Here's another great situation I came across this week....

Let's say your working with an existing unit with only one (1) critical flow transmitter. 

Thru your HAZOP, you've found that this flow measurement is critical for both process control and safety shutdown.  What do you do?  Add another seperate and independent flow signal, as suggested by the high priced safety consultant? 

But what if that ain't an option?  There are some process's that can't take the additional head loss, proper flow conditions (ever hear of laminar flow?), or costs, associated with flow measurement.  

I suggested that if they can't add seperate, they should bring the flow signal directly into their safety system, and then send the signal to their DCS (either hardwired or data highway).  As long as extra care is considered, such that a "stuck" flow xmtr signal can be detetected, I don't see a problem with that.  Just don't try and claim two seperate layers of protections, but you can claim the higher PFD of your SIS than you could with your BPCS..

Let's face it, sometimes, we have to be realistic, and practical.  Then have your high price consultant validate that...