[foxboro] HIC's
- From: "Lowell, Tim:" <tlowell@xxxxxxxx>
- To: "'foxboro@xxxxxxxxxxxxx'" <foxboro@xxxxxxxxxxxxx>
- Date: Tue, 4 Jun 2002 10:04:22 -0700
List,
I have a control problem I'm hoping one of you has solved already. This
refinery of course used to run on board instruments, and we completed a
cutover to I/A over several years that wrapped up last year. In one of the
areas, we have several sets of steam turbine pumps that are alternated in
service every few weeks. On the old board, there were pressure controllers
that were cascaded to HIC's which actually controlled the steam flow to the
turbines. To avoid lots of re-designing and Management-of-Change paperwork,
we decided to cut these schemes over as faithfully as we could to the
original design.
The way we ended up configuring these HIC's is as follows:
PIC (PIDA block) -> HIC #1 (PIDA Block) -> HV#1 (AOUT Block) -> FBM37 or
05 connected to turbine control valve #1
==> HIC#2 (PIDA Block) -> HV#2 (AOUT Block) -> FBM37 or 05 connected
to turbine control valve #2
The HIC#1.MEAS is configured to the HV#1.OUT
The HIC#1.INITI is configured to the HV#1.INITO
The HIC#1.BCALCI and .FBK are configured to the HV#1.BCALCO
The HIC#1.RSP is configured to the PIC.OUT
The HIC#1.LR is configured to always be in REMOTE
The HIC#2.MEAS is configured to the HV#2.OUT
The HIC#2.INITI is configured to the HV#2.INITO
The HIC#2.BCALCI and .FBK are configured to the HV#2.BCALCO
The HIC#2.RSP is configured to the PIC.OUT
The HIC#2.LR is configured to always be in REMOTE
The HV#1.MEAS is configured to the HIC#1.OUT
The HV#2.MEAS is configured to the HIC#2.OUT
We couldn't figure out how to make the make the M/A transition on the HIC's
bumpless, so they aren't. The Console guys just place the HIC that isn't in
service in Manual and line up the HIC output with the PIC remote setpoint
before switching turbines, and then put the newly-in-service turbine HIC in
AUTO after the outside guy switches pumps. This is essentially what they
did on the old board.
What we have been seeing in recent weeks is a weird thing. On one of the
sets of HIC's, and only one, the HIC.MEAS and HIC.OUT do not match, by about
20% difference, and in fact, the HIC.MEAS and HIC.OUT are constantly
switching back and forth between the higher and lower value every
half-second. The valve actually moves up and down in the field, but not as
fast as the HIC is trying to move it, so the pressure stays relatively
stable.
I'm guessing there is some sort of problem with the way we have configured
this scheme, having to do with the way the HIC.OUT = HV.MEAS and HV.OUT =
HIC.MEAS such that some sort of bizarre feedback loop has been created. The
weird thing is, we have 3 other sets of HIC's configured the same exact way,
and they don't exhibit this strange behavior.
Has anybody configured a scheme similar to this? Is there a better way to
handle this? We need to send a Pressure Controller output as a setpoint to
2 or even 3 different Control Valves, but the valve(s) that isn't in service
needs to be able to be controlled manually and shut when it isn't in
service, and then matched up with the Pressure Controller output before it's
put back into service. We don't want to use straight AOUT faceplates,
because the Console guys aren't used to these, and will get easily confused
when they see a different number in the AOUT.MEAS field (which would be the
PIC.OUT) than in the AOUT.OUT field (the actual valve output).
Another question is, what the heck is the deal with this HIC? Why is the
HIC.MEAS and HIC.OUT differing by 20% when they are configured to follow
each other, and all the other ones like this work fine?
Tim Lowell
Control Systems Engineer
Phillips Petroleum Company, Trainer Refinery
Phone: 610-364-8362
Fax: 610-364-8211
tlowell@xxxxxxxx
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