Daniel Arthur
What is Ryznar Stability Index?
Ryznar Stability Index(RSI) was developed by John Ryznar in 1940. It is also called Rznar Index.
The reason behind the development of ryznar index is to get more accurate prediction of calcium carbonate scale than Langalier Saturation Index which was developed in 1936.
Ryznar Index has its own concept & basis of prediction. However the calculation of saturation pH is same as LSI but the difference is in the formula & prediction values.
In Ryznar Index, any value above 6 indicates the water is likely to form calcium carbonate scale while on the other hand any value below 6 water will dissolve calcium carbonate scale and considered the tendency of water is corrosive.
Prediction Based on RSI:
| RSI <5.5 | Heavy scale will form |
| 5.5< RSI <6.2 | Moderate scale will form |
| 6.2< RSI <6.8 | Slight scale will form |
| 6.8< RSI <8.5 | Corrosive water |
| RSI <8.5 | Very corrosive water |
Ryznar Stability Index Calculator:
Ryznar Stability Index Calculation Formula:
| RSI Formula |
|---|
| A =( LOG10 * (TDS)-1) / 10 |
| B = (-13.12 * LOG10(temp +273)) + 34.55 |
| C = LOG(CaH) – 0.4 |
| D = LOG10(M-Alk) |
| pHs = (9.3 + A + B) – (C + D) |
| RSI = 2pHs – pHa |
Where;
TDS = Total Dissolved Solids(ppm)
temp = Temperature(dEG C)
CaH = Calcium Hardness(ppm as CaCO3)
M-Alk = M-Alkalinity(ppm as CaCO3)
pHs = Saturation pH
pHa = Actual pH
Ryznar Index Calculation Example:
| pH | 8.0 |
| Total Dissolved Solids | 500 mg/L |
| Temperature | 30 dEG C |
| Calcium Hardness | 250 ppm as CaCO3 |
| M-Alkalinity | 100 ppm as CaCO3 |
Lets Calculate..
Put cooling water analysis values in above Formula to calculate RSI. You get below values for constants A,B,C & D as per below table:
| A | 0.17 |
| B | 2.02 |
| C | 2.0 |
| D | 2.0 |
| pHs | 7.49 |
Finally, RSI = 2pHs – PHa
RSI = (2 * 7.49) – 8.0 = 6.98
The tendency of this water is slightly scale forming.
