Chemical process plant has possibility of diversity change through the plant lifecycle and change is inevitably occurred. Management of change（ MOC） is absolutely imperative to maintain the function and the safety of the plant, and is one of the most important elements of the PSM. Importance things of MOC are to oversee the flow of change process and to ensure that each activity of PSM is performed appropriately. In this paper, change examples have been traced based on the PSM business process model which represents the framework of PDCA and the resource provision needed for performing each activity. As results of them, it has been confirmed that the logical MOC activities can be emerged from the PSM business process model. The logical MOC activities consist of implementation of change, review of MOC process, record of change history, update of process safety information, etc.

Recent onshore base load LNG plants development projects show a tendency toward larger production capacity. The large capacity plants require large capacity process safety systems, i.e., Pressure Relief Valve（ PRV） and flare system. The flare system requires sufficient capacity to ensure safe shutdown considering simultaneous relieving of PRVs due to common cause failure in accordance with international standards, such as API STD 521. Therefore, identification of potential simultaneous PRV relieving scenario is important, since several PRVs simultaneous relieving may have large relieving rate and require larger flare system capacity in recent large capacity LNG plant. However, although concept of common cause failure is specified in the international standard, criteria to identify possible simultaneous PRV relieving scenario depend on projects, since the simultaneity of relieving by common cause failure depends on assumption or probability. Further, when Safety Instrumented System（ SIS） is used to avoid simultaneous PRV relieving, probability of simultaneous SIS failure should also be taken into account. This paper discusses probability of simultaneous PRV relieving in total power shutdown for LNG plant. The case study is done based on the actual onshore LNG plant data. Further, the results from the case study are generalized as design criteria for flare design capacity considering simultaneous PRV relieving in total power shutdown.