Foundation Fieldbus in Future Safety Instrumented Systems

by | Oct 11, 2007 | Safety

Jim Cahill

Chief Blogger, Editor

Emerson’s Marshall Meier was a very busy person at this year’s Emerson Exchange. In addition to his Web 2.0 in the Plant presentation that I wrote about in an earlier post, he presented on the subject of Foundation fieldbus (FF) technologies emerging in safety instrumented systems (SIS). Key points made in the presentation were that the Foundation fieldbus specification has been enhanced to support SIS, that the Fieldbus Foundation is currently running a demonstration project to validate the FF-SIS specification, and that this specification will begin to emerge in future SIS sensors, final control elements and logic solvers.

The purpose of Marshall’s presentation was to give a look into the technology and process for how this standards effort is unfolding. It was not to say that this technology is ready to apply in your operations.

He first posed the question, “Why use Foundation fieldbus in SIS?” The answer is that Foundation fieldbus provides more computational horsepower and each value has a status associated with it. Conventional 4-20mA analog signals do not provide this goodness indication of data. More advanced diagnostics are also available to be used as part of the safety instrumented function. An example is Rosemount sensors that detect plugged impulse line conditions. Another benefit is with FF-SIS devices, users wouldn’t need to have 4-20mA SIS devices in an otherwise FF installation.

The additional safety-related function blocks specified in phase one of the preliminary specification include analog input and discrete output. Discrete input and analog output blocks are not yet defined in this early specification. Function blocks that can only run in a logic solver include analog comparators and logic blocks. Note that the same device description standard as the process-level FF function blocks will be applicable to FF-SIS blocks.

The Fieldbus Foundation last year announced a demonstration project working with process manufacturers and identified four sites with various suppliers’ safety logic solvers, sensors, and final control elements. These tests are meant to validate the specifications for FF-SIS. These participating companies include Chevron, Shell, BP, and Saudi Aramco.

The FF-SIS specifications will advance in phases. Beyond the function blocks mentioned earlier, phase two will include additional blocks and the potential to have SIS and non-SIS devices on the same segment.

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