Properties of Flammable Substances
Flammable substances are those gases, liquids and solids that will ignite and continue to burn in air if exposed to a source of ignition.
Many flammable and combustible liquids and solids are volatile in nature; that is, they evaporate quickly and are continually giving off vapours. The rate of evaporation varies greatly from one liquid to another and increases with temperature. It is their vapours combined with air, not the liquid or solids themselves, that ignite and burn. In many instances, an increase in temperature creates a more hazardous condition because of the increase in the rate at which vapours are evolved.
This is the lowest temperature of the liquid at which it gives off enough vapour to form an ignitible mixture of vapour and air immediately above the liquid surface.
A liquid is classified as flammable or combustible depending on its flash point. A flammable liquid has a flash point below 37.8 C while a combustible liquid has a flash point greater than 37.8 C.
Example: Flash point of Acetone is - 17.8 C (*closed cup) and that of Aniline is 70.0 C (*closed cup).
The lower the flash point, the greater the potential fire hazard.
Flammable (Explosive) Range
This is the range between the lowest explosive limit (LEL) and the upper explosive limit (UEL).
The LEL is the lowest concentration of vapour in air which will burn or explode upon contact with a source of ignition. Below the LEL, the mixture is too lean (i.e. there is insufficient fuel).
The UEL is the highest concentration of vapour in air which will burn or explode upon contact with a source of ignition. Above the UEL, the mixture is too rich to burn (i.e. there is insufficient oxygen).
The LEL and UEL are usually indicated by the percentage by volume of vapour in air.
Example - For diethyl ether, the LEL is 1.9% and the UEL is 36% by volume of air.
This range becomes wider with increasing temperature and in oxygen-rich atmospheres.
For most solvents the LEL lies in the range 1-5% in air and therefore good ventilation is essential in order to minimize the risk of forming a flammable or explosive atmosphere when such substances are used. However, it is significant that the LEL for most substances is considerably greater than the recommended hygiene standards for the concentration of vapour in the workroom air.
The autoignition temperature of a substance is the minimum temperature required to initiate or cause self-sustained combustion, in the absence of a spark or flame.
- A steam line or a glowing light bulb may ignite carbon disulphide (autoignition temperature is 100 C).
- Diethyl ether (autoignition temperature is 160 C) can be ignited by the surface of a hot plate.
The vapour density is the ratio of the density of the gas or vapour to the density of air (vapour density of air = 1). Generally, vapours from flammable liquids are denser than air and thus tend to sink to ground level where they can spread over a large area.
Example:- Vapour density of ethyl alcohol is 1.59.
Sources of Ignition
A source of ignition represents a sufficiently high enough temperature to ignite a fuel. Common sources of ignition include:- open flames, hot surfaces, static electricity, smoking material, cutting and welding operations, radiant heat, frictional heat, electrical and mechanical (frictional) sparks, spontaneous combustion, and heat-producing chemical reactions.
Examples of generation of static electricity:-
- A rapidly moving belt develops static electricity by coming in contact with the pulley and then moving away from it.
- Paper and cloth take on static charges when passing through machines.
- Paint spraying generates static.
- Static charges are produced by the flow of flammable liquids into or from tanks or other containers through pipes, hose, or even air, or by pouring from one container to another.
Static electricity accumulations sufficient to cause a spark can, however, be prevented by grounding, bonding, or humidification. The danger of fire and explosion presented by flammable liquids, generally, can be eliminated or minimized by strict observance of safe storing, dispensing, and handling procedures.