DI and DO FB in valve positioner

[VickyWu]

Hi All

I am reading a book titled "The Consumer Guide to Fieldbus Network Equipment for Process Control" published by ISA. I do not clearly understand the following statement for how to use the DI/DO FB in control valve positioner. Maybe some FF expert could explain more for me and/or give me some picture. For FF on-off valve positiolner, my last project is DI used for limit switches and DO for open/close on-off valve. But for control valve, I have no experience to use the DI/DO FB.

Page 74:
The DI usually supports limit switches located to sense the fully open and fully closed valve positions.(I understood this.) They may also be used to sense the position of blocking and bypass valves often associated with control valve piping. The DO function block is often provided to allow electronic switching of piping bypass and blocking valves during process startup or shutdown.


Thank you for any advice

[rezabejd]

I think the author is merely speculating on ways in which one might use the DI / DO function blocks in a continuous valve positioner.

For example, I have two shutdown block valves (controlled separately by an emergency shutdown or safety interlock system) in series with a control valve (in throttling service). Without a DI indicating the state of the block valves, a flow controller might casue the throttling valve to saturate wide open, when the shutdown system closes the block valves. I can use the DI to invoke the "track" interlock on the FF PID, to drive it closed when the system trips.

This is just one example.

The text makes some reference to startups, so maybe the controls person designs schemes that force the valves one way or the other based on some startup or shutdown sequencing.

The DI's and DO's do not need to be part of the control valve positioner. They can reside in separate devices and be nearly as effective (costs you one incremental VCR).

HTH
John Rezabek
ISP

[jberge]

Yeah, I agree. I think for control valves the additional DI/DO may often be handled by a separate box since FF positioners usually detect fully opened and fully closed positions in software rather than using external hardware.