Safety integrity level issues

Fire and Gas System and Safety Integrity Level Issues With Short Planning Discussions... 

Dutuit, Y, Innal, R, Rauzy, A. and Signoret, J.-P. 2008. Probabilistic assessments in relationship with safety integrity levels by using Fault Trees, Reliab. Eng. Syst. Safety vol. 93 Issue 12, Dec. 2008. 

SIL stands for safety integrity level. It is a measure of performance of an SIS. It is determined by PFD for SIF (SIS). There are four SIL levels represented by number, viz. SIL 1, 2, 3, 4. Higher the SIL number, the better will be the performance and lower will be PFD value. However, with an increase in SIL number, the cost and complexity of the system increases, but risk level reduces. It is worth noting that there could be individual component PFD but not SIL. SIL is only given to a system (SIS). SIL certification can be issued by the company (self-certification allowed), or other competent authority to indicate that appropriate procedure, analysis, and calculation... 

The safety specification (Clause 7.2) shall include details on both the safety function and the safety integrity level. E/E/PEs safety requirements need to be expressed in clear, precise, unambiguous, verifiable, testable, maintainable, and feasible way so that these are comprehensible. Major issues shall include ... 

For any safety loop comprising several components, the safety integrity level (SIL) achievement is a joint responsibility of end-user and supplier, as will be clear from Table IX/1.0-1. Why discussing this here These are discussed here to show that equipment manufacturer/system integrator or end-user is not only responsible for the same in isolation. In a safety life cycle, there are several phases involving several activities. So, at various stages there will be involvement of either end-user or sup-plier/manufacturers. The same issue has been elaborated in Clause 1.0.1 refer to Fig. IX/1.0-1 also. 

Systematic failure normally occurs on account of design failure, including incorrect specifications, using a component not fit for the operation, and or due to error in software. Safety life cycle is adapted for systematic faults. So safety standards meant for E/E/PEs take care of both. SISs (Ref. Chapter VII) are developed to prevent or mitigate hazardous events to protect people or the environment, or prevent damage to process equipment. In this connection another important issue is SIL (Chapter VIII), which is a discrete level for specifying the safety integrity requirements of safety functions, but is not a measure of risk. SIL provides means for risk reduction to a tolerable level. The fundamental question, in case of functionally safe instrumentation, is how frequently failures of function will lead to accidents. The answers can be ... 

ABSTRACT Economical risks are directly connected with the technological risks. One major problem of a integrated technical and economic risk assessment is the identification of a suitable method, which can combine technological, safety and economical issues in one risk assessment. The Monte-Carlo-Simulation is used in both sciences (engineering economics) and can be adapted to perform a combine risk assessment. The paper explains the Monte-Carlo-Simulation integrated in a V-model process. Via a V-model the different issues can be partitioned, analysed on a low level (component reliability, safety and price) and later combined to a complete system risk assessment. 

Where loss of control could lead to severe consequences, the integrity of the basic process control system and the protective safeguards must be designed, operated and maintained to a high standard. Industry standards such as ANSI/ISA-S84.01 (1996) and IEC 61508 (2000) address the issues of how to design, operate and maintain safety instrumented systems such as high temperature interlocks to achieve the necessary level of functional safety. The scope of these standards includes hardware, software, human factors and management (HSE 2000). 

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