Natural Gas is an environmentally clean, low-cost, plentiful, domestically-produced fuel that is used in more than 70,000,000 homes and businesses throughout the U.S.A. and is rapidly growing as a transportation fuel for motor vehicles. But is it really safe as a vehicle fuel?
Any fuel, including those used in motor vehicles, can be dangerous if handled improperly. Fuels contain energy, which is released when the fuel is ignited. Gasoline is a potentially dangerous fuel, but, by understanding how to handle it, we have learned to use it safely. The same is true of natural gas. Natural gas safely generates our electricity, heats our homes and cooks our meals. But, like gasoline, natural gas must be understood and respected in order to be used safely.
As with all vehicle fuels, natural gas can be used safely if the unique properties of the fuel are understood and common sense procedures are followed. In fact, natural gas has safety advantages compared to gasoline and diesel: it is non-toxic, and has no potential for ground or water contamination in the event of a fuel release. Natural gas is lighter than air and dissipates rapidly when released. An odorant (mercaptan) is added to provide a distinctive and intentionally disagreeable “rotten egg” smell that is easy to recognize. The odor is detectable at one-fifth of the gas’ lower flammability limit.
Natural gas vehicles have an excellent safety record for two primary reasons: the properties of the fuel itself and the integrity of the natural gas vehicle and its fuel delivery system.
Natural gas has a very limited range of flammability. It will not burn in concentrations below about 5% or above about 15% when mixed with air. Gasoline and diesel burn at much lower concentrations and ignite at lower temperatures. Although it takes very little energy to ignite a flammable mixture of air and natural gas, gasoline, or diesel, natural gas burns at a somewhat lower temperature.
From the gas field to the vehicle’s engine, natural gas requires very little processing to make it suitable for use as a fuel. Gasoline and diesel must be processed from crude oil in large and complex oil refineries. After water vapor, sulfur and heavy hydrocarbons are removed, natural gas flows by pipeline (the safest way to transport energy) directly to the fueling station where it is compressed for use.
Alternatively it may be used as Liquefied Natural Gas (LNG) by being cooled to cryogenic temperatures on site or at a central facility and delivered by truck. Gasoline and diesel are also delivered to fueling stations by tank trucks over the highway.
At a compressed natural gas (CNG) fueling station the gas is compressed before being provided to vehicles at 3600 pounds per square inch (psi). Older stations provided CNG at 3000 psi. Although the use of high storage pressures might appear dangerous, compression, storage and fueling of natural gas vehicles (NGV) meet stringent industry and government safety standards. Remember that high-pressure gases are used safely every day in industrial and medical applications.
Natural gas powered vehicles are designed and built to be safe both in normal operation and in accidents. New OEM NGV are subjected to the same federal government crash tests as other vehicles. OEM natural gas vehicle fuel systems must meet Federal Motor Vehicle Safety Standards 303 and 304. Natural gas cylinders are much thicker and stronger than gasoline or diesel tanks. Cylinders are designed not to rupture when fully fueled over six times a day, 365 days a year, far beyond what they will see in service. Industry standards test them far beyond normal environmental and service damage risks. Cylinders must even withstand a bonfire test and penetration by a 30-caliber bullet without rupture! The cylinders are designed for a specific lifetime from 15 up to 25 years and are required to be inspected every 3 years or 36,000 miles.
No matter what the fuel, fueling stations, indoor parking structures and repair garages must be built to ensure high levels of safety. Again recognizing the unique requirements for facilities handling CNG and NGV will differ from those for gasoline or diesel vehicles. For example, leaking diesel and gasoline form puddles on the floor. Natural gas normally rises toward the ceiling and disperses. Therefore the danger of fire would be greatest near the floor for liquid fuels and near the ceiling for natural gas.
Data collected over time has demonstrated natural gas vehicles to be safe in actual operation. Based on a survey of 8,331 natural gas utility, school, municipal and business fleet vehicles (NGVs) that traveled 178.3 million miles:
- The NGV fleet vehicle injury rate was 37% lower than the gasoline fleet vehicle rate.
- There were no fatalities compared with 1.28 deaths per 100 million miles for gasoline fleet vehicles. The collision rate for NGV fleet vehicles was 31% lower than the rate for gasoline fleet vehicles.
- The fleet of 8,331 NGVs was involved in seven fire incidents, only one of which was directly attributable to failure of the natural gas fuel system.
Natural gas vehicles were first commercialized after World War II in Italy. There are now over twelve million in use worldwide. NGVs have been used in the U.S.A. since the early 1970s, with over 130,000 in use today. Yet there has been only one fatality in the US involving a NGV in all that time and it was attributed to human error.
How do natural gas vehicles behave in crashes? The strength of the natural gas cylinders and fuel system generally avoids any leakage or fire. For example, an accident involving a CNG-powered pickup proved to be a testimonial to the safety of CNG tanks. As reported in the May 1995 edition of Automotive Fleet:
“When the 1992 CNG pick-up was broadsided in Midland, Texas, the most vulnerable part of the fueling system bore the brunt of the hit. While the force drove an imprint of the tank safety valve into the side of the truck, the CNG tanks did not rupture, and driver Jimmy Oden walked away.”
And in a tragic 1998 accident, a stopped bi-fueled Honda (a vehicle which could run on either natural gas or gasoline) was impacted by another vehicle moving at nearly 100 mph and a fire fed by gasoline broke out. The 13 gallon CNG fuel tank was intact and remained secured in its support brackets. Nationwide Insurance, in looking at the safety of CNG buses in a fleet, concluded as long ago as 1992 that “…the natural gas powered vehicles will be the safest vehicles in your fleet and (we) have no reservations about insuring them.”
In summary, technical data, appropriate safety regulations and years of experience show NGV to be as safe as, or safer than, conventionally fueled vehicles. However, the following safety measures should be noted:
The US standards for CNG fuel are both 3000 psi (200 bar) and 3600 psi (250 bar). Tanks, fittings, lines and hoses must be listed or approved for use as tested at the specified working pressures. A failure or rupture of any of these fuel system components can cause an explosion of great force. Do not over pressurize 3000 psi vehicle systems by changing the fill receptacle to connect to 3600 psi dispensers. Many imported “kits” use pressure regulators and other components designed for 3000 psi (stamped “200 bar”), if so the vehicle fill receptacle must be of the 3000 psi type even if this means somewhat reduced range on CNG and limited refueling options in the United States. [Ref. NFPA-52 Sections 4.3 / 4.11.2]
Tank Inspection & Expiration
CNG fuel tanks must be U.S.A. Department of Transportation (DOT) approved. As the structural integrity of CNG tanks degrade over time, all tanks are required to have an expiration label. Do not continue to use expired cylinders. Current legislation states that CNG tanks and fuel systems must also be visually inspected after a motor vehicle accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for damage and deterioration per DOT regulation FMVSS 304. Cylinder life cannot be extended. Expired cylinders must be removed from service. [Ref. NFPA-52 Section 4.4]
Pressure Relief Device (PRD)
All CNG tanks must have one or more pressure relief devices which have been bonfire tested and approved by the tank manufacturer so as to rapidly vent gas away to the atmosphere in case of excess temperatures. If a NGV catches on fire, rapidly-expanding CNG in the tank can create internal pressures to the point of tank failure and catastrophic explosion. The PRD is usually located at the tank valve and is designed to activate at a specified temperature so as to rapidly empty the tank via the PRD port on the valve. It is especially critical that CNG tanks in enclosed areas (i.e. car trunk, SUV cargo area) be equipped with secured PRD tubing to vent away the fuel in case of fire. [Ref. NFPA-52 Sections 6.4 & 4.5]
CNG tanks must be secured to the vehicle body, bed, or frame to withstand a force of eight times the weight in all directions of a fully pressurized container (8x G force impact). Only accept the use of brackets and mounting designs which have been approved by the tank manufacturer. Never accept the use of ratchet straps, aluminum bands, etc. on your vehicle. [NFPA-52 Section 6.3]
Tanks must be protected from road hazards, loading, unloading, direct sunlight, exhaust heat, accidental cargo leakage. Shield must not directly contact tank nor trap solid materials or liquids. Cylinders & valves must be shielded with minimum 9 inches road clearance when tires are deflated and cannot be located ahead of the front axle or behind the rear bumper attachment. CNG tanks must be transversely mounted if behind rear axle. [NFPA-52 Section 6.3]
Corrosion-resistant fuel lines (generally stainless steel) must be mounted, braced, and supported to minimize vibration and protected against damage, corrosion, or breakage due to strain or wear. No use of cast iron, plastic, galvanized pipe, aluminum or copper. Fueling connection must be ANSI listed. [Ref. NFPA-52 Sections 4.8 & 4.10]
Venting All Fittings
The neck of the tank and all fittings located in a vehicle compartment (i.e. trunk or SUV cargo area) must be enclosed in a gastight polyethylene enclosure that is vented directly to the outside of the vehicle but not into a wheel well. [NFPA-52 Section 6.4]
A valve that automatically prevents the flow of CNG to the engine when the engine is not running, even if the ignition is switched on must be provided. [NFPA-52 Section 6.6.3]
Fuel Line One-Way Valves
Fuel lines must have two backflow check valves that prevent the return flow of gas from the tank to the fill connector, mounted to withstand breakaway force of 150 lb (68 kg) when applied in any direction that the vehicle would move. [NFPA-52 Sections 6.6 & 126.96.36.199]
Blue “CNG” diamond label made of reflective durable material with minimum size of 4.7″ x 3.3″ must be on vertical surface on the lower right rear of the vehicle, but not on the bumper, to alert first responders of high pressure gas fuel system. Labels at fuel fill receptacle and in engine compartment must provide information as to system working pressure, tank expiration, and the next inspection date. [NFPA-52 Section 6.11]
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