Functional safety—Safety instrumented systems for the process industry sector—Part 3:Guidance for the determination of the required safety integrity levels
1 Scope
This part provides information on
——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
