Gas Detection for Alternate-Fuel Vehicle Facilities

In addition to standard safety requirements set by the local fire department, fueling and maintenance facilities require approvals for their gas detection systems.

REDUCING air pollution emissions from vehicles in urban areas, combined with the desire to reduce dependence upon imported oil, resulted in several new regulations by federal, state, and local regulatory agencies in the 1990s. A focus of these regulations was to encourage conversion of high-polluting vehicles to cleaner vehicles in the various non-attainment areas of the country. Because infrastructure for privately owned vehicles (POVs) was not established, the regulations focused on fleet owners who could provide centralized maintenance and refueling areas.

As a result of these regulations, fleet owners in several urban areas have converted their vehicles from diesel or gasoline power to an alternate fuel, such as CNG (Compressed Natural Gas), LNG (Liquefied Natural Gas), LPG (Liquefied Petroleum Gas), Alcohol fuels, or H2 (Hydrogen).

Federal, State Regulations
The three primary regulations that affected this area are the 1990 Clean Air Act Amendments (CAAA), California's low-emission vehicle and clean fuels regulations, and the National Energy Policy Act of 1992.

Of particular importance to vehicle fleet operators is the CAAA's establishment of the Clean Fuel Fleet Program (CFFP). The CFFP applies to air quality non-attainment areas classified as serious, severe, and extreme for ozone and moderate and serious areas for carbon monoxide. This encompasses 22 cities in 19 states. Both private and government fleets of 10 or more vehicles that are capable of being centrally fueled are affected.

Provisions made in the Clean Air Act allowed California to adopt regulations that require auto manufacturers to produce low-emission vehicles. These regulations spurred the development of alternative fuel vehicles. In 1990, the California Air Resources Board adopted the California Low-Emission and Clean Fuels (LEV/CF) program that made automobile emission regulations more stringent than the national standards of the CAAA.

The LEV/CF regulations took effect in model year 1994 and become increasingly stringent through 2003. Relative to existing new vehicle exhaust emissions standards, the LEV/CF regulations lowered non-methane organic gas emission standards for new vehicles by 40 percent in 1998 and by more than 75 percent in 2003. The clean-burning characteristics of alternative fuels provide manufacturers with additional methods to meet these standards.

The National Energy Policy Act (EPACT) of 1992 also promoted the use of alternative fuels and energy sources. Fleets affected by EPACT include "a fleet that contains at least 20 motor vehicles that are centrally fueled or capable of being centrally fueled, and are used primarily within a metropolitan statistical area or a consolidated metropolitan statistical area, as established by the Bureau of Census, with a 1980 population of 250,000 or more." The purpose of EPACT was to reduce the nation's dependence on imported oil and encourage the use of alternate domestically produced fuels.

The move to alternate fueled vehicles provides benefits in terms of emissions, but it does have a cost. A comparison between natural gas and conventional fuels shows that with natural gas, carbon monoxide emissions are reduced by 90 percent, hydrocarbon emissions are reduced by 80 percent, oxides of nitrogen are reduced by 50 percent, and particulates are reduced by 99 percent. Also, natural gas is non-toxic, non-carcinogenic, and spills rapidly dissipate into the atmosphere. However, vehicle conversions can cost $3,000 and up, and both CNG and LNG have a higher price than gasoline. Much of the increased costs are offset by incentives from utilities and state and federal programs.

Types of Alternate Fuels
There are several types of alternative fuels used by these fleets. Compressed Natural Gas is primarily methane with smaller portions of other gases. CNG burns much cleaner than any other fossil fuel and is a domestic product, not an imported one. Several commercial fueling stations exist in the country, in addition to fleet facilities. CNG dispensed in "fast-fill" systems enables vehicles to be fueled about as fast as a conventional fueled vehicle.

NFPA 52 (Compressed Natural Gas Vehicular Fuel Systems) is the American National Standards Institute-approved standard that applies to the design and installation of CNG engine fuel systems on all vehicles and the installation and operation of their fueling systems. Because natural gas is lighter than air if it is accidentally leaked, it will rapidly disperse. Many fleet operators fuel indoors or perform maintenance indoors on CNG-filled vehicles, so concerns have been raised because natural gas can build up in enclosed area. Appropriately designed safety features, such as ceiling-level ventilation systems actuated by methane detectors, can prevent natural gas buildup.

Liquefied Natural Gas is identical to CNG in raw material composition and clean-burning characteristics. It tends to be less expensive than gasoline, and the storage volume is comparable to gasoline. Because it is chilled to minus 260 degrees F, odorizers cannot be used, making it odorless and colorless.

Liquefied Petroleum Gas, better known as propane, has the unique ability to be stored, distributed, and handled as a liquid while typically being used in a vaporous state. LPG is clean-burning, producing 50 percent less hydrocarbons and 14 percent less oxides of nitrogen than gasoline. Originally it was very popular, but in recent years CNG and LNG have nearly replaced it as a vehicle fuel source. It is in a liquid state, so it can be dispensed as fast as gasoline.

Storage tanks for LPG are typically above ground. NFPA 58, Liquefied Petroleum Gas Code, has been adopted by most states as the model code for LPG storage and safety. LPG is non-toxic and non-carcinogenic. It is heavier than air; LPG vapors tend to settle against the ground and are invisible. It has a narrow range of flammability compared to other transportation fuels and will rapidly dissipate beyond its flammability range in the open atmosphere. It is important that garages housing LPG-fueled vehicles be properly ventilated and monitored.

Facility Safety
Fueling stations and maintenance facilities normally need to be upgraded with the use of alternate fueled vehicles. In the maintenance facilities the potential concern lies with the vehicles themselves, while at the fueling stations the dangers could be the vehicles or the storage/fueling equipment. Thus, at a maintenance facility it is usually necessary to place gas detectors in areas where the vehicles could be located such as work bays, paint shops, chassis wash, tire shop, fuel/dynamometer building, or the CNG compressor area.

In addition to the standard safety requirements established by the local fire department, most facilities require various agency approvals for the gas detection system placed in these areas. The gas sensors and control system normally need approval from FM, CSA, UL, or other NRTL approved agencies. The monitoring philosophy is to react to the hazardous gas leak by providing dispersion and dilute with ventilation. They also require control and management of any ignition source near the location of the potential accidental discharge of gas.

At a low alarm situation (usually at 15-20 percent LEL levels), the following hazard control steps will occur:

  • The exhaust fans will change from low speed to high or from off to on and continue until the alarm clears.
  • The overhead doors open, and alarm strobes are turned on.
  • Visual and audible alarms will warn personnel to evacuate the area.
  • The emergency generator will start but not transfer.

Under a high alarm condition (usually at approximately 25-50 percent LEL), the following steps will occur:

  • Transfer exhaust fans to emergency power and turn on or increase to high speed.
  • Facility-wide audible alarms and evacuation procedures continue.
  • Shunt trips activated to cut power to the selected electrical distribution panels.
  • Local fire department is notified.

Monitoring/Alarm Equipment
The base requirements would include sensors, transmitters, controllers, gas sensor control panel, and a calibration network. This set-up would meet the basic alarm and calibration with relay output from the gas sensor control panel signaling the applicable alarm and ventilation systems.

The calibration network is necessary to maintain the sensor network at peak performance levels. To meet the needs of the local fire department, the system may need to provide a printer or datalog record of calibration and testing to prove due diligence. Many of the sensors might be located in areas difficult to access such as the ceiling, and remote, one-person, non-intrusive calibration would be needed to ease the routine calibration requirements.

Larger and more advanced facilities would go beyond the basic system with a graphical user interface, data recorder, enhanced annunciation devices, and battery backup. These enhancements provide a more comprehensive view of due diligence in maintaining a safe facility. A graphical user interface, whether it is a simple design or part of a plant-wide distributed control system, enables operation and safety personnel to view conditions locally and remotely on a constant basis.

A typical installation will provide a gas risk management system multiplexing a number of combustible gas detectors at various points determined by the fire marshal or engineer. At an LNG refueling station, flame detectors might also incorporated into the system. Due to the two-way communication between the sensors and controller, remote calibration systems are easily added for convenience. The printer output is used to maintain the records demanded by the fire marshal for due diligence. The zoning capability of the system enables operators to control selected doors and fans to react to specific gas alarms.

In several cases, the user depends upon a graphic display panel with serial input from the system to provide the visual overview of the alarm system. In other cases where the user needs a more graphical display of the information, it uses either the Modbus output to link to a GUI or utilizes a Web server to develop its own graphical displays for viewing via a Web browser.

Summary
Alternative fuel vehicles' safety is driven by local, state, and federal regulations in which fleet owners in key metropolitan areas convert much of their fleet to cleaner-burning fuels. Various alternative fuels are available to meet this requirement, each with its own advantages and requirements.

This conversion to alternative fuels leads to special requirements for safety monitoring in the maintenance facilities and refueling stations. A comprehensive gas and flame monitoring system needs to meet the needs of both the user and the local fire marshal.

This article originally appeared in the May 2003 issue of Occupational Health & Safety.

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