Evaluating the Potential for Cold Brittle Fracture in Ethylene Plant through Dynamic Simulation

Cold brittle fracture of process vessels are typically the results of a transient condition in the process that leads to a low temperature excursion below a point where the material of construction of the vessel loses its ductility and becomes fragile. The suitability or ability of process vessels to sustain low temperature conditions can be evaluated by API 579-1 Fitness-For-Service. The use of API 579-1 for establishing operating limit on process vessels has been well discussed by other papers at the Ethylene Producer Conference. However, little has been said on predicting process conditions that may lead to embrittlement. Scenarios believed to cause low temperature excursion are frequently identified during safety reviews such as HAZOP or What-If sessions. Some of those scenarios may involve complex systems such as distillation trains and refrigeration systems. Improper procedures or safety systems may get implemented without properly understanding the transient conditions resulting from those scenarios. This paper discussed the use of dynamic simulation for evaluating the credibility and consequences of scenarios with potential of causing cold brittle fracture. Transient conditions such as startup, sudden de-pressure and loss of heat sources are discussed.