QRA Model is Made Up of These Workbooks

8/17/2019 QRA Model is Made Up of These Workbooks

1/33

1

www.gexcon.com © GexCon AS

Total Raffinage Chemie Explosion Loading and Response Seminar, 26 September 2012, Brussels 1

Gas Explosion Basics

Kees van Wingerden

GexCon ASBergen, Norway



Contents

General

Basic parameters; explosion properties

Flame propagation and acceleration

mechanisms

Factors affecting explosion

effects/consequences

Explosion loads

Summary


2/33

2



Gas explosions: general course of events



Definition explosion

“ An explosion is a chemicalprocess which causes a very fast

and considerable pressure

increase”


3/33

3



Fire triangle and explosion pentagon



Contents

General



mechanisms



Explosion loads

Summary


4/33

4



Explosive part of cloud



Release - dispersion

Gas concentration in

monitoring point


5/33

5



Explosion limits

LEL/LFL = Lower explosion limit / Lower

flammability limit

UEL/UFL = Upper explosion limit / Upper

flammability limit

These values can be found in literature foratmospheric conditions.



Explosion limits

Methane 5.0 – 15 %

Propane 2.1 – 10.1 %

Acetone 2.6 – 13 %

Cyclohexane 1.3 – 7.8 %

Heptane 1.1 – 6.7 %

Hexane 1.2 – 7.4 %

Ethanol 3.3 – 19 % (temp. = 13 – 42 ⁰C)


6/33

6



Flashpoint: ethanol

-40 -20 0 20 40 60 80

Temperatur, 0C.

0

0.2

0.4

0.6

0.8

1

D a m

p t r y k k [ b a r ]

DamptrykkEtanol

Vapour pressure,

Ethanol

V a p o u r p r e s s u r e [ b a r ]

Temperature [degC]



Flash point examples

Methane -188 ⁰C

Propane -104 ⁰C

Acetone -19 ⁰C

Ethanol +12 ⁰C

Petrol ca – 45 ⁰C

Diesel > 55 ⁰C


7/33

7



Ignition energy (electric spark)

Hydrocarbons: MIE = 0.1 – 0.3 mJ



Auto-ignition temperature (hot surfaces)


8/33

8



Auto-ignition temperature The lowest temperature of a hot surface at which

a mixture of fuel and air can ignite

Hydrogen 580 ⁰C

Methane 537 ⁰C

Propane 493 ⁰C

Acetone 535 ⁰C

Ethanol 363 ⁰C

Petrol ca 250⁰C

Diesel ca 220 ⁰C

Main rule; Increasing length of C-chain, - lower ignition

temperature.



Chemical reaction

CH4+2O2+7.52N2

2H2O+CO2+7.52N2 + heat

Simplified equation:

In reality: ≈100 equations

Product composition depends on

mixture, temperature, pressure

CO, CO2, H2O, H2, OH, CH4, N2


9/33

9



Combustion at constant pressureand volume

Unburnt

gas cloud

P/P0=8

V/V0=8

Combustion at

constant volume

Combustion at

constant pressure

Burnt

gas cloud



Pressure development in

deflagrations

In closed vessels:

Pmax/Pin=(nexpl/nin)Tmax/Tin (nexpl/nin = ca.1)

In vented vessels: usually: Pin


10/33

10



Explosion loads

Pressure

Drag (1/2u2)

Blast waves in the surroundings

Flames (direct contact)

Flying debris



Contents

General



mechanisms



Explosion loads

Summary


11/33

11



Flame propagation and accelerationmechanisms

Laminar combustion

Flame instabilities

Explosion generated turbulence dominated flame

propagation

Deflagration-Detonation-Transition (DDT),Detonation

Detonation



Laminar flame propagation

Slow process (dominated by diffusion)

Typical velocities (burning velocities): 0.5

m/s

Expansion velocity: total velocitybecomes: 3-4 m/s

Sf = Su + Sg

Sf = Su u b

=


12/33

12



Laminar flame propagation



Laminar combustion

Flame thickness: typically 1 mm

Typical laminar burning velocities

methane: 0.40 m/s

propane: 0.46 m/s ethylene: 0.75 m/s

acetylene: 1.55 m/s

hydrogen: 3.25 m/s

Expansion ratio typically 7-10 (a=n2T2/n1T1)


13/33

13



Laminar burning velocity



Laminar flame propagation (in real cloud)

Terneuzen 1982: Ignition of vapour clouds resulting from

realistic releases of propane (up to 40 kg/s) into open

area (no congestion) results in low flame speeds and

negligible overpressures


14/33

14



Flame instabilities Flame instabilities will cause laminar flames

to accelerate, especially due to increase of

flame area

Examples:

Intrinsic instability

Chemo-diffusive instability

Acoustically driven flame instabilityTaylor instability



Intrinsic flame instability

Methane-air


15/33

15



Lind-Whitson,1977, various

gases, max. flamespeed 35.4 m/s

(acetylene)

20m diameter balloon hydrogen (Fh-ICT

1982) resulted i max. 60 mbar overpressure

Intrinsic flame instabilities: DDT?



Taylor instability


16/33

16



Turbulent combustion

Turbulence causes an increase ofburning velocities due to mixing ofcombustion products and reactants anddue to an increase of the flame surfacearea

Flame speeds (expansion + combustion)can vary from 5-600 m/s



Turbulent combustion

Flame thickness: dependent onturbulence length scale

Turbulent burning velocity dependent on

turbulence properties

Flame quenching in case of intensiveturbulence and/or low reactivity mixtures


17/33

17



< lt > lt



Explosion generated turbulence

Expansion Flow

interaction

Increasedpressure

Combustion

Turbulence

Positive feedback mechanism of explosion

generated flow and combustion


18/33

18



Explosion generated turbulence

Positive feedback

mechanism of explosion

generated flow and

combustion



Flame acceleration Pressure

1.00.80.60.40.20.0

0

100

200

300

Flame speedMean flow velocity

Distance (m)

V e l o c i t y ( m / s )

Front elevation of test vessel

10001001010

.01

.1

1

10

Maximum Flame Speed [m/s]

M a x i m u m P

r e s s u r e

[ b a r g ]

Planar

Spherical


19/33

19



Experiments in a 50 m3

pipe



Experiments – “3-D corner”


20/33

20



Experiments in 3-Dcorner

P = 0.025 bar

P > 4.0 bar



Experiments – 45 m long array of cross

flow obstructions; no confinement


21/33

21



Experiments – obstructed vs. non-obstructed



Experiments – obstructed vs. non-obstructed


22/33

22



Detonation propagation process High pressure (~20 bar) and high propagation

velocity (~2 km/s)

ZND-theory: shock wave followed by reactionzone

Actual detonation: 3-D shock wave followed byreaction zone



Deflagration to Detonation

Transition (DDT)

Strong deflagrations cantransit into a detonation

Pipes: explosion

generated turbulence atwalls results in flameaccelerations and possiblyDDT


23/33

23



Experiments in congested area –

DDT and detonation



Experiments – DDT and detonation


24/33

24



Experiments – DDT and detonation



Contents

General



mechanisms



Explosion loads

Summary


25/33

25



Obstructions

The strength of gas explosions is strongly related to

the degree of congestion: confinement and presence

of obstructions

Important obstruction parameters are:

Obstruction diameter (Volume)blockage

Dimensions obstructed area (length of flame path in

obstructed area); number of obstructions along flame path



Confinement: flow field divergence

(5 obstacles, BR = 0.5)


26/33

26



Effect of confinement (venting):50 m3 representation of separation module


Total Raffinage Chemie Explosion Loading and Response Seminar, 26 September 2012, Brussels

Confinement

Gas explosions often occur in semi-

confined geometries (mezzanines)

Confinement directs flow

Max. pressure is determined bypressure developed by combustion

and pressure release through

openings

52


27/33

27



Type of Fuel(Wedge-shaped

vessel)



Fuel Concentration(wedge-shaped vessel)


28/33

28





Overpressure – ignition source location

43210

0

100

200

300

400

500

edge ignited

centrally ignited

Distance (m)


29/33

29



Effect of ignition source position(length of flame path)



Effect of gas cloud size

5 5

611

30

11

20

2084

28

54 20

1826

10

18

9

78

6

5 5

178

86

48 26

9137

39

35 63

2218

12

1518

87

8

385

395

405

415

425

435

445

455

465

475

485

75 85 95 105 115 12538575

Scenario 9 Process deck

38 33

3537

206

91

185

216467

505

546 197

446836

189

436

152

122165

43

44 39

4861

427

539 168

470492

543

333 568

788324

302

355558

172142

32

385

395

405

415

425

435

445

455

465

475

485

75 85 95 105 115 12538575


57

62 61

6357

321

138

304

360683

826

902 288

10581424

631

1465

468

388524

72

75 65

59100

628

795 253

689774

1071

553 805

1144951

1292

21081811

548596

385

395

405

415

425

435

445

455

465

475

485

75 85 95 105 115 125385

75



30/33

30


Total Raffinage Chemie Explosion Loading and Response Seminar, 26 September 2012, Brussels

Gas explosions in industrial installations

Main parameters affecting the courseof an explosion:

Number / orientation / location ofequipment, pipes & structuralcomponents ”Degree of congestion”

Vent openings / panels (size andlocation) ”Degree of confinement”

Gas cloud size and location uponignition

Gas concentration, inhomogenieties Gas type (reactivity)

Moment of ignition

Active mitigation measures

59



Contents

General



mechanisms



Explosion loads

Summary


31/33

31



Explosion loads: pressure Pressure affects in general walls / decks / big objects

where a pressure difference can be maintained duringa while

The pressures are highly dynamic and may varystrongly in space (very complicated loading pattern)

P = 0.10 bargP = 10 barg



Explosion loads: drag

Explosion wind (”drag”) = 1/2u2 affects especiallyslender objects (e.g. pipes) where pressuredifferences across the object are quickly diminished.

Also drag pressures andresulting drag forces(1/2u2CD A) are highlydynamic


32/33

32



Explosion loads: blast ”Far field effects”: pressure- or shock wave generated

by initial explosion propagating into surroundings

Maybe important for loads on LQ or fire-screens one.g. neighbouring platform



Contents

General



mechanisms



Explosion loads

Summary


33/33



Thank you very much for yourattention!!

[email protected]

QRA Model is Made Up of These Workbooks

Documents

Transcript of QRA Model is Made Up of These Workbooks