Rory,
Sounds like your loop needs tuning to me. Steve Elwart brought the following
excerpt from Control Engineering to my attention. Perhaps, these values will
put you in the right ball park.
I am by no means a tuning expert, but these should get you in the ball park.
Send any thanks to Steve and Control Engineering.
Regards,
Alex Johnson
The Foxboro Company
10707 Haddington
Houston, TX 77043
713.722.2859 (v)
713.722.2700 (sb)
713.932.0222 (f)
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
Control Engineering readers consistently indicate loop analysis and PID
tuning articles are among the most useful articles we provide. But
establishing valid PID tuning parameters requires determining process
dynamics, and that's seldom possible until the process is up and
operating.
However, that doesn't mean loops must remain in manual until process
dynamic testing is completed. Here's a set of very conservative PID
settings that can be initially used, but will normally result in sluggish
response...but they come with no guarantees.
Gain is in proportional gain (Delta percent output / Delta percent input)
Integral is in repeats per minute
Derivative is in minutes
Loop Type:
Flow Gain = 3 Integral = 30 Derivative = 0
Temperature Gain = 1.25 Integral = 0.2 Derivative = 0.05
Level Gain = 2 Integral = min. Derivative = 0
Gas pressure Gain = 10 Integral = min. Derivative = 0
Note: In "Foxboro-ese",
Proportional Band is 1/Gain * 100 and
Integral is in minutes per repeat (not repeats per minute).
Therefore a gain of 3 is equal to a PBand of 33.
-----Original Message-----
From: Loupe, Rory [SMTP:[EMAIL PROTECTED]]
Sent: Tuesday, May 30, 2000 4:36 PM
To: FOXBORO List (E-mail)
Subject: Foxboro single loop controller to I/A
We recently moved control of a boiler from a Foxboro single loop
controller
to the I/A DCS. The field instrumentation (steam flow transmitter,
water
flow transmitter, level transmitter and water flow control valve)
and piping
remained the same. We tried setting up the DCS control scheme the
same as
the single loop controls with little success.
The control scheme is as follows:
We have a standard PID (setup as a PI controller) level controller
with the
output of the controller scaled the same as the flow transmitter.
The level
tranmitter is setup for 0 to 100 percent. The setpoint of the level
controller is local and set at 50 percent.
The output of the level controller is added to K times the steam
flow out of
the boiler. The K factor is 0 to 1.5. The old single loop
controller had a
K factor of 1 which basically added the output of the level
controller to
the steam flow out of the boiler. This calculated value is the
setpoint of
the flow controller.
We have a standard PID (setup as a PI controller) flow controller
which
controlls the water flow valve. This vavle regulate the amout of
water into
the boiler. The setpoint of the control is equal to the calculated
value
Level_Controller.OUT + (K * Steam_Flow.PNT).
The steam flow stays relatively constant. It's the calculated
setpoint
(using the level controller outpur and steam flow) that seems to be
swinging. I believe this is swinging because the level controller's
output
is swinging.
Has anyone move control of a boiler from a Foxboro single loop
controller to
the Foxboro I/A DCS? Any help or incite would be much appreciated.
Thanks,
Rory Loupe
Process Control Engineer
Rhodia, Inc.
Baton Rouge, LA
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