Sodium Hypochlorite injection Control Help


Thread Starter


Hello Everyone,

I currently have a job with a customer that wants to operate an injection pump for Sodium Hypochlorite injection using a PID as follow.

"The control should depend on the finished water storage tank free chlorine concentration and the permeate flow rate. A setpoint for free chlorine concentration in the finished water storage tank should be made adjustable for the operators, a PID control shall be used to adjust the pump speed so that the desired chlorine concentration is maintained"

From the controls description I'm thinking a cascade PID, but from what I been reading a PID is usually does not work because of the locations of the injection and the location of the analyzer.

I have not done a chemical dosing system does anyone have any samples logic or point me to the right direction.

The application if for a water treatment plant.
[Well], [Iron/Manganese Removal Filter], [Two Stage RO System], [Finished Water Storage Tank], [Distribution System].

The injection points are as follow.

[Well] --> [Filters] ---> [RO] --> [FLOW Transmitter] --> [Injection Points] --> [Finished Storage Tank with CL2 Analyzer]--->[Distribution System]

The permeate water will be injected with Sodium Hypochlorite and Sodium Hydroxide. The CL2 Analyzer is measuring the Free Chlorine inside the storage tank.

Thanks for the Help.

Bob Peterson

I don't see that a PID loop is especially appropriate for this kind of thing.

If you know how much permeate is being added to the storage tank you can calculate how much NaOCl to add to account for the permeate that presumably has no free chlorine in it.

If you know how much water is in the storage tank and its free chlorine level, you can calculate how much additional NaOCl to add to bring the level up to the desired value.

This is not really a programming problem. It is a process problem and should be attacked from that side rather than thinking of it as a programming issue.

I would point out that unless there is considerable movement of the water inside the storage tank that whatever value of free chlorine is being measured is quite possibly not indicative of the free chlorine in the tank as a whole.

This one is interesting enough that I am going to answer it on my blog, as well as here.

I¹ll tell you how to do it, with the caveats that you have already. In the meantime, see if your customer will allow you to redesign the system entirely, so that it will actually work. The way it is currently specified and designed, it won¹t.

I¹ll try to have a detailed answer posted by tomorrow.

Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC