Carbonate Scale Formed

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An index for determining the amount of calcium carbonate scale formed by a water

The purpose of this investigation was to obtain a formula that will give a quantitative index of the amount of calcium carbonate scale that would be formed by a water at any temperature up to 200°F and to predict, if possible, the corrosiveness of waters that are non-scale forming. This information was also desired for waters in which inhibitors of the polyphosphate or molecularly dehydrated phosphate type were present.

of which have been determined. K'2 is the second dissociation constant for carbonic acid, and K's is the activity product of calcium carbonate. The term (K'2 – K's) varies with ionic strength, dissolved solids, and temperature. pCa is equal to the negative logarithm of the calcium ion concentration in moles per liter; pAlk is the negative logarithm of the total alkalinity of the water to methyl orange in terms of titratable equivalents per liter.

Scale formation in distribution mains, domestic hotwater heaters, and various types of cooling equipment is well known. Scale is also a problem in boiler feedwater heaters and feedwater lines. In view of these and other problems caused by undesirable deposits and incrustations, a formula having quantitative significance would be of great value in predicting the behavior of a water and in recommending the necessary corrective treatment.

Larson and Buswell, using more recent data for evaluation of the constants and their variation with temperature and total dissolved solids, express the formula for the pH of CaCO3 as:

The molecularly dehydrated phosphates were studied because of their ability to stabilize a water and thus prevent deposition of CaCO3. It was thought that this quantitative index or formula could be best obtained by using the values and corrections for the different equilibria affecting calcium carbonate solubility. The factors affecting CaCO3 solubility equilibria have been studied by many investigators. Larson and Buswell, in their paper “Calcium Carbonate Saturation Index and Alkalinity Interpretations,” give a good bibliography on this phase of the problem. Prof. W. F. Langelier receives credit for first developing a general expression that takes into consideration the different readily determined variables in a water affecting CaCO3 solution or precipitation. For waters in a pH range of 6.5-9.5, Langelier’s formula for the pH at which a water is in equilibrium with calcium carbonate is: pHs = (pk'2 – pk's) + pCa + pAlk K'2 and K's are apparent constants computed from the true thermodynamic constants K2 and Ks, values

Ks pHs = log – log (Ca+2) K 2

– log (alk) + 9.30 2.5 √ u +

1 + 5.3 √ u + 5.5

In this equation (Ca+2) and (alk) are expressed in parts per million as Ca and CaCO3, respectively. The difference between the actual pH of a sample of water and its calculated pH is the measure of the degree of calcium carbonate saturation and has been called the saturation index or: Saturation Index = Actual pH – pHs A plus value for the saturation index indicates a tendency to deposit CaCO3; a minus value indicates a tendency to dissolve CaCO3. The saturation index is only qualitative, and, as Langelier emphasized, “the saturation index is an indication of directional tendency and of driving force, but it is in no way a measure of capacity.” The saturation index, however, is not always reliable in predicting this because some waters with a positive index may actually be quite corrosive.

Reprint R-20

By John W. Ryznar

Reasons for this are that the Langelier index does not indicate how much calcium carbonate will deposit, nor does it indicate whether a state of supersaturation will be present that will be great enough to produce a precipitate, or whether it is great enough to give a protective film. This can be seen more clearly by assuming that there are two waters with the following characteristics: Water A at 75°C; pHs = 6.0; actual pH = 6.5; Saturation Index = + 0.5 Water B at 75°C; pHs = 10.0; actual pH = 10.5 Saturation Index = + 0.5

The effect of a molecularly dehydrated phosphate on the stability index of a water was studied. The polyphosphates have the ability of stabilizing an otherwise unstable water. In the presence of small amounts of these phosphates, a water that would ordinarily form a heavy deposit of CaCO3 remains stable for extended periods of time.

EXPERIMENTAL WORK

The saturation index would predict both waters to be equally scale forming. Actually, water A would be scale forming, while water B would be quite corrosive. To eliminate misinterpreting a positive saturation index as being non-corrosive or scale forming, a new empirical expression, 2pHs – pH, is proposed. The value obtained by the expression 2pHs – pH is called the stability index to differentiate it from the saturation index. This stability index is not only an index of CaCO3 saturation, but is also of quantitative significance. Using this expression, waters are much more accurately typed to determine whether scale formation or corrosion is to be expected.

In determining the pHs of a water, the constants and corrections for salinity and temperature as given by Larson and Buswell were used. At higher dissolved solids and temperatures, the values were extrapolated using the same formulas as for obtaining pHs at the lower dissolved solids and temperatures. No attempt was made to check experimentally, at the higher temperatures and salinity, the values thus obtained of the constants K'2 and K's. Experimental tests to determine the amount of scale formed by various waters were made on waters having different 2pHs – pH values and on different waters having similar 2pHs – pH values. The equipment for carrying out these tests is shown in Figure 1. In the tests reported, the water was passed through the system only once.

Experimental data have been correlated with this expression so that a semi-quantitative value can be derived for the amount of calcium carbonate scale that will be formed, and a qualitative estimate can be made to indicate whether serious corrosion may be expected. Using waters A and B, the following values would be obtained for the stability index: Saturation Index Stability Index

Water A +0.5 +5.5

Water B +0.5 +9.5

Unlike the saturation indexes for these two waters, the stability indexes are quite different. The stability index will be positive for all waters. Experimental results obtained on various waters indicate that the behavior of natural and treated waters having a stability index of 5.5 will be similar and will give an appreciable amount of calcium carbonate scale. Waters having a stability index of 9.5 will form only a limited amount of calcium carbonate scale and may be severely corrosive, especially at higher temperatures.

Figure 1 — Apparatus for incrustation tests

Two tests were made simultaneously. The coils were carefully cleaned, dried, and weighed before and after each test to determine the amount of scale deposited from the water. The constant temperature baths could be adjusted to give an effluent temperature of the water from room temperature to the boiling point at any desired flow rate. The tests reported here were made at effluent temperatures of 120, 160, and 200°F. All tests were of two hours’ duration with a flow rate of 1 gallon. This apparatus has been used in the laboratory for several years and gives results that can be easily reproduced. The incrustation obtained in these glass coils has been checked with field results. Good correlation has been obtained in all cases, so that a water giving a certain number of milligrams deposit can be predicted either as giving trouble-free conditions or an objectionable scale in a certain length of time. This permits rapid recommendation of the proper stabilizing or corrective treatment where necessary. The results on incrustation on various waters are plotted against the stability index (Figure 2). Without stabilizing treatment, a water having a stability index of approximately 6.0 or less is definitely scale forming, while an index above 7.0 may not give a protective coating of calcium carbonate scale. Corrosion would become an increasingly greater problem as the stability index increased to above 7.5 or 8.0. With polyphosphate, a water with a stability index as low as 4.0 can be used at temperatures up to 200°F with little danger of scale formation. Inasmuch as these phosphates also have definite corrosion inhibiting powers, a water with a stability index around 7.0 – 8.5 may be profitably treated with these phosphates.

Figure 2 — Laboratory results showing relationship between stability index and incrustation

plete and the history of the behavior of the water was known. The values for pHs were calculated, using the formula of Larson and Buswell. In most of the cases very little information was given in the literature regarding the condition of the coldwater mains. The stability index, in most cases, indicated a corrosive tendency was present. The reactions in the cold may have been slow and escaped notice, but it is felt that tuberculation due to corrosion had in too many cases been taken in a matter-of-fact way as something to be expected.

Although the points in the curves lie fairly close along the plotted lines, it is not claimed that any and all waters will fit the curves as well. It is quite possible in some cases that the stability index may be low, indicating a scaling tendency. Yet if the alkalinity is due mainly to hydroxyl ion, there would be very little CO3 present to give CaCO3. This would be especially true of some lime-soda softened waters. However, it is felt that a water having a stability index of 7.5 or more will not form a protective layer of CaCO3.

A New England Water Works Association (NEWWA) report shows that, based on tests of 473 pipelines in 19 different systems, the average loss in capacity of tar-coated cast iron pipe, after 30 years of service, was 52%. Small mains carrying active water may lose over 60% of their capacity in this length of time. This represents a serious economic loss. Although tuberculation due to corrosion is the most frequent cause of loss of flow due to increased friction, in certain cases this could be due to incrustation formed by an unstable water.

To determine how the laboratory results compare with field results, a number of cases were taken at random, in which the analyses of the waters were com-

The data obtained by NEWWA indicated that there was a marked correlation between the pH value of water carried and the rate of capacity loss in the

mains. Waters with a pH value of 6.5 gave twice the loss in capacity in a given length of time as those having a pH value of 8.0. Apparently, a water having a stability index of approximately 6.0 at about 60°F, to which a stabilizing treatment of polyphosphate has been added, should give the best results in terms of minimum corrosion and/or scale in the whole system. Under these conditions, corrosiveness due to aggressive carbon dioxide would be near the minimum in most cases. On the whole, the field data correlate very well with the laboratory results for the different stability indexes (Figure 3). Only those values are plotted where definite information is given for a particular temperature range. With a stability index of 7.5 or higher at 140°F, corrosion is marked. At indexes of 9.0 or higher, corrosion is serious. The stability index should implement the usefulness of the saturation index, and should help to predict more accurately how badly scaling or corrosive a particular water supply may be.

Figure 3 — Field results superimposed on curve A of Figure 2

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