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May 3rd, 1996
Q: How long did it take you to create the
DuPont model?
A: About nine
months
Q: Had you been using VisSim before the
DuPont project?
A: Yes. About two months.
Q: What modeling tools had you used prior
to VisSim.
A: Tutsim and Labview, though Labview isn't really
simulation software.
Q: Could you have done this project with
those tools?
A: No way!
Q: What was the most interesting part of
the project?
A: Good question. I'd have to say when we
verified the model with actual plant operation. I was totally
surprised at how close the model was to the actual plant output.
Many of the unit operations were dead on. I didn't have to retune at
all! There was one case where the subsystem had six inputs and the
actual output looked to be swamped with noise. I figured it was all
stochastic. Then the model mapped exactly onto the crazy output. It
was very gratifying.
Q: What was the worst part?
A:
Converting our 16-bit DLLs to 32-bit was a pain, though we did rely
on Bill at VSI for of lot of that. The other pain was getting the
model to run numerically stable with a 0.5 second step size. We had
to create a custom first-order filter block to solve flow/pressure
loops.
Q: What would you do
differently?
A: On the next job, I'd use predefined
approaches based on our DuPont experience. I'd say the next job
should take about half the time this one did.
Q: Is the approach you took typical in
the industry today?
A: No, I don't think so. A lot of folks
take a pure PDE (partial differential equation) approach, and only
try to solve a small piece of the overall system. Nobody has really
attempted to model the whole system from first principles, nonlinear
warts and all, from an ODE (ordinary differential equation)
standpoint, and get the excellent results in real-time that we have
seen.
Q: So we give you a platform for easy
creation of complex systems?
A: Absolutely!
Q: How long did you spend validating the
model?
A: We're not done yet, but I'd say we spent about 20%
of our time validating the model against measured data sets. It was
very time consuming. We use multiple channels of 16,000 points per
channel. On my old 486, with 16-bit VisSim, it took quite a while to
adjust a parameter and rerun the simulation. Things improved
dramatically with VisSim/32 and my new Pentium.
Q: How did you go about modeling
something like valve?
A: I had the choice of using the
manufacturer's specs or measuring flow and pressure drop, and
calculating the CV curve myself. When I used the latter method, I
used a VisSim map block to do a look-up to get the curve right. I
did have a little trouble getting enough points in the curve so the
piecewise linear look-up would give an accurate answer.
Q: Do you find that actual CV curves
differ from the manufacturer's spec?
A: Yes. Sometimes it
would be off as much as 20-30%.
Q: How applicable is the large scale
system modeling to the process industry?
A: Very. It gives
process control engineers the ability to design controls and
evaluate control changes to a process prior to implementation. It
works both on-line and off-line. Off-line, you can tune up your DCSs
and PLCs with a high degree of confidence with a model like this
one. On-line, if you see unexpected variance between plant output
and model output, you know you have a problem in the plant, and
model will tell you where to look. From there, you just work
backward from the output display that shows the variance and see
where in the model the variance begins. You can then easily find the
plugged line or clogged screen on the plant floor.
Q: What about using "soft sensors" in
your models?
A: Sure. A soft sensor is something that a high
fidelity model like ours can give you. Let's say you have a VisSim
model that realistically represents plant behavior, but there is a
part of the process that you just can't physically get a sensor to.
Just "right-click" on the part of the model that corresponds to the
piece you want to measure. As long as the model is running in
real-time and is being fed actual plant inputs, you can use any of
VisSim's displays to give you the values that a real sensor would
normally provide.
Q: What conclusions can you draw from
this experience?
A: High fidelity modeling can be done on a
large scale! And, from a financial standpoint, it's extremely
worthwhile to the process control community. There are tremendous
cost savings in reduced downtime due to off-line tuning and control
design, as well as operator training. |
