According to a 2009 incident report by the U.S. Chemical Safety Board (CSB), on Feb. 7, 2008, at about 7:15 p.m., a series of sugar dust explosions at the Imperial Sugar manufacturing facility (Fig. 1) in Port Wentworth, Georgia, USA, resulted in 14 worker fatalities and 36 injuries. The explosions and subsequent fires destroyed the sugar packing buildings, palletizer room and silos, and severely damaged the bulk train car loading area and parts of the sugar refining process areas.
Almost any organic material, metal or plastic can become combustible if reduced to small-enough particles—typically less than 420 microns. At this size, the dust becomes the fuel in the classic fire triangle (Fig. 2), which requires only oxygen and heat to burn. However, to create an explosion, two additional components are needed to fulfill the dust explosion pentagon (Fig. 3): dispersion and confinement.
While businesses can substantially reduce the threat of explosions through proper risk analysis and mitigation, the risk persists and has resulted in devastating losses. During a 33-year period ending in 2006, FM Global recorded 166 dust explosion property losses amounting to a gross loss of approximately US$353 million (in 2018 dollars). The woodworking industry experienced the highest number of losses, followed by food processing and metal processing. The number of losses in other industries, including chemical/pharmaceutical, pulp and paper, and minerals were also significant.
FM Approvals and FM Global have long worked to help reduce the property loss potential from flammable or combustible materials, including gases, vapors, dusts and fibers. FM Approvals’ standards and associated test programs help manufacturers verify the effectiveness and applicability of products and systems designed to help mitigate property losses from combustible hazards in the workplace.
Loss prevention investment
Over the past 10 years, FM Global and FM Approvals have invested in new facilities and staff to enable ongoing research and provide the basis for new and updated Approval Standards covering explosion protection products. New research and testing facilities and equipment are enabling FM Approvals to provide manufacturers with more cost-effective and comprehensive certification services that help reduce time-to-market and further differentiate the FM Approved mark from other global certifications.
A key example of this investment is the expansion of the explosion testing capabilities available at the 1,600-acre (647-hectare) FM Global Research Campus in West Glocester, Rhode Island, USA. The expansion included the addition of 2.5, 8 and 25 m3 (88, 283 and 883 ft.3) explosion test vessels, as well as associated diagnostic instruments and data acquisition systems. Within the framework of FM Global’s Strategic Research Program on Explosions and Material Reactivity, FM Global researchers set out in 2010 on a multiphase research effort covering three key explosion protection areas:
- Explosion Venting: mitigates explosion overpressure by providing a pathway for expanding explosion byproducts to escape
- Explosion Suppression: involves detecting and chemically suppressing an explosion in its earliest stages to mitigate explosion overpressure
- Explosion Isolation Research Underway: prevents an explosion from propagating from one part of an industrial process to another through the use of fast-deploying isolation valves and/or chemical suppressants
Companies often use a combination of venting, suppression, containment and/or isolation (Fig. 4) to protect vulnerable facilities or systems. In 2014, FM Approvals introduced Approval Standard 7730, Explosion Venting Devices, encompassing both standard and flameless explosion venting devices. FM 7730 was the culmination of a multiyear, collaborative standards development process that incorporated the new research noted above.
The next step is explosion suppression
Over the past two years, FM Approvals and FM Global Research and Engineering Standards have focused on explosion suppression systems, which are used to detect and suppress a developing dust or gas explosion at its earliest stages.
This latest effort culminated this year in a fully revised Approval Standard 5700, Explosion Suppression Systems, which will replace the original standard issued in 1999. The research at the core of this revised standard has led to a greater understanding of the effect of different factors (e.g., injection-to-ignition location and system activation pressure) on the final reduced pressure due to suppression and has led to the development of a methodology to evaluate the system-level performance of explosion suppression systems.
The new version of FM 5700 provides for the evaluation and certification of complete explosion suppression systems. The new standard includes performance tests to determine full-scale suppression performance, suppressant storage container tests, detector sensitivity, actuation device operation, hydrostatic pressure, corrosion resistance and other factors.
While FM Approvals certification will be based on the evaluation of suppression systems equipped with a single suppressant bottle, certified systems may be installed at larger scales, provided that the maximum protected distance, surface area and volume are all satisfied.
How it works
An explosion suppression system is designed to detect an incipient deflagration (i.e., combustion propagating at subsonic speed) and suppress it to prevent the full impact of the deflagration from developing. Typically, these systems consist of four basic components: the detector(s), a control unit, suppressant storage container(s) and suppressant dispersers.
An explosion is detected in the incipient stage, either by a rise in pressure or the presence of flame within the protected enclosure. Pressure sensors or infrared detectors are typically mounted on the protected equipment, along with the suppressant bottles, and linked to a control unit. An explosion takes only milliseconds to develop, so suppression systems must be able to react almost instantly (Fig. 5) when a spark or other ignition source ignites the combustible dust or gas mixture.
The control unit then actuates the suppressant dispersers to envelop and arrest the explosion. Most suppression systems use one or more suppressant bottles, which are mounted on a vessel or enclosure to be protected such as an industrial dryer, ductwork, conveyor, dust collectors, silos or cyclones. The suppressant bottle is pressurized and uses a fast-acting valve to disperse the suppressant—typically sodium bicarbonate or monoammonium phosphate—and quench the explosion, reducing the overall pressure rise to a level below the enclosure strength.
A closer look at FM 5700
Under the original version of FM 5700—introduced in 1999—an FM Approvals engineer had to travel to a manufacturer’s site to witness system testing, as the necessary explosion test apparatus was not yet available at the Research Campus.
“At that time, we relied on the test facilities of the manufacturers or third-party labs,” notes Brandon Bond, senior engineer for FM Approvals’ electrical group. “Although we witnessed every test, it was a less than ideal way to do it. Now that we have our own test facilities for these systems, we will be conducting full-scale explosion testing as well as component-level performance tests at our own facilities. All manufacturers of explosion suppression systems Approved under the original FM 5700 standard will be required to undergo a re-evaluation against the revised standard within three years.”
The newly revised FM 5700 standard includes the following performance and operations requirements:
- Open air discharge baseline – A fully charged explosion suppressor will be discharged (Fig. 6) and a high-speed video camera (1,000 frames/sec) will be used to record and characterize the growth and velocity of the suppressant cloud.
- Full-scale suppression performance tests – All full-scale suppression tests will be conducted in one of the Research Campus’ three explosion vessels (Fig. 7), depending on the suppressant capacity of the explosion suppression system. A minimum of four full-scale suppression tests will be performed.
- Performance testing of all mechanical and electrical components and subsystems, including:
- Suppressant storage containers (cylinders)
- Actuation device operation
- Cycle operation tests
- Hydrostatic pressure tests
- Hoses, mounting devices and pressure indicators
- Corrosion, vibration and aging
- Power supplies, batteries and controls
- Demonstrated quality control program
- Surveillance audit
- Manufacturing and production tests
- Installation, operating and maintenance manual review
“With the revision of FM 5700, we are providing a consistent and scientifically proven methodology for evaluating these important loss prevention systems,” Bond explains. “The incorporation of full-scale suppression tests in our own explosion vessels based on two years of research is the most significant new aspect of the revised standard.”
He continues, “Explosion suppression systems are composed of complex electro mechanical devices that must work together flawlessly to detect and react almost instantly to an ignition event. We selected and incorporated test methods from our extensive library of standards to accommodate the broadest range of suppression systems. We adapted some of these individual component test criteria to meet the unique requirements of these systems.”
Explosion Isolation research underway
The final phase in the current round of explosion-protection research is underway at the Research Campus and is focused on explosion isolation systems. These systems are designed to detect and react to incipient explosions—often within connecting tunnels or ducts—and cut off the deflagration from traveling to interconnected equipment.
Explosion isolation systems can be chemical systems that discharge a suppressant into pipes or ductwork to quench a deflagration as it tries to move out from the point of origin. These systems can also be mechanically active, using a powered high-speed valve or plate to contain the deflagration; or passive, in which an isolation valve or flap deploys in response to the deflagration pressure.
A new Approval Standard, based on this new explosion isolation research, is planned.