Ryznar stability index

[bobobi-wan]

does any know about finding your ryznar stability index?I googled it and found a great website, unfortunately it is very technical and you would need college chemistry to decipher it.I had 2 pentair heaters lost to corrosion and my readings were kept in range except that my spas always had a.m. chlorine readings which I treated for by lowering them to 3-5 ppm.from what I've researched,this ryznar index would show me what led to the aggressive water.

[chem geek]

You can read more about various saturation indices and the controversy of whether they really do predict corrosion rates here. You can calculate the Langelier Saturation Index (actually the Calcite Saturation Index) using The Pool Calculator. The saturation indices have more to do with whether calcium carbonate, such as found in plaster and grout, will dissolve or will scale, not whether metal will corrode. As you can read from the link, it is not clear whether the "thin film of calcium carbonate scale" really works or not to prevent metal corrosion.

An online calculator for the Ryznar Stability Index is here. An easy way to calculate it is to first calculate the CSI from The Pool Calculator and use the following formula to calculate the Ryznar index:

Ryznar = 2*pHs - pH = 2*(pH - CSI) - pH = pH - 2*CSI

So this basically says that if you want a Ryznar index of 7.0 or below, then at a pH of 7.5 you want the CSI to be +0.5 which in most pools would be on the edge of forming scale. I wouldn't recommend this. Instead, see what your current CSI is at an if it's negative, adjust the Calcium Hardness up to make the index near zero or very slightly positive (assuming your pH is where you want it near 7.5 and your TA is reasonable).

Low pH is the primary source of metal corrosion as is a high oxidizer level which is especially the case if no Cyanuric Acid (CYA) is used.

[bobobi-wan]

I'm not sure how PHs is gotten and at first I was shocking the spas with 6oz of monopersulfate in the 3300 gal spa and 8oz in the 6300 gal spa.my goal was to raise the orp so that the monitor wouldn't add (10 %)chlorine to water already high in chlorine that I was trying to bring back into 3-5 ppm by using thiosulfate.

[bobobi-wan]

I should also add that I now only add 3oz doses of oxidizer to the spas.

[chem geek]

MPS is generally acidic and some brands of MPS are very acidic as they are not pH balanced (they don't have the equivalent of some pH Up in them to help balance their normal acidity). So regularly using MPS could have lowered your pH. A combination of low pH and high FC (especially with little or no CYA) is corrosive. Also, the sulfates added by MPS can also accelerate corrosion, but usually that's for stainless steel or other metals that use a passivity layer and usually such corrosion is when the salt (chloride, specifically) level is higher.

However, unless you were using the MPS extensively and did not check your spa's pH, then I'm not sure this is the cause of the corrosion.

[bobobi-wan]

I noticed the chlor-out lowered the ph but not that the mps did.the ph would get down to 7.3 but would seem to come back up by afternoon altho occaisionally it stayed at 7.3 then I dosed it with soda ash.

[chem geek]

As was noted by Me... in the other thread you started, your chlorine levels may have been too high. If you didn't have Cyanuric Acid (CYA) in the water, then I would say that was definitely too high, but you said the CYA level was 10-30 ppm which seems strange. Are you testing that yourself and see the water in the tube being cloudy, but you can still see the black dot at the top -- is that 30 ppm? There are tubes that go to 20 ppm that might be better.

You should also use a sensitive voltmeter (multimeter) to compare the heat exchanger or bonding wire (since I assume they are electrically connected) to the pool water or ground since there could be a short or stray voltage somewhere making a negative voltage on the bonding wire / heat exchanger. This would rapidly accelerate corrosion.

Richard

[bobobi-wan]

you're right on the money.we had a third party come look at the heater,showed him the damaged exchanger and he immediately said it wasn't chemical and checked the ground which was loose.he also said chemical damage would leave the copper shiny on the inside,not with greenish corrosion.the 2nd heater also had a loose ground at the pump(they were grounded in series).thanks for all your input.

[bobobi-wan]

the guy at our pool supply house said the failure of the "thermostat" flow regulator caused the chemical corrosion of the copper in the heat exchanger. he was saying that a loose ground cause the damage the heater had.have you had any experience in this area? we're still waiting on a evaluation from Az.Boiler.

[Me...]

A bad ground will absolutely contribute to the failure, especially in a Salt Water Pool.

A thermostatic bypass in a pool heater is trying to open and close in order to have the pool water flow maintain a temperature INSIDE the tube bundle of about 120F. If it is lower, you can get condensation and if it is higher it will lead to scaling. You can actually plug those exchanger tubes solid with calcium etc. This has nothing to do with erosion or corrosion inside the tubes.

Another bypass inside will try to maintain a proper velocity inside the tube bundle to prevent erosion. Not corrosion. Manufactures can claim any flow they want but in reality the velocity needs to be contained to around 20-30gpm to prevent excess water flow from eating away the tubes. This other bypass will open when the heater inlet pressure increases in order to help maintain the proper flow.

And green is a sign of copper deterioration. If on the outside of the tube bundle it can indicate bad venting and/or condensation. Products of combustion being trapped on the surface can and will eat the exchanger away from the outside too.