Process Safety, Definition of Safe Failure, Safe Failure Fraction, Architectural limitations
and Hardware Fault Tolerance, Route 2H, SIL 4.
Abstract
IEC 61511-1; 2016 has been published This second edition of the standard has been announced as more consistent with all considerations of IEC-Guide-104 (v 4.0; 2010-08) or in other words, in better alignment with IEC 61508 released in 2010.
Almost immediately, there are in circulation many presentations and white papers comparing the new version with the now outdated 2003 release. They all point out, with different degrees of detail, differences and similarities in both documents, and most coincide in concluding that the new version is an improvement because it clarifies many concepts.
Yet, there are changes that have significantly modified the standard and will affect the way in which it is applied. These modifications need to be understood. One such example is the changes to the Hardware Fault Tolerance (HFT) “unified” new table, which replaces equivalent tables in the old standard. This new table, for example, now allows SIL 4 with HFT of 2.
The intent of this presentation is to discuss implications of the new table, which by no means is a “license” to claim performance levels without the burden of “Prior Use” (“proven in use”) following guidelines stated in the IEC 61508 parent standard.
Starting from our previous white papers conclusion (Route 2H implications on IEC 61511 – Houston, TX, 2014); this paper will show how the new definition of Safe Failures led to the new route, and the need of users to consider standards such as IEC 61649 or IEC 60300 for any claim.
We will finally show how the use of realistic failure rates has become the kernel of the process.
See more of this Group/Topical: Global Congress on Process Safety