National department of Civil protection - Technological risk

Major industrial accidentScenarios and evaluation of the effects of

explosive mixtures. Prevention and protection methods

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vapor cloud explosion A vapor cloud explosion (VCE) results fromthe ignition of a flammable mixture of vapor,gas, aerosol, or mist, in which flame speedsaccelerate to sufficiently high velocities toproduce significant overpressure.

VCEs are generally associated with therelease of a sufficient quantity of flammablegas or vaporizing (flashing) liquid from a: storage tank process or transport vessel piping system

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vapor cloud explosion

Buildings may be damaged and people may be injured by: the blast waveadditional indirect effects from missile generation, crater

formation, ground shock, and fire.Generally: as the blast wave travels farther away from the center of

the explosion it loses energy the magnitude of overpressure and other effects decreases

as the distance increases from the explosion source.

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Five conditions for Vapor Cloud Explosions

1. The released material mustbe flammable and at suitableconditions to form a vaporcloud (the reactivity of theunburned material isimportant )

2. An ignition source is neededto initiate the explosion.Higher energy ignitionsources can lead to a moresevere explosion than dolower energy sources.

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Reactivity of unburned material

Highly unsaturated molecules (es.acetylene, hydrogen)

High flammable range

Low ignition energy

High flame speeds

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Five conditions for Vapor Cloud Explosions

3. Ignition of the flammable vapor cloudmust be delayed until a cloud ofsufficient size has formed.If ignition occurs as the flammablematerial is escaping, a large fire, jet flame,or fireball might occur, but a VCE isunlikely.

The probability of explosion rather thanfire increases with the size of the cloud,since the quantity of the mixture withinthe flammable range increases.

Lenoir and Davenport (1992) analyzed historical data onignition delays and found delay times from 6s to as longas 60 min.

The most probable for generating Vapour CloudExplosion: ignition delays of from 1 to 5 min

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Factors Favoring High Over Pressures

Vapor Cloud Size impacts on:

– probability of finding ignition source

– likelihood of generating any overpressure

– magnitude of overpressure

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Five conditions for Vapor Cloud Explosions

4. Turbulence is required for theflame front to accelerate to thespeeds required for a VCE;otherwise, a flash fire will result.

This turbulence is typically formedby the interaction between theflame front and obstacles such asprocess structures or equipment.

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Five conditions for Vapor Cloud Explosions

The blast effects produced by VCEs can vary greatly and are strongly dependent on flame speed.

In the absence of turbulence, under laminar or near-laminar conditions, flame speeds are too low to produce significant blast overpressure.

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Five conditions for Vapor Cloud Explosions

5. Confinement of the cloud by obstacles canresult in rapid increases in pressure duringcombustion.Absence of confining obstacles limiting thepressure increases.

The degree of confinement in process plants,with their congested equipment layout andbuilt-up structures, is generally high

simulation of the blast wave reflection by a complex of two

buildings

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Effects of obstacles

• Dense concentrations ofprocess equipment inchemical plants or refinerieshave been contributing causesof heavy blast in vapor cloudexplosions in the past.

• Local partial confinement orobstruction in a vapor cloudmay easily act as an initiatorfor detonation, which maypropagate into the cloud aswell

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Typical overpressure profiles

For a given explosion energy, deflagrationsare generally characterized by a gradualincrease to peak overpressure with longdurations, followed by a gradual decreasein overpressure.

Detonations are characterized by a veryrapid rise to peak overpressure followed bya steady decrease of overpressure to form

the more idealized shock front.

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Pressure vs Time Characteristics

In summary an explosion causesoverpressure and dragpressures on buildings and otherstrucures:a) The overpressure produces

the largest loads on the sideof buildings facing theexplosion because ofreflection and lesser loads onthe roof and other sides

b) Drag pressure producesnegative pressure on slenderstrucures such as stacks andtowers

DETONATION

VAPOR CLOUD DEFLAGRATION

TIME

When the blast wave strikes the front of the structure (Figure a),the overpressure rises to a value in excess of the peakoverpressureThe blast wave then bends around the structure, exertingpressures on the sides and roof and finally on the back face(Figures b and c)Then, drag pressure, has a negative value for the backsurfacaces (figure d)

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Factors Favoring High Over Pressures

Weather

– Stable atmospheres lead to large clouds (see table 1)

– Low wind speed encourages large clouds (see table 2)

most unfavorable weather condition F2 (F stable, 2m/sec windspeed)

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Important

Experieces demonstrated the following points: Low-energy ignition of unobstructed propane-air and natural

gas-air clouds does not produce damaging overpressures Combustion of a natural gas-air cloud in a highly congested

obstacle array leads to flame speeds in excess of 100 m/s(pressure in excess of 200 mbar)

There seems to be a minimum threshold below which a UVCEis unlikely to occur: according to some authors the thresholdvalue is in the range of 500-1000 kg to fall to around 100 kgfor highly reactive materials (eg. Acetylene, hydrogen)

Historical analysis shows that almost all of the VCE happenedin the first 10 minutes following the release

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peakoverpressure (psi)

peak overpressure(bar)

Property damage Human injury

0,42 0,03 REVERSIBLE INJURY (italian decree)

0,5 -1 0,34 – 0,068 Glass windows break Knock personnel down

1 0,07 IRREVERSIBLE INJURIES (italian decree)1 -2 0,07 – 1,36 Common siding types fail (corrugated asbestos,

shatters corrugated steel, panel joints fail, wood siding, blows in

2 -3 0,14 – 0,2 Unreinforced concrete or cinder block walls fail

3 – 4 0,2 – 0,27 Self-framed steel panel buildings collapseOil storage tanks rupture

4,2 0,3 DESTRUCTION OF BUILDINGS AND EQUIPMENT HIGH LETHALITY (italian decree)6 0,4 probably of eardrum rupture

10 0,68 Probable total destruction of buildings lung hemorrhage threshold17 1,15 10% probably of fatality from direct blast effect50 3,4 50% fatalities65 4,4 99% fatalities

Human injury and property damage criteria under peak blast ovepressure

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Evaluating Characteristics of Vapour cloud explosions

A TNT equivalence method for predicting VCEoverpressures and duration is often used.In the TNT equivalence approach, the blasteffects from VCEs are correlated with those fromequivalent explosive charges of TNT as a meansof quantifying the intensity of explosions.

It is assumed that only a certain proportion(usually 1%-10% based upon energy) of the fuelreleased contributes to the explosion.

This mass of fuel is converted to an equivalentmass of TNT, taking into account the combustionenergy of the fuel and the detonation energy ofTNT.

Overpressures are determined fromTNT blast curves, which have beenwell established from experimentaldata.

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limitations and deficiencies of TNT equivalence method

The method does have serious limitations and deficiencies, including:1. TNT equivalency is not clearly defined, 2. blast wave characteristics from TNT are very different from those for

a VCE (TNT produces a shorter-duration and higher overpressure blast wave than does a VCE for the same energy)

3. blast attenuation differs between TNT and a VCEThe Multi-Energy method for VCE blast modeling is an alternative for conventional TNT-equivalency method.

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VCE and «DOMINO EFFECT» in current national decree

D.Lgs 26 june 2015, n. 105 «Attuazionedella direttiva 2012/18/UE relativa alcontrollo del pericolo di incidenti rilevanticonnessi con sostanze pericolose. (G.U. 14luglio 2015, n. 161 - S.O. n. 38)» is theimplementation of SEVESO III EUDirective

ANNEX E (art. 19) – «Criteri perl'individuazione degli stabilimenti tra i qualiesiste la possibilità di effetto domino…»deals with «DOMINO EFFECT»

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Seveso III Directive aims at:1) prevention of majoraccidents involving dangerous

substances2) limitation of theconsequences of accidents onman and the environment

For this purpose, the Directivebrings the instruments for themanagement of majoraccidents and ensures thatsafety procedures forprevention and mitigation theeffects of major accidents areput into practice.

Seveso III Directive has been inforce since 1 June 2015 in EU.From this date, Seveso IIIDirective has beenimplemented.

Prevention and protection in seveso III directive

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Explosion Protection Systems

Methods Based on the Prevention of Combustion. (1) Oxidant concentration reduction(2) Combustible concentration reduction

Methods Based on the Prevention or Limitation of Damage. (1) Spark extinguishing systems(2) Deflagration suppression(3) Isolation methods(4) Deflagration pressure containment

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Explosion protection system, can bechosen and dimensioned based on theanalysis of scenarios related tohazardous substances

The following is an analysis of somereference scenarios as a tool for decisionsupport for prevention and protectionmethods.

Impact scenarios: case studies

Substances: Hydrogen, Ammonia, Liquefied Natural Gas (LNG), Liquefied Petroleum Gas (LPG)

Accident conditions1. Maximum impact conditions2. the most likely accident conditions

Critical distances assessed• Maximum distance of source to LEL (m)• Minimum distance of source to LEL (m)• Distance for UVCE Pressure at 0,3 bar (m) (threshold for High lethality and extensive

damage to mechanical and concrete structures)• Distance for UVCE Pressure at 0,07 bar (m) (threshold for irreversible injury and some

damage to concrete structures)The simulations were performed with sotware Effects (TNO)

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IMPORTANT!!!each hazardous substanceshould be analyzed withspecific scenarios that aredependent on severalfactors

Hydrogen: the maximum impact conditions

released overall mass release of liquid in tanks by 100, 50 and 30 m3 with respective amounts of 3000, 1500 and 900 kg at a temperature T = 15 ° C

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 120, 240, 400 sec

F2 Total mass 3000 kg Total mass 1500 kgTime of ignition after start release (sec) 60 240 400 60 240 400

Maximum distance of source to LEL (m) 163,7 592 935 160 579 892Minimum distance of source to LEL (m) 76,3 367 664 70 360 707Maximum distance of source to LEL/2 (m) 182,64 611 954 179 598 911Distance for UVCE Pressure at 0,3 bar (m) 159 146 128 129 117 89Distance for UVCE Pressure at 0,07 bar (m) 505 463 405 407 370 242Explosive mass LEL< < UEL (kg) 2853 2233 1369 1401 936 335

The probability that a cloud of hydrogenleading to an UVCE instead of a Flash Fireis bound to two aspects:1) the mass involved in the release mustbe greater than 100 kg2) there must be a certain degree ofconfinement

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Hydrogen: the most likely impact conditions

Release flow rate (kg / s) Are a function of three historical analysis diameters (25, 50, 10 mm) and are 0.52, 0.13, and 0.021

release time for intervention of security system

120 sec

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 120 180 sec

F2 Mass flow rate 0,52 Kg/s Mass flow rate 0,13 Kg/sTime after start release (sec) 60 120 180 60 120 180

Maximum distance of source to LEL (m) 125 213 235 92,35 94 94Minimum distance of source to LEL (m) 0,11 0,11 50 0,11 0,11 61Maximum distance of source to LEL/2 (m) 144 231 254 111 113 113Distance for UVCE Pressure at 0,3 bar (m) 53 55 53 0 0 0Distance for UVCE Pressure at 0,07 bar (m) 148 152 149 0 0 0Explosive mass LEL< < UEL (kg) 19.53 30.56 22.57 3,18 3,37 0,44

is the continuous release of hydrogen due to ruptureof a pipe or to the failure of control valves Loading

The response times may vary from 1 minute to about 10minutes:- Pneumatic actuators: 1-2 minutes- Electric actuators: 2-5 minutes- Mechanical and manual actuators: up to 30 minutes

The reference broken from the historical analysis havediameters of 10, 25 and 50 mm with intervention times of2 minutes

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LNG: maximum impact conditions

released overall mass release of liquid in tanks of 3000, 500 and 100 m3 with respective amounts of 350.000, 100.00 and 60,000 kg at a temperature T = 15 ° C

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 300, 900 e 1200 sec

F2 Total mass 350.000 kg Total mass 100.000 kgTime after start release (sec) 60 300 1200 60 300 900

Maximum distance of source to LEL (m) 185 765 2671 172 744 1903Minimum distance of source to LEL (m) 54 434 2128 67 455 1696Maximum distance of source to LEL/2 (m) 204 784 2690 191 763 1922Distance for UVCE Pressure at 0,3 bar (m) 214 1137 1317 141 628 166Distance for UVCE Pressure at 0,07 bar (m) 692 3770 4369 451 2073 534Explosive mass LEE< < UEL (kg) 9240 70800 82000 4400 36000 6000

Condition of maximumimpact: instantaneousrelease of fuel as aresult of collapse of atank and explosion ofthe cloud

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LNG: the most likely impact conditions

Release flow rate (kg / s) Sono funzione dei tre diametri da analisi storica (50 25, 10 mm) e sono pari a 1,47 , 0,37 , e 0,06

release time for intervention of security system 120 sec wind speed at 10 meters 2 m/s per class F

5 m/s per class DIgnition time after release 60, 120 150 secondi

F2 Mass flow rate 1,47 Kg/s Mass flow rate 0,37 Kg/sTime after start release (sec) 60 120 150 60 120 150

Maximum distance of source to LEL (m) 98,02 102 102 41 41 0Minimum distance of source to LEL (m) 0,11 0,11 60 0,11 0,11 0Maximum distance of source to LEL/2 (m) 117 121 121 60 60 0Distance for UVCE Pressure at 0,3 bar (m) 41 41 40 0 0 0Distance for UVCE Pressure at 0,07 bar (m) 125 139 135 0 0 0Explosive mass LEE< < UEL (kg) 24,56 24,28 8,86 2,07 2,11 0

continuous release of fuel due to aloss or malfunction with spillageand ignition of the cloud

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Ammonia: the maximum impact conditions

Substance that has a highchemical reactivity in respect ofthe metals and air, has aflammability range from 16% to25%.Instantaneous release of tanksfor liquid flash

released overall mass release of liquid in tanks of 3000, 500 and 100 m3 with respective amounts of 500,000, 90,000 and 19,000 kg at a temperature T = 15 ° C

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 480, 720 sec

F2 Total mass 500.000 kg Total mass 90.000 kgTime after start release (sec) 60 360 720 60 360 720

Maximum distance of source to LEL (m) 129,7 1093 1524 160 784 0Minimum distance of source to LEL (m) 60 829 1366 79 665 0Maximum distance of source to LEL/2 (m) 197 1100 1542 178 802 0Distance for UVCE Pressure at 0,3 bar (m) 205 / 444 100 226 /Distance for UVCE Pressure at 0,07 bar (m) 619 / 1309 314 679 /Explosive mass LEE< < UEL (kg) 13.900 173.400 36.120 4572 16750 0

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Ammonia: the most likely impact conditions

Continuous release into theatmosphere as a result of lossfrom the reservoir due torupture of pipe or a valve.

Release flow rate (kg / s) They are a function of the three diameters from historical analysis (25, 50, 10 mm) and are equal to 33,87; 8.47 and 1.35

release time for intervention of security system

720 sec hypothesis of reservoirdepletion

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 600, 1800 secondi

F2 Mass flow rate 33,87 Kg/s Mass flow rate 8,47 Kg/sTime after start release (sec) 60 600 1800 60 600 1800

Maximum distance of source to LEL (m) 133 368 366 113 145 146Minimum distance of source to LEL (m) / /Maximum distance of source to LEL/2 (m) 160 395 384 131 162 164Distance for UVCE Pressure at 0,3 bar (m) 39 63 63 35 36 36Distance for UVCE Pressure at 0,07 bar (m) 116 208 209 109 110 111Explosive mass LEE< < UEL (kg) 160 1246 1274 59 72 85

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LPG: maximum impact conditions

released overall mass release of liquid in tanks of 3000, 500 and 100 m3 with respective amounts of 978,000, 163,000 and 32,600 kg at a temperature T = 15 ° C

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 1, 60 300 sec

F2 Total mass 978.000 kg Total mass 163.000 kgTime after start release (sec) 1 60 300 1 60 300

Maximum distance of source to LEL (m) 36 209 0 25 132 0Minimum distance of source to LEL (m) 0 0Maximum distance of source to LEL/2 (m) 58 329 0 39 208 0Distance for UVCE Pressure at 0,3 bar (m) / 134 0 / 88 0Distance for UVCE Pressure at 0,07 bar (m) / 399 0 / 282 0Explosive mass LEE< < UEL (kg) 9,2 4660 0 2,1 1600 0

instantaneous release of LPGas a result of tank ruptureaccident

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LPG: the most likely impact conditions

Release flow rate (kg / s) They are a function of three historical analysis diameters (25, 50, 10 mm) and are equal to 30; 7.67 and 0.01

release time for intervention of security system

3600 sec hypothesis of reservoir depletion

wind speed at 10 meters 2 m/s for class F5 m/s for class D

Ignition time after release 60, 600, 1800 sec

F2 Mass flow rate 30 Kg/s Mass flow rate 7,67 Kg/sTime after start release (sec) 60 600 1800 60 600 1800

Maximum distance of source to LEL (m) 178 218 218 97 100 100Minimum distance of source to LEL (m)Maximum distance of source to LEL/2 (m) 280 344 344 153 157 157Distance for UVCE Pressure at 0,3 bar (m) 76 83 83 55 55 55Distance for UVCE Pressure at 0,07 bar (m) 251 269 269 194 195 195Explosive mass LEE< < UEL (kg) 891 1122 1122 181 189 189

Hypothesis based onbreakage of a horizontalpipe or the small rupture ofa tank with horizontal gasjet.

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CRITICAL DISTANCE FOR THE MAXIMUM IMPACT CONDITIONS - F2

H2 (3000kg) 0,3bar

H2 (3000kg) 0,07bar

H2 (1500kg) 0,3bar

H2 (1500kg) 0,07bar

GNL (350.000 kg) 0,3bar

GNL (350.000 kg) 0,07bar

GNL (100.000 kg)0,3bar

GNL (100.000 kg)0,07bar

AMM (500.000kg) 0,3bar

AMM (500.000kg) 0,07bar

AMM (90.000kg) 0,3bar

AMM (90.000kg) 0,07bar

LPG (978.000kg) 0,3bar

LPG (978.000kg) 0,07bar

LPG (163.000kg) 0,3bar

Substance Conditions for maximum impact and ignition timing critical distances of damage from the release pointHYDROGEN high mass (3000kg) after 5 minutes cause damage to buildings and concrete structures up to 1300 meters from the

release pointLNG for large masses (350,000 kg) after 20 minutes the damages of concrete buildings and structures are felt up to 3000 m

AMMONIA: very high masses releases (500.000 kg) produce critical distances of over 1000 meters

LPG very high mass releases produce critical distances of less than 300 m

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HYDROGEN: releases for 120 seconds produces critical distances of a few hundred meters(less than 150 m)LNG: releases for 120 seconds produces critical distances of a few hundred meters (less than 150 m)

AMMONIA: releases for 7200 seconds have critical distances of some hundreds of meters (less than 100 m)LPG: release for 3600 seconds have critical distances less than 200 hundred meters

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CRITICAL DISTANCE FOR THE MOST LIKELY IMPACT CONDITIONS – F2

H2 (0,52 kg/s) 0,3bar

H2 (0,52kg/s) 0,07bar

H2 (0,13kg) 0,3bar

H2 (0,13kg/s) 0,07bar

GNL (1,47kg/s) 0,3bar

GNL (1,47 kg/s) 0,07bar

GNL (0,37kg/s)0,3bar

GNL (0,37kg/s)0,07bar

AMM (33,87kg/s) 0,3bar

AMM (33,87kg/s) 0,07bar

AMM (8,47kg/s) 0,3bar

AMM (8,47kg/s) 0,07psi

LPG (30kg/s) 0,3bar

LPG (30kg/s) 0,07bar

LPG (7,67kg/s) 0,3bar

LPG (7,67kg/s)0,07bar

Use the scenarios in the prevention and protection from UVCE

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FLAMMABLE THREAT ZONESYou can use this scenario to look for potentialsources of ignition and optimize methods based onthe prevention of explosions

Software A.L.O.H.A. (Areal Location of Hazardous Atmospheres) by EPA

OVERPRESSURE (BLAST FORCE) THREAT ZONEYou can use this scenario to test methods ofprotection and damage limitation of overpressureand to check the damage of structures andequipment

A. Windows and gauges broken L. Power lines are severedB. Louvres fail at 0.2-0.5 psi M. Controls are damagedC. Switchgear is damaged from roof collapse N. Block walls failD. Roof collapses O. Frame collapsesE. Instruments are damaged P. Frame deformsF. Inner parts are damaged Q. Case is damaged

G. Brick cracks R. Frame cracksH. Debris - missile damage occurs S. Piping breaksI. Unit moves and pipes break T. Unit overturns or is destroyedJ. Bracing fails U. Unit uplifts (0.9 tilted)K. Unit uplifts (half tilted) V. Unit moves on foundation

Damage Estimates Based on Overpressure for Process Equipment

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Thanks for your attention

Ing. Francesco GERI Presidenza del Consiglio dei Ministri Dipartimento della Protezione Civile Francesco.geri@protezionecivile.itMobile phone 3356193957

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