The history of catastrophic industrial explosions is a long and truly horrible one. In 1921, a silo containing 4500 tons of ammonium sulphate and ammonium nitrate exploded at BASF in Germany, killing 561 workers. The explosion was so massive it was felt 300 miles away and left a crater more than 70 ft deep. In 1971, a flower mill exploded in Germany and resulted in the death of 17 workers. In 1984, an explosion in Bhopal, India immediately killed 2259 people. In 2001, a dust explosion in Toulouse, France resulted in 31 deaths. 
There have been many explosions in the U.S. as well, including at Imperial Sugar in 2008, where 14 died and 60 were injured; Hoeganaes Corp. in 2011, in which five died in three separate incidents in less than five months; West Pharmaceutical, 2003, where six died and 38 were injured. All of these incidents were caused by combustible dust or gas related explosions 
According to the Occupational Safety and Health Administration (OSHA) and the U.S. Department of Labor (DOL), at least 350 explosions caused by combustible dust killed 130 workers and injured more than 800 since 1980. There are a number of statistics available from insurers and others that show the property damage caused by an explosion could range anywhere from an average of $400,000 to $1.6 million in loss per explosion incident . Destroyed buildings and the huge loss of production, as well as the cost of reinvesting in new production equipment, can run into the billions of dollars. The average downtime after an explosion can be estimated to be at least several months, that is, long enough for customers to find another supplier 
The explosion at Imperial Sugar in Georgia in 2008 is the watershed “catastrophic” event, which resulted in a re-issuance of the National Fire Protection Association (NFPA) standards and implementation of a National Emphasis Program -- strict enforcement -- by OSHA. More than 30,000 companies have been made aware of combustible dust hazards and of the need for strict compliance with the NFPA standards.
Some random measures have been undertaken over the years to curtail risks in production. Smoking is no longer allowed in any production facility and welding has to be done with special precaution taken, special permission/permitting and strict direction. Even cell phones are banned in some production facilities.
But to achieve a truly safe production facility, a modern explosion protection solution must consist of
* Prevention of explosive atmospheres
* Avoidance of the ignition of explosive atmospheres
* Mitigation of the effects of an explosion to an acceptable level
Prerequisites for Explosion
Any dust that has a KST value greater than 0 is defined as being a combustible dust under by OSHA Guidance  therefore, measures to avoid or reduce the damage caused by an explosion are mandatory. This dust is not dangerous as long as the other elements of the so-called Explosion Pentagon are absent.
For example, sugar or flour stored in bags in a warehouse or other confined area are harmless products as long the fine dust particles are not airborne and dispersed, creating a concentration in the air which is defined to be combustible.
In addition, a source of ignition has to be present, which can be a mechanically-created spark, a spark created by discharging static electricity, a hot surface, or an open fire.
If you have all of the elements — combustible dust, a confined area, oxygen, an ignition source, and perfect dispersion of dust particles — an explosion would occur. But, the truly catastrophic results of these explosions are actually caused by the maximum explosion pressure build-up in the confined area. This build-up could burst the enclosure which would not be able to withstand the pressure. Both fragments of metal and flames released into work areas would cause deadly injuries and extensive property damage.
And then there is the secondary explosion. Typically, the exploding enclosure/enclosed area is connected to another room or other enclosure (a dust collector might be connected to a mixer or blender, for example). The shock wave of the first explosion “blows” into perfectly dispersed airborne dust in the connecting enclosure or enclosed area, causing catastrophic damage reminiscent of recent combustible dust explosions. Given the explosion risk potential in manufacturing facilities, what should a modern explosion protection solution include?
Prevention of Explosive Atmospheres
Explosion prevention, also known in the past as primary explosion protection, includes all measures to be undertaken to avoid a combustible atmosphere:
Substitution: includes, for example, suspension of highly combustible aluminum powder in oil to avoid reaction with oxygen via a fine layer of oil around each particle
Cleaning and Diluting: This is the housekeeping issue. Avoid dust accumulations in production that otherwise cause a combustible atmosphere when airborne. Cleaning dusty areas with pressurized air is no longer allowed; vacuum systems must be used.  Even vacuum system equipment must be explosion-proof, since the accumulation of dust inside these systems creates an explosion risk if there is a source of ignition.
Conditioning of a combustible into a non-combustible product: This procedure, sometimes as simple as spraying water on powders, is quite often used in coal mines and wood processing industries. NFPA 120,664 Conditioning finds its limitations if moisture changes the worth or handling of product so, for example, pneumatic conveying processes cannot be conditioned.
Blanketing with Inert Gas: This procedure quite often is used in batch operations such as in pharmaceutical production and can be quite costly.
Some measures of explosion prevention are not practical and find their limitations especially in the food processing industry. So, for example, neither water nor oil can be used to produce flour or coffee!
Avoidance of the Ignition of Explosive Atmospheres
This second step to explosion protection includes taking all possible measures to avoid any source of ignition such as mechanically generated sparks, flames, hot surfaces, avoiding static electricity (grounding of all equipment is mandatory), using only intrinsically safe electric/electronic parts. 
Now, in accordance with prevention of explosive atmospheres theory, if you could eliminate ignition sources with 100% certainty, eliminate oxygen with 100% certainty, or control dust concentrations, there would be no need for additional combustible dust explosion protection because you would have eliminated a required element for a combustible dust explosion.
In real processes, control of dust concentration and concentration of oxygen is quite difficult and expensive. Redundant systems increase the cost of investment.
Avoiding ignition sources is mandatory and the first step for explosion prevention, but controlling ignition sources is not enough since an unforeseeable ignition can result from something as simple as a screw or hammer dropping and causing a spark. Prevention and avoidance of ignition of explosive atmospheres alone will not eliminate the risk of an explosion.
In many production processes, fine particles are created, either by grinding, conveying, or any other step in processing. In almost all industries, it is recognized that these fine particulates create an environmental problem or health hazard for workers operating in an enclosed area.
Dust collecting systems are installed to eliminate these issues, but they create a whole new set of risks. Despite the first reported combustible dust explosion in an Italian flour mill in 1785, the knowledge that some fine particulates could mix with air in a confined space and cause a combustible mixture under the right set of circumstances has been ignored or neglected.
Today, thousands of dust collectors fulfill the requirement of providing clean air for a manufacturing environment but many, perhaps most, of these dust collectors do not have proper explosion protection. Many of them do not even have a simple explosion panel. Yet they are installed indoors and cause an even higher risk for workers and property than would be the case without the dust collectors at all.
Mitigation of the Effects of an Explosion: Combustible Dust Explosion Protection
As far back as the 1920s, combustible dust standards were issued by the NFPA. The first standards appeared as recommendations, whereas the newer standards seem to “imply” a mandatory approach to protect against the risk of combustible dust explosions, especially in light of OSHA’s
National Emphasis Program that is essentially enforcing the NFPA standards for combustible dust through its general duty clause.
NFPA 68, 69 and 654 are general standards for a wide variety of industries:
NFPA 68 describes the need for explosion venting and the calculation of the vent area required. NFPA 69 provides a description of how to prevent an explosion. NFPA 654 can be described as the process standard, giving guidance on how to isolate an enclosure to prevent flame propagation and secondary explosions.
Mitigation of Explosion Effects: Indoor Venting As the Third Layer of Modern Explosion Protection
The most commonly used method for deflagration prevention in the U.S. is chemical suppression and, for many years, was the only method used to protect indoor enclosures from the risk of combustible dust explosions where venting to the outside wasn’t an option. With chemical suppression, bottles of an extinguishing agent under pressure of 80 Barg are installed in an enclosure or duct. These bottles are triggered by devices such as spark detectors or pressure sensors. In the event a spark or increase in pressure is detected, the suppression bottles quickly release their agent and extinguish the flame, thereby stopping propagation.
One real drawback to chemical suppression is the “false triggering” of the system due to sudden but small changes of pressure in real world manufacturing processes. After “false triggering,” the process being protected by suppression is contaminated and has to be cleaned. The suppression bottles must be refilled or replaced and the system must be recertified by authorized personnel. Thus, a facility will suffer from downtime and real expenditures due to “false triggering”. 
In addition, in small enclosures, chemical suppression reaction time might not be fast enough to reduce the pressure build-up inside an enclosure below the enclosure’s design pressure. Bursting of the enclosure or deformation could be the result.
Lastly, suppression systems require quarterly maintenance per NFPA 69, by authorized personnel (which is quite often the vendor’s personnel) and create high maintenance costs for the lifetime of production.
For products/dusts with very high Kst values, suppression systems are still the only choice for explosion prevention for enclosures located indoors that cannot be vented to the outside. For the vast majority of applications, however, indoor flameless venting systems are the best solution.
Today, there are options for safely venting indoor enclosures. Explosion vents or panels are the least expensive method of venting an enclosure so long as that enclosure can be safely ducted to the outside. After an event, the vent must be replaced.
NFPA 68 describes the calculation of the vent area necessary to safely vent the deflagration, based on KST value and Pmax, the maximum explosion pressure and the Pred, or reduced design pressure, of the enclosure.
Many dust collectors are installed indoors; they are located close to the source of dust, and frequently away from outside walls. Any duct between the dust collector and the wall that will “lead” explosion pressure to the outside will cause back pressure. This has a huge impact on the strength of the dust collector. 
There are many dust collectors installed with a very low Pred, either because they have been so designed, without taking into consideration the potential of an explosion, or they have weakened after many years of use. The average Pred of rectangular dust collectors today is less than 0.3 Barg.
There are many experts, OSHA inspectors, and publication editorials that have, over the years, recommended that a dust collector be installed outdoors to avoid the catastrophic results of a combustible dust explosion. In fact, this is simply wrong and not practical. This “solution” would cost industry a fortune to relocate dust collectors and install more powerful blowers and fans and result in a tremendous increase in energy costs.
Let us remember that simply placing a dust collector or other enclosure outdoors does not necessarily reduce the risk of property damage or people being injured or killed. If an event occurs, the length of the flame can be 100 ft or longer and, therefore, care must always be taken to be sure that an explosion vent/panel does not face the direction of a neighboring building, a driveway, or other area where people or vehicles could be located. And, for some production, dust collectors must be installed inside. This is true, for example, of sugar-based products. Changes in temperature and humidity would block filter elements. In such cases, and in many others, indoor flameless venting is the best option.
An explosion can be vented safely indoors (as well as outdoors) with a flameless venting system.
Flameless Venting Technology
Flameless venting technology was invented in the 1990s in Europe by REMBE GmbH. The flameless vent consists of a cage, wrapped with a specially-developed mesh material. The bottom of the cage is hermetically sealed with a round burst panel.
Once a deflagration starts, the increasing pressure causes the rupture disc to burst. Flame and dust shoot into the cylindrical body of the. Because the mesh material provides a very large surface area, the heat is dissipated. Dust particles cannot pass through the mesh material and are also kept inside the cylinder.
The increase of pressure outside of the flameless vent is negligible when an event occurs and the rupture disc bursts. There is no pressure wave so no dust will become airborne and create the potential for a secondary explosion.
Some of the flameless venting systems have an ATEX system certificate and can be installed indoors to safely vent an explosion; these systems are also in accordance with NFPA regulations and may be Factory Mutual approved.
Flameless venting systems are passive systems. They don’t need any other device (such as a spark detector or sensor) to be triggered; the explosion pressure will activate the system. While other prevention systems, like suppression systems, do need quarterly maintenance, flameless venting systems do not. The burst panel of the flameless vent is specifically equipped with burst sensors. Once an explosion occurs, the signal is activated to shut down the entire process.
Just as a point of information, in accordance with NFPA 654, an enclosure – in many cases a dust collector – also must be isolated to prevent flame propagation into adjacent enclosures causing secondary explosions .
For companies having converted from suppression systems to flameless venting, the cost savings are enormous. The average downtime after an explosion is suppressed can be more than three days due to the need to clean the entire production line to eliminate the suppression agent. Compare this with a system protected with a flameless vent: the downtime is significantly reduced to just the time it takes to replace the panel and rinse the vent.
During the past 10 years, indoor venting has become more popular in North America, not only because of its effectiveness in eliminating the risk of damage and fatality, but also for economic reasons. Dust collectors don’t have to be relocated or placed outside and downtime caused by false triggering does not occur with passive indoor venting systems. Maintenance costs are dramatically reduced. If an event occurs, replacing the indoor venting system disc is fairly easy and production is able to resume within a short period of time.
Dr. Gerd Ph. Mayer is president, REMBE Inc., For more information, visit www.rembe.us.
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