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Production • Processing • Handling


the valve (flow to close) cannot be considered to cause a safe failure while these forces are too small to move the valve. To be even more specific the IEC introduces so-called no part and no effect failures which are not to be accounted for in the SFF calculation. Tese changes in the definitions have of course a serious impact to the Safe Failure Fraction of the final element. Following version 2010 a SFF of 90 per cent or more can no longer be claimed. In fact it will be significantly lower than 60 per cent normally.


● Partial proof tests are not in line with the standard which sets a target of 100 per cent coverage.


Different routes to HFT What is also new in the IEC 61508 are the different routes to assure a proper hardware fault tolerance is applied for each application or actually each SIL level. Route 1 uses the same table as in the 1998 revision and therefore uses the safe failure fraction as a basis for the required hardware fault tolerance. If we would argue that the safe failure fraction is mainly for E/E/PE safety related system where the standard is mainly written for then actually this route would not be applicable to final elements at all. However, if this route is applied the change in Safe Failure Fraction would have an impact on the hardware fault tolerance. Te new route 2 is based on the prior use/proven in use as found in IEC 61511 version 2003. In applications requiring a SIL3 or SIL2 in the high demand mode a hardware fault tolerance of 1 - and thus a 1oo2 configuration - is required. Although the term ‘proven in use’ itself is quite clear the IEC sets specific requirements (IEC 61508-2 par 7.4.10). Statistical data shall be available for the same application, the same type of process, but for final elements this also means the same closing times. All aspects of the applications and safety mission shall be verified. For example, a valve used in the mining industry closing in 1 second per inch (12 inch valve = 12 seconds) is not ‘proven in use’ on a gas well specifically when closing in 2 seconds. Basically this also applies for route 1 where dependable failure data are required (IEC 61508- 2 par 7.4.9.3 - 5). Dependable meaning that a sufficient confidence level shall exist that the equipment is suitable for the application (e.g. can close in 2 seconds).


Fig. 3. A typical control and safety system consisting of an axial control valve and an axial on-off valve (HIPPS valve) according to IEC 61508. (South Africa).


We also want to note that based on the revised definition it could even be argued that the Safe Failure Fraction is not applicable for a mechanical component like a final element but only to systems having electronics such as the sensors or the voting logic. In conclusion:


● Te diagnostic test interval shall be at best not longer than the MTTR to take credit for diagnostics.


● Part and effect failures shall not be used in the SFF calculations, safe failures are basically only spurious trip failures, reducing the SFF for FE to below 60 per cent.


● SFF can be considered as not applicable for mechanical devices.


Final conclusion


As a final conclusion it can be said that the new IEC 61508 requires mechanical equipment to be ‘proven in use’ (new route 2H) rather than using the Safe Failure Fraction. Te new definitions of safe failures and diagnostics do not seem fit for mechanical equipment or special devices like partial stroking devices. Tese partial stroking devices are not in line with the standard which sets a target of 100 per cent coverage. In general the final result is now more in line with the prescriptive standards requiring redundancy in final elements. ●


Enter 61 or ✔ at www.engineerlive.com/iog Rens Wolters is HIPPS Product Manager, Mokveld Valves BV, Gouda, The Netherlands. www.mokveld.com www.engineerlive.com 61


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