(105d) Cold Brittle Fracture Potential on Cold Box Outlet Piping

A material specification change from stainless to mild steel at the outlet of a cold box has been identified during Process Hazard Review (PHA) as a potential for cold brittle fracture. The cold box outlet has 1 ethane, 1 hydrogen and 3 methane streams, which are warmed up by 40oC propylene in the last core exchanger. A number of scenarios likely to cause a temperature below embrittlement were identified during the PHA resulting in the recommendation of a Safety Integrity Level (SIL) 1 interlock to protect the piping against low temperature. A more thorough analysis of the cold box behavior and review of past plant upsets demonstrated that cold temperatures were more likely to occur during the recovery effort rather than during the upset itself. A set of more realistic conditions likely to cause temperatures below the point of embrittlement was then developed and used in a SIL analyses. This initially suggested that a level 4 SIL would be required but this was later dropped to a level 2 by introducing additional safeguards. The SIL exercise was based on the assumption of pipe failure on the first occurrence of temperature below embrittlement. However, such temperatures have occurred on a number of occasions, which indicates that one of the three elements (cold temperature, sufficient stress and flaw in the material of sufficient size to initiate a fracture) required for brittle fracture to occur was absent. Based on this latest observation a parallel exercise has been initiated to assess fitness for service of the existing piping as described in API Recommended Practice 579 Fitness For Service.