Workplace and Building Safety
In Module 1 we discussed the difference between "health" and "safety"
issues. While "toxic wastes" usually refer to wastes that are health
hazards, the term "hazardous wastes" includes both health hazards
and safety hazards. Many are both. This module focuses on safety hazards. We
will break this discussion into three areas: Fire, Process Safety, and Ventilation.
In the U.S. the chief standards setting body for fire safety is National Fire Protection Association (NFPA). Here is an ACS (American Chemical Society) web site with an excellent page on the Hazard Identification System ("the fire diamond"). Look it over.
Liquids are often divided into Flammable Liquids and Combustible Liquids. There are finer divisions as well. Most are based on the concept of flash point. The flash point is a Temperature. It is lowest temperature at which the liquid gives off enough vapor to form an ignitable mixture with the air above the liquid. This temperature is determined by closed-cup and a variety of test methods. Briefly, they feature a pan with the liquid in question, and a spark making device above the pan. The liquid is stirred and heated. The temperature at which the vapor goes "poof" is the flash point.
With that knowledge, go back to the ACS site, take a quick look at the red section and note the temperatures.
You should know about the Flammable Rangewhich is also known as the Explosive Range. If there is a leak in a natural gas line and the flammable gas fills a room that has an open flame in it, at first nothing happens. There is too little gas for combustion. (In the analogous situation in a gasoline engine, we say the mixture is too lean.) As the natural gas fills the room, it reaches a concentration known as the Lower Explosive Limit or LEL. That's when the boom happens. If there was no open flame in the room however, and the gas concentration continued to rise, it would reach a point where if you were to introduce a spark or flame, nothing would happen either. There is too little oxygen to sustain combustion, and that level is called the UEL, Upper Explosive Limit. (In the gasoline engine we say the mixture is too rich.) For some explosive gases, workers would be asphyxiated at concentrations above the UEL. But for other gases, there is enough oxygen to survive. This is what can happen, a room or closed space becomes filled with a flammable gas above the UEL, no explosion takes place. Then the workers, perhaps noticing the gas odor, ventilate the room, the gas concentration drops below the UEL, a spark, perhaps from a tool, happens and.....
LEL-UEL are typically expressed as vapor % of atmosphere. For many liquids
(or gases), the LEL is 6 to 12 %. A few common ones are in 1% to 2% range. Let's
digress a moment and contrast LEL concentrations with TLVs. Most TLVs are 100s
or 1000s PPM, while 1% is 10,000 PPM. So if the vapor concentration is less
than the TLV, the concentration is usually much less than LEL.
Here are some terms that your book explains:
Explosion: might be from over pressure, a chemical reaction or both
Deflagration: Progress less than the speed of sound
Detonation: Shock wave progress faster than sound
Here are two key points about Fire extinguishers
They must be specific to material being extinguished - one size does not fit all.Here's a nice power point with some clear explanations.
They must be inspected and tested -
Now go back to ACS page and look at the Fire Diamond in some detail, including notes on toxic, reactive, flammable, oxidizers, and water reactive. You do not need to memorize the chemicals on those sites, but should know the general idea. If you were writing a fire triangle, how might you get this information .
In the U.S. the safety of buildings is regulated by local governments. The local government typically adopts a building code. The building code may contain the fire safety and life safety codes, or these may be adopted separately. Typically the building code is administered by the "Building Official" who inspects construction. Sometimes the Fire Code is administered by a "Fire Inspector." Many jurisdictions have a "Fire Marshall" who has law enforcement powers respecting fire safety. In Alaska, outside of the cities, there is no general building code. The state government however, has adopted the codes for all public buildings, industrial facilities, and buildings of mass occupancy. The State Fire Marshall is responsible for enforcing the fire codes for those types of buildings.
The chief problem with the system is that once a facility is constructed, there is little inspection inside the cities and none outside the cities. For example, a gasoline service station must be built according to the code for gasoline dispensers and will have the proper burial, tank type, hoses and such. However an industrial facility, say a warehouse, might store steel beams or TNT, and the Fire Marshall will not know about it unless the building owners notify the marshal. There is no inspection service to check. If there is a fire, the fire fighters called to the scene will not know what is inside the warehouse. There are laws requiring notification of storage of hazardous material, we will discuss some of these, but without inspection, there is little certainty
The most important aspect of the safe storage of materials is know what you are storing. If you have that pinned down, there is ample advice, direction, and regulations to assure safe storage.
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