E. SIF Device Selection - PIU - MHFT

IEC 61511 and ANSI/ISA S84.01-2003 require that components and subsystems (sensors, logic solvers and final elements) for use as part of a SIS for SIL 1 to SIL 3 applications, be designed in accordance with IEC 61508-2 and IEC 61508-3, as appropriate, or else comply with the Proven-In-Use (PIU) requirements of IEC 61511.

Additionally, the standards require that sensors, logic solvers and final elements selected for use as part of a SIS for SIL 1 to SIL 3 applications conform to a Minimum Hardware Fault Tolerance (MHFT) criteria.

The MHFT has been defined to alleviate potential shortcomings in SIF design that may result due to the number of assumptions made in the design of the SIF, along with uncertainty in the failure rate of components or subsystems used in various process applications.

IEC 61511 and ANSI/ISA S84.01-2003 have further design requirements regarding the independence of the SIS and the BPCS (sensors, logic solver and final elements). IEC 61511-2 clause 11.2.4 deals with the special concern for SIS-BPCS Separation, Independence, Diversity, Hardware common cause, Systematic (software) common cause and Human errors.

Premier Consulting Services provides expert consulting in the selection of components and subsystems (sensors, logic solvers and final elements), addressing the requirements of "proven-in-use" and "minimum hardware fault tolerance" in IEC 61511 and ANSI ISA S84.01-2003. Specific emphasis is made on determining the adequacy of field devices with "prior use" records, including the number of these devices with sufficient operating experience in a similar operating profile and process application environment. PCS provides further guidance and analysis of test results (i.e. FMEDA’s) or third party certifications (i.e. TÜV, FM, etc) for field devices with certain SIL claim limits and their adequacy for the SIS application, including any application guidelines and/or restrictions.

Bearing in mind that the logic solver is normally shared by a number of safety functions, selection of the safety PLC technology is crucial to a safe and reliable SIS.

Premier Consulting Services expertise can prove invaluable in the analysis of logic solvers manufacturers’ claims for "safety availability", "reliability", "fault tolerance", "safe failure fraction" as it relates to "demand mode" or "continuous mode" of operation. Furthermore, an analysis of any third party (i.e. TÜV, FM, etc.) certification guidelines and restrictions, as well as an analysis of the manufacturer’s "safety manual" becomes an essential review process in the selection of the logic solver technology.

Premier Consulting Services recognizes that third party certifications (i.e. TÜV, FM, etc) to IEC 61508 and other applicable standards are focused exclusively on a "fail safe" mode of operation of the device. Premier Consulting Services also recognizes the importance of "process up-time" and therefore provides the expertise for the selection of SIS devices that will issue not only safety, but a high degree of "reliability" and low "spurious trip" rate.

There are some devices and PLCs on the market that have "low fault tolerance" and low redundancy but high "safe failure fraction", and thus get certified to even a SIL 2 or SIL 3 rating. PCS expert analysis and recommendations build towards avoiding the trap of designing a "safe" but "unreliable" SIS.

SIF Device Selection - PIU - MHFT
Inputs:	Deliverables:
Field equipment performance data Site environmental data Process up-time requirements List of SIF with individual SIL targets Project data gathered during study	Proven-in-use device analysis Fault Tolerance device analysis Third party certification analysis Application restrictions analysis Device safety & reliability analysis BPCS-SIS independence analysis