Chlorine Feed Pump Location

[jartgo]

So, I stumbled across a recommendation in AWWA's "Water Treatment Plant Design" (p10.27 4th edition) that says in order to inhibit air binding of the pump, the hypochlorite pump should be located below the level of liquid in the tank. Further that the suction piping should be sloped upwards so that bubbles can flow back into the tank.

I've only been in four or five small treatment plants, but have never seen the chlorine pump located below the feed tank. Is this commonly done? From what I've seen it's usually sitting on top of the tank or mounted on the wall next to it.

 

[quark]

I have been using diaphragm pumps that sit on top of NaOCl tank and feed into the lines. We do free chlorine estimation after mixing and total viable count before and after the feed.

We never checked if the trapped air was an issue.

 

[hydrae]

In small systems, it is typical to install the pump above the solution tank, the reason, in case of suction tube failure, the bleach does not make a big mess. In some cases there may not be a containment structure.

I larger facilities there are alarms and personnel to respond to suction tube/pipe failures before all the solution is spilled. There is also a typically a containment structure in chemical areas in the larger plants. Also the suction lift capacities are easily exceeded when using larger tanks. Most Diaphragm pumps have a suction lift of only 5 ft water.

Trapped gas can be a big issue particularly if the pump is injecting to high pressure, a bubble of gas develops in the tubing, when it gets to the pump the pump losses prime pumping significantly less gas than it does when pumping liquid. Resulting is inconsistent treatment.

Hydrae

 

[bimr]

Sometimes when you visit facilities you see all sorts of things that defy logic and scientific explanation. However, you can rest assured that in the final analysis, the scientific principles are correct.

Many plants using hypochlorite complain about problems with vapor lock. The operators then find a way to make things work.

Many reocommend peristatlic pumps over diaphragm pumps. Diaphragm pumps tend to cause bubble to be created on the suction stroke which may or may not cause vapor lock.

You would be best served if you follow recommended design guidelines.

 

[ch81pc]

Another complicating factor is temperature. As the ambient temperature gets hotter the vapour locking issue will get worse. I had a system where we had a 1m +ve head of hypo feeding a diaphragm pump and even with the positive head the instantaneous low pressure pulse of the suction stroke would lead to gaslock issues in the summer. You can get round it, you just need to run your storage tank at a higher level.