In relation to the proof test intervals for Safety Instrumented Systems (SIS) Instruments, a recent Reliability Centered Maintenance (RCM) analysis was conducted. The analysis determined that field transmitters should have a 2 to 3 year preventive maintenance (PM) frequency, compared to the current 1 year PM frequency. However, it is important to note that SIS Instruments must be proof tested at the frequency assumed in the Safety Integrity Level (SIL) calculation, typically every 1 to 2 years, to ensure safety. For further guidance, please email pattomks@yahoo.com. Thank you for your input. Best regards.
Many people, like myself, are having difficulty understanding your terminology. Can you clarify what SIS instruments are? Your statement has piqued my interest. I am curious as to why there are discrepancies in results between RCM and IEC 61508. Despite both being risk-based methodologies, it is surprising to see varied outcomes. - V.Narayan.
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Hello, both Reliability Centered Maintenance (RCM) and Instrumented Protected Functions (IPF) are risk-based methodologies outlined in IEC 61508 and IEC 61511. These methods should result in similar test intervals for acceptable risk levels. However, it is important to note that there may be variations in the calculation of Safety Integrity Level (SIL) and acceptable probability of failure on demand between RCM and IPF. This could be due to differences in failure rates or the possibility of incomplete RCM processes lacking reliability calculations. It is recommended to verify the assumptions and failure data used in both methods. Additionally, ensure that when implementing RCM and IPF, the complete loop as a subsystem is considered, not just the sensing elements. Thank you, V. Narayan.
How do proof test intervals impact the acceptable Probability of Failure on Demand (PFd) of a Safety Integrity Level (SIL) Loop? The acceptable PFd for a SIL 2 Loop typically falls within the range of 1/100 to 1/1000. If we confirm that the PFd remains within these limits, can we proceed with adjusting the testing frequency based on Reliability Centered Maintenance (RCM) guidelines? Maintenance is considering reducing the testing frequency of an SIL Loop from annually to once every 2 to 3 years following RCM analysis. Thank you, Pattom.
It seems that there is a discrepancy between the SIL study and the RCM study, as they are providing significantly different results. This suggests that one or both of the studies may be utilizing incorrect data or methodologies. The PFD, failure rates, configuration, and test frequencies are all interconnected mathematically, but a detailed explanation is beyond the scope of this text. For further information on this topic, various books by authors such as Trevor Kletz, Hoyland & Rausand, Edwards, and others delve into the mathematics involved. Chapter 3 of my book also covers this subject. If you require further assistance, please feel free to reach out to me directly at eml@effective-maintenance.com. V.Narayan.
Thanks for sharing this valuable information. It seems the RCM analysis findings suggest a longer time interval between maintenance operations, which could potentially reduce costs and be logistically favorable. However, I fully concur with the emphasis on maintaining the proof testing frequency dictated by the SIL calculation for safety considerations. It's insightful to see where economic efficiency and safety requirements intersect in this context. I look forward to emailing pattomks@yahoo.com for more guidance.
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Answer: - The RCM analysis suggests a preventive maintenance (PM) frequency of 2 to 3 years for field transmitters, as opposed to the current 1 year PM frequency.
Answer: - Proof testing at the frequency assumed in the Safety Integrity Level (SIL) calculation, typically every 1 to 2 years, is crucial to ensure safety and reliability of SIS Instruments.
Answer: - For additional guidance, you can email pattomks@yahoo.com.
Answer: - Conducting an RCM analysis helps in determining optimal maintenance frequencies that can improve the efficiency and effectiveness of maintenance activities for SIS Instruments.
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