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Study on the harm effects of releases from liquid hydrogen tank by consequence modeling
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 6 2 4 e1 7 6 2 9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Study on the harm effects of releases from liquid hydrogen tank by consequence modeling Zhiyong Li a, Xiangmin Pan b,c, Xi Meng b,c, Jianxin Ma b,c,* a
Institute for Energy Conservation, Jiaxing University, Jiaxing 314001, PR China Clean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, PR China c School of Automotive Studies, Tongji University, Shanghai 201804, PR China b
article info
abstract
Article history:
The accidental releases of hydrogen from liquid storage and the subsequent consequences
Received 27 December 2011
are studied from a harm perspective rather than a standpoint of risk. The cold, thermal and
Received in revised form
overpressure effects from hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud
20 April 2012
explosion are evaluated in terms of two kinds of effect distances based on lethal and
Accepted 27 May 2012
harmful criteria. Results show that for instantaneous release, the sequence of effect
Available online 30 June 2012
distances is vapor cloud explosion > flash fire > cold cloud > fireball, and for continuous release, the sequence is vapor cloud explosion > flash fire > jet fire > cold cloud. An overall
Keywords:
comparison between instantaneous and continuous release reveals that the catastrophic
Hydrogen releases
rupture, rather than leakages, is the dominant event. Besides, the effect distances of liquid
Liquid hydrogen tank
hydrogen tank are compared with those of 70 MPa gaseous storage with equivalent mass.
Harm effects
Compared with 70 MPa gaseous storage, the liquid hydrogen storage may be safer under
Consequence modeling
leak scenarios but more dangerous under catastrophic rupture scenario. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In order to make hydrogen-powered vehicles travel as long distance as the gasoline-powered vehicles, more hydrogen needs to be carried onboard. Liquid hydrogen can be stored and transported in much larger quantities than compressed hydrogen and may be considered as an alternative for hydrogen vehicles, especially for those which require long driving range or long hours of operation. Despite the complexity of the storage and refilling systems, automobile producers like BMW and GM have applied liquid storage technology in their hydrogen cars, internal combustion engines and fuel cells, respectively. Besides issues of hydrogen boil-off, liquefaction, weight, volume and tank cost need to be addressed, safety issues are
also critical concerns for the use of liquid hydrogen storage. Compared with compressed hydrogen, liquid hydrogen has much lower storage pressure. Thus the risk caused by high pressure may be reduced to some extent. However, new hazards such as cold effects are introduced by cryogenic hydrogen release with extremely low temperature. The safety issues on cryogenic liquid hydrogen for on-board hydrogen storage need further investigation. This paper studies the accidental releases of hydrogen from cryogenic liquid storage tank and calculates the subsequent consequences such as hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud explosion. The cold effects, thermal effects and overpressure effects from the hydrogen consequences are evaluated in terms of two kinds of effect distances based on lethal and harmful criteria. The effect
* Corresponding author. Clean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, PR China. Tel.: þ86 21 69589480; fax: þ86 21 69589121. E-mail address: [email protected] (J. Ma). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.141
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 6 2 4 e1 7 6 2 9
distances of liquid hydrogen tank are also compared with those of 70 MPa gaseous hydrogen storage with equivalent mass.
2.
Modeling
2.1. Possible consequences of releases from liquid hydrogen tank Releases from liquid hydrogen tank can be either instantaneous or continuous. The instantaneous release is a sudden burst of a storage tank. The result is the release of the entire hydrogen and the subsequent evaporation and dispersion of the hydrogen. Immediate ignition of the released hydrogen may result in a fireball. If without immediate ignition, the released cryogenic hydrogen can cause a significant reduction in ambient temperature, which may cause harm to people. The released hydrogen can rapidly mix with air and form a hydrogen flammable cloud. Ignition of the hydrogen cloud will result in a flash fire (vapor cloud fire). A vapor cloud explosion may occur if the released hydrogen accumulates in a confined area. The consequences of continuous release also depend on the time of ignition. Direct ignition results in a jet fire, while delayed ignition results in a flash fire or an explosion. Without ignition, the cold cloud may also cause harm to people. To explain these consequences more clearly, an event tree is drawn in Fig. 1. To conclude, five typical consequences of hydrogen release are considered in our study: cold cloud, fireball, jet fire, flash fire and vapor cloud explosion. The thermal effects including both direct flame contact and heat radiation are calculated with fireball model by Martinsen et al. [1] and jet fire model by Cook et al. [2], respectively. The explosion overpressure of a vapor cloud explosion is calculated with BakereStrehlow method [3].
2.2.
Harm effects and harm criteria
There are three kinds of harm effects for releases from liquid hydrogen tank: cold effects from cryogenic hydrogen, thermal effects from hydrogen fires, and overpressure effects from hydrogen explosions. For cold effects, the reduction in temperature caused by the released hydrogen may result in cryogenic burns to people. For thermal effects in a fire event,
Without ignition Instantaneous release
Liquid hydrogen
the direct contact with hydrogen flames and the radiant heat fluxes may cause burn to people. For overpressure effects in an explosion event, the blast wave overpressures and possible fragments may result in eardrum rupture, lung hemorrhage, internal injuries, head and body impact, etc. The severity of these effects can be divided into two levels in general: lethal effect and harmful effect. According to IGC Doc 75/07/E/rev [4], the criteria for harmful effect are defined as an approximation value of 1% probability of fatality to a general population.
2.2.1.
Criteria for cold effects
According to IGC Doc 75/07/E/rev [4], the harm criterion for cold effect could be a temperature below 40 C, corresponding to 1% probability of fatality to a general population. As for lethal effect, it is not likely that high lethality will result from cryogenic burn as the total amount of hydrogen released from a fuel cell car is not large.
2.2.2.
Criteria for thermal effects
In a fire event, exposures to hydrogen flames or radiant heat flux may result in burn injury or lethality. Direct contact with hydrogen flames can be conservatively assumed to result in lethality, regardless of whether it is a prolonged direct flame contact (e.g. engulfment in a jet fire) or an instant direct flame contact (e.g. engulfment in a flash fire). Therefore, the fireball radius in a fireball event, the jet flame length in a jet fire event and the 4% hydrogen concentration envelope in a flash fire event can be considered as the criteria for lethal effects. For exposures to radiant heat flux, the consequences are dependent upon both the heat flux and the exposure duration, so harm criterion can be expressed in term of a thermal dose unit which combines the heat flux intensity with the exposure time. The initial form of measuring thermal dose, so as to relate it to injury or fatality factors, was to take a simple product of radiation intensity and time (I t). However, Lees stated that this can understate the effect of radiation at higher intensities and a better correlation should beIn t. Eisenberg et al. proposed that index, n, should take the value of 1.33(4/3) when correlating fatality data and 1.15 for non-fatal injuries. Various authors have adopted this approach and Hymes proposed that a value of 1.33 provides adequate correlation in both fatal and non-fatal cases, which has been widely followed [5].
Cold cloud
Direct ignition
Fireball Flash fire
With ignition Delayed ignition Without ignition Cold cloud
Continuous release
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Direct ignition
Vapor cloud explosion
Jet fire Flash fire
With ignition Delayed ignition
Vapor cloud explosion
Fig. 1 e Event tree of releases from liquid hydrogen tank.
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According to TNO Green book [6], a thermal dose of 520 (kW/m2)3/4 s is suggested for the harm criterion of 1% probability of fatality. This thermal dose will be used as the criterion for harmful effects from fireball and jet fire. As for flash fire, the criterion of 1% lethality is not found in literature. However, a hydrogen concentration level of 2% is suggested as a starting valve for lethal effect in Seveso Directive [7]. This criterion may be conservatively considered as the one close to 1% lethality and will be used as the criterion for harmful effects from flash fire.
2.2.3.
Atmospheric temperature is assumed to be 15 C. Wind velocity is assumed to be 5 m/s. Pasquill stability is assumed to be neutral stability (D). Pasquill stability describes the amount of turbulence in the atmosphere. The stability depends on the wind speed, time of day, and other conditions such as solar radiation and cloud cover. Pasquill stability-D refers to a neutral atmospheric condition which usually occurs on less sunny and windier days or overcast and windy nights. Details of Pasquill stability can be found in Atmospheric Diffusion [10].
Criteria for overpressure effects
In Seveso Directive [7], 30 kPa is recommended as a high lethality value and will be used as the criterion for lethal effects. As for harmful effects, according to IGC75/07E [4], 7 kPa is suggested as the harm criterion for harmful effects, corresponding to 1% lethality. A summary of these criteria is listed in Table 1. Based on these harm criteria, the effect distance can be divided into two categories: “harmful distance” and “lethal distance”, corresponding to harmful effects and lethal effects, respectively. All effects are calculated at one meter height instead of the ground level. One meter, rather than zero meter, is considered to be a good average height for exposure to people, which include both adults and children.
3.
2.3.
3.1.
Harm effect distances of each consequence
3.1.1.
Catastrophic rupture
Other modeling assumptions and parameters
For reasons of conservatism, all continuous releases are assumed to be horizontal and downwind. Delayed ignition points are assumed to be located at the furthest downwind location at which the concentration is equal to the lower flammable limit. As for release pressure, liquid hydrogen is usually transported in a cryogenic tank at approximate 0.7e1.4 bar (10e20 psi) [8], therefore the medium value 1 bar is selected as the storage pressure in modeling. As for release hole size, the leak diameter is conservatively assumed to be 10 mm, corresponding to the full bore rupture of 14.3 mm (9/16 inch) pipe or connection parts. Leakages usually occur in connectors, valves and fittings [9] and generally the leak diameter is not expected to be larger than the hole size of full bore rupture. Other modeling parameters such as release height and weather conditions are listed in Table 2. Release inventory is assumed to be 3.5 kg. Release height is assumed to be 1 m.
Results and discussions
Calculations are carried out with Process Hazard Analysis Software Tool from Det Norske Veritas. The software is usually used to assess situations which present potential hazards to life. The calculations are done in three steps including discharge calculations, dispersion calculations and flammable effects calculations. These steps are executed sequentially. First the discharge (hydrogen released from storage), then the dispersion (cloud formation) and finally the flammable effects (fireball, jet fire, flash fire, and vapor cloud explosion). The cryogenic effects (the temperature drop) are already calculated in discharge and dispersion calculations.
The harm effect distances of catastrophic rupture of liquid hydrogen tank are shown in Fig. 2. It can be seen that for instantaneous release from liquid hydrogen tank, the cold cloud produces a harmful distance of 21.0 m to people. If immediate ignition occurs, the direct contact with a fireball will result in a lethal distance of 3.3 m. The thermal dose of the fireball cannot reach the harm criterion of 520 (kW/m2)3/4 s because of the short duration, indicating that the harm effect of the heat radiation from short-duration fireball may be neglected. If delayed ignition occurs in open space, the flammable cloud may lead to a flash fire with a lethal distance of 34.2 m and a harmful distance of 36.6 m. If delayed ignition occurs in confined space, the flammable cloud may lead to a vapor cloud explosion, resulting in a lethal distance of 35.6 m and a harmful distance of 42.2 m.
Table 1 e Harm criteria used in modeling. Consequences Cold cloud
Harm effect Cold effects
Fireball
Thermal effects
Jet fire
Thermal effects
Flash fire
Thermal effects
Vapor cloud explosion
Overpressure effects
Lethal Harmful Lethal Harmful Lethal Harmful Lethal Harmful
Harm criteria (1% lethality) (1% lethality) (1% lethality) (1% lethality)
Lethal Harmful (1% lethality)
High lethality is not likely 40 C Fireball radius Thermal dose of 520 (kW/m2)3/4 s Jet fame length Thermal dose of 520 (kW/m2)3/4 s LFL (4% concentration) 1/2 LFL (2% concentration) (start valve for lethality, be conservatively considered as close to 1% lethality) 30 kPa 7 kPa
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Table 2 e Modeling assumptions and parameters.
12
Item
10
Release inventory (kg) Storage pressure (bar) Release direction Release hole size (mm) Release height (m) Atmospheric temperature ( C) Wind velocity (m/s) Pasquill stability Result output height (m)
3.5 1 e e 1 15
Leak from tank
Horizontal, downwind 10 mm
Leakage
45 Effect distances (m)
35
Harmful distances Lethal distances
30 25 20 15
520(kW/m2)3/4s Not reached
10 5 0
Cold cloud
Fireball
6 4 2
Flash fire
Cold cloud
Jet fire
Flash fire
Vapor cloud explosion
Fig. 3 e Harm effect distances of 10 mm leakage of liquid hydrogen tank.
5 D (neutral) 1
The harm effect distances of a leakage with hole size of 10 mm are shown in Fig. 3. It can be seen that for continuous release from liquid hydrogen tank, the cold cloud produces only 1.6 m harmful distance to people, much shorter than the cold effect distance of 21 m of catastrophic rupture. This result may be attributed to the much smaller release rate of the leakage. At smaller release rate, the ambient air has sufficient time to warm the released hydrogen, resulting in smaller cold cloud. If immediate ignition occurs, the direct contact with a jet flame will result in a lethal distance of 4.3 m, longer than the lethal distance of 3.3 m caused by a fireball. Moreover, the thermal dose of the jet fire is much greater than that of the fireball because of the much longer duration. As shown in Fig. 3, the thermal dose of 520 (kW/m2)3/4 s caused by the jet fire can reach 5.7 m. If delayed ignition occurs in open space, the flammable cloud can lead to a flash fire with a lethal distance of 5.5 m and a harmful distance of 7.7 m, much shorter than those caused by catastrophic rupture. The flammable cloud of the leakage cannot reach as far as that of tank rupture because of its smaller mass flow rate, which results in easier dilution and smaller flammable cloud. If delayed ignition occurs in confined space, the flammable cloud may lead to a vapor cloud explosion, resulting in a lethal
40
Lethal distances
8
0
The overall sequence of effect distances for instantaneous release from liquid hydrogen tank is vapor cloud explosion > flash fire > cold cloud > fireball. The vapor cloud explosion is the leading consequence for instantaneous release from liquid hydrogen tank.
3.1.2.
Harmful distances Effect distances(m)
Catastrophic rupture of tank
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Vapor cloud explosion
Fig. 2 e Harm effect distances of catastrophic rupture of liquid hydrogen tank.
distance of 6.1 m and a harmful distance of 11.0 m. These effect distances are also much shorter than those caused by catastrophic rupture because of the smaller flammable cloud of the leakage. An overall comparison between Figs. 2 and 3 tells that the longest effect distance of a catastrophic rupture is much longer than that of the worst leakage with hole size of 10 mm. From a pure harm perspective, the catastrophic rupture, rather than leak scenario, is the dominant event. One might argue that the sudden burst of cryogenic hydrogen tank is not likely to occur unless in some extreme event such as a car crash, and the leak scenarios should be the more frequent events likely to occur. The TNO Purple Book [11] usually suggests that the frequency of catastrophic failure of pressure vessel is one order of magnitude lower than that of leakages, and for non-pressure vessel the rupture frequency should be much smaller. The argument is reasonable and may be right from a standpoint of risk. However from a pure consequence perspective, the catastrophic rupture, rather than leak scenario, should be considered as the dominant event. The overall sequence of effect distances for continuous release from liquid hydrogen tank is vapor cloud explosion > flash fire > jet fire > cold cloud. The vapor cloud explosion is the leading consequence for continuous release from liquid hydrogen tank.
3.2. Comparison of compressed gas cylinder and liquid hydrogen tank with equivalent mass Compared with 70 MPa gaseous hydrogen, the liquid hydrogen storage has much lower pressure and temperature. The lower pressure may improve safety while the cryogenic temperature may introduce new hazard. Which of the storage options are safer needs further investigations. A safety comparison between liquid storage and 70 MPa storage is performed as following based on a consequence perspective. The developments of consequences for compressed gaseous hydrogen release are physical explosion, jet fire, flash fire and vapor cloud explosion, having been studied in our previous research [12]. Comparisons will be drawn under the worst conditions including catastrophic rupture and the worst leakage with a hole size of 10 mm. For catastrophic rupture, the effect distance comparison between 70 MPa storage and liquid hydrogen storage are shown in Fig. 4. It can be seen that physical explosion of
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45
Effect distances (m)
40 35 30
Harmful distances(Liquid) Harmful distances(70MPa) Lethal distances(Liquid) Lethal distances(70MPa)
25
60
50
Effect distances (m)
70 MPa storage produces longer effect distances than cold cloud from liquid storage rupture. The physical explosion of 70 MPa storage produces a harmful distance of 26.2 m and a lethal distance of 11.0 m, while the cold effect of liquid hydrogen tank leads to a harmful distance of 21.0 m and no lethal effect. It may be concluded that for instantaneous release without ignition, the liquid hydrogen storage is safer than 70 MPa storage. In contrast, for flammable effects, it can be seen from Fig. 4 that all ignition consequences of releases from liquid hydrogen storage including fireball, flash fire and vapor cloud explosion produce longer harmful distances or lethal distances than those from 70 MPa storage. It may be concluded that for instantaneous release with ignition, liquid hydrogen storage is more dangerous than 70 MPa storage. As for continuous release, the effect distance comparison between liquid hydrogen storage and 70 MPa storage is shown in Fig. 5. It can be seen that the cold effect is the only nonflammable effect, so for continuous release without ignition, liquid hydrogen storage is more dangerous than 70 MPa storage. In contrast, for flammable effect, it can be seen from Fig. 5 that all ignition consequences of releases from liquid storage including jet fire, flash fire and vapor cloud explosion produce shorter harmful distance and lethal distance than those from 70 MPa storage. It may be concluded that for continuous release with ignition, liquid hydrogen storage is safer than 70 MPa storage. To sum up, for non-flammable effect, liquid hydrogen storage is safer than 70 MPa storage for instantaneous release, but more dangerous than 70 MPa storage for continuous release. In contrast, for flammable effects, liquid hydrogen storage is more dangerous than 70 MPa storage for instantaneous release, but safer than 70 MPa storage for continuous release. Considering that flash fire and vapor cloud explosion are the leading consequences, as shown in Figs. 4 and 5, we may conclude that liquid hydrogen storage may be safer than 70 MPa gaseous storage under leak scenario but more dangerous than 70 MPa storage under catastrophic rupture scenario. From a pure consequence perspective, it is difficult to tell which storage is safer in general. Further investigations need to be made from a standpoint of risk, which combines both consequences and the likelihood of release scenarios.
Harmful distances(Liquid) Harmful distances(70MPa) Lethal distances(Liquid) Lethal distances(70MPa)
40
30
20
10
0
Cold cloud
Non-flammable effects
Jet fire
Flash fire
Vapor cloud explosion
Flammable effects
Fig. 5 e Harm effect distances of 10 mm leakage for two different storages.
4.
Conclusions
In this paper, the accidental release of hydrogen from cryogenic liquid storage tank and the subsequent consequences are studied including hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud explosion. The cold effects, thermal effects and overpressure effects from the consequences are evaluated. Besides, these effects of releases from liquid hydrogen tank are compared with those from 70 MPa compressed hydrogen storages with equivalent mass. The main conclusions can be summarized as following. (1) For instantaneous release from liquid hydrogen tank, the sequence of harm effect distances is vapor cloud explosion > flash fire > cold cloud > fireball. For continuous releases from liquid hydrogen tank, the sequence of harm effect distances is vapor cloud explosion > flash > fire jet fire > cold cloud. The vapor cloud explosion is always the leading consequence of releases from liquid hydrogen tank. (2) The longest lethal and harmful distances under catastrophic rupture scenario are much longer than those under the worst leak scenario. From a pure harm perspective, the catastrophic rupture, rather than leakages, is the dominant event. (3) The liquid hydrogen storage may be safer than 70 MPa gaseous storage under leak scenario but more dangerous under catastrophic rupture scenario. From a pure consequence perspective, it is difficult to tell which storage is safer in general. Further investigations need to be made from a standpoint of risk, which will combine both consequences and the likelihood of release scenarios.
20 15 10
Acknowledgment
5 0
Cold cloud
Physical explosion
Non-flammable effects
Fireball
Flash fire
Vapor cloud explosion
Flammable effects
Fig. 4 e Harm effect distances of catastrophic rupture for two different storages.
This work is supported by: (1) the High-technology Research and Development Program of China (863 Program), Project Number: 2011AA11A284; (2) Program of International S&T Cooperation, Project Number: 2010DFA64080; (3) the 111 Project, Project Number: B08019
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 6 2 4 e1 7 6 2 9
references
[1] Martinsen WE, Marx JD. An improved model for the prediction of radiant heat from fireballs. In: International Conference and Workshop on Modelling Consequences of Accidental Releases of Hazardous Materials. San Francisco California; 1999. p. 605e21. [2] Cook J, Bahrami Z, Whitehouse RJ. A comprehensive program for calculation of flame radiation levels. Journal of Loss Prevention in the Process Industries 1990;3:150e5. [3] Baker QA, Doolittle CM, Fitzgerald GA, Tang MJ. Recent developments in the Baker-Strehlow VCE analysis methodology, vol. 17. Process Safety Progress; 1998. p. 297e301. [4] IGC Doc 75/07/E/rev. Determination of safety distances. European Industrial Gases Association; 2007. [5] Daycock JH, Rew PJ. Thermal radiation criteria for vulnerable populations. Health and Safety Executive contract research report 285/2000; 2000.
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[6] CPR 16E (Green Book). A model for the determination of possible damage. TNO; 1992. [7] Decreto Ministeriale 9 maggio. Requisiti minimi di sicurezza in materia di pianificazione urbanistica e territoriale per le zone interessate da stabilimenti a rischio di incidente rilevante. Gazzetta Ufficiale 16 giugno 2001: 138; 2001. [8] California Energy Commission. California hydrogen fueling station guidelines; 2004. [9] Barthe´le´my H. Compatibility of metallic materials with hydrogen-review of the present knowledge. The 16th World Hydrogen Energy Conference; 2006. [10] Pasquil F, Smith FB. In: Atmospheric diffusion. 3rd ed. Ellis Horwood; 1983. [11] CPR 18E (Purple Book). Guidelines for quantitative risk assessment. Committee for the Prevention of Disasters; 1999. [12] Zhiyong Li, Xiangmin Pan, Jianxin Ma. Harm effect distances evaluation of severe accidents for gaseous hydrogen refueling station. International Journal of Hydrogen Energy 2010;35:1515e21.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Study on the harm effects of releases from liquid hydrogen tank by consequence modeling Zhiyong Li a, Xiangmin Pan b,c, Xi Meng b,c, Jianxin Ma b,c,* a
Institute for Energy Conservation, Jiaxing University, Jiaxing 314001, PR China Clean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, PR China c School of Automotive Studies, Tongji University, Shanghai 201804, PR China b
article info
abstract
Article history:
The accidental releases of hydrogen from liquid storage and the subsequent consequences
Received 27 December 2011
are studied from a harm perspective rather than a standpoint of risk. The cold, thermal and
Received in revised form
overpressure effects from hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud
20 April 2012
explosion are evaluated in terms of two kinds of effect distances based on lethal and
Accepted 27 May 2012
harmful criteria. Results show that for instantaneous release, the sequence of effect
Available online 30 June 2012
distances is vapor cloud explosion > flash fire > cold cloud > fireball, and for continuous release, the sequence is vapor cloud explosion > flash fire > jet fire > cold cloud. An overall
Keywords:
comparison between instantaneous and continuous release reveals that the catastrophic
Hydrogen releases
rupture, rather than leakages, is the dominant event. Besides, the effect distances of liquid
Liquid hydrogen tank
hydrogen tank are compared with those of 70 MPa gaseous storage with equivalent mass.
Harm effects
Compared with 70 MPa gaseous storage, the liquid hydrogen storage may be safer under
Consequence modeling
leak scenarios but more dangerous under catastrophic rupture scenario. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In order to make hydrogen-powered vehicles travel as long distance as the gasoline-powered vehicles, more hydrogen needs to be carried onboard. Liquid hydrogen can be stored and transported in much larger quantities than compressed hydrogen and may be considered as an alternative for hydrogen vehicles, especially for those which require long driving range or long hours of operation. Despite the complexity of the storage and refilling systems, automobile producers like BMW and GM have applied liquid storage technology in their hydrogen cars, internal combustion engines and fuel cells, respectively. Besides issues of hydrogen boil-off, liquefaction, weight, volume and tank cost need to be addressed, safety issues are
also critical concerns for the use of liquid hydrogen storage. Compared with compressed hydrogen, liquid hydrogen has much lower storage pressure. Thus the risk caused by high pressure may be reduced to some extent. However, new hazards such as cold effects are introduced by cryogenic hydrogen release with extremely low temperature. The safety issues on cryogenic liquid hydrogen for on-board hydrogen storage need further investigation. This paper studies the accidental releases of hydrogen from cryogenic liquid storage tank and calculates the subsequent consequences such as hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud explosion. The cold effects, thermal effects and overpressure effects from the hydrogen consequences are evaluated in terms of two kinds of effect distances based on lethal and harmful criteria. The effect
* Corresponding author. Clean Energy Automotive Engineering Center, Tongji University, Shanghai 201804, PR China. Tel.: þ86 21 69589480; fax: þ86 21 69589121. E-mail address: [email protected] (J. Ma). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.141
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 7 6 2 4 e1 7 6 2 9
distances of liquid hydrogen tank are also compared with those of 70 MPa gaseous hydrogen storage with equivalent mass.
2.
Modeling
2.1. Possible consequences of releases from liquid hydrogen tank Releases from liquid hydrogen tank can be either instantaneous or continuous. The instantaneous release is a sudden burst of a storage tank. The result is the release of the entire hydrogen and the subsequent evaporation and dispersion of the hydrogen. Immediate ignition of the released hydrogen may result in a fireball. If without immediate ignition, the released cryogenic hydrogen can cause a significant reduction in ambient temperature, which may cause harm to people. The released hydrogen can rapidly mix with air and form a hydrogen flammable cloud. Ignition of the hydrogen cloud will result in a flash fire (vapor cloud fire). A vapor cloud explosion may occur if the released hydrogen accumulates in a confined area. The consequences of continuous release also depend on the time of ignition. Direct ignition results in a jet fire, while delayed ignition results in a flash fire or an explosion. Without ignition, the cold cloud may also cause harm to people. To explain these consequences more clearly, an event tree is drawn in Fig. 1. To conclude, five typical consequences of hydrogen release are considered in our study: cold cloud, fireball, jet fire, flash fire and vapor cloud explosion. The thermal effects including both direct flame contact and heat radiation are calculated with fireball model by Martinsen et al. [1] and jet fire model by Cook et al. [2], respectively. The explosion overpressure of a vapor cloud explosion is calculated with BakereStrehlow method [3].
2.2.
Harm effects and harm criteria
There are three kinds of harm effects for releases from liquid hydrogen tank: cold effects from cryogenic hydrogen, thermal effects from hydrogen fires, and overpressure effects from hydrogen explosions. For cold effects, the reduction in temperature caused by the released hydrogen may result in cryogenic burns to people. For thermal effects in a fire event,
Without ignition Instantaneous release
Liquid hydrogen
the direct contact with hydrogen flames and the radiant heat fluxes may cause burn to people. For overpressure effects in an explosion event, the blast wave overpressures and possible fragments may result in eardrum rupture, lung hemorrhage, internal injuries, head and body impact, etc. The severity of these effects can be divided into two levels in general: lethal effect and harmful effect. According to IGC Doc 75/07/E/rev [4], the criteria for harmful effect are defined as an approximation value of 1% probability of fatality to a general population.
2.2.1.
Criteria for cold effects
According to IGC Doc 75/07/E/rev [4], the harm criterion for cold effect could be a temperature below 40 C, corresponding to 1% probability of fatality to a general population. As for lethal effect, it is not likely that high lethality will result from cryogenic burn as the total amount of hydrogen released from a fuel cell car is not large.
2.2.2.
Criteria for thermal effects
In a fire event, exposures to hydrogen flames or radiant heat flux may result in burn injury or lethality. Direct contact with hydrogen flames can be conservatively assumed to result in lethality, regardless of whether it is a prolonged direct flame contact (e.g. engulfment in a jet fire) or an instant direct flame contact (e.g. engulfment in a flash fire). Therefore, the fireball radius in a fireball event, the jet flame length in a jet fire event and the 4% hydrogen concentration envelope in a flash fire event can be considered as the criteria for lethal effects. For exposures to radiant heat flux, the consequences are dependent upon both the heat flux and the exposure duration, so harm criterion can be expressed in term of a thermal dose unit which combines the heat flux intensity with the exposure time. The initial form of measuring thermal dose, so as to relate it to injury or fatality factors, was to take a simple product of radiation intensity and time (I t). However, Lees stated that this can understate the effect of radiation at higher intensities and a better correlation should beIn t. Eisenberg et al. proposed that index, n, should take the value of 1.33(4/3) when correlating fatality data and 1.15 for non-fatal injuries. Various authors have adopted this approach and Hymes proposed that a value of 1.33 provides adequate correlation in both fatal and non-fatal cases, which has been widely followed [5].
Cold cloud
Direct ignition
Fireball Flash fire
With ignition Delayed ignition Without ignition Cold cloud
Continuous release
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Direct ignition
Vapor cloud explosion
Jet fire Flash fire
With ignition Delayed ignition
Vapor cloud explosion
Fig. 1 e Event tree of releases from liquid hydrogen tank.
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According to TNO Green book [6], a thermal dose of 520 (kW/m2)3/4 s is suggested for the harm criterion of 1% probability of fatality. This thermal dose will be used as the criterion for harmful effects from fireball and jet fire. As for flash fire, the criterion of 1% lethality is not found in literature. However, a hydrogen concentration level of 2% is suggested as a starting valve for lethal effect in Seveso Directive [7]. This criterion may be conservatively considered as the one close to 1% lethality and will be used as the criterion for harmful effects from flash fire.
2.2.3.
Atmospheric temperature is assumed to be 15 C. Wind velocity is assumed to be 5 m/s. Pasquill stability is assumed to be neutral stability (D). Pasquill stability describes the amount of turbulence in the atmosphere. The stability depends on the wind speed, time of day, and other conditions such as solar radiation and cloud cover. Pasquill stability-D refers to a neutral atmospheric condition which usually occurs on less sunny and windier days or overcast and windy nights. Details of Pasquill stability can be found in Atmospheric Diffusion [10].
Criteria for overpressure effects
In Seveso Directive [7], 30 kPa is recommended as a high lethality value and will be used as the criterion for lethal effects. As for harmful effects, according to IGC75/07E [4], 7 kPa is suggested as the harm criterion for harmful effects, corresponding to 1% lethality. A summary of these criteria is listed in Table 1. Based on these harm criteria, the effect distance can be divided into two categories: “harmful distance” and “lethal distance”, corresponding to harmful effects and lethal effects, respectively. All effects are calculated at one meter height instead of the ground level. One meter, rather than zero meter, is considered to be a good average height for exposure to people, which include both adults and children.
3.
2.3.
3.1.
Harm effect distances of each consequence
3.1.1.
Catastrophic rupture
Other modeling assumptions and parameters
For reasons of conservatism, all continuous releases are assumed to be horizontal and downwind. Delayed ignition points are assumed to be located at the furthest downwind location at which the concentration is equal to the lower flammable limit. As for release pressure, liquid hydrogen is usually transported in a cryogenic tank at approximate 0.7e1.4 bar (10e20 psi) [8], therefore the medium value 1 bar is selected as the storage pressure in modeling. As for release hole size, the leak diameter is conservatively assumed to be 10 mm, corresponding to the full bore rupture of 14.3 mm (9/16 inch) pipe or connection parts. Leakages usually occur in connectors, valves and fittings [9] and generally the leak diameter is not expected to be larger than the hole size of full bore rupture. Other modeling parameters such as release height and weather conditions are listed in Table 2. Release inventory is assumed to be 3.5 kg. Release height is assumed to be 1 m.
Results and discussions
Calculations are carried out with Process Hazard Analysis Software Tool from Det Norske Veritas. The software is usually used to assess situations which present potential hazards to life. The calculations are done in three steps including discharge calculations, dispersion calculations and flammable effects calculations. These steps are executed sequentially. First the discharge (hydrogen released from storage), then the dispersion (cloud formation) and finally the flammable effects (fireball, jet fire, flash fire, and vapor cloud explosion). The cryogenic effects (the temperature drop) are already calculated in discharge and dispersion calculations.
The harm effect distances of catastrophic rupture of liquid hydrogen tank are shown in Fig. 2. It can be seen that for instantaneous release from liquid hydrogen tank, the cold cloud produces a harmful distance of 21.0 m to people. If immediate ignition occurs, the direct contact with a fireball will result in a lethal distance of 3.3 m. The thermal dose of the fireball cannot reach the harm criterion of 520 (kW/m2)3/4 s because of the short duration, indicating that the harm effect of the heat radiation from short-duration fireball may be neglected. If delayed ignition occurs in open space, the flammable cloud may lead to a flash fire with a lethal distance of 34.2 m and a harmful distance of 36.6 m. If delayed ignition occurs in confined space, the flammable cloud may lead to a vapor cloud explosion, resulting in a lethal distance of 35.6 m and a harmful distance of 42.2 m.
Table 1 e Harm criteria used in modeling. Consequences Cold cloud
Harm effect Cold effects
Fireball
Thermal effects
Jet fire
Thermal effects
Flash fire
Thermal effects
Vapor cloud explosion
Overpressure effects
Lethal Harmful Lethal Harmful Lethal Harmful Lethal Harmful
Harm criteria (1% lethality) (1% lethality) (1% lethality) (1% lethality)
Lethal Harmful (1% lethality)
High lethality is not likely 40 C Fireball radius Thermal dose of 520 (kW/m2)3/4 s Jet fame length Thermal dose of 520 (kW/m2)3/4 s LFL (4% concentration) 1/2 LFL (2% concentration) (start valve for lethality, be conservatively considered as close to 1% lethality) 30 kPa 7 kPa
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Table 2 e Modeling assumptions and parameters.
12
Item
10
Release inventory (kg) Storage pressure (bar) Release direction Release hole size (mm) Release height (m) Atmospheric temperature ( C) Wind velocity (m/s) Pasquill stability Result output height (m)
3.5 1 e e 1 15
Leak from tank
Horizontal, downwind 10 mm
Leakage
45 Effect distances (m)
35
Harmful distances Lethal distances
30 25 20 15
520(kW/m2)3/4s Not reached
10 5 0
Cold cloud
Fireball
6 4 2
Flash fire
Cold cloud
Jet fire
Flash fire
Vapor cloud explosion
Fig. 3 e Harm effect distances of 10 mm leakage of liquid hydrogen tank.
5 D (neutral) 1
The harm effect distances of a leakage with hole size of 10 mm are shown in Fig. 3. It can be seen that for continuous release from liquid hydrogen tank, the cold cloud produces only 1.6 m harmful distance to people, much shorter than the cold effect distance of 21 m of catastrophic rupture. This result may be attributed to the much smaller release rate of the leakage. At smaller release rate, the ambient air has sufficient time to warm the released hydrogen, resulting in smaller cold cloud. If immediate ignition occurs, the direct contact with a jet flame will result in a lethal distance of 4.3 m, longer than the lethal distance of 3.3 m caused by a fireball. Moreover, the thermal dose of the jet fire is much greater than that of the fireball because of the much longer duration. As shown in Fig. 3, the thermal dose of 520 (kW/m2)3/4 s caused by the jet fire can reach 5.7 m. If delayed ignition occurs in open space, the flammable cloud can lead to a flash fire with a lethal distance of 5.5 m and a harmful distance of 7.7 m, much shorter than those caused by catastrophic rupture. The flammable cloud of the leakage cannot reach as far as that of tank rupture because of its smaller mass flow rate, which results in easier dilution and smaller flammable cloud. If delayed ignition occurs in confined space, the flammable cloud may lead to a vapor cloud explosion, resulting in a lethal
40
Lethal distances
8
0
The overall sequence of effect distances for instantaneous release from liquid hydrogen tank is vapor cloud explosion > flash fire > cold cloud > fireball. The vapor cloud explosion is the leading consequence for instantaneous release from liquid hydrogen tank.
3.1.2.
Harmful distances Effect distances(m)
Catastrophic rupture of tank
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Vapor cloud explosion
Fig. 2 e Harm effect distances of catastrophic rupture of liquid hydrogen tank.
distance of 6.1 m and a harmful distance of 11.0 m. These effect distances are also much shorter than those caused by catastrophic rupture because of the smaller flammable cloud of the leakage. An overall comparison between Figs. 2 and 3 tells that the longest effect distance of a catastrophic rupture is much longer than that of the worst leakage with hole size of 10 mm. From a pure harm perspective, the catastrophic rupture, rather than leak scenario, is the dominant event. One might argue that the sudden burst of cryogenic hydrogen tank is not likely to occur unless in some extreme event such as a car crash, and the leak scenarios should be the more frequent events likely to occur. The TNO Purple Book [11] usually suggests that the frequency of catastrophic failure of pressure vessel is one order of magnitude lower than that of leakages, and for non-pressure vessel the rupture frequency should be much smaller. The argument is reasonable and may be right from a standpoint of risk. However from a pure consequence perspective, the catastrophic rupture, rather than leak scenario, should be considered as the dominant event. The overall sequence of effect distances for continuous release from liquid hydrogen tank is vapor cloud explosion > flash fire > jet fire > cold cloud. The vapor cloud explosion is the leading consequence for continuous release from liquid hydrogen tank.
3.2. Comparison of compressed gas cylinder and liquid hydrogen tank with equivalent mass Compared with 70 MPa gaseous hydrogen, the liquid hydrogen storage has much lower pressure and temperature. The lower pressure may improve safety while the cryogenic temperature may introduce new hazard. Which of the storage options are safer needs further investigations. A safety comparison between liquid storage and 70 MPa storage is performed as following based on a consequence perspective. The developments of consequences for compressed gaseous hydrogen release are physical explosion, jet fire, flash fire and vapor cloud explosion, having been studied in our previous research [12]. Comparisons will be drawn under the worst conditions including catastrophic rupture and the worst leakage with a hole size of 10 mm. For catastrophic rupture, the effect distance comparison between 70 MPa storage and liquid hydrogen storage are shown in Fig. 4. It can be seen that physical explosion of
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45
Effect distances (m)
40 35 30
Harmful distances(Liquid) Harmful distances(70MPa) Lethal distances(Liquid) Lethal distances(70MPa)
25
60
50
Effect distances (m)
70 MPa storage produces longer effect distances than cold cloud from liquid storage rupture. The physical explosion of 70 MPa storage produces a harmful distance of 26.2 m and a lethal distance of 11.0 m, while the cold effect of liquid hydrogen tank leads to a harmful distance of 21.0 m and no lethal effect. It may be concluded that for instantaneous release without ignition, the liquid hydrogen storage is safer than 70 MPa storage. In contrast, for flammable effects, it can be seen from Fig. 4 that all ignition consequences of releases from liquid hydrogen storage including fireball, flash fire and vapor cloud explosion produce longer harmful distances or lethal distances than those from 70 MPa storage. It may be concluded that for instantaneous release with ignition, liquid hydrogen storage is more dangerous than 70 MPa storage. As for continuous release, the effect distance comparison between liquid hydrogen storage and 70 MPa storage is shown in Fig. 5. It can be seen that the cold effect is the only nonflammable effect, so for continuous release without ignition, liquid hydrogen storage is more dangerous than 70 MPa storage. In contrast, for flammable effect, it can be seen from Fig. 5 that all ignition consequences of releases from liquid storage including jet fire, flash fire and vapor cloud explosion produce shorter harmful distance and lethal distance than those from 70 MPa storage. It may be concluded that for continuous release with ignition, liquid hydrogen storage is safer than 70 MPa storage. To sum up, for non-flammable effect, liquid hydrogen storage is safer than 70 MPa storage for instantaneous release, but more dangerous than 70 MPa storage for continuous release. In contrast, for flammable effects, liquid hydrogen storage is more dangerous than 70 MPa storage for instantaneous release, but safer than 70 MPa storage for continuous release. Considering that flash fire and vapor cloud explosion are the leading consequences, as shown in Figs. 4 and 5, we may conclude that liquid hydrogen storage may be safer than 70 MPa gaseous storage under leak scenario but more dangerous than 70 MPa storage under catastrophic rupture scenario. From a pure consequence perspective, it is difficult to tell which storage is safer in general. Further investigations need to be made from a standpoint of risk, which combines both consequences and the likelihood of release scenarios.
Harmful distances(Liquid) Harmful distances(70MPa) Lethal distances(Liquid) Lethal distances(70MPa)
40
30
20
10
0
Cold cloud
Non-flammable effects
Jet fire
Flash fire
Vapor cloud explosion
Flammable effects
Fig. 5 e Harm effect distances of 10 mm leakage for two different storages.
4.
Conclusions
In this paper, the accidental release of hydrogen from cryogenic liquid storage tank and the subsequent consequences are studied including hydrogen cold cloud, fireball, jet fire, flash fire, and vapor cloud explosion. The cold effects, thermal effects and overpressure effects from the consequences are evaluated. Besides, these effects of releases from liquid hydrogen tank are compared with those from 70 MPa compressed hydrogen storages with equivalent mass. The main conclusions can be summarized as following. (1) For instantaneous release from liquid hydrogen tank, the sequence of harm effect distances is vapor cloud explosion > flash fire > cold cloud > fireball. For continuous releases from liquid hydrogen tank, the sequence of harm effect distances is vapor cloud explosion > flash > fire jet fire > cold cloud. The vapor cloud explosion is always the leading consequence of releases from liquid hydrogen tank. (2) The longest lethal and harmful distances under catastrophic rupture scenario are much longer than those under the worst leak scenario. From a pure harm perspective, the catastrophic rupture, rather than leakages, is the dominant event. (3) The liquid hydrogen storage may be safer than 70 MPa gaseous storage under leak scenario but more dangerous under catastrophic rupture scenario. From a pure consequence perspective, it is difficult to tell which storage is safer in general. Further investigations need to be made from a standpoint of risk, which will combine both consequences and the likelihood of release scenarios.
20 15 10
Acknowledgment
5 0
Cold cloud
Physical explosion
Non-flammable effects
Fireball
Flash fire
Vapor cloud explosion
Flammable effects
Fig. 4 e Harm effect distances of catastrophic rupture for two different storages.
This work is supported by: (1) the High-technology Research and Development Program of China (863 Program), Project Number: 2011AA11A284; (2) Program of International S&T Cooperation, Project Number: 2010DFA64080; (3) the 111 Project, Project Number: B08019
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[6] CPR 16E (Green Book). A model for the determination of possible damage. TNO; 1992. [7] Decreto Ministeriale 9 maggio. Requisiti minimi di sicurezza in materia di pianificazione urbanistica e territoriale per le zone interessate da stabilimenti a rischio di incidente rilevante. Gazzetta Ufficiale 16 giugno 2001: 138; 2001. [8] California Energy Commission. California hydrogen fueling station guidelines; 2004. [9] Barthe´le´my H. Compatibility of metallic materials with hydrogen-review of the present knowledge. The 16th World Hydrogen Energy Conference; 2006. [10] Pasquil F, Smith FB. In: Atmospheric diffusion. 3rd ed. Ellis Horwood; 1983. [11] CPR 18E (Purple Book). Guidelines for quantitative risk assessment. Committee for the Prevention of Disasters; 1999. [12] Zhiyong Li, Xiangmin Pan, Jianxin Ma. Harm effect distances evaluation of severe accidents for gaseous hydrogen refueling station. International Journal of Hydrogen Energy 2010;35:1515e21.