Research on Architectural Constraints Analysis and an Improvement Method for Safety Instrumented Systems

The problems of architectural constraints should be considered when verifying the safety integrity level (SIL), since over optimistic failure rate data can result in a higher SIL without a proper redundancy design. Firstly, based on the related parameters of architectural constraints, the requirements of architectural constraints between IEC61508 (IEC, International Electrotechnical Commission) and IEC61511 were analyzed and compared in this paper, and the particular applications in practice were pointed out. Secondly, the shortages of the IEC61508 are proposed, and an improvement principle is presented as follows. The combined different types of elements or subsystems are considered as type B, Then, according to the SIL of elements or subsystems of type B and parallel hardware fault tolerance, the SIL of a system is determined. The SIL of the combined same element or subsystem with different safety failure factor (SFF) is determined according to the SIL of subsystem with lower SFF and parallel hardware fault tolerance. Finally, the influence on verification of SIL by the uncertainty of SFF is discussed. The improved method for the architectural constraints can carry out the verification of architectural constraints for SIL more accurately, make up for the deficiency of the PFD
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computation method, and improve the safety of the system hardware architecture.