AGT uses quantitative methods to perform SIL verification as specified in Safety Requirement Specifications. Reliability block diagrams are used for the calculation of total Probability of Failure (PFD) and suitable modifications to the sensor and/or field device configuration and/or proof test interval are determined to comply with the SIL requirement for a SIF or SIS. Failure data from the source or from industry accepted reliability databases such as OREDA are used. IEC 61508 defines SIL4 has the highest level of safety integrity (or lowest probability of failure on demand) and SIL1 has the lowest level of safety integrity (or highest probability of failure on demand). Probability of failure on demand (PFD) for sensors, logic solvers and actuators or field devices are determined based on the various configurations. SIL verification simply involves determination of PFD for the entire system under study which is the sum of PFDs of the subsystems comprising of sensors, logic solvers & actuators.
