I’ve been a member of the A-List at Control.com pretty nearly since it was founded in, IIRC, 1994. In that time I’ve answered dozens of questions from the list.
The other day, the following question was asked, and I thought it was interesting enough and important enough to answer it here, as well as on the list.
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.
First, as fellow list member Bob Peterson noted, this is about application design, first and foremost, and then automation design.
As the questioner has described the system, it just won’t work well. The Cl2 analyzer is located way downstream of the injection points, which makes the use of PID problematic. But even before that, there are problems.
The chemical processes here are:
[2Fe^(+2) + Cl2 –> 2Fe^(+3) + 2Cl-] and [Mn^(+2) + Cl2 –> Mn^(+4) + 2Cl-] The iron and manganese precipitate out and can be removed by the filter.
The oxidation of both Fe and Mn take time, especially since HOCl is not very water soluble. The answer is typically to inject the chlorine into the line before a mixing tank, and measure the chlorine residual at the outlet of the mixing tank. Sometimes, to reduce the carryover of chlorine, either SO2 or something that dissociates into SO2 is injected prior to another mixing tank. All of this is before the filter. Either two analyzers are used, one after each mixing tank, or one analyzer with two sample lines and a sample-and-hold function can be used.
Personally, what I’d do is to run the metering pumps from the flow meter and use the single analyzer for dosage trim. Metering pumps can be had which can do that function without special function blocks in the PLC.
Measuring the chlorine residual after the storage tank and expecting the control loop to work is plain silly.
So what should you do? You have three choices. One, do it the way they said to, and be prepared to be blamed when it doesn’t work. Two, give the project back now, rather than later. Three, try to get them to properly design the system so that the control loops actually work.
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