Re: [foxboro] TRKENL and HOLD on PIDA block
- From: "Johnson, Alex P (IOM)" <Alex.Johnson@xxxxxxxxxxxx>
- To: "foxboro@xxxxxxxxxxxxx" <foxboro@xxxxxxxxxxxxx>
- Date: Tue, 16 Feb 2010 11:05:15 -0600
Richard,
Look at the diagram as the start of the PIDA block section of B0193AX Vol 3.
In it you can see that the Output is driven by the TRACK input when the block
is in TRACK mode. The value of the TRACK input is passed through the output
limits, but is otherwise not altered.
Setting that aside, you can setup a simulation fairly easily.
Build an AIN, PIDA. Use the IOMOPT on the AIN to specify counts (0-64,000 is
normal).
If you enter more than 64,000 the AIN will go BAD. You can watch what the PIDA
block does when you set TRKENB and TRACK=20%.
Hope this helps.
Regards,
Alex Johnson
Invensys Operations Management
10900 Equity Drive
Houston, TX 77041
+1 713 329 8472 (desk)
+1 713 329 1600 (operator)
+1 713 329 1944 (SSC Fax)
+1 713 329 1700 (Central Fax)
alex.johnson@xxxxxxxxxxxx (current)
alex.johnson@xxxxxxxxxxxxxxxx (good until September 2010)
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Targosky, Richard S.
Sent: Tuesday, February 16, 2010 10:57 AM
To: foxboro@xxxxxxxxxxxxx
Subject: [foxboro] TRKENL and HOLD on PIDA block
Hello List-
We have a PIDA block which is a Flow Control Loop. (FIC45) The MEAS for this
block is FT45.PNT. The CEOPT is set to a 1 to put the PID into HOLD if there is
an IO error.
The problem is this - this flow goes into a heat exch - which can plug-off. So
- the PID block responds by driving the valve open. Eventually - the plug
breaks free - and the FT goes off-scale hi (BAD_IO). This puts the PID block
into HOLD and the valve output stays at its high value - this keeps the FT
off-scale hi - and all is bad!!
Best solution - re-range the xmitter so you do not create the off-scale
condition. But - we cannot do this right now.
Proposed solution - I will tie the TRKENL of the PID block FIC45.TRKENL back to
the High Out-of-Range (HOR) of the flow transmitter FT45.HOL.
I will set the TRACK value to 20 % as was suggested by operations.
The effect of this change (I think) is this --
If the Flow Transmitter goes to an Off-Scale high value - the block will goto
TRACK enable. This will drive the block to the 20% output value.
When the flow comes back into range (which should be rather quickly) - The HOR
bit should go back to 0 and PID action (AUTO) should take over in a bumpless
fashion. If the operator needs to put the loop into MANUAL to move the valve
they can - MAN takes precedence to TRACK.
My question to the list -
HOLD (thru CEOPT) or TRKENL - which one takes precedent?? Looking thru the
Foxboro documentation - I do not see an answer.
My concern is that even if I tie the TRKENL back - that the BAD_IO may still
force the loop into HOLD.
Rick Targosky
PPG Industries
Natrium, WV
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