Preventing Dust Explosions

Household items, such as breakfast cereal, flour, corn starch and sugar are so common that many were left surprised after the 2008 combustibledust explosion at a sugar refinery in Georgia. Heard in much of the early media coverage was the question “How could sugar explode?” Plant explosions are, thankfully, not routine occurrences. However, when they do occur, it is the unusual event — often a combination of abnormal events — that is typically the trigger. This article outlines some of the guidelines that are available to help prevent dust explosions. It also offers a list of factors to consider when choosing explosion vent technology to minimize combustible-dust-explosion hazards. Its focus includes the following three technical areas of risk management:
1) Dust control
2) Ignition control
3) Injury and damage control

Dust control To summarize, dust control is achieved by the implementation of appropriate combinations of housekeeping and dust collectionand- filtration measures. These measures are combined with inspection to ensure that they remain effective. A layer of dust that is only 1/32-in. thick is a concern when it covers 5% or more of a workplace floor area that has dust-laden structural members, such as joists and I-beams. Successful dust control reduces fugitive emissions of combustible dusts, which in turn cuts the dust explosion risk. Quantification of a combustible dust risk requires that the characteristics of the material be identified by test. Key parameters include the maximum explosion pressure (Pmax), the [You must be registered and logged in to see this link.] and the minimum ignition energy (MIE) of the dust. Prevention and protection strategies must also be based on knowledge of process conditions, such as dust concentration, airflow velocity, operating pressure, temperature and humidity. The higher the Kst value, the faster the rate of pressure rise due to combustion. Kst values above 600 are considered extremely explosive. Most common plant dusts have Kst values between 100 to 150. Particle size can profoundly affect explosive properties, as the finer the dust, the higher the Kst value. For example, sugar has a recorded Kst value of about 138. Of more concern with regard to the potential damage to process equipment is the fact that a fine-sugar-dust explosion will generate a pressure in excess of 100 psi within an enclosed volume in less than 100 milliseconds. Other key parameters include particle size, with smaller particles equating to more rapid combustion and easier ignition. The reason is that more surface area is freely accessible by the surrounding air to support combustion.

Ignition control Understanding and monitoring for potential ignition hazards at a plant provides a first layer of protection to the facility. Essential measures include proper grounding and bonding of equipment and ducting, appropriate wiring of electrical equipment, the control of static electricity (see Avoiding Static Sparks in Hazardous Atmospheres, CE, June 2009, pp. 44–49) and monitoring of process equipment hot spots. Preventive maintenance programs ensure that such design safety measures remain effective. Review of dust-explosion loss history proves that it is important to consider “normal” and potential “abnormal” circumstances while evaluating processes within a facility. Many times a dust explosion is the result of an abnormal event, such as when an automated process fails and it is replaced by a temporary, manual activity. Other times it could be a consequence of a change in product packaging.
For example, an operator at a manual bag-emptying station receives a shrink wrapped pallet of bags. Instinctively, he walks toward the pallet, across a concrete floor, while wearing appropriately specified conducting shoes. The surrounding metal equipment is grounded. The operator stands on shrink-wrap-packaging material (an insulator) as it is unwrapped, tears open a bag of material and empties it into an adjacent hopper. As the dry powder flows, static charge builds on the now insulated operator until there is sufficient electrical potential to release a spark having sufficient energy to ignite the dust present in the emptying station.

Injury and damage control Referenced techniques fall into the following four areas:
• Prevention: Detection of sparks and embers traveling through a dust collection system can lead to their quenching before reaching an area of high dust-explosion risk, such as a filtration unit
• Isolation: Introducing a barrier to flame propagation can prevent a primary dust explosion in one piece of equipment from amplifying into a typically more-severe secondary event in a connected item of equipment. Solutions include chemical isolation (particularly well-suited to large or irregular-shaped ducting) and mechanical isolation barriers (Figure 2), such as pinch valves, knife gate valves and rotary airlocks
• Venting: Pressure relief is provided to process equipment and to building structures by releasing the products of combustion to the atmosphere in a safe trajectory. Flameless venting provides for over-pressure protection without the release of flame or particulates to the atmosphere
• Suppression: Explosion effects can be minimized by injecting a flame quenching agent into process equipment to arrest the combustion process — explosion suppression equipment responds rapidly to prevent the full development of a dust explosion, preventing a destructive overpressure from developing (Figure 3) In addition, an emergency action plan with properly maintained exit routes ensures the right safety response for protection of personnel.

Understanding the risks

The course of action to minimize combustible- dust-explosion hazards begins, preferably, in the planning stages of a process by identifying the risks, determining the explosive reactivity of the combustible process dust, and implementing a combination of both prevention and protection measures.
Economic considerations favor the use of explosion vent technology in terms of cost of equipment and installation. However, many factors must be considered before choosing explosion vents.
Consider the following application factors: Can the flame ball that is ejected from an open vent be accepted? It will usually extend 30 to about 100 ft in length and about half this in diameter. An 1,800-ft3 vessel venting from a single position will produce a flame ball over 100 ft in length and 50 ft in diameter. Simple free venting must be directed to a safe location where personnel will not be present and other equipment cannot be damaged (Figure 4).
If venting equipment is installed indoors, can a vent duct to a safe outdoor location be provided? Vent ducts will always increase the required vent area, and their use may not allow smaller process volumes at higher Kst values to be protected by venting at all.
Can the required vent area be accommodated? As well as requiring the space for vent installation, can the reaction forces during venting be sustained and, for tall equipment, can a nearthrust, neutral vent arrangement (topsidewall- mounted vents) be achieved to prevent collapse during relief? If there are process inlets and outlets to the protected equipment, are these protected to prevent propagation of the dust explosion to other equipment or work areas? Standards are now very clear in requiring isolation of vented equipment to prevent secondary explosions. Secondary explosion risks are typically much greater in their potential for damage and destruction.
Can the clean up of a vented explosion be accepted? Depending upon the design basis adopted, a vented explosion may require replacement of capital equipment components that have become damaged by the pressure wave, resulting in loss of production while delivery is awaited. Most vented equipment is designed with a pressure relief area that prevents failure of the equipment structure. A higher vent area is always required to protect equipment within its design pressure.
What will the neighbors think? A vented explosion is a spectacular event that will draw considerable community attention.
What if the process material is toxic or hazardous? A vented release must simply be avoided for certain materials. As illustrated by the previous series of questions posed for a vented-dustexplosion application, each process needs to be considered both alone and as a component of a production facility to ensure that the right explosion protection and prevention technology is implemented. There are always options for dust-explosion-risk management. The technical solutions adopted must take proper account of the practical needs of each process and the consequences of safety system operation.
Some final rules of thumb include: plan for the abnormal; and maintain a strict management of change policy that will catch the potential consequences of product material changes, hardware changes, and procedural changes. Dust-explosion-risk management requires periodic detailed review, even at the best-protected facility.