Chemical dosing controllers 1

[RSTENG]

Please list/describe equipment you use for chemical dosing (SBS, alum, chlorine) either you use PLC or Single Loop Controller. What is your experience, especially negative side of used solution and suggestions. What algorithm you use for this type of control?
Thanks for your courtesy and time.
Filip

 

[bimr]

Most water treatment chemicals are fed at a fixed rate that is proportional to flow. The fixed rate is subject to daily adjustment based on bench scale testing.

You can use any type of PID controller to do this.

 

[RSTENG]

Thanks bimr.
Please let me rephrase my question to make it more clear.
Some plants use Single Loop Controllers (SLC), i.e. Yokogawa YS170 with intention that could carry on dosing if there is a problem with PLC and then pump supplying i.e. water could run locally and dosing could be done from SLC in local mode.
Does anyone who use PLC instead of SLC can mention any problem?
Is there any known reason that could justify use of SLC?
Also, how you deal with problem of location of injection point and measuring point?
Regards,
Filip

 

[bimr]

Don't think it make much difference between PLC and SLC. It depends on what the plant preference is.

You would want to design the system such that you can operate manually with the control system.

Don't know what your process is, but some processes do not have a measuring point. For a clarifier application, you would feed 10 mg/l of alum. You set the alum rate on the controller and the controller would pace the chemical feed to the flow rate. No feedback.

What do you mean by SBS?

What do you mean "by how you deal with problem of location of injection point and measuring point?" Are you referring to reaction time?

 

[RSTENG]

Thanks again bimr.
SBS is Sodium Bisulphite used in dechlorination.
And, yes, my question is related to reaction time. In particular to maintaining ORP level.
Regards,
Filip

 

[bimr]

Not following your problem. Have you looked into the use of
Ascorbic Acid/Vitamin C?

http://www.vita-d-chlor.com/faq.asp

 

[Impdrew]

Filip,

Not sure exactly what you are asking here ?

We have been a fan of using PLC controllers (AB mainly) where we utilise a number of dosing modes.
For SBS, we would tend to have a feed forward sample providing a chlorine residual signal and feedback sample providing also providing a chlorine residual signal. We would then use the feedback signal to trim the error correction.
Dose Modes:
1: Flow proportional.
2: 1 with residual feedback
3: 2 with feedforward residual trim

For algorithms we run loop timers that are equivalent to the time it takes for a change in dose rate at the dosing pumps to be registered fully at our online analysers(which includes the time not only in the pipe mains but also in any dosing and sampling pipework). We only register the analyser signal and calculate the new dose correction at each expiration of that timer. Works well as it does not rely on how well the pump is calibrated, it will just keep trimming.

Andrew