Evaluation of Safety Distances Related to Unconfined Hydrogen Explosions Sergey Dorofeev FM Global 1 st ICHS, Pisa, Italy, September 8-10, 2005

Motivation H 2 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 H 2 explosions can also be a significant safety problem  Releases in obstructed areas (refuelling stations, hydrogen production units, etc.)  Relatively fast dilution of H 2 -air mixtures at open air and inefficient FA without confinement  On the other hand: large quantities of H 2 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

Methodology Flame folding due to obstacles Plus Bradley correlation for turbulent burning velocity: Flame speeds b a  x x y R R

Methodology Experimental data Flame speeds

Methodology Correlation Flame speeds

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 reaches maximum in the centre and decreases linearly with radius Stoichiometric H 2 /air – unrealistic and overconservative! Model for gas distribution r C max

Methodology Variable: maximum concentration in the centre, C max ‘Worst case’: maximum of =, averaged between UFL and LFL Properties of ‘worst case’:  C max = 88% vol.  = 0.1  max  = 60% of total chemical energy ‘Worst case’ distribution LFL C max UFL

Methodology Calculations of blast parameters are based on our method published in 1996 Dimensionless overpressure and impulse are functions of flame speed, V f Blast parameters

Methodology An assessment of damage potential is made here using pressure-impulse (P, I) damage criteria Damage potential Damage descriptionP a, PaI a, Pa∙sk, Pa 2 ∙s Total destruction of buildings Threshold for partial destruction; 50 to 75% of walls destroyed Threshold for serious structural damage; some load bearing members fall Border of minor structural damage

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

Results Example for medium congestion Radii for selected levels of damages

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

Results The same method applied to: hydrogen, ethylene, propane, methane – medium congestion Safety distances – fuel comparison

Results The same as a function of total combustion energy of released gas Safety distances – fuel comparison

Conclusions A simple approximate analytical method for evaluation of blast effects and safety distances for unconfined H 2 explosions has been presented Potential blast effects of unconfined H 2 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