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. 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. Example: For diethyl ether, the LEL is 1.
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. Under WHMIS , a product that is considered to be flammable in the truck is also considered to be flammable when used in the workplace- this was not always the assumption under WHMIS 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.
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. 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.
A rapidly moving belt develops static electricity by coming in contact with the pulley and then moving away from it. Gasoline also has carbon and hydrogen available — and it's volatile, so gasoline can evaporate easily, making it highly combustible. Related: How do wildfires start? In contrast, a fire resistant material is one that doesn't burn easily. One example of this is the artificial stone used in kitchen countertops, like the DuPont brand Corian.
The plastic of a Corian countertop is filled with finely ground rocks made of hydrated aluminum oxide, a chemical compound that doesn't burn.
These rocks lower the fuel value the amount of carbon available for combustion of the countertop, making it more fire resistant, Carroll said. Although the rock attracts and holds water molecules, it does not get wet enough to form a puddle. Water keeps the countertop cool and helps block heat from getting any fuel. If there were a heat source for instance, a lit cigarette resting on a Corian countertop , it would need to boil away the water surrounding the aluminum oxide first in order to then heat up the fuel, or the plastic molecules, enough to burn.
Moreover, countertops like Corian don't have much plastic — there's just enough to hold the rock together, Carroll said. Fuel and heat are two sides of the fire tetrahedron, a triangular pyramid in which each side represents the elements necessary for fire. The other two sides are oxygen and a sustainable chemical reaction, Carroll explained.
Most materials — aside from granite and asbestos, which are among the rare materials that are actually, literally fireproof — can be made more or less combustible only by eliminating one or more sides of the fire tetrahedron, he said.
Unlike fire resistance — the properties that make it hard for a fire to either start in the first place or keep going — chemicals known as flame retardants can help to slow or extinguish an already-burning fire.
They can be hard to detect unless special instruments are used. Most flammable and combustible liquids flow easily. A small spill can cover a large area of workbench or floor. Burning liquids can flow under doors, down stairs and even into neighbouring buildings, spreading fire widely.
Materials like wood, cardboard and cloth can easily absorb flammable and combustible liquids. Even after a spill has been cleaned up, a dangerous amount of liquid could still remain in surrounding materials or clothing, giving off hazardous vapours. Vapours can flow from open liquid containers.
The vapours from nearly all flammable and combustible liquids are heavier than air. If ventilation is inadequate, these vapours can settle and collect in low areas like sumps, sewers, pits, trenches and basements.
The vapour trail can spread far from the liquid. If this vapour trail contacts an ignition source, the fire produced can flash back or travel back to the liquid. Flashback and fire can happen even if the liquid giving off the vapour and the ignition source are hundreds of feet or several floors apart. The most obvious harm would be the danger of a fire or explosion.
After the immediate danger of a fire, there are sometimes other properties of these liquids that may be hazardous to the body. Some flammable and combustible liquids are corrosive. Many undergo dangerous chemical reactions if they contact incompatible chemicals such as oxidizing materials, or if they are stored improperly.
The Material Safety Data Sheet and the supplier's labels on the containers should tell you about all the hazards for the flammable and combustible liquids that you work with.
An example is 2-propanol also known as: dimethylcarbinol, isopropanol, or isopropyl alcohol. It is a colourless liquid with a sharp odour like rubbing alcohol or resembling that of a mixture of ethanol and acetone. It is flammable liquid and vapour. Vapour is heavier than air and may spread long distances. Distant ignition and flashback are possible.
It is also considered to be a mild central nervous system depressant.
0コメント