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SIPI61508 Soft computing based qualitative method for determination of SILs

Learn about qualitative methods using soft computing for assessing Safety Integrity Levels (SILs) based on IEC 61508 standards. Explore detailed steps for risk analysis, reduction, and classification.

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SIPI61508 Soft computing based qualitative method for determination of SILs

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  1. SIPI61508Soft computing based qualitative method for determination of SILs István Ajtonyi1 – László Ormos2 1 University of Miskolc, Institute of Electric Engineering, Department of Automation 2 College of Nyíregyháza, Technical and Agricultural Faculty, Department of Electrotchnics and Automation

  2. SIPI61508 “Functional safety is that part of the overall safety of a plant that depends on the correct functioning of its safety related systems.” (from IEC 61508 part 4.)

  3. SIPI61508 Quantitative method for determining safety integrity level The quantitative method is used to assist in development of the safety requirements specification (SRS) and the defining of the safety integrity level (SIL) by historical data. The steps of quantitative method are: • evaluation of hazard event rate without protection, definition of • target risk frequency, and record of all details of the SIL, • addition of external and non-SIS protection and evaluation of • effect on risk frequency, • proposition of an SIS risk reduction measure which reduces the • hazard event rate and hence the risk frequency, • conclusion of a practical risk reduction factor for the SIS • consistent with being below the target risk frequency, • conversion of the risk reduction factor to an SIL value for the SIS, • draft the SRS with a reference to the calculation sheet and risk • reduction model, • finalization SIS detail SRS.

  4. Control System Operating Equipment Protection System SIPI61508 Hazard and risk analysis: • recognizing and understanding the safety problem to be solved, • separation of safety control from process control.

  5. Safety integrity level Low demand mode of operation (average probability of failure to perform its design function on demand) 1 3 4 2 10-5 to 10-4 10-3 to 10-2 10-2 to 10-1 10-4 to 10-3 SIPI61508 Risk reduction and classification • safety integrity level from the fallowing table: • basic data • estimated event frequency, • chance of occurrence for an event, • event likelihood. • unprotected risk frequency, • protected risk frequency, • risk reduction factor, • safety availability, • probability of failure on demand,

  6. SIPI61508 Qualititative method for determining safety integrity level The qualitative method is a very attractive alternative for arriving at SILs because it avoids the need to place actual quantitative figures on the hazard demand rules, risk frequency and the consequences. In practice the process industries there are separate versions for three categories of hazard: • harm to persons, • harm to environment, • loss of assets (production and equipment losses/repair costs).

  7. SIPI61508 Thom’s catastrophe theory - the conditional catastrophe

  8. C Extent of damage CA slight injury, CB severe irreversible injury to one or more persons or death of person, CC death of several persons, CD catastrophic consequences, multiple deaths F frequency and exposure time FA seldom to relatively frequent, FB frequent to continuous events. P hazard avoidance/ mitigation PA possible under certain conditions, PB hardly possible. W occurrence probability W1 very low, W2 low W3relatively high. W1 W2 W3 a - - 1 a - 2 1 a PA PB FA FB 4 3 2 Starting point FA 3 2 1 PA FB PB b 4 3 - no safety requirement a no special safety b a single E/E/PES is not 1,2,3,4 safety integrity CA CC CD CB PA FA PA PB PB FB SIPI61508 Risk parameters charts based on IEC 61508 Function for SIL determination by risk graph

  9. SIPI61508 The safety layer matrix method for SIL determination • Basic requirements for safety layers before the logic of the matrix diagram can be used: • independent SIS and non-SIS risk reduction facilities, • each risk reduction facility is to be an independent • protection layer, • each protection layer reduces the SIL by 1, • only one SIS is used.

  10. SIL1 SIL1 1 SIL1 SIL1 SIL1 SIL1 SIL1 SIL3 2 SIL1 SIL1 SIL2 SIL1 SIL2 SIL3 SIL3 SIL3 SIL3 3 Independent protection layer low med. high low med. high low med. high Event likelihood Event likelihood Event likelihood Severity Minor Serious Extensive SIPI61508 Hazardous event severity matrix method for SIL determination Function for SIL determination by severity matrix

  11. SIPI61508 Conclusion • qualitative method requires professional experience, • qualitative method does not require the collection of historical • data, • qualitative method can take into consideration information • which might not be quantified, • qualitative method carries out information from information.

  12. SIPI61508 Thank you for your paying attention.

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