Re: [foxboro] PH controller in Waste Water Treatment
- From: "Johnson, Alex P (IPS)" <alex.johnson@xxxxxxxxxxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Wed, 22 Mar 2006 09:27:56 -0500
Robert,
I'm not control expert but I've lived with, worked with, and sat next to
lots of them. With that in mind, I'd like to make a couple of comments on
the threads of response that you have received that I think deserve a
comment:
1) There was a comment about the suitability of the PIDA algorithm and our
control block set for various types of process control problems - large dead
times, long lags, and combinations of both.
The PIDA algorithm when used with the PIDTAU option is quite capable of
dealing with long dead-times. This option is seldom used and probably not
well documented, but if for some reason your process has a long dead time we
do have a solution in our block.
Long lag processes aren't too bad in general if the DT is short. [Yes, this
is a general statement and there may be exceptions.]
The real problem is long lags and large dead times. Those problems need
extra help beyond our PID algorithm.
2) General comment on pH control
Here's what I've learned from those who really do understand pH control:
It is really, really easy to design a physical system for pH control that
simply cannot control pH regardless of the algorithm used.
The real issue with pH control is almost always related to the process
equipment. At the end of the day, if the pH in the inflow can vary 2:1, you
need a range of neutralizing flow that can support a 100:1 change in flow.
That's a bit wide for most flow valves and generally requires separate tanks
with separate neutralization flows and lots of mixers and so on.
No control algorithm is going to control the pH if the physical equipment
can't meet the process demands.
Are you certain that your physical plant can meet the need?
Regards,
Alex Johnson
Invensys Systems, Inc.
10900 Equity Drive
Houston, TX 77041
713.329.8472 (voice)
713.329.1700 (fax)
713.329.1600 (switchboard)
alex.johnson@xxxxxxxxxxxxxxxx
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of Balmer, Robert D.
Sent: Tuesday, March 21, 2006 12:42 PM
To: Foxboro DCS Mail List
Subject: [foxboro] PH controller in Waste Water Treatment
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Does anyone out here on the list have a good I/A controller solution for
PH control in dealing with waste water?
One of our local talents has come across a MFA PH controller and thinks
it is the answer to our problems. I just personally have never heard of
this type of controller.
We have struggled for years using stand alone controller on our effluent
and are looking for a good solution to stop the struggling.
Thanks in advance for any help.
=20
Robert Balmer
=20
Applications Analyst/Programmer
ISA CCST
Fort Madison Iowa
319 463 2206
rbalmer@xxxxxxxxxxxxxxx
=20
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