Saturation Index (Langelier, etc.)

[waterbear]

Richard,
I think your theoretical discussions of pool chemistry are extremely important! They provide a good basic model that we can then apply to real world instances and in doing so discover what other factors are playing a role in the outcome.....keep up the good work!

[medvampire]

Richard
Well, I certainly hope that my looking at the pool and taking measurements is not causing a quantum state reduction that affects my results.

Richard
If I find a worm hole in my back yard its your fault.
Steve

[DaveD]

Saturation or scaling indices - puckorius, Ryzner or Langlier are used extensively in the water treatment industry to prevent scaling on heat exchangers (heaters) and we also use them on the industrial pad type humidifers (Munters). I am no expert on them though I do know that using them will prevent excessive scale (calcium) build up. While everyone dumps on pool stores that use them improperly, they can help in a situation where scaling is an issue. An example is my fathers hot tub and his unsoftened well water with a Calcium level of 1000 from the tap. He has scale rings foring in the fiberglass tub and is concerned about his heater scaling. So who knows how to use properly and can tell me the right combination of Alkalinity, pH, etc. to stop his scaling?

[medvampire]

The previous discussion from months ago did not totally dismiss the saturation indexes but did explore the non predictability of a pool environment. Even after my discussion with Richard (chemgeek) I do feel there are merits to predicting the saturation index using his calcations. I can’t find his spreadsheets he created readily but he did some great work in prediction scaling and I would recommend using his calculations. Down and dirty answer. Lower pH and ALK may prevent the precipitation of calcium but may damage the heater if the heater core is copper.
Steve

[chem geek]

The spreadsheet is still accessible at the PoolEquations.zip link found near the end of the first post in this thread. The corrections or enhancements I made to the Langelier saturation index were not huge (to the temperature and TDS portions of his index), but it is just based on the solubility of calcium carbonate. The theory is that having calcium carbonate just "in balance", that is in full saturation, means that the plaster will not have a tendency to dissolve nor will it have a tendency to form additional scale. In practice, one needs to be well away from the "0.0" balance of the index to see problems. Some people don't notice issues until +0.7 while others don't see problems until +1.0 (on the positive side for scaling -- don't know for corrosion or pitting of plaster, but may also be this same order of magnitude).

With regard to copper and other metal pipes, the theory is that having near-saturation calcium carbonate forms a thin film or other properties on the metal that helps to inhibit corrosion. Too much saturation will cause scale and too little prevents the film from forming. However, even without the thin film, corrosion of metal requires other factors including lower pH and dissolved oxygen and conductivity (ionic strength). The pool environment and the above neutral (> 7.0) pH usually mean that metal will not corrode. It is generally only when one uses Tri-Chlor tablets and doesn't carefully montor the pH that one risks getting the pH lowered. Storing chlorine in non-airtight containers in the pump house can also lead to corrosion of pumps and other metal.

Richard

[DaveD]

To clarify, my Dad's issue is with a fiberglass hot tub. He has white rings forming just above the water line and is concerned about the heater getting fouled. His tap water is 1000 Calcium (from a well). When he called the spa manufacturer, they recommended a sequestering (also know as chelating?) agent such as Leisure Time Spa Defender. Am requesting MSDS sheets for it. They also suggested the ultimate solution would be to use 50% salt softener water with 50% well. My suggestion to him was to lower his pH from 8 to 7.4 and his alkalinity from 80 to 60. He uses bromine. Will look at the calculator you mention for langlier. Doesn't temp have a major influence on the scaling indices? Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.

[DaveD]

To clarify futher - a pool heater is often gas fired into a heat exchanger. Differences in the tube or exchanger to water temp would most likely be in the 30F area as Ben suggests depending on the water flow rate over the heat exchanger. In an electric water heater, the heater is essentially a calrod like an electric stove and some have a sheath over the element made of S.S. Looking at a watlow info sheet on one of their heaters, the surface temp difference to the fluid can be as high as 100-200F. A very minor build up on the element sheath (same with a gas fired unit) can reduce transfer efficiency and cause higher gas or electric usage. For electric heaters, the buildup can be so severe as to cause the element to overheat and burn out. Since a pool heater exchanger material can be materials other than S.S. that do not lend themselves to being isolated and chemically cleaned with acid, it would seem to me to be important to keep the exchanger or elements clean - energy will only get more expensive!

Below is a question about just such an issue:
"Thank you for your replies. I am in New Zealand and investigating why we cannot get domestic water heater elements that last.We can get 304 s/s and Incoly sheathed elements (at a price) but as they are machine made the element leg diameter is rather small to give a good watts/sq.in.ratio. Corrosion of the sheath and/or resistance wire failure occures prematurely because of lime scale buildup which raises the surface and internal temp.of the leg.I also believe that current element design of two or more element legs in parallel causes a hot spot between the legs which encourages the deposit of lime in this area. It soon builds up into a solid mass and
increases corrosion in this area.I am looking at a design for possible manufacture using a single tube of a material which will at least help solve the corrosion problem."

[chem geek]

To clarify, my Dad's issue is with a fiberglass hot tub.
:
Since the "surface temp" of a heater element is much higher than the surrounding water, doe this effect the optimum point to prevent scaling?

My other suggestion to him was that he may be better off just getting the water to where it won't form the white rings and not worry about the heater. It is supposedly S.S. Suggested he find out how difficult it is to remove and clean it since S.S. will easily tolerate acid cleaning.

So with the fiberglass hot tub you don't need to worry about corrosion or pitting of plaster/gunite surfaces (unless you've got tile grout around a top edge or something like that). So you can tend to run your water in a "corrosive" state with regard to the normal indices. Just don't have it be too low in pH. And yes, having a SS heater means you are probably even less likely to corrode than copper (iron would be worse, of course).

The problem, of course, is that your father's source water is so high in hardness to begin with. We've talked about this issue of high hardness source water in other threads and pretty much come to the conclusion that the most effective way to deal with this is to use a water softener (the type where water flows through an ion exchange resin or similar device) which will essentially exchange the calcium ions with sodium (or potassium).

So assuming the source water is high in calcium hardness but isn't very high in TA (otherwise it would be very cloudy and percipitating calcium carbonate unless it's pH was also high), you could just run the hot tub water on the low side of TA (about 50) to compensate for the high CH. The main downside with the lower TA will be that the pH will be harder to control. Also, even with a TA of 50 and a CH of 1000, at hot tub temperatures you will still be in a scaling state though much less so. You could use some Borax (compensated with acid) for a borate buffer (see this thread for more info on this), but such a buffer tends to help prevent a rise in pH and isn't as good in preventing a drop in pH. All in all, I think getting the CH lowered from the source water is the way to go and using the water softener will let you do that.

As for temperature, yes you are right that the scaling is a function of temperature so when a heater is used you want to take that into account. In practice, the heater isn't on all of the time so while some tendency towards scaling might occur while heating, it may have a tendency to dissolve such scale when not heating. Ben tends to run his pools slightly on the corrosive side for this reason (and possibly others) but in your specific case you can safely run more on the corrosive side without harm because the hot tub is fiberglass and has no plaster/gunite/grout.

Your advice of running at lower pH (still above 7.0, however) and lower TA is good. It's just that with a hot tub it is much harder to control pH to begin with so with the lower TA and lower pH operating conditions it's trickier to prevent going too low. Fortunately, the bubbling action in a hot tub would tend to outgass CO2 and make the pH rise (though less so at lower pH and lower TA), but any chemical additions (including bromine) could affect the pH. You'll have to experiment to find that optimum balance of non-scaling, stability and control.

Richard