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

This Project is funded by the European Union

Project implemented by Human Dynamics Consortium

This project is funded by the European Union

Projekat finansira Evropska Unija

TOP EVENTSCONSEQUENCE ANALYSIS MODELS

Antony ThanosPh.D. Chem. [email protected]



• Consequence analysis framework

Releasescenarios Release

scenarios Accident

typeAccident

typeEvent

trees

Releasequantification

Releasequantification

Hazard

Identification

Release models

Consequenceresults

Consequenceresults

Domino effectsDomino effectsLimits of

consequence analysis

Dispersion models

Fire, Explosion Models



• Pool fire Ignition of flammable liquid phase

Liquid fuel tank fire

Main consequenceThermal radiation




• Pool fire characteristics Pool dimensions (diameter, depth)

o 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

bund

p

AD

4



• Pool fire characteristics (cont.) Pool dimensions (diameter, depth)

(cont.)

o 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

Combustionrate

Release to pool



• Pool fire characteristics (cont.) Pool dimensions (diameter, depth) (cont.)

o 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)

DepthVA

AmMM

poolpool

combcombin

/



• 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.)

o Example :

v

Cab

abpv

Cab

H

HmgasesliquefiedTT

TTCH

HmTT

001.0)(

)(

001.0

m = specific comb.rate (kg/m2.sec) Tb = Boiling point βρασμού (Κ) Ta = ambient temperature (K) ΔHc = Combustion heat (J/kg) ΔHv = latent heat (J/kg) Cp = liquid heat capacity (J/kg.K)



• 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.)

o Example, Thomas correlation :

o Big pools : Hf/Dp in the range of 1-2

61.0

42

pap

f

Dg

m

D

H

Hf= flame height Dp= pool diameter m= specific comb.rate ρa= air density g=gravity constant



• Pool fire models (cont.) Point source model (cont.)

o No flame shape taken into account

o A fraction of combustion energy is considered to be transmitted by ideal point in pool center

Thermal radiationtransmittedsemi spherically



• Pool fire models (cont.) Point source model (cont.)

o Increased inaccuracies near pool end (important for Domino effects)

24 x

tHMfq aC

q = thermal radiation flux at “receptor” (kW/m2) f = thermal radiation fraction (0.1-0.4, depending on

substance and pool size. Big pools, low values) Μ = combustion rate (kg/s) ΔHc= combustion heat (kj/kg) ta = transmissivity coeff., x = distance of pool center from “receptor”



• Pool fire models (cont.) Solid flame radiation model,

radiation emitted via flame surface

Pool diameter

Flame height

Pool depth



• Pool fire models (cont.) Solid flame radiation model (cont.)

o Calculation based on : flame shape (usually

considered cylinder -tilted or not-),

distance from flame (View Factor),

emissive power (thermal radiation flux at flame surface)




o Calculation equation :

atEVFq q = thermal radiation flux at “receptor” (kW/m2) VF = view factor for flame shape at receptor Ε = emissive power (kW/m2) ta = transmissivity coefficient




o View Factor : function of distance of receptor from flame and flame dimensions. Different equation for different flame shapes

o 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 coefficientMore important for far-field effects (due

to increased distance)




o Emissive power : Depending on pool size, substanceFor big pools, soot formation (20

kW/m2), masking of flame, significant reduction of average flame emissive power




o Emissive power : (cont.)




o 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)




o Emissive power : (cont.)One example of correlations

available for max emissive power :

p

f

D

HfHcm

E41

max

Ε= emissive power (kW/m2) m= specific combustion rate (kg/m2.sec) ΔHc= combustion heat (kJ/kg) f = thermal radiation ratio Hf = flame length Dp = pool diameter




o 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)

)1(20140

)1(12.012.0

max

pp DD

smoke

eeE

sEsEE




o 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



• Pool fire models (cont.) UK HSE suggestions for LPGs :

o Emissive power : 200 kW/m2 over half flame height

Some conservative assumptions o 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)

o Low relative humidity examined (high transmissivity coefficient)



• Pool fire models (cont.) Example results for propane pool fire

Dp=10 m, wind speed 5 m/sec T=25 °C (confined fire, Aloha),

o flame height Hf : 21 m

o combustion rate M : 400 kg/min



• 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)

o pool diameter Dp = 27 m

o flame length : 11 m



• Fireball, BLEVE (Boiling Liquid Expanding Vapour Explosion)

Rapid release and ignition of a flammable under pressure at temperature higher than its normal boiling point

LPG BLEVE (Crescent City)



Secondary consequences: oFragments (missiles)oOverpressure



• 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) :

o t, duration (sec)

o m, mass (tn)

o No significant deviations for various correlations available, example results for full propane tank BLEVE (100 m3)

tnmmt

tnmmt

mD

306.2

3045.0

8.5

6/1

3/1

3/1

ADL TNO AIChE

Aloha

Radius (max), m

122 112 110 106

Duration, sec

16 14 16 13



• Fireball/BLEVE characteristics and models (cont.) Radius and duration from

correlations with tank content, example (AIChE CCPS) :

o t, duration (sec)

o m, mass (kg)

tnmmt

tnmmt

mD

306.2

3045.0

8.5

6/1

3/1

3/1



• Fireball/BLEVE characteristics and models (cont.) Mass in fireball calculations :

o Typically whole tank content (worst case approach.)

o 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



• Fireball/BLEVE characteristics and models (cont.) Solid flame model

o radiation emitted via fireball surface,

o Usually fireball considered as sphere touching ground (conservative approach, adopted by UK HSE)

Evolution of fireball/BLEVE



• Fireball/BLEVE characteristics and models (cont.) Solid flame radiation model (cont.)

o Calculation based on : sphere shape at contact with

ground, distance from fireball (sphere View

Factor), fireball emissive power (thermal

radiation flux at fireball surface) atEVFq

q = thermal radiation flux at “receptor” (kW/m2) VF = view factor ar receptor for sphere shape fireball Ε = emissive power (kW/m2) ta = transmissivity coefficient



• Fireball/BLEVE characteristics and models (cont.) Solid flame radiation model (cont.)

o View Factor : function of distance of receptor from flame and fireball radius

o Transmissivity coefficient : as in pool fire case



• Fireball/BLEVE characteristics and models (cont.) Emissive power in fireball calculations :

o Correlations are available for emissive power calculation based on :

o vapour pressure at failure conditions (AIChE CCPS)

Pv, vapour pressure at failure (MPa)

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

o Experimental data provide values up to 350 kW/m2

o UK, HSE suggestion >270 kW/m2

39.0235 vPE



• Fireball/BLEVE models (cont.) Example results (full 100 m3 propane

tank BLEVE, Aloha)

o 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



• Fireball/BLEVE models (cont.) Example results (full 100 m3

propane tank BLEVE, Aloha) (cont.)

o 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



• Jet flame Ignition of gas or two-phase

release from pressure vessel

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 lengthd= release point diameterCt= fuel content per mole in stoichiometric mix of fuel/airΜWa= air molecular weightMWf= fuel molecular weight

f

a

MW

MW

Ctd

L 15



• Jet flame models (cont.) Dimensions from empirical

equations. Example of simplified Considine-Grint equation for LPGs

L= jet flame lengthM= release rate (kg/sec)W= jet radius at flame tip (m)

LW

ML

25.0

1.9



• Jet flame models (cont.) Point source models

o Single point : all energy is released from flame “center”. Similar to relevant point source model for pool fires

o Multipoint source : several point along jet trajectory taken into account



• Jet flame models (cont.) Solid flame radiation model

o Radiation emitted via flame surface

o Calculation based on : shape (cylinder, tilted or not) distance (View Factor)emissive power



• Jet flame models (cont.) (cont.) Solid flame radiation model (cont.)

o Calculation equation :

atEVFq q = thermal radiation flux at “receptor” (kW/m2) VF = view factor for flame shape at receptor Ε = emissive power (kW/m2) ta = transmissivity coefficient



• Jet flame models (cont.) Solid flame radiation model (cont.)

o View Factor : function of distance of receptor from flame and flame dimensions for shape assumed

o Transmissivity coefficient : as in pool fires, fireball/BLEVE

o 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)

11.021.0 00323.0

jus

jets

eF

A

cMFE



• Jet flame models conservative approaches Examination of horizontal jet

o Produce more extended thermal radiation zones

o 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)



• Jet flame models example results (cont.) Example results, 2 in hole in top of

propane tank/gas phase, vertical jet (Aloha)



END OF PART AEND OF PART A



• Vapour cloud dispersion (cont.) Extent of cloud : dimensions,

downwind/crosswind till specific endpoints (concentration)



• Vapour cloud dispersion (cont.) Endpoints :

o Toxics : several toxicity endpoints (e.g. IDLH, LC50)

o 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)



• Vapour cloud dispersion (cont.) Endpoints : (cont.)

o 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



• Vapour cloud dispersion (cont.) Example results for LPG dispersion (SLAB)

at ground level centerline

0%

1%

10%

100%

0 50 100 150 200 250 300 350 400

Downwind distance, x (m)

Cen

terl

ine

"gr

oun

d"

con

cen

trat

ion

(v/

v)

UFL

LFL

1/2 LFL



• Vapour cloud dispersion (cont.) Example results for LPG dispersion (SLAB)

(cont.) at ground level

-100

-50

0

50

100

0 50 100 150 200 250

Downwind distance (DW), x(m)

Cro

ssw

inf

dis

tan

ce (

CW

) 44 DW(UFL)

20 CW(UFL)

128 DW(LFL)

67 CW(LFL)

204 DW(1/2LFL)

94 CW(1/2LFL)

LFL

1/2 LFL

UFL



• Vapour cloud (gas) dispersion (cont.) Release conditions affecting

dispersion :o substance properties (Boiling Point

etc.)

o pressure, temperature at containment

o release rate and area

o release point height

o release direction (upwards –PSV-, horizontal)



• Vapour cloud (gas) dispersion (cont.) Meteorological conditions affecting

dispersion :o atmospheric stability class (A-F),

o wind speed,

o air temperature,

o humidity (for some substances reacting with water as for example HF or other polar substances : SO2, NH3 etc.)

o Type of area : rural/industrial/urban, roughness factor



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

o Expression of turbulent mixing in atmosphere. Related with atmospheric vertical temperature gradient (dT/dz)



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




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

A : unstable, in combination with high winds favors dispersion

D : neutralF : stable, minimum mixing in

atmosphere

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



• Vapour cloud (gas) dispersion (cont.) Wind speed :

o Wind speed referred in meteorological data usually refer to measurement at 9-10 m height

o Boundary layer effect (variation of speed with height)



• Vapour cloud (gas) dispersion (cont.) Wind speed : (cont.)

o Simplified function :

p, function of :stability classsurface roughness

p

refrefz z

zuu



• Vapour cloud (gas) dispersion (cont.) Wind speed : (cont.)

o Variation with stability class for rural environment :

0

4

8

12

16

20

24

28

32

0 5 10 15 20 25 30 35

Height, m

Win

d v

elo

city

, m/s

ec

A C D E F



• Vapour cloud (gas) dispersion (cont.) Meteorological conditions :

o 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



• Vapour cloud (gas) dispersion (cont.) Type of surroundings :

rural/industrial/urbano Refers to variation of height in

elements of surrounding

o Usually attributed via “roughness factor”



• Vapour cloud (gas) dispersion (cont.) Type of surroundings (cont.)

o Conservative approach : open country (rural)

Averaging time :

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

speed

direction



• Vapour cloud (gas) dispersion (cont.) Averaging time : (cont.)

o For continuous releases, concentration at constant location (x, y, z) is not constant

y

x

x

T=t

T=0

not exposed

exposed

y

average winddirection




o increase of averaging time :plume boundaries widenconcentration distribution

flattens




o Very important to use averaging time in models, suitable to exposure time under interest

o Models may use parameters for certain averaging time, which might not be suitable for application in Safety Report. PLEASE ALWAYS CHECK !!!PLEASE ALWAYS CHECK !!!

o Gaussian models use implicit 10-min averaging time but…




o Toxics exposure usually under interest for period of 30 min (due to LC50 30 min endpoints etc.)

o Aloha-DEGADIS (Heavy Gas Dispersion) uses 5 min for toxics

o Ignition of flammable cloud is related with very low exposure time (time just for ignition to happen).

o Aloha-DEGADIS uses 10 sec for flammables (e.g. LPGs, no matter if toxic effect is examined)




o Example results for propane release from liquid phase piping (SLAB)

-100

-50

0

50

100

0 50 100 150 200 250


Cro

ssw

inf

dis

tan

ce (

CW

) 44 DW(UFL)

20 CW(UFL)

128 DW(LFL)

67 CW(LFL)

204 DW(1/2LFL)

94 CW(1/2LFL)

LFL

1/2 LFL

UFL

-100

-50

0

50

100

0 50 100 150 200 250


Cro

ssw

ind

dis

tan

ce (

CW

) 28 DW(UFL)

10 CW(UFL)

86 DW(LFL)

47 CW(LFL)

136 DW(1/2LFL)

72 CW(1/2LFL)

LFL

1/2 LFL

UFL

Averaging time = 1 sec Averaging time = 30 min




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

o ½ LFL reporting contributes to uncertainty of averaging time (conservative approach)



• Vapour cloud (gas) dispersion (cont.) Passive (neutral) dispersion

(Gauss) :

o Release of gas with density equal or higher than air




(Gauss) :

o Basic characteristics: Maximum concentration at

centrelineConcentration reducing with

increasing distance from sourceIf release at ground level,

maximum concentration at ground level




(Gauss) :

o Basic equation for point source continuous release

u, wind speed at z (m/sec)M, release rate flow (kg/sec)Hd, active release height (m)

2

2

2

2

2

2

2

)(

2

)(2

2),,( z

e

z

e

y

HzHzy

zy

eeeu

MzyxC




(Gauss) : (cont.)

o 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

2.0

min10 min10

aver

y

ty taver




(Gauss) : (cont.)o Example results for dispersion for NH3

release by 2 in hole in 6 bar gas vessel, D5 (Aloha)




(Gauss) : (cont.)

o Equations provided for point stationary source (no momentum)

o But jets of releases have significant momentum due to high velocity… modifications needed in model to take into account release momentum




(Gauss) : (cont.)o For jets of releases modifications are

needed, typical example : plume rise parameter to modify the release source point at downstream location

Original

Modified



• Vapour cloud (gas) dispersion (cont.) Flue gases dispersion

o Example : pool fire combustion products, e.g., SO2

o 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 gasesRelevant plume rise equations provided for

stacks (Briggs, Holland equations), provide unrealistic plume rise height

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



• Vapour cloud (gas) dispersion (cont.) Flue gases dispersion (cont.)

o Special atmospheric condition to be considered (temperature inversion conditions, trapped plume)



• Vapour cloud (gas) dispersion (cont.) Plume rise effects

o Concentration at centrelines not continuously decreasing with distance

o Max concentration at centreline appears at distance from source



• Vapour cloud (gas) dispersion (cont.) Plume rise effects (cont.)

o 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)

0,0

0,2

0,4

0,6

0,8

0 1000 2000 3000 4000 5000 6000

Downwind distance (m)

Gro

und leve

l ce

ntr

eline c

once

ntr

ati

on

(mgr/

m3)



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion

o Special complex modelsCFDBox models (instant releases)Grounded plume models

(continuous releases)



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion (cont.)

o Maximum concentration expected at centerline

o Concentration decreases with increasing distance

o More extended plume compared to neutral dispersion



• Vapour cloud (gas) dispersion (cont.) Heavy gas dispersion (cont.)

o Meteorological dataF2 produce more extended

cloud

o Propane/butane cases same release source (e.g.

same hole size) will produce more extended cloud for propane due to higher release rate



• Vapour cloud (gas) dispersion models Heavy gas dispersion (cont.)

o Example results for propane release from 2 in hole in liquid phase of tank (D5) (Aloha)



• Vapour cloud (gas) dispersion models Heavy gas dispersion (cont.)

o Example results for propane release from 2 in hole in liquid phase of tank (F2) (Aloha)



END OF PART BEND 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 consequenceOverpressure

Main consequenceOverpressure

VCE results (Flixborough)Secondary consequences: oFragments (e.g. broken glasses)



• Vapour Cloud Explosion (VCE) characteristics Very short duration (<1 sec)

Models, high uncertainty due to several assumptions used in every model

Type of models:o CFD (FLACS, PHOENIX etc.)

o TNT blast charge (TNT equivalency)

o Air-fuel charge blast (Multi-Energy, Baker -Strehlow -Tang etc.)



• Vapour Cloud Explosion (VCE) models TNT equivalency model

o Simple, based on analogy with explosives effects

o A fraction of combustion energy released in cloud is attributed to produce overpressure

o The former energy fraction is recalculated as equivalent (on energy basis) mass of TNT

o The effects are defined based on known correlation of overpressure with TNT mass



• Vapour Cloud Explosion (VCE) models (cont.) TNT equivalency model (cont.)

o αe, refers to part of combustion energy released producing overpressure (1-10%)

o High uncertainty in both αe value and quantity of flammables (released mass –till what time ???-, mass within LFL-UFL) to be used

o Review on topic by TNO Yellow Book and AICheJ CCPS Guideline

TNT

ffeTNT Hc

HcWW

Wf= flammable in cloud WTNT= TNT equivalent mass (combustion energy basis) αe = TNT equivalency coefficient (energy basis) Hcf = combustion energy of flammables (kJ/kg)




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

αe must be selected along with suitable flammable mass

for αe 1-5%, 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




o 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

o Explosives blast and VCE present differences, as explosives have short duration higher shock wave peak values.

o TNT equivalency model is approximation of phenomenon based on statistical analogies




o Overpressure calculated by diagram

for distances required

o Uncertainty on centre of explosion

to be considered

o Similar diagrams for positive phase

duration, impulse

3TNTW

RR

HcTNT= combustion energy for TNT (4,680 kJ/kg) R = Hopkinson distance (m/kg0.33) R= distance from explosion centre



• Vapour Cloud Explosion (VCE) (cont.) TNO Multi-Energy model :

o Only confined areas of cloud are considered

o Partial explosions from confined areas expected

o 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



• Vapour Cloud Explosion (VCE) (cont.) TNO Multi-Energy model : (cont.)

o Overpressure from Berg graph

using Sachs distance

3

0PE

RR

E= combustion energy R = Sachs distance R= distance from explosion centre



• Vapour Cloud Explosion (VCE) (cont.) TNO Multi-Energy Model : (cont.)

o Similar graphs for positive phase duration, dynamic pressure

o Blast strength 10 : detonation, explosives case, not valid for VCEs as propagation of blast via detonation requires high homogeneity in cloud



• Vapour Cloud Explosion (VCE) (cont.) TNO Multi-Energy Model : (cont.)

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

definition of confined areas definition of successive or

simultaneous blast in confided areas

selection of blast strength (confinement increase, increases blast strength

o HSE suggests blast strengths 2 and 7



• Vapour Cloud Explosion (VCE) (cont.) Baker-Strehlow-Tang model

o Similar principles as TNO Multi-Energy model

confined areas only taken into accountstoichiometry of air with fuel in confined

areas

o Gas type “reactivity” (susceptibility to flame front acceleration) taken also into account along with obstacle density

o methane, CO : low reactivity

o H2, acetylene, ethylene/propylene oxide : high reactivity

o other substances : medium



• Vapour Cloud Explosion (VCE) (cont.) Baker-Strehlow-Tang model (cont.)

o Flame speed defined by table on gas reactivity and confinement type



• Vapour Cloud Explosion (VCE) (cont.) Baker-Strehlow-Tang model (cont.)

o 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)



• 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.

o 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



• 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 :

o Combustion products (e.g. SO2, HCl, NO2 etc.)

o Unburned pesticide active substance



• Pesticides fires and dispersion (cont.) UK HSE suggests stoichiometric

conversion of S, Cl to SO2 and HCl.

Conversion ratios :

o C to CO : 5%

o 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 fraction for 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 :

o what is plume rise for flue gases ??

o which is the combustion rate ?? Plume rise in warehouse flue gases

o For fire in full development plume rise might be high, but potentially low in initial phase of fire

o Typical equations fail, as producing unrealistic plume rise



• Pesticides fires and dispersion (cont.) Combustion rate, affected by type of fire

o Roof collapseAs in open area, fuel controllingHigh flue gas temperature

o Ventilation controlledroof intact, some window breakage

(limited release area)fire rate controlled by availability of

oxygen in warehouselow temperature of flue gases, low

plume rise



• Pesticides fires and dispersion (cont.) UK HSE suggests :

o plume rise set at max 50 m

o calculations for source via a few m2 area of window (nevertheless, recognized as pessimistic), (NTUA methodology refers to 3 m2)

o special models UK HSE suggests meteorological condition

to be examined as worst case ones :

o F2

o D5, D15 with low inversion height (400 m)



• Pesticides fires and dispersion (cont.) NTUA methodology suggests the following

cases :

o Roof collapse : flue gas rate 8 kg/m2.sec (per warehouse area), T=500 °C

o Ventilation controlled (roof intact) : flue gas rate 5 kg/m2.sec (per opening area, assumed 3 m2), T=140 °C



• Pesticides fires and dispersion (cont.) NTUA methodology classification of fires

(as per HSE FIRE Pest II computer program)

CombustiblesCombustion rate

Duration

High intensity fire

Flammable liquids (product solutions in solvents) purring in floor and pool fire.Inert “technical” substances or products containing significant percentage of flammable solvents

High 6500 sec

Medium intensity fire

Inert “technical” substances or products containing significant percentage of flammable solvents

Medium 7000 sec

Low intensity fire

Inert “technical” substances or products in flammable packaging

Low13000

sec



• Pesticides fire and dispersion (cont.) Survivor fraction according to NTUA

methodology (as per HSE, Risk Assessment Method for Warehouses 1995)

Solid substance

Liquid substance

Liquid substance

(2)

Liquid substance

particles <2 mm, high storage height (1)

cans < 10kg, large

storage height (1)

cans < 10kg, small storage

height (1)

metal drums

High intensity fire

10 10 0.5 10

Medium intensity fire

5 5 0.5 4

Low intensity fire

2 2 0.5 1

(1) Medium and large storage height : > 2 m, small storage height: < 2 m

(2) Same for particles <2 mm and small storage height



• Accidents with effects to environment No mature and wide-used quantitative

models for estimation of effects to environment

Qualitative models (applied some times, examples :

o Energy Institute (ex. IP) Screening Tool

o Belgium (Flanders) Richtlijn Milieurisicoanalyse

o 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




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




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)

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

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