Dispelling CNG Safety Myths

By: Rob Minton

Published in Automotive Fleet Magazine, Sept. 2015

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There are many misconceptions of how safe compressed natural gas is as a vehicle fuel. This is the same fuel used to heat houses, clothes dryers, stoves, etc., but compressed to be used as a motor fuel, usually to 3600 psi. And while there are several factors to consider when deploying natural gas vehicles (NGVs), safety of the fuel and the vehicles that use it should not be one of them.

According to the U.S. Department of Energy’s Alternative Fuels Data Center, natural gas vehicles actually are safer than those powered by gasoline or diesel. The fact that CNG is lighter than air further enhances its safety. Other fuels such as diesel, gasoline or LPG, which are heavier than air, pool on the ground creating a fire hazard. Should a CNG leak occur, the gas will disperse rapidly upwards into the atmosphere and dissipate. CNG has a high ignition temperature, about 1,200 degrees Fahrenheit, compared with about 600 degrees Fahrenheit for gasoline. It also has very narrow flammability limits, that is, in concentrations in air below about 5 percent and above about 15 percent, natural gas will not burn. The high ignition temperature and limited flammability range make accidental combustion of CNG unlikely.

Get The Facts

Data collected over time has demonstrated natural gas vehicles to be safe in actual operation. Based on a survey by the Federal Transit Administration of 8,331 natural gas utility, school, municipal and business fleet vehicles (NGVs) that traveled 178.3 million miles on CNG:

• 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.

Pressure

Another misperception of CNG is that the high pressure storage of the product makes it more hazardous than other fuels. CNG fuel cylinders are much, much stronger than gasoline tanks. Natural gas fuel tanks are up to a half-inch thick and are made of steel or a composite designed to be stronger than steel, and valves are made to handle high pressure.

One of the most common worries about natural gas vehicles has to do with leaks. Actually, natural gas is a naturally occurring product made up primarily of methane. In its natural state, it’s odorless. However, for practical purposes, it’s odorized to enable easy leak detection. According to a safety article in the Natural Gas Vehicle Knowledge Base, the average person can detect the odor at concentrations as low as 0.3%. As mentioned earlier, for combustion to occur, natural gas requires a concentration of at least 5%. In other words, a driver will detect the smell long before combustion conditions are met. While a high pressure gas leak tends to make a lot of noise while the gas is escaping, the high pressure forces the gas to escape very rapidly causing high concentrations of gas to build up in the immediate vicinity of the leak. The resulting high concentrations of gas and its tendency to dissipate upwards make it less likely for the gas in the immediate vicinity of the leak to ignite. Naturally, the high pressure does cause the gas to escape with some force so obvious precautions are necessary.

The design of the cylinders is subject to a number of "severe abuse" tests such as heat and pressure extremes, gunfire, collisions and fire. The systems are also fitted with valves and other safety devices to prevent leakage and eliminate the risk of an explosion - actually making them safer than the legal requirements stipulate.

Some vehicle OEMs work with an approved vehicle modifier who will add the CNG fuel system before it is offered for first sale. In this situation, The National Highway Safety Administration (NHTSA) requires an additional alterer’s label at the driver’s door that certifies that the alterations do not take the vehicle out of compliance with Federal Motor Vehicle Safety Standards. This label assures you that the original crashworthiness of the gasoline system is matched by the added CNG system.

So in addition to CNG’s benefits of producing less greenhouse gases than gasoline and diesel, being less expensive than gasoline or diesel, and having an abundant supply right here in the U.S., you can add one more reason to deploy natural gas vehicles in your fleet: CNG is safer than gasoline.