At the risk of stirring up a hornets nest, I would like to get some feedback on the long held belief that adding acid via the skimmer may damage some equipment with metal. In most cases this probably not a wise thing to do but for some pools under some situations could this method actually be beneficial? Let me explain why I think so using my situation as an example.
First, I have a Northstar pump which happens to use a synthetic impeller which is, as far as I know, not subject to corrosion. Some sort of polymer or fiberglass.
Second, the only other components in my setup which could be damaged by acid are the heater's heat exchanger and the swg.
Third, my fill water is quite high in PH, TA and CH. Over time CH builds up to a point where I basically have to do a refill about once a year. My CH starts off at 200 ppm but a year later is well over 600 ppm. Also, because of evaporation and the SWG, each week I add about 1/2 gallon of acid. And yes, I tried adding Borax but I think the fill water trumps any benefit Borax may have and I still have had to add the same amount of acid.
Fourth, by the time acid hits the metal equipment, I estimate that it is diluted to a ratio of about 40:1. The flow rate of my system is about 88 GPM and I can pour acid at a rate of about ½ gallon in 15 seconds or about 2 GPM, so my dilution ratio is higher than about 40:1. In addition, the low PH would only occur for the duration of the pour which is about 15 seconds.
So since I probably have a decent coating of calcium on the SWG plates and the heat exchanger, intuition would tell me that the low PH for 15 seconds would probably not cause any damage. In fact, the occasional dosing of acid would help keep calcium from building up on the plates of the SWG as well as the heat exchanger.
One more side benefit for a plaster pool is that there is no chance of acid pooling at the bottom of the pool that can occur even if poured near a return.
In addition, the SWG manual suggests using 2:1 water to acid solution for cleaning the cell once every three months for several minutes. So in terms of damage to the cell would 4 times a year at 2:1 for several minutes be better than 52 times a year at 40:1 for 15 seconds? Again, I would think more often at lower acid concentrations would be less destructive to the cell but that is just a hunch.
So my question to the chemists out there, would subjecting metal to 15 seconds of 40:1 acid dilution once a week cause any appreciable corrosion especially when there is a high calcium concentration in the water as well?
So am I missing something here or is this a viable method of adding acid? Maybe, just not for everyone.