Downsides to salt pools

[giroup01]

Sure.

Thanks! ...I'll have to chew on that, but yes, thank you!

When you say:

Having CYA in the water helps make this happen because it combines with the hypochlorous acid that is produced so that the reaction of chlorine dissolving in water can continue at a rapid pace. So the lower buildup of products in the chlorine reaction, due to CYA binding with the generated hypochlorous acid, means that this reaction becomes even more favored compared to the oxygen generation.

Would this explain why SWG manufacturers seem to specify a somewhat higher than normal CYA? To increase the ratio of chlorine to oxygen?

[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]

Brad,

OK, so with an off-line chlorinator he won't have the problem of the floating dispenser, but given this guy's history of not monitoring his water chemistry (which may have gotten the chlorine to a high level) he could now be in the position of getting his pH very low in his pool if he uses Trichlor exclusively and doesn't check the pH once and a while and add pH Up (Sodium Carbonate or Washing Soda). If the pH gets too low, then he can end up corroding his stainless steel again. Given his history, it would be a good idea to keep the TA on the high side, at least 120 ppm, as long as he's using Trichlor. Also keep in mind that with the CYA from the Trichlor he will have to maintain higher Free Chlorine levels as the CYA level rises in order to have proper disinfection and to prevent algae. See Ben's Best Guess CYA chart as a rough guide for how much FC is needed at various levels of CYA.

By the way, if you can get contact info for the metallurgist, then you can give that to me (via E-mail or Private Message) and I'll contact him to see where he got his 500 ppm salt limit for this type of stainless steel.

Thanks,
Richard

[Poolsean]

There's a company in North Carolina, Bradford Industries, that makes these Stainless Steel pools and spas and cost into the Tens of thousands of dollars! Most of the ones they display and are on their brochures show most of the pool/spa being tiled. However they do have some very large custom built ALL Stainless Steel commercial projects.

Very unusual looking with all straight sides. We have one of these all stainless steel spas in Boca Raton Florida at what was formerly the Mission Bay Aquatic Complex. This is a non-salt chlorinated spa. They use sodium hypochlorite from a Stenner Chemical Feed Pump. Most of the corrosion appear at the welds.

Just the fact that this spa was already showing signs of corrosion was reason enough for me to not propose a salt system on it. I'm sure salt would have been blamed for any more damage.