|See part two of this series, "Act Now to Prevent Fires and Explosions in Your Industrial Dryer. Certain conditions contribute to an environment that allows unplanned and uncontrolled fires and explosions in industrial drying equipment. Learn how to control them.|
Uncontrolled and unplanned fires and explosions in the plant are obviously undesirable. The most important reason to take measures to reduce and, hopefully, eliminate fires and explosions in a dryer system is personnel safety. At the end of the day, you want all employees to leave the plant in the same condition as they entered. While a slow fire may give personnel time to react, an explosion could kill or severely injure someone who happens to be in the wrong spot at the wrong time.
The longevity of your equipment is another reason to reduce fires and explosions. All equipment is designed to operate under set conditions. The high heat produced by a fire can easily degrade the structure of your equipment, making it more prone to failure. Similarly, explosions can catastrophically damage equipment, ductwork, supporting structures, piping systems and other key elements.
As we now live in a litigious society, any adverse situation in a plant setting can result in legal accusations. It is important to note that any fire or explosion in the system that injures personnel or damages equipment could cost your company serious money in a lawsuit.
For any or all of these reasons, unplanned and uncontrolled fires or explosions will significantly affect the bottom line on your plant’s ledger. But large costs aside, at the very least, unplanned and uncontrolled fires and explosions can cause downtime, potentially cost maintenance dollars and likely damage a portion of the product. For most plants running on tight margins, any of these losses can be problematic. For some, such a loss closes the plant permanently.
In order to have a fire or explosion, three elements — oxygen, a fuel source and an ignition source — must be present at the same time in the same location. Remove any one of them and you cannot have a fire or explosion.
Three central causes create the conditions that allow fires and explosions to occur in dryer systems:
- High oxygen content in the dryer system.
- Dirty ductwork.
- Problems from operations and controls such as poor shutdown and startup sequences, upset conditions, operator error and erratic inlet or infeed conditions.
Most of the conditions described below that could lead to fires and explosions affect direct-fired, rotary drum dryers, but many apply to almost any type of dryer system. (Make sure to see the April 2015 issue of Process Heating, where Part 2 of this series will discuss the specific measures to take to prevent fires and explosions.)
Too Much Oxygen
It is in the very nature of a dryer system to have two of the three elements required for a fire to occur. The product being dried is the fuel, and the heat present for drying serves as the ignition. Therefore, when a fire starts in an industrial dryer, the most common reason for it to occur is high oxygen content. A fire can be started with the right mix of fuel, oxygen and ignition. For this reason, your dryer system will be at risk for a fire or an explosion when free oxygen is introduced.
Oxygen can enter your system in many ways. For instance, air leaks allow oxygen into the system. Explosion doors or manway covers may not seal properly. Also, it is not uncommon to have cracks in equipment or ductwork. However, the number one cause of air leaks in a rotary drum dryer system is poor drum sealing surfaces with incorrectly designed drum seals. The seal rings can get divots or become out-of-round. If your drum flexes while rotating and resembles a water balloon rolling down the sidewalk — always wider than tall, on a slightly less visible scale — the seals will never match up to round inlet and outlet rings. This mismatch will create gaps that the seals cannot cover. Thick rubber seals, hardened with age, are not flexible enough. Metal leaf seals may work for a short period of time, but their effectiveness is drastically reduced through the wear of metal-to-metal contact.
Neglected airlock maintenance can lead to poor airlock conditions that also contribute to higher oxygen content. Gaps between the rotors and housing allow air and combustible dry dust back into the system. If the system infeed or outfeed is a trough auger or drag chain, it is nearly impossible to seal those against air being drawn into the system.
Rise of the VOCs
The second condition that can lead to fires and explosions — or worsen the situation, should one occur — is dirty ductwork. For instance, the black tar buildup that tends to accumulate in the ductwork of dryer systems used for distillers dried grains with solubles (DDGS) — and likely others — is condensable volatile organic compounds (VOCs). VOCs can be created when a product is thermally damaged by excessive and radiant heat.
To help prevent buildup, industry experts recommend keeping ductwork at temperatures above 220°F (104°C) for DDGS dryers and most wood product dryers. As the temperature in the ductwork falls below this threshold, water and VOC vapors will condense on the sides of the ductwork. Keep in mind that dryer systems for other products may produce different VOCs that have condensate temperatures above or below those for DDGS and wood products. To determine whether your system is susceptible to VOC buildup within the ductwork, consult with your dryer manufacturer.
Buildup inside of the ductwork is flammable and could have a lower autoignition temperature than the material you are drying. A fire could even start when you are operating within specifications if your oxygen (O2) content is high. If a fire starts elsewhere and spreads to tar-encrusted ductwork, the ductwork or more could be destroyed.
Thermal damage also leads to the creation of fine particulate (<30 microns). Separation devices, including outlet hoppers and cyclones, are not efficient enough in removing these finer particles. The unseparated particles remain in the gas stream, if exhaust gas recycle (flue gas recycle) is being utilized, and get burned or partially burned, increasing the VOC load and buildup on ductwork.
Operations and Control Systems
Lastly, operations and control systems can create conditions that cause fires and explosions. Poor shutdown and startup sequences, the way in which upset conditions are handled, variations in infeed moisture content and operator error all can result in a fire event. The majority of fires and explosions occur during startup or shutdown of a dryer system. The system is full of oxygen (atmospheric air) during startup. It is not until the system is up and running that this oxygen has been safely burned by the system. Until then, it is available to interact with fuel that may be present, just waiting for a spark to ignite.
During rapid shutdowns such as those experienced with the use of an emergency-stop switch or caused by an upset condition, gas temperatures and volumes drop immediately. This creates negative pressure voids that pull fresh air into the system. If a high temperature alarm triggers burner shutdown, that action immediately creates an inrush of oxygen-rich air to make up the void created by the burner. This oxygen will only aid any fire that might have created the high temperature event in the first place. Therefore, proper programming — capable of handling upset conditions without introducing conditions that intensify the problem — is required.
Inconsistent product infeed moisture conditions and mass flows can throw a dryer’s control system into a cat-and-dog chase where no one wins. All refractory, metal and the high volume of gases hold a great amount of inertia — a thermal flywheel, so to speak — that must be overcome should an adjustment in running rates be needed. When an infeed product is wetter than what was previously introduced, the burner is required to spike its output to compensate for the lower drum temperatures that are caused by the introduction of that wetter material. When the normal moisture-content product returns, there is still energy left over by the thermal flywheel from the spike. As a result, there will not be enough moisture to remove the extra energy. This creates an overtemperature situation and overdried product because the incoming drier product does not have the moisture required to remove the extra energy. This causes the burner output to drop off, which could result in a void where oxygen can be introduced. This oscillation of burner output will continue for hours until the system reaches steady state, all the while creating elevated risks of fire and explosions.
Sometimes, an error is a matter of a lack of operator training. Should an upset condition occur and the operator does not know how to alleviate the potential risks by putting the dryer system into manual to stop or minimize the swings of the control system, the dryer system will be at risk for a fire or explosion.
In the words of a trusty bear, only you can prevent unwanted fires and explosions. Fortunately, each potentially hazardous condition has a simple solution. Reduce the risk of unwanted fires and explosions by:
- Reducing the amount of oxygen in your system by minimizing air leaks.
- Keeping ductwork clean to reduce the amount of volatile fuel in your dryer system.
- Renovating operations training and control systems such that startup and shutdown sequences ensure the dryer is not susceptible to a massive influx of oxygen or energy. As much as is possible, operate the dryer in a steady-state manner. Implement operator training for upset conditions and amend the control system to take care of most common upset conditions.
As discussed, direct-fired rotary dryer systems have fuel, ignition, oxygen and the potential for pressure to build up. Therefore, fires and explosions are a real risk. There is no manner of preparation a plant can do to fully eliminate the risk of unwanted fires and explosions. Therefore, your system should be equipped with pressure-relief panels or doors properly located to relieve the pressure without endangering plant personnel. Pressure-relief panels and doors will go a long way toward protecting the equipment and personnel in an adverse situation.
The next part of this series will discuss the specific measures that must be taken to reduce the risk of fires and explosions in your dryer system.