Evaluation of Safety Distances Related to Unconfined Hydrogen Explosions

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

2. Motivation Confined versus unconfined • 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

3. Motivation Consequences • 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

4. Motivation Analysis strategy • 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

5. 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

6. Methodology Flame speeds • 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

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

8. Methodology Flame speeds • Experimental data

9. Methodology Flame speeds • Correlation

10. Methodology Hydrogen distribution • 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

11. Methodology Model for gas distribution • 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! r Cmax

12. Methodology ‘Worst case’ distribution • 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. • < > = 0.1max • <E> = 60% of total chemical energy LFL UFL Cmax

13. Methodology Blast parameters • Calculations of blast parameters are based on our method published in 1996 • Dimensionless overpressure and impulse are functions of flame speed, Vf

14. Methodology Damage potential • An assessment of damage potential is made here using pressure-impulse (P, I) damage criteria

15. Results Characteristic obstacle geometry • 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)

16. Results Flame speeds • Obstacle geometry affects significantly flame speeds • To reach 300 m/s: 1 kg, 40 kg, and 1000 kg high, medium, and low congestion

17. Results Radii for selected levels of damages • Example for medium congestion

18. Results Safety distances – contributing factors • Scenarios • Consequences • Pressure • Thermal • Fragments • Acceptance criteria • Population • Regulations • Costs

19. Results Safety distances - example • Defined, as an example, by minimum building damage criterion for unconfined H2 explosions

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

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

22. 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