25 May 2005
PRACTICAL (PUCKORIUS) SCALING INDEX (PSI)
DEVELOPMENT OF PSI
1- have been used to predict the tendency of water to form or dissolve scale. Both the
LSI and the RSI are based on the pH of saturation (pHs) for calcium carbonate under a
specific water condition. Both indices use the measured pH of the cooling water.
1. LSI = (measured pH) - (pHs). A positive value indicates scale; a
negative value indicates no scale.
2. RSI = (2 pHs) - (measured pH). A value below 6 means scale; above 6
means no scale.
PSI. Use of LSI and RSI sometimes gave inconsistent or even conflicting
results. Based on a long-term study relative to calcium carbonate scale formation when
applying both the LSI and RSI, P.R. Puckorius and J.M. Brooke developed a new index
that gives much better and more consistent indication of cooling water-scaling
conditions. The new index, the Practical Scaling Index (PSI), sometimes called the
Puckorius Scaling Index, is based on correcting the system pH to match the total
alkalinity of the water being evaluated. This is necessary because the pH is often
buffered, causing the pH to relate incorrectly to the bicarbonate alkalinity, one of the
factors in the pHs calculation.
CALCULATING PSI. To calculate the PSI, the pHs is determined in the
same manner as for the LSI and RSI. An adjusted or equilibrium pH (pHeq) is obtained
from a total alkalinity/pH chart (Table B-7). The value of pHeq rather than measured pH
is used in the following formula:
PSI = (2 pHs) - (pHeq)
A value below 6 means scale; above 6 means no scale.
Calculating pHs. The pHs can be determined from the relationship
between various characteristics of water. The following factors and formula are used in
determining the pHs:
a. Factors needed to calculate pHs :
A = TDS (ppm) (Table B-1)
B = Temperature (oF) (Table B-2)
C = Calcium hardness (ppm as CaCO3) (Table B-3)
D = Total alkalinity (ppm as CaCO3) (Tables B-5 and B-6)