A SIL assignment method for the design and rectification of SIS used for preventing failure action and spurious activation is presented in this paper. The method has two stages, including SIL assignment based on experience and SIL assignment based on risk quantitative calculation results. SIS experience information, stored in a SIS experience knowledge database, comprises of equipment, SIS names and SIL information, which is summarized from many different SIS design packages of petrochemical units and SIS evaluation reports. In the stage of the SIL assignment based on experience, the SIL of SIS for a unit could be assigned by common SIS information retrieved from the experience knowledge database. In the stage of the SIL assignment based on risk quantitative calculation results, according to IEC 61511, a risk matrix is established to determine tolerable risks. A SIS initial event failure data table and a protective layer failure data table are established and the data could be obtained from certain common databases, such as OREDA, CCPS. The data will be used to calculate and identify residual risks after applying safeguards according to the Layer of Protection Analysis (LOPA) method. The SIL of SIS can be acquired from the calculation results of residual risks and tolerance risks. The combination use of the two stages can generate a method which could be applied in the SIS design of new-built or old-improved units. A SIL assignment program for the design and rectification of SIS is developed to help SIL-assigning. The software include three modules, a module of SIL assignment based on experience, a module of SIL assignment based on risk quantitative calculation results, and a module of SIL verification. Experience data and risk quantitative calculation results will contribute to the design of SISs for equipment systems. A residual oil hydrogenation unit is taken as an example to illustrate the design and rectification method for SIS. (C) 2016 Elsevier Ltd. All rights reserved.