Miscellaneous Short Discussions

[chem geek]

Watermom,

In this thread you mention how pouring bleach down the skimmer with the pump running is not a problem and from a practical point of view I would tend to agree, but do want to be more specific about the potential for harm.

If we assume a GPM rate of 40 when the pump is running, then 6% bleach is around 60,000 ppm FC and if it is poured slowly at a rate of about 5 seconds per cup, then that's 1.33 minutes per gallon (0.75 GPM) so this is a dilution ratio of 53 to 1 so an FC level hitting the pump, filter, etc. of about 1000 ppm FC (it's higher in the PVC line from the skimmer, but I'm assuming that all gets mixed together with the water from the other return lines; the GPM rate at the skimmer will be lower, but it will get diluted later so it nets out to the overall GPM rate of the system). This isn't all disinfecting chlorine since the pH will be high. I calculate that with normal pool conditions (pH 7.5, TA 100, CYA 30) that the pH will rise to about 9.3 and the disinfecting chlorine level will be 13.8 ppm. For comparison, the recommendation for the mid-point of Ben's Min/Max is 0.05 ppm and for shocking is around 0.3 ppm so the disinfecting chlorine level is indeed quite high and is similar to having 27 ppm FC with no CYA.

The time for exposure is probably only around 30 seconds to a minute depending on how much is being added, but this could happen every day.

I am bringing this up not only for the issue of how to pour chlorine into the pool, but to address concerns with using a liquid form of chlorine in vinyl pools. Some pool builders swear that using a sodium hypochlorite will damage vinyl pools and some say to use lithium hypochlorite instead. Though more expensive, it dissolves quickly and can be broadcast around the pool so is not so concentrated in one place. I'm guessing that pouring chlorine quickly in one place may have the chlorine settle briefly near the bottom (it's denser than water so may settle until it mixes). Clearly this can happen with Cal-Hypo which is why it should get mixed in a bucket before adding to a vinyl pool. I was hoping that pouring sodium hypochlorite slowly in front of a return would be the best approach, especially in the deep end where the water flow is strong from return to skimmer and return to floor drain(s). It's almost impossible to calculate what happens, but I can imagine that slowly pouring in front of a return such that very little chlorine avoids hitting the flow would cause the chlorine to rapidly disperse into a larger volume of water and therefore get diluted rather quickly and therefore be safer.

No conclusions here -- just putting out some info.

As for Cyanuric Acid dissolving in the skimmer, that's a whole other matter since it dissolves so slowly. In this case, it may take many hours to dissolve so if it took as long as one turnover of water, then it essentially would not cause any higher concentration of acid or CYA at all. Of course, the pump must be running continuously or else there can be a localized buildup of acidity.

A Trichlor tablet in the skimmer may also be safe for the same reason, so long as the pump is continually running. I don't remember how long it takes for a tablet to dissolve, but I do think it's hours.

Richard