March 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
ment to chlorine which is applied in intermittent dosages of various durations and strength.
453
LITERATURE CITED *
(1) Dobson, J. G., Tmns. Am. Soc. Mech. Engrs., 68, 247-65 (April
1946).
S.Bur. Fisheries, 38, 127-259 (1921-22). (3) Matthews, Annie, J . Marine Biol. Assoc. United Kingdom, 9, No. 4, 557-60 (1913). (4) Reynolds, D. M.9 and Redfield, A. c., 6th Rept. to Bur. of Ships, Paper 14 (1943), unpublished. ( 5 ) Reynolds, D. M., and Redfield, A. C., Interim Rept. 7 to Bur. of Ships (1943), unpublished. (6) Turner, H. J., Jr., Interim Rept. 11 t o Bur. of Ships (1945), unpublished. (2) Field, I. A,, BUZZ. [J.
ACKNOWLEDGMENT
The pioportionating pump used in the chlorine experiments a t Woods Hole was loaned by % proportioneers, %, providence, I., through the kindness of Jeff CorYdon. The equipment used a t Kure Beach was supplied by Wallace and Tiernan, Inc., through the kindness of R. B. Martin, and consisted of their type MSP chlorinators. The location and facilities were supplied by the Ethyl Dow Corporation and were arranged for by F. L. LaQue of the Interna&nal Nickel Compani The sea water was ~h~ drawn from the intake canal of the Ethyl D~~ Dom7icide Was by John v. Grebe Of the Company.
RECDIVED Ami1 9, 1947.
C o n t r i h t i o n No. 380 of the Woods Hoke Oceanographic Institution. This work was done under a contract between the Institution a n d the Bureau of Ships, Navy Department. T h e interest of H. A. Ingram, U.S.N., in initiating the s t u d y is gratefully acknowledged. The opinions presented here are those of the authors and do not necessarily reflect the official opinion of the Navy Department or the naval service at large.
Corrosion Prevention by Controlled Calcium Carbonate Scale SHEPPARD T. POWELL, H. E. BACON, AND E. L. KNOEDLER Professional Building, Baltimore 1, M d . T h e use of controlled calcium carbonate scale for corrosion prevention in cooling tower systems serving steel equipment was discussed in an earlier paper. Conditions were described in which rising temperatures caused the actual pH of the water to decrease at the same rate as the calcium carbonate saturation pH; this produced scale of nearly uniform thickness over the entire temperature range. New data for the ionization constants of carbonic acid have been used to recalculate the pH temperature curves shown in the former paper to bring them up to date.
I
N A recent publication (8, 9) the theory and experience in the
formation and control of a calcium carbonate scale to act as a dam between a metal surface and a corrosive medium (water) were discussed. Explanations and corrective measures were suggested for differences between operating experience and theory as formulated in the Langelier saturation index. In particular, it was pointed out that calculation of the true theoretical index over a range of temperature rise required recognition of the decrease in actual pH throughout the same range. Subsequent to publication of the previous discussion (a), Langelier published two papers (6, 6) in which he expanded his original work in this field. He further compared his results with those of the authors and noted that a discrepancy of 0.2 to 0.4 existed between the two sets of data in which corrections were made for change of p H with temperature. Attention has been called to newer values for the ionization constants of carbonic acid published by Harned and Davis ( 3 ) and by Harned and Bonner (2). A review of these data led to the conclusion that the older values used by Amorosi (1) should be replaced by these new constants for purposes of practical application of the Langelier index. The original data presented in the authors' paper, have been recalculated using this latest information, and the results are included in this paper. The purpose of this discussion is not to review the earlier work, but to make available the adjusted data and curves with illustrative problems to meet the demands of a continuing need which exists in this field. More detailed discussions are given in the former paper. Other papers on the subject have also been published (4, 7 , 10).
EFFECT O F TEMPERATURE ON pH
As was explained earlier, the pH value of a water solution varies with temperature, the magnitude of the change depending on the initial alkalinity. Figures 1, 2, 3, 4, and 5 show the variation of actual pH with temperature fqr waters containing 25, 50, 100, 200, and 300 parts per million (p.p.m.) of methyl orange alkalinity. These were calculated according to the method of AmQrosi and McDermet but using the data of Harned and Bonner. To simplify the presentation of this paper, the same examples are employed as were used before (8). The discrepancy between expected performance and actual performance is explained and compensated partially, when the effect of temperature on actual pH is taken into consideration, and when the change in saturation index is plotted over a normal operating range, in this case 80" to 150" F. Such discrepancies were illustrated in Figure 4 of the previous paper. It was observed that a falling index (case A) predicts increased aggressiveness at high temperatures, a rishg index (case B) predicts heavier scale, and a nearly horizontal index (case C) predicts the formation of a uniform protective film throughout the temperature range considered. Also, it was indicated that the conventional saturation index would predict increased scaling in each of these three instances. Figure 0 shows the decrease in saturation p H (pH,) with temperature. This is an expression of the change in constant C of the Langerier index, as illustrated by the difference between the parallel lines of Figure 1 in the previous paper. Figure 7 shows four curves for actual p H (pH,) against temperature at several alkalinities. These are essentially similar t o the curve of Figure 6. As previously explained, waters having these approximate relations will produce a scale of nearly uniform thickness throughout this temperature range. Thus, light scaling on a cold surface will not be accompanied by formation of a thick scale on a hot surface, a condition which occurs often in practice, These data, when plotted as shown in Figure 8, indicate the desirable pH-alkalinity relations to produce a scale relatively uniform in thickness over a temperature range of 80' to 150' F. This graph shows the relations that might be expected to produce a uniform scale, but should not be confused with the saturation index which indicates the rate and amount of scale deposited,
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
454
Vol. 40, No. 3
10.0
.e 6 4
.2 9.0 .8
.6 2
0
A .2 (Lo
.e .6 A
.2 7.0
80
I00
I20
180
140
TEMPERATURE,
loo
120
140
TEMPERATURE,
OF
160
180
OF
Figure 2. Actual pH (pHa) us. Temperature for Water Containing 50 P.P.M. Methyl Orange Alkalinity
Figure 1. Actual pH (pH,) 2;s. Temperature for Water Containing 25 P.P.M. Methyl Orange Alkalinity IO0
iao
.8
a AT 8 0 O F s 9 5 AT 140°F:908
.6
AT 14O0F~900
.6
I
Bo
180
A
A
.2
.2
90
9.0
.8
.e .e
.6 1
2 0
.4
A
.2
.2
8.0
ao
.e
.e
.6
.e
A
4
.2
2
7.0
r.0 Bo
100
120
140
I60
180
TEMPERATURE, OF
Figure 3. Actual pH (pH.) us. Temperature for \Water Containing 100 P.P.M. Methyl Orange Alkalinity
80
100
120 TEMPERATURE
140
160
180
O F
Figure 4. Actual pH (pH,) us. Temperature for Water Containing 200 P.P.M. Methyl Orange Allzalinity
March 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
455
0
10.0
.2
.e I
Q rz
.6
4
."
A
.e
3:
g s .e k!?
.2
a 1.0 9.0
1.2
'
IO0
80
.8
120
TEMPERATURE
.6
180
140
160
OF
Figure 6 . Temperature us.. Saturation pH Decrease
z P A 6
.2 4
80
2 10.0
A
7.0
BO
00
I20
I40
TEMPERATURE,
160
100
OF
Figure 5. Actual pH (pHa)os. Temperature for Water Containing 300 P.P.M. Methyl Orange Alkalinity
a
$ 4
If the saturation index (S.1.) changes in value from one temperature to another, the increase or decrease will be found to predict whether scale will be heavier or lighter, depending on whether the change in the index is positive or negative. This change in saturation index with temperature has been called the coefficient of uniformity of scaling (AS.1.) defined as:
AS.1. = S.I. ta where tz
- S.I. tl
> tl
To illustrate the results obtainable by this method, Figure 9 was prepared and the examples used in the previous paper were re-evaluated and plotted on this curve. As in the former case, data, for those water systems in which successful results were obtained by controlled scale formation, generally fall in a relatively small area. Points outside of this shaded area produced unsatisfactory results. Thus a zone of probable good performance is indicated by the shaded region. If the point of intersection between the ApH value and the uniformity coefficient falls within the shaded area, the uniformity of scale deposition is likely to be satisfactory. These areas were shaded to assist the reader in evaluating the general range in which the results appear to be applicable. The open circles represent cases in which the results were reported to be satisfactory. The full black dots represent the conditions under which unsatisfactory results were obtained, and the triangles represent conditions which showed corrosion occurring. The enclosed area was estimated and cannot be fixed too firmly, but it is believed to be representative in view of the operating experience to date. The shaded region indicates that satisfactory results could be obtained with low alkalinities, but, in actual practice, control of the scaling characteristics appears to have been more easily accomplished in the regions of medium alkalinity.
.2 8.0 8
7.8 80
100
120
140
TEMPERATURE
I60
180
OF
Figure 7. Temperature us. Actual pH for Four Alkalinity Conditions Which Make These Changes. ' Equal as Temperature Is Increased
For systems in which the water temperature differential is no€ greater than about 70" F., the additional chemicals and effort required to control the treatment to achieve zero ApH and AS.1. values are not always justified, although this is the point where most perfect results would be expected. An inlet temperature of 80" F. and an outlet temperature of 150" F. were used as the basis for calculation of Figure 9. Similar calculations were made for an inlet temperature of 80" F. and an outlet temperature of 105' F. The results are showi on Figure 10 for the same examples as were used for the 80 O to 150' F. temperature range. EXAMPLES
The following example was taken from the authors' earlier paper to illustrate the use of Figures 1, 2, 3, 4, and 5 in determining the conventional saturation index and the corrected saturation index. Assume a water of the following composition: P.p.m..as CaCOa Calcium hardness Methyl orange alkalinity Total dissolved solids pH. at SOo F.
200
100 500
8.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
456
TABLEI.
Plant Protection Aa Good B Excellent C Good D Good E Good F Excellent Gb Good H Fair J Poor KC Poor L Good M Good N Poor P Poor Good Q
.
Vol. 4q NO. 3
SATURATIOX INDEX D A T A FOR \'ARIOUS C O O L I N G W A T E R SYSTEMS (Temperature range 80' to 150° F.) Methyl Calcd. Orange Calcium S.I. AlkaHardat Marble Optimum Actual Uniformity linity ness SOo F. Test DH DH' 'uH
Satisfactory Excellent Good Satisfactory Excellent Excellent Unsatisfactory Unsatisfactory Unsatisfactory Poor Good Good Poor Poor Good
95 90 125 136 135 107 188
112 119 224 204 150 105 110 170 250 90 49 152
210 200
67 106 106 18 25 66
+0.80 +0.70
f0.46 +0.70 +l.OO f0.69 f0.69 1.30 +i.io 0.83 f1.04 4-0.83 4-0.59 f0.31 +0.27
25
57
44
nd 4-0.30 +0.15 +0.50 +0.50
. ... .... +o:io
+0.26 +0.20 +0.82
-0.30 f0'27
3S.I
9.65 9.60 9.95 10.05
10.05 9.80 10.40 10.65 10.50 9.30 9.77 9.77 8.47 8 . 55 9.27
9.11 8.47 9.70 9 0 8.60
C0.66 +0.10 1-1.30 +0.31 -1 23 -0.21 -0.45 -0.18 f0.67 +0.14
Plants A to E', and L, M , a n d Q obtain protection of b o t h hot and cold surfaces with slightly heavier scale on hot, surfaces t h a n a t those marked satisfactory. b Plants G €1 and J obtain very slight corrosion of cold equipment and light t o heavy scale in hot equipment; a higher satuliatibn index and Ion-er coefficient of uniformity are indioadd, C Plants I
