Determination And Verification Of SIL Of Safety Instrument System For QRC Sulfur Recovery Unit

Safety instrument system is important for the safe operation of petrochemical plants.In recent years,national standards and regulations require safety instrument system to carry out safety integrity level evaluation to determinate the required SIL and verify whether the system meets the requirements.In accordance with the GB/T20438,GB/T21109 and NO116,this paper carried out SIL determination and SIL verification work in the design phase of the safety instrumentation system of Qingdao Refining&Chemical.Sulfur recovery unit used HAZOP/LOPA assessment method to determine SIL.First of all we carefully analyzed the QRC sulfur recovery unit process principle and process flow diagrams,on the basis of HAZOP report,Combined with the experience of the design unit to identify the twelve safety instrument function.Then,the initial risk of the twelve SIF circuits,the risk of protective layer reduction and the LOPA gap were calculated by using the LOPA method.The SIL grade of each circuit was determined.The results were as follows:three loops SIL is above SIL1,The other nine loops have no SIL requirements.Finally,compared with the SIL evaluation results of the first sulfur recovery unit in the company,it was found that there was interlocking overprotection in No1 sulfur unit,and there was a large gap between the two evaluation results due to the different evaluation criteria.To ensure that the safety instrumentation system meets the need for device risk control,SIL verification was performed for the three loops with SIL1 or above.We used the risk assessment software PSMSuite of Tsinghua University to work.We first collected the relevant data.General data collection referenced to a sulfur unit.Equipment structure and selection referenced site and design experience.Equipment failure data used internationally recognized equipment failure database.The data entry software automatically calculates the PFD_avg.The verification results show that the original design of the three SIF loops does not meet the SIL requirements.And then analyzed the three SIF loop improvement measures,put forward the appropriate optimization program.Finally,this paper compared the SIL evaluation results of the No.1 sulfur unit,the SIL nonconformance rate of the No.1 sulfur SIS was also high,and it was found that the multiple operation of the actuator has a great influence on the result.So the new and in-service devices are required to carry out SIL assessment to detect security risks.