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Recipe for a dust explosion

01 November 2008

Industrial explosions caused by seemingly unknown reasons have been a hazard that manufacturers and risk managers have dealt with for countless years. Stefano Tranquillo looks at the mechanism of dust cloud explosions

An excellent tool for preventing a dust explosion is the dust explosion pentagon. You can prevent an explosion by eliminating any one of the pentagon’s five sides
An excellent tool for preventing a dust explosion is the dust explosion pentagon. You can prevent an explosion by eliminating any one of the pentagon’s five sides

Under the right circumstances, dust explosions can be forceful enough to collapse an entire building. For organisations, such as companies in the food, forest products, rubber, fibre, tobacco, metal powder handling and pharmaceutical industries, whose operations by their very nature produce combustible dust, the property damage from a dust explosion can be catastrophic, resulting in business operations being interrupted or shut down entirely.

The financial implications of a dust explosion can in fact be devastating to a business. Over a recent 10 year period, organisations insured by one insurance organisation alone experienced over 450 fires or explosions involving combustible dust or fibre deposits, totalling an estimated $580 million in damage to their facilities. The average gross loss for dust fires was $1.2 million, and the average gross loss for dust explosions was $1.9 million.

If combustible dust led to an explosion at your facility, how would your company continue to produce product, get supplies, and maintain the flow of goods or services to its customers?

Although you may not be able to totally eliminate combustible dust from your process or your facility, there are prevention measures that your company’s personnel can take. Such measures can reduce the frequency and magnitude of a dust fire or explosion, protect property, jobs and business profits as well as prevent business interruption. These property loss prevention measures would be in addition to the in-house process safety reviews that are required under the European ATEX directive.

Understanding the hazard posed by a dust explosion and preventing such incidents from occurring are two of the most challenging tasks facing many risk managers overseeing production operations today. That is because essentially all dust will burn when suspended in air or distributed on surfaces. Additionally, the sources of ignition for many dust fires and explosions are unknown because finding evidence of the ignition source after a loss is often difficult.

Dust layer
How much dust is too much? If you can write your name in it with your finger, your facility could be at risk because all it takes is a dust layer as thin as 1.6 mm to contribute to an explosion.
Industries producing dust as a product tend to be more aware of the hazards than industries that produce dust as a by-product. Unfortunately, it’s easy for personnel to overlook the hazards.

For example, the following such disaster could have been prevented:
Without warning, a series of wood dust explosions destroyed a particleboard manufacturing plant in a few seconds, causing an estimated $16 million in property damage and business interruption. Sparks erupting from a broken sander belt ignited a cloud of dust particles inside some nearby ductwork. The burning cloud swept down the duct into an outdoor dust collector, causing a small explosion. The explosion then flashed back into the building through a warm air return system causing additional explosions that blew out improperly designed walls and collapsed the roof. In addition, the force broke the building’s automatic fire sprinkler system, preventing it from controlling ensuing fires. By the time firefighters arrived, the blaze and explosions had already delivered the worst blow.

Four things could have sparked the incident: lax dust removal; poor building construction; impaired sprinkler protection; and a poorly designed air handling system. A dust fire occurs when a fuel, in this instance a combustible dust, comes in contact with an ignition source in the presence of oxygen, contained in the air. A dust explosion requires two additional conditions: suspension of the dust so that it burns more quickly in air, and confinement that provides resistance, creating pressure buildup.

Fuel
Dust fuels the explosion, so it is the most important element to eliminate or contain. Ensure that properly designed dust-handling systems combined with manual housekeeping keep accumulations to an acceptable level – usually a millimetre or two but this depends on the dust involved. Establish a comprehensive housekeeping programme, assign accountability, and commit time and resources regularly. Provide a cleaning schedule and review it periodically.

Accumulations should be removed from all surfaces like floors, piping, vents and furniture, including areas that seem harmless. Dust at higher locations is much more hazardous because it can become airborne and create explosive clouds once it is disturbed. Use vacuum cleaners to remove dust from equipment, ledges and beams. First, focus on the highest levels of a facility where dust gathers. Do not use compressed air to blow dust accumulations away. This can disperse dust into the air and form dust clouds.
Regularly inspect any operation that produces combustible dust, either as a product or by-product. If a new process that generates dust is being planned or a new material is about to be introduced, look to your process safety review and property loss prevention consultant for guidance in planning safeguards.

If you have a dust-handling system or plan to install one, properly design and maintain it and prevent dust from escaping and accumulating outside of the handling equipment. If your housekeeping programme cannot keep accumulations below the acceptable level, then handling systems should be improved to reduce the amount of dust escaping.

Ignition
Open flames, friction, mechanical sparks, static electricity and electrical equipment can ignite dust. It’s essential to identify and strictly control all ignition sources. Evaluate the combustibility of the dust your operations produce. Some dust can self ignite spontaneously without a spark or a hot surface. Make sure electrical equipment complies with proper codes and standards and inspect processing equipment regularly. Be aware that grinding and sanding operations or misaligned, rotating equipment can create mechanical sparks. Eliminate static electricity by bonding and grounding electrically conductive components.

Provide sprinkler protection in areas involving combustible equipment, operations or building construction. Install spark-extinguishing systems in ductwork above frequent spark-producing sources like particleboard sanders. In the previous case study, if a spark-extinguishing system had been installed, burning dust would not have travelled into the dust collector.

A fire needs oxygen to sustain itself. While you can’t eliminate oxygen from a room, you can install a gas system that reduces the oxygen level in certain areas of your facility. The process, called inerting, pumps non-combustible gas -- like nitrogen -- into a dust-handling system to keep the oxygen level surrounding the dust below the minimum concentration needed to support combustion.

Dispersion
Dust clouds can form suddenly from vibrating equipment, a small explosion, an earthquake or leaking high-pressure fittings. Dust accumulating on equipment and on building structures – like beams and supports – is fuel just waiting to be shaken loose. Once suspended in the air and surrounded by plenty of oxygen, it is an even greater threat. To eliminate dust clouds, make sure process equipment does not allow dust to escape through holes and loose fittings in equipment, pipes and ducts. Keep equipment in good repair and inspect it regularly. Avoid using warm air return systems that recycle air back into the building. Design shelves and ledges with smooth surfaces that are usually easier to clean. Locate dust-handling equipment outdoors, if possible.

Confinement
A cloud of dust cannot produce a damaging explosion unless it is confined inside a piece of equipment or a room. Confinement allows explosive forces to build up until equipment ruptures or the walls – or roof – of the room give way.

Since explosive concentrations are inevitable within dust handling equipment it is critical that protection is provided. This could involve designing the equipment to withstand the pressure of an explosion or by providing suitable venting. The latter uses lightweight blow-out panels or doors that open at relatively low pressure allowing an explosion to vent most of its force outside the equipment. Obviously, it is not desirable to vent an explosion into the building. If equipment cannot be located outdoors, it should have its vent ducted to outside the building. Ducts should be short and straight or the efficiency of venting can be greatly reduced.

Another option for protecting equipment is the use of explosion-suppression systems that detect an explosion using high-speed detectors. Within milliseconds, the system discharges an extinguishing gas or powder into the equipment during the earliest stages of the developing explosion to stop it before damaging pressures develop. This is particularly useful where venting is impractical or vents cannot be ducted to outside the building.

To stop explosions propagating from one piece of equipment to another, “explosion isolation” may be needed. This separates equipment from the damaging effects of an explosion in other parts of the system. Examples of isolation are rotary star valves located below dust collectors. These heavy-duty steel valves have interior vanes inside an enclosed housing that continuously feed product from the collector into a pipe or a bin. If they are designed properly, they can prevent burning materials or pressure from passing from one vessel to another.

Defusing the bomb
Looking back at the previous case study, perhaps you see that the particleboard plant was a ticking time bomb. Dust particles in the duct above the sanders were waiting for the right circumstances. When the dust explosion propagated through the warm air handling system, it shook widespread building dust into the air, fuelling more, stronger explosions.

If plant personnel had known about the five ingredients of an explosion, they could have taken action to prevent the accident before it occurred.

Unfortunately for many manufacturers, the ingredients for a dust explosion are already there. Combine complacency with poor housekeeping and you have the perfect recipe for disaster.
If your facility operations produce dust, it is wise to seek the advice of a loss prevention consultant to help you evaluate combustibility, potential damage scenarios and the consequences to your operation.

Even with the best loss prevention and control efforts, be sure to develop plans to ensure business continuity. Having a business continuity plan can keep a dust explosion from becoming a catastrophe that halts your business operations. The plan should extend well beyond making sure insurance cover is adequate to focus on maintaining supply to customers.

Footnote
Stefano Tranquillo is UK Vice President and Operations Manager, FM Global



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