- Email: [email protected]
Risk assessment for liquid hydrogen fueling stations
international journal of hydrogen energy 34 (2009) 1135–1141
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Risk assessment for liquid hydrogen fueling stations Shigeki Kikukawaa,*, Hirotada Mitsuhashia, Atsumi Miyakeb a
Japan Petroleum Energy Center, New Fuels Department, 3-9 Toranomon 4-Chome, Minato-ku, Tokyo 105-0001, Japan Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
b
article info
abstract
Article history:
In recent years, consumers calling for the protection of the environment on a regional and
Received 2 October 2008
global scale are demanding the use of vehicles that do not emit harmful exhaust. It is
Received in revised form
anticipated that one response to this demand is the widespread use of fuel cell vehicles
23 October 2008
(FCVs). In order to achieve this, it is necessary to provide hydrogen fueling stations where
Accepted 23 October 2008
FCVs can refuel.
Available online 13 December 2008
A liquid hydrogen fueling station was selected for the purpose of this study as liquid hydrogen can be transported and stored in much larger quantities than compressed
Keywords:
hydrogen gas. The facility of hydrogen fueling stations must be safe. In order to gauge the
Risk assessment
safety measures necessary for liquid hydrogen fueling stations we used a risk assessment
Liquid hydrogen fueling stations
approach. A large number of accident scenarios were identified using FMEA and HAZOP.
Safety measures
The consequence level for each accident scenario was evaluated using data from liquid
Codes and standards
hydrogen explosion experiments. The size of the risk of the accident scenario was evaluated using a risk matrix and, in order to reduce that risk, a study was made into the necessary safety measures for liquid hydrogen fueling stations. As a result of this study, we were able to gauge the safety measures required to guarantee a high level of safety for liquid hydrogen fueling stations. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1. Compressed hydrogen fueling stations and liquid hydrogen fueling stations In order for fuel cell vehicles to run as long as the travel distance of gasoline-powered vehicles, the fuel cell vehicle must carry as much hydrogen as possible. In order to achieve this the hydrogen gas would either have to be stored in highpressure hydrogen gas cylinders or stored as liquid hydrogen. At normal temperature and pressure hydrogen takes a gaseous form but at temperatures below 252.6 C hydrogen gas liquifies. Fueling stations that dispense liquid hydrogen to fuel cell vehicles are known as ‘Liquid Hydrogen Fueling Stations’. Compared to compressed hydrogen fueling stations, liquid hydrogen fueling stations are low pressure and, as there are
fewer machines to use, simple in construction. However, in order to handle the cryogenic temperatures required by liquid hydrogen, they differ widely in that they require a double-wall storage tank, pipes and dispensing hoses and that the dispenser must be equipped to handle cryogenic liquids. A Liquid Hydrogen Fueling Station and a dispensing hose are shown in Figs. 1 and 2, and the simple flowcharts for liquid hydrogen fueling station shown in Fig. 3.
2.
Outline of risk assessment
The introduction of liquid hydrogen fueling stations, which are the subject of this study, should run parallel to that of the fuel cell vehicle. They must be as safe as gasoline stations.
* Corresponding author. Tel.: þ81 3 5402 8513; fax: þ81 3 5402 8527. E-mail address: [email protected] (S. Kikukawa). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.10.093
1136
international journal of hydrogen energy 34 (2009) 1135–1141
being negligible. As a result of this type of safety assessment of all potential accident scenarios, it is considered that the risk at liquid hydrogen fueling stations can be sufficiently reduced, therefore allowing the establishment of safety measures which ensure their safety.
3. Definition of liquid hydrogen station model
Fig. 1 – Liquid hydrogen fueling station [1].
However, at the present time, the liquid hydrogen fueling stations of an appropriate size that is expected to spread in the future are not yet in existence. Then it was decided that in this study, safety inspections would be conducted using ‘Risk Assessment Method [2]’ which is widely accepted as being an effective method of safety assessment. Fig. 4 shows a flowchart detailing the investigation using the Risk Assessment Method. Firstly, the model of the liquid hydrogen station that is expected to spread in the future will be defined based on the anticipated demand for liquid hydrogen fueling stations. Next, all possible accidents that could occur at the model of liquid hydrogen station will be identified and a potential accident scenario list drawn up. It is extremely important that at this point all potential hazards are identified. Next, the magnitude of risk of each potential accident scenario on the list will be calculated and the result entered into the risk matrix and evaluated. Should the risk be assessed as being negligible, then the investigation into that potential accident scenario shall be terminated. If that is not the case, then safety measures will be considered to reduce the risk and the risk re-assessed. The potential accident scenario shall be investigated until the risk can be assessed as
Fig. 2 – A dispensing hose.
The liquid hydrogen station model being used as the subject of this investigation was defined as being of a style that is expected be widely used in the future and one that can feasibly be constructed and correctly equipped. Specific capabilities and specifications are based on the anticipated demand of the spread of the fuel cell vehicle shown in the results of investigations into liquified hydrogen fueling stations by organizations such as the World Energy NETwork (WE-NET) [3]. It was estimated that the liquid hydrogen storage tanks would be comparatively large as it was anticipated that both liquid hydrogen and vaporized compressed hydrogen would be used for refueling. Furthermore, the scale of vaporization loss needed be taken into account. The specifications for the liquid hydrogen station model are shown in Table 1.
4.
Hazard identification
In risk assessment it is extremely important to thoroughly investigate all accident scenarios relating to the model. Understanding each accident scenario requires close examination of the details of the investigation. Accident scenarios were identified using the HAZOP (Hazard and Operability Studies) and FMEA (Failure Mode and Effect Analysis) methods that are widely considered to be effective ways of identifying the source of a risk. Further reference was made to Refs. [4,5] in the accident database enabling scenarios that had been overlooked to be added and thus increasing the thoroughness of the investigation into accident scenarios. At liquid hydrogen fueling stations, double-wall vacuum insulating pipes are used for parts that need to be maintained at low temperatures such as pipes running from the storage tank to dispensers. Parts that do not need to be maintained at low temperatures use single-wall pipes. In accident scenarios involving single-wall pipes, it is conceivable that hydrogen in liquid form or vaporized hydrogen could leak out from a hole (Fig. 5). However, where double-wall pipes are concerned, the location of the hydrogen leak differs depending on whether the hole is in the outer or inner tube. For example, if a pinhole was on the outer layer of the double-wall pipe, air would leak in through the hole, the degree of vacuum would decrease, there would be an increase in the amount of heat infiltrating the double-wall pipe and the liquid hydrogen inside the pipe would vaporize. This would in turn cause a rise in pressure inside the liquid hydrogen storage tank resulting in hydrogen gas be discharged from the vent line (Fig. 6). If, on the other hand, there were a pinhole in the inner layer of the doublewall pipe, the leaked liquid hydrogen would vaporize inside the vacuum between the inner and outer layers causing the
international journal of hydrogen energy 34 (2009) 1135–1141
Liquid Hydrogen Storage Tank
Liquid Hydrogen Lorry
Dispenser
1137
FCV
Fig. 3 – Simple flowchart showing a liquid hydrogen fueling station.
pressure to rise. As there is a vacuum valve on a double-wall pipe allowing for the release of air from the vacuum, hydrogen gas would jet from the valve (Fig. 7). In order to visualize a hydrogen leak from a double-wall pipe, a CFD simulation was used for cases where there was a microscopic hole in the inner layer of the pipe and hydrogen leaked out from a vacuum valve. An example of this can be seen in Fig. 8. As the liquid hydrogen storage tank is also double-walled, it was assumed that similar cases would occur where holes and points from which hydrogen was discharged differed. In accident scenarios involving liquid hydrogen leaks where no ignition or explosion occurred, it was hypothesized that there could be cases where hydrogen in liquified form sprayed out causing frostbite or where, following the leak, liquid hydrogen vaporized and the gas caused asphyxiation. Once all conceivable accident scenarios had been recorded and scenarios that appeared more than once removed from the list, 131 cases remained.
5. Risk assessment of liquid hydrogen fueling stations 5.1.
equipment found at liquid hydrogen fueling stations is based on current technologies, it has not seen much practical use and as such there is not a sufficient accumulation of detailed data pertaining to the probability of breakdowns and accidents at the present time. A thorough investigation was conducted for the identification of these types of accident scenarios and the risks associated with each scenario evaluated and defined as per the risk matrix in Fig. 9, which allows for categorization by level of satisfaction of the safety measures [6].
5.2.
Estimation of consequence levels in risk assessment
As it is difficult to accurately ascertain whether, should the hydrogen leak and ignite, there would be an explosion, fire, or both, the consequence levels of blast pressure and flame were calculated separately allowing for impact on humans and materials to be assessed. For the purposes of risk estimation the event having the largest level of consequence has been reflected in the table.
5.2.1.
Risk matrix
Safety of personnel, customers and public must be achieved as first priority. But excessive safety measures for liquid hydrogen fueling stations will be obstacle to the widespread use of them. This study requires the compatibility of the hydrogen safety with the proper rules. In addition, while
Consequence of blast pressure
There are some forms of explosion like BLEVE or VCE. Authors assumed two forms of explosion that might be happened at liquid hydrogen fueling stations. One is a diffusion explosion where leaked liquid hydrogen vaporizes and mixes with the air and ignites resulting in a diffusion explosion, and the other is a premixed explosion where leaked liquid hydrogen remains on the residual area and mixes with the air and ignites resulting in a premixed explosion. The appropriate
START Definition of H2 station model
Experiments, Simulations, Surveys, etc. By Project Partners
Hazard Identification
Risk Estimation Risk Reduction Risk Evaluation
N
Tolerable Risk? Y
Output of the study : Safety requirements for H2 station
END Fig. 4 – Sequence of verification procedures in the risk assessment.
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international journal of hydrogen energy 34 (2009) 1135–1141
To vent line
Table 1 – Specifications of liquid hydrogen fueling station model. No. of cars refueled/h Refuel capacity/car Amount of liquid hydrogen refueled/h Liquid hydrogen storage tank
10 38 L (30 Nm3) 380 L/h (300 Nm3/h) ( ¼ 10 cars/h 38 L/car) Pressure: 0.35 MPa, Capacity: 17,000 L 2
No. of dispensers
LH2 Storage Tank
Heat infiltrating Air
Vaporized hydrogen gas form of explosion is selected according to the state of leakage in each accident scenario and, once the blast pressure for each had been calculated, the consequence level was assessed. Because it was understood that, depending on the ignition timing blast pressure would differ, the experiments and simulations used an ignition timing that would result in maximum blast pressure. Fig. 10 shows the relationship between the distance from the point of ignition with the blast pressure when using a hole size of 1 mm [7].
5.2.2.
Fig. 6 – A pinhole was on the outer layer of the double-wall pipe.
5.3.2.
Estimates gained as a result of experiments
Where possible, experiments were conducted for the purpose of obtaining estimates and the results of those experiments recorded. For example, recording the results of experiments conducted on kinks in the dispensing hose or material fatigue [8].
The impact of a jet flame
As it is not visible to the naked eye, in order for the length of the flame to be measured, the hydrogen flame had to be made visible using flame photometry techniques and filmed (Fig. 11). The results were as follows: a 14-mm wide hole produced a 10-m long jet flame, a 1.0-mm wide hole produced a 1.7-m long jet flame and a 0.2-mm wide hole produced no jet flame. It is thought that as the hole was small and considering relatively low pressure, only a small amount of hydrogen leaked out and that it dispersed before ignition could occur [7].
5.3.3.
Other assessments
Where the experiment itself is difficult to conduct and where present data is insufficient, a qualitative assessment was carried out based upon estimates provided by an expert.
6. Safety measures for liquid hydrogen fueling stations
5.3.
Estimation of probability levels in risk assessment
As a result of this study, we got sixty-seven safety measures that must be installed to liquid hydrogen fueling stations. Table 2 shows the main safety measures taken from the list.
5.3.1.
Use of a similar accident database
6.1.
General liquid hydrogen fueling stations
6.1.1.
Policies concerning materials used
Where there have been similar accidents at existing gasoline stations and factories that handle high-pressure gas, the probability of the occurrence of an accident was estimated based on the number of those cases. For example, the number of cases of similar accidents at gasoline stations [4] was used for accident scenarios where, as the result of fuel cell vehicles accidentally moving away during refueling, the dispensing hose was severed or the dispenser was pulled down.
As storage tanks and pipes at liquid hydrogen fueling stations are both used outdoors and come into contact with cryogenic liquid hydrogen, if appropriate measures are not in place, rainwater will corrode metals and hydrogen or low temperature embrittlement may result in cracks and ruptures which may result in a large liquid hydrogen leak. The impact of corrosion and hydrogen embrittlement differs depending on the type of steel. As the impact on SUS316L in particular is extremely small [9], it has been decided that this type of steel shall be used for equipment at liquid hydrogen fueling
Hydrogen
Hydrogen gas would jet from the vacuum valve.
Pinhole
Fig. 5 – Leakage from single-wall pipe.
Fig. 7 – A pinhole was on the inner layer of the double-wall pipe.
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international journal of hydrogen energy 34 (2009) 1135–1141
Peak Over Pressure (Pa)
100000
10000
1000 z=1.0m Forward z=1.0m Backword
Fig. 8 – Example of a CFD simulation where a pinhole was on the inner layer of the double-wall pipe.
stations. We need more evidence that the materials have a performance of anti-hydrogen embrittlement.
100 0.1
1
10
100
Distance from point of ignition (m) Fig. 10 – Relationship of the distance between the point of ignition with blast pressure (d [ 1 mm; P [ 0.4 MPa, continuous leak).
6.1.2. Installation of a fire protection wall along site boundaries It has been decided that the liquid hydrogen fueling stations shall be surrounded by 2 m high fire protection walls (excluding the approach) and that a barrier be erected on the site. The purpose of the fire protection wall is there to prevent a fire at the fueling station from adversely affecting the area outside the boundaries and to prevent any fires in the vicinity from affecting the station. In addition, a barrier shall be erected between the area of the fueling station in which the compressor and accumulator have been installed and the area which houses the dispenser and from which gas is pumped in order to minimize the impact on customers on site in the unlikely event of hydrogen leak.
crack in an earthquake, the resulting difference in ground levels may cause the pipes to rupture. Therefore, it has been decided that in order to prevent cracks generated by earthquakes and differing ground levels, the area of the station from the storage tanks to the dispenser be built on a single foundation.
6.2.
Liquid hydrogen storage tanks
6.2.1.
Design of quakeproof storage tanks
Rather than laying the pipe running between the liquid hydrogen storage and the dispenser underground, it has been decided to lay them in trenches. This enables prevention of damage to the pipes if vehicles drive over them and allows for the pipes to be inspected.
It is necessary to take measures that will prevent a large leak in the event of a strong earthquake. Therefore, it has been decided that liquid hydrogen storage tanks should be designed to be quakeproof and that they should be constructed on a firm surface as determined by subsurface investigations. Further measures would entail the installation of a seismometer that, in the event of an earthquake, would allow for monitoring of tremors and automatic shut down of the hydrogen fueling station.
6.1.4. Liquid hydrogen equipment to be installed on a common foundation
6.2.2. Monitoring pressure of the vacuum-insulated layer inside storage tanks
As double-wall pipes in liquid hydrogen fueling stations are inflexible, should the ground on which the station is built
Should, for some reason, the outer storage tank of the liquid hydrogen storage be damaged, and the vacuum-insulated
6.1.3.
Laying pipes in trenches
Probability Level Consequence Severity Level 1
Extremely Severe Damage
2
Severe Damage
3
Damage
4
Limited Damage
5
Minor Damage
A
B
C
D
Improbable
Remote
Occasional
Probable
H M M L L
H H M L L
H H H M L
H H H H M
Fig. 9 – Risk matrix.
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international journal of hydrogen energy 34 (2009) 1135–1141
6.3.2. Installation of a break-away device (under development) Should a fuel cell vehicle accidentally move away from the dispenser while being refueled with liquid hydrogen, the dispenser hose will be pulled off resulting in a large quantity of liquid hydrogen leaking out. It has been decided that a break-away device be installed to reduce the risk. However, where this device is in use with gasoline dispenser hoses and compressed hydrogen gas hoses, those for use with liquid hydrogen are still being developed. It is hoped that they will be commercially available in the very near future.
7. Fig. 11 – Experiment of a jet flame (d [ 1.0 mm, P [ 0.4 MPa, Mitsubishi Heavy Industry).
layer rupture allowing heat infiltration, the quantity of vaporized liquid hydrogen would increase and the quantity of hydrogen discharged from the vent line would increase. As there is a high risk that the hydrogen gas discharged from the vent line would be over a certain amount, it has been decided that should the degree of vacuum in the vacuum-insulated layer be observed to be abnormal.
6.3.
Dispenser
6.3.1.
Installation of a collision guard
In order to prevent slow-moving vehicles from colliding with the dispenser (e.g. such as those where the driver has mistaken the accelerator for the brake), a collision guard shall be installed around the dispenser.
Table 2 – Main safety measures. Location (1) General
Safety measures Policies concerning materials used (corrosion, hydrogen embrittlement, low temperature embrittlement) Installation of a fire protection wall along station boundaries Laying pipes in trenches Liquid hydrogen equipment to be installed on a common foundation
(2) Liquid hydrogen Design of quakeproof storage tanks storage tank Monitoring pressure of the thermally insulated vacuum layer in storage tanks (3) Dispenser
Installation of a collision guard Installation of a break-away device under development
(4) Vent line
Installation of a heater and orifice in the secondary covers of the valves and manual line valves Connection of the purge valve to the vent line
Summary of results of risk assessment
The results of the risk assessment on liquid hydrogen fueling stations can be seen in Fig. 12. There were 131 identified accident scenarios pertaining to the model liquid hydrogen fueling stations. The results of assessments of all accident scenarios that took place before the adoption of safety measures can be seen in the table on the left. The results of assessments of all accident scenarios that took place after the adoption of safety measures can be seen on the right. The figures in the tables show the number of potential accident scenarios and the corresponding risk level. As potential accident scenarios that enabled intrinsic measures to be devised are incompatible with the risk matrix, they were not entered on the right side of Fig. 12. When the results of risk assessment following the application of safety measures were compared to those gained prior to the application of safety measures, the risk at the liquid hydrogen filing station was seen to have been significantly reduced. This study produced sixty-seven safety measures which are considered necessary for liquid hydrogen fueling stations. In other words, the safety measures that were accurately applied at liquid hydrogen fueling stations reduce the risk as much as possible and provide a high level of safety. However, even after the application of the safety measures there were still forty-five potential accident scenarios listed in the medium risk. This is because, as the probability of ignition and the presence of humans and buildings were conservatively set at 1, it was assessed that station personnel would come into contact with a jet flame from the leak and that the human impact within the site boundaries would be high. Unfortunately, even after application of safety measures, thirteen potential accident scenarios that had been assessed as being high remained. These were: (a) a helicopter/airplane crash landing on the storage tanks, (b) a crane on a neighboring site falling on and damaging the storage tanks, (c) a moving vehicle colliding with the dispenser, (d) an FCV moving away while being refueled, and (e) an earthquake occurring during refueling causing the vehicle to move violently thereby rupturing the hose. (a) and (b) are difficult for the hydrogen fueling station to deal with independently and should be dealt with by means of other regulations. The measures corresponding to the (c) scenario could not be specified as the weight and speed of the moving vehicle are
international journal of hydrogen energy 34 (2009) 1135–1141
Before Application of Safety Measures
After Application of Safety Measures
Probability Level
A
B
C
D
I
8
1
12
29
II
0
0
0
6
III
8
1
10
8
IV
2
0
4
14
V
0
1
5
13
Consequence severity Level
Probability Level
Consequence severity Level
1141
A
B
C
D
I
13
0
0
0
II
5
0
0
0
III
31
8
0
0
IV
13
2
1
0
V
19
5
1
0
Fig. 12 – Risk map.
unknown factors. The share valve used in some of the measures is still being developed. The others were put into practice following the doubling and tripling of safety measures which resulted in the mitigation of likelihood to an extremely low level but, as the safety measures could not be adopted for the purpose of reducing consequence levels, they had to remain in the high category. Safety measures for (d) and (e) in particular require the development of a break-away device for liquid hydrogen. As a result of the above investigation, this study has succeeded in revealing the effectiveness and suitability of safety measures for liquid hydrogen fueling stations through use of the proposed risk matrix. In addition, it has enabled the establishment of a framework for the risk assessment of liquid hydrogen fueling stations.
Acknowledgements This study represents a part of the study that JPEC was entrusted by the Independent Administrative Agency ‘‘New Energy and Industrial Technology Development Organization (NEDO)’’. It was conducted as part of the ‘‘Establishment of Codes & Standards for Hydrogen Economy Society – Study on the Safety Technologies of Hydrogen Infrastructure’’. The authors gratefully acknowledge the valuable guidance provided to us for this study by members of the committee, affiliated organizations and intellectuals and those various
organizations that cooperated with the experiments, analysis, and safety inspections.
references
[1] Showa Shell Sekiyu Website. Available from: http://www. showa-shell.co.jp/products/hydrogen/station.html. [2] ISO/IEC Guide 51. Safety aspects-guidelines for their inclusion in standards. 2nd ed.; 1999. [3] World Energy Network. Phase 2, Task 1. Investigation and study for system evaluation, vol. 4. The Institute of Applied Energy; 2002. [4] Technical know-how database for troubles related to hazardous materials. Hazardous Materials Safety Techniques Association; 2005. [5] Accident examples database. The High Pressure Gas Safety Institute of Japan; 2005. [6] Shigeki Kikukawa, et al. Risk assessment of Hydrogen fueling stations for 70 MPa FCVs. International Journal of Hydrogen Energy 2008;33:7130. [7] Committee on the Safety Technologies of Hydrogen Infrastructure, Japan Petroleum Energy Center, March 2, 2007, file06-2-2. [8] Yoshiharu Matsuoka, et al. Safety technology for a hydrogen supply infrastructure. Final report from 2004 to 2005, 2005. p. 458. [9] World Energy Network. Phase 2, Task 10, development of cryogenic materials technologies. Japan Research and Development Center for Metals (JRCM); 2002.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Risk assessment for liquid hydrogen fueling stations Shigeki Kikukawaa,*, Hirotada Mitsuhashia, Atsumi Miyakeb a
Japan Petroleum Energy Center, New Fuels Department, 3-9 Toranomon 4-Chome, Minato-ku, Tokyo 105-0001, Japan Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
b
article info
abstract
Article history:
In recent years, consumers calling for the protection of the environment on a regional and
Received 2 October 2008
global scale are demanding the use of vehicles that do not emit harmful exhaust. It is
Received in revised form
anticipated that one response to this demand is the widespread use of fuel cell vehicles
23 October 2008
(FCVs). In order to achieve this, it is necessary to provide hydrogen fueling stations where
Accepted 23 October 2008
FCVs can refuel.
Available online 13 December 2008
A liquid hydrogen fueling station was selected for the purpose of this study as liquid hydrogen can be transported and stored in much larger quantities than compressed
Keywords:
hydrogen gas. The facility of hydrogen fueling stations must be safe. In order to gauge the
Risk assessment
safety measures necessary for liquid hydrogen fueling stations we used a risk assessment
Liquid hydrogen fueling stations
approach. A large number of accident scenarios were identified using FMEA and HAZOP.
Safety measures
The consequence level for each accident scenario was evaluated using data from liquid
Codes and standards
hydrogen explosion experiments. The size of the risk of the accident scenario was evaluated using a risk matrix and, in order to reduce that risk, a study was made into the necessary safety measures for liquid hydrogen fueling stations. As a result of this study, we were able to gauge the safety measures required to guarantee a high level of safety for liquid hydrogen fueling stations. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1. Compressed hydrogen fueling stations and liquid hydrogen fueling stations In order for fuel cell vehicles to run as long as the travel distance of gasoline-powered vehicles, the fuel cell vehicle must carry as much hydrogen as possible. In order to achieve this the hydrogen gas would either have to be stored in highpressure hydrogen gas cylinders or stored as liquid hydrogen. At normal temperature and pressure hydrogen takes a gaseous form but at temperatures below 252.6 C hydrogen gas liquifies. Fueling stations that dispense liquid hydrogen to fuel cell vehicles are known as ‘Liquid Hydrogen Fueling Stations’. Compared to compressed hydrogen fueling stations, liquid hydrogen fueling stations are low pressure and, as there are
fewer machines to use, simple in construction. However, in order to handle the cryogenic temperatures required by liquid hydrogen, they differ widely in that they require a double-wall storage tank, pipes and dispensing hoses and that the dispenser must be equipped to handle cryogenic liquids. A Liquid Hydrogen Fueling Station and a dispensing hose are shown in Figs. 1 and 2, and the simple flowcharts for liquid hydrogen fueling station shown in Fig. 3.
2.
Outline of risk assessment
The introduction of liquid hydrogen fueling stations, which are the subject of this study, should run parallel to that of the fuel cell vehicle. They must be as safe as gasoline stations.
* Corresponding author. Tel.: þ81 3 5402 8513; fax: þ81 3 5402 8527. E-mail address: [email protected] (S. Kikukawa). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.10.093
1136
international journal of hydrogen energy 34 (2009) 1135–1141
being negligible. As a result of this type of safety assessment of all potential accident scenarios, it is considered that the risk at liquid hydrogen fueling stations can be sufficiently reduced, therefore allowing the establishment of safety measures which ensure their safety.
3. Definition of liquid hydrogen station model
Fig. 1 – Liquid hydrogen fueling station [1].
However, at the present time, the liquid hydrogen fueling stations of an appropriate size that is expected to spread in the future are not yet in existence. Then it was decided that in this study, safety inspections would be conducted using ‘Risk Assessment Method [2]’ which is widely accepted as being an effective method of safety assessment. Fig. 4 shows a flowchart detailing the investigation using the Risk Assessment Method. Firstly, the model of the liquid hydrogen station that is expected to spread in the future will be defined based on the anticipated demand for liquid hydrogen fueling stations. Next, all possible accidents that could occur at the model of liquid hydrogen station will be identified and a potential accident scenario list drawn up. It is extremely important that at this point all potential hazards are identified. Next, the magnitude of risk of each potential accident scenario on the list will be calculated and the result entered into the risk matrix and evaluated. Should the risk be assessed as being negligible, then the investigation into that potential accident scenario shall be terminated. If that is not the case, then safety measures will be considered to reduce the risk and the risk re-assessed. The potential accident scenario shall be investigated until the risk can be assessed as
Fig. 2 – A dispensing hose.
The liquid hydrogen station model being used as the subject of this investigation was defined as being of a style that is expected be widely used in the future and one that can feasibly be constructed and correctly equipped. Specific capabilities and specifications are based on the anticipated demand of the spread of the fuel cell vehicle shown in the results of investigations into liquified hydrogen fueling stations by organizations such as the World Energy NETwork (WE-NET) [3]. It was estimated that the liquid hydrogen storage tanks would be comparatively large as it was anticipated that both liquid hydrogen and vaporized compressed hydrogen would be used for refueling. Furthermore, the scale of vaporization loss needed be taken into account. The specifications for the liquid hydrogen station model are shown in Table 1.
4.
Hazard identification
In risk assessment it is extremely important to thoroughly investigate all accident scenarios relating to the model. Understanding each accident scenario requires close examination of the details of the investigation. Accident scenarios were identified using the HAZOP (Hazard and Operability Studies) and FMEA (Failure Mode and Effect Analysis) methods that are widely considered to be effective ways of identifying the source of a risk. Further reference was made to Refs. [4,5] in the accident database enabling scenarios that had been overlooked to be added and thus increasing the thoroughness of the investigation into accident scenarios. At liquid hydrogen fueling stations, double-wall vacuum insulating pipes are used for parts that need to be maintained at low temperatures such as pipes running from the storage tank to dispensers. Parts that do not need to be maintained at low temperatures use single-wall pipes. In accident scenarios involving single-wall pipes, it is conceivable that hydrogen in liquid form or vaporized hydrogen could leak out from a hole (Fig. 5). However, where double-wall pipes are concerned, the location of the hydrogen leak differs depending on whether the hole is in the outer or inner tube. For example, if a pinhole was on the outer layer of the double-wall pipe, air would leak in through the hole, the degree of vacuum would decrease, there would be an increase in the amount of heat infiltrating the double-wall pipe and the liquid hydrogen inside the pipe would vaporize. This would in turn cause a rise in pressure inside the liquid hydrogen storage tank resulting in hydrogen gas be discharged from the vent line (Fig. 6). If, on the other hand, there were a pinhole in the inner layer of the doublewall pipe, the leaked liquid hydrogen would vaporize inside the vacuum between the inner and outer layers causing the
international journal of hydrogen energy 34 (2009) 1135–1141
Liquid Hydrogen Storage Tank
Liquid Hydrogen Lorry
Dispenser
1137
FCV
Fig. 3 – Simple flowchart showing a liquid hydrogen fueling station.
pressure to rise. As there is a vacuum valve on a double-wall pipe allowing for the release of air from the vacuum, hydrogen gas would jet from the valve (Fig. 7). In order to visualize a hydrogen leak from a double-wall pipe, a CFD simulation was used for cases where there was a microscopic hole in the inner layer of the pipe and hydrogen leaked out from a vacuum valve. An example of this can be seen in Fig. 8. As the liquid hydrogen storage tank is also double-walled, it was assumed that similar cases would occur where holes and points from which hydrogen was discharged differed. In accident scenarios involving liquid hydrogen leaks where no ignition or explosion occurred, it was hypothesized that there could be cases where hydrogen in liquified form sprayed out causing frostbite or where, following the leak, liquid hydrogen vaporized and the gas caused asphyxiation. Once all conceivable accident scenarios had been recorded and scenarios that appeared more than once removed from the list, 131 cases remained.
5. Risk assessment of liquid hydrogen fueling stations 5.1.
equipment found at liquid hydrogen fueling stations is based on current technologies, it has not seen much practical use and as such there is not a sufficient accumulation of detailed data pertaining to the probability of breakdowns and accidents at the present time. A thorough investigation was conducted for the identification of these types of accident scenarios and the risks associated with each scenario evaluated and defined as per the risk matrix in Fig. 9, which allows for categorization by level of satisfaction of the safety measures [6].
5.2.
Estimation of consequence levels in risk assessment
As it is difficult to accurately ascertain whether, should the hydrogen leak and ignite, there would be an explosion, fire, or both, the consequence levels of blast pressure and flame were calculated separately allowing for impact on humans and materials to be assessed. For the purposes of risk estimation the event having the largest level of consequence has been reflected in the table.
5.2.1.
Risk matrix
Safety of personnel, customers and public must be achieved as first priority. But excessive safety measures for liquid hydrogen fueling stations will be obstacle to the widespread use of them. This study requires the compatibility of the hydrogen safety with the proper rules. In addition, while
Consequence of blast pressure
There are some forms of explosion like BLEVE or VCE. Authors assumed two forms of explosion that might be happened at liquid hydrogen fueling stations. One is a diffusion explosion where leaked liquid hydrogen vaporizes and mixes with the air and ignites resulting in a diffusion explosion, and the other is a premixed explosion where leaked liquid hydrogen remains on the residual area and mixes with the air and ignites resulting in a premixed explosion. The appropriate
START Definition of H2 station model
Experiments, Simulations, Surveys, etc. By Project Partners
Hazard Identification
Risk Estimation Risk Reduction Risk Evaluation
N
Tolerable Risk? Y
Output of the study : Safety requirements for H2 station
END Fig. 4 – Sequence of verification procedures in the risk assessment.
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To vent line
Table 1 – Specifications of liquid hydrogen fueling station model. No. of cars refueled/h Refuel capacity/car Amount of liquid hydrogen refueled/h Liquid hydrogen storage tank
10 38 L (30 Nm3) 380 L/h (300 Nm3/h) ( ¼ 10 cars/h 38 L/car) Pressure: 0.35 MPa, Capacity: 17,000 L 2
No. of dispensers
LH2 Storage Tank
Heat infiltrating Air
Vaporized hydrogen gas form of explosion is selected according to the state of leakage in each accident scenario and, once the blast pressure for each had been calculated, the consequence level was assessed. Because it was understood that, depending on the ignition timing blast pressure would differ, the experiments and simulations used an ignition timing that would result in maximum blast pressure. Fig. 10 shows the relationship between the distance from the point of ignition with the blast pressure when using a hole size of 1 mm [7].
5.2.2.
Fig. 6 – A pinhole was on the outer layer of the double-wall pipe.
5.3.2.
Estimates gained as a result of experiments
Where possible, experiments were conducted for the purpose of obtaining estimates and the results of those experiments recorded. For example, recording the results of experiments conducted on kinks in the dispensing hose or material fatigue [8].
The impact of a jet flame
As it is not visible to the naked eye, in order for the length of the flame to be measured, the hydrogen flame had to be made visible using flame photometry techniques and filmed (Fig. 11). The results were as follows: a 14-mm wide hole produced a 10-m long jet flame, a 1.0-mm wide hole produced a 1.7-m long jet flame and a 0.2-mm wide hole produced no jet flame. It is thought that as the hole was small and considering relatively low pressure, only a small amount of hydrogen leaked out and that it dispersed before ignition could occur [7].
5.3.3.
Other assessments
Where the experiment itself is difficult to conduct and where present data is insufficient, a qualitative assessment was carried out based upon estimates provided by an expert.
6. Safety measures for liquid hydrogen fueling stations
5.3.
Estimation of probability levels in risk assessment
As a result of this study, we got sixty-seven safety measures that must be installed to liquid hydrogen fueling stations. Table 2 shows the main safety measures taken from the list.
5.3.1.
Use of a similar accident database
6.1.
General liquid hydrogen fueling stations
6.1.1.
Policies concerning materials used
Where there have been similar accidents at existing gasoline stations and factories that handle high-pressure gas, the probability of the occurrence of an accident was estimated based on the number of those cases. For example, the number of cases of similar accidents at gasoline stations [4] was used for accident scenarios where, as the result of fuel cell vehicles accidentally moving away during refueling, the dispensing hose was severed or the dispenser was pulled down.
As storage tanks and pipes at liquid hydrogen fueling stations are both used outdoors and come into contact with cryogenic liquid hydrogen, if appropriate measures are not in place, rainwater will corrode metals and hydrogen or low temperature embrittlement may result in cracks and ruptures which may result in a large liquid hydrogen leak. The impact of corrosion and hydrogen embrittlement differs depending on the type of steel. As the impact on SUS316L in particular is extremely small [9], it has been decided that this type of steel shall be used for equipment at liquid hydrogen fueling
Hydrogen
Hydrogen gas would jet from the vacuum valve.
Pinhole
Fig. 5 – Leakage from single-wall pipe.
Fig. 7 – A pinhole was on the inner layer of the double-wall pipe.
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Peak Over Pressure (Pa)
100000
10000
1000 z=1.0m Forward z=1.0m Backword
Fig. 8 – Example of a CFD simulation where a pinhole was on the inner layer of the double-wall pipe.
stations. We need more evidence that the materials have a performance of anti-hydrogen embrittlement.
100 0.1
1
10
100
Distance from point of ignition (m) Fig. 10 – Relationship of the distance between the point of ignition with blast pressure (d [ 1 mm; P [ 0.4 MPa, continuous leak).
6.1.2. Installation of a fire protection wall along site boundaries It has been decided that the liquid hydrogen fueling stations shall be surrounded by 2 m high fire protection walls (excluding the approach) and that a barrier be erected on the site. The purpose of the fire protection wall is there to prevent a fire at the fueling station from adversely affecting the area outside the boundaries and to prevent any fires in the vicinity from affecting the station. In addition, a barrier shall be erected between the area of the fueling station in which the compressor and accumulator have been installed and the area which houses the dispenser and from which gas is pumped in order to minimize the impact on customers on site in the unlikely event of hydrogen leak.
crack in an earthquake, the resulting difference in ground levels may cause the pipes to rupture. Therefore, it has been decided that in order to prevent cracks generated by earthquakes and differing ground levels, the area of the station from the storage tanks to the dispenser be built on a single foundation.
6.2.
Liquid hydrogen storage tanks
6.2.1.
Design of quakeproof storage tanks
Rather than laying the pipe running between the liquid hydrogen storage and the dispenser underground, it has been decided to lay them in trenches. This enables prevention of damage to the pipes if vehicles drive over them and allows for the pipes to be inspected.
It is necessary to take measures that will prevent a large leak in the event of a strong earthquake. Therefore, it has been decided that liquid hydrogen storage tanks should be designed to be quakeproof and that they should be constructed on a firm surface as determined by subsurface investigations. Further measures would entail the installation of a seismometer that, in the event of an earthquake, would allow for monitoring of tremors and automatic shut down of the hydrogen fueling station.
6.1.4. Liquid hydrogen equipment to be installed on a common foundation
6.2.2. Monitoring pressure of the vacuum-insulated layer inside storage tanks
As double-wall pipes in liquid hydrogen fueling stations are inflexible, should the ground on which the station is built
Should, for some reason, the outer storage tank of the liquid hydrogen storage be damaged, and the vacuum-insulated
6.1.3.
Laying pipes in trenches
Probability Level Consequence Severity Level 1
Extremely Severe Damage
2
Severe Damage
3
Damage
4
Limited Damage
5
Minor Damage
A
B
C
D
Improbable
Remote
Occasional
Probable
H M M L L
H H M L L
H H H M L
H H H H M
Fig. 9 – Risk matrix.
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6.3.2. Installation of a break-away device (under development) Should a fuel cell vehicle accidentally move away from the dispenser while being refueled with liquid hydrogen, the dispenser hose will be pulled off resulting in a large quantity of liquid hydrogen leaking out. It has been decided that a break-away device be installed to reduce the risk. However, where this device is in use with gasoline dispenser hoses and compressed hydrogen gas hoses, those for use with liquid hydrogen are still being developed. It is hoped that they will be commercially available in the very near future.
7. Fig. 11 – Experiment of a jet flame (d [ 1.0 mm, P [ 0.4 MPa, Mitsubishi Heavy Industry).
layer rupture allowing heat infiltration, the quantity of vaporized liquid hydrogen would increase and the quantity of hydrogen discharged from the vent line would increase. As there is a high risk that the hydrogen gas discharged from the vent line would be over a certain amount, it has been decided that should the degree of vacuum in the vacuum-insulated layer be observed to be abnormal.
6.3.
Dispenser
6.3.1.
Installation of a collision guard
In order to prevent slow-moving vehicles from colliding with the dispenser (e.g. such as those where the driver has mistaken the accelerator for the brake), a collision guard shall be installed around the dispenser.
Table 2 – Main safety measures. Location (1) General
Safety measures Policies concerning materials used (corrosion, hydrogen embrittlement, low temperature embrittlement) Installation of a fire protection wall along station boundaries Laying pipes in trenches Liquid hydrogen equipment to be installed on a common foundation
(2) Liquid hydrogen Design of quakeproof storage tanks storage tank Monitoring pressure of the thermally insulated vacuum layer in storage tanks (3) Dispenser
Installation of a collision guard Installation of a break-away device under development
(4) Vent line
Installation of a heater and orifice in the secondary covers of the valves and manual line valves Connection of the purge valve to the vent line
Summary of results of risk assessment
The results of the risk assessment on liquid hydrogen fueling stations can be seen in Fig. 12. There were 131 identified accident scenarios pertaining to the model liquid hydrogen fueling stations. The results of assessments of all accident scenarios that took place before the adoption of safety measures can be seen in the table on the left. The results of assessments of all accident scenarios that took place after the adoption of safety measures can be seen on the right. The figures in the tables show the number of potential accident scenarios and the corresponding risk level. As potential accident scenarios that enabled intrinsic measures to be devised are incompatible with the risk matrix, they were not entered on the right side of Fig. 12. When the results of risk assessment following the application of safety measures were compared to those gained prior to the application of safety measures, the risk at the liquid hydrogen filing station was seen to have been significantly reduced. This study produced sixty-seven safety measures which are considered necessary for liquid hydrogen fueling stations. In other words, the safety measures that were accurately applied at liquid hydrogen fueling stations reduce the risk as much as possible and provide a high level of safety. However, even after the application of the safety measures there were still forty-five potential accident scenarios listed in the medium risk. This is because, as the probability of ignition and the presence of humans and buildings were conservatively set at 1, it was assessed that station personnel would come into contact with a jet flame from the leak and that the human impact within the site boundaries would be high. Unfortunately, even after application of safety measures, thirteen potential accident scenarios that had been assessed as being high remained. These were: (a) a helicopter/airplane crash landing on the storage tanks, (b) a crane on a neighboring site falling on and damaging the storage tanks, (c) a moving vehicle colliding with the dispenser, (d) an FCV moving away while being refueled, and (e) an earthquake occurring during refueling causing the vehicle to move violently thereby rupturing the hose. (a) and (b) are difficult for the hydrogen fueling station to deal with independently and should be dealt with by means of other regulations. The measures corresponding to the (c) scenario could not be specified as the weight and speed of the moving vehicle are
international journal of hydrogen energy 34 (2009) 1135–1141
Before Application of Safety Measures
After Application of Safety Measures
Probability Level
A
B
C
D
I
8
1
12
29
II
0
0
0
6
III
8
1
10
8
IV
2
0
4
14
V
0
1
5
13
Consequence severity Level
Probability Level
Consequence severity Level
1141
A
B
C
D
I
13
0
0
0
II
5
0
0
0
III
31
8
0
0
IV
13
2
1
0
V
19
5
1
0
Fig. 12 – Risk map.
unknown factors. The share valve used in some of the measures is still being developed. The others were put into practice following the doubling and tripling of safety measures which resulted in the mitigation of likelihood to an extremely low level but, as the safety measures could not be adopted for the purpose of reducing consequence levels, they had to remain in the high category. Safety measures for (d) and (e) in particular require the development of a break-away device for liquid hydrogen. As a result of the above investigation, this study has succeeded in revealing the effectiveness and suitability of safety measures for liquid hydrogen fueling stations through use of the proposed risk matrix. In addition, it has enabled the establishment of a framework for the risk assessment of liquid hydrogen fueling stations.
Acknowledgements This study represents a part of the study that JPEC was entrusted by the Independent Administrative Agency ‘‘New Energy and Industrial Technology Development Organization (NEDO)’’. It was conducted as part of the ‘‘Establishment of Codes & Standards for Hydrogen Economy Society – Study on the Safety Technologies of Hydrogen Infrastructure’’. The authors gratefully acknowledge the valuable guidance provided to us for this study by members of the committee, affiliated organizations and intellectuals and those various
organizations that cooperated with the experiments, analysis, and safety inspections.
references
[1] Showa Shell Sekiyu Website. Available from: http://www. showa-shell.co.jp/products/hydrogen/station.html. [2] ISO/IEC Guide 51. Safety aspects-guidelines for their inclusion in standards. 2nd ed.; 1999. [3] World Energy Network. Phase 2, Task 1. Investigation and study for system evaluation, vol. 4. The Institute of Applied Energy; 2002. [4] Technical know-how database for troubles related to hazardous materials. Hazardous Materials Safety Techniques Association; 2005. [5] Accident examples database. The High Pressure Gas Safety Institute of Japan; 2005. [6] Shigeki Kikukawa, et al. Risk assessment of Hydrogen fueling stations for 70 MPa FCVs. International Journal of Hydrogen Energy 2008;33:7130. [7] Committee on the Safety Technologies of Hydrogen Infrastructure, Japan Petroleum Energy Center, March 2, 2007, file06-2-2. [8] Yoshiharu Matsuoka, et al. Safety technology for a hydrogen supply infrastructure. Final report from 2004 to 2005, 2005. p. 458. [9] World Energy Network. Phase 2, Task 10, development of cryogenic materials technologies. Japan Research and Development Center for Metals (JRCM); 2002.