Corrosion and scaling - related to SWG?

[Chas]

I have a customer with a 10,000 gallon 10x30 rectangle indoor vinyl liner IG pool using a Zodiac Duo-Clear SWG, heated with a Lochinvar ERL 152-8009 gas fired pool heater. My only involvement here was in 2010 when I replaced the liner, installed the SWG and heater. I looked at the pool this past Monday 9/14/15 because the liner is pulling out of the track at one of the 2' radius corners and the heater is boiling water and banging. My observations are:

Water tested with a TF100 test kit purchased from this website in June of 2015:
FC 3.0
CC .5
TC 3.5
CH 425
TA 110
CYA < 5 (barely present, she added 1/2 pound because the bottle said "conditioner"

A local pool store has been selling her the salt and testing the salt level for her. I don't have a test kit for salt and really don't know salt systems. I read the stick re; SWG, that didn't answer my questions.

The liner is pulled out of the track for about 16" in one of the radius corners because there's nothing left of the lip that the liner bead locks into. The inside of the track is packed full of with crystals, from the size of sand to some as large as 1/4" in diameter, irregularly crystalline shaped, off white slightly brownish in color. I tasted one, it tastes a little salty but not biting like a piece of rock salt. The coping is anodized extruded aluminum and there are actually 6 - 8 small holes eaten through the coping along that 16" section of the 2' radiused corner, from 1/32" to 1/8" in size. This coping corner sounds hollow when tapped on, the other three corners do not. There is no other signs of corrosion to the coping other than in this corner. She adds salt to the pool at the end that's opposite this corner.

The owner gave me a small bag of scale pieces, some as large as 1/4" at about 1/32" thick, slightly curved (like the inside of a heat exchanger tube) that she collected from the bottom of the pool below the returns, she said it only sends this scale into the pool when the heater is running. She hasn't run the heater at all this summer because of the banging.

There is evidence of light brown staining to the liner and especially at the white returns. The owner agrees that she has a high mineral (iron) content to her makeup water.

1) Is SWG appropriate for an indoor pool?
2) Is CYA appropriate for an indoor pool when used with SWG?
3) Is the salty water corroding the aluminum?
4) If the water is corrosive, can it also be scale forming in the heater's tube bundle heat exchanger?
5) The heater Mfg says CA up to 350 is acceptable. Change some (or most) of the water to drop it from it's present 425?
6) Is there any way to measure for or determine electrolysis, ie; possibly the pool and coping isn't grounded or bonded correctly?
7) Any tips on cleaning a heat exchanger cupronickel tube bundle in or out of a Lochinvar ERL 152-8009?

Thanks for any input! This is a tough one for me.

 

[Texas Splash]

Chas, any chance you have the pH reading handy as well? That's going to be quite important when analyzing this issue.

 

[Chas]

Ouch! I guess I spaced listing the pH as I was listing readings. The pH was a high 7.6.

Thanks Texas Splash for catching that!

 

[chem geek]

1) Is SWG appropriate for an indoor pool?

Yes, but you can't have things like aluminum tracks on coping for pool covers due to splash-out, but that has little to do with being indoors. Outdoor pools would have the same problems unless they get regular summer rains to dilute the splashed-out salt.

2) Is CYA appropriate for an indoor pool when used with SWG?

CYA is essential in an indoor pool regardless of whether or not an SWG is used. With no CYA in the water, the chlorine is not moderated in its strength so 3 ppm FC with no CYA is 50 times higher in active chlorine (hypochlorous acid) level than the minimum FC/CYA ratios we recommend. This will corrode metal faster as well as pump seals and be harsher on swimsuits, skin, and hair. You are lucky that the gas heater was cupro-nickel because if it were just copper it would probably have corroded due to the VERY high active chlorine level combined with SWG salt levels.

3) Is the salty water corroding the aluminum?

See this post. Salt splash-out and evaporation concentrates salt and salt is bad for any metal, but particularly for aluminum. Though non-SWG pools also have some salt, it's usually much lower than in SWG pools so the build-up takes longer. One can mitigate this damage by regularly washing away the salt by hosing with tap water.

4) If the water is corrosive, can it also be scale forming in the heater's tube bundle heat exchanger?

The higher corrosion from salt has nothing to do with being scale forming. Assuming the pH of 7.6 you later wrote and assuming 3000 ppm salt and 88ºF water temperature, the Calcite Saturation Index (CSI) is +0.20 in the bulk pool water, but in the heater that is 30ºF hotter at the surface of the heat exchanger the CSI would be around +0.4 which might scale some but usually not a lot. I suspect that perhaps the pH has been even higher in the past because getting to 7.9 would have the CSI be +0.7 when scaling is more likely.

5) The heater Mfg says CA up to 350 is acceptable. Change some (or most) of the water to drop it from it's present 425?

It's not the CH alone that is the issue but the COMBINATION of higher pH, TA, and CH. So you have different ways of fixing this. Since the pH tends to rise in SWG pools, lowering the TA level would help reduce that rate of rise if some of it is from carbon dioxide outgassing. If the pool is covered most of the time, then that outgassing should be minimal. At any rate, since it's a vinyl pool there's no need for the higher CH so yes you could replace some of the water to lower the CH as well.

6) Is there any way to measure for or determine electrolysis, ie; possibly the pool and coping isn't grounded or bonded correctly?

If the issue is from splash-out, then no amount of electrical protection is going to overcome that. If the problem is exposure to salt water (not splash-out), then electrically connecting a zinc anode to the bonding wire and burying it in moist soil would protect all metal connected to that bonding wire.

7) Any tips on cleaning a heat exchanger cupronickel tube bundle in or out of a Lochinvar ERL 152-8009?

Someone else can answer this one.

 

[Chas]

Hey chem geek -
Thanks a bunch for the info!! I'll need to do some thinking and discussion with the owner.

I'm still waiting for a reply from her as to what the salt level was. I'm leaning toward changing the bulk of the water but that brings its own set of problems with iron.

I'm in the HVAC field as my regular gig, I'll be able to handle the heater cleanup. Just never did that model heater before, thought if there were any tips on that model it will be appreciated.

Thanks again! I'll be back......

 

[Chas]

For what it's worth......the customer called me back today to say that the salt level 3 months ago was at 3,000. I bet that chem geek is right and the pH has been higher in the past. We'll service the heater tomorrow. I'm thinking that it's likely that chem geek is also right regarding splashout, possibly the aluminum coping at that corner (the only place with significant corrosion and sounds hollow when I tap on it) is rotting from the backside as splashout seeps behind the hollow coping. A caulk seal may be in order there, I'll have to look at this again.

I looked at this post that chem geek recommended. Wow! That's it! Those pics show exactly the irregularly crystalline shaped substance I found.

I am surprised to see the recommendation of CYA for the indoor pool. I've always been told (by pool "pros") no CYA for indoor pools. After reading and studying chem geek's comments re; CYA, I think I understand and we'll need to adjust that. I think we'll begin with a water changeout, then work with what we get for new water. I'm thinking Metal Magic or similar should be added right away as the pool is refilled with the iron filled tap water. That tap water will likely have a TA of around 280, after reading the comments regarding a lower TA, lower than the 110 it is now, I wonder what my target should be?

 

[chem geek]

As I'm sure you've figured out by now, there's a lot of advice from "pool pros" and the pool and spa industry in general that is either just plain wrong or not the best advice. The industry doesn't look at the fundamental chemistry of what is going on, especially when doing so would conflict with product sales. It's an old industry with little incentive to innovate, explore, or understand.

Just note that the CYA level for an indoor pool should be on the low side, 20-30 ppm, since it is just there to moderate chlorine's strength. Also, since there is no sunlight, it may be necessary to have a higher FC/CYA ratio than we normally recommend, up to a 20% FC/CYA ratio (so 4 ppm FC with 20 ppm CYA or 6 ppm FC with 30 ppm CYA). This may not be necessary, but if there tends to be a buildup of CCs then the higher chlorine level may help keep that in check. Indoor pools are where a supplemental UV or ozone system may be helpful to control CC.

As for your TA target, it only needs to be lowered if you find that the pH rises over time. If a pool cover is used, that may not happen so the TA may be OK where it is. Just remember that the combination of pH, TA, and CH (plus some other lesser factors) determine whether calcium carbonate scaling may occur so with the high CH it's probably best to lower the TA level, especially given the history in this pool. Our SWG recommendations in the Pool School are for a TA of 60-80 ppm so you can see how it is with 70 ppm in terms of pH rise over time.

 

[Chas]

It's an old industry with little incentive to innovate, explore, or understand.

Boy, you got THAT RIGHT! That fact is a big reason I got out of the biz full time and went into HVAC back in the mid '90s. HVAC has its share of head-in-the-sand (or somewhere else) as well, but nothing like the old guard pool guys......

So - CO2? Where does that come from? Yes, she does keep a blanket on the pool.

Thanks for the excellent info!

 

[chem geek]

Pools are intentionally over-carbonated to provide pH buffering and to protect plaster surfaces, providing carbonate. You add calcium to the pool and the combination of calcium and carbonate saturates the water to prevent dissolving of calcium carbonate from plaster surfaces.

So the carbon dioxide is generally in tap water and if the TA is very low it is even added to pool water, usually from baking soda (Alkalinity Up; sodium bicarbonate). The problem is that this TA is a SOURCE of rising pH because the amount of carbon dioxide in the water is more than that which would naturally be their from being exposed to air. So that excess carbon dioxide outgases from the pool. It outgases faster with more aeration and at lower pH.