when the system is off. In addition, the hydrogen in the engine is at lowpressure while the hydrogen in the storage cylinders and associated manifold is at highpressure. The likelihood of a leak increasing with increasing pressure Leakage or permeation sites include fuel lines, connections without welding, and nonmetal seals like gaskets, O-rings, pipe thread compounds, and packings.
The fuel cell stacks are designed to prevent hydrogen and air from mixing directly when used in a fuel cell engine. Seals are used to accomplish this. A heavyduty power module motor might have great many seals. Fuel cell stacks leak either internally (between flow paths) or externally (to the surrounding environment) over time. To bargain
with this expected spillage, power module stacks are ordinarily encased and the nook is vented with constrained air to
forestall hydrogen aggregation. Other symptoms of fuel cell stack leaks include poor electrical performance of individual fuel cells.
A hydrogen leak is not a danger in and of itself; however, when combined with air at appropriate concentrations, it can cause a fire and cause asphyxiation if it displaces the oxygen in the air (Section 6.1.2).
Only if hydrogen leaks into an enclosed space can there be enough hydrogen to cause asphyxiation. Outside, the hydrogen dissemination is fast to such an extent that the gamble is irrelevant. However, humans cannot detect hydrogen’s presence because it is colorless, odorless, and tasteless, and there are no warning signs before unconsciousness occurs.
Spillage gases might be hot and represent a consume risk as definite in Segment 6.3. As described in Section 6.4, leakage gases may present a highpressure risk.
Detecting Hydrogen Leaks There will always be hydrogen leaks. The majority of hydrogen accidents are the result of leaks that have not been discovered. A leak detection system is included in transit bus applications to accomplish this.
A collection of sensors that are connected to the control system of the vehicle make up the leak detection system. The sensors are typically calibrated to sound an alarm at 25% of the LFL of hydrogen and to sound warnings at 5 and 15% of the LFL of hydrogen, respectively, at strategic locations around the vehicle (such as beneath the roof canopies and in the engine compartment). These warnings and Hydrogen Fuel Cell Engines and Related Technologies: Hydrogen’s LFL is 4% hydrogen in the air. Hydrogen Fuel Cell Engines, Version 0 (December 2001) Module 6: The alarm Key Points & Notes thresholds on FUEL CELL ENGINE SAFETY PAGE 6-3 represent hydrogen concentrations of 0.2, 0.6, and 1%, respectively. Accordingly, the hole recognition framework
demonstrates a caution before gas focuses arrive at a perilous level.
The control system notifies the driver via dashboard lights, a message display center, or other means when a sensor triggers, and if an alarm concentration occurs, the engine is turned off. On specific leak indicators, measured gas concentrations may be displayed simultaneously. Section 5.12 provides a comprehensive description of a typical leak detection system.
As a rule, the on-board spill discovery framework is as it were
dynamic at whatever point the vehicle is on. Since the vehicle is unoccupied when it is turned off, dashboard announcements are of little use. As a result, any parking or maintenance facility into which the vehicle is driven while fueled must include a leak detection system.
The requirements for the facility’s gas leaks are further explained in Section 10.1.
Leaks of hydrogen can be large or small. A triggering of the leak detection system, an audible gas rushing sound, or a gradual or abrupt drop in fuel pressure are all signs of leaks. Leak tests are the only way to find very slow leaks, which may not be apparent through any of these methods.
Leak tests are a regular part of maintaining the fuel system, and fuel cell stack leak tests are an important part of maintaining the fuel cell engine. There are generally three types of leak tests: Pressure drop (or “leak-down”) observations over time, the application of a leak detection solution, or the use of a hand-held leak detector (or “sniffer”). In most cases, these procedures are carried out sequentially and provide increasingly precise methods for locating hydrogen leaks.
Pressure drop tests indicate whether a leak exists in a
large, generalized area.
• Hand-held leak detector tests indicate whether a leak
exists within a localized area. Hand-held leak detectors
only work on circuits pressurized with a flammable gas;
they do not work on circuits pressurized with air or inert
gases such as nitrogen.
• Leak detection solution tests indicate whether a leak
exists at a precise location. These solutions work on any
gas, flammable or not.
The most common way to stop hydrogen leaks is to tighten or replace the leaking part or fitting.
Leaks must be fixed in a safe location free of ignition sources and where hydrogen cannot accumulate. Preferably, this is outside away from above impediments, or
inside a hydrogen-safe support office. Only those working on the leak should have access to the personnel. It is strictly forbidden to smoke.
The remaining hydrogen is safely stored on board the vehicle in the event of a leak. To move the bus into a facility that is not certified as hydrogen-safe or to repair a component on the fuel storage circuit, venting the remaining fuel is the only option.
Before a break can be fixed, it should be found utilizing spill tests
also, spill identification hardware as demonstrated previously. In the event that the leak occurs in a hydrogen-powered vehicle, leak tests that are typically performed during routine maintenance may be utilized to assist in locating the location of the leak. When the components are pressurized, a leak can only occur.
Even when the vehicle is off, the fuel storage circuit maintains its pressurized condition. The fuel delivery circuit, which is downstream of the motive pressure regulator, is one of the other circuits that is only pressurized when the vehicle is running. Open (disconnect) the battery knife switches if the vehicle can be turned off to fix the leak while the vehicle is electrically dead.
Tighten the connections once you find a leak, and if the leak doesn’t go away even after you tighten them, replace the parts around it. Only tighten fittings at ambient or low pressure: When a fitting is tightened under high pressure, it could break, resulting in serious personal injury. Therefore, before tightening the fitting, if a leak is discovered in the high-pressure circuit, the circuit should be vented to near atmospheric conditions by following the prescribed venting procedures and equipment.
In a similar vein, prior to opening the circuit, if a component needs to be replaced, vent the circuit to atmospheric pressure: A fitting could be propelled with extreme force if it is loosening under pressure.
Never tighten a fitting when it is already compressed; doing so runs the risk of serious personal injury and the fitting breaking. Never use pressure to loosen a fitting; doing so may result in a significant forceful propulsion of the fitting or component.
Before fueling, a fuel circuit must undergo nitrogen and hydrogen purges after being exposed to air and vented to atmospheric pressure. This usually only applies to hydrogen storage cylinders, but it may also apply to other parts if the manufacturer specifies them. Re-pressurize the component, conduct a leak test once more, and purge as necessary using facility equipment and procedures after a leak has been fixed
Hydrogen Fires
The properties of hydrogen that contribute to its flammability hazard are:
• it has the widest flammability range of any fuel
• it has the lowest ignition energy of any fuel
• it has the greatest energy per weight of any fuel
• it burns invisibly and without smoke
• it can potentially generate electrostatic charges that result in sparks through flow or agitation
Hydrogen, once released, is flammable at a wide concentration range and mixes with air. Once ignited, this flammable mixture burns with great vigor and is very simple to ignite.
In the light, the flame is almost impossible to see. The danger of combustion and explosion increases if hydrogen leaks into an enclosed environment. The risk of fire is reduced when hydrogen leaks into an open environment because it quickly rises and spreads. Fires that already exist burn vertically and typically last for brief periods of time.
Most mixtures of hydrogen and air are potentially flammable and explosive, and can be easily ignited by a spark
or hot surface. Hydrogen flames are almost invisible in
daylight.
The risk of fire is reduced to some extent by design. The building’s materials are resistant to fire and completely grounded to prevent the accumulation of static charge.
Pressure relief devices are included in high-pressure fuel storage cylinders to prevent explosive pressure buildup within the cylinders by releasing the contents when submerged in fire. The discharge from the pressure relief device is directed to vents that protrude from the bus canopy’s top, allowing for unimpeded airflow.
Personnel must exercise extreme caution when directly handling hydrogen, such as when venting or fueling. Proper facility equipment and procedures must be followed when venting or fueling, and all ignition sources must be eliminated.
The fueling and venting facility must be grounded together with the vehicle to prevent static electrical discharge. Suitable facility fire detection, fire extinguishing
equipment and procedures must be in place.
Detecting Hydrogen Fires
Hydrogen fires are difficult to detect and pose a serious threat to personnel due to their near-invisibility.
Large or small hydrogen fires are possible. The intensity of a fire is directly proportional to the pressure at the source of the leak. Flames, smoke from nearby equipment engulfed in flames, heat waves, a burning odor, an explosion, component damage, or the activation of the fire suppression system are all signs of fire. It’s possible that none of these methods will reveal very small fires.
To detect and put out fires, some transit bus applications include a fire suppression system. A collection of sensors that are connected to the control system for the vehicle make up the fire suppression system. The sensors are designed to sound an alarm in the event of a fire and are positioned strategically all over the vehicle, such as beneath the roof canopies and in the engine compartment.
A few kinds of sensors can likewise identify high intensity. Thermal wire that is designed to short when melted and serve as a signal to the control system may be used in fuel cell applications.
The control system shuts down the engine and informs the driver via dashboard lights, a message center, or other means when a sensor trigger occurs. Single-shot fire retardants may be released into one or more zones associated with the triggered sensor after the vehicle is turned off. Section 5-12 provides a comprehensive description of a fire suppression system. Fire retardants do not leak into the passenger compartment of a vehicle.
When a retardant discharge occurs, expect a high noise
level. A cloud of dry chemical retardant dust may exit
the vehicle from the discharge areas. Avoid breathing
the dry chemical dust as it will irritate throat and lungs
Unless the vehicle battery knife switches are open (disconnected), the fire suppression system is almost always working. As a result, any parking or maintenance facility into which the vehicle is driven must have a second fire detection system. Office necessities
are additionally portrayed in Segment
Extinguishing a Hydrogen Fire
The steps to take to put out a hydrogen fire are the same as those to put out any other gas-fueled fire. Eliminating the fuel source is the primary action. Allow the fuel to exhaust itself under controlled conditions if this is not an option. The goal is to keep people and equipment from getting hurt or in danger, as well as from getting damaged. As with any fire, evacuate everyone except the firefighters, get in touch with the local fire department if necessary, and fight the fire as far away as possible.
The only way to put out large fires is to cut off the fuel supply. Little flames can be battled with a dry powder retardant (suggested), carbon dioxide or halon quencher.
By removing the fog nozzle from carbon dioxide extinguishers, the fire can be extinguished rather than contained. A fire cover may likewise be utilized. However, if a hydrogen fire is put out without shutting off its fuel supply, an explosive or flammable mixture may re-form and re-ignite from hot surfaces or other ignition sources in the area.
Standard firefighting procedure is to stop the fire from spreading while it burns out when the hydrogen source cannot be turned off. Use a lot of water to cool the equipment around you; also, proceed with the water stream until
well after the fire is out. If it is safe to do so, remove flammable materials from the surrounding area. Avoid the ends of storage cylinders when fighting a hydrogen fire. If you hear a rising sound coming from a safety device for venting or if a storage cylinder starts to look different because of the fire, stop using it right away.
Use a hose holder that is not manned, monitor the nozzles, or leave the area and let the fire burn.
Avoid inhaling the vapors by remaining upwind. In the event of unstoppable fires, evacuate to a radius of 450 meters (1500 feet). Think about evacuating the downwind area.
Shut down a hydrogen-powered vehicle as soon as it is safe to do so in the event of a fire. This effectively isolates the fuel in the fuel storage system by closing the solenoid valves connected to each cylinder and the high-pressure solenoid. The fire suppression system should automatically release retardants into the fire area when an alarm is shut off. Utilize the aforementioned standard fire fighting methods if the blaze persists. Tow the vehicle back to the maintenance facility and notify the manufacturer once the fire is out.
High Temperature
In a transit bus, water, glycol solutions, oils, and gases flow through pipes and other vessels. The turbocharger compressor outlet, which can reach more than 200 degrees Celsius, is the highest temperature that any of these streams can reach. Uncovered surfaces can cause serious consumes whenever contacted.
Covers for the engine compartment prevent heat transfer during normal operation; However, operating the engine with the engine covers open is possible. Avoid coming into contact with any internal surfaces that have circulating liquids or gases if the covers are open, such as during maintenance procedures. After the engine has been turned off, internal components may continue to get hot for some time.
Avoid contact with internal surfaces that are in contact
with liquids or gases. Heed all warning decals
High Pressure
A transit bus’s hydrogen is typically stored in roof-mounted cylinders at operating pressures of up to 5000 psig (345 barg) and up to 3600 psig (250 barg). When filling these cylinders with hydrogen, staff members work with it under high pressure. If a leak or component rupture occurred, this high pressure is extremely dangerous and could produce an explosive force. In any event, when drained,
the hydrogen chambers are frequently at a leftover tension of
300-500 psig (21-35 barg).
Never loosen or crack any high-pressure component’s fittings; doing so may result in a significant forceful propulsion of the fitting or component. When under pressure, never tighten a high-pressure fitting; doing so runs the risk of serious personal injury and the fitting breaking. Follow all fueling and venting instructions precisely.
As certified pressure vessels, all high-pressure components must be regularly inspected and replaced in the event of damage or fault. According to Section 2.2.1, hydrogen storage cylinders must pass a series of tests to be certified.
Hydrogen capacity chambers have different wellbeing highlights.
There is a solenoid valve in each cylinder that automatically closes when the engine is turned off or when a bus impact causes a collision sensor to trip. Additionally, there is an internal excess flow valve in each cylinder that closes when the gas flow leaving the cylinder is too high (for example, when a pipe bursts). The same function is performed by an additional excess flow valve located on the common cylinder manifold. When exposed to fire, the contents of each cylinder are released by pressure relief devices mounted at each end.
When working with rooftop components, always refer to the warnings and cautions in Section 7.1.1.
The filling box and the roof contain all high-pressure components. The intermediate (or “motive”) pressure of approximately 178 psig (12 barg) is used to regulate the flow of hydrogen out of the cylinders. Within the fuel delivery circuit, this is further regulated to pressures of up to 30 psig (2 barg).
The motive and low-pressure circuits are shielded from fuel overpressure by additional components. If the pressure rises above 200 psig (14 barg), a pressure relief valve is installed downstream of the motive pressure regulator and opens to the atmosphere. Hydrogen escapes from the fuel delivery through a burst disk.
if the pressure within the circuit is greater than 46 psig (3 barg), the circuit should be connected to the atmosphere.
Within the fuel cell engine, hydrogen, air, water, and coolant operate at a maximum pressure of approximately 35 psig (2.4 barg). The hydraulic fluid and lubrication oil operate at a maximum pressure of approximately 6.2 barg, or 90 psig.
The air brakes on the bus chassis operate at up to 125 psig (9 barg). When working with any pressurized component, extreme caution is required due to the potential for danger posed by these pressures.
When a fuel cell engine shuts down, the following circuits
vent or depressurize:
• motive pressure fuel circuit
• fuel delivery circuit
• air delivery circuits
• humidification water circuits
• hydraulic circuits
• lubrication circuits
When a fuel cell engine shuts down, the following circuits
remain pressurized for some time, but depressurize slowly:
• stack coolant circuit
• bus coolant circuit
When the a fuel cell engine shuts down, the following circuits do not depressurize:
• fuel storage cylinders
• high-pressure fuel circuit
• bus chassis air system
Even when the engine is turned off, the high-pressure circuit and hydrogen cylinders remain pressurized.
Additionally, fire-resistant tanks that maintain constant pressurization are found in fuel cell vehicles equipped with a fire suppression system.
Electrical Shock
There are a number of high- and low-voltage components in a fuel cell-powered transit bus.
The voltage produced by fuel cell stacks is proportional to the number of fuel cells, and for a heavy-duty engine like a transit bus, the total levels can exceed 1000 VDC (open circuit voltage). An inverter converts this into AC power to run the drive motor. To achieve a predetermined torque setpoint, the inverter adjusts the AC output frequency and current, and the output voltage floats as necessary up to 460 VAC.
Occasionally, some of this AC power is channeled into one or more “dump choppers,” or bleed resistors, which are cooled by water or coolant and convert it to heat. These high-voltage components can remain charged for up to five minutes even after the engine has been turned off, posing a serious risk of shock or electrocution.
The DC and AC voltages in fuel cell engines are very high. To avoid electrocution or shock, proceed with caution when gaining access to electrical components.
The engine compartment houses the fuel cell stacks, inverter, and other high-voltage components individually. These barriers must be in place to protect against shock and electrocution during normal operation.
Try not to work a power module motor except if all high voltage
hindrances are set up.
The reactant gases are automatically vented when the engine is turned off, and one or more dump choppers are activated. Any power produced by the stacks’ residual reactant gases is continuously absorbed by these dump choppers. The dump chopper is no longer connected, and a residual voltage may develop in as little as a few minutes if groups or individual fuel cell stacks have been removed from the vehicle. In addition, a fuel cell stack’s zero volts (0 V) reading does not guarantee that all cells are uncharged.
Always make the assumption that the stacks of fuel cells are electrically charged. Until you verify that there is no voltage present, do not touch the fuel cell stack, its graphite cells, or the cell voltage monitoring wires.
