The work reported in this document was performed by Technology & Management Systems, Inc. ("TMS"), under Contract DTRS57-97-P-81040 from the U.S. Department of Transportation, John A. Volpe National Transportation Systems Center ("Volpe Center"). Mr. William T. Hathaway was the Project Technical Monitor at the Voipe Center. Dr. Phani K. Raj of TMS was the principal investigator on this project.

Our thanks are due to Dr. James Hansel of Air Products, Inc. for providing background material, valuable technical discussions on hydrogen infrastructure and safety issues. Our thanks are also due to Mr. Ronald Kangas and Mr. Jeffrey Mora of the Federal TransitAdministration for overall direction on this project and for their valuable technical comments.

Figure 2-2 Flowsheet of a Typical Hydride Storage System for Use in Vehicles

Figure 2-3 Schematic of a Metal Hydride Storage Vessel Used on a Bus

Figure 2-4 Decision Tree for Selecting the Type of Hydrogen Refueling Station

Figure 2-5 Classified Area In and Around Dispensers

Table 2-1 Comparison of the Properties of Hydrogen, Methane, and Gasoline

Table 2-2 Comparison of the Characteristics of Metal Hydrides

Table 2-3 Diesel Equivalent Values

Table 2-4 Hydrogen Fuel Concentration Specifications for Use in a Fuel Cell

Table 2-5 Volume Concentrations of Hydrogen and Trace Impurities in Hydrogen Fuel

Table 2-6 Training Topics for Various Personnel

Table 3-1 Risk Assessment

Table 3-2 Frequency Categories

Table 3-3 Hazard Categories

A large number of transit buses in revenue service in the United States are using alternative fuels (alternative to gasoline and diesel). At the end of 1997, over 2,300 transit buses in service were using Compressed Natural Gas (CNG), Liquefied Petroleum Gas (LPG), Liquefied Natural Gas (LNG), and alcohol fuels (methanol and ethanol). CNG is by far the fuel of choice; over 1500 buses (i.e., about 65% of the alternative fuel bus fleet) use CNG. The use of hydrogen as an alternative bus fuel began in early 1998 with Chicago Transit Authority placing three hydrogen-powered buses in revenue service. The hydrogen in these buses is maintained in a compressed state and is used to power a fuel cell which in turn generates electricity to drive the wheel motors.

One of the key issues in the use of alternative fuels in transit buses is safety. Over the past two years, the Federal Transit Administration (FTA) has published a series of guideline documents for the safe design and operation of transit facilities in which buses powered by alternative fuels are stored or maintained. So far the alternative fuels covered by these design guidelines include CNG, LPG, LNG, or alcohol fuels. TheFTA recognized the need to address the safety of hydrogen use as an alternative fuel in transit vehicles, andtherefore initiated the work reported in this document.

The Clean Air Act Amendments of 1990 mandate the reduction in tailpipe emissions of air pollutants from mobile sources including heavy duty vehicles or engines. In addition, the National Energy Policy Act of 1992 set a national goal to replace the use of up to 30% of petroleum fuel with alternative fuels by the year 2010 and mandates the use of alternative fuels in the nation's federal, state, and fuel provider fleets at a rate not less than the promulgated phase-in rate. In addition, several states have promulgated statutes encouraging or requiring the use of alternative fueled vehicles by fleet operators.

The increasing use of alternative fuels in the nation's transit bus fleet is a consequence of the above statutes and by the FTA's Alternative Fuels Initiative (AFI) initiated in 1988. The AFI includes field testing, demonstration and assistance in revenue service placement of buses powered by CNG, LNG, LPG, alcohol fuels, and hydrogen fuel cells.

Each of these alternative fuels has unique physical and chemical properties which differ from those of traditional diesel fuels in common use in transit bus fleets operating in the U.S. Transit agencies have decades of knowledge and experience on the use, handling and storage of diesel fuels. However, the use of these alternative fuels in buses is relatively new. The unique properties of the fuels affect usage, storage, handling and response to emergencies.

A number of transit agencies are already operating fleets of alternative fuel buses. However, the transition has been somewhat difficult because of the absence of adequate guidelines to address the issues involved in the design of facilities and vehicles to ensure a safe and smooth transition. The industry as a whole is learning from the experience of some of the pioneers in the transit industry who have successfully converted to operating alternative fuel buses. There is, however, an urgent need to provide guidance to other transit systems that are either contemplating transitions or are initiating the process in the near future. This document is intended to provide some guidance to these transit agencies in their efforts to make the transition to alternative fuel safe and efficient.

The purpose of this document is to: discuss the various safety issues related to the use of hydrogen; provide guidance and information on safe practices followed in the industrial gas and chemical industries (which produce, store, transport, and use hydrogen in a number of industrial processes); discuss applicable regulations, codes and standards (both national and regional, if any); and, in general, discuss issues to be considered when converting an existing diesel facility, or building a new facility, to service hydrogen fueled vehicles.

The scope of this document is limited to discussing issues related to bus facilities, e.g., bus fueling, storage, and maintenance facilities. The overall safety of a hydrogen use facility depends not only on safety systems designed into the fixed facilities but also on the safety systems included in the bus design and the knowledge and training of personnel. Therefore, this document also addresses bus fuel systems and personnel training issues.

In Chapter 2, issues and practices related to the use of hydrogen as an alternative fuel are considered. The topics discussed include:

• Relevant hydrogen properties
• Approaches to storing hydrogen on board the vehicle
• Design issues related to
• Fueling facility
• Vehicle storage/parking facility
• Repair/maintenance facility
• Bus fuel system and safety features
• Personnel training and operational procedures
• Emergency preparedness and other special issues

An approach to performing a system safety analysis using the methods conforming to Military Standard (MIL Std) 882(3 is described in Chapter 3. The system safety process is applicable when guidance on a specific design approach is not available or when a unique design issue warrants the use of detailed hazard analysis. The hazard resolution process requires giving full consideration to all elements of the alternative fuels system, including the vehicle. In addition, this assessment procedure may be beneficial when a transit authority initially begins operation with a small number of alternative fuel vehicles.

This document is organized into three chapters, and each chapter number is represented by a numeral. Verbatim quotations from regulations or technical standards are italicized. Other information related to report organization include the following.

Glossary. A Glossary is provided at the end of the document in which the definitions of several terms used are indicated. When a term defined in the glossary is used in the text, it is highlighted (bolded and italicized) for clarity. All acronyms when first used are expanded in the text. Also, a list of acronyms is provided at the end of this document.

Graphic Symbols or Icons. Informational icons are used to draw a reader' s attention to some important aspect of the discussion. Where a statute, regulation or standard is being cited verbatim, the source of the work is acknowledged with an appropriate icon. The section numbers indicated in these citations refer to the section number in the original code or standard. The other type of icon used is intended to provide additional information or raise a cautionary flag to the reader. A list of icons and their definitions is provided at the end of this document. Very important communications to the readers are enclosed in boxes.

Units. All units of measure identified in this document are in Standard International units (meters,

kilograms, seconds, Kelvin, etc.). Values in conventional British units are provided in parenthesis. Where verbatim citations are provided, the units used in the codes or standards are cited without any change.

Footnote. In many cases, footnotes are included to expand the information content of a sentence. One can read this document without reading the footnotes and not lose any important information.

Acronyms. References to regulations or standards are made by providing the acronym for the regulation/standard followed by the relevant section number. All references to the National Fire Protection Association (NFPA) standards are to their latest published editions. These may not necessarily be the version adopted by the local or state regulatory authorities. Readers are encouraged to identify the current adopted vintage of the standards in their community. The information provided in this document can then be used to refer to the appropriate chapters/sections in the currently adopted standards to obtain current requirements. It is cautioned, however, that because of the differences in editions, some requirements may vary from those discussed in this document. However, the requirements of currently adopted standards take precedence.

Listed below are several statutes, regulations, codes, and standards that are relevant to the use of alternative fuel in buses. Not all of these have been cited or referenced in the text to follow. They are included as sources of additional information.

• Clean Air Act Amendments, 1990, Title U[, "Provisions Relating to Mobile Sources," PL 101-549.
• Energy Policy Act of 1992 (EPACT), Public Law 102-486.
• Alternative Motor Fuels Act of 1988 (AMFA), Public Law 100-494.

• Code of Federal Regulations, 29 CFR, Subpart H, Section 1910. 103: "Hydrogen".

The following NFPA standards can be obtained from the National Fire Protection Association, 1

Batterymarch Park, P.O. Box 9101, Quincy MA 02269-9101 or by calling 1-800-344-3555.

• NFPA 50A -- Standard for Gaseous Hydrogen Systems at Consumer Sites (1994 Edition).
• NFPA 50B -- Standard for Liquefied Hydrogen Systems at Consumer Sites (1994 Edition).
• NFPA 54 -- National Fuel Gas Code. This code is a safety code that shall apply to the installation of fuel gas piping systems, fuel gas utilization equipment, and related accessories.
• NFPA 70 -- National Electric Code. The purpose of this code is the practical safeguarding of persons and property from the hazards arising from the use of electricity.
• NFPA 88A -- Standard for Parking Structures. This standard covers the construction and protection of,as well as the control of hazards in, open, enclosed, basement, and underground parking structures. This standard does not apply to one- and two-family dwellings.
• NFPA 88B -- Standard for Repair Garages. This standard covers the construction and protection of, as well as the control of hazards in, garages used for major repair and maintenance of motorized vehicles and any sales and servicing facilities associated therewith.
• NFPA 497A - Recommended Practice for Classification of Class I Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. This recommended practice applies to locations where flammable gases or vapors, flammable liquids or combustible liquids are processed or handled and where their release to the atmosphere may result in their ignition by electrical systems or equipment.

The primary standards that apply specifically to hydrogen systems are NFPA 50A and NFPA 50B. These industry consensus standards stipulate the requirements for the design of hydrogen systems (including containers, pressure relief devices, equipment assembly, marking, etc.), location of hydrogen storage areas, operation and maintenance, fire protection and other safety requirements. No specific standards exist for vehicular hydrogen fueling systems, requirements for operation of vehicles, specification for the vehicle storage and maintenance buildings.

The focus of the Occupational Safety and Health Administration (OSHA) regulations in 29 CFR, g 1910. 103 is primarily the safety of the worker in a plant that stores or uses hydrogen. Substantial parts of these regulations are adaptations of the various requirements in NFPA 50A and NFPA 50B. In this document, the various requirements from NFPA Standards are referred to and the exact language from the relevant sections is cited where necessary.

Hydrogen is used as a transportation fuel in three different forms, namely, gaseous hydrogen stored in tanks at elevated pressure, liquid hydrogen held in cryogenic dewars, and chemically bound in metal hydrides. Discussed below are the properties of hydrogen and issues of safety in each form of the hydrogen fuel for vehicular use.

Table 2- 1 provides some important data on the properties of hydrogen. For purposes of comparison, corresponding properties of methane and gasoline vapors are also indicated.

Flammability. Hydrogen has the widest range of flammable concentrations in air among all common gaseous fuels. This range is between the lower limit of 4% to an upper limit of 75% by volume. Because of this wide range, a given volume of hydrogen release will present a large flammable volume, thus increasing the probability of ignition. However, other properties such as high buoyancy and high diffusivity in air tend to reduce the duration over which a large volume of the gas-air mixture is in the flammable concentration range, resulting in a decrease in the ignition potential with time.

Sources: Herd (1976), McCarty(l975), ASHRAE (1969)

Notes(1) The condition of 293 K temperature & one atmosphere pressure referred to as Standard Temperature & Pressure (STP)

(3) Measured diffusion flame temperatures (Burgess & Hertzberg, 1974)

Ignitability. A flammable concentration of hydrogen-air mixture can be ignited either by a spark or by heating the mixture to its autoignition temperature. The minimum spark energy required for ignition of hydrogen in air is about an order of magnitude (i.e., by about a factor of 10) less than that for methane or gasoline vapor. However, in absolute terms, the ignition energy for all three gases is low so that exposure to weak sparks, hot surfaces, or flames (matches) is sufficient to ignite them. A static electricity spark produced by the human body (in dry conditions) is sufficient to ignite hydrogen.

Hydrogen has a higher autoignition temperature compared to that of gasoline (see Table 2-1). However, the low ignition energy characteristic makes it more readily ignitable. The hot air-jet ignition temperature for hydrogen is 943 K (compared to 1,493 K for methane and 1,313 K for gasoline). Hence, hydrogen can easily get ignited by a jet of hot combustion products emitted from an adjacent enclosure.

Explosion/Detonation. Hydrogen has a wide concentration range in air over which it is detonable. In long tubes and tunnels, an ignition of hydrogen-air mixture in the detonable range will result in an initial "deflagration" flame which will transition to a "detonation" front after a certain induction" length. The induction distance is least for hydrogen compared to the induction lengths in methane-air or gasoline-air mixtures. The detonation velocity in a hydrogen-air mixture can be as high as 1.5 km/s to 2.2 km/s. The energy of' explosion of hydrogen, expressed in kg of TNT per kg of fuel is 24, compared to 11 for methane and 10 for gasoline. Hence, on an equal mass basis, hydrogen explosions will be more destructive than those from gasoline.

Characteristics of Leaks and Dispersion. The relatively small molecular diameter of hydrogen (1.58 D ), compared to those of all other gaseous fuels, makes fuel lines and fuel systems more susceptible to a hydrogen leak (Hansel et al., 1993). The occurrence of hydrogen leaks from properly mated metal-to-metal seals (flared or compression joints) in a hydrogen line is remote. However, non-metal seals, such as gaskets, packings, and pipe thread compounds, present a high probability for leaks. While catastrophic failure of metal joints is highly unlikely, large sudden failures in non-metal seals can occur due to the embrittlement of the materials.

If the leak rate is small due to the hydrogen passing through tortuous paths in the seals, mixing of the released hydrogen with air is determined by buoyancy and molecular diffusion in the absence of air currents. Hydrogen has very high buoyancy (its gas density is only 7% that of air). Because both the diffusivity and buoyancy are high, release of hydrogen at a small leak rate results in its rapid mixing with air. In the case of a large leak rate (e.g., from large failure of a gasket), the exiting jet of hydrogen will be turbulent and result in very rapid mixing with air. Hansel et al., calculate that a turbulent hydrogen jet will be diluted to near air concentrations within a distance of 500-1,000 jet hole diameter. Therefore, if the jet is ignited, the flame length will be less than about 500 diameters from the hole (e.g., for a 1 mm diameter leak, the flame length will be less than 0.5 m).

Hydrogen Flame Properties. A hydrogen flame (or fire) is nearly invisible even though the flame temperature is higher than that of hydrocarbon fires. This is because of the absence of soot in a hydrogen fire. The presence of a hydrogen fire can be inferred, however, from the air density stratification along and above the flame ("mirage" effect). The near invisibility of hydrogen flame poses serious hazards to personnel in an industrial setting. A small leak from a gasket in a pipeline may have been ignited by static electricity but will be invisible to a casual inspection of the pipeline, and it is possible to get burned by the "torch fire."

Hydrogen Embrittlement. Constant exposure of certain types of ferritic steel s to hydrogen results in the embrittlement of the metals. Such embrittlement can cause cracks in pipes, welds, and metal gaskets. The sensitivity of embrittlement of a metal depends upon, in addition to the material characteristics, the purity of hydrogen, the metal stress and plastic deformation, temperature, pressure, and the periodicity in pressure variations, among other items (Ringland, 1994). Stainless steel has a better embrittlement resistance property than ordinary steels. Pure aluminum and most of the aluminum alloys are considered to have better resistance to embrittlement than that of stainless steel, provided the gas is dry (Ringland, 1994). Therefore, when considering the material for critical (hydrogen) systems, it is very important to evaluate

the susceptibility of the metal to embrittlement when exposed to hydrogen.

2.1.3 Chemically-Bound Hydrogen in Metal Hydrides

One of the methods of storing hydrogen for use in ground transport vehicles is in the form of metal hydrides. Certain metals, such as magnesium, nickel, iron, titanium, and certain metal alloys (iron-titanium, lanthanum-nickel-aluminum, etc.) combine with hydrogen under conditions of high pressure (15 to 20 atmospheres) and relatively moderate temperature ( 330 to 600 K) to form metal hydrides (Hueng, 1997; Waide et al., 1975). The metal hydrides dissociate and release hydrogen when the pressure is reduced or the temperature is increased. Figure 2-1 shows the typical variation of the amount of hydrogen chemically bound as a function of pressure and temperature. The hydrogen "holding" property of metal hydrides depends on the type of the hydride and other conditions.

Table 2-2 illustrates the properties of typical hydrides. As seen from this table, the mass of hydrogen held in the hydrides is between 1C/o and 2% by weight of the base material. A high value of 7.3% by weight absorbed hydrogen is indicated in the case of magnesium hydride. The temperature at which hydrogen desorption occurs is relatively low.

In general, the total weight of the hydride together with the hydrogen absorbed is relatively large even for moderate hydrogen mass. For example, a gallon of gasoline is equivalent (in heat energy) to 1 kg of hydrogen. Therefore, to store 20 kg of hydrogen (equivalent of 20 gallons of gasoline) in hydride form will require 275 kg of magnesium hydride (MgH₂) at 7.3% hydrogen absorption efficiency. The weight of "fuel" and the hydride will be 295 kg (about 650 Ibs). If other hydrides are used, the total weight will be substantially higher.

2.1.4 Hydrogen Storage on Vehicles

As indicated earlier, there are three practical methods for storing hydrogen in "fuel tanks" on board transit and other road vehicles. These methods are discussed below.

Compressed Hydrogen Gas (CHG). This mode of storage is very similar to the current practice of storing CNG in pressure vessels. CNG is stored in lightweight composite material cylinders at pressures up to 28 MPa (4,000 psig). It is, however, important to evaluate the compatibility of the storage vessel material with hydrogen before it can be used as a pressure vessel. Also, because of the consequences of leaks from tanks and because the valve stem represents the weakest part of the storage system, generally, the valve assembly in the tank is protected by a metal sleeve. The tanks are provided with Pressure Relief Devices (PRDs). Also, the pressure regulator(s) used in the fuel line are provided with a heat source to prevent icing caused by the low temperature due to gas expansion.

Liquefied Hydrogen Storage (LH₂). Hydrogen liquid at its normal boiling point (20.3 K) is stored at or close to ambient pressure in a vacuum jacketed (double walled) container suitable for vehicle use. Notwithstanding the vacuum jacket and the thermal insulation for the primary storage tank, heat will leak in and increase the tank pressure. These vessels are typically called dewers. Current best technology can limit the heat leak to a boiloff of about 1.8% of the tank inventory per day (Hansel et al., 1993). The tank pressure will have to be relieved by venting the excess boiloff hydrogen. Relief valves are therefore provided on these tanks. Large vessels, such as tanker truck tanks transporting LH₂ have, in addition to a vacuum jacket, a liquid nitrogen jacket to further reduce heat leak into LH₂.

Metal Hydride Storage. A typical hydrogen storage system in metal hydrides consists of solid

2.1.5 Fuel Economy and Diesel Equivalence

The heat of combustion of hydrogen, on a mass basis, is about 2.8 times the heat of combustion of common hydrocarbon fuels (gasoline, diesel, etc.). Therefore, for a given load duty, measured in energy units, the mass of hydrogen required is only about one-third the mass of hydrocarbon fuel needed. However, the volume of the hydrogen "fuel tank" depends on the type of storage of hydrogen on board a vehicle. Table 2-3 shows the diesel equivalent volumes for different hydrogen storage strategies.

Table 2-3 shows that if the hydrogen is stored as a compressed gas at 25 MPa (3,600 psig), then, on an energy equivalent basis, about 16 times the normal diesel tank volume is needed to store the compressed hydrogen on board the vehicle. That is, for a bus with a 500 liter (125 gallon) diesel tank, about 2,110 gallon volume of compressed hydrogen storage tank volume is required. Of course, this assumes that the hydrogen engine and the diesel engine have the same overall efficiency. Higher engine efficiencies are expected to result from the use of hydrogen fuel. Hence, the overall tank volume required for storing an equivalent amount of hydrogen in a compressed state may be somewhat less than 2,110 gallons, though still large.

The use of liquid (cryogenic) hydrogen in a vehicle will require about 4.2 times increase in liquid storage volume compared to diesel. That is, a 500 liter (125 gallon) diesel tank will have to be replaced by a cryogenic tank volume of about 2,100 liters (550 gallons). However, the weight of the fuel will be reduced by a factor of approximately 2.8. That is, a 500 liter (125 gallon) diesel with 400 kg of diesel will be replaced by about 150 kg of liquid hydrogen.

The use of metal hydrides presents a severe weight penalty; every kilogram of diesel is replaced by about 4.5 kg of metal hydride to obtain the same hydrogen/diesel energy equivalence. For example, a 500 liter (125 gallon) diesel tank with a fuel weight of 400 kg will have to be replaced by a metal hydride tank containing 1,900 kg (i.e., 1.9 tons) of "fuel."

2.1.6 Hydrogen Fuel Quality

The quality of hydrogen required depends on the type of vehicular power plant in which it is used. For example, if hydrogen is used in an internal combustion engine, a purity level of 95% (called "fuel Type 1") may be sufficient. There are, at present, no specific hydrogen fuel standards for use in an internal combustion engine. If hydrogen is used in fuel cells, a very high purity is necessary in order to avoid "poisoning" the cells. Hydrogen used in a fuel cell has to be essentially free of CO and CO₂. as these have detrimental effects on the electrolytic cells. Table 2-4 shows the gas specification required for a hydrogen-oxygen

fuel cell.

Moisture content in hydrogen gas may have deleterious effects on the container (fuel tank and associated plumbing) metal. It is known that moisture is one of the contributing factors for hydrogen embrittlement of metals (Ringland, 1994) due to its effect on the acceleration of the formation of fatigue cracks.

2.2 FUELING FACILITIES

The physical and operational requirements of hydrogen refueling facilities are discussed in this section. Since hydrogen refueling can involve either a compressed gas or a cryogenic liquid, depending on the type of hydrogen storage on the vehicle, both systems are discussed below.

Source: Ballard Power Systems (1996)

Source: Hansel (1996)

* Total Hydrocarbons

The widely used industry standard for designing industrial systems for gaseous hydrogen storage and/or transfer is in NFPA 50A. Similarly, NFPA 50B describes the standard for liquefied hydrogen systems. The materials presented in this section are based on NFPA 50A and NFPA 50B standards. Information from other sources on safe industrial practices is also presented. Gaseous hydrogen refueling and liquefied hydrogen refueling are discussed in separate subsections.

2.2.1 Compressed Hydrogen Gas (CHG) Fueling Facility

A compressed hydrogen facility may be supplied with gaseous hydrogen from on site tube trailers ("a temporary facility") or a pipeline or by the vaporization of Liquefied Hydrogen(LH,) from a storage tank and compression of the vapors. Tube trailer based hydrogen supply may be sufficient if the transit system is servicing fewer than 5 buses (see Figure 2-4). In the case of a temporary storage facility (principally tubetrailers), an auxiliary compressor and a gas dispensing station will be needed in addition to the hydrogen from the tube trailers. In the case of a larger dispensing facility, hydrogen gas is obtained from either a gas pipeline or by evaporating liquid hydrogen. This gas is then compressed on site with a compressor and supplied to the dispensing station (similar to the process in most CNG dispensing stations). One other design involves the compression of liquid hydrogen and then evaporating the compressed liquid to generate

Normally, the dispenser in a CHG dispensing station should be rated to fill a bus tank up to a pressure of 25 MPa (3,600 psig). The rate of discharge of the gas from the dispenser will depend on whether a slow fill or a fast fill facility is being designed. Normal fast fill times range from 5 to 15 minutes for a 40 ft bus.

2.2.1.1 General Siting Requirements. In general, CHG refueling facilities are to be located outdoors or in separate special buildings (NFPA 50A, §3-2). The location is dependent on the size of the operation. NFPA 50A Table 3-2.2 indicates the minimum separation distances to be provided between gaseous hydrogen systems and other equipment and buildings. These criteria should be followed. The following requirements for the location of different sizes of hydrogen systems are excerpted from NFPA 50A.

2.2.1.2 Electrical Equipment. All electrical equipment and machinery that have a potential for exposure to hydrogen should conform to NFPA 70, National Electrical Code requirements. In addition, NFPA 50A, §4-1.2 stipulates that all equipment within 4.6 m (15 ft) of hydrogen equipment should be classified Class I, Division 2. In addition, the guidelines and recommended practices indicated in the American Petroleum Institute's Publications 500 and 505 should be reviewed. These guidelines refer to the design and operation of electrical machinery and equipment operating in environments where the potential for the occurrence of flammable vapor mixtures with air exists.

There are no specific guidelines in NFPA 50A for electrical classification of the hydrogen dispenser area. However, because of similarities between CHG and CNG the requirements for CNG dispenser should, at a minimum, be made applicable. Figure 2-5 shows, schematically, the electrical classification that should be used around a hydrogen dispenser.

The design requirements for hydrogen containers indicated in §2-1 and for pressure relief devices and piping, tubing, and fittings in §2-2 and §2-3 of NFPA 50A should be followed.

2.2.1.4 Detection Instruments. NFPA 50A does not indicate specific requirements for detecting hydrogen leaks and fire detection. It is, however, prudent to install hydrogen leak detection instruments. Instruments to detect the leak of hydrogen gas from the fueling system or the presence of hydrogen gas in the atmosphere in the fueling area should be provided. The presence of hydrogen in the atmosphere can be detected with flammable gas detectors. If provided, these detectors should be installed over the dispenser area and on the underside of the ceiling of a weather protection canopy provided over the dispenser (since hydrogen gas released is lighter than air it will tend to rise and accumulate under the canopy). Hydrogen leaks can be detected remotely by ultraviolet or infrared frequency based sensors. The field of view of these sensors should be such that they can "see" the gas leaks from the fueling side of the bus or from the roof mounted tanks. If the released hydrogen ignites, these sensors will detect the fire rapidly. Note that a hydrogen fire is undetectable in the visible range, i.e., it cannot be "seen" by a human being.

2.2.1.5 Other Important Design Considerations. Consideration should be given to implementing all of the following design items. There are no current standards or guidelines specific to each of the requirements indicated below; however, many are used as a matter of routine in hydrogen and other flammable gas industries leg., CNG). Therefore, it is prudent to implement these in the design of a hydrogen fueling station.

1. The design of the fueling system (hoses, lines, and nozzle) should be such that no air leaks into the bus tank. The consequence of a failure of this "air free fueling" protection can be disastrous.

2. The bus being refueled, the dispensing hose, and the dispenser station should be electrically grounded to prevent static electrical discharge during a refueling operation.

3. A lightning arrestor should be provided in a fueling facility. Correct sizing of the cable and its footing in the earth are required to ensure proper grounding.

4. The fueling lines should be designed to ensure that under no circumstances will the external air enter the fuel tank. The fuel dispenser should incorporate a feature that prevents fuel transfer to the bus if the bus fuel tank pressure is less than 200 kPa (`14.7 psig). Low tank pressure may indicate that the tanks contain air (e.g., tanks are new or are in use and the relief device leaked all of the hydrogen out of the tanks and air diffused back into the tanks). The presence of air in a hydrogen tank poses extreme danger.

5. A proper venting system has to be designed to vent hydrogen from (pressurized) storage and from a bus in the case of an emergency. The vent height should be such that the hydrogen released will not be sucked into adjoining buildings, air intakes, or other facilities. Also, the vent should be located far from any ignition source.

7. The designs of the fuel dispensing control panel and other instrument panels should be based on sound human engineering principles. That is, the display panels should be bright and the display lettering should be visible and readable by a person standing close to a bus and fueling the bus. The keyboard on the control panel should be large so as to be useable by a person wearing thick winter gloves. The dispensing area should be brightly lit so that all parts of the facility are visible under the worst weather and/or nightitme conditions. The following guidelines applicable in the design of a CNG fueling station should also be considered as

being applicable to a CHG dispensing station.

8. The lines from the pressurized storage container(s) or compressor to the dispenser should be equipped with both automatic fast closing valves and manual valves at the storage end and at the dispenser end. If the fueling area is indoors, then each compressed gas pipeline entering the building should be equipped with a manual isolation valve. (Refer to NFPA 52, §4-11.3, §4-11.5, and §4-11.10.)

9. NFPA 52 requires that dispensers have a breakaway connection on each hose to limit the breakaway force to 660 N (150 Ibs) in the event of vehicle drive away. A manual isolation valve should be provided immediately upstream of this breakaway and a bleed off device should be located between the breakaway and the manual valve. (Refer to NFPA 52, §4-11.7, §4-11.8, and §4-11.11).

10. NFPA 52 §4-11.4 alludes to excess flow valves, but does not require the provision of these valves. If local or state codes require their use, it should be noted that current technology indicates that they are impractical, particularly with the large flow rates typical of transit compressed gas fuel stations. In the absence of these valves, the dispenser should be equipped with electronic shutdown for excess flow and should be designed to limit the total fill of any one vehicle to the maximum amount of fuel that the vehicle would use. In the event either of these conditions (excess flow rate or fueling continuing even when the bus tanks are full) occurs, it is desirable to immediately and automatically shut down fueling at all other islands in the area (all fuels) and shut down the compressors. Sounding a remote alarm should also be considered.

11. The requirements for the materials of the dispensing hose and their testing are specified in NFPA 52 §4-9.3 and §4-10, respectively. It is, however, prudent to use a hose manufactured of manmade material and braids of stainless steel. All hoses must be electrically conductive. Hoses must be designed for CHG use and must be certified by the manufacturer as such. The hose ends should be installed by the original hose manufacturer, who should pressure test the hose to 1.5 times the rated pressure. Hoses should be equipped with a manufacturer tag indicating the serial number of the hose, the test pressure and the test date. This tag should be marked on site with the first date of service.

Hoses should be discarded after 12 months of use--for hose with carbon steel braid, after six months of use or immediately after any mechanical abuse, such as a breakaway incident.

Hoses should be equipped with retractors or counterweights to ensure that fueling connections do not fall to the ground.

12. The fueling connector should be equipped with a sealed vent back connection which is connected by hose to a pipe to convey this vent back gas to a safe area away from the fueling operation. This hose must decouple so as not to interfere with the breakaway on the main dispenser hose. (Refer to §4-14.7 of NFPA 52.)

13. The dispenser is required by NFPA 52 to include a temperature compensation system, (Refer to §4-14.1, §4-14.2, and §4-14.3.) This system is generally electronically controlled and its function is to adjust the fill pressure to account for variations in ambient temperature as well as the heating effect which is common in vehiclecylinders during a fueling cycle. This system must be carefully designed and calibrated to ensure fills as close as possible to complete while not allowing overfills to occur. If this system includes a safety relief valve, it should be piped to a safe location in a separate line from the fueling connection ventback (See §4-9.2.).

2.2.1.6 Dispensing Area Operations and Procedures. Existing codes do not address, in detail, the operational aspects of the fueling area. NFPA 52 for CNG specifies some signage requirements as do several of the local codes. It is prudent practice to establish the following procedures, as a minimum, in a hydrogen fueling facility.

2.2.1.7 Safety Control Systems. It should be noted that the current practice in the hydrogen industry is to perform hydrogen transfer operations only outdoors. (The industrial gas and chemical industry recommends that all hydrogen transfers be made outdoors). However, if a hydrogen refueling station is located within a separate building, it is prudent practice to institute the following procedures and alarm systems to enhance safety in the fueling area.

2.2.1.8 Maintenance of Safety Equipment. Regular maintenance of instruments and equipment should be performed. This maintenance, as a minimum, should include:

Facilities should also undergo a fire prevention inspection. This inspection should include all devices which are commonly inspected in transit garages, such as:

• All sprinkler valve assemblies (monthly inspection).

• Yard hydrants and hoses, inside hoses and portable fire extinguishers (monthly inspection). Electrical equipment and storage of flammable liquids should be checked monthly.
• Housekeeping, as well as cutting and welding, smoking regulations, sprinkler alarms and doors at cut-off walls (monthly inspection).
• Operational capability and readiness of systems to verify triggering, interlocks, and automatic controls (threshold should be checked with a simulated gas release event). Weekly inspections should include:
• General condition of automatic sprinkler heads
• Dry pipe valves
• Water supplies
• Locked valve shutoffs

In addition to the above, the electrical systems controlling ventilation should be checked monthly. A calibration of flammable gas detectors and other remote hydrogen sensors (infrared and ultraviolet) should be done on a six-month interval or per the manufacturer's recommendations, whichever is more frequent. The fire sensors should be inspected regularly and any time there is a suspicion of malfunction.

2.2.1.9 Defueling Connections Design. There may arise circumstances in which the hydrogen inventory on the bus will have to be removed. These circumstances may include a small leak in the fuel system or components, scheduled maintenance of the fuel system or components, or other emergencies. The transit facility must have a well thought out plan of action to defuel a hydrogen bus and develop the facility and bus fuel system designs accordingly. The following approaches should be considered.

• 1 . Downloading the fuel through a certified hose from the bus to be defueled to empty tanks of another hydrogen bus of the same type. Of course, no additional defueling is possible when the tank pressures in each bus equalize.

• 2. Connecting the tank of the bus to be defueled to the suction side of a compressor (if present) and using the inventory to refuel other buses. Caution must be exercised so that low pressure (risk of air intrusion) is not created in the bus that is undergoing defueling. An air leak into a hydrogen tank will pose an extreme danger.

• 3. Designing an atmospheric vent stack system located far from ignition sources and buildings to which the bus containing a residual gas in a tank can be connected and hydrogen gas released safely into the atmosphere. The height of the vent stack should be greater than the height of any building within a radius of 30 m (100 ft).

All of the above defueling strategies require proper design of the bus fuel system so that proper and safe defueling connectors are provided on the system.

2.2.2 Liquid Hydrogen Fueling Facility

numerals indicate the order of preference.)

Table 3-2.1 Maximum Total Quantity of Liquefied Hydrogen Storage Permitted

2.2.2.2 Electrical Equipment. NFPA 50B does not specify special requirements for electrical equipment used in LH, service other than to stipulate that the requirements of Article 501 of NFPA 70 be met (02-9, NFPA 50B). However, in keeping with the requirements for the dispensing units of other similar flammable gases (CNG, LPG, etc.) it is prudent to design the electrical equipment in and around dispensingunits for LH, service to conform, as a minimum, to the requirements discussed in Section 2.2.1.2 of thisdocument. These requirements are schematically illustrated in Figure 2-5 of this document.

2.3 BUS STORAGE FACILITY

A bus storage facility is a building where buses are parked for long periods of time, i.e., 12 or more hours (dead vehicle storage). Issues relating to design considerations for such buildings for storage of hydrogenfueled buses are discussed in this section. In certain climates, transit buses are stored outdoors. Outdoor parking does not present a significant area of safety concern, and therefore is not discussed in this document.

NFPA 88A standard is applicable to vehicle parking structures. The standard is not intended to be applied to alternative fuel bus parking garages. However, since this is the only standard available, and many of its requirements are equally applicable to a transit bus garage, some of its provisions are indicated below. A transit agency should, however, review NFPA 88A completely and determine which sections may be applicable to its facility. Garages without physical barriers between parking and maintenance of vehicles (not for vehicle dead storage) are discussed in Section 2.4 of this document.

2.3.1 Design Overview

The principal design considerations for an indoor bus storage facility should include:

It may be a prudent practice to park hydrogen fuel buses outdoors until sufficient operational data are obtained on the reliability of hydrogen fuel system components. This will ensure that if hydrogen leaks occur, the gas will be dispersed in the atmosphere without any potential for a hazard. If indoor storage of hydrogen fueled buses is necessary due to climate or other considerations, the transit system should evaluate a bus fuel system design in which all CHG tanks on the bus, except one tank, are automatically shut off as soon as the bus enters the garage.

2.3.2 Operations in an Indoor Storage Facility

A transit system should implement other (passive) safety practices in hydrogen bus parking garages as a part of routine operations. Associated with these practices should be the training of personnel-including bus operators--to understand the various safety issues and to inculcate a "safety first" attitude in day-to-day operations. These passive safety enhancement procedures should include but not be limited to:

2.4 BUS MAINTENANCE FACILITY

A bus maintenance facility is generally a partial or fully enclosed building within which repairs and routine servicing of buses are performed. In many transit systems, this facility consists of one or more bays consisting of either a lift or a pit over which the bus to be serviced is parked. Also, in transit systems which operate mainly diesel fuel buses and a limited number of alternative fuel buses, the same maintenance facility is used for servicing alternative fuel and diesel buses. In large facilities several buses may be serviced at the same time. Where alternative fuel buses are serviced in existing diesel bus maintenance facilities, a common practice is to designate a section of the facility to service alternative fuel buses only with or without walled partitions between the alternate fuel service bays and diesel service bays; The section of the facility in which alternate fuel buses are serviced is upgraded with the provision of gas sensors, alarms, and special equipment. A shared maintenance facility may not be the bes