SWCG in Plaster Pool

[BlueWave8]

I would very much like to convert my copper/silver ion generator system to SWCG.

Part of my job is designing guardrail safety fixtures for highways, so I am very familiar with the effects of salt, chlorine, and moisture on steel (even galvanized or stainless steel). My main concern is for the rebar in my concrete/plaster pool. With concrete being essentially porous, should I be concerned? I know that 3,000 ppm is not much compared to the ocean, but with the presence of chlorine it can still cause or accelerate stress corrosion cracking (as far as the rebar goes-though I have limited knowledge of concrete, so I am not sure if this should be a concern). I am also slightly concerned about the tile and the plaster itself. Since it is an indoor pool, I am also slightly concerned about the metal pieces of the HVAC system (ducting, heat exchanger).

Can anyone ease my mind or contribute some personal experience or technical knowledge? Is corrosion increased over pool water with chlorine only? If so approximately how much or how big of a risk is it?

 

[PoolGuyNJ]

The steel should be near the outside of the shell. The plaster finish is the water proofer. The bond wire will, in most cases dissipate any stray currents but adding a zinc anode after the cell and attaching it to the bonding wire is neither difficult and will absolutely minimize any strays from the cell.

Calcium chloride is a different salt than sodium chloride and tends to be more reactive. The salt spray from melting ice of a road mix used for de-icing use can be substantially higher in concentration too and since it doesn't get washed off, sticks around more on guard rails and sign posts, compound that set of issues.

The scale of the faced challenges is substantially lessened in a pool.

The stainless used in pools is typically a high grade and very resistant to rust.

 

[BlueWave8]

Thank you for the very informed response. I was looking for just this kind of information.

So you would not worry about running an indoor plaster pool with a SWCG? Have you ever seen signs of swimming pool corrosion that can be contributed to salt use?

I am just trying to make sure its not going to cause my pool to need refurbishing 10 or 20 years sooner than it would otherwise.

 

[dmanb2b]

Ease your mind

Just kidding

All chlorine pools have salt in them as one of the main byproducts of liquid or stabilized chlorine is salt. In a typical pool, it is not uncommon to find 500-1500ppm. Although 3000-5000ppm is much higher, salt pools have been in use for well over 40 years.

As far as HVAC is concerned, I would assume the main corrosion risk there is condensation from the pool water, but regardless of what's in your pool water, condensation itself is mineral free.

Hope this helps

 

[dmanb2b]

Sorry just saw your 10-20 year comment. A regular plaster pools avg. finish life span is 10-15 years or so...maybe a little longer in an indoor pool, while pebble-tec may last around 20 years. Either way, it's not the salt, but pools do need to be refurbished every 10-25 years...your mileage will vary, but the biggest threat to pool finishes is improper water chemistry vs. salt. Just make sure your CSI is in range for a plaster pool and you should be all set

 

[BlueWave8]

Thanks for the info, I am very new to pool ownership, but not new to water chemistry! (salt water aquariums....)

I did not know what the expected life of a plaster pool was, I was hoping it was longer than that though! Will the pool surface visibly degrade when it needs replacing?

Unfortunately I do not know what the maintenance patterns of the pool have been in the past, but I will be getting everything into shape over the next couple months.

Am I correct in that a truly optimum CSI would be 0?

 

[dmanb2b]

No worries...I pulled this from the poolcalculator.com website (very useful tool/website)...also, do check out pool school for recommended chemistry (button on upper right of this page)

"The calcite saturation index is a tool for estimating the likelihood of plaster corrosion or calcite scaling. The LSI, Langelier Saturation Index, is a very similar but slightly less accurate measure. The CSI uses pH, TA, CH, CYA, temperature, Borate, and Salt levels to estimate the likelihood of problems. A low saturation index means the water is likely to dissolve calcite out of plaster, pebble, tile, stone, and concrete surfaces (and perhaps fiberglass) which will eventually cause damage. A high saturation index means the water is likely to deposit calcite scale on the walls of the pool and in the plumbing.

CSI is most sensitive to changes in pH. With a plaster pool, it is best to try and get your CSI a little below zero, so that changes in pH won't shift your pool too far towards corrosion or scaling. With a vinyl pool the CSI can be kept more negative, which makes it very unlikely that pH changes could get the CSI into the range of scaling risk."

http://www.poolcalculator.com/chemistry.html

http://www.poolcalculator.com

 

[duraleigh]

Hey, Blue Wave,

I know you didn't ask this but I'll toss it out there for your consideration.

I would manually chlorinate an indoor pool with Clorox. Your FC loss will be so small that a jug of Clorox may last you a week or so and, to me, the convenience of an SWG would be negated.

And other than convenience, SWG's hold only small advantages over manual chlorination.

 

[BlueWave8]

Thank you both for the responses.

Dave, I may consider that option. I will have to see where the FC levels stay...

I'll be making my first trip to the pool tomorrow morning, test kit won't be here until later next week.

 

[JamesW]

I think that the two primary concerns regarding the HVAC system would be humidity and trichloramines (nitrogen trichloride).

Proper air handling should address these concerns. Humidity sensors should be used to help manage humidity.

Nitrogen trichloride is primarily the result of the oxidation of urea. Making sure that bathers are clean and do not pee in the pool will minimize the formation of trichloramine.

Low bather load pools will not typically develop trichloramine problems. High bather loads pools should consider the use of supplemental oxidation to reduce the formation of volatile disinfection byproducts.

Using about 20 to 30 ppm of cyanuric acid should greatly reduce the formation of trichloramines (nitrogen trichloride).

Maintaining consistent chlorine levels will also help reduce trichloramines. Shocking increases the rate of trichloramine formation due to the increased chlorine to nitrogen ratio.

 

[Ranger987]

To add a few thoughts... The places where "salt damage" most often show up are on fixtures outside the pool. Items such as diving boards, deck chairs, sandstone coping, handrails, etc., where the salt water will come in contact due to normal pool use, should be rinsed regularly to keep the salt level from building up with repeated exposure. Generally speaking, you will maintain around 2500-4000ppm in your pool. The saltwater doesn't get really corrosive until around 6000ppm. (I'm using very general numbers) When you climb out of the pool, the saltwater splashes on the fixtures, water evaporates & salt remains. Without rinsing, the level of salt will increase until it becomes far more damaging than "normal". Just rinse with fresh water regularly and you should be fine.

Having said that, duraleigh's comments regarding the indoor pool are valid. As he suggested, your chlorine demand should be so low that the ECGs tend to overchlorinate, unless closely monitored. I personally worked on a pool only slightly smaller than yours with that issue. The chlorine level was so high that we couldn't accurately measure it. We drained the pool 50%, refilled, and still had extremely high CL level, and ended up dumping all the water & starting over.

 

[chem geek]

As was just noted, splash-out with evaporation to concentrate salt can be particularly damaging as shown in this post regarding aluminum rails used for an automatic pool cover for an indoor pool. However, such issues can be avoided or mitigated just as other metal corrosion can be reduced through the use of a zinc sacrificial anode connected to the bonding wire as was also mentioned.

The use of a small amount of CYA in the pool (around 20 ppm) will reduce chlorine's strength to reduce the rate of metal corrosion from chlorine as well. Some pools with 3-5 ppm FC with no CYA were reported to have more rapid stainless steel corrosion in saltwater chlorine generator pools.

The main problem you will have with an indoor pool is the lack of sunlight helping to break down the buildup of slow-to-oxidize organics such as urea which can then lead to higher chloramines that can smell. An SWG can help a bit through it's super-chlorination of a portion of the water in the cell, though usually people install UV systems to help. Another approach is to use a non-chlorine shock (MPS) on occasion. If the bather-load is relatively low, then this latter approach is probably the easiest to do, if necessary. Some people use enzymes to help break down organics, but I'm not so certain that they do anything with urea -- I think they are mostly for oils such as from lotions and such.