Vapor Density
Vapor density refers to the weight of a gaseous fuel. Vapor density measures the weight of the gas molecules compared to air. The weight of air is assigned the value of 1.0. A gas with a vapor density of less than 1.0 will rise to the top of a confined space or rise into the open atmosphere. A gas with a vapor density greater than 1.0 is heavier than air and will settle close to the ground.
In situations where a flammable gas is present, knowing the vapor density of the product allows you to predict whether the danger of ignition is at a high level or at a low level within an area. Hydrogen gas has a density of 0.07, so it is very light and will rise in the atmosphere. Propane gas has a vapor density of 1.51, so it will settle to the ground.
Carbon monoxide has a density of 0.97, which is very close to air. Carbon monoxide mixes readily with all layers of the air since it has almost the same density.
Flammability Limits
Mixtures of flammable gases and air will burn only when they are mixed in certain concentrations. If there is too little fuel (vapor) present in the mixture, there will not be enough fuel to support the combustion process. This mixture is known as being too lean. If too much fuel vapor is present in the mixture, there will not be enough oxygen present to support the combustion. This mixture is known as being too rich.
The range of mixtures that will burn varies from one fuel to another. Natural gas must be mixed with air in concentrations of between 4.5% and 15.0% in order to burn. These values are known as the lower and upper flammability limits. The terms flammability limits and explosive limits are used interchangeably in this book. Under most conditions, if the flammable gas and air mixture can be ignited, it has the ability to explode. Test instruments are available to measure the percent of fuels in gas and air mixtures and to determine when an emergency scene is safe.
The lower explosive limit (LEL) refers to the minimum amount of gaseous fuel that must be present in a gas and air mixture for the mixture to be flammable or explosive. In the case of carbon monoxide, the LEL is 12.5%.
The upper explosive limit (UEL) of carbon monoxide is 74%. These two values tell us that carbon monoxide can burn or explode when the concentration is at least 12.5% and no greater than 74% in air.
Boiling-Liquid, Expanding-Vapor Explosion (BLEVE)
You must understand one potentially deadly set of circumstances involving liquid and gas fuels. This is a boiling-liquid, expanding-vapor explosion or BLEVE. A BLEVE can occur when a liquid fuel is stored in a vessel under pressure. A propane tank is an example; the vessel is partly filled with the liquid propane and the rest of the vessel is taken up by propane in the form of a vapor.
The most common cause of a BLEVE is a fire that is exposed to the tank. The fire heats up the liquid in the tank, causing it to generate more vapors. This increases the internal pressure of the tank to a point where the tank can rupture catastrophically. When this happens, large pieces of the tank can be propelled significant distances, injuring and even killing fire fighters. In addition, any remaining flammable liquid is immediately released from the tank. Because the temperature of the liquid is at or above its boiling point, the liquid immediately turns into vapor and creates a rapidly expanding cloud. The fire can then ignite the escaping vapors, creating a fireball. All of this can happen extremely rapidly, in a matter of a second or two.
Over the years dozens of fire fighters have been killed by BLEVEs that occurred while they were trying to fight fires that involved tanks of liquefied gaseous fuels. By understanding the characteristics of flammable gas fuels and the mechanism of a BLEVE, we can help to prevent injuries or deaths in emergency situations.
Will a gas with a vapor density of 2.6 rise, fall or have neutral buoyancy in air?
A. Neutral
B. Rise
C. Fall
D. It depends on what gas it is
The weight of a gaseous fuel is the:
A. gas mass.
B. vapor density.
C. explosive limit.
D. BLEVE.
