This article appears in the November/December 2018 issue of Sugar Producer.
Sugar presents a combustion hazard in its dry, dusty form. A fine sugar dust explosion can generate a pressure in excess of 100 PSI within enclosed process equipment in less than one-tenth of a second.
Where sugar dust and air are mixed together in a confined space, the consequences of a dust explosion are the greatest. Filtration equipment handling a combustible dust almost certainly contains a hazardous concentration of material under normal operating conditions. Other items of equipment may only temporarily carry a “combustible load” of hazardous material, perhaps during start up, shut down, loading or unloading. However, while that combustible load is present, the risk is no less severe.
If the potential for a combustible load cannot be eliminated, protection measures are required. However, when this is the case, many operators of sugar dust processing or handling facilities may not fully understand the steps that must be taken to mitigate those risks.
Because these safety precautions typically represent some capital expenditure, many owners and operators are seeking information regarding available technologies on the market.
Explosion Protection Devices
To protect process equipment and personnel, a hybrid of technical measures is often required. Among the options are passive devices like vents or containment systems along with active devices such as explosion suppression or spark detection and extinguishing systems. Chemical or mechanical isolation devices are required to protect connected equipment and piping from propagating to a secondary event, which can often be more dangerous and destructive than the initial event.
Explosion Vents
During the early stages of a sugar dust or gas explosion, explosion vents open rapidly at a predetermined burst pressure, allowing the combustion process to escape to the atmosphere and limiting the pressure generated inside the process equipment to calculated safe limits. Venting is the most popular and widely adopted protection mechanism, in part because it is economical and requires little attention or maintenance once installed.
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, a result of the hole pattern cut into the sheet metal. Over time, however, this type of vent is prone to tears in the plastic film, which is the seal between the process conditions and the atmosphere. With this type of technology, the holes and slots in the stainless steel sheets can admit particulates and debris over time. In addition to being unsanitary, particularly for food processors, the buildup can eventually affect the functionality of the vent.
“A vent that becomes heavier in weight due to buildup will open slowly and less efficiently,” says Geof Brazier, president of BS&B Pressure Safety Management, a manufacturer of a broad range of dust explosion prevention and protection technologies. “Ultimately, the pressure in the equipment will be higher than expected, which is a safety concern.”
In 1990, a single-section explosion vent was patented, comprised of a single 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. This design produces a vent that is lighter and more robust and largely eliminates the potential for buildup or contamination.
Despite their popularity, explosion vents will not work for every application. With venting, the combustion process will release a large ball of flame into the atmosphere that might be 10 times the size of the protected equipment for a few milliseconds. While this might be an acceptable consequence for outdoor equipment at a remote location, for applications within a plant compound or inside a building, it could endanger personnel or equipment, and even lead to a secondary explosion.
In cases where a flameball must be avoided, flame arrestors can be deployed. These devices are designed to absorb the pressure wave, flame and at least some of the dust that would normally be ejected by a vented explosion.
Three-dimensional flame arrestors applied downstream of explosion vents are heavy and can cause damage when installed directly on top of a vent. To address this concern, flameless systems have been designed with the vent installed inside the flame arrestor. In this process, the heavier flame arrestor is mechanically mounted directly to equipment, reducing the weight load transmitted through the more sensitive vent. It also serves the dual purpose of allowing for easy inspection of the vent and arrestor while installed.
Suppression Equipment
For processes where an explosion would ideally be prevented altogether, suppression systems are the best alternative. Explosion suppression equipment detects a dust explosion in the first milliseconds of the event and then signals extinguishing modules to release a flame-quenching medium into the process equipment. This effectively stops the explosion in its infancy and only low pressure that is safe for the protected equipment is produced.
“Suppression can be preferable for indoor equipment, simply because the explosion doesn’t really propagate; it starts, but it never evolves into a full-blown event,” says Brazier.
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. Nitrogen is often used to provide the motive power. Although some available systems combine nitrogen and extinguishing agent in a single canister, this means the cannon must be oriented vertically downward to discharge properly. If not, the extinguishing powder could be left behind.
In some suppression systems, the nitrogen and extinguishing chemical are kept separate until the instant of activation, allowing discharge cannons to be installed facing down, horizontally, or even upwards.
Separating the extinguishing agent and providing it in an easily replaced canister also means that on-site personnel can refit the system in case of an event. This can be a significant advantage given that some systems require a service technician to travel to the facility to reset the system.
Containment Systems
To “contain” a sugar dust explosion, process equipment is built to resist the pressures generated by the combustion occurring inside a sealed container. With this type of system, there is a high cost of construction, since the design pressure has to be typically over 100 PSIG, and sometimes over 150 PSIG, which is far above the typical required operating pressure conditions of the equipment.
In this type of system, the isolation devices play a critical role in preventing damage to connected equipment and piping. For this, a fast-activating knife gate or pinch valve device can be used, as well as chemical isolation systems. With containment systems, however, isolation equipment must also be able to withstand greater pressures, up to 150 PSIG, unlike similar equipment required with vents or suppression devices that must survive only 3 to 10 PSIG.
Regardless of the type and combination of equipment installed, the appropriate solution for each application is a hands-on, collaborative endeavor.
“There is always more than one way to achieve combustible dust safety,” says Brazier. “The expertise is in reviewing each option for a particular industrial process and arriving at a combination of technologies that is technically effective as well as cost-effective.”