cpp SPECIAL POWTECH Pictures: Rembe Everything you need to know about explosion safety SAFETY SYSTEMS AND METHODS AT A GLANCE Whenever a production process generates dust, the plant concerned is potentially at risk of dust explosion regardless of whether it involves screening, drying, milling or filtration. The article gives a brief but comprehensive description of the basics of explosion safety and the protective systems available for this purpose. 8 cpp 03-2017
Stage 3 of a risk assessment: a matrix showing the potential risks of individual plant components is set up based on the first two stages Explosion vents differ from one application to another and precautions mostly involve the use of rectangular products Explosion safety covers a broad range of applications. To decide whether and to what extent precautions are required, you must initially conduct a systematic risk assessment. The first step is to gauge the probability that an explosive atmosphere might be created and determine whether any potential sources of ignition exist. Next, the effect that an explosion might have has to be classified. Both assessments can be merged into a matrix. The resulting indicators tell you whether or not the plant or parts of it have to be protected and if so, how. A green box means that no precautions are necessary. A red box, on the other hand, signifies that precautions are urgently needed. The higher the numerical rating, the more extensive these precautions should be. How explosions occur An explosion requires the availability of oxygen from the air, a source of ignition and a combustible substance. In the event of a dust explosion, the distribution of dust is another factor. However, not every dust-air mixture is explosive. What matters is the mixing ratio. Specific explosion limits have been identified for every common type of dust, and within those limits an explosive mixing ratio can be expected. The lower explosion limit is the minimum concentration required to create explosive atmosphere. The upper explosion limit specifies the point at which the mixture becomes too rich, so that the atmosphere is no longer explosive. There also needs to be an effective source of ignition (TRBS 2152 Part 3, “Hazardous explosive atmospheres – avoidance of the ignition of hazardous explosive atmospheres”, page 2). Among the common sources of ignition are hot surfaces, electric sparks and glowing embers arising during the process. Explosion safety precautions Depending on the results of the hazard and risk analysis, a number of proven precautions need to be taken. They are divided into explosion prevention and explosion protection. Preventive precautions are designed to prevent explosive atmosphere and therefore to reduce the probability of an explosion. Wherever possible, combus - tible substances are replaced by substances that cannot produce an explosive mix. In addition, it is possible to overlay the substance-air mix with inert gases. This has the effect of lowering the content of oxygen from the air, so that no explosion can occur. Preventive precautions concentrate on the avoidance of effective sources of ignition. This includes, for instance, the use of suitable equipment to protect the product flow from impurities and monitor the system earthing with a view to preventing electronic discharges. Protective precautions involve reducing the impact of a possible explosion to a more moderate level, so that the resulting damage is less severe. Conventional venting using explosion vents, flameless venting, explosion isolation and explosion suppression are just a few examples. This kind of explosion safety is a vital necessity in virtually all plants for various reasons: • It is in the nature of the relevant processes that there can almost never be absolute or complete avoidance of effective sources of ignition. • Inerting tends to be too expensive and/or impossible due to the characteristics of the processes involved. Other preventive precautions may be helpful in certain respects, though they cannot normally eliminate the risk of explosion completely. Conventional venting If a plant is situated outside a building or if parts of it are next to an outer wall, one frequent safety precaution is to install explosion vents. Such precautions are usually applied, for instance, to stationary silos, filters and elevators located outdoors. If an explosion occurs, the explosion vent protects the system by opening. This reduces any overpressure within the vessel and the explosion is released to the outside. As virtually no two processes are the same, there are numerous types of explosion vents which differ in shape and material as well as in their resistance to temperature, pressure and vacuum. Even processes with complicated hygiene requirements can be equipped with explosion vents today. The EGV HYP explosion vent from Rembe, for example, was highly successful in passing the EHEDG cleanability test. This test determines the inplace cleanability of plant components where this is a practical requirement for manufacturing absolutely hygienic products. cpp 03-2017 9
