| CHEMICAL LABORATORY SAFETY THINGS THAT BURN: FLAMMABLE AND COMBUSTIBLE CHEMICALS |
 |
| CHEMICAL LABORATORY SAFETY THINGS THAT BURN: FLAMMABLE AND COMBUSTIBLE CHEMICALS |
|
THINGS THAT BURN : FLAMMABLE AND COMBUSTIBLE CHEMICALS: American Chemical Society chemical safety video courses
Flammable vs. Combustible Chemicals
Flammable and combustible chemicals are represented in almost all classes of compounds. The National Fire Protection Association (NFPA) categorizes chemicals by a standard numbering system that indicates the conditions under which a particular chemical will burn. Current NFPA ratings are:
| 4 | Extremely flammable |
| 3 | Ignites at ambient temperature |
| 2 | Ignites when heated moderately |
| 1 | Must be preheated to burn |
| 0 | Will not ignite below 1500oF (8155oC) |
The NFPA and the U.S. Department of Transportation (DOT) distinguish between flammable and combustible chemicals on a property known as the flash point (Tf).
Specific terminology has been established to characterize the ratio of fuel vapor and air: the lower flammability limit (LFL) and the upper flammability limit (UFL). The LFL and the UFL are defined as the lowest and highest fuel-air ratios, respectively (expressed as a percent of vapor), at 25oC and 1 atm that will allow ignition of the vapor.
The flash point is defined as the lowest temperature (under controlled laboratory conditions) of a liquid at which vaporization is sufficient to create an ignitable fuel-air mixture at the surface of the liquid when a momentary ignition source is introduced into the vapor-air mixture.
Fire Triangle
Ignition of a flammable or combustible fuel requires three components: a fuel, an oxidizer, and an ignition source. When the three components (sides of the triangle) are brought together and the fuel-oxidizer ratio is appropriate, the fuel can ignite. Conversely, you can prevent ignition of the fuel by eliminating one of the components.
Flammable and combustible fuels are divided into four primary categories: carbonaceous solid fuels, liquid fuels, gases, and metals.
Occasionally, the reaction of fuel and oxidizer will yield a product that is also flammable.
A hazardous material may also form unexpectedly.
Oxidizers (oxidizing agents) are elements or compounds containing at least one highly electronegative atom that can easily accept electrons from a fuel in an exothermic oxidation-reduction reaction. An oxidizer can be organic or inorganic and can exist as a solid, liquid, or gas.
Occasionally, a flammable or combustible fuel may also contain an oxidizing agent as a functional group. In other words, one material may supply two sides of the fire triangle (fuel and oxidizer) and thus can ignite in the presence of any ignition source.
As in any oxidation-reduction reaction, the reaction between a fuel and an oxidizer involves the breaking and forming of chemical bonds, a process that requires energy. The reaction is exothermic because the energy produced by the bond formation in the products exceeds the energy required to break the bonds in the reactants. Ignition sources provide the activation energy that must be added to an atomic or molecular system for a reaction to occur. Energy from an ignition source is commonly in the form of heat, light, or electrical charge. Some sources of ignition are obvious and some are quite insidious.
Obvious sources of ignition include open flames, matches, spark igniters, and sparks from tools. Less obvious ignition sources include friction heat, hot motor surfaces, hot bearings, lasers, hot plates, wall switches, rotary telephones, energized incandescent light bulbs, and static electrical charge.
Another ignition source is static electrical charge, which is generated by friction between two nonconductors or a nonconductor and a conductor.
Catalysts increase the rate of a reaction by lowering its activation energy and thereby increasing the probability that an ignition source will supply sufficient activation energy to initiate a reaction. Sometimes a catalyst can lower the activation energy so much that the flammable or combustible chemical will ignite at ambient temperature.
Self-Ignition
Another source of ignition energy for a flammable fuel-oxidizer mixture is the heat generated by the exothermic reaction between the fuel and the oxidizer. If the energy produced by the heat of reaction exceeds the activation energy of the reactants, the fuel-oxidizer mixture may ignite. This concept is referred to as "self-ignition" and includes the phenomena of spontaneous combustion, hypergolic mixture ignition, and pyrophoric ignition.
Heat energy generated by a relatively slow oxidation reaction in a poorly ventilated area can spontaneously ignite a flammable or combustible chemical. The classic example is a pile of oily rags igniting in a closed garage.
Hypergolic mixtures are pairs of flammable and oxidizing chemicals whose reaction can generate enough energy to ignite the mixture.
The final form of self-ignition involves compounds that react so rapidly with air that they ignite at ambient temperature.
Fire and Spill Prevention - General Procedures
Many of us handle flammable and combustible chemicals not only in the laboratory, but also when we order, receive, store, transport, and dispose of the chemicals. As expected, the more people who handle a particular flammable or combustible chemical, the greater the potential for an accident. You can minimize the probability of a fire or hazardous spill by careful research, planning, and communication.
- KNOW THE CHEMICAL.
- HANDLE MINIMAL QUANTITIES.
- AVOID THE FIRE TRIANGLE.
- KNOW AND APPLY ADMINISTRATIVE CONTROLS.
Fire and Spill Prevention -- Common Operations
Ordering is the first (indirect) step in the handling of flammable or combustible chemicals. Before ordering a chemical, determine the minimum quantity necessary for your work and the available storage space at the facility. Then, order only the quantity of chemical that can be used in a single day or safely stored.
When you receive a flammable or combustible chemical in your laboratory's receiving area, it may have suffered abuse and mishandling that make it more susceptible to fire and explosion. Therefore, always keep the receiving area free of external ignition sources. Make sure that you or anyone receiving a flammable or combustible chemical is trained in chemical segregation and storage requirements, spill and fire emergency procedures, and documentation methods.
Several hazards may arise when a flammable or combustible chemical is transported or transferred. Breakage is the primary danger, with potential consequences ranging from an unmanageable spill to a fire to injury from broken glass. For this reason, never carry more of a flammable or combustible chemical (or any chemical) than you can safely handle. Use stable carts, rubber buckets, or the original DOT outer package to assist you in transporting any flammable or combustible chemical, and never attempt to carry a container with wet hands or wet gloves.
Another hazard is the buildup of static electrical charge as the chemical moves within the container or between containers. To avoid the danger of sparks during the transfer of the liquid from one metal container to another, bond the two containers with a bonding wire to equalize the two potentials, and also ground them to an earth ground.
The main purpose of all storage containers for flammable or combustible chemicals is safe containment and prevention or confinement of fires. Safety cans, laboratory-safe refrigerators, flammables storage cabinets, and reinforced storage rooms are all appropriate storage facilities for various types and quantities of chemicals. Make sure you know the hazards and storage requirements of the chemicals you are working with and communicate them to your co-workers.
Safe use of flammables and combustibles begins BEFORE you enter the laboratory. Consult all available resources to learn as much as possible about the required reactants, expected products and by-products, reaction mechanisms, and potential hazards of the chemicals you plan to use.
Know and understand the required waste disposal procedures before beginning your experiment. Always remember that the waste in your reaction vessel can still react and present numerous hazards.
Emergency Response -- General Procedures
A major element in the safe handling of flammables and combustibles is knowing what to do in case of an emergency. Never enter a flammable or combustible chemical work area unless you have the knowledge and training to manage an emergency. Because time is critical in any emergency response, plan your emergency actions before beginning your work with flammables and combustibles, and practice these actions (e.g., extinguisher use, spill cleanup, evacuation) before you actually encounter an emergency.
When you enter your work area, immediately check the location and condition of all exits, fire alarms, extinguishers, spill cleanup materials, telephones, safety showers, and eye wash stations. Never block or impede access to any of these emergency response devices. The following information should be posted near the telephones:
- Emergency phone numbers
- Detailed dialing instructions (e.g., dialing "9" for an outside line)
- Specific instructions if calls must be received by an internal operator
- Name and exact street address of the workplace
- Specific instructions to assist a response agency in locating a particular room
- Phone numbers of internal emergency response team personnel
- Locations of other phones that have outside access. Verify that all telephone information is correct, legible, and clear, and that all safety devices have been recently inspected.
Spill Emergency Response
Many flammable and combustible spills present both a toxic and flammable hazard. Even a small spill can develop into an unmanageable fire or release toxic vapors that quickly exceed the permissible exposure limit (PEL), short term exposure limit (STEL), and even levels immediately dangerous to life and health (IDLH). Spill response can generally be divided into two categories:
- spills too large to safely absorb with towels but too small to require evacuation, and
- large spills requiring evacuation.
Before attempting any cleanup, know the hazards of the chemicals involved and the correct disposal procedures.
Attempt cleanup of only very small spills of nontoxic materials.
If you believe the spill is too large to safely treat with an adsorbent, or that cleanup might entail toxic fume inhalation, evacuate the area and follow your facility's emergency response plan. Never attempt to manage an unmanageable spill.
Fire Emergency Response
Fires are divided into four classes depending upon the fuel involved, and extinguishers are rated accordingly. The extinguisher material specified for a particular fire class will remove one or more of the components of the fire tetrahedron and will not react with the fuel to further perpetuate the fire.
- Class A fires, which involve the burning of carbonaceous solid fuels such as paper or wood, can be extinguished by elimination of the oxidizer. Fire extinguishers for Class A fires use water or monoammonium phosphate, which forms a crusting sealant on the surface of the fuel.
- Class B fires, the fuels are water-soluble and water-insoluble liquids. Recommended extinguisher materials are carbon dioxide, monoammonium phosphate, and powders, such as potassium carbonate and sodium carbonate, that inhibit the free radical chain reaction.
- Class C fires involves a Class A or Class B fire complicated by the presence of energized electrical equipment and requires an extinguisher specifically designed for Class C fires. Monoammonium phosphate or, preferably, carbon dioxide can be used to extinguish a Class C fire, but never use water or other conducting materials.
- Class D fires are metal fires and are quite different from other fire classes because of the high fire temperatures (over 1000oC) and the high specific reactives of the fuels. Class D extinguishers spray a salt/polymer mixture on the burning metal to seal the fuel from the oxidizer.