Variation of the explosion risk in a hybrid collector during revamping operations

Dusts removal from gaseous streams is a very common operation carried out in a number of industrial plants, e.g. those ones dedicated to the manufacturing of cement and steel, in order to meet the more increasingly stringent law requirements. Therefore, in the last decades, such industries have been to face more and more often the necessity to improve their de-dusting systems inspired by the principle of the “maximum safety technologically feasible”, but also pushed by the increased penetration of environmental issues into the public opinion. Unfortunately, the budget dedicated for all these improvements is often very low because de-dusting is, in the major part of the practical cases, an operation having a low influence on production performances; therefore it is seen more as a “necessary cost” than a profit generator. The unavoidable consequence is that plant managers investments are dedicated to other more fruitful processes than de-dusting, addressing the low budget available for gas stream purification to the so-called revamping strategies: that is, the reuse of existing de-dusting plants, either by enhancing their efficiencies through the use of various technical tricks or by adding another pieces of equipment, rather than install new and more technologically advanced plants. This solution is considered very interesting in terms of profit since the costs for the decommissioning of the obsolete equipments does not exist (in fact the old plant remains). Obviously, all the economic benefits derived from these strategies are counterbalanced by a series of technical disadvantages. In particular, a systematic risk assessment of safety of the whole new configuration of the revamped plant is necessary. In fact such an evaluation can not be limited to a safe design of the new installed equipment, but it has to be extended to the already existing equipment also considering the impact that changes in process conditions (induced by the new equipment) can have on the whole plant. In this work, the relevant problem of an explosion in the de-dusting section of a cement plant, due to the establishment of an overpressure inside the apparatus, has been considered. Particularly, using fault tree analysis, it has been evaluated the changing in the overall risk (considering, for simplicity, one year of mission time) of explosion referring to a hybrid-like collector realized by introducing a Fabric Filter (FF) downstream with respect to an Electrostatic Precipitator (ESP). Results have shown that a chain of failures in the FF section may affect relevantly the explosion risk in the collector leading to the unavoidable need for the introduction of mitigation actions into the system.