RECURSIVE OPERABILITY ANALYSIS OF SYSTEMS WITHMULTIPLE PROTECTION DEVICES

摘要

Recursive operability analysis (ROA) is a powerful hazard evaluation method particularlyrnsuitable for the safety analysis of systems with many protection levels activated by the samernprocess variable, since it enables the propagation of deviations of this variable along thernprocess lines to be readily determined, and hence the correct sequence in which the means ofrnprotection must intervene. In situations of this kind, indeed, it is more versatile than a classicrnOA with guide words, especially when such means are designed to operate at differentrndeviation levels. This paper illustrates ROA's versatility through its application to a 900 kg/hrnliquid chlorinated wastes furnace consisting of a primary and a post combustion chamberrnfitted with temperature and oxygen control devices.rnThe system as a whole is controlled by a DCS, which governs the auxiliary fuel flow rate andrnthe primary air flow rate set point in function of the furnace temperature, and regulates thernsecondary air flow rate to control the oxygen percentage in the flue gas. All these operationsrncan also be performed manually.rnInterlock systems are installed for two high and low temperature levels in the post combustionrnchamber and two low oxygen percentage levels in the flue gas. High and low temperatures inrnthe primary chamber are monitored by an alarm with an interlock system and an alarmrnrespectively. The paper describes the application of ROA in the safety analysis of thisrnincinerator through an assessment of deviations of the following process variables: primaryrnair flow rate, flow rate and lower calorific power of the chlorinated hydrocarbons (CHC),rnand furnace temperature. It is shown that analysis requires a thorough knowledge of thernsystem and the thermodynamics of its processes, and precise determination of how thernvariables evolve, especially when they are correlated, as in the case of temperature andrnoxygen concentration. The ways in which deviations can develop in quantitative terms and therncorrect sequence in which the means of protection must intervene are deduced from a correctrnstudy of the process thermodynamics.