Dec.. 1921
T H E JOURNAL OF INDUSTRIAL A N D ENGINEEBINQ CHEMISTRY
1149
The Determination of Moisture in Insulating Oils'a By RESEARCHDEPARTMENT. wESTINGHOUS&
C. J. Rodman ELECTRIC AND MANUPACTURINQ CO., EASTPITTSBURQH,
Many methods of determining water in oils have appeared in the literature, but no reliable simple method has come to the author's notice. By examination of the literature in detail, a classification of methods has been made. This classification includes five qualitative, a dozen approximate, and few quantitative methods. I n dealing with moisture in insulating oils, a method cannot be called quantitative unless an accuracy of closer than 0.002 per cent is obtained. The approximate methods include those which determine moisture content to an accuracy of 0.01 to 0.05 per cent. The qualitative tests are those which have been recommended here a i d there for the use of the electrical engineer as a rapid means of finding the presence of water. No insulating oil should be allowed to take up enough moisture to show positive tests by qualitative methods, because the dielectric strength of an oil becomes less considerably before the moisture can be thus detected. QWALITATIVS TESTS I-Addition of anhydrous CuSO4. Turus blue with HzO.,%*,* &Hot nail "spdts," indicating Hz0.L' 3-Addition of certain water-soluble, b u t oil-insoluhlc dyes.' 4-Centrifugation.4 5-Electrical precipitation:b APPRVXIMATEANALYSIS l-Loss of weight by heating. For nonvolatile oils and greases.6.1* !&-Diluting with solvent and settling by gravity. Applicable t o thin oils, but a diluent is t o be avoided. Centrifugation may be used to hasten.' &Use of color comparator tube.6 4-Tleating with normal acids.6 +Treating with CaCn. This is convenient, h u t with petroleums accurate t o 3 per cent of t h e Hz0 value only, as C L H Zis absorbed by them. T h e 3 pcr cent accuracy holds only above 0.01 per Sent content.' 6-2'1eating with sodium. One cc. Hz represents 0,0016 g. HzO in t h e sample used. This is accurate and convenient. Complicated apparatus necessary t o insure thorough contact of sodium with oil and t o eliminate quantitatively all of t h e hydrogen.6 7-Distilling with a immiscible liquid. Water-saturated xylene, a mixture of xylene and toluene, or benzene and toluene are used. Accurate t o approximately 0.033 g. of Hz0 per 100 cc. of distillate.' 8-Directly distilling off the water. This is accurate t o within 0 . 0 3 per cent, and does not apply to light oi1s.a Inert gases used t o d r y oils.2 9-Water in oils may be tested by conductivity providing no other decomposition products are present. Only approximate results obtainable.0 1O-Oil mixture with organic solvent t o produce cloud. A more general case than No. 7. 11-The Tyndall effect is used t o determine finely divided water in SUSpension. QUANTITATIVE METHODS I-By special application of No. 3 (qualitative tests) less than 0.003 per cent Hz0 can be detected. Necessitates color scale, clear oils-standardization for quantitative work. 2-Vacuum treatment while shaking sample, heating, and collecting H z 0 in PZOStube. Method quantitative if oil is free from nitrogenous volatile compounds, as is usually the case. B y special method we are able to determine water t o f 0.001 per cent. 3-Same treatment as No. 2, using freezing mixture to collect HpO and volatile components, afterwards estimating water by No. 4 method. Good results obtainable, b u t complicated apparatus must be employed. 4-Vacuum treatment of thin film of oil running over heated surface. Vapor eliminated and measured by oil-water vapor pressure difference. 5-Mixing oil with dry ether (solvent), treating with CaCz, drawing off CzHz by vacuum, precipitating out in alcoholic AgNO8 solution and noting change in resistance with precipitation of silver acetylide.8 This gives time-rate curve and is very desirable where both hygroscopic and constitution moisture ere involved. It is accurate t o less than 0.01 per cent and is chie5y applicable t o certain insulating materials and oils. '
Inasmuch as the dielectric strength of an oil is greatly influenced by the moisture present,lo it is highly desirable to 1 Preqented before the Section of Petroleum Chemistry a t the 61st April 26 t o 29, Meeting of the American Chemical Society, Rochester, N. Y., 1921 2 Published a s Scientific Paper 92 of the Westinghouse Electric and Manufacturing Company. * Numbers refer t o Bibliography.
PA.
determine the moisture content accurately. The different forms in which the moisture is present within the oil may vary greatly. This is dependent upon the composition of the oil and its impurities. Water may be in finely divided suspension, condensed nuclearly upon dust particles, absorbed by fibers or soaps, in colloidal form or in true solution. The true dielectric strength of pure oil is most strongly influenced by water in solution, but the usual conductivity takes place by the alignment of particles within the oil. Very pure mineral oil of the paraffin series does not dissolve more than 3 X per cent H20.11 The unusually high percentage of moisture sometimes found in oils is attracted by impurities which are soluble in oil. Such impurities (unsaturated compounds, fatty acids, etc.) possess a greater coefficient of solubility for water than do the pure paraffins
.
A Sample bulb C :Tnermometer
E = E l e c t r i c a l w ndinq F - F r e e z i n q mixture G =M a n o m e t e r
FIG.1
Of the numerous possible methods of determination of water in transformer and similar oils only two or three may be depended upon for the accuracy desired. A modification of Method 2 (Quantitative Methods) has given excellent results. PRINCIPLE OF METHOD Water is fractionated from the oil a t reduced pressure and elevated temperature, during rapid agitation of the sample. Under these conditions the partial pressures of oil and water are far apart and fractionation is rapid. The water is collected with some oil distillate in a trap cooled by liquid air. Redistillation of the water and collection in a P20hweighing bottle take place upon the removal of liquid air. The small amount of oil vapor distilling over passes on through the phosphorus pentoxide. The method depends for its success upon the production of a great number of thin films within the oil sample. A low freezing mixture may be substituted fac: the liquid air. APPARATUS AND PROCEDURE Fig. 1 shows diagrammatically the arrangement of the apparatus. The weighing bulb* is of special design consisting of two glass parts, an inner and outer chamber. The inner chamber (2 cm. in diameter X 10 em. long) is slightly constricted a t the lower end and has a stopcock handle a t the upper end, This fits concentrically within the outer chamber (2.5 cm. in diameter x 10 cm. long), and is made airtight by the ground joint near the top. An inlet allows gas to enter the outer chamber first. The gas passes down-
*
Modification of bulb used by Dr. H. C. P. Weber, Westinghouse Electric and Manufacturing Company.
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
1150
Vol. 13, No. 12
ward through the outer chamber, entering the inner from NOTESON THE METHOD the bottom. Pure P205 packed between two plugs of glass I n determining the moisture content to ~ 0 . 0 0 care 1 must wool within the central chamber is used as the moisture be exercised a t every point in the determination. The oil absorbent. The gas passes up through the PZOSand out sample must be representative of that which is to be examined. through a small tube directly opposite the inlet. By turning Any apparatus used in connection with the oil should be the inner member through 90" the bulb is sealed to any carefully cleaned and dried and then rinsed with some of tJhe gas pressure difference. Such a bulb holds a vacuum many oil sample. Humidity changes influence the water content weeks. appreciably if the sample is open to the air. Care should The sample bulb, which fits within the heater shaker, is of be exercised in keeping the breath away from the sample, as about 50-cc. capacity. A tube, closed a t the bottom and surface condensation of water vapor may grent'ly influence just large enough to accommodate a small thermometer the result.* The time of a run should not exceed 20 min. (range 0" to 150" C.), passes down into the bulb. The bulb This gives ample time to insure complete elimination of the can be kept a t any desired temperature by the electric heater moisture. surrounding it. The motor speed can be controlled so that Factors which give high results, and hence are additive, the shaker runs from 50 to 250 per minute. The liquid air may be briefly summarized as follows: trap consists of a closely bent U-tube of 1-cm. tubing, sur1-Surface condensation of water from humid air. rounded by a Dewar flask containing liquid air. of any of the apparatus by water or dust. 2-Contamination 3-Volatile nitrogenous and basic substances within the oil. PRocEnvRE-Before making a run the apparatus is thor4-External leaks of any kind. oughly cleaned and dried. A few cc. of oil to be tested are 5-Increase of pressure during run so t h a t excess gas is trapped in PzO, used to rinse the sample bulb before pipetting in the 25 to bulb. 40 cc. of the oil. The bulb is connected to the liquid air 6-Adsorption of some oil vapor in weighing bulb. trap .by means of a heavy, pure rubber, pressure tube. The Subtractive errors are as follows: Pz06tube is fastened in place also by means of a rubber 1-Slimination of water not complete as a result. of high viscosity of oil connection. Rubber cement prevents gas leakage a t the sample. 2-Bath temperature too low. joints. The vacuum is now turned on and the pressure 3-Shaking not vigorous enough to expose all t.he oil t o thin film condition. very quickly reduced to a millimeter. The manometer 4-Decrease in pressure over the initid pressure within t h e bulb. reading is taken and the P206 bulb closed. Closing a screw 5-Time of Bhaking oil too brief. pinchcock tightly a t either connection to the PZOEbulb Weighing errors may be positive or negative. suffices to hold the vacuum in the system when taking the Using the representative transformer oil little difficulty P20a bulb out for weighing. The Pz05bulb is replaced and is experienced in the determination of water due to its visscrew cocks opened. While shaking and heating the bulb cosity, impurities present, or to the volatility of the sample. to 140' C . the liquid air trap is covered with COz snow and Leaks are readily taken care of by using rubber cement acetone or liquid air contained in a silvered Dewar flask. between the glass-rubber connections. Other possible leaks The water vapor is quickly and completely eliminated from are prevented by .proper construction of apparatus. the oil and froeen in the trap, which also condenses any Freshly sublimed P205 free from organic matter will not ablight oil distillate. Upon removing the freezing mixture sorb and hold any oil under low vacuum conditions. Should. the frozen water is rapidly vaporized and caught quantita- any oil distillate come over in appreciable quantities it is tively within the P& bulb. By closing the Pz06 bulb a t readily eliminated by surrounding the Pa05 tube by a temperthe same pressure as in the blank weighing one can readily ature bath of 100" to 150' C.I2 Though the possible errors obtain the eliminated water by difference. Phosphorus seem numerous, it is only necessary to recognize them to pentoxide does not absorb any oil vapors at the pressures eliminate them by following out carefully the procedure used. recommended, and little difficulty will be experienced in obTwo methods of procedure may be followed. The more taining an accurate result. rapid method does not account entirely for the moisture in BIBLIOGRAPHY the air within any part of the apparatus. As shown in Table 1--Elect. Club J., U. S . A., May 1904. I, it gives comparatively good results, varying *0.002 from 2--E!ect., 66 (1911), 490. the mean. A single determination is not to be recommended 3-A. H. Gill, "Oil Analysis,'' 6th Ed., 1913, p. 22. P-Chem.-Ztg., 33 (l909), 1259. where the best accuracy is desired. Water Added Per cent 0.0042 0.0194 0.0110 O.OOS8 0.0074
TABLE I Water Found Per cent 0.0047 0.0216 0.0092 0.0078 0.0086
d dl
+ dz, etc.'
Difference +O ,0005
+o.
0022 -0.0018 -0.001 +o .0012
-
0.0012
In this case the vacuum is cracked without shaking $he sample bottle, thus taking out the air above the oil sample when the weighing tube or freezing mixture is not yet in place. A pinchcock is immediately cloeed. The freezing mixture is then raised around the trap and the weighing thbe put in place, and the determination is made as described above. The second method of procedure takes into account the amount of moisture in the air. Two blank runs are first made to determine the humidity. Several runs made for humidity moisture check the theoretical values. With the addition of this correction to the first procedure, the moisture in hydrocarbon oils may be determined accurately.
5-Bureaii of Mines, Technical Paper 26 (1912). See also patent literature. 6-Orig. Com. 8th Intern. Congr. A g p l i e d Chem., 10, 17. 7-J. Ind. Ens. Chem., 10 (191S), 357; Direccion General d e Mines, Buenos Aires, Bol. SB (1915). S-U. S . Dept. Agriculture, Bureau of Chemistry, Circular 97; J. SOC. Chem., 29 (1910). 197; Chem.-Ztg., 6 (1892j, 29. 9-J. SOC. Leather Chem., 3, 101-4; 14, 654-74; 3, 2fJ6-8; J. I n d . Eng. Chem., 12 (19LO), 486-98 1 0 - ' E l ~ ~ t .66, , 490; Elect. J.: 16, 74-6. 11-See also Elect., 82, 103, 67, 81s; Z. QnoYg. Chem., 81, 24-39; 2. Elektvochcm., 17, 346. 12-Z. anovg. Chcm., 81, 24-30. SOC.Chem. I n d . , 39 (1920), 305. 13-J. * T h e rubber connections used in the vacuum line do not give up any appreciable amount of moisture after having been oncc subjected t o low vacuum, if care is taken subsequently t o keep out humid atmosphere.
A committee of eleven has been appointed to draft tentative plans for an organization of oil men, to comprise refiners, jobbers, producers, supply and tank men, which it is expected will embrace the entire oil industry of the southwest, probably with headquarters in KanSas City. Part of the plan includes the formation of an arbitration board to settle differences between conflicting interests, and an oil exposition to be held yearly.
