Downsides to salt pools

[Gunslinger]

Richard, your explanations of SWG chemistry are truly awesome, but I don't believe for NY second that the reason for the manufacturers' 70-80ppm CYA recommendation (I've actually seen 80-100ppm recommended) has anything to do with real-world chemistry. Rather, I suspect it has more to do with a slightly different interpretation of the acronym "CYA", rooted in the early days of the SWG evolution when reliable operation was not up to par with current devices. In a loss-of-power event, for example, high CYA levels would help to ensure that all your residual FC doesn't disappear on a sunny day before you've noticed something is amiss. I believe there is no longer any rationale justification for this recommendation other than my suspicion that the manufacturers have no interest in pursuing the matter and are content to leave it unchanged.
Personally, I barely manage to keep my CYA level above 25ppm, and have experienced no consequences from that during the four seasons I have operated my Pool Pilot SWG -- although I do test my FC and Ph quite often.

[chem geek]

Gunslinger,

You may be right, but users of this forum, including Evan (waterbear) noticed that the output efficiency of the salt cell fell dramatically at lower CYA levels. That is, you needed to increase the time and/or power of the salt cell to achieve the same FC level when the CYA was below the 70-80 recommended range and this effect was MUCH larger than the sunlight protection effect of the additional CYA (which is quite minimal at higher CYA -- see this graph) . Now this specific CYA level probably varies by manufacturer, but this effect is absolutely real as it was reported by multiple users. Maybe your salt cell is one of the lucky ones that operates well at lower CYA, or maybe you never got to the 80 ppm CYA where the efficiency dramatically improves for some salt cells.

So at least some SWG manufacturers may be recommending higher CYA in order to have higher efficiency. The issue then becomes whether efficiency of chlorine generation is the only factor or whether the proportion of oxygen generation is also affected. I don't know the answer to that and was just speculating based on chemistry -- we don't have real-world data on SWG output (i.e. measuring the gasses) to know for certain.

giroup01,

I believe the high CYA recommendation is primarily for SWG cell efficiency as mentioned above. I do not know if the oxygen effect is real (there might be coatings on the plates to inhibit oxygen generation, if that's even possible) or if the manufacturers know about it if it is or if they recommend higher CYA partly for that reason. Basically, I don't know a lot. But if the chlorine generation rate goes down at lower CYA but the conductivity stays the same and there is the alternate reaction of generating oxygen available, then it seems reasonable to assume that more oxygen is generated at lower CYA. The extreme example of this occurs if you remove all of the chloride and just have a different electrolyte (sulfates, carbonates, etc.) in which case you get pretty much ONLY oxygen and hydrogen. I had fun as a kid making such concoctions and carefully exploding them (this is definitely a "don't try this at home" scenario where you must not use glassware and should wear glasses).

Richard

[Waterworks]

I was just speaking with my customer that has the indoor Stainless Steel pool. (This is not the same pool that I spoke of last week with the massively rusted ladders and +100 ppm chlorine.) Once his pool was up and running with the SWG he quickly noticed corrosion issues with the walls and floor of the pool. He is now using pucks, and I had him add a little bit of extra CYA. The build up of CYA over time won't be an issue because he drains the pool periodically. He has not noticed any corrosion since getting rid of the SWG and switching to pucks.

I told him that I thought that high chlorine was the cause of the corrosion and that I thought that by adding some CYA and more closely monitoring chlorine levels we could avoid the corrosion while allowing him to use his SWG again. The pool is completely 316L grade Stainless and is grounded properly. When the initial corrosion happened he brought a metallurgist out to look at the problems. The metallurgist told him that 316L stainless could not handle sodium chloride in concentrations above 500 ppm. This is vastly different from everything I've read about 316L steel.

Let me know what you think.

Brad

[chem geek]

Brad,

I can only refer you to this same EPA document that I referenced before where it says that the 316 stainless steel is more resistant to chlorine pitting than 304. However, it does show in at least some studies that 316 is superior to 316L, but I would find it surprising that salt levels above 500 ppm are simply corrosive to 316L. The chlorine appears to be far more corrosive to stainless steel than the salt level, though the salt level may accelerate such corrosion. So my take would be that using CYA would cut down the corrosion from chlorine to the point where the higher salt levels of an SWG pool wouldn't be a problem with 316 type steel.

However, this is all just speculation on my part based on the fact that we haven't had any reports of stainless steel corrosion in SWG pools that have been using CYA (and haven't had horribly high chlorine levels). We did have the one report of the indoor pool with 3-5 ppm FC, but that had no CYA.

So if your customer is up to it, they can bring up the CYA level in their pool to at least the 10-20 ppm range (if not 30 ppm) and then try the SWG (with the 3000 ppm salt). My prediction is that they won't see corrosion. If they had 304 stainless steel, then they *might* see corrosion eventually, but even then I'd bet it would take a few years.

Now having CYA in an indoor pool is going to present a different set of problems since indoor pools tend to get combined chlorines that do not break down as readily as outdoor pools (due to the lack of sunlight) so you might need to periodically use a non-chlorine shock (potassium monopersulfate) for the indoor pool. First try monitoring (or have the customer monitor) the combined chlorine level -- if it's not a problem, then you're done. If not, then use the non-chlorine shock.

By the way, you mentioned that the customer saw corrosion issues with the walls and floor of the pool. What does that mean? Isn't the pool plaster/gunite? Is he now talking about corrosion as in roughness of the plaster finish? Or is this pool made out of stainless steel?! Or do the walls have some stainless steel bars and by "the floor" he means metal floor drains? I'm confused.

That's my two cents (again, for what it's worth),
Richard

[Waterworks]

The pool is a one-piece shell made entirely of 316L stainless steel and was set in place by a crane; like a one-piece fiberglass pool, but made of steel. Don't ask how much it cost because I have no idea. I won't be able to convince him to try the SWG on that pool again because if I'm wrong it's an awfully expensive mistake. After the first round of corrosion the metalurgist applied some sort of clear corrosion proof sealant and somehow got rid of the original corrosion. I'm guessing that this was expensive as well.

Brad

[chem geek]

Brad,

Yikes! I had no idea such a thing as an all stainless steel pool existed. You are absolutely right that you don't want to take any chances with that kind of pool. Not using salt and adding CYA would be wise, just to play it absolutely safe. Since the customer is using Trichlor tabs (pucks), you should remind him that it is important for the feeder (if it's a floating feeder) not to park itself near the side of the pool with the circulation off. Otherwise, the acidity from the Trichlor will corrode the stainless steel in the vicinity. This is what happened to me in my own pool with stainless steel bars along the side of the pool -- the bars didn't corrode, but the mountings closest to the floating feeder did.

Richard

[Waterworks]

I don't think that Stainless Steel pools actually do 'exist' per se. As far as I know one of his companies builds SS tanks for agriculture or something and he had them build him this one as a one-of-a-kind thing.
He uses an off-line chlorinator, so the floating thing won't be a problem.

Brad

[chem geek]

Richard, your explanations of SWG chemistry are truly awesome, but I don't believe for NY second that the reason for the manufacturers' 70-80ppm CYA recommendation (I've actually seen 80-100ppm recommended) has anything to do with real-world chemistry. Rather, I suspect it has more to do with a slightly different interpretation of the acronym "CYA", rooted in the early days of the SWG evolution when reliable operation was not up to par with current devices. In a loss-of-power event, for example, high CYA levels would help to ensure that all your residual FC doesn't disappear on a sunny day before you've noticed something is amiss. I believe there is no longer any rationale justification for this recommendation other than my suspicion that the manufacturers have no interest in pursuing the matter and are content to leave it unchanged.
Personally, I barely manage to keep my CYA level above 25ppm, and have experienced no consequences from that during the four seasons I have operated my Pool Pilot SWG -- although I do test my FC and Ph quite often.

I want to apologize to you (and others) for very likely being wrong about the higher CYA improving SWG cell efficiency. The experiments Mark describes in the later posts in this thread have pretty much confirmed that SWG efficiency is not affected by CYA level (at least not from 45 to 80). There is some effect on efficiency from pH.

I incorrectly assumed that the industry charts showing the half-life of chlorine vs. CYA level were correct and based on experiments, but apparently they are wrong and based on incorrect assumptions or too few data points. Mark's tests plus feedback from other users shows that higher CYA levels protect chlorine by more than accounted for by simply forming more stable compounds. The CYA itself blocks some of the UV and apparently does so over rather shallow depths. I'm reworking the theoretical model to account for this new data in this post.

This is what science is all about. Finding real-world data and developing a model that explains it or changing (or expanding or replacing) an older model to account for this data. I'm sorry I didn't catch that or question the industry data earlier. Maybe now we will be able to make rational recommendations on CYA levels, including using higher CYA levels in very sunny areas, though keeping correspondingly higher FC levels, and no longer thinking that this is less efficient (i.e. uses more chlorine) because we now know that it probably doesn't.

Richard

[waterbear]

well, something is going on. I have seen in my own pool and my customer's pools a definite increase in SWG efficiency when the CYA is at the higher level of the recommended range based on cell output percentage and FC levels for a given pump run time. In my own pool, for exampe, when my CYA falls to about 50 ppm (I get a lot of spashout when I run my water features and have a small pool) I have to turn my generator output up to about 30% but when I get the CYA back up to around 70-80 I run the output at about 8-15% to maintain the same FC level of 4 ppm. I have seen similar effects in customer's pools with some of them having to run their cells at close to 100 % when the CYA was in the 40 ppm range and once the level was increased to the higher end the cell was running below 50% to maintain the FC. I still think there is something more than just chlorine retention going on here since the efficiency of the systems seem to rise greatly as the CYA level is increased. Also, I find it interesting that some of the higher salt system like PoolX recommend higher CYA levels (80-100 ppm) along with higher salt levels . Not really sure why.

[chem geek]

If the ability to lower the SWG output at higher CYA levels is during the day when there is sunlight, then the CYA protection mechanism would account for that. It's not necessary for the SWG to be more efficient -- if chlorine usage dropped due to more protection from CYA (even at higher FC levels) so less breakdown from the sun, then the SWG output can be turned down.

The reason I originally didn't think that CYA protection was the reason was that I trusted the industry chlorine half-life vs. CYA graph which did not account for the significant lowering of SWG on-time one can have at higher CYA levels. However, if this graph is wrong and CYA protection dramatically increases at higher levels, then that will result in the same ability to lower SWG output (level or time).

You can do a similar experiment as Mark did by running the SWG at night and seeing the rate of increase in the FC level at different CYA levels -- essentially just repeat what you are seeing during the day, but do it at night instead (or during the day with an opaque cover that protects the pool from sunlight). If you find that the rate of FC increase doesn't change with CYA level, then there is no change in SWG efficiency with CYA and the reason for the ability to lower the SWG level is just due to a lower rate of chlorine usage. Definitely let us know if you come up with different results as that would indicate a more complex scenario -- possibly a dependency on specific model of SWG.

Richard