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Evaluation of Safety Distances Related to Unconfined Hydrogen Explosions
Sergey DorofeevFM Global
1st ICHS, Pisa, Italy, September 8-10, 2005
Motivation
• H2 releases in confined and semi-confined geometries (tunnels, parking, garages, etc.) represent a significant safety problem Possibility of hydrogen accumulation, Promoting role of confinement for FA and pressure build-
up
• Unconfined H2 explosions can also be a significant safety problem Releases in obstructed areas (refuelling stations,
hydrogen production units, etc.) Relatively fast dilution of H2-air mixtures at open air and
inefficient FA without confinement On the other hand: large quantities of H2
Confined versus unconfined
Motivation
• Potential consequences of unconfined hydrogen explosions important for safety distances Blast effects Thermal effects Effects of explosion-generated fragments
• Blast effects are usually of the prime interest for safety distances
• May be especially important for hydrogen because of their potential severity
• Unconfined hydrogen explosions and their blast effects are the focus of the present study
Consequences
Motivation
• A detailed analysis of blast effects should include Hydrogen release and distribution Flame propagation and blast generation in complex 3D
geometry Blast wave propagation and its effect on the surrounding
objects
• This would generally require an application of 3D CFD simulations Limited variety of the cases / applications
• A simple approximate analytical tool should be useful Screening tool to select the cases where detailed
analysis may be necessary
Analysis strategy
Objective
• Develop a simple approximate method for evaluation of blast effects and safety distances for unconfined hydrogen explosions Model for evaluation of hydrogen flame speeds in
obstructed areas Model for properties of “worst case” hydrogen distribution Model for blast parameters Set of blast damage criteria
Methodology
• Pressure effect of a gas explosion essentially depends on the maximum flame speed
• It is important to have a reliable estimate for the flame speed
• Flame speed increases due to: Increase of the flame area in an obstacle field Increase of the turbulent burning velocity during flame
propagation
Flame speeds
R
fTf A
ASV
Methodology
• Flame folding due to obstacles
• Plus Bradley correlation for turbulent burning velocity:
Flame speeds
3/12
2
)(3
41)1(
T
Lf
L
x
R
x
ySbaV
ba
x
x
yR
R
Methodology
• Experimental data
Flame speeds
10
100
1000
0.01 0.1 1 10 100
Distance, m
Exp
eri
men
tal f
lam
e s
peed
, m/s
x=45, y=4 mm H2
x=33, y=4 mm H2
x=31, y=4 mm H2
x=18, y=1 mm H2
x=12, y=1 mm H2
x=10, y=1 mm H2
x=9, y=0.65 mm H2
x=7, y=0.65 mm H2
x=6, y=0.65 mm H2
x=39, y=5 cm C2H4
x=22, y=5 cm C2H4
no obstacles H2
no obstacles C3H6
Methodology
• Correlation
Flame speeds
0
50
100
150
200
250
300
0 50 100 150 200 250 300
Model flame speed, m/s
Exp
eri
men
tal f
lam
e s
peed
, m/s
x=45, y=4 mm H2 x=33, y=4 mm H2
x=31, y=4 mm H2 x=18, y=1 mm H2
x=12, y=1 mm H2 x=10, y=1 mm H2
x=9, y=0.65 mm H2 x=7, y=0.65 mm H2
x=6, y=0.65 mm H2 x=39, y=5 cm C2H4
x=22, y=5 cm C2H4 no obstacles H2
no obstacles C3H6
Methodology
• There is clearly a variety of release scenarios, which can affect the resulting hydrogen distribution
• Continuous release Slow: jet or plume with size of flammable volume break
size Fast: jet with size of flammable volume >> break size
• Instantaneous release – most dangerous Pressure vessel rupture LH2 release or vessel rupture
• Other scenarios
Hydrogen distribution
Methodology
• Instead of considering specific scenarios here, a simple general model for instantaneous releases is analysed
• This model assumes that the released hydrogen forms a cloud with a non-uniform concentration
• The form of the cloud is assumed to be semi-spherical, for simplicity
• Hydrogen concentration reachesmaximum in the centre and decreases linearly with radius
• Stoichiometric H2/air – unrealistic and overconservative!
Model for gas distribution
r
Cmax
Methodology
• Variable: maximum concentration in the centre, Cmax
• ‘Worst case’: maximum of < >=<(-1)SL>, averaged between UFL and LFL
• Properties of ‘worst case’: Cmax = 88% vol.
< > = 0.1max
<E> = 60% of total chemical energy
‘Worst case’ distribution
LFL
Cmax
UFL
Methodology
• Calculations of blast parameters are based on our method published in 1996
• Dimensionless overpressure and impulse are functions of flame speed, Vf
Blast parameters
),min( *2
*1
* PPP ),min( *2
*1
* III 3*2*3/4**
1 )/(0033.0)/(062.0)/(34.0 RRRP 968.0**
1 )/(0353.0 RI
))/(14.0/83.0(1 2**
20
2*2 RR
c
VP f
))/(0025.0)/(04.0/06.0(1
4.011 3*2**
00
*2 RRR
c
V
c
VI ff
Methodology
• An assessment of damage potential is made here using pressure-impulse (P, I) damage criteria
Damage potential
kPPII aa ))((
Damage description Pa, Pa Ia, Pa∙s k, Pa2∙s
Total destruction of buildings 70100 770 866100
Threshold for partial destruction; 50 to 75% of walls destroyed
34500 520 541000
Threshold for serious structural damage; some load bearing members fall
14600 300 119200
Border of minor structural damage 3600 100 8950
Results
• High congestion, x = 0.2 m; y = 0.1 m: a technological unit with multiple tubes / pipes.
• Medium congestion, x = 1 m; y = 0.5 m: a technological unit surrounded by other units / boxes.
• Low congestion, x = 4 m; y = 2 m: a large technological unit surrounded by other large units (e. g., refueling station)
Characteristic obstacle geometry
Results
• Obstacle geometry affects significantly flame speeds
• To reach 300 m/s: 1 kg, 40 kg, and 1000 kg high, medium, and low congestion
Flame speeds
0
100
200
300
400
500
600
0.1 1 10 100 1000
m, kg
Fla
me
spee
d, m
/s
Low congestion
Medium congestion
High congestion
Results
• Example for medium congestion
Radii for selected levels of damages
0
50
100
150
200
250
300
1 10 100
m, kg
R,
m
Full destruction (buildings)
50-75% destruction
Significant damage
Minimum damage
Results
• Scenarios
• Consequences Pressure Thermal Fragments
• Acceptance criteria Population Regulations Costs
Safety distances – contributing factors
Results
• Defined, as an example, by minimum building damage criterion for unconfined H2 explosions
Safety distances - example
1
10
100
1000
0.1 1 10 100 1000
m, kg
R, m
Low congestion
Medium congestion
High congestion
TNT equal energy
Results
• The same method applied to: hydrogen, ethylene, propane, methane – medium congestion
Safety distances – fuel comparison
1
10
100
1000
1 10 100 1000 10000
m, kg
R, m
CH4
C3H8
C2H4
H2
Results
• The same as a function of total combustion energy of released gas
Safety distances – fuel comparison
1
10
100
1000
100 1000 10000 100000 1000000
E, MJ
R, m
CH4
C3H8
C2H4H2
Conclusions
• A simple approximate analytical method for evaluation of blast effects and safety distances for unconfined H2 explosions has been presented
• Potential blast effects of unconfined H2 explosions strongly depends on the level of congestion
• Certain threshold values of the mass of hydrogen released may be defined as potentially damaging
• This minimum mass varies by several orders of magnitude depending on the level of congestion
• In terms of potential blast effects, hydrogen may represent a significantly high threat as compared to ethylene, propane, and methane
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