pH Loop tuning

Mehdi

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Join Date
Dec 2005
Location
Tehran
Posts
53
Is there anyone here who has experienced a loop for controlling pH by dosing some chemicals upstream of sample point? In one case the sample point is far away from dosing point and a long sample line is used to bring sample to sensor. I use a PI controller to adjust injection of dosing pump. But it never works and always bouncing.

In my opinion, this is long sample line cause a bad lag in system which can not be removed easily.

2pH.JPG
 
Do you have a constant flow in the line between the dosing point to the sample point? If not, PI controllers will be a pain. Some of the difficulties can be overcome with an accurate flow-meter, and adjusting the loop time based on flow.

Still, a PI loop looks like it will be difficult here. Is your dosing pump a fixed displacement pump? is the dosing rate variable? are you able to control both acid and base dosing? what is your deadband? is the lag in the sample line dependent on pressure? temperature? do you get adequate mixing in the 8" line between the inflow and the dosing agent?

I have way to many questions to even hazard a viable solution.
 
pH is very non-linear. Take a look at using look-up tables or similar non-PID approaches to this control loop.

The line may or may not be too long. It is a function of lag time. If you have a high velocity in the line, the 20 m may only induce a delay of a few seconds. If the velocity is low, the delay may be significant.
 
Tom, Looking at the drawing, I'm not as concerned with the main line lag, as I am with the sense line lag... What kind of flow is coming back through the sensing line? I normally mount Ph sensors either directly in-line, or in a short parallel branch around the mainline.
 
My memory is scetchy becouse it was a few years ago that i worked with someone on this problem..But..We had a Loop (used to clean the pipeline) that acid was pumped into..we had a setup similar to yours above and had the same problem..EcoLab came in made suggestions, One was to put the sensor before the injection point..What would happen is the chemical was injected by the pump and then seen by the sensor almost immediatly the pump would shut off after a few mins the PH would drop and pump would start again..This gave us the same problem..Now we inject some acid in the line then wait x mins (X is figured by volume and length of line)then take a sample..if it low inject some more wait x mins and test again..Its a little slow to come up but once it is at the ph level we want (We have an upper and lower range) it is fairly easy to maintain..I will see if i can get a copy of the graph we use to track chemical usage..
 
rdrast said:
Tom, Looking at the drawing, I'm not as concerned with the main line lag, as I am with the sense line lag... What kind of flow is coming back through the sensing line? I normally mount Ph sensors either directly in-line, or in a short parallel branch around the mainline.

I agree that the sensor should be in the main line. However, the concern is still transit time for the sample. If the flow in the 45' line @ 1/2" diameter is 0.5 gpm the delay is going to be about a minute and a half - surely a problem. If the flow in the 1/2" sample line is 5.0 gpm the delay is about eight seconds - perhaps not a significant problem.
 
Be careful in putting the pH in the main line.

Couple of suggestions...
- pH sensors typically have a maximum velocity of fluid flow specified. It is not very high and will effect the reading. This is probably why there is a sample leg. Each manufacturer has different capabilities.
- pH is logarithmic (sp). You should linerize the signal prior to applying control. Often it is done in a two stage process if the starting pH is too high or low relative to the desired pH.
- Is there a static mixer on the line. With out good agitation and reaction time, you may not get a representative sample.
- Often times a loop like this has a compound control where chemical is added proportional to flow and then trimmed based on actual pH measured.
- Buffering and alkalinity (sp) also come into play, but I am not a chemist. I do suggest you talk with one if there is some one handy.

Some links to help with pH
http://www.omega.com/techref/ph.html

http://www.omega.com/techref/phtour.html


Best of luck.
 
rdrast said:
Do you have a constant flow in the line between the ....

Usually there is a constant flow in the line and just chemical characteristics of fluid (water in this case) vary. Dosing pump is a micro-processor based MiltonRoy which has a 4-20 mA input to adjust its pulse/min. Dosing rate must be variable because chemicals in the water vary. Adequate mixing happens in 8" line.



What I think is the problem, is long delay between sample point and sensing element which should be compensated some way.
 

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