Preventing dust explosions
February 1, 2008
While there are no guarantees when it comes to dust explosion prevention, the grain storage and processing industries can improve their chances of protecting against such a devastating accident by taking a few practical measures.
It is likely that the completed risk assessment (see part one of this series in January 2008 issue of World Grain) will have highlighted a number of deficiencies throughout the plant where explosion protection is inadequate.
The busy plant manager, having already spent considerable time and effort on the risk assessment, and struggling with finite resources and day-today plant management demands, can feel overwhelmed by the prospect of the task ahead.
At this stage, further progress toward achieving adequate explosion protection can slow or stop completely. To ensure progress, it is important to develop an Explosion Protection Plan consisting of a three-step approach that includes the following:
• Identify the areas of highest risk to personnel and your business by categorizing them as low, medium and high risk.
• Determine the probability of an explosion occurring into low, medium or high probability.
• Prioritize based on the areas of highest risk and highest probability being tackled first.
Examine the improvement options available and determine which option is best, taking into account local circumstances, complexity of the solution, and cost. As mentioned before, some solutions may only require a change in procedures or working practices, such as improving housekeeping or better maintenance planning. The reasoning and rationale behind the decision should be recorded.
It is recommended that this step is carried out with the support of a competent person that is experienced and knowledgeable in the field of explosion protection and your particular industry. In a nutshell, the objective is to identify solutions that will achieve a basis of safety, providing maximum safety and simplicity at minimum cost. You then must decide how and when the solutions will be implemented. The timing will, of course, depend on local circumstances and resources available.
Draw up a plan that will include targets and milestones to be achieved. Aim to stick to the plan until you achieve your goals, remembering at each step you are improving safety. Remember to regularly review your progress against the plan.
PREVENTION AND PROTECTION MEASURES
Ideally, formations of dust clouds and sources of ignition should be prevented completely. In practice, however, this is often not possible or practical in the grain industry.
Usually the best that can be practically achieved is a reduction in dust clouds by good plant design and housekeeping and control of sources of ignition by technical measures, such as bearing temperature and belt slip detectors, as well as procedural measures.
Despite controlling sources of ignition, there will be areas in the plant where there is a residual risk of an explosion. Therefore measures must be taken to protect against the worst effects of an explosion should one occur. The protective measures employed include:
• Containment — The vessel or plant is built strong enough to withstand the effects of an explosion. High pressures of approximately 118 psi with cereal dusts are typical. Containment can be a useful and practical option for small vessels and vessel connections but is rarely a practical or cost-effective option for standard equipment in grain plants, although heavily built equipment such as hammermills and rollermills will likely be strong enough to withstand the effects of an explosion within the machine itself.
• Isolation — Explosions are isolated from other parts of the plant by mechanical or chemical means to prevent explosion propagation. Equipment used for isolation are rotary valves, screw conveyors with baffles and material plugs, fast acting shut-off valves, passive barrier valves and chemical barriers.
• Suppression — Suppressant material, usually in powder form, is rapidly injected into an explosion as soon as possible after ignition is detected to quench the explosion at its source. Optical and/or pressure sensors are used to detect the explosion and activate the release of suppressant. It is important that the suppression system is well designed for the particular application to avoid false activation.
• Venting — Extremely common in grain handling facilities, venting consists of weak panels designed to rupture in the event of an explosion. These panels are fitted to vessels to relieve the explosion to a safe area. Where vents cannot relieve to a safe area, flameless vents can be usefully employed. These vents are fitted with membranes or diaphragms that are de- signed to relieve the pressure in the event of an explosion, quenching the flame as it passes through the membrane.
Usually a combination of containment, isolation, suppression and venting will be employed throughout the plant. The precise selection of equipment will depend on a detailed assessment of the plant to be protected. Some countries have standards, codes and guidance for explosion protection, and these should be followed.
HIGH-RISK PLANT ITEMS
Two very common but potentially high-risk plant items that are often associated with dust explosions are the bucket elevator and the dust filter. These items are often inadequately protected, but protection measures can be employed to significantly improve safety in these key pieces of equipment.
The potentially explosive dust concentration within the elevator casing combined with a potential ignition source from the mechanical movement of pulleys, belts and buckets can be a lethal combination. To improve safety, it is recommended that the following steps be taken:
• Fitting of sensors — To detect belt slip, belt/bucket misalignment and bearing temperature, these sensors should be incorporated into the plant control system to take action if a fault is detected. Stand-alone control systems are also available.
• Explosion venting — In addition to the sensors, the bucket elevators should be adequately vented to a safe area. The position, sizing and burst characteristics of the vents and the vent ducts will depend on the elevator size, design and the flammability characteristics of the dust. The vents should be fitted with burst switches.
• Explosion Propagation — To prevent an explosion in the bucket elevator propagating to other areas of plant, both through the product stream (via the elevator inlet and outlet) and through any connected dust control pipe work, it is recommended on the product inlet and outlet to fit rotary valves or screw conveyors with plugs/baffles. On the dust control pipe work, fitting of fast-acting shutoff slides is recommended. Alternatively, it may be more cost-effective to fit independent spot filters on the bucket elevator itself.
Within dust filters, there is a high risk of an explosive atmosphere. In centralized dust control systems, sources of ignition can be introduced through the dust control pipe work into the filter from other items in the plant. Preventive measures to be taken include:
• Explosion venting — As with bucket elevators, filters must be adequately vented to a safe area with vents of the correct size and bursting pressure. The vents must be fitted with burst switches. It is important that filter bags do not block the explosion vent. Many filters are now fitted with vent panels located beneath the filter bags.
• Explosion propagation — Explosions can propagate with devastating consequences from the filter itself through the air inlet pipe work to the interconnected plant, for example, in a centralized dust control system. One simple and effective method to prevent this is with a passive barrier valve fitted in the dirty air inlet pipe work close to the filter. The design of the valve allows air to flow in one direction only (into the filter). In the event of an explosion the valve closes, preventing the explosion and flame front from traveling out of the filter to other areas of plant.
A rotary valve of the correct type with close-fitting vanes fitted to the filter hopper outlet will prevent an explosion propagating from the product side of the filter. The rotary valve should be stopped in the event of an explosion.
By combining a structured, methodical, step-by-step approach with the application of specialist equipment and practical advice, the plant manager can effectively manage the threat posed to personnel and businesses from dust explosions.
Ged Begley is a freelance grain handling and milling technologist based in Scotland. Ged has worldwide experience in cereal milling and has specialist knowledge and extensive experience in wheat debranning, wheat gluten processing and the Scotch Whiskey Industry. He can be contacted at Begleyconsulting@aol.com . Tel: + 44 1292 280723