It’s rare. It doesn’t happen often, but when it does, a dust explosion or fire can be catastrophic. In the food and agriculture industries, perhaps none in recent memory was more tragic than what happened at the Imperial Sugar refinery located just outside of Savannah, GA. The massive explosions — there were more than one — claimed 14 lives and injured 36 others, sparking a second look at how bakers, snack producers and other food companies need to prevent combustible dust hazards in the workplace, following that 2008 event.
During the past decade, the grain-based foods industry has averaged about 10 agricultural dust explosions per year, according to the 2014 annual report by Kansas State University (KSU). Last year, there were only seven incidents resulting in 14 injuries and no deaths. Overall, most of the explosions occurred in grain elevators and feed plants, said Kingsly Ambrose, PhD, the KSU assistant professor who compiled the report. Dr. Ambrose added that the statistics cover only reported explosions, but not fires, which can be more common in the baking industry. (See “Grain Dust Explosions Remain Hazard” in the March 2015 issue of World Grain.)
Preventing or mitigating dust explosions or fires requires a comprehensive strategy with many components. Among these are a familiarity with federal Occupational Health and Safety Administration (OSHA) regulations and National Fire Prevention Association (NFPA) standards, proper documentation of ingredients and processes, and employee evacuation procedures and proper training, according to Jason Stricker, national accounts manager, Shick USA, Kansas City, MO.
Cleaning to higher levels
Controlling combustible dust fundamentally comes down to prevention and protection. Key among the prevention procedures? The overwhelming consensus strongly suggests keeping your house in order.
“It all comes down to maintenance and good housekeeping,” Mr. Stricker said. “If there is dust around, you need to make sure that you are cleaning it up quickly.”
When it comes to sanitation and maintenance, expectations have changed dramatically over the years, noted John Hunter, national sales manager, bakery and ingredient handling, Buhler, Inc., Plymouth, MN. In many ways, worker safety has benefitted from the Global Food Safety Initiative and the food industry’s emphasis on enhancing sanitation and ensuring food safety.
“As for cleaning standards, what was acceptable a few years ago is no longer acceptable from a hygiene and safety perspective,” Mr. Hunter said. “Now, if you have dust on the floor, you need to properly vacuum it on a regular basis. When cleaning it up, you have to control dust so there is no personnel exposure. When you go into a new bakery now, they’re really clean compared with how they were in the past.”
An effective preventive maintenance schedule often involves monitoring filter media or checking bag houses or cartridges for possible clogging or rupture, according to James Toole, product manager, bulk handling systems, Gemini Bakery Equipment/KB Systems, Philadelphia. “Ensure that combustible ingredients are stored correctly and properly maintained,” he advised. “Train personnel on proper handling and transferring of flammable ingredients. Incorporate properly designed pneumatic systems when conveying large quantities of dust-prone ingredients. Always repair any damaged equipment or correct any equipment issues in a timely manner.”
In addition to cleaning filters and monitoring flexible and bolted connections, the filter systems’ flexible connections and seals on storage bins need to be routinely examined. “Those are your biggest contributors to combustible dust in a bakery’s ingredient handling system,” Mr. Stricker said.
Mounting a preventive defense
Good housekeeping also requires preventing dust from building up on equipment, pipes, the floor or even the ceiling so that even a small oven fire doesn’t turn into a larger disaster. Kevin Pecha, sales manager, AZO, Inc., Memphis, TN, explained that dust clouds can develop as a result of normal conveying or they can be generated through faulty material handling, creating a potential condition to cause such an explosion. “Secondary conflagrations can occur by allowing dust collecting on flat surfaces to become airborne,” he said. “Proper ventilation as well as diligent sanitation, in conjunction with care to prevent electrical or static discharges, should be of great importance.”
In some cases, unsafe conditions in bakery ingredient systems may result from poor initial design, misuse, inadequate maintenance and poor housekeeping, observed Bill Kearns, vice-president of engineering, Fred D. Pfening Co., Columbus, OH.
“Most dust explosions that reach the news have poor housekeeping as one element,” Mr. Kearns said. “Typically, what happens is that a small primary event shakes loose dust from building rafters and overhead pipes and provides an ignition source for a much larger secondary explosion. A good management plan includes preventing both the primary event, by having proper equipment and maintaining it, and with good housekeeping to avoid any dust accumulations.”
A deflagration event — which can be a fire or explosion — requires an ignition source, a confined space, ample oxygen and flammable dust. Most bakery ingredients, such as flour, have fairly low combustibility. However, if the conditions are right — a light cloud of fine dust could envelop the area — the risk is there. That’s especially true with the many ignition sources present in a typical bakery, ranging from ovens and fryers to welding and static electricity.
“Virtually all organic bakery ingredients are combustible and thus carry varying degrees of fire and explosion risk,” Mr. Kearns said. “Non-organic ingredients, such as salt and calcium sulfate, are generally non-combustible and do not have similar risks. In general, the more finely divided and drier the ingredient, the greater the fire and/or explosion risk.”
There are several variables to determine an ingredient’s combustible dust potential. “Certain ingredients rank higher on the risk scale using the minimum ignition energy (MIE),” Mr. Pecha said. He noted such measurements as the speed of the pressure rise (Kst), as well as the maximum pressure of a dust explosion (Pmax), can assist bakers and equipment manufacturers to determine the proper level of protection needed for the equipment.
The higher the Kst value, the greater the speed at which a dust deflagration can build, according to Mr. Stricker. Granulated sugar may be rated the lowest level on the ST Class system chart, which the US Chemical Board uses to rank explosion risks, but others like starches may have a ST2, or moderate, rating while materials such as aluminum powder get the highest risk or ST3 rating.
In addition to relying on the ST Class chart for determining a dust’s deflagration index, technicians can select from a variety of test kits to determine a dust’s explosive potential.
Bakers and millers, however, should exercise caution in using published Kst values, according to Lisa Arato, sales engineer, Zeppelin Systems USA, Odessa, FL. They simply don’t tell the whole story regarding the raw material parameters and characteristics that bakers are actually dealing within inside their plants.
“This value should be based on the actual particle size, moisture content and chemical properties of the ingredient after it has gone through the process,” Ms. Arato explained. “You can look up a Kst value for any ingredient or find it on the Internet. However, that doesn’t necessarily mean that it’s the ingredient that you’re using as they come from different sources and may tend to degrade and get smaller after going through the conveying lines. You’re not going to be able to have that exact Kst value that you need unless you run through a test that determines what you are going to see out in the field, which is difficult to replicate.
“You can take granulated sugar and test it, and it won’t ignite,” she continued. “But then you can take that granulated sugar and run it through a conveying system, and the particle size becomes a lot smaller. In a high-concentration dust cloud, it will ignite easily. Those are two very different conditions.”
Specialty ingredients such as those used in gluten-free products — especially if they are mishandled — can pose a higher risk than flour.
“If you manage things correctly, they’re not a problem. They’re not dangerous, but you need to take care in handling some ingredients, especially gluten-free ingredients like corn starch, rice starch or rice flour — all of which can be a potential issue if not handled correctly,” Mr. Hunter said. “If you design a system for flour and change it over to rice starch and you don’t do a risk assessment to see if there are any changes that you need to do, then that potentially could introduce some risk.”
According to OSHA, particles smaller than 420 microns are considered dust or explosive, noted Dr. Ambrose. “Mostly, in the baking industry, we deal with flour or powder particles in the size range from 10 to 100 microns,” he noted. “This smaller size poses additional danger of creating dust clouds.”
Going beyond prevention
Investing in safety systems to buy peace of mind can be an expensive proposition and often may require advice from an expert in the field to determine what systems are necessary and what might be optional.
“To be compliant with combustible dust regulations doesn’t mean you have to go out and put protection equipment on every piece of process equipment that you have,” Mr. Stricker said. “You need to work with a qualified adviser to determine what needs to be protected, what can be protected, and if something can’t be protected, you need to go into prevention mode.”
Mr. Kearns, a member of the NFPA 61 Technical Committee, noted that there is a great deal of misinformation about what is required by the codes. In general, the intent of the codes is to apply appropriate measures to mitigate real, reasonable risks. “The decision to rely on implementing advanced explosion protection should be based on a reasonable risk evaluation,” he said. “This can be done internally or by outside insurance or risk consultants. It is important to select a consultant with relevant experience and without a hidden agenda to provide a reasonable evaluation. Larger, multi-plant companies with many years of experience will be able to assemble an internal team to assess the risk.”
Technology has come a long way in improving safety during the last decade, noted Ken Johnson, Gemini’s president. “The cost of controls and monitoring equipment has dropped dramatically, making more comprehensive system monitoring possible. This technology combined with standard equipment safeguards adds redundancy and choices to a client’s equipment configuration,” he said.
Like other ingredient handling manufacturers, Gemini/KB Systems manufactures equipment that complies with industry standards and NFPA codes. “Indoor and outdoor bulk storage bins and silos typically have high-level alarms, overfill safety systems and weak-seam roofs as standard equipment,” Mr. Johnson stated. “Occasionally, a customer requires added protection like chemical suppression, flame arresters on weigh hoppers, rupture panels on outdoor silos and particle monitors in silo penthouses.”
Rapid advances in information technology now make it easier to provide real-time operating information to help bakeries control their ingredient systems based on temperature, pressures, motor amperages and other data and thus avoid dangerous conditions. “Triboelectric sensors are available to monitor emissions from bin vents and dust collectors to avoid dust clouds,” Mr. Kearns said. “These units can be used in unoccupied areas to detect airborne dust before a dangerous situation develops.”
Eliminating ignition sources is critical. Although bakers and snack producers cannot do anything about ovens and fryers, they can control circumstances around welding, a big factor in many combustible dust fires. “You need to eliminate these credible ignition sources such as making sure your equipment is properly grounded and you have properly rated electrical devices so you can’t have a static discharge,” Mr. Stricker said.
Arc flash, Ms. Arato said, is another source that should be avoided at all costs.
As Mr. Hunter pointed out, “If you have no spark or dust, an event is unlikely as both are required for an event to occur. The ideal course of action is to eliminate one or both of these components.”
Venting for relief
For protection, silos and storage bins typically contain a secured weak-seam roof. “The goal of this strategy is to simply, safely and cost-effectively prevent excessive damage to equipment in the unlikely event of a rupture,” said Michael Palmer, applications manager, Gemini/KB Systems.
The company’s indoor storage bin system employs a filter membrane over the entire area of the roof that also serves as a weak-seam safety device. “Pressure safety switches, gauges and other instruments monitor bin and hopper pressures for added redundancy,” Mr. Palmer noted. “Selection of suppression, containment and/or venting systems is typically part of a larger project strategy and often tailored to meet not only NFPA requirements but also insurance requirements and local fire codes. In the end, it’s often a mixed bag of industry requirements and risk preferences.”
In many cases where silos are located outside, the more significant risks may involve smaller receiving and storage bins — as well as bag dumping stations — inside the facility, according to Mr. Hunter, who added there are plenty of options for protecting facilities, equipment and employees. “Flame quenching or explosion vents can be put almost anywhere,” he said. “It’s more of a budget issue than an issue of whether or not it can be done.”
A plant’s layout will determine if passive venting is the most economical option. Ms. Arato noted venting through a sidewall or a roof must be within 6 meters or about 20 ft from the storage bin or other systems. “A rupture disc and associated duct work will relieve the pressure to a safe location. That’s usually the most affordable way, but if you have to make changes to your building and layout to accommodate that vent, it might not be cheap,” she explained. “Instead, installing a flameless vent can offer a way to extinguish the flame from an explosion if the equipment is not located near an outside wall.”
Bulk ingredient handling systems commonly use inline magnets, filters and screens to capture foreign materials when transferring flour, sugar or other materials from a truck or rail car. “You need to make sure you prevent introduction of metal into rotating equipment that gets into the conveying system that could cause a spark,” Ms. Arato said.
Interlocks also can prevent dust from escaping air locks, lump breakers or other rotating equipment. When a door is opened at a bag dumping station, a fan will pull air and create negative pressure to keep ingredients from escaping and creating a dust cloud, Ms. Arato noted. “It will draw the dust away from the operator and a potential for an explosion,” she said.
Evaluating risks — and taking necessary precautions — are the safest paths in the long run.