A mixture of hydrocarbon gas and air cannot be ignited and burn unless its composition lies within a range of gas in air concentrations known as the Flammable Range. The lower limit of this range, known as the Lower Flammable Limit (LFL), is that hydrocarbon concentration below which there is insufficient hydrocarbon gas to support and propagate combustion. The upper limit of the range, known as the Upper Flammable Limit (UFL), is that hydrocarbon concentration above which there is insufficient air to support and propagate combustion.
The flammable limits vary somewhat for different pure hydrocarbon gases and for the gas mixtures derived from different petroleum liquids. Very roughly, the gas mixtures from crude oils, motor or aviation gasolines and natural gasoline-type products can be represented respectively by the pure hydrocarbon gases propane, butane and pentane. Table 2.1 gives the flammable limits for these three gases. It also shows the amount of dilution with air needed to bring a mixture of 50% by volume of each of these gases in air down to its LFL. This type of information is very relevant to the ease with which vapors disperse to a non-flammable concentration in the atmosphere.
In practice the lower and upper flammable limits of oil cargoes carried in tankers can, for general purposes, be taken as 1% and 10% by volume respectively.
|Gas||Flammable limits % vol. Hydrocarbon in air||Number of dilutions by air to reduce 50% by volume mixture to LFL|
Table 2.1 Flammable Limits of Propane, Butane and Pentane
There are many schemes for dividing the complete range of petroleum liquids into different flammability classes based on flashpoint and vapor pressure and there is a considerable variation in these schemes between countries. Usually, the basic principle is to consider whether or not a flammable equilibrium gas/air mixture can be formed in the space above the liquid when the liquid is at ambient temperature.
Generally, in this guide it has been sufficient to group petroleum liquids into two categories entitled non-volatile and volatile, defined in terms of flashpoint as follows:
Flashpoint of 60ºC or above, as determined by the closed cup method of testing. These liquids produce, when at any normal ambient temperature, equilibrium gas concentrations below the lower flammable limit. They include distillate fuel oils, heavy gas oils and diesel oils. Their RVPs are below 0.007 bar and are not usually measured.
Flashpoint below 60ºC, as determined by the closed cup method of testing. Some petroleum liquids in this category are capable of producing an equilibrium gas/air mixture within the flammable range when in some part of the normal ambient temperature range, while most of the rest give equilibrium gas/air mixtures above the upper flammable limit at all normal ambient temperatures. Examples of the former are jet fuels and kerosene and of the latter, gasolines and most crude oils. In practice, gasolines and crude oils are frequently handled before equilibrium conditions have been attained and gas/air mixtures in the flammable range may then be present.
The choice of 60ºC as the flashpoint criterion for the division between non-volatile and volatile liquids is to some extent arbitrary. Since less stringent precautions are appropriate for non-volatile liquids, it is essential that under no circumstances is a liquid capable of giving a flammable gas/air mixture ever inadvertently included in the non-volatile category.
The dividing line must therefore be chosen to make allowance for such factors as the misjudging of the temperature, inaccuracy in the flashpoint measurement and the possibility of minor contamination by more volatile materials. The closed cup flashpoint figure of 60ºC makes ample allowances for these factors and is also compatible with the definitions adopted internationally by the International Maritime Organization (IMO) and by a number of regulatory bodies throughout the world. (See Section 1.6 for information on the relationship between the flashpoint and flammability of residual fuel oils.)