3.3.2 Operation and Maintenance
When working on high-voltage systems, always ensure that the main switch is in the “off” position and tag it and/or lock it in that position so that no one else can turn the vehicle on.
Always assume that high-voltage electrical components are live, even when the vehicle main switch is in the off position. Use a volt meter to check before touching any electrical component.
Always use appropriate personal protective equipment, such as high-voltage gloves, when working on high-voltage systems. Follow the safety recommendations in the manufacturer’s service manual.
Never wear jewelry when working around high-voltage systems.
Always reinstall high-voltage covers and barriers. Reapply high-voltage labels if they are removed or damaged.
Do not ignore warnings from the ground fault monitoring system. Follow instructions in the service manual to isolate and repair the source of the fault. Unrepaired ground faults can cause shocks or electrocution to vehicle occupants or service technicians.
High-voltage electrical cables are designed with special shielding and insulation, and the exterior cover should be made from orange colored material. Always replace high-voltage wiring and harnesses with parts as specified by the manufacturer. Do not manufacture your own high-voltage harnesses or substitute non-approved parts.
Remember that high voltage will always be present inside a traction battery or ultracapacitor pack (if vehicle has a hybrid-electric drive system) even when the main switch is off or automatic shutdown has occurred. Special tools and procedures are required to work safely inside the battery/ultracapacitor pack. Do not open the battery/ultracapacitor pack cover unless you have been trained to work safely with live high voltage.
Monday, June 9, 2014
3.3.1 Design
All electrical cables that carry greater than 30 VAC or 60 VDC should be considered high-voltage cables and should be permanently identified with orange covering material, and be located to preclude casual contact by vehicle operators and maintenance personnel. No high voltage cables should be routed through the passenger compartment.
Any part or connector energized with high voltage during normal operations should be located behind a cover or in an enclosure labeled with the hazardous voltage symbol. See Figure 18. Removal of the cover should require removal of special fasteners and/or should trigger an interlock system that de-energizes the high-voltage components. If the cover or enclosure is made of conductive material, it should be grounded to the vehicle chassis.
The electrical system should include an automatic disconnect function that isolates both poles of any high-voltage source (fuel cell stack and/or high voltage battery pack) from other system components.
The connectors used should be “fail safe” so that they will open if the control signal is lost due to a system fault. This automatic disconnect function should be triggered by the following:
• Turning off the main switch
• By automatic system shutdown (due to a hydrogen sensor detecting a leak, or other system fault that shuts off the hydrogen fuel supply)
All high-voltage components should be designed with adequate electrical isolation to prevent “current leakage” between them and other electrical components or the chassis. The vehicle control system should include a ground fault monitoring system that can detect leakage current and set a warning light and/or take other action (up to automatic system shutdown) when a current above a set threshold is detected.
All electrical cables that carry greater than 30 VAC or 60 VDC should be considered high-voltage cables and should be permanently identified with orange covering material, and be located to preclude casual contact by vehicle operators and maintenance personnel. No high voltage cables should be routed through the passenger compartment.
Any part or connector energized with high voltage during normal operations should be located behind a cover or in an enclosure labeled with the hazardous voltage symbol. See Figure 18. Removal of the cover should require removal of special fasteners and/or should trigger an interlock system that de-energizes the high-voltage components. If the cover or enclosure is made of conductive material, it should be grounded to the vehicle chassis.
The electrical system should include an automatic disconnect function that isolates both poles of any high-voltage source (fuel cell stack and/or high voltage battery pack) from other system components.
The connectors used should be “fail safe” so that they will open if the control signal is lost due to a system fault. This automatic disconnect function should be triggered by the following:
• Turning off the main switch
• By automatic system shutdown (due to a hydrogen sensor detecting a leak, or other system fault that shuts off the hydrogen fuel supply)
All high-voltage components should be designed with adequate electrical isolation to prevent “current leakage” between them and other electrical components or the chassis. The vehicle control system should include a ground fault monitoring system that can detect leakage current and set a warning light and/or take other action (up to automatic system shutdown) when a current above a set threshold is detected.
3.3 HIGH VOLTAGE SYSTEMS
Any voltage greater than 30 VAC or 60 VDC can harm humans through electric shock and is considered “hazardous voltage” (SAE, 2002).
Hydrogen fuel cells produce electricity at nominal voltages of 300 to 600 VDC, which is used to power an electric drive system that operates at similar voltages (often after conversion to alternating current). The drive system may be designed with a hybrid-electric configuration. If so, there will also be a high-voltage battery pack installed on the vehicle.
Hydrogen fueled ICEs do not produce electricity directly, but hydrogen ICE vehicles may also be designed with a hybrid-electric drive configuration. If so, they will also include a high-voltage battery pack and other high-voltage electrical components.
Any voltage greater than 30 VAC or 60 VDC can harm humans through electric shock and is considered “hazardous voltage” (SAE, 2002).
Hydrogen fuel cells produce electricity at nominal voltages of 300 to 600 VDC, which is used to power an electric drive system that operates at similar voltages (often after conversion to alternating current). The drive system may be designed with a hybrid-electric configuration. If so, there will also be a high-voltage battery pack installed on the vehicle.
Hydrogen fueled ICEs do not produce electricity directly, but hydrogen ICE vehicles may also be designed with a hybrid-electric drive configuration. If so, they will also include a high-voltage battery pack and other high-voltage electrical components.
Tuesday, June 3, 2014
3.2.2 Operation and Maintenance
Anyone who will operate or maintain liquid hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen and liquid hydrogen, operation of onboard safety systems, liquid hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with liquid hydrogen. Lines that will carry liquid hydrogen must be well-insulated with the outer layer of insulation vapor sealed. O-rings or other seals used in connections between liquid hydrogen lines must be made of special materials that can withstand liquid hydrogen temperatures without breaking. Substituting seals made of different materials can result in liquid hydrogen leaks.
When working on the liquid hydrogen fuel system, always wear personal protective equipment, including safety glasses and a full face shield, loose fitting insulated or leather gloves, leather boots ankle height or higher, a long-sleeved shirt, and long pants without cuffs. Pant legs should be worn outside of the boots.
Never loosen any joint in the hydrogen fuel system while the connected components are under pressure (gaseous hydrogen lines) or contain liquid hydrogen. Never disturb the insulation on liquid hydrogen lines or cryotanks while they contain liquid hydrogen. Shut down the system, isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the manufacturer’s service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, any component that will carry liquid hydrogen (including cryotanks and lines) must be purged with helium before being refilled with liquid hydrogen. Nitrogen must not be used because the residual nitrogen in the lines could liquefy and freeze when exposed to liquid hydrogen. This could potentially plug pressure relief valves and other system valves. Alternatively, components can be purged of air with nitrogen and the nitrogen can be purged with gaseous hydrogen before refilling with liquid hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak.
Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Periodically check the exterior surface of liquid hydrogen cryotanks and fuel lines for cuts or damage to the exterior insulation layer. All damage that breeches the vapor barrier must be repaired.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans, in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or liquid hydrogen storage system also disconnect the vehicle’s 12/24-VDC battery.
Do not walk on liquid hydrogen cryotanks or expose them to damage from impact or abrasion.
Always electrically ground and bond the vehicle when fueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
The manufacturer of the liquid hydrogen cryotank will specify a minimum ullage13 space required in each tank. During fueling, this ullage space must be maintained and tanks should not be overfilled.
Anyone who will operate or maintain liquid hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen and liquid hydrogen, operation of onboard safety systems, liquid hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with liquid hydrogen. Lines that will carry liquid hydrogen must be well-insulated with the outer layer of insulation vapor sealed. O-rings or other seals used in connections between liquid hydrogen lines must be made of special materials that can withstand liquid hydrogen temperatures without breaking. Substituting seals made of different materials can result in liquid hydrogen leaks.
When working on the liquid hydrogen fuel system, always wear personal protective equipment, including safety glasses and a full face shield, loose fitting insulated or leather gloves, leather boots ankle height or higher, a long-sleeved shirt, and long pants without cuffs. Pant legs should be worn outside of the boots.
Never loosen any joint in the hydrogen fuel system while the connected components are under pressure (gaseous hydrogen lines) or contain liquid hydrogen. Never disturb the insulation on liquid hydrogen lines or cryotanks while they contain liquid hydrogen. Shut down the system, isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the manufacturer’s service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, any component that will carry liquid hydrogen (including cryotanks and lines) must be purged with helium before being refilled with liquid hydrogen. Nitrogen must not be used because the residual nitrogen in the lines could liquefy and freeze when exposed to liquid hydrogen. This could potentially plug pressure relief valves and other system valves. Alternatively, components can be purged of air with nitrogen and the nitrogen can be purged with gaseous hydrogen before refilling with liquid hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak.
Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Periodically check the exterior surface of liquid hydrogen cryotanks and fuel lines for cuts or damage to the exterior insulation layer. All damage that breeches the vapor barrier must be repaired.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans, in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or liquid hydrogen storage system also disconnect the vehicle’s 12/24-VDC battery.
Do not walk on liquid hydrogen cryotanks or expose them to damage from impact or abrasion.
Always electrically ground and bond the vehicle when fueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
The manufacturer of the liquid hydrogen cryotank will specify a minimum ullage13 space required in each tank. During fueling, this ullage space must be maintained and tanks should not be overfilled.
Wednesday, May 28, 2014
3.2.1 Design
The exterior of the vehicle should be marked with diamond-shaped labels that say “Liquid 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.
All cryotanks used to hold liquid hydrogen must be permanently marked “hydrogen,” securely mounted to the vehicle, and protected from damage by road debris.
All liquid hydrogen cryotanks must have a safety pressure relief valve installed. The outlet(s) from the valve(s) should empty into a hydrogen diffuser, whose outlet is located at or above the top surface of the vehicle. 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.
Each liquid hydrogen cryotank should have a manual shutoff valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
Each liquid hydrogen cryotank should be equipped with a liquid level gauge that can be read from the vehicle cab and a pressure gauge that can be read locally on or near the tank.
While certification standards for on-vehicle liquid hydrogen tanks have not yet been finalized, at a minimum, tanks should be tested/certified in the same way that current liquefied natural gas (LNG) tanks are tested.
For each tank design, this includes a 10-foot and a 30-foot drop test of a full tank to ensure that the tank will not leak even if subjected to a severe crash, and a 20-minute flame test to ensure that the tank will not immediately vent even if impacted by a fire (SAE, 1997).
The fuel system should include one or more electrically activated valves that will isolate the hydrogen cryotank(s), 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 liquid and gaseous 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 that will come into contact with liquid hydrogen, including cryotank(s), fill lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with the low temperatures of liquid hydrogen. All gaseous hydrogen fuel lines and valves (downstream of the cryotank heat exchanger) shall 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 liquid or gaseous hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the buildup of static electricity.
All components of the fuel system that will carry or contain liquid hydrogen must be well-insulated, labeled, and be located to prevent casual contact by vehicle operators or maintenance personnel. The outer insulating layer must be vapor sealed to prevent air infiltration. Any fuel line that may be isolated between two closed valves with residual liquid hydrogen still inside (i.e., a fill line) must contain a pressure relief valve to vent hydrogen that vaporizes as the line heats up. These valve(s) should vent to a common plenum, with the pressure relief valve(s) on the liquid hydrogen cryotank(s).
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 gaseous 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 that can automatically trigger a vehicle shutdown when a crash is detected. 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 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.
The control system should 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 the hydrogen fuel supply (isolating all hydrogen in the liquid hydrogen cryotank(s)). This switch should be located in the passenger cab easily accessible to the operator, similar to a conventional ignition switch.
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 on board liquid hydrogen 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 port, 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 safely remove hydrogen fuel from the liquid hydrogen cryotank(s) and purge them with nitrogen or helium as required for maintenance.
After system shutdown, hydrogen will typically be vented from the vehicle’s low-pressure gaseous 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 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The same hydrogen diffuser can be used for both this function and to diffuse hydrogen released through the fuel system pressure relief valve(s).
The exterior of the vehicle should be marked with diamond-shaped labels that say “Liquid 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.
All cryotanks used to hold liquid hydrogen must be permanently marked “hydrogen,” securely mounted to the vehicle, and protected from damage by road debris.
All liquid hydrogen cryotanks must have a safety pressure relief valve installed. The outlet(s) from the valve(s) should empty into a hydrogen diffuser, whose outlet is located at or above the top surface of the vehicle. 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.
Each liquid hydrogen cryotank should have a manual shutoff valve installed that will allow that cylinder to be isolated from the rest of the fuel system for maintenance.
Each liquid hydrogen cryotank should be equipped with a liquid level gauge that can be read from the vehicle cab and a pressure gauge that can be read locally on or near the tank.
While certification standards for on-vehicle liquid hydrogen tanks have not yet been finalized, at a minimum, tanks should be tested/certified in the same way that current liquefied natural gas (LNG) tanks are tested.
For each tank design, this includes a 10-foot and a 30-foot drop test of a full tank to ensure that the tank will not leak even if subjected to a severe crash, and a 20-minute flame test to ensure that the tank will not immediately vent even if impacted by a fire (SAE, 1997).
The fuel system should include one or more electrically activated valves that will isolate the hydrogen cryotank(s), 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 liquid and gaseous 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 that will come into contact with liquid hydrogen, including cryotank(s), fill lines, valves, and sealing materials should be constructed of materials that have been tested to be compatible with the low temperatures of liquid hydrogen. All gaseous hydrogen fuel lines and valves (downstream of the cryotank heat exchanger) shall 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 liquid or gaseous hydrogen should be electrically grounded and bonded to the vehicle chassis to preclude the buildup of static electricity.
All components of the fuel system that will carry or contain liquid hydrogen must be well-insulated, labeled, and be located to prevent casual contact by vehicle operators or maintenance personnel. The outer insulating layer must be vapor sealed to prevent air infiltration. Any fuel line that may be isolated between two closed valves with residual liquid hydrogen still inside (i.e., a fill line) must contain a pressure relief valve to vent hydrogen that vaporizes as the line heats up. These valve(s) should vent to a common plenum, with the pressure relief valve(s) on the liquid hydrogen cryotank(s).
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 gaseous 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 that can automatically trigger a vehicle shutdown when a crash is detected. 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 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.
The control system should 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 the hydrogen fuel supply (isolating all hydrogen in the liquid hydrogen cryotank(s)). This switch should be located in the passenger cab easily accessible to the operator, similar to a conventional ignition switch.
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 on board liquid hydrogen 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 port, 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 safely remove hydrogen fuel from the liquid hydrogen cryotank(s) and purge them with nitrogen or helium as required for maintenance.
After system shutdown, hydrogen will typically be vented from the vehicle’s low-pressure gaseous 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 percent hydrogen concentration), the hydrogen should vent through a hydrogen diffuser. The same hydrogen diffuser can be used for both this function and to diffuse hydrogen released through the fuel system pressure relief valve(s).
3.2 LIQUID HYDROGEN SYSTEMS
Liquid hydrogen storage on vehicles is much less common than high-pressure compressed hydrogen storage. If your vehicle does store liquid hydrogen onboard, many of the design and operating considerations listed in Section 3.1 will apply, but since liquid hydrogen is stored at very low temperatures, the additional considerations listed below will also apply.
Liquid hydrogen storage on vehicles is much less common than high-pressure compressed hydrogen storage. If your vehicle does store liquid hydrogen onboard, many of the design and operating considerations listed in Section 3.1 will apply, but since liquid hydrogen is stored at very low temperatures, the additional considerations listed below will also apply.
3.1.2 Operation and Maintenance
Anyone who will operate or maintain hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen, operation of onboard safety systems, hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with hydrogen (for example, lines, valves, and regulators designed for use with natural gas).
While they may look and function the same, they may be subject to hydrogen embrittlement. In addition, compressed natural gas fuel systems typically operate at lower pressures (maximum 3,600 psi) than hydrogen fuel systems.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak. Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Never loosen any joint in the fuel system while the connected components are under pressure. Shut down the system and isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, some components, particularly hydrogen fuel cylinders, must be purged with nitrogen before being refilled with hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check the exterior surface of hydrogen fuel cylinders for nicks, dents, and cuts that could weaken the structure. See the manufacturer’s service manual for information on the allowable level of wear and damage before cylinders need to be replaced. The Federal Motor Vehicle Safety Standards applicable to natural gas fuel cylinders (FMVSS 304, 49 CFR 571.304) specify that a visual inspection by a “qualified container inspector”11 must be conducted “at least every 36 months or 36,000 miles or at the time of re-installation.” The inspection procedures for damage assessment are outlined in pamphlet C-612 from the
Compressed Gas Association. While standards specifically applicable to hydrogen cylinders have not yet been developed, at a minimum, the requirements applicable to natural gas fuel cylinders should be followed.
Local laws and regulations may require more frequent cylinder inspections, for example, in conjunction with annual registration safety inspections. The fuel system, including the high-pressure storage tanks, should also be visually inspected after any accident, and be retested or replaced as required.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans in accordance with
the schedule and procedures in the manufacturer’s service manual.
The fuel system will likely include a coalescing filter to remove any oil that might carry over into the hydrogen fuel from the fuel station compressor. Check and empty or replace this filter periodically in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off of railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or gaseous hydrogen storage system, also disconnect the vehicle’s 12/24-VDC battery and close the manual fuel valves to isolate hydrogen in the storage cylinders.
Do not try to repair damaged fuel lines—replace them.
Do not walk on hydrogen fuel cylinders or expose them to damage from impact or abrasion. Do not allow strong chemicals, such as battery acid or metal cleaning solvents, to contact the hydrogen fuel cylinders.
Always electrically ground and bond the vehicle when fueling and defueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
If the vehicle must be defueled for servicing of the hydrogen fuel system, the rate of fuel release must be carefully controlled. Follow the instructions in the manufacturer’s service manual. Unless the hydrogen storage tanks will be removed, always leave a small amount of pressure in the tanks so that the internal pressure is a few psi above atmospheric pressure. Any time tank pressure falls below atmospheric, it is possible for air to enter, and the tank must be purged with nitrogen before refilling with hydrogen.
Anyone who will operate or maintain hydrogen-fueled vehicles should receive hydrogen safety training. At a minimum, this training should cover the characteristics of hydrogen, operation of onboard safety systems, hydrogen fueling operations, and actions to take in an emergency.
During maintenance, never substitute fuel system replacement parts that have not been specifically tested and certified for use with hydrogen (for example, lines, valves, and regulators designed for use with natural gas).
While they may look and function the same, they may be subject to hydrogen embrittlement. In addition, compressed natural gas fuel systems typically operate at lower pressures (maximum 3,600 psi) than hydrogen fuel systems.
Periodically check all connections in the hydrogen fuel system for leaks using procedures outlined in the manufacturer’s service manual. Tighten or repair all leaking joints, no matter how small the leak. Leak checks should also be conducted after repair or replacement of any fuel system lines or valves.
Never loosen any joint in the fuel system while the connected components are under pressure. Shut down the system and isolate and vent components as directed in the manufacturer’s service manual. Torque all joints to the levels specified in the service manual. Do not over tighten. Overtorquing can cause leaks.
Air must never be allowed to enter the hydrogen fuel system. If exposed to the atmosphere, some components, particularly hydrogen fuel cylinders, must be purged with nitrogen before being refilled with hydrogen. See the manufacturer’s service manual for specific purging procedures.
Periodically check the exterior surface of hydrogen fuel cylinders for nicks, dents, and cuts that could weaken the structure. See the manufacturer’s service manual for information on the allowable level of wear and damage before cylinders need to be replaced. The Federal Motor Vehicle Safety Standards applicable to natural gas fuel cylinders (FMVSS 304, 49 CFR 571.304) specify that a visual inspection by a “qualified container inspector”11 must be conducted “at least every 36 months or 36,000 miles or at the time of re-installation.” The inspection procedures for damage assessment are outlined in pamphlet C-612 from the
Compressed Gas Association. While standards specifically applicable to hydrogen cylinders have not yet been developed, at a minimum, the requirements applicable to natural gas fuel cylinders should be followed.
Local laws and regulations may require more frequent cylinder inspections, for example, in conjunction with annual registration safety inspections. The fuel system, including the high-pressure storage tanks, should also be visually inspected after any accident, and be retested or replaced as required.
Periodically check and calibrate hydrogen sensors in accordance with the schedule and procedures in the manufacturer’s service manual.
Periodically check operation of the fan in the hydrogen diffuser and any ventilation fans in accordance with
the schedule and procedures in the manufacturer’s service manual.
The fuel system will likely include a coalescing filter to remove any oil that might carry over into the hydrogen fuel from the fuel station compressor. Check and empty or replace this filter periodically in accordance with the schedule and procedures in the manufacturer’s service manual.
Do not ignore warning lights or alarms. Do not attempt to override automatic system shutdown unless absolutely necessary (e.g., to move vehicle off of railroad tracks).
Always make sure that the main switch is off before servicing the vehicle. Before working on the fuel cell system or gaseous hydrogen storage system, also disconnect the vehicle’s 12/24-VDC battery and close the manual fuel valves to isolate hydrogen in the storage cylinders.
Do not try to repair damaged fuel lines—replace them.
Do not walk on hydrogen fuel cylinders or expose them to damage from impact or abrasion. Do not allow strong chemicals, such as battery acid or metal cleaning solvents, to contact the hydrogen fuel cylinders.
Always electrically ground and bond the vehicle when fueling and defueling. Connect the ground strap or cable at the fuel station if one is provided.
Before fueling, check that the onboard fuel port is free of dirt and debris. Always replace the fuel port dust cover after fueling.
Do not smoke or use a cell phone when servicing or fueling the vehicle.
If the vehicle must be defueled for servicing of the hydrogen fuel system, the rate of fuel release must be carefully controlled. Follow the instructions in the manufacturer’s service manual. Unless the hydrogen storage tanks will be removed, always leave a small amount of pressure in the tanks so that the internal pressure is a few psi above atmospheric pressure. Any time tank pressure falls below atmospheric, it is possible for air to enter, and the tank must be purged with nitrogen before refilling with hydrogen.
Sunday, May 18, 2014
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.
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.
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.”
2.4.1 Labeling
As with other alternative-fueled vehicles, all commercial vehicles that store hydrogen fuel onboard should carry a label that identifies the type of fuel used in order to alert emergency response personnel to the types of hazards they might face if the vehicle is involved in an accident. These labels should conform with SAE J2578, a fuel cell safety recommended practice developed by the Society for Automotive Engineers (SAE, 2002).
This practice recommends a diamond-shaped label with a blue background and white lettering (see Figure 16). It also specifically recommends that the words “Compressed Hydrogen” or “Liquid Hydrogen” be contained in the blue diamond, depending on the form in which the hydrogen is stored on the vehicle.
SAE J2578 does not specify the location of the hydrogen label on the vehicle. For hydrogen-fueled cars, the manufacturer usually affixes the label to the rear of the vehicle, as shown in Figure 16. For commercial tractor-trailer units, a label on the rear of the power unit would likely be obscured by the trailer. For that reason, all commercial vehicles should include the hydrogen label on the rear of the vehicle and on each side of the power unit cab, below the DOT numbers mandated by 49 CFR 390.21. As with these DOT numbers, the hydrogen labels should be legible from fifty feet in daylight.
Any commercial trailer unit that will store hydrogen (for example, to fuel a transportation refrigeration unit) should also have SAE J2578-compliant hydrogen labels affixed. These labels should be located on the rear of the trailer and on each side of the trailer, in the vicinity of where the hydrogen fuel tank is located.
As with other alternative-fueled vehicles, all commercial vehicles that store hydrogen fuel onboard should carry a label that identifies the type of fuel used in order to alert emergency response personnel to the types of hazards they might face if the vehicle is involved in an accident. These labels should conform with SAE J2578, a fuel cell safety recommended practice developed by the Society for Automotive Engineers (SAE, 2002).
This practice recommends a diamond-shaped label with a blue background and white lettering (see Figure 16). It also specifically recommends that the words “Compressed Hydrogen” or “Liquid Hydrogen” be contained in the blue diamond, depending on the form in which the hydrogen is stored on the vehicle.
SAE J2578 does not specify the location of the hydrogen label on the vehicle. For hydrogen-fueled cars, the manufacturer usually affixes the label to the rear of the vehicle, as shown in Figure 16. For commercial tractor-trailer units, a label on the rear of the power unit would likely be obscured by the trailer. For that reason, all commercial vehicles should include the hydrogen label on the rear of the vehicle and on each side of the power unit cab, below the DOT numbers mandated by 49 CFR 390.21. As with these DOT numbers, the hydrogen labels should be legible from fifty feet in daylight.
Any commercial trailer unit that will store hydrogen (for example, to fuel a transportation refrigeration unit) should also have SAE J2578-compliant hydrogen labels affixed. These labels should be located on the rear of the trailer and on each side of the trailer, in the vicinity of where the hydrogen fuel tank is located.
2.4 HYDROGEN SAFETY PRINCIPLES
The most important safety principle in any situation is education—making anyone who will come into contact with a vehicle aware of a potential hazard. For hydrogen and other alternative-fueled vehicles, this is done with appropriate labeling, to let users, emergency responders and the public know that hydrogen is present.
As with other motor fuels, fire and explosion are the most significant everyday hazards associated with hydrogen. Also as with other fuels, a hydrogen leak from a vehicle’s fuel or engine system, or from a fueling station, provides the starting point for all fire and explosion hazards.
Safe design for using hydrogen therefore requires attention to these safety principles:
• Properly label all vehicles that use hydrogen fuel. Avoid fire and explosion by: Avoiding leaks through proper design and maintenance, Providing leak detection systems to detect leaks and, if a leak is detected, shut off the fuel system as soon as possible,
• Removing ignition sources from areas where leaked hydrogen might be present, and
• Properly ventilating all enclosed spaces where leaked hydrogen might accumulate.
These are essentially the same principles that apply to the use of any gaseous fuel (e.g., natural gas), but their application may be slightly different based on the properties of hydrogen. Each of these principles, as applied to hydrogen, is discussed further below.
The most important safety principle in any situation is education—making anyone who will come into contact with a vehicle aware of a potential hazard. For hydrogen and other alternative-fueled vehicles, this is done with appropriate labeling, to let users, emergency responders and the public know that hydrogen is present.
As with other motor fuels, fire and explosion are the most significant everyday hazards associated with hydrogen. Also as with other fuels, a hydrogen leak from a vehicle’s fuel or engine system, or from a fueling station, provides the starting point for all fire and explosion hazards.
Safe design for using hydrogen therefore requires attention to these safety principles:
• Properly label all vehicles that use hydrogen fuel. Avoid fire and explosion by: Avoiding leaks through proper design and maintenance, Providing leak detection systems to detect leaks and, if a leak is detected, shut off the fuel system as soon as possible,
• Removing ignition sources from areas where leaked hydrogen might be present, and
• Properly ventilating all enclosed spaces where leaked hydrogen might accumulate.
These are essentially the same principles that apply to the use of any gaseous fuel (e.g., natural gas), but their application may be slightly different based on the properties of hydrogen. Each of these principles, as applied to hydrogen, is discussed further below.
Thursday, May 8, 2014
DAZO HHO PWM ( Pulse Width Modulators ) FOR FREEZING TEMPERATURES
The PWM has become the new standard for current control for Advanced DAZO HHO Kit enthusiasts. Rather than just straight current flowing into your generator, these PWM boards control current and the resulting heat that can sometimes be a problem, by pulsing off and on rapidly. Some pulse at a fixed frequency, with current adjustable. Others allow both current and frequency to be controlled like our Heavy Duty 30 amps.
DO I NEED A PWM? That is a common question. If your HHO kit is cheaply made of acrylic or PVC that melts at 180-190 degrees F, then YES! If you don't control the current it will melt around the terminals and leak. Ours are designed to withstand even constant currents of 50 amps+ without damage. They don't need one to regulate current for protection like cheaper models.
FREEZING TEMPS? YES! You should get one in Extreme Freezing Areas, because the electrolyte concentration can be increased with a good PWM preventing freezing to - 40 degrees F!
30 AMP PWM Assembled in a Project Box! They are built using the Heavy Duty 30 AMP PWM and come with a fan and dial controls for adjusting the current AND frequency (Actual product may vary from picture). Current boxes do NOT come with an additional on/off switch as shown in photo. They DO come with a single fan, dual mosfets and heat sinks and two controls for current (amperage) control and frequency. Size is approximately 3"W x 4-1/2"L x 2-1/2" H. Powered by 12 volts- can be set up for 24 volt by special order. each connection is split into 2 terminals for power in + and -, and power out to load + and - for a total of 8 terminals to divide up the current.
These are nice units and come with ventilation slots cut into the sides to allow air to be drawn up and over the cooling heat sinks. Holes are pre-drilled in the back for the wires and connectors to attach to the board inside.
Tuesday, May 6, 2014
2.3.2 Fires and Explosion
Hydrogen has higher energy content (per pound) than diesel fuel and is very reactive, which results in a very vigorous fire. However, the buoyancy of gaseous hydrogen means that it rises rapidly and diffuses quickly, resulting in a very vertical and localized flame. Hydrogen fires also tend to burn out fairly quickly. By contrast, diesel fuel and gasoline fires tend to burn longer and spread over a much larger area, as the liquid fuel puddle expands.
There is virtually no risk of a thermal “explosion” of hydrogen stored in a closed tank unless oxygen has been allowed to enter the tank during fueling operations, which is virtually impossible if the fueling system has been designed properly. Hydrogen gas that leaks into an enclosed space, whether on a vehicle or in a building, can create an explosive mixture, depending on the volume of hydrogen relative to the volume of air in the space.
Diesel fuel is not very volatile, and the risk that accumulated vapors will result in an explosion is small, even for large leaks. Gasoline, however, is very volatile and vapors from leaks can create the risk of an explosion if they collect in an enclosed space on the vehicle or in a building.
Hydrogen has higher energy content (per pound) than diesel fuel and is very reactive, which results in a very vigorous fire. However, the buoyancy of gaseous hydrogen means that it rises rapidly and diffuses quickly, resulting in a very vertical and localized flame. Hydrogen fires also tend to burn out fairly quickly. By contrast, diesel fuel and gasoline fires tend to burn longer and spread over a much larger area, as the liquid fuel puddle expands.
There is virtually no risk of a thermal “explosion” of hydrogen stored in a closed tank unless oxygen has been allowed to enter the tank during fueling operations, which is virtually impossible if the fueling system has been designed properly. Hydrogen gas that leaks into an enclosed space, whether on a vehicle or in a building, can create an explosive mixture, depending on the volume of hydrogen relative to the volume of air in the space.
Diesel fuel is not very volatile, and the risk that accumulated vapors will result in an explosion is small, even for large leaks. Gasoline, however, is very volatile and vapors from leaks can create the risk of an explosion if they collect in an enclosed space on the vehicle or in a building.
2.3.1 Leaks
In some ways a gaseous hydrogen fuel leak is less dangerous than a leak of diesel fuel or gasoline. Leaking diesel fuel and gasoline can puddle and spread over a large area, and the puddles will persist because they evaporate slowly (Amerada Hess, 2001).
Gaseous hydrogen leaks tend to be vertical with only a relatively narrow area/volume in which a flammable mixture exists;9 the hydrogen quickly dissipates in open air to nonhazardous levels.
If designed properly, the most likely location of a major hydrogen leak from a vehicle will be through the PRD, which should vent away from the occupied area of the vehicle. PRDs are designed to vent the entire contents of a hydrogen tank in only a few minutes, after which there is no lingering risk of hydrogen fire or explosion if the release was in the open air. Large hydrogen leaks inside buildings are more dangerous unless the facility has been designed to evacuate the leaked gas and to minimize ignition sources at ceiling level.
Leaking liquid hydrogen can pool and spread, but will quickly evaporate as it is heated by the surrounding air. As it evaporates, the cloud of gaseous hydrogen formed over the spill may move horizontally as it rises and dissipates.
While diesel fuel and gasoline leaks are easily visible and accompanied by a strong characteristic smell, gaseous hydrogen leaks are invisible and odorless (see Table 5). The only indication of a gaseous hydrogen leak may be a whistling noise similar to escape of other high-pressure gases. A liquid hydrogen leak may be accompanied by an area of fog surrounding the leaking hydrogen and/or the formation of frost on the tank or lines in the vicinity of the leak, because the super cold hydrogen cools the surrounding air and causes water vapor to condense.
In some ways a gaseous hydrogen fuel leak is less dangerous than a leak of diesel fuel or gasoline. Leaking diesel fuel and gasoline can puddle and spread over a large area, and the puddles will persist because they evaporate slowly (Amerada Hess, 2001).
Gaseous hydrogen leaks tend to be vertical with only a relatively narrow area/volume in which a flammable mixture exists;9 the hydrogen quickly dissipates in open air to nonhazardous levels.
If designed properly, the most likely location of a major hydrogen leak from a vehicle will be through the PRD, which should vent away from the occupied area of the vehicle. PRDs are designed to vent the entire contents of a hydrogen tank in only a few minutes, after which there is no lingering risk of hydrogen fire or explosion if the release was in the open air. Large hydrogen leaks inside buildings are more dangerous unless the facility has been designed to evacuate the leaked gas and to minimize ignition sources at ceiling level.
Leaking liquid hydrogen can pool and spread, but will quickly evaporate as it is heated by the surrounding air. As it evaporates, the cloud of gaseous hydrogen formed over the spill may move horizontally as it rises and dissipates.
While diesel fuel and gasoline leaks are easily visible and accompanied by a strong characteristic smell, gaseous hydrogen leaks are invisible and odorless (see Table 5). The only indication of a gaseous hydrogen leak may be a whistling noise similar to escape of other high-pressure gases. A liquid hydrogen leak may be accompanied by an area of fog surrounding the leaking hydrogen and/or the formation of frost on the tank or lines in the vicinity of the leak, because the super cold hydrogen cools the surrounding air and causes water vapor to condense.
2.3 COMPARISON OF HYDROGEN TO OTHER MOTOR FUELS
This section directly compares the properties of gaseous and liquid hydrogen to those of diesel fuel to contrast the safety issues of hydrogen to those of more common motor fuels. Two specific areas will be highlighted: the behavior of fuel leaks, and the characteristics of fire and explosions with these fuels.
This section directly compares the properties of gaseous and liquid hydrogen to those of diesel fuel to contrast the safety issues of hydrogen to those of more common motor fuels. Two specific areas will be highlighted: the behavior of fuel leaks, and the characteristics of fire and explosions with these fuels.
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