PID control advice?

[sparkotronic 0]

Hi,

I'm looking for some advice or opinions on a control loop that I have been asked to look at and possibly modify.

The present system has been there for 10+ years and as far as I am aware it's never successfully worked.

My employer has the current set up which is shown in the attached drawing.

Using raw water, an acid is added which is then circulated round a contact loop, after the loop, an Alkali is added.
The expected end result of the loop is to achieve a specific alkali Ph.

The loop has 2 Ph probes which are selectable via SACDA and 2 dosing pumps.
The acid dosing pump is controlled by a PID loop where the SP is entered in the SCADA and the PV is measured from the selected Ph probe.
The Alkali dosing pump is purely proportional to flow.
In the PID block, the derivative is switched off.

One probe is just after the acid dose and the 2nd is just after the Alkali dose.
My employer wants to be able to control the acid dose by monitoring the Ph probe which is just after the Alkali dose and is also over 10 minutes process time after the acid dose. 
I've been told that the system was designed to work this way, even though they have had no successes in running it this way.

My wee head can't process how I can control an acid dose while monitoring the after an Alkali has been added, the Alkali will raise the value by 5.
There is also only one set of PID parameters.

Looking at it, firstly I'd assume that each probe would require its own PID parameters.
I have had a play with it, but can't seem to get the loop to react to change and then recover in an acceptable time scale.

Could someone give me an opinion on whether the system could operate from the Alkali probe with some PID tweaking?

Thanks in advance,

Colin

[Jairus 5]

Base on your diagram ,it is possible but not quite accurate, since 10 minutes is long time  feedback. If "NO" other issue why not bring your acid dose closer to Alkaline dose. to reduce the response time.

[dFx 0]

Had to work on such a slow loop in the past. We started with PID and failled to go with it.

Then we realized that knowing the flow was a major issue, and Added some formula to offset PID's setpoint correctly. We went through a tough comissionning, but ended well adding some weight factor on the formula setpoint with the real setpoint.

To start, as you know your flow, and your Customer likely know at constant flow how much acid is needed, start building some basic formula for acid ( acidDose = Flow * RawWaterPhError)

Note I use raw Water Ph, cause your middle probe is only usefull to limit the ph drop.

Once you get it stable (it should be easy), then slowly add some weigth to your PID, adding almost only integrative action (Proportional should be small in my mind), 

PID role is just to correct the error using slow integrative action, while your formula should presets almost correctly the acid dosing. 

so your loop should look like : formula * weigth + Setpoint * (1 - weight) -> PID setpoint -> PID -> PID output -> Dosing pump setpoint

 

Last but not least, make sure that the alkali probe is correctly setup (not too close from alkali injection point, and is sending some pretty stable signal).

And also have a look at how is builded the pump setup (big issues can comes from bad installation).

 

@Jairus  : generaly, you need an agitator system and even some more time to the acid to properly diffuse in the water. Shortening the distance can be a big issue for the final probing.

Edited 8 Oct 2018 by dFx
Typo

[waynes 8]

A long dead time is typical in process plants, but the incoming raw water wouldn't change too much inside a minute.

I like the way of adding an offset and applying a weight to the SP, but would it not be easier to use the flow rate as a feed forward factor.

In other words, tune it at a flow rate of x. Calculate the delta in flow rate and apply the change to the dosing pumps on the output of the PID, instead of the SP.