DO Control + Pressure Control

Hi everybody,

Next week seems my company wants to send me to another site to take a look into a system with DO control to aid digested bacteria.

For sure they do not expect miracles with me, (on the beginning I said I do not want to go..but..) also I do not know if I will be with them next week.. , however always I wanted to get involved with this type of control.

Anyway the complaint is that on automatic mode the fans are running much more then in manual, with same result.

System components (as I know them from report)

3 aeration tanks

Each one has two oxygen sensors (1 inlet, 1 outlet, but only one is giving measurement, as per operator choice) and a Flow Control Valve

Each FCV connected to an air pipe, with a pressure transmitter and 4 blowers with VFD keeping the pressure + one spare. I do not know if we have any air dampening tank.

As I have been told there is one PID controlling each FCV according with sensor reading, and for air pressure we have a PI controller starting in sequence all blowers.

S7-400 as controller.

Any input as to point me in right direction for learning will be more than appreciated.

Thank all in advance
Marian

What is your question? PID tips?

Cheers

Good point.

Read a lot of PID lately, and learned that actually for this type of control PID is not best, however this is what I got.

1.What kind of problems anyone encountered with DO
2.What kind of problems anyone encountered with pressure regulated by fans
3.In this case how these two loops should be corelated
4.PID tips related to DO control
5.PID tips related to pressure control.
6.Do the air pipe is enough, or we need a dampening air tank?
7.How many days will take for bacteria to develop

Pressure has to be kept between 6,2 to 8psi
Oxygen level over 2mg/l

Thanks

My business is dissolved oxygen (DO) control. I won't tell all I've learned over the last 25 years, but here are a few pointers.

PID is not a good algorithm for this, because the process is inherrently slow response. Tuning must be very slow to avoid hunting.

DO is inherrently a "noisy" signal. That is, the DO value fluctuates quite a bit even at steady state. Your tuning and logic must account for this.

The "fans" are actually referred to more commonly as blowers. Fans have a discharge pressure of a few inches of water. With a discharge pressure of 1 to 15 or 20 psi the machine is usually called a blower.

You don't need a dampening tank. Usually the air flow rate is so large that the volume required would be huge.

The bacteria usually develop and stabilize within one or two weeks of starting the process. Often the process is "seeded" with a tank truck full of waste sludge (bacteria) from a nearby plant to speed the process.

Good luck!

Thanks a lot,

A little is much more than nothing,
I've read previous post with the formulas... but I will definitely not try to learn something like that over this period.

Hope tomorrow I will have a chance to see how they implemented exactly

Best regards