Distance between Chlorine Injector and Analyzer 1

[polyroly]

I am wondering if there is a spec or a rule of thumb for the minimum pipe distance between the chlorine injector and residual analyzer to ensure adequate mixing?

 

[bimr]

Water Treatment Plant Design By American Water Works Association, American Society of Civil Engineers states 10 pipe diameters for turbulent flow when no mixing device is used. Chlorine residuals are stable after only a few seconds for chlorine contact.

If you are using a static mixer, then the fluid should be mixed at the exit of the static mixer. The face-to-face length of a mixer unit is short (1-5 pipe diameters)

http://www.sulzerchemtech.com/Portaldata/11/Resources/brochures/mrt/Static_Mixer_for_Water-e.pdf

Westfall static mixers recommends 10 pipe diameters after the westfall mixer:

http://www.westfallmfg.com/sim.htm

 

[polyroly]

Thanks bimr. I found a copy of this text from a co-worker....where exactly is this stated in the text book?

Also, under low flow situations(in my case 600 mm pipe) is there an issue with having the analyzer 6 m away to cause too much delay between the analyzer signal and the output from the chlorinator to cause high/low amounts of chlorine being injected?

 

[bimr]

Look under oxidation and disinfection page 247.

It is generally recommended to have the sample line as short as possible.

The chlorine dosage is typically paced in proportion to the water flow rate past the chlorine injection point, not to the actual chlorine concentration.

 

[ch81pc]

What type of controller are you using to interface between the analyzer and the injector?
Shortest is always best, but if this isn't practicle beacuse of poor mixing and streaming in low flows, and a longer distance is needed, run the plant at minimum flow and then time how long it takes from you increasing the dose manually to the analzyer picking it up. Add a few more seconds (whatever your comfortable with) and put that in as your dwell time on the controller. It all depends on how robust your plant is and if you have you have a CPP system to shut the plant if your residual goes hihi or lolo. I've had a plant with a 20m sample line (from sample point to analzyer) which was 10m down stream of the injection point that worked fine once we got the dwell time right, otherwise it was hunting all over the place.