In the presence of ignition sources, combustible and flammable liquids are potential sources of risk. First, the closed or open vapour space above such liquids provides a fire and explosion hazard. Combustion, and more frequently explosion, might occur if the material is present in the vapour-air mixture in suitable concentration. From this it follows that burning and explosion in the zone of combustible and flammable liquids may be prevented if:
· the ignition sources, air, and oxygen are excluded; or
· instead of oxygen, inert gas is present in the surrounding; or
· the liquid is stored in a closed vessel or system (see figure 41.3); or
· by proper ventilation, the development of the dangerous vapour concentration is prevented.
Figure 41.3 Common types of tanks for storage of flammable and combustible liquids
In practice, a large number of material characteristics are known in connection with the dangerous nature of combustible and flammable liquids. These are closed-cup and open-cup flash points, boiling point, ignition temperature, rate of evaporation, upper and lower limits of the concentration for combustibility (flammable or explosive limits), the relative density of vapours compared to air and energy required for the ignition of vapours. These factors provide full information about the sensitivity for ignition of various liquids.
Nearly all over the world the flash point, a parameter determined by standard test under atmospherical conditions, is used as the basis to group the liquids (and materials behaving as liquids at relatively low temperatures) into categories of risk. The safety requirements for storage of liquids, their handling, the technological processes, and the electrical equipment to be set up in their zone should be elaborated for each category of flammability and combustibility. The zones of risk around the technological equipment should also be identified for each category. Experience has shown that fire and explosion might occur—depending on the temperature and pressure of the system—within the range of concentration between the two flammable limits.
