersion begins from flame end

- Flue gases dispersion

- Vapour cloud (gas) dispersion (cont.)
- Flue gases dispersion (cont.)
- Special atmospheric condition to be considered (temperature inversion conditions, trapped plume)

- Flue gases dispersion (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Plume rise effects
- Concentration at centrelines not continuously decreasing with distance
- Max concentration at centreline appears at distance from source

- Plume rise effects

- Vapour cloud (gas) dispersion (cont.)
- Plume rise effects (cont.)
- Example results from calculation of SO2 dispersion from dike fire of heating oil tank (D5, release rate 0.25 kg/sec SO2, dike equivalent diameter 66 m)

- Plume rise effects (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Heavy gas dispersion
- Special complex models
- CFD
- Box models (instant releases)
- Grounded plume models (continuous releases)

- Special complex models

- Heavy gas dispersion

- Vapour cloud (gas) dispersion (cont.)
- Heavy gas dispersion (cont.)
- Maximum concentration expected at centerline
- Concentration decreases with increasing distance
- More extended plume compared to neutral dispersion

- Heavy gas dispersion (cont.)

- Vapour cloud (gas) dispersion (cont.)
- Heavy gas dispersion (cont.)
- Meteorological data
- F2 produce more extended cloud

- Propane/butane cases
- same release source (e.g. same hole size) will produce more extended cloud for propane due to higher release rate

- Meteorological data

- Heavy gas dispersion (cont.)

- Vapour cloud (gas) dispersion models
- Heavy gas dispersion (cont.)
- Example results for propane release from 2 in hole in liquid phase of tank (D5) (Aloha)

- Heavy gas dispersion (cont.)

- Vapour cloud (gas) dispersion models
- Heavy gas dispersion (cont.)
- Example results for propane release from 2 in hole in liquid phase of tank (F2) (Aloha)

- Heavy gas dispersion (cont.)

- END OF PART B

- Vapour Cloud Explosion (VCE)
- Delayed ignition of flammable vapour cloud under partial confinement (obstacles within cloud) producing overpressure during flame front propagation

Main consequence

Overpressure

Secondary consequences:

- Fragments (e.g. broken glasses)

VCE results (Flixborough)

- Vapour Cloud Explosion (VCE) characteristics
- Very short duration (<1 sec)
- Models, high uncertainty due to several assumptions used in every model
- Type of models:
- CFD (FLACS, PHOENIX etc.)
- TNT blast charge (TNT equivalency)
- Air-fuel charge blast (Multi-Energy, Baker -Strehlow -Tang etc.)

- Vapour Cloud Explosion (VCE) models
- TNT equivalency model
- Simple, based on analogy with explosives effects
- A fraction of combustion energy released in cloud is attributed to produce overpressure
- The former energy fraction is recalculated as equivalent (on energy basis) mass of TNT
- The effects are defined based on known correlation of overpressure with TNT mass

- TNT equivalency model

- Vapour Cloud Explosion (VCE) models (cont.)
- TNT equivalency model (cont.)
- αe, refers to part of combustion energy released producing overpressure (1-10%)
- High uncertainty in both αe value and quantity of flammables (released mass –till what time ???-, mass within LFL-UFL) to be used
- Review on topic by TNO Yellow Book and AICheJ CCPS Guideline

- TNT equivalency model (cont.)

- Vapour Cloud Explosion (VCE) models (cont.)
- TNT equivalency model (cont.)
- Some comments/examples on selection of mass and αe :
- αe must be selected along with suitable flammable mass
- for αe1-5%, mass must not contain only the part of cloud in LFL-UFL section
- flammable mass must take into account not only gas but also liquid droplets (aerosol) in 2-phase releases
- Dow approach : mass defined by release rate and time to maximise LFL distance

- Some comments/examples on selection of mass and αe :

- TNT equivalency model (cont.)

- Vapour Cloud Explosion (VCE) models (cont.)
- TNT equivalency model (cont.)
- Some comments/examples on selection of mass and αe : (cont.)
- mass defined by time to reach ignition source or time to stop release (time for energizing isolation valves)
- HSE suggests TNT mass double the gas mass in confined areas

- Explosives blast and VCE present differences, as explosives have short duration higher shock wave peak values.
- TNT equivalency model is approximation of phenomenon based on statistical analogies

- Some comments/examples on selection of mass and αe : (cont.)

- TNT equivalency model (cont.)

- Vapour Cloud Explosion (VCE) models (cont.)
- TNT equivalency model (cont.)
- Overpressure calculated by diagram
for distances required

- Uncertainty on centre of explosion
to be considered

- Similar diagrams for positive phase
duration, impulse

- Overpressure calculated by diagram

- TNT equivalency model (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- TNO Multi-Energymodel :
- Only confined areas of cloud are considered
- Partial explosions from confined areas expected
- Energy released assuming stoichiometric combustion, based on air contained in areas taken into account (average 3.5 MJ/m3 of air for most hydrocarbons) uniform concentration of flammable in confined areas assumed

- TNO Multi-Energymodel :

- Vapour Cloud Explosion (VCE) (cont.)
- TNO Multi-Energymodel : (cont.)
- Overpressure from Berg graph
using Sachs distance

- Overpressure from Berg graph

- TNO Multi-Energymodel : (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- TNO Multi-EnergyModel : (cont.)
- Similar graphs for positive phase duration, dynamic pressure
- Blast strength 10 : detonation, explosives case, not valid for VCEs as propagation of blast via detonation requires high homogeneity in cloud

- TNO Multi-EnergyModel : (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- TNO Multi-EnergyModel : (cont.)
- Disadvantage : complex empirical rules for (TNO Yellow Book, Assael) :
- definition of confined areas
- definition of successive or simultaneous blast in confided areas
- selection ofblast strength (confinement increase, increases blast strength

- HSE suggests blast strengths 2 and 7

- Disadvantage : complex empirical rules for (TNO Yellow Book, Assael) :

- TNO Multi-EnergyModel : (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- Baker-Strehlow-Tang model
- Similar principles as TNO Multi-Energy model
- confined areas only taken into account
- stoichiometry of air with fuel in confined areas

- Gas type “reactivity” (susceptibility to flame front acceleration) taken also into account along with obstacle density
- methane, CO : low reactivity
- H2, acetylene, ethylene/propylene oxide : high reactivity
- other substances : medium

- Similar principles as TNO Multi-Energy model

- Baker-Strehlow-Tang model

- Vapour Cloud Explosion (VCE) (cont.)
- Baker-Strehlow-Tang model (cont.)
- Flame speed defined by table on gas reactivity and confinement type

- Baker-Strehlow-Tang model (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- Baker-Strehlow-Tang model (cont.)
- Overpressure from graph using Sachs distance and flame speed
- Energy to be used double to actual as graph presents free air blast (not surface blast)

- Overpressure from graph using Sachs distance and flame speed

- Baker-Strehlow-Tang model (cont.)

- Vapour Cloud Explosion (VCE) (cont.)
- Example results for propane release from 2 in hole in liquid phase of tank (D5) (Aloha, Baker-Strehlow-Tang method)

Ignition time 2 min

Composite for unknown ignition time

- Pesticides fires and dispersion
- Variation of stored substances quantities within year due to seasonal production of some products.
- Evaluation of stored quantities distribution could be required to evaluate quantities to be taken into account in calculations.

- Some times, active substance stored in powder form
- Specific combustion rate rather low (TNO Green Book) : in the range of 0.02 kg/m2.sec

- Variation of stored substances quantities within year due to seasonal production of some products.

- Pesticides fires and dispersion(cont.)
- Special characteristic of pesticides : when burnt, not all substance is consumed, flue gases contain unburned pesticide substances (“survivor” fraction)
- In case of fire, dispersion of flue gases must examine :
- Combustion products (e.g. SO2, HCl, NO2 etc.)
- Unburned pesticide active substance

- Pesticides fires and dispersion(cont.)
- UK HSE suggests stoichiometric conversion of S, Cl to SO2 and HCl.
- Conversion ratios :
- C to CO : 5%
- N to HCN and NO2 : 5%

- According to TNO Green Book, formation rate of NO2, HCN and NO decreases with this order, Taking into account the similar toxicity of the former, conversion of N to NO2 only is conservative

- Pesticides fires and dispersion (cont.)
- Survivor fraction in flue gases : 0.5-10% of combustion rate of substance at source
- Lower survivor fractionfor high boiling point substances
- UK HSE suggests survivor fraction 10% otherwise justification must be provided

- Pesticides fires and dispersion (cont.)
- Especially in closed warehouse cases :
- what is plume rise for flue gases ??
- which is the combustion rate ??

- Plume rise in warehouse flue gases
- For fire in full development plume rise might be high, but potentially low in initial phase of fire
- Typical equations fail, as producing unrealistic plume rise

- Especially in closed warehouse cases :

- Pesticides fires and dispersion (cont.)
- Combustion rate, affected by type of fire
- Roof collapse
- As in open area, fuel controlling
- High flue gas temperature

- Ventilation controlled
- roof intact, some window breakage (limited release area)
- fire rate controlled by availability of oxygen in warehouse
- low temperature of flue gases, low plume rise

- Roof collapse

- Combustion rate, affected by type of fire

- Pesticides fires and dispersion (cont.)
- UK HSE suggests :
- plume rise set at max 50 m
- calculations for source via a few m2 area of window (nevertheless, recognized as pessimistic), (NTUA methodology refers to 3 m2)
- special models

- UK HSE suggests meteorological condition to be examined as worst case ones :
- F2
- D5, D15 with low inversion height (400 m)

- UK HSE suggests :

- Pesticides fires and dispersion (cont.)
- NTUA methodology suggests the following cases :
- Roof collapse : flue gas rate 8 kg/m2.sec (per warehouse area), T=500 °C
- Ventilation controlled (roof intact) : flue gas rate 5 kg/m2.sec (per opening area, assumed 3 m2), T=140 °C

- NTUA methodology suggests the following cases :

- Pesticides fires and dispersion (cont.)
- NTUA methodology classification of fires (as per HSE FIRE Pest II computer program)

- Pesticides fire and dispersion (cont.)
- Survivor fraction according to NTUA methodology (as per HSE, Risk Assessment Method for Warehouses 1995)

- Accidents with effects to environment
- No mature and wide-used quantitative models for estimation of effects to environment
- Qualitative models (applied some times, examples :
- Energy Institute (ex. IP) Screening Tool
- Belgium (Flanders) Richtlijn Milieurisicoanalyse
- IPC Guidance Note on Storage and Transfer of Materials for Scheduled Activities, Irish EPA

- No unique approach in EU members (in many countries no specific approach) in relevant requirements

- Literature for Top Events Consequence Analysis Models
- Lees’ Loss Prevention in the Process Industries, Elsevier Butterworth Heinemann, 3nd Edition, 2005
- Methods for the Calculation of Physical Effects due to Releases of Hazardous Materials (Liquids and Gases), Yellow Book, CPR 14E, VROM, 2005
- Methods for the Determination of Possible Damage to People and Objects Resulting from Releases of Hazardous Materials , Green Book, CPR 16E, TNO, 1992
- Guidelines for Chemical Process Quantitative Risk Analysis, CCPS-AICHE, 2000
- Guidelines for Consequence Analysis of Chemical Releases, CCPS-AICHE, 1999
- Guidelines for Evaluating the Characteristics of Vapour Cloud Explosions, Flash Fires and BLEVEs, CCPS-AICHE, 1994

- Literature for Top Events Consequence Analysis Models (cont.)
- Safety Report Assessment Guides (SRAGs), Health and Safety Executive, UK
- Risk Assessment Methods for Warehouses - Computer Program FIREPEST II, Health and Safety Executive, 1997
- Assael M., Kakosimos K., Fires, Explosions, and Toxic Gas Dispersions, CRC Press, 2010 Benchmark Exercise in Major Accident Hazard Analysis, JRC Ispra, 1991
- Rew P., Humbert W., Development of Pool Fire Thermal Radiation Model, HSE Contract Research Report 96, 1996
- McGrattan K., Baum H., Hamins A. Thermal Radiation from Large Pool Fires, National Institute of Standards and Technology, NISTIR 6546, Nov 2000
- Taylor J., Risk Analysis for Process Plant, Pipelines and Transport, E&FN SPON, 1994

- Literature for Top Events Consequence Analysis Models (cont.)
- Drysdale D., Fire Dynamics, J. Wiley and Sons, 2nd Edition, 1999
- Beychok M., Fundamentals of Stack Gas Dispersion, 3rd Edition, 1994
- C. Delvosalle, F. Benjelloun, C. Fiévez,, A Methodology for Studying Domino Effects, Faculté Polytechnique de Mons,Ministere Federal de l’;Emploi et du Travail, July 1998
- RIVM, Reference Manual Bevi Risk Assessments, 2009
- ALOHA, Users Manual, US EPA, 2007
- ALOHA Two Day Training Course Instructor's Manual
- Environmental risk assessment of bulk storage facilities: A screening tool, EI, 2009
- Richtlijn Milieurisicoanalyse, 2006

- Literature for Top Events Consequence Analysis Models (cont.)
- IPC Guidance Note on Storage and Transfer of Materials for Scheduled Activities, Irish EPA, 2004
- N. Markatos, NTUA, Chemical Engineering Department, Methodology of Assessment of Consequence from fire in Pesticide installations, 2001 (in Greek)