Fieldbus Reliability and Availability Studies for Single and Redundant Systems - Part I

In Fieldbus applications, Reliability and Availability are two terms which are of prime concern to the end user. Mean Time Before Failure (MTBF) is a factor of the Reliability of the product and this figure is rarely mentioned in the manufacturer's product data sheet, simply for marketing reasons. The other parameter which is always ignored is the Availability. The question is, which one is more important; Reliability or Availability? As far as the instrument is concerned, there is no preference between them; it actually depends in the end on the criticality of the process. Fieldbus products are widely used in the process industries for monitoring and control functions, some of which may be for critical loops and include safety related applications. In 2006 Foundation Fieldbus (FF) announced that TUV Rheinland Industrie Service GmbH had approved it's protocol specification for FF Safety Instrumented Functions (SIF) on the basis of compliance with IEC 61508 up to and including SIL3. Subsequently, the first demonstration of the FF-SIF at Shell,Aramco, Chevron, in 2008 showed how SIFs can be designed up to SIL3, and hence make full use of the certified protocol. Dr Hassan El-Sayed, Consultant for Sira Consulting Ltd and Mr Paul Reeve, Functional Safety Manager for Sira Certification, examine the Reliability and the Availability of Fieldbus architectures covering redundant power supplies on a single segment and the fault tolerance in