Flammability ranges

Flammability limits. A flammable gas 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 burn, i.e., lacks fuel. Above a certain concentration, the upper flammability limit, it is too rich to burn, i.e., lacks oxygen. Concentrations between these limits constitute the flammable range. 

Combustion of a flammable gas-air mixture occurs if the composition of the mixture lies in the flammable range and if there is a source of ignition. Alternatively, combustion of the mixture occurs without a source of ignition if the mixture is heated up to its autoignition temperature. 

Flammability limits are also affected by temperature. An increase in temperature usually widens the flammable range. 

Flammable Limits Flammable limits, or the flammable range, are the upper and lower concentrations (in volume percent) which can just be ignited by an ignition source. Above the upper limit and below the lower hmit no ignition will occur. Data are normally reported at atmospheric pressure and at a specified temperature. Flammable hm-its may be reported for atmospheres other than air and at pressures other than atmospheric. 

Various types of rapid, adiabatic compressions have caused explosions. With propane at an initial temperature of 25°C, To = 432°K (I59°C) for compression and specific heat ratios of 25 and I.I3, respectively. Assume that now air enters a compressor to bring propane into the flammable range at 5 percent by volume. The mixture then will be mostly air with k = 1.47. The same compression ratio of 25 will elevate the final temperature T2 to 834°K (56I°C), i.e., above the published autoignition temperature of 450°C for propane and perhaps high enough to cause an explosion. 

Conclusions The use of an inert atmosphere can virtually eliminate the possibility of explosions and fire with flammable materials. However, inerting systems can be quite expensive and difficult to operate successfully and can be hazardous to personnel. Before using inert systems, alternatives should be explored, such as using nonflammable materials or operating below the flammable 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. 

B). They also increase the flammable range, particularly the UFL. An additional effect of elevated pressure is to suppress static discharges (2-6.4.1). 

FLAMMABLE RANGE The Concentrations of flammable gas or vapour between the LEE and UEL at a given temperature. 

Prevention of a mixture forming within the flammable range. 

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. 

Flammability Range - Defined as the difference between the UEL and LEL. This difference is an indication of how wide the flammability limits of a chemical are. The wider this range, the more hazardous the chemical may be considered from a fire standpoint. 

Ignition Temperature - This is defined as the minimum temperature at which a chemical substance will ignite without a spark or flame being present. Along with the values of flash point and flammability range, it provides and indication of the relative fire potential for the chemical. 

In some cases, however, only the part of the cloud which is within the flammable range is considered to burn. This may be a factor of 10 less than the total cloud. For further discussion of explosion efficiency see CCPS (1989) or Feeds (1986),... 

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. 

LFL decreases, which broadens the range over which the mixtnre is flam-mahle. In general, pressnre has little effect on the LFL, except at low pres-snres (e.g., ahont 100 inmHg for methane) where comhnstion is not possible. As the pressnre increases, the UFL generally increases, broadening the flammability range. Eqnations are available for calcnlating the effect of temperatnre on the LFL and UFL, and pressnre on the UFL (Coward and Jones 1952, Zabetakis 1965). 

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. 

Since this mixture contains 3.3% hy volumne total comhustihles, it is above the LFL ,, and helow the UFL ,, thus is within the flammable range. 

Flammable Range The range of concentrations between the lower and upper flammability limits. 

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