Recursive Operability Analysis of systems with multiple protection devices

Recursive operability analysis (ROA) is a powerful hazard evaluation method that is particularly suitable for the safety analysis of plants with multiple protection levels activated by the same process variable. The propagation of deviations of the variable along the process lines can be readily determined with this method, and hence the correct sequence in which the protection means must intervene can be appropriately followed. This paper illustrates the versatility of a ROA through its application to a 900kg/h liquid chlorinated waste furnace. The following process variable deviations were assessed: primary air flow rate, flow rate and lower calorific power of the treated wastes and furnace temperature. It has been shown that the analysis requires a precise determination of how the variables evolve, especially when they are correlated, as is the case with the temperature and oxygen concentration. The ways in which deviations can develop in quantitative terms and the correct sequence in which the protection means must intervene are deduced from a correct study of the process thermodynamics.