25 May 2005
J.W. Ryznar later devised a more sensitive formula for predicting calcium
carbonate scale. This formula is known as the Ryznar Index or Ryznar Stability Index
(RSI) . The formula is: 2pHs - pHactual. A value of 6 indicates "stable" water, a value less
than 6 indicates a scale-forming tendency, and a value greater than 6 indicates a scale-
dissolving tendency. The indices have also been used to try to estimate the degree to
which calcium carbonate scale will form in drinking water and in cooling water. The
more positive the LSI value, the greater the scale formation; however, for the RSI, the
smaller the index, the greater the scale formation. The LSI and RSI can give conflicting
predictions based on the same water quality information.
Practical (Puckorius) Scaling Index (PSI). Paul R. Puckorius and J.
Maxey Brooke developed a modified version of the RSI that gives a more accurate and
consistent indication of the calcium carbonate scaling potential of cooling water. Known
as the Practical Scaling Index (PSI), and also known as the Puckorius Scaling Index, it
takes into consideration the effect of the type of total alkalinity of the cooling water on
the measured pH (pHactual) value. The measured pH does not always relate correctly to
bicarbonate alkalinity because of the buffering effect of other ions. Rather than using the
measured pH in calculating the PSI, an adjusted or equilibrium pH (pHeq) is used: PSI =
2pHs - pHeq. As with the RSI, a PSI value of 6 indicates stable water and a value lower
than 6 indicates a scale-forming tendency. Without scale-control treatment, a cooling
tower with a PSI of 6 to 7 should operate scale free. However, a PSI of greater than 6
indicates that scaling may occur. Information on calculating the PSI is provided in
Appendix B. Use of the PSI is most applicable when cooling water pH is above 7.5.
Scale-Control Methods. Three basic methods are used to prevent the
formation of scale in cooling water systems:
a) Remove the water scaling ingredients from the water before use. This
includes softening, RO, and other technologies described in Chapter 2.
b) Keep the scale-forming ingredients in solution. This is the most common
scale-control method used for cooling water, and it can be achieved by
use of either or both of the following two methods: adding acid, which
lowers the pH of the recirculating water, or adding a scale inhibitor
(phosphonate or specific polymer), which allows higher COC to be
maintained without scaling. Acid neutralizes (destroys) mineral alkalinity,
one of the constituents forming calcium carbonate scale; however,
because of the hazards associated with handling strong acids and the
potential damage from an acid spill, the use of acid in cooling towers is not
c) Allow the water-scaling ingredient to precipitate as sludge. Modern
chemical treatment can distort or modify scale crystals such that they
cannot adhere to each other to form a hard deposit; instead, they become
a sludge that can be removed through filtration or blowdown.