ISDI;;STRIilL AAVDE,VGINEERISG CHEXISTRY
April. 1925
409
Apparatus for Control of Boiler Water Treatment Based on Chemical Equilibrium’.’ By R. E. Hall,3 H. A. Otto,4and H. A. Jackson’ PITTSBURGH EXPERIMENT STATION, BUREAU OF MINES,A N D HAGAN CORP..PITTSBURGH, Pa.
OSSIDERATION of the temperature-concentration diagrams of the salts in boiler waters forming adherent scale or loose sludge has led to an understanding of the merhanism of boiler scale formation. On this basis a system of boiler water treatment has been proposed, in which the criterion for preventing the formation of adherent scale on the evaporating surfaces and drums of the boiler is the continuous maintenance in the boiler water of the following condition:
C
(
(
Concentration of favor-)’> able ion
favorable K b s d . prod. substance of
Kcsol. prod. of un-
)
favorable substance
(Concentra- ) d tion of unfavorable ion, ~of ~ ~
T boiler-water
The exponents of the various terms of the inequality depend upon the specific substances involved. The terms “favorable ion” and “favorable substance” refer to materials which, when precipitated in the boiler water, form nonadherent sludge. The concentration of favorable ion in the boiler water should be maintained but slightly greater than the value indicated by the right member of the inequality. No stipulation is made as to whether the favorable ion used for treatment shall be introduced into the feed or boiler water. In the great majority of cases the deposition of hard adherent scale is prevented by not allowing calcium sulfate as solid phase to be in equilibrium with the boiler water. For this purpose, if the operating pressure of the boiler is not too high, soda ash is both economical and convenient. A pressure limit to its use exists because the carbonate concentration required increases with pressure increase, while the rate of decomposition of carbonate varies likewise with the pressure. Dependent upon various factors, this limit may be 200 to 250 pounds gage pressure; a t higher pressures than this, the more stable phosphate radical may be used for favorable ion. This paper describes simple apparatus adapted to making with dispatch and suitable precision sulfate and alkalinity determinations on the boiler water. Since soda ash is most frequently used for treatment, the discussion will be limited thereto. In this case the maintenance of the condition in the boiler mater
’’’’rn’ ’s5 ( K
so!. prod. CaC03
x
sol. prod. CaSOc
SO4 P . P . ~ .
boiler water
is necessary to prevent growth of adherent scale.6 Carbonate Determination
The cabinet consists of a metal case 20 inches high, 1l1/*inches wide, and 33/8 inches deep, as shown in Figure 1,
and is arranged to hang on the wall of the boiler room. Procedure in the determination of the favorable ion concentration is as follows: A sample of water is drawn from the boiler. If it is taken I Received April 17, 1924 Presented by R E. Hall under the title “Simple Apparatus for the Control of Boiler Water Conditioning Based on Chemical Equilibrium” before the Division of Industrial and Engineering Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D . C . , April 21 to 26, 1924. Published by permission of the Director, U.S. Bureau of Mines. 1 Pittsburgh Experiment Station, Bureau of Mines. Hagan Corporation. 6 Hall, THISJOURNAL, 17, 283 (1924).
from the gage glass, sufficient water must be blown out prior to taking the sample so that any difference in concentration due to condensation in the gage glass and water column will be eliminated. (Unless the sample is taken by means of a cooling coil, or analogous device, considerable concentration is effected by vaporization during cooling. However, both carbonateandsulfateareaffected alike, so that the ratio of their concentrations is undisturbed.) This sample of water is cooled quickly by immersing its container in cold water, and is then filtered as it is passed through funnel a. The pipet b is designed with an overflow tube, c, so that overflow begins when b contains 100 cc. The flask d is supplied for catching this overflow. In the whole procedure it must be remembered that we are dealing with the concentrated boiler water, hence errors of several tenths of a cubic centimeter in the pipet b have very little significance. A 250-cc. Erlenmeyer flask, e (or f), is now set under b, and the 100-cc. sample drawn into it. From the indicator bottle, nz. two or three drops of phenolphthalein are added, and the flask is set under buret g, which is made with a V-cock and two barrels, one calibrated to 1 cc., and the other to 5 cc. It is arranged so that acid blown over from h by application of pressure to the tube IC automatically comes to the zero level by siphoning back through tube i. The sample of boiler water in f is now titrated to the end point of phenolphthalein, with the acid in the larger barrel. Two or three drops of bromophenol blue from n are now added to the sample in f, and acid drawn from the smaller barrel of the buret is added until the end point appears. (This indicator is preferable to methyl orange, because the coloration frequently present in boiler waters is yellow to brown, and because its end point is far more readily seen if an excess of indicator is used.) If soda ash is used in maintaining the boiler water in its appropriate condition, then it is convenient to use N / 3 0 sulfuric acid for the titration. I n this case the reading of the smaller buret in cubic centimeters times 20 gives directly the parts per million of COS radical. The larger buret reading minus that of the smaller, if positive, times 5.67, gives the concentration of OH radical in the same unit. Usuallyfive times the larger buret readingisasufficiently close approximation. Rarely, if ever, will the value be negative, owing to the rapid decomposition of bicarbonates a t boiler temperatures; but it may be necessary to control the OH concentration if the decomposition of the carbonate is rapid enough to make its proportions large. For refilling h, the small stopper j is removed and a funnel inserted through which standard acid may be introduced. Filter papers of suitable size are provided in a container on the front of the door of the cabinet. I n case it is advisable to use phosphate as the favorable ion, the concentration in the boiler water can be determined in the same way by titration and with the same two indicators. The only thing that must change is the factor by which the buret readings are multiplied. Sulfate Determination
A sulfate determination with an accuracy of 100 to 200 p. p. m., which is all that is required in this work, is exceedingly simple. Figure 2 shows the apparatus that the writers are using for this purpose. d sample of water is drawn
.
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ISDL’STRIAL AND EnTGINEERISG CHEXISTRY
II
from the boiler and filtered (best while hot because of the changing concentrations in the water if allowed to cool in contact with the solid phase). A 10-cc. sample of this water, when cool, is placed in the specimen jar with plane glass walls, and to this are added approximately 10 cc. of a 2 to 5 per cent solutionof barium chloride, which is roughly tenth normal with respect to hydrochloric acid. This amount of acid will far more than neutralize any alkalinity present if the boiler is maintained even remotely near to the conditions that have been laid down, and the barium chloride will precipitate the sulfate. The principle underlying the determination is the fact that a bright line observed through a turbid solution of this sort becomes distinctly outlined t o the eye only when the dilution has been made great enough so that a certain amount of light can be directly transmitted. The side of the specimen jar toward the lamp is entirely darkened except for the light which enters a plane surface of a 60-degree prism, and passes through the opposite edge, thus forming a bright line. Sulfate-free water is added to the suspended barium sulfate, the turbid solution is stirred, and an observation is made to see whether the line is yet visible. This process is repeated until such time as the eye is able to distinguish the
Vol. 17, No. 4
1
line through the turbid solution. On the front side ( A ,Figure 2) of the specimen jar has been placed a graduated scale which reads in parts per million of sulfate radical. Hence, as soon as the proper dilution is reached, the concentration of the sulfate radical can be read directly from the calibrated scale marked
A , F r o n t View Figure 2-Sulfate
E , Rear View C o n c e n t r a t i o n Gage
April, 1923
I S D C S T R T I L A S D ESGISEERISG CHEXISTRY
“10 cc.” If the concentration of sulfate is low, a 20-cc. sample of boiler water may be taken and treated as indicated for the 10-cc. sample. I n this case the scale marked “20 cc.” is used to indicate the sulfate concentration. With this type of apparatus an inexperienced obserrer can duplicate results to 200 p. p. ni., which is all the accuracy that can be desired for the determination. Of course, a sulfur photometer as described by Parr,6 or the Jackson turbidimeter could be adapted to this determination, but the degree of accuracy required is not sufficient to justify such exactness. ”Fuel, Gas, IVater and Lubricants,” 1922,p 174
41 1
Application of Analytical Data
There are two methods of maintaining the carbonate and sulfate concentrations in the correct ratio. The first is to adjust the inflow of soda ash solution so that the carbonate concentration in the boiler water will be in the proper ratio t o the sulfate as determined. The second is to maintain a fixed satisfactory sulfate concentration by blow-down, and hence a uniform carbonate concentration. Reference to a curve based on the solubility product ratio for the pressure of operation, in either case will give information as to the proper concentrations to maintain.
Laboratory Tests on Finishes’ By H. C. Mougey GENERAL A l O T O R S RESEARCH C O R P . , DAYTON, OHIO
HE testing of an automobile finish in the laboratory 116 days. The short life of the varnish with the 180 kauri rewith the view of predicting the life of the finish in ser- duction is due to the poor undercoat of the rubbing varnish vice is a very difficult matter. The writer is in hearty failing and taking the finishing varnish with it. I n other sympathy with attempts to establish such tests, but a t the words, a finishing varnish will not give its full life unless expresent time he feels that, in addition to laboratory tests of the posed over proper undercoats, and if exposed over undermaterials used in the seDarate coats, all new automobile coats such as are ordinarilly used in automobile finishing, a long oil finishing varnish finishes should be tested c y may give poor results as exposing them on test racks Laboratory tests on finishing materials cannot be dicompared with a short oil, under standard exposure rectly interpreted into terms of service unless the entire harder drying finishing varconditions, supplemented system of coats is studied as a whole. This requires a nish. with practical tests such as consideration of the properties of each coat, the relation Pulsifer3 recognizes the actual service on cars. of the coats to each other, the methods of application necessity of proper coats There are several reasons and drying time, and the service expected of the finish. directly beneath the finishwhy laboratory tests do not So-called accelerated tests are really identification tests ing varnish, and this is the always give results that in most cases. Instead of subjecting the material to be are proportional to service fundamental fact behind tested to a series of forces tending to destroy the material tests. Pulsifer2 has called the system, recommended and having both these forces and the power of resistance by him, of increasing elasattention to the fact that of the sample changing in exactly the same manner as the tests on finishing varticity of the individual they would in actual service, the tests, as usually connishes should not be concoats as the finishing varducted, simply show the way in which the sample reacts sidered alone, but that such nish coat is approached. to the particular test. factors as elasticity, moisUnless this method is folture resistance, and film lowed there is no definite must all be considered toconnection between laboragether in estimating service durability. This estimate is tory tests and actual service, and unfortunately it frenecessarily based on the assumption that other factors are quently happens that this method is not followed. On favorable for best results, but it frequently happens that account of these facts, the entire system of undercoats and topcoats, dried as they would be in production, must be they are unfavorable. For example, Figure 1 shows the appearance of two finish- tested as a unit to determine the life of the finish. ing varnishes of very different average durabilities, after DRYING 4.5 months’ exposure. One of these varnishes ( A ) has kauri reduction of 180 and under proper conditions is very Another factor affecting durability is the thoroughness of durable. The other ( B ) has a kauri reduction of 40, but drying regardless of the undercoats used. As an illustration even under favorable conditions it never shows a high dura- of this, a black color varnish made by grinding black pigbility. In this particular case both finishing varnishes are ment in a long-oil, ester-gum, wood-oil varnish was exposed applied over the same undercoat on different sections of the over bare steel. On the first panel, the varnish was airsame test panel. The rubbing varnish (C) under these fin- dried 2 days, on the second panel it was baked 2.5 hours a t ishing Yarnishes is of the kind ordinarily used in automobile 135” C., and on the third panel it was baked 1 hour a t 204.5’ finishing, and unprotected with finishing varnish it has a C. The life of these panels was, respectively, 3 . 5 , 9, and 12 very short life on outdoor exposure. In this particular months. In a similar manner, if two varnishes of the same test the unprotected rubbing varnish failed in 47 days, t,he kauri reduction are exposed over suitable undercoats, the finishing varnish with a kauri reduction of 40 failed in 123 one that dries better will usually be more durable, and this days, and the varnish with a kauri reduction of 180 failed in difference in drying may overcome a very great difference in the kauri test. For example, a certain hard-drying Presented as discussion a t t h e symposium on “ T h e Physical Testing finishing varnish having a kauri reduction of 120 is on the of Varnishes” before the Section of Paint a n d Varnish Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C . , April 21 t o average slightly more durable than another slower drying
T
26. 1924. 2
J . S o < . .1ulomoiise Eng., 10, 12 (1922).
a
J. SOC.Auromofioe Eng, 12, 89 (1923).
