Preventing Dust Explosions
On January 29, 2003, an explosion and fire destroyed the West Pharmaceutical Services plant in Kinston, NC, causing six deaths, dozens of injuries, and hundreds of job losses. The facility produced rubber stoppers and other products for medical use. Fuel for the explosion was a fine plastic powder that accumulated above a suspended ceiling over a manufacturing area of the plant. That same year, dust would cause explosions in 25 different U.S. plants, including two others in North Carolina and three in Kentucky. The following year, 28 other plants in 19 states would have dust explosions. In all, federal investigators say there have been 281 explosions nationwide over the past 25 years.
Many died or were injured in these explosions. Following the West Pharmaceutical disaster, 15 more people died and 119 were injured in similar accidents around the country. Yet every death could have been prevented.
“You can prevent dust explosions for almost nothing,” says professor Bill Kauffman at the University of Michigan, a leading expert on dust explosions. His prescription is simple: “Remove the dust.” All kinds of common items such as flour, nondairy creamer, wood, and aluminum are far more lethal than gun powder if they are reduced to a dust and suspended in the air. All it takes is a single spark , Kauffman warns.
|Dust explosions create their own self sustaining "domino effect," continuing as long as there is fuel.|
Grain silos, coal-unloading facilities and refineries, for example, all have places and circumstances where if a spark occurs at the wrong time it could be a recipe for disaster. All these sites have areas that could be considered hazardous due to the presence of explosive material in the atmosphere. A spark from activating a switch or heat produced from a pump could be all it takes to ignite the fuel and create an explosion or fire. In spite of the death toll, there has been far less interest in dust explosions and their prevention than one would expect.
The Chemical Safety Board (CSB) is a federal agency that investigates industrial chemical accidents and hazards. In a report issued in November 2006, it reports that there are lax or nonexistent government regulations, a haphazard warning system, and widespread ignorance about the dangers that dust poses.
What is a dust explosion?
A dust explosion occurs when fine dust suspended in air is ignited, causing a very rapid burning with a release of gaseous products and subsequent pressure rise of explosive force that can damage plant, property, and people. Dust explosions can be categorized as either primary or secondary.
A primary explosion takes place in a confined atmosphere, such as a cyclone, storage silo, or enclosed part of the manufacturing plant. After detonation, the shock wave can damage and often rupture walls, allowing burning dust and gases from the explosion to be expelled into the surrounding area, disturbing settled dust that may have accumulated. Once airborne, this dust can support a larger secondary explosion. Secondary explosions can cause severe damage to surrounding plant buildings. All large-scale dust explosions result from chain reactions of this type.
For a dust explosion to take place, several key conditions must be present:
The dust must be combustible and fine enough to be airborne;
The dust cloud must be of explosive concentration, i.e. between the lower and upper explosive limits for that particular dust;
There must be sufficient oxygen in the atmosphere to support and sustain combustion; and,
There must be a source of ignition.
While the range of characteristic values within a group is wide, there are not substantial differences among groups. Whether considering metallic, carbonaceous, plastic, chemical, or agricultural dusts, the values reflected in the table are typical. The table shows a significant difference between Class I gas or vapor hazards and Class II dust hazards. Though other characteristics are similar, the ignition energy of easily ignited industrial dusts is 20 times greater than typical Class I Group D materials. Only a few dusts, such as zirconium and thorium hydride, ignite at energies below 10 mJ. These dusts will ignite spontaneously at room temperature under some conditions.
Besides removing the dust, there are ways to reduce the chances of an explosion. One is to reduce available oxygen. Another is to dilute the fuel by adding moisture or dry inert material. The diluent used to reduce oxygen content is usually carbon dioxide, which is more effective than nitrogen. At high temperatures, water vapor is as effective as carbon dioxide. However, many metal dusts react with water, and moisture may increase explosion severity. Argon and helium are preferred as diluents for most metal dusts, since many metals react with carbon dioxide or nitrogen. Still, hydrides of thorium, uranium, and zirconium are preferably inerted with carbon dioxide. The key is to find the right combination of product and diluant for effective protection.
Electrical devices; prevention
There are two objectives in Class II locations: Keep the dust away from ignition sources, and prevent ignition of dust that accumulates on the device. There are five commonly used methods to accomplish this:
Dust-ignition-proof enclosures (Division 1);
Dust-tight enclosures (Division 2);
Purging (Division 1 and 2);
Sealing (Division 2); and,
Intrinsic Safety (Division 1).
Dust-ignition-proof, and dust-tight enclosures are described in detail in the 2005 NEC (National Electric Code).
Purging to reduce hazard in Class II areas is recognized in the NEC, which refers to NFPA 496 2003 for design and installation details. Purging involves supplying enclosures with compressed air or inert gas at the proper flow and pressure in order to reduce the hazardous gas or combustible dust inside the enclosure. Most purging applications require a minimum enclosure pressure of 0.10 inches (2.5mm) of water. (One psi is equal to 27.7 inches of water.) In some circumstances, a minimum enclosure pressure of 0.50 inches (12.7mm) of water is required to protect against ignitable dust. But in all cases, a higher enclosure pressure should be maintained to create a reasonable safety factor. In rare circumstances, enclosure pressures as high as 2.5 inches (63.6mm) of water may be required to offset sudden atmospheric pressure fluctuations, such as those created near missile launchings or offshore drilling platforms.
For Division I applications, the loss of pressurization requires the disconnection of power to the enclosure. For Division 2, loss of pressurization allows power to remain on provided an audible or visual alarm notifies the operator of the condition. Motors, transformers, and other devices that are subject to overload must have automatic means to be de-energized if temperature exceeds the design limits. Use of cooling devices in the enclosure should also be considered, and vortex coolers provide a very inexpensive solution. However, the pressurization gauge on the pressurization system must be specified or reprogrammed to compensate for the added induced air pressure within the enclosure.
According to NEC 2005 Article 500, the philosophy of the requirements is that if a Division 1 location device is dust-tight and the circuits leading to it are intrinsically safe for Class I Group D locations, the device is intrinsically safe for Class II Group F and G locations. For Group E the circuits are only Division 1 locations. There has never been a documented accident or explosion where intrinsic safety technology has been properly implemented.
Various explosion protection methods are permitted, allowing combinations of measures that present the best engineering and financial solution. The most commonly used protection methods are explosion relief venting (rupture disks) and explosion suppression. Others include containment and oxygen concentration reduction. Many rupture disk manufacturers monitor the disks electronically though intrinsic safety barriers and purged cabinets in Class II, Class III, and Class I areas. This way an alarm signal can be activated and automatically shutdown equipment to end the supply of combustible material.
Avoid the dust
Preventing and controlling the dust hazard involves basic practices and training:
Maintain effective housekeeping. If dust is not there it cannot ignite as a layer or be dispersed as a cloud. Maintain handling equipment to reduce leaks and keep dust inside. Clean up any dust that escapes. Even small accumulations of dust (as little as 0.031 in.) can create an explosion hazard if spread over sufficient surface area;
Conduct workforce training and education regarding recognition and control of combustible dust hazards;
Design machinery and plant layout to minimize damage if an explosion occurs. Use flame arresters to prevent flame spread and vents to relieve pressure and reduce structural damage;
Use intrinsically safe wiring practices;
Locate electrical equipment that could cause problems in purged cabinets; and
Detect the early pressure rise when an explosion occurs in a closed system and quench it with an inerting material.
These simple and common-sense approaches can cut and virtually eliminate the risk of explosion, injury, and damage.
|Source: Pepperl+Fuchs with values reported by U.S. Bureau of Mines.|
|Cloud ignition energy||5 mJ and higher|
|Minimum explosive concentration||0.02 oz/ft3 and higher|
|Maximum pressure developed||30-150 psi|
|Rate of pressure rise||less than 15,000 psi/sec|
|Ignition temperature-cloud||200°C and higher|
|Ignition temperature-layer||150°C and higher|
|Joseph A. Kaulfersch is a market analyst for Pepperl+Fuchs Inc. Reach him via email@example.com .|