Natural gas can be as safe as other transportation fuels, and in many circumstances, safer. In the minds of some potential users, it has an undeserved reputation as being unsafe particularly for marine vessels. This stems largely from unfamiliarity, lack of understanding of the physical characteristics of natural gas, unfounded preconceived notions and misinformation.
Natural gas is sold as a combustible gas meeting a commercial specification. Methane, the largest constituent of natural gas, is colorless, odorless (a compound that produces an odor is normally added to gas for domestic use to aid in leak detection), flammable and, most importantly, lighter than air. Other combustible gases that may occur in natural gas in minor quantities are Ethane, Butane and Propane. These have higher heating values than Methane. Natural gas has a very high autoignition temperature, in excess of 1200°F, making it more difficult to ignite than other fuels, adding to its safety characteristics.
The flammable range of natural gas has been reported to be 5 to 15% gas in air at ambient temperatures and pressures. If the gas concentration is less than 5% (lower flammability limit-LFL), it cannot be ignited. Higher than 15% (higher flammability limit-HFL), there is too little oxygen to support combustion.
Natural gas is a simple asphyxiant. A person in a room filled with natural gas would be overcome from lack of Oxygen, but would not be poisoned by the gas and could be fully resuscitated if found in time. Prolonged exposure does not cause cancer. Many bacteria consume methane naturally in the environment.
Natural gas can be stored and used safely onboard marine vessels. An important property of natural gas is that it is lighter than air at ambient conditions. If gas leaks, it will dissipate upward, disbursing very rapidly at the same time. Small leaks dissipate too rapidly to become flammable. A leak cannot form pools on the floor of an engine room as can diesel fuel, gasoline and its vapors, or even heavier-than-air propane gas.Good safety engineering takes advantage of the physical properties of natural gas.
Minimizing the Danger of Fire
Three elements are required to initiate combustion:
A successful safety program prevents combining the three elements.
Methane detectors are designed to measure the level of methane concentration. If a leak is detected, a mitigation plan to avoid the danger of fire can be initiated to prevent a flammable mixture from developing.
If a gas leak is detected in a vessel compartment one or more of the following alternatives can be exercised:
Concern has been expressed that natural gas stored in cylinders as compressed natural gas (CNG) or in cryogenic tanks as liquefied natural gas (LNG) could explode. However, without the presence of oxygen, the gas in the containers cannot burn. A safety goal is to keep the combustible gas away from oxygen, at least until the gas is delivered to the engine or turbine where the operator wants it to burn. When a gas container is filled for the first time or after any repairs, it is normally first purged with Nitrogen or other inert gas to remove all air. In subsequent fillings, only natural gas is refilled in the tank. If any repairs are to be made, the tank is again purged with inert gas to remove any flammable gas, after which air can be introduced allowing repairs to proceed safely.
Note that natural gas does not explode, it deflagrates. In an explosion the flame front exceeds the speed of sound significantly. In a deflagration the flame front is slower than the speed of sound so that energy is released over a longer period of time. This characteristic can be utilized in design to enhance the safe use of natural gas as a marine fuel.
Controlling Sources of Heat or Spark
Fire can be avoided in the presence of a combustible mixture if there is no source of ignition to ignite the gases. "Explosion proof" electrical equipment is available for use in areas where combustible mixtures may be occur. The equipment will not cause sparks or get hot enough to cause ignition.
Fire Safety Codes for Marine Vessels
There are several regulatory or advisory bodies in the United States that have issued standards and codes pertaining to design practices and procedures for using natural gas aboard marine vessels. Among the groups are:
U. S. Department of Transportation
Other countries have there own versions of these agencies or refer to the US codes and standards.
Local authorities such as a city's Fire Marshall can also exert jurisdiction.
Brett & Wolff can provide more information about the relevant codes and design issues to interested parties. Contact us.
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