METHANE/NATURAL GAS (MARSH GAS)
A common hazard in many manholes is the presence of marsh gas— also known as natural gas, coal damp, and swamp gas. Natural gas is a mixture of several gases and the end product of anaerobic (in the absence of oxygen) decomposition of organic material. It is found in concentrations or deposits by itself, in conjunction with petroleum, and in deposits near coal. Its presence in manholes usually is the result of decomposition of organic material in the sewer.
Methane—a flammable, nontoxic, colorless, odorless, and tasteless gas—constitutes from 70 to 95 percent of natural gas, depending on the location of the deposit. Although natural gas and methane have some technically important differences, they are very similar in the properties pertinent to emergency responders. Although methane is nontoxic, the organic material in the manhole may break down into some gases, such as hydrogen sulfide, that are toxic.
Used as a fuel, natural gas is the most commonly compressed gas used in the world. Its most common container is a pipeline, which may be under relatively high pressure or at pressures as low as four ounces psi.
Methane and natural gas each can be used to produce carbon black (by burning them under specific conditions). They burn relatively cleanly.
PROPERTIES
Methane, a flammable gas, has an ignition temperature of 1,000°F and a flammable range of from 5 to 15 percent in air. Because it is a gas and gases already are in the proper physical state to burn, it technically has no flash point. Methane can be liquefied and as a liquid has been cited to produce vapors sufficient to form an ignitable mixture with the air at
— 306°F. It will be ready to burn at any situation and temperature at which human life exists. Methane’s vapor density is 0.55, and its molecular weight is 16. Its boiling point is -259°F, its freezing point is
296.5°F, and it is slightly soluble in water.
Natural gas is a flammable gas with an ignition temperature of between 900°F and 1,100°F. Its flammable range is from 3-8 to 17.0 percent in air.
HAZARDS
Flammability is the major hazard of methane and natural gas; the flammable range is from 3-8 to 17.0 percent. Fire is the secondary hazard. The primary’ hazard is the tremendous explosion that occurs when an accumulation of flammable gas is ignited within its flammable range. Most loss of life and property occurs with the explosion. Emergency responders must be aware of the danger of a killer explosion when dealing with a released flammable gas, and care should be taken to eliminate all possible ignition sources. Methane has an extremely low vapor density of 0.55, which makes it considerably lighter than air and able to disperse very quickly.
FIRE SCENARIO
Stopping the flow of gas is the only proper way to fight any gas fire, but any flame resulting from leaking natural gas or methane should not be extinguished unless the flow of gas can be stopped immediately. Extinguishing the flame of burning natural gas as it escapes from a leaking pipeline can cause the gas to build up, and a devastating explosion could result. If the leaking pipe is isolated from people and property, no system is threatened, and the environment is not being harmed, it usually may be best to allow the fire to continue to bum until the product is used up or the flow can be stopped safely.
PROTECTIVE CLOTHING AND EQUIPMENT
No special clothing is required for contact with methane and/or natural gas, but if an emergency responder enters an atmosphere containing either of these gases within the flammable range, clothing that will protect from burning gas is needed. No protective clothing will prevent harm from an explosion of either gas. SCBA should be worn even though the gases are nontoxic so that responders will have the proper oxygen level
