INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 17, No.12
Methods of Testing Transformer Oils‘ By Hans C . Staeger BROWNBOVERI& Co.,BADBN,SWITZERLAND
from molecular weight determinations the formula C&240rp which is in agreement with the writer’s investigations.6 Brauent calculated the acids as dibasic, with an average molecular weight To fulfil these conditions certain electrical and physical of 600. According to Charits&koff, however, they are dihyproperties are required of the material. This article, however, droxycarboxylic acids. According to Rodman?they are supposed will deal only with the chemical behavior of transformer oils to be acids of the type CnHzn-202. Concluding from the aboveinvestigations of Staeger and Bohnenblust, however, when exposed both to high temperaturesand the oxidizing mentioned they are of an entirely different type. A long series of investiinfluence of the air. gations carried out in this laboratory supports the statement of Charitschkoff. For this reason they are regarded as highly unConstitution of Transformer Oils saturated compounds which are to be designated as dihydroxy monocarboxylic acids. The only exceptions are the acids formed In the early art of high-voltage transformer design resin by the oxidation of mineral oils in presence of lead as catalyst. oils, produced by distillation of colophony, were applied. In this case monohydroxy monocarboxylic acids with a lower mo 1e c u l a r w e i g h t result. Afterwards, owing to some These hydroxy acids transd i s q u a l i f y i n g properties form very easily into internal (rapid polymerization when anhydrides and polymerizaA brief outline of the chemical constitution of oils is heated), these oils were retion products. In this way followed by an outline of the type of reactions occurring neutral combinations of high placed by mineral oils, which under the influence of elevated temperatures i n t h e m o l e c u l a r weight develop a t the present time are used presence of oxygen. Several standard tests of insulatwhich are only partly saponiexclusively. Mineral oils fiable. ing oils are then critically reviewed. Data are offered differ in composition accordWATER-In such reactions to support the contention t h a t the result of no single water also forms.8 It is deing to the oil fields from test can be safely employed to indicate the quality sired especially to emphasize w h i c h they come. They a n d probable service life of an insulating oil. The BBC the fact that’themost unsatumay contain the following rated combinations are not test is described and defended as being m o s t nearly the most reactive. Waters. chemical groups: representative of service conditions a n d as taking acproved that oils with the highcount of all phases of oil deterioration. est content of unsaturated hyI-Unsaturated hydrocardro carbons were the least bons oxidized. The oxidation it( A ) Cyclic self is disturbed bv simul( a ) Aromatic hydrotaneous autoxidation reactionsagThese reactions develop supercarbons (benzene, naphthalene) ( b ) Alicylic hydrocarbons (terpenes, polyterpenes, di- oxides of the type hydroxy, tetrahydroxy combinations)2 R’CH=CHR “ OP-R’CH-CHR ” ( B ) . Aliphatic (olefins, polyolefins) \O/ 11-Saturated hydrocarbons ( A ) Cyclic (naphthenes, polynaphthenes, condensed naph- which decompose easily by losing oxygen. In the nascent state thenes)2 the oxygen is highly reactive, which enables it to oxidize quite ( B ) Aliphatic (paraffins) stable compounds. This phenomenon will be considered later.
T
HE oil in a transformer has two functions-to serve as an insulating material and as a cooling medium.
+
These different tain not products
main groups react differently on account of their constitutions. Since the usual transformer oils cononly one but several of these groups, the reaction are not uniform. The main groups are:
It is thus seen clearly that the reactions and the resulting products are of a very complicated nature. Contrary ta various statements, it is emphasized that acids originate a s the first reaction products. As previously’o shown, these are unsaturated and, as hydroxy acids, very reactive; MINERALOIL REsINs-According to Holde and E i k ~ ~ a n n acids ,~ mineral oils contain, besides asphalts and pitch in a colloidal they catalyze further sludging. This formation of acids is solution, soft to brittle resins. They are to be regarded as gradual, as was proved in the writer’s investigation of lOO@ intermediate products between mineral oils and asphaltenes. hours’ heating. The mineral oil resins contain hydrocarbons of cyclic structure, not of chain structure. Marcusson defines them as “transformaCatalytic Effect of Metals in Transformer tion products of mineral oils, which are’produced by oxidation, polymerization, and condensation, They change by intramolecuAttention is called to the possible catalytic influence of lar oxygen transfers and oxygen additions into asphaltenes.” ASPHALTENES-upon long heating and exposure to air the min- the different metals with which the oil comes in contact in a eral oil resins change very easily into compounds containing transformer. It has been found in this laboratory that more oxygen. Mineral oil resins and asphaltenes are cyclic certain metals-such as copper, brass, rheotan, constantan, hydrocarbons which contain sulfur and oxygen in bridge bond, tin, and zinc-accelerate the sludging, whereas iron, nickel, besides carbon and hydrogen. In the molecule they show a partly unsaturated bond. The parent substances of these groups and aluminium have no great influence. Reaction products of compounds are the terpenes, which are chemically related to developed by these metals are the same as originate in the transformer, but lead forms other products. Figure 1 shows certain ingredients of the oils. CARBENES AND CAmoms-These compounds are formed upon the formation of acids during 1000 hours, and Figure 2 the introduction of air into mineral oils heated to 120” C. At higher sludging of the same oils. The results show that the area temperatures they form in great quantities, whereas at lower of the immersed metal has an influence, but that there temperatures more asphaltenes are formed. ASPHALTOGENIC Acms-These acid products of oxidation does not exist any proportionality. The sample corresponddiffer from naphthene acids by their higher molecular weight. 6 Staeger and Bohnenblust, Bull. Schweie. elektroteck. Verein. 1924. Charitschkoff4 produced such acids experimentally and deduces 6
1
* 8 4
Received June 2, 1925. Marcusson, Chcm. Ztg., 86, 729 (1911). Mitt. Matcrialgrufungsamt, as, 148 (1907). Oesterr. Chcm. Tcth. Ztg., 13, 125 (1910).
7 8 0 10
Elcktrofech. Z.,96, 145 (1914). Elcc. World, ‘79, 1271 (1922). Waters, Bur. Standards, Bull. 7 , 305 (1911);Circ. 49 (1920). angler, “Vorgange der Autoxydation,” Braunschweig, 1904. J . I n s t . Elec. Eng. (London),61, 661 (1923).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1925
ing to curve Cu, Figures 1 and 2, contained copper, the surface of which was one hundred times as large as in case of Cub. Therefrom it can be concluded that copper has the greatest catalytic influence of the metals investigated.
Testing Methods Considering the large field of application of high-voltage transformers and the high degree of safety required, only the most suitable oils can be used for this purpose. I n order to select the proper oil from the great number of refined
/’”
70 hours with introduction of oxygen. The sludge thus formed is weighed. LIFETEST-The shortcomings of these two and several other methods were recognized in America and the so-called life test was recently proposed.14 Different oils are put in an apparatus and simultaneously heated up to 120’ C. until formation of sludge is observed. I n this manner the “life” of a transformer oil is determined. BBC TEST-Several years ago the SociW Anonyme Brown Boveri observed from experiment with transformer oils in service that oils with a good tar figure were not always excellent in service; in other words, it was proved that no absolute relation exists between tar figure and sludging. The same can be said of oils tested by the Michie test. A method was worked out in which the oils were exposed to air under conditions similar to those of service and the resulting reaction products, such as sludge, acids, and superoxides, were determined. The oil samples were heated in a copper beaker exposed to air a t 112’ C. (former maximum temperature plus tolerance) for 300 hours and the resulting sludge was determined gravimetrically. The oil-soluble acids were determined by titration, and the probable formation of superoxides was measured by the decrease in the tensile strength of a cotton yarn. The scientific investigations of the Bl3C test carried out in this laboratory16 have shown that the results of this test agree best with the results observed under actual service conditions.
I
‘9 OL 1
8t
1
I
300 500
Time in
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100 //Out-5
cuH
7
6rossfi
Figure 1
mineral oils, various testing methods have been brought forward. Certain electric corporations have elaborated specifications for the acceptance of such oils. A review and comparison of the different tests are therefore of value. GERMANTEST METHOD--I(iSSling investigated the .influence of air (oxygen) and heat on mineral oils.I1 He found that some of the reaction products can be precipitated by petroleum ether and some are of resinous character and can be isolated by alkali. The latter gives, according to Kissling, the so-called “tar figure.” In addition, carbonization products are formed, which may be determined analytically as the “coke figure.” From these investigations the German test method for transformer oils was developed. The test temperature in Kissling’s investigations was originally 150’ C. and the time 60 hours. The method now used1* specifies the use of a stream of oxygen for 70 hours a t 120’ C. The resulting ‘(tar products” are to be isolated with Kissling’s alkali, then precipitated and weighed. Besides this, there is a quick method employing sodium peroxide for the oxygen supply. The values determined are always lower than by the first method. MICHIE TEsT-In England all the sludge is included, and a standard method, the Michie sludge test, was therefore deve10ped.l~ I n this method the oil is heated a t 150’ C. for 11 Chcm. Ztg., SO, 932 (1906); 31, 328 (1917);81, 938 (1908); SS, 531 (1909). 1’ ’4
1916.
“Transformer and Schaltetoele,” Berlin, 1913. Pollard Digby, Report on Switch and Transformer Oils, London,
6
I
h4;FB,Zn, cum Sn, A1 Consf tu 8 Uheof.
5
4
None AID pb=
3 2
I I
I
/ooo
300 500
Pme in Hours Figure 2
Criticism of Testing Methods MICHIETEST-The most important cause of disagreement is the temperature. As stated, Michie uses 150’ C. as the test temperature. Also in the earlier French specifications the same temperature was required.I6 Aside from the consideration that it does not agree with service conditions, recent investigation proved that this temperature is, without Snyder, Proc. A m . SOC.Teslina Materials, 14, 11, 954 (1924). Staeger, Hcluctica Chim. Acta, 6 , 6 2 , 386, 893 (1923). 16 Cahier des charges pour la fourniture des huilea de transforrnateura, Union de syndicat de I’electricitb, Paris, 1914. 14 1)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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any question, too high. The sludge formation is an oxidative polymerization, in which compounds of high molecular weight are formed under the oxidizing influence of atmospheric oxygen. As mentioned above, asphaltongenic acids have higher molecular weight than the original oil, calculated by the saponification value (Brauen) as well as by the direct molecular weight determination by the cryoscopic method (Staeger). That is, the values found in the writer's investigations vary from 350 to 800 with the different oils. That an oxidation is actually proceeding was proved, not only by the acid formation, but also by direct determination of oxygen. 'A sample taken from a transformer that had been in service for several years gave the following analysis: C 76.8, H 7.05, S 0 16.54 per cent; whereas a sludging test carried out in the laboratory 0 16.54 per cent. The oil showed C 76.46, H 7.05, S before the test analyzed: C 86.24, H 13.32, S 0.31, 0 0.23 per cent.I0 Judging from the results of investigations now available, this kind of oxidation has a more or less sharp temperature limit for oils as they are applied in transformers. The writer believes that this depends upon the degree of refining, highly refined oils being very sensitive to temperature and oxidation. This temperature limit is between 115' and 120" C. Above this region the oil starts to decompose with the formation of easily volatilizable acids of lower molecular weight, and ordinarily no sludge is precipitated. If sludge is formed, however, it has a carbene-like composition (coke-like) and is materially different from that formed below this temperature. The writer designates this reaction as "destructive oxidation." This phenomenon has been known for a long time. Z a l ~ z i e c k i ,as ~ ~well as Engler and Bock,'* proved that acetic acid and other fatty acids of low molecular weight are formed. Such procedures were used during the war for the production of fatty acids from mineral oils. In this laboratory up to 10 per cent of such acids were detected in Michie oils, which contained acetic acid. No trace of sludge was present. As an indication of the intensive decomposition, consideration is also directed to the large amount of water found (up to 5 per cent). The decomposing influence of temperature is thus plainly shown. This test method is to be condemned also because it employs wrong test conditions, aside from the consideration that the blast of air and oxygen is disadvantageous. The reaction is not to be hastened by rise of temperature or blast of air. The blasting of a mineral oil facilitates the oxidation and also produces acids of low molecular weight, even a t temperatures as low as 120' C. For example, a certain oil after 70 hours' blast of oxygen showed a neutralization value of 2.8 and with blast of air 1.9, while on heating in air without blasting, only 0.4. It is emphasized that these behaviors are dependent upon the constitution of an oil and cannot be generalized. It is impossible to evaluate a transformer oil by considering only one group of reaction products. It was proved that with a blast of oxygen, under certain circumstances, large quantities of peroxides and superacids are formed which impair cotton. I n the oxidation of cellulose (cotton) oxycellulose is formed, which according to recent investigations'g is a combination of aldehyde alcohols. The aldehyde groups reduce Fehling's solution and the oxycellulose can be determined by the copper figure.20 I n order to carry out the tests, the temperature could not be held a t 150' C., cotton being destroyed completely a t this temperature. Oils were chosen which in the Michie test were found to be free from sludge, and these were oxidized a t 112" C. in a copper
+
+
17 18
Hauser, "Lehrbuch d u Cellulose Chemie," Berlin, 1922. Schwalbe and Zieber, "Chemische Betriebskontrolle in der Zellstoff Industrie," Berlin, 1922. 3)
20
*
2.angew. Chcm., 4, 416 (1891). Chcm. Z l g . , 16, 596 (1892).
Vol. 17, No. 12
beaker both in a blast of oxygen and one of air and, for comparison, by simply heating in open air. The results are shown in the following corrected copper figures, the so-called cellulose figures being deducted: Oxygen blast Air blast Open air
Copper figure 9 . 2 (extremely high) 7.9 0.6
The corresponding acid figures, given below, were not high enough to justify the assumption that decomposition by acid had occurred. From the above it can be concluded that oils found to be excellent by the Michie test decompose with formation of superoxides even a t relatively low temperatures, as can also be shown by the decreasing tensile strength of cotton yarn (90/2): Initial After 100 hours' heating in oil at 112' C. with oxygen blast After 100 hours' heating in oil at 112' C. in open air After 100 hours' heating in a good oil a t 112' C. in open air
Grams 240 0 *1 56 225 *2
Further, oil-soluble acids of low molecular weight were formed. The oils showed the following neutralization values: Initial After Michie test After oxygen blast, 112' C., 100 hours After open-air oxidation, 100 hours A good oil, after open-air oxidation, 100 hours
A 0.11 12.5 8.2 4.6
B 0.09
0.15
It can therefore be concluded that the Michie test assumes conditions that do not exist in operation and yields products that are never formed in the transformer. The reactions differ from those a t air temperatures below 115" C. This test applies only to highly refined oils which are very sensitive to oxidation and temperature. The sludge is not the only product of the decomposition of the oil and cannot be considered as the only qualifying figure of the oil. TAR FIGuRE-The tar figure also considers only one group of reaction products-namely, the acids. At the beginning of the decomposition of the oil acids are formed. (Even naphthenes can be transformed by oxidation to chain-like hydrocarbons and these in turn oxidize to acids.) By internal anhydride formation the acids, as hydroxycarboxylic acids, result in lactones and transformed lactide-like polymerization products of neutral and unsaponifiable character. For example, in a transformer oil sludge after 1000 hours' heating a t 112' C. about 60 to 70 per cent saponifiable compounds were still found, and in sludge from a transformer which had been operated for about ten years, about 50 per cent. Besides these acids there still exist in the sludge oil-soluble acids which were determined in the writer's investigation of the neutralization value. As mentioned above, the rate of acid formation is not uniform, and it is inadmissible to interrupt the process a t a certain point. It has been proved that oils oxidize with difficulty a t the start, but that the first formed reaction products catalyze the sludge formation with the result that the sludge content after a certain time does not agree a t all with the tar figure. This can be seen from the following values: Tar figure Sludge, per cent by weight after 1000 hours
0.07 2.67
0.07
0.14
Oils that are good by the Michie test may have bad tar figures because they incline to acid formation on account of their highly refined character. LIFE TEST-Whereas in the tar test only the acid products are determined, the life test shows only the sludge. The temperature is lower than in the Michie test and is within sdmissible limits. The same oils as in the first test will be found satisfactory, as they are regarded from the single viewpoint
December, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-of a long “life.” Regarding the decomposition and formation of dangerous reaction products, the same is to be said of this test as in the criticisms of the Michie test, as is shown by the following figures: Life test Neutralization value Decreasing tensile strength of cotton yarn
110 days 32
142 days 44.2
100 per cent 100 per cent
Further, it is not advisable to heat several samples in the Same compartment. An extensive series of tests showed that under these circumstances condensed reaction products catalyze the decomposition of the others. It is well-known that traces of polynaphthenic acids or unsaturated compounds of low molecular weight facilitate the decomposition extremely. Such products are always liable to come in touch with the samples. The exclusive determination of the sludge products may be considered as the main fault of this test, and it will therefore never be able to indicate the oils most suitable for operation, especially since some oils that form large quantities of sludge are soluble in hot transformer oil, which is not taken into consideration in this method. BBC TEST-A~~ the different reaction products are determined and considered in the BBC test. Snyder makes the criticism that this test determines the sludge after a certain time which is not properly fixed. A long series of comparative tests between laboratory and field, carried out over several years, have shown that the results are comparable
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and that the fixed time corresponds to a certain operating time. Snyder also objects to the use of copper. It should be remembered, however, that the reaction products which arise from the catalytic influence of copper in this test method are absolutely the same as those developed in a transformer, and in evaluating the quality of an oil for service this is the most important consideration. Furthermore, in certain oils there occur reactions which cannot be measured by analytical methods, but can only be estimated by their effect. These reactions are those which destroy the insulating materials of the transformer. To estimate their effects, cotton yarn is used as described above. The results yielded by this test method are satisfactorily illustrated by the test results cited above. It has been objected that, contrary to the BBC test, no unprotected cotton and copper are present in a transformer, only impregnated cotton being used, and that for the elimi, nation of oxidation transformers are provided with oil conservators or filled with nitrogen. To this the writer would reply that the fundamental principle of material testing is to stress the material under the worst operating conditions in a limited time and in evaluat,ing the quality of the material to take into consideration all reaction products liable to be formed in operation. I n accordance with these principles the specifications of the BBC test were developed as they appear above.
How to Make a Sanitary Survey of Your Plant’ B y C. L. Ferguson TAB SELBY Saos Co., PORTSMOUTE, OHIO
B
EFORE attempting t o make a sanitary survey famil-
iarize yourself as thoroughly as possible with the standard requirements of sanitation. These come under the heading of codes or laws which either recommend or demand that certain conditions shall or must be as stated. With this information a t hand it is comparatively easy to make a report which excludes the highly technical data. All such technical problems should be considered just as medical and surgical problems should be considered. If you have a surgical problem, call the surgeon. If you have a case of trachoma or a foreign body in the anterior or posterior chamber of the eye, call an eye specialist. In other words, if you have health hazards with which it is beyond your ability to cope, seek the services of one who is competent, or learn how similar problems are solved elsewhere, The fundamental standard requirements for an industrial plant consider general cleanliness, drinking water, ventilation, lighting, exhaust systems, toilets, cuspidors, wash =rooms, dressing rooms, rest rooms, and restaurants. In order to become familiar with these factors it is imperative that you know your plant. This can only be accomplished by frequent visits through it.
Cleanliness In taking up fundamental standard requirements we find that it is general cleanliness which first attracts and pleases t h e eye. A clean plant is always impressive. While making a survey do not forget to interview some of the employees. When they have been thoroughly sold they take pride in showing you through the plant. Recently the writer lis]
Received April 23. 1925.
tened to an enthusiastic report by a foreman who had visited a factory in a small, isolated inland town. His enthusiasm was a manifestation of the enthusiasm demonstrated to him by the employees. He reported the plant free from dirt and the employees ever ready to explain how their plant. was kept so clean. This foreman is not a sanitary engineer, nor did he visit this plant with the idea of making even a casual inspection, but general cleanliness stood out so prominently that he apparently saw nothing else. Drinking Water
The supply of drinking water must he pure. First, d e termine its source. If water from the city mains is used the necessity for its inspection is not so urgent, becausc this is daily examined by a chemist a t the water-works plant under supervision of the local or state health departments. If water from a privately owned driven well is used, it should be inspected a t frequent intervals and a sample of this water should be collected by the inspector and sent away for an examination. The drinking of plenty of water is highly essential; therefore, note the number of water fountains. Are they conveniently located and are they of the sanitary type? Are the conditions around the fountains conducive to free drinking?
Ventilation Another important requirement of a sanitary plant is good ventilation. See that the air is in motion. Does fresh air enter the room constantly while the bad air is being taken out? I n the summer time it is comparatively easy to determine whebher or not the air is moving. Witness the lifting of the
