Establishment of setpoints

Setpoints for SIFs should be selected such that actions can be taken to achieve or maintain the safe state of the process within the allocated process safety time. 

The Process Parameter Limit (PPL) is the least conservative value of a given process variable at which the initiation of the SIF will reasonably protect the process equipment for each postulated event. The PPL and its basis should be managed as process safety information. 

Setpoints are chosen to assure that a safety actuation occurs before the process reaches the PPL. Trip setpoints are aiso ohosen to assure that the plant can operate and experience expected operational transients without unneoessary trips or safeguard actuations. 

The choice of an MSP requires determining the Total Segment Uncertainty (TSU). The TSU represents the expected performance of the SIS process measurement under any applicable process and environmental conditions. The trip or actuation is only required to prevent or mitigate certain postulated events, so only the process and environmental conditions that occur during those postulated events should be considered. 

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