Getting Technical
An LSI value of 0 is neutral water which has no scaling or corrosion potential . Negative LSI values mean potential corrosion which increases as LSI values become more negative .
As the LSI became increasingly adopted worldwide by water technicians responsible for treating heating and cooling water circuits , it became a reasonably reliable indicator of how different types of waters would behave in heating and cooling circuits and the severity of scaling or corrosion to be expected . LSI guidelines for HVAC water circuits indicate scaling at + 1.5 or greater and corrosion at 0.0 getting more severe as the LSI becomes more negative .
As the LSI expanded in popularity , particularly amongst municipalities who applied it to many thousands of water distribution lines , more inaccuracies in scaling and corrosion predictions began to appear as well as some totally wrong predictions . Further investigations revealed that water containing unusual ratios of dissolved substances along with the ubiquitous calcium carbonate were the main sources of these LSI anomalies as well as water containing small amounts of strong alkalis such as caustic soda [ NaOH ] which cause higher than normal pH values .
In 1944 John Ryznar had seen that it was , surprisingly , possible for low hardness and high hardness waters to both have the same LSI . To get around this problem he created what is now known as the Ryznar Stability Index RSI = 2pHs – pH By reversing the positions of the two variables ( pH and pHs ), Ryznar obtained values which more accurately predicted the severity of scaling tendencies and , in addition , were always positive , falling within the range of + 1 to + 13 which is close enough to be consistent with the 0 to 14 range of pH .
The RSI became generally adopted in place of the LSI as a better indicator of potential corrosion of mild steel .
Practical guidelines for applying the RSI to both closed and evaporative water circuits adopted the following RSI values -
< 5.5 definitely scaling 5.5 to 7.5 very small amounts of scaling or corrosion > 7.5 definitely corrosive
Through ease of use and improved accuracy , the RSI became the practical successor to the LSI .
40 years went by until any alteration was made to the RSI mainly to give it greater accuracy over a wider range of water types . The change was replacing pH by another calculated pH known as the equilibrium pH or pHeq , an index derived from water alkalinity analyses . This new index became known during the early 1980s as the Puckorius Scaling Index [ PSI ] where PSI = 2 ( pHs ) – pHeq . The PSI has an identical structure to the RSI and the calculated values result in numbers of similar magnitude . While the PSI appears to be an improvement over the LSI and RSI for many applications , it is not completely free of inaccuracies .
LSI , RSI and PSI are still used by water technologists today because of simple quick calculations which are sufficiently accurate to run the majority of heating and cooling water circuits as close to their cost-efficient optimums as is practically possible . A more rigorous assessment of the potential for calcium carbonate scale to form under varying conditions of heat transfer has been developed for improved coverage of water circuit profiles over their complete operating ranges .
“ Scaling indexes give little or no attention to chloride and sulphate content which are among the most widely recognised contributors to corrosion .”
This is the Calcium Saturation Index [ CSI ] which requires a large number of separate calculations therefore software tools such as the popular WaterCycle are used for rapid calculation of CSI values and often of other relevant water parameters at the same time .
One other area in which a more specialised scaling prediction index has been developed is where waters contain very high amounts of sodium chloride , specifically sea water widely used in marine engine cooling and other heat transfer circuits such as those in nuclear power stations .
For high salt [ sodium chloride ] content the Stiff & Davis Stability Index ( S & DSI ) is used to express the scaling potential for calcium carbonate . The data needed to calculate the S & DSI is the same as the data needed to calculate the LSI .
RSI and LSI are still the most widely used assessments for anticipated corrosion enabling counter-measures to be specified at an early stage of a project . Calculating possible corrosion rates has never been possible on a scientific basis so it has become generally accepted that proven anti-corrosion treatment should be used in any water circuits where corrosion is a possibility .
Scaling indexes give little or no attention to chloride and sulphate content which are among the most widely recognized contributors to corrosion . Paul Puckorius , co-developer of the PSI has suggested that cooling water – and presumably also heating water in HVAC installations – is always corrosive , mainly because of the presence of dissolved oxygen and dissolved solids . This is true even when the water is scale-forming , and particularly true if it is not scale-forming . RACA
www . hvacronline . co . za RACA Journal I May 2021 37