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Old March 22nd, 2006, 12:07 PM
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rezabejd rezabejd is offline
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In any SIL calculation, there are elements that take into account the PFD and spurious trip rate. Safe failure percentage is another variable. So if using a bus, the aspects of the communication protocol that contribute to these elements would affect the risk reduction and spurious trip rate of the given SIF.

How is this different from a point-to-point wired system? Can't we achieve a similar level of diagnostic coverage on a "field" network, as the current SIS suppliers achieve on IO cards in a chassis? Aside from speed, how is 8 transmitters on a single non-redundant FF network different from 8 HART-smart transmitters connected to a conventional IO card?

Depending on the target SIL, a disconnected transmitter = a FF transmitter that isn't communicating, and is either a vote to trip (higher spurious trip rate) or "not" a vote to trip (e.g., logic degrades to 2oo2 instead of 1oo2). Same for both instances, isn't it?

Is the difference - at the moment - more than the diagnostic coverage? Once the diagnostic coverage on FF is comparable to what we have now on TUV qualified conventional IO, will a field network "look" any different to the logic?

More often than not, the contribution of sensors and logic solver to the PFD is small compared to the final element(s). FF will only make this better.
 

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