| Imagine how nasty it would be if some of the dirty water draining into your sink was allowed to leak back into the fresh water coming from your faucet. Yuck! Now imagine looking up at a factory’s exhaust stack and noticing that it’s located just inches from the intake pipe, the one that’s supposed to suck in fresh air for the workers inside. Even a novice can see that this is an unhealthy situation. Some of the airborne contaminants exiting the building are sure to be sucked right back in.
Last week we discussed the importance of location with regard to intake and output pipes, an integral part of a local exhaust ventilation system. The placement of these stacks is governed by various industry standards that present guidelines to insure that ventilation systems work properly and protect the health of people in the workplace. These guidelines have been determined by scientific study to equate to a safe minimal standard, as determined by a body of experts that have come together to form a consensus committee on the subject. Generally speaking, the standards set recommend that exhaust stacks extend upward a minimum of 10 feet above the highest point of the roof. As for discharge velocity, the rate at which contaminated air blows out of the stack, the standards recommend that operation take place at a minimum of 3,000 feet per second. Separation distances between exhaust stacks and air intakes vary according to dilution requirements set out in the standards, but basically the separation must be great enough so that airborne contaminants leaving the exhaust stack get safely diluted by outside air so they will pose no hazard should they ever reach the air intake ducts. This combination of height, velocity, and distance factors allows contaminated air to be dispersed far enough from the building to avoid downdrafts created by wind passing over the roof, thereby preventing undesirable consequences like the smoke that re-entered my house through its fireplace on windy days. One device that is sometimes incorporated into the scenario to keep workplace air clean is the inclusion of rain caps on the roof. These look like conical shaped hats, and they’re supposed to keep rain from falling into the exhaust stack. It seems like a good idea, but they unfortunately do not work very well. To begin with, they don’t do a good job of keeping out rain, especially when it’s driven by strong winds. Another drawback is that they can actually direct contaminants exiting from exhaust stacks back down to the roof and into the building’s fresh air intake ducts. Yet another drawback of rain caps is that they often result in the local exhaust ventilation system fan working harder than it has to because the contaminated air slams into the rain cap, thereby slowing its rate of exit and causing it to lose velocity energy. This means a fan must be selected to work harder to compensate for the resistance. Well, that’s it for our series on local exhaust ventilation systems. Next time we’ll switch gears and discuss how those outlet covers in your home with the cute little red and black buttons work to protect you against death by electrocution. They’re usually positioned near water sources and are known as “Ground Fault Circuit Interrupters,” or GFCI. I’ll be discussing topics like this on an upcoming show to be featured on The Discovery Channel, where I’ll be acting as a subject matter expert. The series, titled “Curious and Unusual Deaths,” will cover a wide range of potential threats that are present in our everyday environments. _____________________________________________ |
Posts Tagged ‘NFPA’
Industrial Ventilation – Local Exhaust Ventilation Exhaust Stack Design
Sunday, June 5th, 2011Industrial Ventilation – Local Exhaust Ventilation Exhaust Stacks
Sunday, May 29th, 2011| I like to bring the outdoors inside by the inclusion of natural elements, lots of wood, stone, and gurgling water. I once lived in a house with a very impressive looking natural stone fireplace. On calm days it was a pleasure to throw on a few logs and start a nice crackling fire. But shortly after moving in I discovered that under certain conditions the smoke would back up in the chimney and actually flow back down into the house, creating a smelly, sooty mess. This usually resulted in me having to open all the doors and windows to vent the place out. The first time it happened I thoroughly investigated. Was anything blocking the chimney? If not, what was the problem? A little outdoor surveying brought the issue to light. The fireplace chimney was not built high enough above the roofline, so that when the wind blew, it created downdrafts along the roof that worked against the smoke, forcing it back down into the chimney.
The phenomenon at play with my stone fireplace is similar to one sometimes facing industrial ventilation applications. A fireplace chimney functions very much like an exhaust stack on a local exhaust ventilation system, its function being to efficiently discharge contaminants from the building, most typically in a vertical fashion. At a minimum, exhaust stacks must be designed to provide sufficient dilution of airborne contaminants when they are released into the atmosphere, while adhering to applicable environmental standards. Dispersion into the atmosphere scatters contaminating molecules into a huge playing field, the sky, thereby reducing concentrations to safe levels. Just as the vast ocean is capable of absorbing enormous amounts of pollutants from oil spills and the like, the atmosphere at large is equally capable. To keep contaminated air moving out of the exhaust stack while achieving the highest amount of atmospheric dispersion, the following factors must be taken into consideration during the ventilation system design process:
These factors are addressed for various types of airborne contaminants through standards published by the National Fire Protection Association (NFPA), the American National Standards Institute (ANSI) in conjunction with the American Industrial Hygiene Association (AIHA), and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). Next time, we’ll take a closer look at their recommendations and the standards they’ve set up to prevent undesirable incidents such as the one I encountered with my natural stone fireplace. _____________________________________________
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Coal Fired Boiler Explosions
Sunday, July 25th, 2010|
Try this for a tongue-twister: Coal fired electric utility power plant boiler… If you’ve been reading along with us for the last couple of weeks, you now have a pretty good idea of what these are and what they do. These boilers are contained within furnaces in coal fired power plants. The furnace’s job is to combine coal and air to create a combustion process. It is like a big, insulated enclosure that keeps the heat energy from the combustion process from escaping before it can be absorbed by the water and steam in the boiler tubes. The heat energy is then funneled to the steam turbine to spin an electrical generator, creating the energy which will eventually find its way into our homes and businesses. During the operation of the boiler, coal and air must be introduced into the furnace at carefully measured rates to maintain a proper fuel-to-air ratio which will enable the release of heat energy from the coal at a safe, controlled rate. Fuel-air ratio is the amount of coal entering the furnace divided by the amount of air entering the furnace. If this ratio isn’t precisely maintained, conditions may be right for an explosion to occur. Specifically, the ratio has to fall within an “explosive range.” Once within this range, all that is needed is an ignition source, such as hot ash, or even mere static electricity, and the result may be a furnace explosion. There are certain times at which furnace explosions are more likely to occur than others, such as when the boiler is being started, operated at less than full capacity, or shut down. When a furnace explodes, a pressure wave moves out from the center of the blast. This pressure wave will bear up against the sides of the furnace with great force, and if the pressure is high enough the sides of the furnace, which are made of heavy steel components, will actually bend and split open. Boiler tubes may even rupture, releasing high pressure steam and water into the power plant and furnace. At the very least, the boiler will be down for expensive repairs and no electricity can be produced by its turbine generator. This down time can last for many months and results in lost revenue to the energy producer. Aside from an explosive fuel-to-air ratio, there are other potential causes of furnace explosions. For example, poor coal quality can lead to incomplete combustion, or the flame going out completely, encouraging unburned coal particles to settle and accumulate in the furnace. The accumulation of coal can grow to the point where it forms an explosive mixture when combined with the right amount of air. So how can boiler explosions be prevented? The National Fire Protection Association (NFPA) looked into the problem and developed an industry standard. This standard is known as NFPA 85, Boiler and Combustion Systems Hazards Code. Its purpose is to contribute to operating safety and prevent uncontrolled fires, explosions, and implosions of coal fired boilers. NFPA 85 lays out guidelines to follow when designing, building, and operating boiler fuel handling systems, air handling systems, and combustion control systems. Following its guidelines will certainly significantly decrease the probability of explosions occurring. Another means of explosion prevention includes implementing a boiler operator training program. These enable attendees to better understand operating procedures and equip them with the knowledge to safely control the combustion process, particularly when a furnace explosion is most likely to occur. This training can be done with a combination of classroom instruction along with time on a simulator and may be followed up with hands-on training in the plant itself. Lastly, boiler explosions can be prevented by implementing an effective inspection and maintenance program to locate and repair or replace boiler components, averting the possibility of a potential disaster occurring. Things such as check lists can be used to ensure that nothing is missed. This is a strategy that all pilots must use before starting their planes, and it is now being used in hospitals as well to cut back on the rate of patient infection due to carelessness on the part of hospital staff. Hey, we’re all human, and humans are not perfect. But remember that an ounce of prevention is truly worth a pound of cure, and then some. A properly placed check on the list could mean lives will be saved. _____________________________________________
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Arc Flash Dangers
Sunday, September 13th, 2009|
Imagine being hit by a bolt of lightning. Like lightning, an arc flash can unexpectedly release tremendous amounts of energy, resulting in serious injuries and even death. An arc flash is the result of a short circuit or electrical fault in energized equipment. Current flows through the air and creates an electrical arc, very much like the phenomenon of lightning. But unlike lightning, arc flash dangers are present in a myriad of circumstances that do not require storm conditions to manifest. Over 80% of electrically related injuries involve some type of arc flash. They can be caused by a wide variety of factors, including: equipment malfunctions, inadequate safety procedures, carelessness, lack of training, dropped tools, etc. The amount of energy released by the electrical arc depends on the amount of electrical current flowing through the arc and how long the current will flow before it is interrupted by a circuit breaker or fuse. The radiation released in an arc flash can be so intense and so rapid that it can instantly burn skin and ignite clothing. Temperatures at the electrical arc can rapidly climb to tens of thousands of degrees. At temps this extreme metal becomes liquid, then vaporizes, and the air surrounding the arc becomes superheated to approximately 30,000°F. The superheated air and metal vapor together expand with explosive force. This creates a dangerous and potentially lethal pressure wave of hot gas, molten metal droplets, and solid metal shards that can create burns and shrapnel wounds.
The Occupational Safety and Health Administration (OSHA) requires employers to assess the workplace for arc flash hazards that are present or that are likely to be present. Assessment is done using standards developed by the National Fire Protection Association (NFPA) and the Institute of Electrical and Electronics Engineers (IEEE) to specifically address arc flash hazards. If hazards are identified, employers must mitigate risks by adhering to a six point compliance plan such as the following: 1. Implement a worker safety program with defined responsibilities. 2. Perform engineering studies that include calculations to determine the degree of arc flash hazards. These studies also must be updated when any changes to the electrical system are made by the employer or the electric utility. 3. Provide workers with the correct personal protective equipment (PPE) based on the study results. These PPE must then be maintained on site to protect workers. 4. Provide worker training on the hazards of arc flash. This training must be documented and workers must demonstrate proficiency through testing. Worker training must also be updated whenever any changes occur to the electrical system. 5. Provide appropriate tools for safe working. 6. Place conspicuous warning labels on equipment to warn workers about potential arc flash hazards. OSHA, like other government agencies, expects employers to keep up with regulations and take the necessary steps to remain compliant. For example, OSHA won’t send notices out to employers to inform them that they must implement an arc flash program in their plant. It’s up to the employers to know that and institute the necessary precautions on their own. It must also be noted that OSHA doesn’t walk employers through the steps of setting up an effective worker safety program. This means that workers can be exposed to arc flash hazards simply because their employer is ignorant of regulatory requirements and is operating based on misconceptions. Some workers are unfortunate enough to work for employers that don’t take potential dangers like arc flash seriously enough to implement an effective safety program. When their wakeup call comes, it’s often too late. _________________________________________________________________ Do you want to know more about implementing an effective Arc Flash Program in your facility? Phil O’Keefe developed and presents a 90-minute webinar entitled: “Arc Flash Program Fundamentals.” Contact him for more information about conducting this webinar for your organization. |
