3.1.1 Design
The exterior of the vehicle should be marked with diamond-shaped labels that say “Compressed Hydrogen” in white letters on a blue background (see Figure 16). For commercial vehicles, one label should be located on the rear of the power unit and one label should be located on each side of the power unit cab below the DOT numbers. The hydrogen labels should be legible from fifty feet in day light.
The hydrogen fuel system will likely include two pressure regulators, which reduce the gas pressure in stages from the fuel storage cylinders to a fuel cell or hydrogen ICE. The three stages are:
• The fuel storage system at up to 5,000 psi,
• The motive pressure circuit at up to approximately 175 psi, and
• The low pressure circuit at approximately 15 psi.
Each stage will include pressure relief devices, isolation valves, and other valves to regulate the flow of gas under all conditions. All components should be designed to withstand at least three times their expected maximum working pressure (NFPA, 2005).
Storage cylinders used to hold compressed hydrogen gas must be tested and certified by the manufacturer to withstand the normal forces expected during vehicle operation (such as working pressure, pressure shocks from fueling, and vibration) over a 15-year life, as well as certain extreme events such as would be encountered in a vehicle crash. These cylinders should be permanently marked “hydrogen,” securely mounted to the vehicle with the certification label visible, and protected from damage by road debris (i.e., if mounted between the frame rails, they should be protected by a cover).
All hydrogen storage cylinders must have a PRD or TRD installed. The outlets from each PRD/TRD should be connected to a common manifold that exits at or above the top surface of the vehicle, with outlet flow directed away from vehicle occupants and pedestrians.
Each hydrogen storage cylinder should have a manual shut-off valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
The fuel system should include one or more electrically activated valves that will isolate the hydrogen storage cylinders, individually or as a group, from the rest of the system when closed. These valve(s) should “fail safely” so that they will close if the control signal is lost due to a system fault.
All hydrogen fuel lines should be securely mounted to the vehicle and routed away from heat sources. To the extent possible, fuel line connections should be minimized since leaks are most likely at joints. Fuel lines should not be routed through the passenger compartment.
All components of the fuel system, including cylinders, lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with hydrogen and not subject to hydrogen embrittlement.
All components of the fuel system and engine system that will carry or contain hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the build up of static electricity.
Any compartment into which hydrogen could leak (from a fuel line connection or valve or from the fuel cell stack) should be ventilated such that hydrogen cannot collect in concentrations greater than 25 percent of hydrogen’s lower flammable limit. Hydrogen carrying components should not be located such that hydrogen can leak into the passenger compartment under any circumstance.
Because fuel cell stacks can develop internal leaks over time, they will likely be installed in their own enclosure, which will have both ventilation holes and a ventilation fan to force air through the enclosure to flush out any leaked hydrogen so that it can not collect.
One or more hydrogen sensors should be installed on the vehicle. The number and location of these sensors will depend on the hydrogen fuel and engine system design. These hydrogen sensor(s) should be connected to the vehicle control system to provide an alarm and automatic system shutdown if a hydrogen concentration greater than a preset threshold is detected. This threshold could be anywhere from 25 percent to 50 percent of the lower flammable limit for hydrogen (1–2 percent hydrogen concentration).
The fuel system may also have an excess flow valve installed that is designed to close off fuel flow and trigger an automatic system shutdown when flow in excess of a set threshold is detected. The threshold is set to be greater than the maximum flow that could be used by the fuel cell or hydrogen ICE at full power. Flows greater than this amount indicate that there is probably a leak in the system.
The vehicle may also have an inertial crash sensor installed, which can automatically trigger a vehicle shutdown when a crash is detected.
The vehicle control system should be configured so that automatic system shutdown can be triggered by detection of leaked hydrogen, excess fuel flow, a vehicle crash, or other system fault. Automatic system shutdown should include closing valve(s) to isolate hydrogen in the hydrogen storage cylinders, disconnecting traction power, and de-energizing high-voltage equipment. During system shutdown, hydrogen should be vented from all other fuel and engine system components. Some vehicles may include a switch to override automatic shutdown and allow the vehicle to continue to operate for a short time. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high-speed traffic or off of a railroad track.
The control system should also include a single main on/off switch that allows the vehicle operator to shut down the fuel cell system, disconnect traction power, de-energize high-voltage equipment, and shut off hydrogen fuel supply (isolating all hydrogen in the hydrogen storage cylinders). This switch should be located in the operator’s cab easily accessible to the operator, similar to a conventional ignition switch. Some vehicles may also have one or more secondary means of shutting down the system, for example, by opening a battery disconnect switch accessible from outside the vehicle.
The vehicle control system should include an interlock to the vehicle fueling port such that fueling cannot begin unless the fuel cell system is shutdown and the vehicle traction system is de-energized so that the vehicle cannot move.
The onboard fuel filling receptacle must be electrically bonded to the vehicle chassis, and some method must be provided to electrically connect the vehicle chassis to the fuel station ground during fueling. This can be done through the fueling nozzle (preferred) or with a separate ground strap.
A dust cap permanently mounted to the vehicle should be provided for the onboard fuel filling receptacle, to keep out dirt and debris when the vehicle is not being fueled.
The vehicle fuel system should include fittings and other provisions necessary to de-fuel safely from the hydrogen storage cylinders and purge the cylinders with nitrogen, as required for maintenance.
After system shutdown, hydrogen will typically be vented from the low-pressure sections of vehicle’s fuel system and fuel cell stack. The outlet for this venting hydrogen should be at or above the top surface of the vehicle. If, under normal operations, venting hydrogen will achieve concentrations greater than 25 percent of the lower flammable limit (1.0 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The hydrogen diffuser should be designed to mix the exiting hydrogen gas with enough air that under normal operations the resultant flow will have a hydrogen concentration less than 25 percent of the lower flammable limit.
Sunday, May 18, 2014
3. GUIDELINES FOR DESIGN AND OPERATION OF HYDROGEN SYSTEMS ON VEHICLES
Safe use of hydrogen as an alternative fuel requires that vehicles be designed and operated with the physical and chemical properties of hydrogen in mind. These properties are discussed in chapter 2. In most aspects, commercial vehicles powered by hydrogen will be quite similar to those powered by diesel fuel, but the propulsion system will be significantly different and some hydrogen-specific design elements are required. Likewise, operation of these vehicles will be similar to operation of diesel-fueled vehicles, with a few specific exceptions.
This chapter briefly discusses the most important hydrogen-specific aspects of vehicle design and operation. The issues discussed are of necessity generalized and are based on current codes and standards and best practices (see Appendix B). Each vehicle manufacturer will develop their own specific designs, which are likely to vary significantly in their details, while adhering to the same over-all design principles.
The information in this chapter is intended to familiarize commercial vehicle operators with the types of safety systems they are likely to see on hydrogen-fueled vehicles and the general operating principles they will need to use with them. Vehicle operators should look to the manuals provided by the manufacturer with each hydrogen-fueled vehicle for specific operating and maintenance instructions.
Safe use of hydrogen as an alternative fuel requires that vehicles be designed and operated with the physical and chemical properties of hydrogen in mind. These properties are discussed in chapter 2. In most aspects, commercial vehicles powered by hydrogen will be quite similar to those powered by diesel fuel, but the propulsion system will be significantly different and some hydrogen-specific design elements are required. Likewise, operation of these vehicles will be similar to operation of diesel-fueled vehicles, with a few specific exceptions.
This chapter briefly discusses the most important hydrogen-specific aspects of vehicle design and operation. The issues discussed are of necessity generalized and are based on current codes and standards and best practices (see Appendix B). Each vehicle manufacturer will develop their own specific designs, which are likely to vary significantly in their details, while adhering to the same over-all design principles.
The information in this chapter is intended to familiarize commercial vehicle operators with the types of safety systems they are likely to see on hydrogen-fueled vehicles and the general operating principles they will need to use with them. Vehicle operators should look to the manuals provided by the manufacturer with each hydrogen-fueled vehicle for specific operating and maintenance instructions.
2.4.2.3 Ventilating Enclosed Spaces
Hydrogen leaking into open air poses very little danger to anyone—it will quickly dissipate to nonflammable levels. Hydrogen that leaks into an enclosed space potentially presents a much greater hazard. When designing a hydrogen fueled vehicle, it is important to minimize all potential for hydrogen to leak into the passenger compartment, trunk, cargo space, wheel wells, and other enclosed spaces. This is done through careful placement of fuel tanks, lines, and connections. It may also be advisable to provide ventilation openings in locations that might not otherwise require them, specifically to vent any leaked hydrogen.
Another important consideration is placement of the outlet for any PRDs/TRDs. These outlets should be at the top surface of the vehicle and pointed away from the passenger or cargo compartment.
Buildings that will be used to house or maintain hydrogen-fueled vehicles should be designed so that there are no dead pockets at the ceiling where leaked hydrogen might collect and not be swept in by the building’s ventilation system.
Hydrogen leaking into open air poses very little danger to anyone—it will quickly dissipate to nonflammable levels. Hydrogen that leaks into an enclosed space potentially presents a much greater hazard. When designing a hydrogen fueled vehicle, it is important to minimize all potential for hydrogen to leak into the passenger compartment, trunk, cargo space, wheel wells, and other enclosed spaces. This is done through careful placement of fuel tanks, lines, and connections. It may also be advisable to provide ventilation openings in locations that might not otherwise require them, specifically to vent any leaked hydrogen.
Another important consideration is placement of the outlet for any PRDs/TRDs. These outlets should be at the top surface of the vehicle and pointed away from the passenger or cargo compartment.
Buildings that will be used to house or maintain hydrogen-fueled vehicles should be designed so that there are no dead pockets at the ceiling where leaked hydrogen might collect and not be swept in by the building’s ventilation system.
2.4.2.2 Removing Ignition Sources
Hydrogen is very easily ignited. A spark from static electricity, a vehicle tailpipe, electrical device, or even a hot surface can all ignite a mixture of air and leaked hydrogen within its flammable range.
It is important to minimize potential ignition sources in areas where hydrogen might leak and collect.
On a vehicle, static electricity is removed by proper grounding and bonding of electrical components. Fuel tanks, lines, and connections should be deliberately placed so that they avoid surfaces that might be hot or a source of ignition. In a building that will be used to house or maintain hydrogen fueled vehicles, any leaked hydrogen will quickly rise. The only area in which it is likely to collect in flammable concentrations is within a few feet of the ceiling. Electrical lines and equipment and heating equipment should not be located in this area near the ceiling. If electrical equipment must be located near the ceiling, it should be sealed so that it is “explosion proof” or intrinsically safe.
Hydrogen is very easily ignited. A spark from static electricity, a vehicle tailpipe, electrical device, or even a hot surface can all ignite a mixture of air and leaked hydrogen within its flammable range.
It is important to minimize potential ignition sources in areas where hydrogen might leak and collect.
On a vehicle, static electricity is removed by proper grounding and bonding of electrical components. Fuel tanks, lines, and connections should be deliberately placed so that they avoid surfaces that might be hot or a source of ignition. In a building that will be used to house or maintain hydrogen fueled vehicles, any leaked hydrogen will quickly rise. The only area in which it is likely to collect in flammable concentrations is within a few feet of the ceiling. Electrical lines and equipment and heating equipment should not be located in this area near the ceiling. If electrical equipment must be located near the ceiling, it should be sealed so that it is “explosion proof” or intrinsically safe.
2.4.2.1 Removing the Fuel Source—Avoiding and Detecting Leaks
Good design for a fuel system involves two major principles:
• Avoiding leaks of hydrogen fuel
• Detecting leaks of hydrogen fuel
The most likely locations for a hydrogen leak are at joints and connections in the high-pressure hydrogen fuel system. Hydrogen gas is the smallest of all molecules and can, therefore, move more easily through joints than other gases. However, most hydrogen leaks can be avoided by designing hydrogen systems using appropriate materials and minimizing connections.
Proper maintenance practices, in accordance with the manufacturer’s instructions, are also critical. This includes the use of the proper tools when making and breaking connections, tightening to the correct torque as specified by the manufacturer, and use of only approved replacement parts.
In a properly designed and maintained hydrogen fuel system, the most likely location for a hydrogen release will be through the PRD/TRD. If the PRD/TRD is properly oriented, a release will pose little danger to the vehicle, the operator, or the public. Hydrogen-fueled vehicles should enter only buildings designed to handle hydrogen.
All vehicles that use hydrogen fuel should also be equipped with one or more sensors to detect hydrogen leaks.
These sensors should be linked to the vehicle control system. If hydrogen levels approaching the lower limit of flammability are detected, the system will automatically shut down the vehicle and close valves to isolate the hydrogen within the high-pressure tank. In most cases, this will stop the source of the leak and remove any hazard. Some vehicles may include an “override” switch that will allow the vehicle to operate for a short time, even after a hydrogen leak has been detected. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high speed traffic or off of a railroad track.
Buildings used to store or maintain hydrogen-fueled vehicles should also generally be equipped with hydrogen sensors.
These sensors will be hooked into a building alarm system, as well as the building’s ventilation system. If hydrogen levels approaching the lower limit of flammability are detected, the system will sound an alarm to warn people in the area to evacuate and will increase the ventilation rate in the building to help remove any accumulated hydrogen. Some buildings designed for hydrogen vehicles may also be equipped with ultraviolet flame detectors to detect hydrogen fires. If so, they will also be hooked to the building alarm system.
Good design for a fuel system involves two major principles:
• Avoiding leaks of hydrogen fuel
• Detecting leaks of hydrogen fuel
The most likely locations for a hydrogen leak are at joints and connections in the high-pressure hydrogen fuel system. Hydrogen gas is the smallest of all molecules and can, therefore, move more easily through joints than other gases. However, most hydrogen leaks can be avoided by designing hydrogen systems using appropriate materials and minimizing connections.
Proper maintenance practices, in accordance with the manufacturer’s instructions, are also critical. This includes the use of the proper tools when making and breaking connections, tightening to the correct torque as specified by the manufacturer, and use of only approved replacement parts.
In a properly designed and maintained hydrogen fuel system, the most likely location for a hydrogen release will be through the PRD/TRD. If the PRD/TRD is properly oriented, a release will pose little danger to the vehicle, the operator, or the public. Hydrogen-fueled vehicles should enter only buildings designed to handle hydrogen.
All vehicles that use hydrogen fuel should also be equipped with one or more sensors to detect hydrogen leaks.
These sensors should be linked to the vehicle control system. If hydrogen levels approaching the lower limit of flammability are detected, the system will automatically shut down the vehicle and close valves to isolate the hydrogen within the high-pressure tank. In most cases, this will stop the source of the leak and remove any hazard. Some vehicles may include an “override” switch that will allow the vehicle to operate for a short time, even after a hydrogen leak has been detected. This switch should only be used in case of extreme emergency, for example, to move the vehicle out of high speed traffic or off of a railroad track.
Buildings used to store or maintain hydrogen-fueled vehicles should also generally be equipped with hydrogen sensors.
These sensors will be hooked into a building alarm system, as well as the building’s ventilation system. If hydrogen levels approaching the lower limit of flammability are detected, the system will sound an alarm to warn people in the area to evacuate and will increase the ventilation rate in the building to help remove any accumulated hydrogen. Some buildings designed for hydrogen vehicles may also be equipped with ultraviolet flame detectors to detect hydrogen fires. If so, they will also be hooked to the building alarm system.
Wednesday, May 14, 2014
2.4.2 Avoiding Fire and Explosion
As shown in Figure 17, a fire or explosion requires three things:
• Fuel (i.e., a hydrogen leak),
• An ignition source, and
• Oxygen (from the air).
Safe design for use of any gaseous fuel, including hydrogen, involves paying attention to all three legs of the “fire triangle.”
As shown in Figure 17, a fire or explosion requires three things:
• Fuel (i.e., a hydrogen leak),
• An ignition source, and
• Oxygen (from the air).
Safe design for use of any gaseous fuel, including hydrogen, involves paying attention to all three legs of the “fire triangle.”
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