Risk-based design of a regenerative thermal oxidizer

Following an explosion in a piece of equipment [a thermal oxidizer (Loss Prev. Bull. 2003, 170, 8-10)] employed for the purification of gas streams contaminated by volatile flammable compounds, a similar plant that is under construction was redesigned with a risk-based approach, to increase both the reliability and operational safety of the regenerative thermal oxidizer (RTO) and venting collection system (VCS). This paper, in particular, describes the methodologies that were used and the results that were obtained in terms of the reduction of the frequency of occurrence of the top event (TE) "Explosion in the venting collection system". A recursive operability analysis (ROA) was performed to identify "primary events" that could lead to a possible explosion in the RTO or VCS, with their evolution toward the TE. Fault trees were then directly derived from the ROA tables, through an intermediate logic diagram, the incidental sequence diagram, and then quantified in order to assess the frequency of occurrence of a possible explosion. The modifications that were suggested, thanks to the risk-based approach, should reduce this frequency from 1.03 to 5.27 x 10(-3) occurrences/year.