PoolOwnerNumber9 said:
I have seen many instances of heaters being destroyed by people who used Trichlor in their skimmer, and who ran their pump continuously.
Though I can certainly understand how there could be a problem from having the pump cycle on and off, I don't see how from the chemistry that Trichlor in the skimmer with continuous 24/7 flow would lead to a problem. Of course, we don't recommend putting Trichlor in the skimmer (unless it's the kind that do not dissolve when there is no flow such as
BioGuard Smart Sticks). The pH simply does not drop enough and the chlorine levels do not rise enough to be an issue if the flow is continuous so something else must be going on. Perhaps the tablet doesn't dissolve continuously and once in a while "falls apart" with a piece that can then potentially get lodged somewhere downstream and cause a problem with local physical contact -- but that's just speculation on my part trying to jibe with your experience.
When researching corrosion in SWG pools, I was sent this PDF file about studies that showed that high chlorine of 20 ppm FC with no CYA in the water was highly corrosive, and 30 ppm extremely corrosive, at least to stainless steel. This study was comparing chlorine levels vs. salt levels and didn't look at pH as a factor (presumably the pH was maintained near 7.5, but the study doesn't explicitly state that), but it does indicate, as you say, that high levels of hypochlorous acid (so high levels of chlorine with no CYA or very high levels of chlorine with some CYA as found with Trichlor) can be highly corrosive. Consider the source when looking at studies, however (Eltech provided technology for SWG systems). Even so, we have seen stainless steel corrosion in salt pools at higher chlorine levels with no CYA (around 5 ppm FC with no CYA) after about a year, but not consistently. Stainless steel corrosion is a "runaway" type of corrosion since it is competing between rapid corrosion of steel vs. reforming of a passivity layer of chromium oxide (which is interfered by chloride ion). Copper corrosion, on the other hand, is just direct corrosion and slower than seen in plain (not stainless) steel and does not have the same "runaway" effect.
I know from personal experience that a Trichlor feeder parked near stainless steel poles in my pool rusted the mounts and also rusted a stainless steel thermometer in the skimmer when I briefly had Trichlor in there but not run continuously (these were the early days when I first owned a pool and didn't learn pool water chemistry). So there is no question that Trichlor can be a serious problem in the region where it dissolves if there is not good circulatory flow at all times.
Anyway, the debate isn't that important since we know that both acidic conditions and high hypochlorous acid levels are a problem and should be avoided. If someone wants to do some experiments with some copper pennies (actually copper coated zinc unless the penny is older than 1982) comparing acidic conditions with low chlorine vs. high chlorine levels at normal pH, that would be interesting.
Richard