Flammable and combustible liquids are found stored in almost every facility at every industrial plant. A flammable liquid is defined as one having a flash point below 100 deg F (37.8 deg C) with a vapor pressure not exceeding 40 psi (276 kPa). They are volatile, constantly giving off heavier-than-air vapors that cannot be seen with the naked eye. Combustible liquids have a flash point at or above 100 deg F (37.8 deg C).
One main difference between flammable and combustible liquids has to do with the behavior of their vapors. Vapors from flammable liquids tend to flow along slopes and collect in low areas. Vapors from combustible liquids, on the other hand, are not prone to travel far from the source of leak unless temperature remains above flash point.
When determining gas sensors location, these distinctions must be taken into account, because liquids themselves do not burn or explode, but rather the flammable vapors resulting from evaporation.
Flash Point: The lowest temperature at which a liquid gives off sufficient vapor at its surface to form a flammable or explosive mixture. Some hazardous liquids have flash points at or below room temperature and are covered by a layer of flammable vapors that will ignite as soon as exposed to an ignition source.
Liquids hazardous potential also depends on factors as ignition temperature, explosive range, vapor density and rate of evaporation, but flash points remains as the most important factor. Table below is an example of properties for some common found flammable liquids.
| Name | Formula | flash point (F) | LEL % | UEL % | Ignition temperature (F) | Specific Gravity | Vapor Density |
|---|---|---|---|---|---|---|---|
| Ethyl Alcohol | CH3CH2OH | 55 | 3.3 | 19.0 | 685 | 0.79 | 1.59 |
| Acetic Acid (glacial) | CH3COOH | 103 | 4.0 | 19.9 | 867 | 1.05 | 2.07 |
| Benzene | C6H6 | 12 | 1.2 | 7.8 | 928 | 0.88 | 2.77 |
References:
- NFPA 30, Flammable and Combustible Liquids Code
- General Monitors Staff, Fundamentals of Combustible Gas Detection