Natural Gas Fuel Safety
Like any fuel, natural gas is flammable. The fuel storage and delivery systems for natural gas vehicles (NGVs) are governed by the National Fire Protection Association (NFPA). NFPA 52, the Vehicular Natural Gas Fuel Systems Code, defines specific safety requirements for NGVs and their fueling facilities. In addition, NFPA 30A applies to facilities that perform maintenance and repair of NGVs and NFPA 88A applies to parking garages.
Compressed Natural Gas
Natural gas is odorless and colorless in its natural state. As a safety precaution, chemicals (odorants) are intentionally added when the gas is pumped into the local distribution network of pipelines to give it a distinctive, pungent smell, similar to rotten eggs. The odor signals a potential compressed natural gas (CNG) gas leak. Owners who notice this lingering odor coming from their vehicle should close the vehicle's manual shut-off valve, if it has one. They should then contact a qualified repair facility and request guidance on how to proceed. Note that a slight odor may be detected when the fueling nozzle is being connected or disconnected during the refueling process. This is normal and should quickly dissipate when fueling has been completed.
Natural gas is lighter than air, so leaking natural gas from vehicles parked outside will generally rise and disperse safely; however, natural gas leaks in an enclosed garage could pose a danger, as there is limited area for the gas to disperse. Owners noticing a rotten egg odor coming from their garage should keep clear of the area and contact their fleet manager and fire-safety officials.
CNG is also stored at very high pressures, presenting different safety issues than gasoline or diesel fuel present. Repair facilities need to take precautions to secure CNG cylinders while they are being serviced and follow standard safety procedures while working. When vehicles and fueling stations are operating well and are properly maintained, the high-pressure gas is unlikely to present any danger.
For more information on all aspects of indoor CNG vehicle maintenance facility protection, see the Compressed Natural Gas Vehicle Maintenance Facility Modifications Handbook. Also review an ongoing Sandia National Laboratories’ project that focuses on modeling unexpected natural gas releases in vehicle maintenance facilities, which will inform the development of safety codes and educational materials.
Liquefied Natural Gas
Liquefied natural gas (LNG) is a cryogenic liquid stored at around -260°F. LNG is also odorless in its natural state and does not contain an odorant; therefore, an LNG leak is difficult to detect. This is why LNG vehicles and garages include electronic methane sensors to detect leaks. The cold natural gas vapors are heavier than air when they initially leak from a vehicle, so they may cling to the ground or pool, causing a potential fire hazard, as well as an asphyxiation hazard in enclosed spaces. For these reasons, gas detectors should be installed near the ground and ceiling in areas where the fuel itself or LNG vehicles are stored. LNG storage or vehicle maintenance facilities should be equipped with both floor- and ceiling-level ventilation to exhaust any potential leaks.
Unlike CNG-filled tanks, LNG-filled tanks may occasionally vent off natural gas if stored unused for a long period of time. LNG tanks are typically designed to hold a full tank of LNG for a week or more without venting. Once the fuel warms, the LNG begins to vaporize, and the pressure will rise in the tank until the relief valve opens to release or "vent" some natural gas. For this reason, LNG vehicles should be parked either outside or in a facility equipped with proper ventilation to safely accommodate any vented LNG. LNG should also be used in applications where the vehicles are used regularly to avoid the need for venting.
Another safety concern resulting from LNG is caused by the very cold temperatures at which it is stored. Cryogenic or freeze burns can be caused from coming into contact with LNG liquid, LNG vapor, or even the cold surfaces of pipes or tanks containing LNG. While LNG refueling hoses are well-insulated and designed to avoid accidental leaks, anyone working with LNG should be aware of the hazards and, if necessary, wear personal protective gear. LNG fueling systems and tanks require minimal maintenance but should be inspected regularly for leaks and to ensure proper functioning of the tank's pressure gauge and LNG level indicator. For more information, see:
- NFPA 52, the Vehicular Natural Gas Fuel Systems Code
- NFPA 30A, Code for Motor Fuel Dispensing Facilities and Repair Garages
- NFPA 88A, Standard for Parking Structures
- NFPA 59A, Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG)
- NFPA 57, Liquified Natural Gas (LNG) Vehicular Fuel Systems