A material specification change from stainless to mild steel at the outlet of a cold box hasbeen identified during Process Hazard Analysis (PHA) as a potential for cold brittlefracture. The cold box outlet has one hydrogen and three methane streams, which arewarmed up by 40oC propylene in the last core exchanger. A number of scenarios likelyto cause a temperature below embrittlement were identified during the PHA withrecommendation of a Safety Integrity Level (SIL) 1 interlock to protect the piping againstlow temperature. A more thorough analysis of the cold box behavior and review of pastplant upsets demonstrated that cold temperatures were more likely to occur during therecovery effort rather then during the upset itself. A set of more realistic conditions likelyto cause temperature below embrittlement was then developed and used in a SILanalysis. A level 3 SIL has first been evaluated and then dropped to a level 2 byintroducing additional safeguards. The SIL exercise was based on the assumption ofpipe failure on the first occurrence of temperature below embrittlement. However, suchtemperatures have occurred in a number of occasions, which indicates that one of thethree elements (cold temperature, sufficient stress and flaw in the material of sufficientsize to initiate a fracture) require for brittle fracture to occur was absent. Based on thislatest observation a parallel exercise has been initiated to assess fitness for service ofthe existing piping as describe in API Recommended Practice 579 Fitness For Service.
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