(88d) Risk Analysis for Operation of Aluminum Heat Exchangers Contaminated by Mercury

Brazed aluminum plate-fin heat exchangers are extensively used in gas separation processes including LNG, LPG, NGL, nitrogen rejection and olefins manufacture. In situations where mercury is a trace component of feed gas or liquid feeds to crackers, condensation of liquid or precipitation of solid mercury can occur in heat exchanger passes, even with functional mercury removal systems in place. Mercury in liquid phase causes, under certain well-defined conditions, liquid metal embrittlement of susceptible metallurgy or amalgam corrosion of core fins, both of which can lead to sudden loss of pressure containment. Mercury contaminated aluminum heat exchangers require close scrutiny and quantitative risk assessment to allow safe operation, remediation or to justify replacement. The risk analysis procedure involves computational prediction of mercury deposition, inspection of critical areas, detailed assessment of metallurgy and fabrication, strain analysis of temperature changes during trips and shutdowns and oxide fatigue analysis. Differentiation of leak and rupture failure modes can be accomplished based on calculated amount of deposition and on location of mercury deposits as determined from focused inspection. Failure statistics are essential to assignation of probability-based risk factors. A description of currently practiced risk assessment procedures will be offered along with case examples.