If your FT is a "Smart" Transmitter you may already have a pressure signal
coming to the DCS. If your FT is not Smart, replacement transmitters are
just about as cheap as non-smarts or a new PIC Loop. This pressure signal
can be used as a limit to your FT.
If the vessel you are trying to control has the capacity and the span
(inches of H2O) to smooth out the signal to your VSD you can skip the FIC
and use the level signal as a speed Remote Setpoint with the pressure as a
limit.
Daryl Reynolds
Reynolds Technical Services Inc.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Lance Dusing
Sent: Friday, July 06, 2001 9:12 AM
To: Foxboro DCS Mail List
Subject: Process Control
Hi everyone,
Here is a process control question I have been pondering.
Under normal process conditions, the control looks like this...
|--|
|FT|--
|--| |
|
|--| |---| ->|---| |---|
|LT|--->|LIC|--->|FIC|--->|VSD|
|--| |---| |---| |---|
However, if the pressure in the line with the VSD gets to large, corrective
action needs be taken.
How is the best way to control this?
Limit the FIC hi output limit with logic?
Switch to a PIC?
Anything else?
Lance
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