By Roberto Hajnal and Luis Carlos Lagos
We all know that security in grain storage facilities is a very important issue, particularly as the processes associated with their operation include all the types of accidents that could occur in any industry, as well as the monumental risk of a dust explosion and its resulting serious damages by fire or secondary explosion.
The risks associated with handling combustible dust particles, such as grain dust, is less known and understood than the operating risks associated with flammable gases, such as hexane — a solvent used in the oil industry.
Grain elevators in South America have not been exempt from dust explosions, and the region has recently suffered several accidental explosions with great loss of life and materials, as briefly illustrated below.
On Friday, April 26, 2002, the ACA Terminal (Asociation de Cooperativas Argentina C.L.) exploded in San Lorenzo, Santa Fe, Argentina. The result was tragic: three deaths, 19 injured and the total destruction of its infrastructure with millions of dollars lost in materials.
But that was not an isolated incident. In October of 2001, a similar explosion left three dead in the terminal of A.C. Toepfer in Puerto San Martin, Argentina. A month later a similar disaster destroyed the port terminal of Coinbra (Louis Dreyfus’ Brazilian grain subsidiary) in Paranaguá, Brazil. Fortunately, this time it was without casualties but it inflicted complete material damage.
Other recent explosions, although less damaging, were the explosions in the terminal of Productos Sudamericanos in Punta Alvear, near Rosario, Argentina, mid-2000 and in Louis Dreyfus’ Terminal in General Lagos, in the Tarapaca region of Chile in 1999, which sadly killed one person. Another at Molino Argentino, Buenos Aires, Argentina, killed three in 1995.
It is not exceptional that these explosions occured, rather their frequency in recent years is what is remarkable. In fact, in the previous 15 years, South America had not suffered any serious explosions in its region. Still, Argentina has had its share of accidents — many less serious but some worse. One of its largest accidents was a little more than 10 years ago when we were surprised by the explosion of Genaro Garcia in Puerto Rosario with 10 deaths. Possibly Argentina’s worst explosion was the tremendous tragedy that occurred in 1985 at the silos of Junta Nacional de Granos in Bahia Blanca that killed 22 and injured more than 10.
WITH AND WITHOUT GOVERNMENT CONTROL
Unfortunately, there are no formal statistics kept on dust explosions in South America, a formulation of which might raise awareness of this serious issue, as well as encourage education about planning methods and/or equipment to prevent such disasters.
These statistics are readily available in the U.S., where grain dust explosions are a cause of great concern. In the last 12 years, there have been more than 150 explosions causing damage valued at more than $100 million dollars, as well as hundreds of injured and more than 30 deaths.
Comprehensive legislation has been introduced in the U.S., regulating the design of plants and conveyors, as well as requiring monitoring devices, management safety processes, training and security procedures for grain facilities.
The incidence of grain dust explosions in Europe is very rare, owed in part to better designs, better maintenance and climatic fac-tors. However they are not exempt. In France they revised already strict legislation after the explosion at the silos of Blaye in Gironde, France, in 1997, which resulted in 11 deaths.
Today in Argentina and neighboring countries, there is no specific legislation for the security of silo plants, such as exists in practically all the rest of the world. Nonetheless, designers and operators working with or without specific regulations should be alert and take special precautions against grain dust explosions.
THE BASIC FACTORS
Grain dust is explosive, and by far the biggest risk in the grain storage and processing industries is a dust explosion.
By the very nature of processed products and the operating activities, dust will always be present and the potential for a source of ignition will never be totally eliminated. Wherever grains are handled, there is a potential to generate dust and consequently, the risk of explosion.
The most important preventive practices to limit the dangers of dust explosion are to avoid the formation of explosive air-dust concentrations and limit any source of ignition that could ignite a primary explosion, thereby minimizing the risk of expansion that would generate secondary explosions.
Experience and many studies have demonstrated that the dust of practically any organic product is flammable and can cause an explosion under the correct set of circumstances.
The following elements must be present simultaneously for rapid combustion: a suspended cloud of flammable dust (combustible); a source of ignition with sufficient energy; and sufficient oxygen (more than 8%). These elements together form what is known as a "fire triangle."
The combustible (dust) must conform to a certain range of concentration in the air. The greater the density of dust in the air and the smaller the dust particles, the more violent and uncontrollable the explosion will be. This mix must be produced in a confined space, and if the dust is in a completely closed space, the pressure that accumulates during the explosion will greatly increase its intensity.
Modern plants are constructed to avoid confined spaces where possible, but some will always exist out of operational necessity: bucket elevators, tunnels under silos, linking tunnels, etc. Nowadays, grain silos are frequently not made of concrete — instead they are made of sheet metal or a beam structure of concrete columns and metal floors without walls or side metallic closings. These are lighter and easy to expel so that the expansive wave of an explosion is easily dissipated.
Bucket elevators are generally enclosed with grain that is continually moving and generating dust. Almost 60% of the dust explosions in the U.S. started in bucket elevators. A bucket elevator resembles a turbine moving at high speed, displacing large quantities of air and combustible material that is agitated by the buckets that rotate, creating a cloud of dust particles in the air confined in the same elevator box. This creates the risky equation of a combustible plus oxygen plus confinement, which can easily result in an explosion.
All that is necessary to detonate this is a source of ignition, specifically, the heat and sparks that can be generated by various forms of the following:
· Slipping of a belt over the pulley,
· Misalignment of a belt,
· Overheating of bearings,
· Welding or cutting too close to operating equipment,
· Flammable materials and residues,
Dust deposited on the floor or in corners causes additional risks because it is disturbed by the expansive wave of a first explosion generating more combustible material for a secondary explosion. The subsequent explosion is normally much more violent than the first, and can create a third explosion. In this way it produces a chain reaction of growing intensity that culminates in total destruction and fire.
CHANGING THE EQUATION
The explosiveness of dust depends on multiple factors: its concentration; the size of its particles; the composition; the moisture content and environmental temperature, among other factors.
The increased pressure that is produced in a first explosion can be limited by means of windows, doors or openings designed to release pressure. But during the secondary explosion, the expansive wave is so strong that the panels designed to suppress the explosion generally are not sufficient and can cause the collapse of more resistant structures.
In order to assure that an explosion is not produced, the equation "dust + oxygen + ignition" must be altered. The elimination of any one of these requirements will prevent the reaction. This can be done by neutralizing ignition sources or eliminating or reducing the emissions of dust.
Dust is eliminated by controlling its generation or installing vacuum systems that collect the dust in filter sleeves at each point of emission. Cyclones, although they are relatively inefficient systems for eliminating the most explosive particles, are very widely used in South America.
Some modern systems use an application of mineral oil on grains to prevent the formation of dust, thereby avoiding environmental contamination and the generation of explosive dust throughout the entire process.
No system can replace the need for complete manual or automated cleaning; this is the principal method to effectively avoid secondary explosions.
Sources of ignition can be eliminated principally by preventing electrical failures, by use of adequate non-explosive materials (cables, lighting, electrical components and motors) in equipment, and plants that are well designed and maintained. Mechanical failures, like the temperature of bearings and slipping and misalignment of the belts, can be controlled by the installation of adequate electronic monitors. These automatic monitors add a higher level of security as they are connected to an alarm system and are strategically located so they can shut down a conveyor and/or the entire facility if necessary.
It is very difficult to prevent an explosion. However, it is always possible to minimize the damage by designing robust structures and machines that can resist the pressure created by a first explosion and by using openings that act like vents to alleviate the explosion, in order to prematurely dissipate it and avoid its destructive and lethal propagation.
Dust explosions can also occur in well-built and maintained facilities, operated by ideal and expert personnel. Therefore, it is important to insist in establishing means of prevention and security that may have an additional cost but that minimize the risk, providing more reliable and safer facilities.
In Argentina, the Asociación Argentina Poscosecha de Granos APOSGRAN, with headquarters in the Rosario Stock Exchange, is an institution that provides training for the prevention of this type of accidents through journals, courses and a technical magazine. Contact APOSGRAN at email@example.com.
The U.S. Grain Elevator and Processing Society has a series of safety videos and programs, including one dedicated to preventing grain dust explosions, covering topics such as controlling fuel and ignition sources. It also has guidebooks and audio cassettes on grain dust hazards and prevention methods. Find more information or ordering details at www.geaps.com/geapsstore or call 1.612.339.4625.
Engineer Roberto Hajnal is a consultant to the grain processing industry and is a member of the Board of Directors of APOSGRAN (Asociación Argentina Poscosecha de Granos). E-mail him at firstname.lastname@example.org.Luis Carlos Lagos is c.e.o. of SGL Corp., dedicated to the suppression of dust with mineral oils in the storage process and is also a member of APOSGRAN’s board of directors. Contact him at email@example.com.