Chlorine Gas

[schrody]

Anyone have experience with it? How much would the recommended levels of other chemicals change while using it?

I'm asking because I found a local pool that uses it and their website states these recommended levels:
pH7.8 - 8.2
Alkalinity 90 -130 PPM
Chlorine 4.0 + PPM
Conditioner 100 PPM

The two I don't understand are pH and CYA. Why so high?

 

[chem geek]

The pH is high because the addition of chlorine gas is very acidic. The CYA is high because they are probably only adding chlorine once a week so to prevent chlorine loss during the day from sunlight the higher CYA is used.

As an example, let's start with 4 ppm FC, 100 ppm CYA, 120 ppm TA and a pH of 7.8. If I add enough chlorine gas to increase the FC by 10 ppm to get to 14 ppm, then the pH will drop to 7.4. As the chlorine then gets used up during the week and drops to 4 ppm FC, the pH would drop to 7.2 (and the TA would drop by 14 ppm to end up at 106 ppm) except there will probably be some carbon dioxide outgassing depending on the amount of aeration so the pH may stay near 7.4 or so, perhaps rising even higher.

So now they come by the next week and repeat the process and add some pH Up and/or baking soda in combination to get the pH back to 7.8 and the TA to 120 ppm. Then they add the chlorine gas. If the TA is higher or 50 ppm Borates are used, then the swing in pH is less.

Though some algae could potentially just start to grow in the day or two at the end of the week when the chlorine is low, it gets killed when the chlorine is then raised to the higher level. So the pool remains clear and algae free. The 4 to 14 ppm FC swing is still within normal chlorine limits with the FC staying below 15% of the CYA level so would not be irritating or noticeable.

The only thing to be careful of with this approach is that the application of chlorine gas into the pool needs to be done carefully. The flow rate for the chlorine gas needs to be such that it all dissolves into the pool and fairly evenly, so not all at the bottom of the pool (they usually put the hose into the deep end and have it pointed/bubbling upwards). This helps to prevent the acidity from damaging plaster which is the most common problem from pool services using chlorine gas that don't know how to apply it correctly. An example of a pool service that does this well is Pool Chlor whom I visited when I went to the NEHA conference in Arizona last year (Que Hales of Pool Chlor also runs JSPSI).

Chlorine gas is the cheapest form of chlorine so hopefully the pool service is passing on some of this savings to you.

Richard

 

[Ohm_Boy]

the application of chlorine gas into the pool needs to be done carefully

I recall a municipal gas chlorination system that somehow pumped a vacuum on the water, and the chlorine gas injector operated below atmospheric pressure. I think that this was A) supposed to help prevent gas escape and B) get more complete dissolution of the chlorine into the water. My recollection is foggy ,as I didn't deal much with the chlorine system at the time. I was part of a Bromine Prototype Team doing a benchmark test there, so most of my stuff was in a different section. But the operators seemed pretty proud of their vacuum system.

 

[schrody]

Thank you, Richard. That definitely explains the CYA. It also explains why they would want that pH range before adding gas...but the site reads more like they expect your pH to remain high. Maybe I read it wrong.

In any case, I'm not currently using them or gas (if you could only see all the bleach bottles around the house). I am, however, considering asking if they need part time help. I appreciate your information and clear explanation of the process!