Chapter 4: Fires And Explosions PDF

FIRES AND EXPLOSIONS  We have been talking about source models for the release of materials and about dispersion models if the material is a toxicant. (Chapter 3)  Another concern is a release of flammable materials where we need to worry about fires and explosions.  In order for a fire t...

FIRES AND EXPLOSIONS  We have been talking about source models for the release of materials and about dispersion models if the material is a toxicant. (Chapter 3)  Another concern is a release of flammable materials where we need to worry about fires and explosions.  In order for a fire to start or be sustained you need to have a Fuel, an oxidizer and an ignition source.  If one of the three components is eliminated, then there will not be a fire (or explosion)  Fuel must be present in certain concentrations.  Typical cases where fuel occur are if there is a leak, during filling operations, transfer operations, or excessive dusts.  Although we often cannot always eliminate these sources we can help by having good ventilation to keep vapors from building up.  Often we locate things out-doors, use grating on floors so vapors don’t build up. Grating floor The concentration of fuel needed to start a fire or explosion varies depending on the type of fuel. Each flammable substance has a specific range of concentrations in air, known as the flammable range, within which it can ignite or explode if an ignition source is present. This range is defined by two critical limits: 1.Lower Explosive Limit (LEL): The minimum concentration of the fuel in air below which the mixture is too lean to burn. 2.Upper Explosive Limit (UEL): The maximum concentration of the fuel in air above which the mixture is too rich to burn. Here are examples of the LEL and UEL for some common fuels: 1.Methane (Natural Gas): - LEL: 5% - UEL: 15% 2. Propane (LPG): - LEL: 2.1% - UEL: 9.5% 3.Gasoline Vapors: - LEL: 1.4% - UEL: 7.6% Factors Determining Burning vs. Explosion: 1. Concentration of Fuel:· Burning: Can occur over a range of concentrations but typically within the lower range of the flammable limits.· Explosion: Requires the fuel concentration to be within the explosive range, which is often a narrower band where rapid combustion can occur. 2. Mixing of Fuel and Oxidizer:· Burning: Often occurs in open air where fuel and oxidizer mix gradually. ·Explosion: Requires a well-mixed fuel and oxidizer combination, often in a confined space. 3. Ignition Source:· Burning: A continuous ignition source can sustain the reaction. ·Explosion: A single, often small, ignition source can trigger a rapid reaction. 4. Confinement:· Burning: Typically occurs in open or semi-open environments.· Explosion: Often occurs in confined spaceswhere pressure can build up rapidly.  Oxygen is the most common oxidizer, especially that found in ambient air.  For oxygen, we often use “Inerting (inert gas)” with nitrogen, helium blankets over flammable materials to reduce O2 content below that where you can have combustion.  Heat is a common ignition source.  “Ignition sources are free!!!” (It implies that these sources can be found easily and without cost, which underscores the potential danger they pose if not managed properly.)  Although we can eliminate ignition sources, it is almost inevitable (unavoidable ) that an ignition source will be available if there is a large release of flammable material that cannot be diluted quickly.  The fire tetrahedron or fire pyramid adds a fourth component— chemical chain reaction—as a necessity in the prevention and control of fires.  The free radicals formed during combustion are important intermediates in the initiation and propagation of the combustion reaction.  Fire suppression materials scavenge these free radicals  IMPORTANT DEFINITIONS  Combustion – a chemical reaction in which a substance combines with an oxidizer and releases energy.  Explosion – rapid expansion of gases resulting in a rapid moving pressure or shock wave.  Mechanical Explosion – due to failure of vessel with high pressure non reactive gas.  Confined explosion – an explosion occurring Confined explosion within a vessel or a building. Usually results in injury to the building inhabitants and extensive damage.  Unconfined explosion – an explosion occurring in the open. Usually results from spill of a flammable gas spill. These explosions are rarer than confined since dilution occurs.  Dust Explosions - This explosion results from the rapid combustion of fine solid particles. Many solid materials become very flammable when reduced to a fine powder. Dust Explosions  IMPORTANT DEFINITIONS  BLEVE – Boiling Liquid Expanding Vapor Explosion – when liquid is at a temperature above its atmospheric boiling point. Vessel ruptures – flammable liquid flashes and results in a fire/explosion  Detonation – explosion (chemical reaction) with shock wave greater than speed of sound  Deflagration – explosion (chemical reaction) with shock wave less than speed of sound Flash Point (FP)  a property of material used to determine the fire and explosive hazard. The lowest temperature of a liquid at which it gives off enough vapor to form an ignitable mixture with air.  Needs to be determined experimentally.  Different methods to determine, open cup and closed cup. Open cup is usually a few degrees higher.  Flammable IA – Flash point < 73°F, boiling point < 100 °F  Flammable IB – Flash point < 73°F, boiling point > 100 °F  Flammable IC – 73°F < Flash point < 100 °F  Combustible II – 100 °F < Flash point < 140 °F  Combustible IIIA – 140 °F < Flash point < 200 °F  Combustible IIIB – Flash point > 200 °F  Minimum energy input needed to initiate combustion  Most hydrocarbons have low MIE~0.25 mJ  Whereas the “spark” from walking across the room is 22mJ (almost 100X too much)  Again, we always assume that an ignition source will exist  Tables give MIEs for some substances  Ignition sources are free!!!  Table 6-3 gives the results of a study by Factory Mutual Engineering Corporation who studied over 25,000 industrial fires to determine the source of ignition.  Fires and explosions kill more than 200 and injure more than 5,000 workers each year  OSHA requires employers to provide proper exits, fire fighting equipment, and employee training to prevent fire deaths and injuries in the workplace  There are approximately 6,000 office fires in the United States every year  Major causes of office fires: Arson Smoking Wiring and Appliances  Arson (‫)الحريق المتعمد‬  Pay close attention to security measures  Lock doors and windows after business hours  Keep areas around the building well-lit and clear of combustibles  Pay attention to housekeeping within the building as well  Smoking Materials  Use large, non-tip ashtrays in areas where smoking is allowed  Make sure ashtray contents are cold before emptying  Make sure no one leaves smoldering cigarettes in wastebaskets  Prohibit smoking in hazardous areas  Wiring & Appliances  Designate an employee to turn off and unplug all appliances at the end of the day (including coffee makers)  Do not overload outlets  Replace broken or cracked electrical cords  Missing or broken fire safety equipment  Burned out exit lights  Accumulated trash Propped open fire doors  Propped open fire doors  Blocked stairways Blocked stairways FIRE CLASSES (DEPENDS ON TYPE OF FIRE CAUSE) [USA STANDARD BY NFPA (A, B, C, D AND K)] Class A - Wood, paper,cloth, trash, etc… (A - Ash) Class B - Flammable liquids, oil, gas, grease, etc... (B - Barrel) Class C - Energized electrical equipment (C - Circuit) FIRE CLASSES (DEPENDS ON TYPE OF FIRE CAUSE) [USA STANDARD BY NFPA (A, B, C, D AND K)] Class D - Used for combustible metal fires - Heat from fire causes powder to “cake” and form a barrier - A Class D fire is characterized by the presence of burning metals. - Only certain metals are flammable and examples of combustible metals include sodium, potassium, uranium, lithium, plutonium and calcium - The most common Class D fires involve magnesium and titanium. FIRE CLASSES (DEPENDS ON TYPE OF FIRE CAUSE) [USA STANDARD BY NFPA (A, B, C, D AND K)] Class (K or F) (Kitchen) - It is a new type of fire that was recently added to the types of fires and is specialized in fires that occur with vegetable oils in kitchens. - The extinguisher liquid quickly cools down the grease, knocks down the flames and forms a vapor securing blanket. Class K (Kitchen) FIRE CLASSES (DEPENDS ON TYPE OF FIRE CAUSE) [EUROPEAN STANDARD BY CEN (A, B, C, D AND K)] [European classification] [USA classification] (DEPENDS ON TYPE OF EXTINGUISHERS FILLING MATERIAL) The 5 most common: All Purpose Water - Carbon Dioxide - Wet Chemical- Dry Powder- Foam Fire Extinguishers comparison based on applications [European classification] In OSHA Pull the pin Aim at the base of the fire Squeeze the handle Sweep side to side NEVER fight a fire if: You do not have the proper extinguisher or equipment The fire has spread beyond its point of origin Your instincts tell you to “GET OUT!!”  Inspect:  Monthly (Employee)  Annually (Competent Professional)  Mount in designated/conspicuous locations  Mount near exits Not Mounted Obstructed Access Not Labeled Not Mounted  Types of fire extinguishers How to use an extinguisher Location of fire extinguishers Extinguisher limitations HANDS ON TRAINING IS REQUIRED! Hazard Identification Exercise