Typical Plant Layout
Installation
The SCM should have its sampling point after the rapid mixer where the coagulant is dosed. It will work with both aluminium and iron salts.The exact position of the sampling point will depend on your plant layout, but about 5-10 pipe diameters downstream from the mixer will probablywork. The monitor output goes into a PID controller (or PLC) which controls the dosing pump speed. Follow the recommendations about installation in the manual.
Set Averaging Time
The Accufloc has an adjustable averaging time to partially compensate for incomplete mixing of the sample. If this makes the reading unstable, (eg it varies by more than +/-0.10 in less than 10sec) increase the filtering time. The default of 2 seconds will work for a reasonably well mixed sample. If you need to set this higher than about 20 seconds to get a stable reading your installation may have a problem.This should be as low as practical as it will interfere with the controller's response to sudden changes.
Calibration of the Span
It is not actually necessary to calibrate the span of an SCM (some SCMs don’t even have this option). The Accufloc’s factory calibration will be acceptable for most plants.However if you calibrate the unit so the -5.00 point corresponds to the most under-dosed raw water that is likely to be encountered, it will make the display more convenient to read.
The way to do this by pouring a sample of raw water into the inlet of the SCM and setting the span to –10.0 at this sample.
Setting the Zero Point
The zero point (set-point) should be set to correspond to the optimal coagulant dose. The only way to determine this is to try it, jar tests are the only practical way to do this.Use the controller to manually set the pump speed to the optimal dosage. Let the change reach the SCM, then set the SCM's zero point to this.
Tuning the Controller
For plants where the mixer retention time is large compared to the sample line delay PID controllers may be able to auto-tune successfully. However many PLCs and PID controllers use an auto tuning algorithm which is not well suited to the coagulant dosing process, and may produce dangerously unstable settings. Manual tuning is pretty easy since controller tuning does not have to be very tight. Normally things take hours to change. The Accufloc manual has a simple, one-step, tuning formula in the back ( Try this method of Manual Tuning )Verifying That it Works
Plants which respond quickly can be checked out manually by disturbing the system in some way, eg manually changing Alum dose, and see how well it returns the plant to normal.For large slow plants you just have to keep an eye on it.