Functional safety—Safety instrumented systems for the process industry sector—Part 3:Guidance for the determination of the required safety integrity levels
——the underlying concepts of risk, the relationship of risk to safety integrity, see Clause 3;
——the determination of tolerable risk, see Annex A;
——a number of different methods that enable the safety integrity levels for the safety instrumented functions to be determined, see Annexes B, C, D, E, and F.
In particular:
a) applies when functional safety is achieved using one or more safety instrumented functions for the protection of either personnel, the general public, or the environment;
b) may be applied in non-safety applications such as asset protection;
c) illustrates typical hazard and risk assessment methods that shall be carried out to define the safety functional requirements and safety integrity levels of each safety instrumented function;
d) illustrates techniques/measures available for determining the required safety integrity levels;
e) provides a framework for establishing safety integrity levels but does not specify the safety integrity levels required for specific applications;
f) does not give examples of determining the requirements for other methods of risk reduction.
Annexes B, C, D, E, and F illustrate quantitative and qualitative approaches and have been simplified in order to illustrate the underlying principles. These annexes have been included to illustrate the general principles of a number of methods but do not provide a definitive account.
Note: Those intending to apply the methods indicated in these annexes shall consult the source material referenced in each annex.
Figure 1 shows the overall framework for GB/T 21109 and indicates the role that this part plays in the achievement of functional safety for safety instrumented systems.
Figure 2 gives an overview of risk reduction methods.
Foreword I Introduction II 1 Scope 2 Terms, definitions and abbreviations 3 Risk and safety integrity – general guidance 3.1 General 3.2 Necessary risk reduction 3.3 Role of safety instrumented systems 3.4 Safety integrity 3.5 Risk and safety integrity 3.6 Allocation of safety requirements 3.7 Safety integrity levels 3.8 Selection of the method for determining the required safety integrity level Annex A (Informative) ALARP and tolerable risk concepts Annex B (Informative) Semi-quantitative method Annex C (Informative) The safety layer matrix method Annex D (Informative) Determination of the required safety integrity levels – a semi-qualitative method: calibrated risk graph Annex E (Informative) Determination of the required safety integrity levels - a qualitative method: risk graph Annex F (Informative) Layer of protection analysis (LOPA) Figure 1 Overall framework of GB/T 21109 IV Figure 2 Typical risk reduction methods found in process plants (for example, protection layer model) Figure 3 Risk reduction: general concepts Figure 4 Risk and safety integrity concepts Figure 5 Allocation of safety requirements to the safety instrumented systems, non-SIS prevention/mitigation protection layers and other protection layers Figure A.1 Tolerable risk and ALARP Figure B.1 Pressurized vessel with existing safety systems Figure B.2 Fault tree for overpressure of the vessel Figure B.3 Hazardous events with existing safety systems Figure B.4 Hazardous events with redundant protection layer Figure B.5 Hazardous events with SIL 2 SIS safety function Figure C.1 Protection layers Figure C.2 Example safety layer matrix Figure D.1 Risk graph: general scheme Figure D.2 Risk graph: environmental loss Figure E.1 DIN V 19250 Risk graph – personnel protection (see Table E.1 ) Figure E.2 Relationship between GB/T 21109 series, DIN 19250 and VDI/VDE 2180 Figure F.1 Layer of Protection Analysis (LOPA) report Table A.1 Example of risk classification of incidents Table A.2 Interpretation of risk classes Table B.1 HAZOP study results Table C.1 Frequency of hazardous event likelihood (without considering PLs) Table C.2 Criteria for rating the severity of impact of hazardous events Table D.1 Descriptions of process industry risk graph parameters Table D.2 Example calibration of the general purpose risk graph Table D.3 General environmental consequences Table E.1 Data relating to risk graph (see Figure E.1) Table F.1 HAZOP developed data for LOPA Table F.2 Impact event severity levels Table F.3 Initiation Likelihood Table F.4 Typical protection layer (prevention and mitigation) PFDavg
