Vapor flammability range

When a tank is emptied of Class I liquid, there is left a mixture of vapor and air, which can be, and often is, within the flammable range. When such a tank is refilled with a Class I liquid, any charge that reaches the tank shell will be bled off by the required bond wire. Also, there will be no flammable mixture at the surface of the rising oil level because the Class I liquid produces at its surface a mixture too rich to be ignitable. 

Flammable or Explosive Limits — the upper and lower vapor eoneentrations at whieh a mixture will bum or explode. The lower explosive limit of p-xylene is 1.1 pereent by volume in air, whereas the upper explosive limit is 7.0 percent in air. A mixture of p-xylene vapor and air having a coneentration of <1.1 pereent in air is too lean in p-xylene vapor to bum. Conversely, a mixture containing more than 7.0 percent is too rieh in p-xylene to bum. By subtraetion (7.0 - 1.1), p-.xylene is said to have a flammable range of 5.9. Materials having low explosive limits and wide flammable ranges are extremely dangerous. 

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

Where there is reasonable potential for ignition of a vapor mix in the flammable range, means shall be provided to stop the propagation of flame through the vapor collection system. The means chosen shall be appropriate for the conditions under which they will be used. 

Unfortunately, this was not the case for the plant in this example, and the worker wrongly chose to maintain production. By the time he arrived at the furnace, some of the fuel oil from burner "A" was deposited on the furnace tubes. Due to the heat from burner "B," the oil had vaporized and had been carried into the furnace stack. An explosion occurred when the mixture of air and unburned fuel came into the flammable range. 

When mixed with air, LPG can form a flammable mixture. The flammable range at ambient temperature and pressure extends between approximately 2 per cent of the vapor in air at its lower limit and approximately 10 per cent of the vapor in air at its upper limit. Outside this range, any mixture is either too weak or too rich to propagate flame. However, over-rich mixtures resulting from accidental releases can become hazardous when diluted with air. At pressures greater than atmospheric, the upper limit of flammability is increased but the increase with pressure is not linear. 

The released material must be flammable and at suitable conditions to form a vapor cloud. Some portion of the resulting cloud must mix with air such that concentrations are within the flammable range for the material. 

Flammability limits. A flammable gas or vapor will bum in air only over a limited range of composition. Below a certain concentration of the flammable gas, the lower flammability limit, the mixture is too lean to bum. Above a certain concentration, the upper flammability limit, it is too rich to bum. Concentrations between these limits constitute the flammable range. 

In general, the flammability range increases with temperature.5 The following empirically derived equations are available for vapors ... 

The UFL increases significantly as the pressure is increased, broadening the flammability range. An empirical expression for the UFL for vapors as a function of pressure is available 6... 

Flammable mixture A mixture of a gas, vapor, mist, or dust in air which is within its flammable range. 

Commercial Liquefied Petroleum Gas (LPG), is a mixture of the liquefied gases of propane (C3) and butane (C4). It is obtained from natural gas or petroleum. LPG is liquefied for transport and then vaporized for use as a heating fuel, engine fuel or as a feedstock in the petrochemical or chemical industries. It has a flammability range of 1.8% to 10% and the vapor has a density of 1.5 to 2.0 that of... 

Upper Explosive Limit Also known as Upper Flammable Limit. Is the highest concentration (expressed in percent of vapor or gas in the air by volume) of a substance that will burn or explode when an ignition source is present. Theoretically above this limit the mixture is said to be too rich to support combustion. The difference between the LEL and the UEL constitutes the flammable range or explosive range of a substance. That is, if the LEL is one ppm and the UEL is five ppm, then the explosive range of the chemical is one ppm to Five ppm. (See also LEL)... 

Vapor Dispersion Vapors from certain materials can be dispersed or moved using water spray or air movement. Reducing the concentration of the material may bring the material into its flammable range. 

Third, the fuel-air ratio of a sufficient amount of the vapor cloud must be in the flammable range. The more uniform the fuet-air mixture, near the stoichiometric fuel-air ratio, the stronger the explosion. 

In considering the ignition of vapors, two important concepts are flammability limit and flammability range. Values of the vapor/air ratio below which ignition cannot occur because of insufficient fuel define the lower flammability limit. Similarly, values of the vapor/air ratio above which ignition cannot occur because of insufficient air define the upper flammability limit. The difference between upper and lower flammability limits at a specified temperature is the flammability range. 

Even if these vapors are in the flammable range, they are rarely ignited if the vents terminate as described above. 

Flammable vapor burns in air only over a limited range of fuel-to-air concentrations. The flammable range is defined by two parameters the Lower flammable limit (LFL) and the upper flammable limit (UFL). These two terms are also called the lower explosive limit (LEL) and the upper explosive limit (UEL). 

---