defcon.klaxon
Lifetime Supporting Member
Hi all,
I'm working on a wastewater plant with oxidation ditch aerators. The aerators are on VFDs and the operators want the dissolved oxygen in a really tight control, typically we're talking 1.0-1.5mg/L. There are a few things that make this loop challenging. First, there are two aerators and it seems that one's response affects the other and vice versa. Second, the response is very slow; dead time is around 2 to 3 minutes and while in theory the response should be self regulating, it almost appears to be integrating; a control output change of 6% took over an hour for the DO to settle out and even then, it was very noisy.
Also, if I compare aerator speed command to the DO, the DO lags the speed command by about 90 degrees so it slowly hunts and hunts and hunts. So I'm in an odd spot where I have a sensitive process variable that can vary when the aerator changes speed by just one percent, but if I try to tune the loop so that it's slow to change speed, it can't respond quickly.
Wondering if anyone had some suggestions for how to tackle this loop. One question I have is, how does the Loop Update Time come into play?
Oh and for reference, I'm using the independent mode.
Thanks guys!
I'm working on a wastewater plant with oxidation ditch aerators. The aerators are on VFDs and the operators want the dissolved oxygen in a really tight control, typically we're talking 1.0-1.5mg/L. There are a few things that make this loop challenging. First, there are two aerators and it seems that one's response affects the other and vice versa. Second, the response is very slow; dead time is around 2 to 3 minutes and while in theory the response should be self regulating, it almost appears to be integrating; a control output change of 6% took over an hour for the DO to settle out and even then, it was very noisy.
Also, if I compare aerator speed command to the DO, the DO lags the speed command by about 90 degrees so it slowly hunts and hunts and hunts. So I'm in an odd spot where I have a sensitive process variable that can vary when the aerator changes speed by just one percent, but if I try to tune the loop so that it's slow to change speed, it can't respond quickly.
Wondering if anyone had some suggestions for how to tackle this loop. One question I have is, how does the Loop Update Time come into play?
Oh and for reference, I'm using the independent mode.
Thanks guys!