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
b
a
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 gtgt 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 lt? gtlt?(?-1)SLgt,
  averaged between UFL and LFL
• Properties of worst case
• Cmax 88 vol.
• lt? gt 0.1?max
• ltEgt 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

Damage description Pa, Pa Ia, Pas k, Pa2s
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
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