-Turbine foot of combustor as the 1-16 and has less deposition trouble with a given fuel, If it had been designed for it, the 5-33 might have burned poor fuel without encountering deposition. However, design compromises favored specific output rather than deposition resistance, so high quality fuels are still required. Similarly, the continuing need for increased specific output will probably absorb the benefit of most future improvements in combustor design, Hence, the need for high quality jet fuels will probably continue. LITERATURE CITED
(1) Arthur, J. R., Kapur, P. K., and Yapier, D. H., Nature, 169,
372 (1953).
Fuels-
( 2 ) Bass, E. L., Lubbock, I., and Williams, C. G., Shell Aviation News, 156 (1951). (3) Comerford, F. &I.,Fuel, 32, 67 (1953). (4) IIadai, D,, Ibid,, 32, 112 (1953). cHEM., 45, 602 (1953). (5) Hunt, R. A., J ~ .rZrD, , (6) Parker, W. G., and Wolfard, H. G., J. Chem. SOC., 1950, p. 2038. ( 7 ) Schalla, R. L., and RIcDonald, G. E., IXD.ENG.CHEX., 45,
-
1497, (1953).
(8) Sharp, J. G., Aircraft Eng., 23, 2 (1951). (9) Thorp, K,, Long, R., and Garner, F. H., Fuel, 30, 266 (1951). (10) Ibid.,32,116 (1953). (11) Williams, C. G., Shell Aviation S e w s , 105, 106 (1947). RECEIVED for review April 24, 1954.
AccmrEn J u n e 26, 1954.
Corrosion and Deposit in Gas Turbines B. 0. BUCKLAND General Electric Co., Schsnectady, N . Y .
Use of residual fuels in gas turbines has been limited by the fact that the ash from these fuels is often very corrosive to high temperature turbine parts and also tends to form slag deposits that reduce turbine efficiency. Vanadium and sodium are two of the most corrosive ash constituents usually encountered, and methods of inhibiting both by chemical additives have been developed. Chromium compounds, which are needed to inhibit sodium, however, augment slag formation, and methods have been developed to remove the sodium from residual fuels by washing and centrifuging. When this process is used it is only necessary to introduce an additive that prevents vanadium corrosion and at the same time forms a nonsticking ash. A water solution of magnesium sulfate, thoroughly mixed with the oil to form an emulsion, is an economical additive which is satisfactory for this purpose. ,4s a result of tests and operating experiences, a fuel specification has been prepared that defines residual fuels suitable for use in gas turbines.
c
OhlhfERCIALLY acceptable gas turbines using natural gas
fuel are a t present available. By Dee. 1, 1953, 36 General Electric gas turbines using this fuel had accumulated 140,000 hours of satisfactory operation. Three power generation units had operated 50,000 hours, and 33 gas pipeline pumping and compressor drive units had operated 90,000 hours. The usefulness and market for these prime movers could be considerably increased if they could be made to operate satisfactorily on both coal and residual fuel oil. For this reason, the General Electric Co. has undertaken the development of the residual fuel oil burning gas turbine, and this paper deals with the problems involved in using this fuel. As of December 1, there were 13 General Electric residual fuelburning gas turbines in use which had accumulated an operating time on this fuel of 50,000 hours. This service experience has shown that two basic problems exist in burning residual fuelslag-forming substances that are present in the oil corrode the metallic parts, and these same substances deposit on the nozzles and buckets, thus producing an accumulation of material in the gas path of the turbine and causing a loss in efficiency and capacity. As much as 2 tons of ash is fed through one of these machines during operation a t full load for 1000 hours. Since the first stage nozzle, for example, cannot tolerate an accumulation of more than 1 to 2 pounds of this material without a substantial
October 1954
loss of efficiency and capacity, these deposits are a source of trouble. Shortly after the failure by corrosion of the first stage nozzle of a locomotive unit while on factory test in 1948, a program of laboratory investigations was started to learn more about the nature of oil ash corrosion and its prevention. This program has continued from that time. Two basic tools that were developed for use in the study are the so-called crucible tests and the small burner tests. The results of some of the work on the crucible. tests and the small burner tests have been reported ( 3 ) and a report on corrosion experience during operation has been presented (@.
Work by the General Electric Go. on the corrosion phase of the bunker C problem, was first undertaken because it was far more important than the deposit phase and affected the operation of the turbines before any substantial interference occurred due t o the ash deposition. Furthermore, it was necessary to discover the nature of the possible cures for the corrosion before an effective attack on the deposit problem could be made. Work on the corrosion phase of the problem led to the prediction that satisfactory life could be obtained by a proper adjustment of the constituents of the ash by means of additives, together with a proper choice of alloys of construction. A fuel specification defining the ash constituent relationships, together
INDUSTRIAL AND ENGINEERING CHEMISTRY
2163
with some of the supporting evidence, was proposed (5). In this specification, calcium is used as an additive to inhibit vanadium pentoxide corrosion, and the ratio of sodium to vanadium in the fuel is maintained a t 0.3 or less t o prevent corrosion due to sodium sulfate. E M U L S I O N BREAKER
.OD5% TO .02
NOZZLE TYPE CENTRIFUGES
PUMP
Figure 1. Oil W;ashing S y s t e m
Thus, early in 1953, wags had been discovered arid applied to prevent corrosion due to vanadium pent'oxide and soldium sulfate, the two principal corroding constituents of the ash, by the use of additives. A sufficient number of useful additives had been tried, the more effective ones being the alkaline earths, to define the nature of the cures for corrosion and to allow an effective attack on the deposit phase of the problem. This u-orlr has been started and is continuing. It is not finished, but sufficient information and experience are available 2300 concerning the deposit feature as well as some new evidence concerning t,he corrosion feature of the problem to warrant a modification of the proposed fuel specification. 0 REMOVAL OF SODIUSI
tial amount at the point of use was undertaken. This has beer1 done on a plant scale, and several methods of removing the salt and reducing the sodium coiltcnt of the oil to in the neighborhood of 4 or 5 p.p.m. have been developed. They consist of mising water or a water solution of a salt intimately with the oil and centriEuging the mixture. Two centrifuges of the selfcleaning nozzle type are used in the manner shown in Figure 1. In the prowss, the wash solut,ion picks up most of the sodium in the oil. I t is nccessary to have a deneity difference of about 3 or 47, between the oil and the wash solutiori a t centrifuging temperatures. Since some bunker oils are about as dense as water, the requirement in euc,h cases is fulfilled by using a water solution of a salt to increase the density of the wash. A magnesium sulfate solution has been found satisfactory for this purpose. Table I shows the approximate Podi'um content and calcium content after washing one type of bunker C oil. Initially, this oil contained 70 to 100 p.p~m.of sodium and 200 to 250 p.p.m. of calcium. The use of oils desalted in this manner has shown that both sodium and calcium promote the accumulation of nozzle and bucket deposit. It has been demonstrated by plant scale tests that a reduction in the sodium content of the oil from approximately 100 to about 5 to 7 pap.m. reduces the rate of deposit accumulation several fold with an oil containing approximately 150 p.p.m. of calcium. Furthermore, reducing the calcium in the same oil froin 150 to approximately 5 p.p.m. further reduces the deposit accumulat>ion. One machine has been operating with oil cont'aining about 6 p.p.m. of calcium and 4 p.p.m. of sodium. In 800 hours of operation, the effective flow area of the first stage nozzle decreased only about 3%. This decrease occurred in the first 350 hours and remained substantially constant for the
W X
2200
Oil delivered on the eastern seaboard has been W very bad from a corrosion as l\-ell as a deposit standpoint, both because of the naturally-contained sodium and because of contamination with fj salt water which occurs in tanker transportation. 2100 This problem also plagues marine boiler users, the Navy, and utilities burning bunker C because of the accumulation of boiler and superheater deposits and, in some cases, because of superheater cor0 rosion. Although vanadium, vhen uninhibited, causes corrosion, it also inhibits corrosion due t o Figure 2. sodiumsulfate. This is the reason for proposing the requirement that the sodium to vanadium ratio is 0.3 or less. Since this requirement was difficult to attain in some oils, particularly n-hen salt water contamination occurred, additives were sought to inhibit sodium sulfate corrosion when the sodium content of the oil was high. The addition of chromic acid t o the fuel was tried on a plant scale and, although this mas satisfactory from a corrosion standpoint, the treatment m s not useful and was abandoned because it produced extremely hard and rapidly accumulating deposit. After this attempt to inhibit the sodium sulfate corrosion by means of chromic acid failed, and as it became clear that sodium in the oil also markedly increased the rate of accumulation of deposit, reduction of the sodium content of the oil by a substan-
2164
ico
200
300 400 500 600 708 HOURS FIRED OPERATION O N RESID. FUEL
800
900
Effect of Sodium and Calcium Reduction on Deposit .4ccumulation
TABIXI.
SODILJll A S D r A L C I U M IS O I L AFTER ~~'.4SHIR.G Bunker C oil Sodium, 70-100 p.p.m. Calcium, 200-280 p.p.m. API gravity, 12.0° at 60 F. Viscosity, 200 Sagholt see. Furol a t 122 O F.
Kash Tap ,,.ater Tap water plus 10% No. 2 oil Calcium nitrate solution LIagnesium sulfate solution
Metal Concentrations after Wash, P.P.11. Sodium Calcium e-10 90-120 60-100 2 5 5-10 120-170 20-30 4-8
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 10
-
-Turbine
remaining 500 hours of operation. Before the reduction of sodium and the calcium in the oil, the decrease in effective area was approximately 30% in some 60 hours of operation. I t had not reached a st,eady-state value in this time. After the reduction of the sodium but not the calcium it was about 10% in 60 hours. A nozzle area index number,, which is proportiona~ to
v, 3000-
H u cc
2 -I &
z I
2
2000-
-
500-
0 400300-
;
200-
(3 W
3
TABLE 11. CHXRACTER OF ASH General Electric Go. Fuel Specification, O c t . 21, 1Y53 (a). The ratio of the weight of sodium in the ash t o t h e weight of vanadiiiii: in the ash should not be greater than 0.3. If this ratio is not satisfied b y tile original oil it can be obtained by removing sodium by means of washing, g p i i . u g i n g , filtering, electrostatic precipitating, or by a n y other desalting (b). The ratio of the weight of magnesium t o the weight of vanadium in the ash should not be less than 3.0. Oil soluble materials can be added to the fuel t o obtain this ratio, in case i t does not satisfy the condition naturally. I n cases where the oil is modified a t the point of use, water solutions of suitable magnesium salts, such a s magnesium nitrate or magnesium sulfate, may also be added t o obtain the desired ratio b u t they must be thoroughly mixed with the fuel t o obtain a fine and uniform dispersion, and the oil must be burned sufficiently soon after treatment t o avoid partial separation or settling out, of the mater solution. Wh$n t h e vanadium content is 2 p.p.ni. or less, the foregoing weight ratio of magnesium to vanadium need not be maintained. ( e ) . The sodium content of the oil should not exceed 10 p.p.m. and a value of 5 or less is preferred. When the sodium content is 5 p.p.m. or less, the foregoing weight ratio of sodium to vanadium need not be maintained. (d). The oalcium content should not exceed 10 p.p.m. and a value of 5 or less is preferred. (e). After Items 2(a), (b). ( c ) , and (d) are satisfied, the total ash content in the oil should not exceed 2000 p,p.m.
UNTREATED
1000800600-
Fuels-
100-
z W
2
50-
MAGNESWM
0
w
a
in
L
lo
1500
16b0
17b0
lsb0
TEMPERATURE, O F.
Figure 3.
Effect of Additives on Corrosion by Vanadium
100-Hr. corrosion b u r n e r tests Residual oil c o n t a i n i n g 350 p.p.m. of v a n a d i u m 25 Cr-20 N i (AISI Type 310) specimens
the effective flow area, is plotted against hours of operation on bunker C fuel in Figure 2. Until these desalting experiments, calcium had been a mainstay as an inhibitor for vanadium pentoxide corrosion but, because of turbine deposits, it has been found desirable to remove it. This removal requires another method for inhibiting vanadium attack. During the plant turbine tests, magnesium was used as the inhibitor. This illustrates the fact that some knowledee of the corrosion Dhase of the problem was necessary in order to make an effective attack on the deposit problem. Magnesium does not cause deposit, or a t least does so to a much less degree than calcium. Fortunately, in the temperature range below about 1650" F. magnesium is a better inhibitor for vanadium corrosion than calcium. This is shown in Figure 3, in which the weight loss of a 25 Cr-20 Ni specimen tested in the small burner rig is plotted against temperature when the fuel is an oil with approxinlately 350 p.p.m. of vanadium. The three lower curves show results for the same oil with calcium and magnesium additives used separately and in combination. At approximately 1675" F. the calcium and the magnesium additives are equivalent, but a t 1500' F. the magnesium is considerably more effective.
content of the ash to the neighborhood of 5 p.p.m. in order to reduce the deposition rate to acceptable values, and substitutes magnesium a8 ah inhibitor against vanadium pentoxide corrosion for the calcium formerly used. The new specification is written to include both modified residual fuels and distillate fuels. The distillate fuels contemplated me, in general, also low ash fuels with ash quantities ranging from 3 to 100 p.p.m. It is expected that the important factors in preventing corrosion with distillate fuels, as with any other fuel, will be the ratios of the ash constituents, as limited in the specification, even when the total quantities of these metallic materials are very low. Evidence supporting this expectationthe effect of varying vanadium concentration o n ,the corrosion rate as measured by the small burner tests for four residual oilsis shown in Figure 4. -4 rapid increase in corrosion rate occur6 as the vanadium content increases, and any amount of vanadium, uninhibited, causes a corrosion rate higher than vanadium inhibited in the proper ratio by means of, say, magnesium. T h a . it is believed that the best results will be obtained when the ash constituents have the ratio specified for all quantities of total ash,
-
FUEL SPECIFICATION
The foregoing experience is sufficient to warrant a modification in the proposed fuel specification ( 3 ) . This modified sDecification was issued in October
1953. The specification concerning the character of the ash is given in Table 11. Basically, the specification limits the sodium and calcium
October 1954
Figure 4.
Effect of Vanadium Concentration on Corrosion 100-Hr. small burner tests at 1600" F. 25 Cr-20 Ni (AISI T y p e 310) specimens
INDUSTRIAL AND ENGINEERING CHEMISTRY
2165
regardless of hov- small. Some distillate-type fuels mag contain as much as 2 p.p.m. of vanadium. However, since the rate of corrosion at these concentrations and below n-ithout inhibitors is low, the specified magnesium-vanadium ratio need not be maintained for concentrations of vanadium of 2 p.p.m, or less. Data showing the way in which the corrosion rates increase with increasing sodium content in low concentration ranges are being obtained but are not yet available. Also corrosion by sodium sulfate is influenced strongly by the presence or absence of carbon as well as, perhaps, by the presence of chlorides. Thus, there is at present no very accurate way of specifying a value of sodium that will ensure safety xithout the protection afforded by a suitable sodium-vanadium ratio However, in Borne cases little or no corrosion has occurred with 5 p.p.m. of sodium Then the sodium to vanadium ratio was not maintained; therefore a t concentrations of 5 p.p.m. of sodium or less the sodium to vanadium ratio need not be maintained. I n borderline cases, where this discontinuity in the specification would be unreasonably discriminating, it may be possible to rate the oils in question by the small burner tests. CONCLUSIONS
Efforts by the General Electric Co. to develop a completely satisfactory bunker C burning gas turbine by means of laboratory experiments and experience in the field are continuing. I n addition, a number of other investigators are also working on the problem, particularly in Europe and England. Sulzer Brothers of Switzerland suggested the use of silicon as an additive and this has been tried in a series of tests by the Shell Oil Co. ( 1 ) . These tests showed that silicon added to the fuel in the form
of ethyl silicate practically eliminates the deposit. They also showed that zinc and magnesium effectively reduce the rate of deposition. A short test by the General Electric Co. on a plant scale showed that aluminum added to the fuel almost eliminates the deposit. Additives in a solid form may be the cheapest and most satisfactory solution to the problem Then suitable means of adding them and pumping the fuel containing them are developed. U o s t desirable would be the discovery of a single additive that would prevent corrosion and eliminate the deposit at the bame time As investigations continue and as knowledge is gained it seems likely that an inexpensive additive will be found to solve the coirosion-deposition problem. Fuel treated R ith such an additive \I-ould he useful not only for gas turbines but also for other applications iyhere corrosion and deposit have been encountered, such as marine and stationary boilers. As the use of modern steam tempeiatures of 1050" and 1100" F. incieases, the problem of superheater and reheatei corrosion in large power generation boiler plants may be expected to increase. One solution to the problem will, no doubt, be the adjustment of the ash forming constituents of the fuel by means of additives, as proposed for residual oil burning gas turbines. LITERATURE CITED
(1) Bowden, A. T., Draper, P., and Rowling, H., presented a t the meeting of the Inst. Mech. Engrs. (London), April 1953.
(2) Buckland, B. O., and Gardiner, C. M., presented at the American
Power Conference, March 1953. (3) Buckland, B. O., Gardiner, C. M.,and Sanders, D. G., presented at the meeting of the ASRIE, December 1952, Paper A-52-161. RECEIVED for review March 18, 1954.
ACCEPTED .iUgUSt
9, 1954.
Stabilitv of Aircraft Turbine Fuels J
C. R. JOHNSOY' AND D. F. FINK
A. C. KIXON
Shell Oil Co., N e w York 20, !V. Y .
Shell D e v e l o p m e n t Co., Emeryville, Calif.
A l l the knowledge gained from research on gasoline and furnace oil is not sufficient to solve the many stability problems encountered with aircraft turbine fuels. Long-time storage is only one of the factors requiring study. Idditional work is necessary to solve low, temperature filter plugging and high temperature fuel system problems. Although the long-time storage performance of a fuel may be generally characterized by the methods used in its manufacture, this performance cannot be correlated with detailed hydrocarbon composition as determined by the usual laboratory tests. The desire of designers to use fuel as a heat sink in their attack on the thermal barrier introduces severe problems of high temperature stability. Fuel performance under these conditions appears unaffected by oxidation inhibitors, but is influenced by certain detergents.
I'
ir estimated that peacetime demands for turbine fuels mill ISbe 112,000,000 barrels in 1956. Wartime requirements would, of course, be much greater. Wartime demands require the utilization of both straight-run and cracked components boiling in the 150" to 500" F. range. It is essential that these fuels remain stable when stored for long periods of time and also under conditions imposed by aircraft operation, All the knowl-
2166
edge gained from research on gasoline and furnace oil has not been sufficient to solve the many stability problems encountered with aircraft turbine fuels. Introduction of new fuels results in problems that require still further study. This paper outlines pertinent ideas and information from many laboratoiies. Specific references are given for some of the work; some of the investigations are under military sponsorship.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol, 46, No. 10
