Sugar is combustible and therefore presents an explosion hazard when it is finely divided and dispersed as a dust cloud in air. All that is needed is an ignition source.
Sugar fines from the handling of crystalline sugar can trigger an explosion. The finer the sugar, the greater the risk. So, confectioner/powdered sugar (i.e.- 10X), which has a particle size in the order of 60 microns, can be a particular hazard. The milling of the sugar to achieve this fine particle size presents a potential ignition source as well.
In facilities that handle or produce sugar, fines can accumulate in a number of areas such as production floors; along conveyor belts and on machinery; in hot rotary dryers used to dry sugar; in steel storage and conditioning silos; in dust collectors; and on beams, rafters, light fixtures and other horizonal surfaces.
When sugar is moved or disturbed—such as during startup, shutdown, loading or unloading—it is prone to become airborne with ample exposure to oxygen in the air. Add any ignition source, and the combination can result in devastating explosions.
In dust explosions, there is often a smaller initial explosion, followed by a larger secondary explosion. In such cases, the first explosion creates pressure waves that add turbulence and can increase dust loading, followed by a large ignition source.
Sugar dust fueled the Imperial Sugar refinery explosion in 2008. After the explosion, OSHA proposed the Combustible Dust Explosion and Fire Prevention Act of 2008, a bill that aimed to reduce dust explosion risk.
When viewing a plant or process it is helpful for plant operators and managers to ask three central questions:
- How can I prevent a deflagration event from starting?
- How can I mitigate the pressure created from a deflagration?
- How can I prevent a deflagration from propagating to another piece of equipment, or into the environment around the equipment?
The relevant NFPA codes that apply to sugar processing are NFPA 68 Standard on Explosion Protection by Deflagration Venting; NFPA 69 Standard on Explosion Prevention Systems; NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. NFPA 61 Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities also protects lives and property from fires and dust explosions in facilities handling, processing, or storing bulk agricultural materials, their by-products, or other agricultural related dusts and materials. Additionally, NFPA 652 Standard on the Fundamentals of Combustible Dust provides the minimum requirements to be met to achieve duct explosion protection, and includes the requirement for a Dust Hazard Analysis to be performed.
To protect process equipment and personnel, a hybrid of technical measures is often required. Among the options are passive devices such as explosion vents along with active devices such as explosion suppression equipment.
In addition, explosion isolation devices are vital to protect connected equipment and piping from propagating resulting in a secondary event, which can often be more dangerous and destructive than the initial event.
Dust explosion prevention
Explosions result from an ignition of dust when mixed with air during processing, handling, or storage operations. A rapid rise in pressure occurs in the containing structure, and if it is not of adequate strength to withstand the pressure, extensive damage and injury to personnel can occur.
Good housekeeping is essential since even relatively small amounts of combustible sugar dust can pose a dangerous explosion hazard. According to the NFPA, 1/32 of an inch of such dust covering just 5 percent of the surface area of a room “presents a significant explosion hazard.”
Equipment where airborne dust can accumulate includes mechanical conveyors. Whenever an enclosed conveyor is being filled or emptied, there is a potential dust cloud at that point that could be ignited.
Dust collection equipment such as baghouses are particularly subject to potential explosions since they typically handle the driest, finest dust in a process. Dust can also accumulate when conveyed or when product is discharged into silos.
Additionally, process equipment such as bucket elevators and hammermills can generate not only dust, but also provide an ignition source for a dust explosion.
The first step in mitigating the risk for a dust explosion is preventing an event from occurring. Careful housekeeping keeps the area free of dust and is a vital activity to protect both the building structure and personnel.
Identifying and controlling potential ignition sources is also critical. While ignition sources cannot be totally eliminated, they can be significantly reduced. Techniques include monitoring belt alignment, slippage, motor drive overloading, and bearing temperatures. Preventative maintenance on equipment such as rotary airlocks as well as checking for product accumulation within processing equipment can also help to reduce sources of ignition.
Dust explosion venting
During the early stages of a dust explosion, explosion vents open rapidly at a predetermined burst pressure, allowing the rapidly expanding combustion gases to escape to the atmosphere and limiting the pressure generated inside the process equipment to calculated safe limits.
Venting is the most widely adopted protection mechanism because it provides an economical solution and is often considered as a fit-and-forget solution. However, it is important to note that vents need to be regularly inspected per NFPA 68. Product build-up from materials such as solidified sugar can prevent explosion relief devices from operating effectively.
For decades, explosion vents have traditionally been designed using a “composite” approach that sandwiches plastic film between more resistant stainless-steel sheets with holes, or slots, cut into it. These vents are designed to “open” at typically 1 to 1.5 PSI set pressure.
With this type of technology, the holes and slots in the stainless-steel sheets can admit particulates and debris over time. The build-up can eventually affect the functionality of the vent. A vent that becomes heavier in weight will open slowly and less efficiently.
A better solution is a single-section explosion vent, comprised of a solitary sheet of stainless steel in a domed configuration. Perforations around the perimeter aid opening at the desired low set pressure and are protected with gasket materials.
The single-section domed design produces a vent that is more robust, lighter in weight and also largely eliminates the potential for build-up or contamination.
Venting is not limited to process equipment. It should be considered for building volumes in which it is not possible to adequately control fugitive dust.
Despite its popularity, explosion vents will not work for every application. With venting, the combustion process releases a large ball of flame into the atmosphere.
While this might be an acceptable consequence for outdoor equipment such as silos, for applications within a plant it could endanger personnel or equipment, and even lead to a secondary explosion.
In cases where a flame ball must be avoided, flameless venting can be deployed. Flameless vents are designed to absorb the pressure wave and eliminate the flame that would normally be projected by a vented explosion.
To address this need, companies like BS&B Pressure Safety Management provide a flameless system designed with the vent installed inside a housing which incorporates a flame arrestor. In dusty, sugar environments, a cover is recommended to prevent dust from infiltrating the stainless-steel mesh through which the pressure is released, in this style of vent.
Explosion suppression equipment
For processes where an explosion would ideally be prevented altogether, suppression systems are the ideal alternative. Explosion suppression equipment detects a dust explosion in the first milliseconds of the event and then signals explosion suppressors to rapidly release a flame quenching medium, such as sodium bicarbonate, into the process equipment. This effectively stops the explosion in its infancy and results in a reduced explosion pressure that is safe for the protected equipment.
For a 24/7 process, a suppression system can be very desirable, because the speed of clean up and refit allows for a quick return to production. With venting or flameless venting, the explosion fully develops in the process equipment, requiring cleanup, fire-related damages and other consequences that take time to get the process back into operation.
A typical suppression system consists of sensors and several explosion suppression “cannons” which propel an extinguishing agent, such as sodium bicarbonate, into the process equipment. Pressurized nitrogen is typically used to provide the motive power.
The third consideration is explosion isolation, which protects interconnected equipment in the event of an explosion. Ducting and piping connecting process equipment can propagate an explosion of even greater intensity and this is why isolation is called for in NFPA 654. If unprotected, the ducting, piping, as well as all the connected vessels and equipment are at risk.
Broadly, explosion isolation can be categorized as passive or active per NFPA 69. A common example of passive isolation is a flap valve that is essentially a one-way valve installed on the inlet duct to a dust collector. The flap is open during normal operation and latches closed against a seat in response to the cessation of airflow and a pressure wave travelling in the opposite direction. These valves must be mounted horizontally and due to pressure piling, must be mounted on a duct having a strength twice that of the reduced explosion pressure for the equipment it is isolating.
Flap valves are not suitable for applications where the product handled can stick, cake, or otherwise accumulate. Such accumulations can prevent the flap mechanism from moving freely and seating properly in the event of a deflagration. For this reason, flap valves are not the ideal solution for sugar applications. Another example of a passive isolation device is a suitably designed airlock to isolate a hopper in the event of a deflagration. These devices must conform to the requirements of NFPA 69 to provide adequate isolation. Not all rotary airlocks are equal in this regard.
Chemical isolation systems overcome the application limitation of flap style valves. Chemical isolation is an active isolation method that typically consists of an explosion pressure detector that triggers a chemical suppressor. Chemical isolation is not limited to horizontal ducts or air flow direction. Furthermore, chemical isolation can be used on rectangular ducts and casings with moving internals such as drag conveyors. Additionally, this method of isolation provides the more economical solution for large ducts.
Dust explosion prevention and mitigation systems must be tailored to the application and the specific equipment used. Careful attention to prevention, mitigation and isolation will ensure the protection of both personnel and plant while limiting the potential for preventable business interruptions.