National Academies Press: OpenBook

Emergency Response Procedures for Natural Gas Transit Vehicles (2005)

Chapter: Chapter Two - Considerations Related to Use of Natural Gas

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Suggested Citation:"Chapter Two - Considerations Related to Use of Natural Gas." National Academies of Sciences, Engineering, and Medicine. 2005. Emergency Response Procedures for Natural Gas Transit Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/23328.
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Suggested Citation:"Chapter Two - Considerations Related to Use of Natural Gas." National Academies of Sciences, Engineering, and Medicine. 2005. Emergency Response Procedures for Natural Gas Transit Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/23328.
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Suggested Citation:"Chapter Two - Considerations Related to Use of Natural Gas." National Academies of Sciences, Engineering, and Medicine. 2005. Emergency Response Procedures for Natural Gas Transit Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/23328.
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4PROPERTIES OF NATURAL GAS FUEL Table 1 lists the key properties of natural gas. Clearly, many of these properties, particularly those related to the physi- cal state of natural gas, differ considerably from those of diesel fuel. A number of existing resources provide more comprehen- sive descriptions of the properties of natural gas (both CNG and LNG). Those resources also provide detailed comparisons between the properties of natural gas and those of other conventional and alternative fuels and information on engi- neering considerations related to the use of natural gas use as a vehicle fuel. The following list cites some of these resources. (References that are primarily of a scholarly nature or that mainly list benefits of natural gas or alternative fuels have been omitted.) 2004 Emergency Response Guidebook, U.S. Department of Transportation, Washington, D.C., 374 pp. Chernicoff, W.P., T. Balon, and P. Raj, Design Guidelines for Bus Transit Systems Using Electric and Hybrid Elec- tric Propulsion as an Alternative Fuel, Report FTA-MA- 26-7071-03-1, Federal Transit Administration, Washing- ton, D.C., 2003, 43 pp. Compressed Natural Gas: A Collection of Resources, DOE/ GO-102003-1776, Freedom Car and Vehicle Technologies Program, U.S. Department of Energy, Washington, D.C. DOE Alternative Fuel Data Center (maintains a list of resources and documents that may be found at www.eere. energy.gov/afdc/index.html) [Nov. 2004]. Hemsley, G.V., TCRP Synthesis of Transit Practice 1: Safe Operating Procedures for Alternative Fuel Buses, Trans- portation Research Board, Natural Research Council, Washington, D.C., 1993, 48 pp. Fire Protection Handbook, 19th Ed., A.E. Cote, ed., National Fire Protection Association, Quincy, Mass., 2003. (Sec- tion 14, Chapter 2, “Alternative Fuels for Vehicles,” con- tains a discussion on the fire implications of alternative fuel vehicles.) Murphy, M.J., H.N. Ketola, and P.K. Raj, Summary Assess- ment of the Safety, Health, Environment, and System Risks of Alternative Fuel: Clean Air Program, Report FTA-MA- 90-7007-95-1, Federal Transit Administration, Washington, D.C., 1995, 144 pp. NFPA 52: Compressed Natural Gas (CNG) Vehicular Fuel Systems, National Fire Protection Association, Quincy, Mass., 2002. NFPA 57: Liquefied Natural Gas (LNG) Vehicular Fuel Systems, National Fire Protection Association, Quincy, Mass., 2002. Raj, P.K., W.T. Hathaway, and R. Kangas, Design Guide- lines for Bus Transit Systems Using Compressed Natural Gas as an Alternative Fuel: Clean Air Program, Report FTA-MA-26-7021-96-1, Federal Transit Administration, Washington, D.C., 1996, 94 pp. Raj, P.K., W.T. Hathaway, and R. Kangas, Design Guide- lines for Bus Transit Systems Using Liquefied Natural Gas (LNG) as an Alternative Fuel, DOT-FTA-MA-26- 7021-97-1, Federal Transit Administration, Washington, D.C., 1997, 84 pp. Reference Guide for Integration of Natural Gas Fuel Sys- tems, Report GRI-02/0013, Gas Research Institute, Park Ridge, Ill., 2002. SAFETY ISSUES Natural gas has a different hazard profile than traditional liq- uid fuels such as gasoline and diesel fuel. Two properties that affect its hazard profile and consequent emergency response are its gaseous state and its storage at high pressure or low temperature. In normal transit operations, the risks from these hazardous properties have been mitigated through effective design. Gaseous State The physical state of a fuel has significant implications for the degree of its fire hazard. Only gases burn; therefore, liquid or solid fuels must first either vaporize or decompose, respec- tively, before burning. These are relatively slow processes whose rate tends to limit the rate of heat release. However, in the case of a gaseous fuel, the fuel is already in a form suit- able for burning, and the rate of heat release is limited only by the rate of release of the gas. Owing to the high pressure of CNG and the high volatility of LNG, both can be released very quickly. Should a fire occur, the significance for emergency response is threefold. First, fires fed by natural gas may attain large heat release rates quickly. Second, unlike with fires fed by solid and liquid fuels, the size of the fire is generally not reduced by cooling the fuel supply with water. Third, if a fire CHAPTER TWO CONSIDERATIONS RELATED TO USE OF NATURAL GAS AS A TRANSIT FUEL

5fed by a natural gas leak is extinguished, but the gas is still escaping, the gas can re-ignite and, because unburned gas has accumulated, lead to an even larger rate of heat release. Storage Conditions Compressed Natural Gas CNG is stored under high pressure, generally at 25 MPa (3,600 psig). Gases at this pressure contain a considerable amount of stored energy. The significance for emergency response is that under accident conditions hazards related to high-pressure are poten- tially present. Such hazards include the following: • Rapid venting of fuel cylinder contents. Cylinders are designed to vent under conditions of excessive temper- ature, but the resulting release of fuel can present a fire hazard. • Mechanical forces resulting from the action of high- pressure gas jets. • Pressure vessel rupture. Because cylinders are well pro- tected from rupture by numerous design features and extensive testing, cylinder rupture would be an extremely rare occurrence. • A rapid release of gas during fueling, resulting from a failure in a hose or fitting, or for some other reason. Liquefied Natural Gas LNG is stored under moderate pressure at a temperature of about −162°C. The hazard implications are: • LNG can vaporize quickly in contact with a warm sur- face, leading to a very rapid release of natural gas. • Odorants are solid at these (–160°C) temperatures, so LNG cannot be odorized effectively. • Many materials become brittle at cryogenic tempera- tures and lose their structural strength. • LNG tanks are designed to vent under conditions of excessive pressure caused by overfilling or overheating, but the resulting release of fuel can present a fire hazard. • If LNG warms up while being confined, very high inter- nal pressures will be generated that can cause a rupture or explosion. SECURITY ISSUES To identify security issues related to the use of natural gas as a transit fuel, it is useful to examine the FTA Top 20 Security Program Action Items for Transit Agencies. Of these action items, those most relevant to the use of natural gas as a tran- sit fuel are listed in Table 2. Consideration of these action items leads to the identifi- cation of any issues that are specific to natural gas. For exam- Property Value for Natural Gas Comparable Value for Diesel Fuel Nominal chemical formula CH4 a C14H25 Typical molecular weight 17 ~200 Boiling point, °C − 162 200–350 Density, at 20°C, 1 atm, kg/m3 Density, at 20°C, 25 MPa, kg/m3 Density, liquid at −162°C 0.70 217 424 850 Fuel energy value, mJ/kg 47–50 43 Flammability limits of vapor, vol. % 5–15 No flammable vapors exist below 50°C Minimum ignition energy, mJ 0.28 0.24 Autoignition temperature, °C 450 b 230 Adiabatic flame temperature, K 2157 2275 Odor threshold, ppm 10,000 c ~0.5 Occupational exposure limit, ppm ~15,000 d 14e aFormula is for methane. Natural gas is typically 85% to 99% methane. bValue includes the effect of higher hydrocarbons present in natural gas. The autoignition temperature of pure methane is higher. cNatural gas itself has essentially no odor. In commerce, natural gas is required to be odorized with sulfur compounds so the odor is detectable at 20% of the lower flammability limit. dMethane is considered nontoxic and a simple asphyxiant, but higher hydrocarbons are considered more toxic. Value is for natural gas assuming a reasonable level of higher hydrocarbons. eNo value has been established for diesel fuel. Shown is the National Institute for Occupational Safety and Health recommended value for kerosene. TABLE 1 PROPERTIES OF NATURAL GAS AND DIESEL FUEL

ple, with respect to fueling infrastructure availability, if a transit fleet is fueled by natural gas, then without the avail- ability of natural gas in compressed or liquefied form, there can be no transit operations. Inasmuch as there may not be nontransit sources of CNG or LNG available (or at least no other source that can dispense fuel to transit buses), a dis- ruption to or destruction of the natural gas fueling facilities 6 would render the fleet inoperative. Another issue is whether or not the use of natural gas as a transit fuel presents a secu- rity hazard. Because the properties of natural gas and the design of equipment for natural gas use do differ in several ways from those for diesel fuel, the question about a security hazard is multidimensional, and additional research is needed to define the outcomes. FTA Top 20 Goal Questions for Natural Gas No. 8—Threat and vulnerability assessment Is the threat and vulnerability assessment any different for natural gas vehicles and facilities? No. 13—Ongoing training programs on safety, security, and emergency procedures Do these training programs cover natural gas vehicles and facilities? Nos. 15 and 16—Audits and drills Do these audits and drills include natural gas vehicles and facilities, if appropriate? No. 20—Protocols to respond to Office of Homeland Security Threat Advisory Levels Do these protocols include appropriate responses for natural gas vehicles and facilities? Source: Adapted from “FTA Top Security Program Action Items for Transit Agencies” [Online]. Available: http://www.transit-safety.volpe.dot.gov/Security/SecurityIniatives/Top20/ [Nov. 2004] (2). TABLE 2 RELATION OF NATURAL GAS TO FTA TOP 20 SECURITY ISSUES

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TRB’s Transit Cooperative Research Program (TCRP) Synthesis 58: Emergency Response Procedures for Natural Gas Transit Vehicles identifies and documents the state of the practice on emergency response protocols to incidents involving natural gas-filled transit buses. The report is designed to assist first responders to natural gas incidents—emergency response professionals such as police officers and fire-fighters; transit agency operations and maintenance employees, police, and security guards; and certain members of the general public.

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