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 b u t 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 a n 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 i t seems likely that a n inexpensive additive will be found to solve the coirosion-deposition problem. Fuel treated R ith such a n 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
-Turbine hlilitary logistics require t h a t fuels be stored for long periods of time. T o be suitable for use these fuels must be limited in both soluble and insoluble gum content-they must pass through the fuel system to the combustion chambers without plugging filters or causing malfunctioning of control systems. Work is under way to determine engine tolerances and ways and means of restricting gum formations within tolerable limits. A great deal of confusion exists today with respect to the importance of the problem of low temperature filtration. Subsonic aircraft operating for long periods of time at high altitudes draw fuel from their storage tanks a t very low temperatures. The fuel must have a freezing point, pour point, and cloud point below operating temperature if it is t o pass through 10-micron filters. This phase of the problem is clear. Confusion exists with respect to ice formation. Ice crystals from dissolved water can plug filters at temperatures slightly below 32' F. To overcome this problem certain aircraft have in the past been designed with alcohol injection equipment. Supersonic flight involves adiabatic compression of air that results in a boundary layer temperature rise which is a function of the Mach number. For example, speeds oi Mach 2 result in a surrounding air temperature of about 200" F., and RIach 3 a temperature of about 580" F. The dissipation of heat a t these temperatures is one of the major problems of the attack on the thermal barrier. The fuel, as a heat sink, is eyed enviously by the designer as the most economical solution to the problem. Electronic equipment, hydraulic systems, engine lubricants, and even the missile skin itself, are included in the designers' plans for use of the fuel as a coolant. These applications result in new problems of high temperature fuel stability. I n some applications, fuel in the tanks is heated to moderate or high temperatures for a significant period ot time. I n other applications, the fuel is heated t o very high temperatures for a few seconds as i t passes through a heat exchanger and into the engine. A number of investigators were rather surprised to learn t h a t what might be considered reasonably stable fuels, when heated t o 300" F. for 3 seconds, formed sufficient insoluble matter to plug nozzle screens. A solution t o this particular problem is most urgently needed. STORAGE STABlLITY
Because jet fuels have appeared on the scme only recently and their specifications have been changed rather frequently, there is relatively little information in the literature on the storage fitability of modern jet fuels (JP-3 and 4). Sltman ( 1 ) reports the slight effect of aging on JP-1 type fuels under laboratory, accelerated, and desert conditions; a Texas Co. article (6) suggests that the 16-hour accelerated test ma) be too stringent for JP-3 fuels containing cracked materials; while Murray (6) stresses the need for good stability in both military and commercial fuels and makes a plea for better accelerated tests. A considerable amount of work has been done by Shell Development Co. under contract to first the Navy and later the Air Force, but publication of most of this work is pending. A cooperative investigation is presently being conducted under the auspices of the Coordinating Research Council, b u t information from this study is not yet available. A considerable amount of work has been done on the stability of the chief components of jet fuelnamely, gasoline and Diesel fuels-and some of this work is outlined briefly in this paper. For the purposes of this discussion, gasoline is defined as a product boiling between 100' and 400" F., diesel fuel between 350' and 600" F., and jet fuel between 100' and 600' F. (although in view of recent specification changes the range is more nearly 150" to 500' F.). All three products may be made up of straight-run, thermally cracked, and catalytically cracked components. All three materials are susceptible t o oxidative polymerization which builds u p in time soluble and insoluble
October 1954
Fuels-
gums containing mainly carbon, hydrogen, and oxygen with various amounts of nitrogen and sulfur. I n all cases the reaction appears to proceed through the intermediate formation of peroxides, although this is by no means certain for the higher boiling components. All three materials are generally stable in the absence of air or oxygen a t temperatures below about 700" F., where cracking reactions begin. Much work has been done on the stability of gasoline (13-16); this has shown that the formation of gum, at, least in a gasoline containing thermally cracked components, proceeds on the average a t a rate of about 2 mg. per deciliter per month at 110' F. (in the presence of natural or synthetic inhibitors) and has a heat of activation of about 25,000 calories per mole (increasing the temperature 10" F. in the vicinity of 110" F. increases the rate by a factor of 2). Doubling the partial pressure of oxygen also increases the rate of gum formation by about lo%, on the average. There are indications t h a t these relat'ions do not hold to the same degree for gasolines containing catalytically cracked product's even though such products are just as stable and have as good inhibitor susceptibilities according t o accelerated tests as the thermally-cracked gasolines (3). The stability of diesel fuels has not been as widely investigated as the stability of gasolines. However, recently the U. S. Bureau of Mines a t Bartlesville started an extensive investigation of the effect of aging on t8heproperties of straight-run, thermally cracked, and catalytically cracked and blended diesel-t,ype fuels. This study has revealed that the stability characteristics of these fuels with respect t,o the formation of color bodies, soluble gum, and lnsoluble gum decreases in the order-straight run, catalytically cracked, thermally cracked ( 8 ) . Work on t'he stability of fuel oil has shown the importance of minor component's with regard to this property. Thompson and associates (IO,11) demonstrated that the presence of cert.ain sulfur compounds and pyrroles greatly increases the tendency of the fuel to precipit,ate insoluble mat'erials. However, the higher boiling fuels have not shown the same susceptibilit,y to amine- and phenol-t,ype inhibitors as have gasolines. The behavior of jet fuel is consistent with the properties of its chief components. Measurement of Stability. The methods of measuring and predicting the storage stability of jet fuels have mainly been adapted from those used for gasolines with appropriate variations t o allow for the higher end points of the fuels. hlilitary specifications dealing with gum content and stability involve evaporation of the fuel in a stream of air a t 400" F. rather than at 320' F. as is used for gasoline (ASTM D-381-50). Present specifications allow the presence of 10 mg. per deciliter of gum in t,he fuel. However, there is good evidence that the air jet procedure promotes the formation of gum while the test ie in progress, so the ASTM method is being modified to allow the u8e of steam instead of air as the vaporizing medium. The temperat,ure is also being increased t o 450' F., which allows complete vaporization within 15 minutes ( 7 ) . The accelerated aging procedure has also been adapted from gasoline practice. The fuel is aged at, 100" C. and 100 pounds per square inch gage oxygen for 16 hours as (ASTM D-873-49), during which time the fuel must not build up more than 20 mg. per deciliter of total gum. Insoluble gum formed during this test and also during storage can be determined by the filtration procedure outlined in the above $STM method. Soluble gum may also be determined b y the acid flocculation method, but the significance of this procedure for jet fuels has not been established. The formation of color bodies usually accompanies aging of petroleum fractions and this is true also for jet fuels. The usual methods for color determination, Saybolt (ASTM D-156-49) and Union (ASTM D-155-45T), may be employed, as well as phot,oelectric colorimeter. The significance of changes in color is not well defined, but it has been observed that the rate of change of color (based on some final value) is more rapid in the early stages than is the rate of formation of gum. The aging
INDUSTRIAL AND ENGINEERING CHEMISTRY
2167
1 1 O'C.
I
26 I O
26.34
25 5 8
,
105'C. ,
,
Le 8 2 1 ' " K n IC'
, 2: Cb
9c
,
2:
c
U
Lb.139.Inch 30 100 150
Temp. Coefficient ( R ) 3810 4280 3790 3900 'nv.:
t?
-
A)
I 30
20
I 40
Thev:r.al V s U.V . Aging 100
M i d - C o n t i n e n t Composite JP-4
Heat of hctivation ( E ) , Cal./hIole 17,500
W i r e (0 3) /
,
19,800 17,400 18.600 ( a v . )
of jet, fuels is also accompaiiied hy the production of peroxides, although litt,le parallelism Iias been obscryed between the rate of production of peroxides and that of gum. Peroxides are commonly determined by the well knoun Yule and Wilson method of reaction with ferrous ion ( l e ) , although the newly developed arsenious acid method is also applicable ( 1 2 ) . Extrinsic Factors. TEMPERATL-RE, PRESSL-RE, A N D DESERT STOR-~GE. The rate of degradation of jet fuel appears, in general, to be a normally temperature sensitive reaction with a heat of activation of about 21,000 calories per mole (0). This reaction was evaluated by det>erminingthe effect of varying the temperature in the neighborhood of 100" C. (Figure 1). The reaction is also sensitive to oxygen prrssure, with an oxygen pressure coefficient of - 0.52, determined b. varying the oxygen pressure near 100 pounds per square inch gage. The combination of two effects can be espressed by the equation f Log 2 = 4500
I 10
Gum. rngjdl,, U. V . , 20 and 40 hr. Aging
Figure 1. Effect of Temperature on Jnduction Period of California Catal>-ticallF Cracked .Jet Fuel Oxygen Pressure,
I
0
27 8 4
1)
6
12
M o n t h s a: 1 1O'F.
Effec! of Copper
I
___ Aging Conditions IIO'F., 1 a:mA:r
TC
P P?
- 0.52 log -2
r_(
1 he corresponding equation for gasolines has coefficients of 5500 and 0.18, indicating t h a t jet fuels are somewhat more susceptible to the effects of temperature and less susceptible t o t#he effecbs of oxygen pressure than are cracked gasolines. Although the equation was determined for a variet'y of jet fuels, comparison of the results of calculated stabilities with the results of actual desert storage tests indicat>est h a t the correlation considerably underrates t,he stabilities of fuels containing cracked components and slightly overrates the stability of straight-run fuels. This means t h a t t,he present accelerat,ed aging t,est in the military specifications tends to exclude unnecessarily fuels containing cracked componcnte, at least on the basis of normal storage stability. The equat.ion represenh only the average behavior OF the fuels investigated and this, toget'her with the fact that the extrapolations involved are rather long, m e m s t h a t individual fuels can depart widely from the predicted behavior. Light, particularly ultraviolet light, has a marked accelerating effect on the deterioration of jet. fuel. I n a standard Atlas Fadeometer 20 hours of exposure t,o ultraviolet light (2900 to 400 A.) was equivalent to more than 8 months storage at 110" F. (Figure 2). Ordinarily, jet fuels are not exposed t80light during storage. One important factor, however, is the materials t o which the fuel is exposed either during manufacture or storage. This ia usually steel which, fort,unately, does not generally have 2188
INDUSTRIAL
1
t
!
California Composite J P - 3 80
-
(0.I%w,S)
I I
60
B
i4 0 a
17 20
I
I
7
I
14 Days al 7O'C.
I
J
21
Effect of S and N Compounds
Figure 2.
Factors Affecting Fuel Stability
AND ENGINEERING CHEMISTRY
Vol. 46, No 10
-Turbine a dclctcrious effect on stability, excacpt in the presencc of sea water. Copper is the only coinmon inetal that has a generally adverge effect on jet fuel stability, even a t concentrations in the order of parts per million. Fortunately, this action can be controlled by metal deactivstors, compounds that tie up the copper ion in solution by chelation and so prevent its catalytic effert. However, copper surfaces have rven more deleterious long-time effects than soluble copper, and this action is not as controllable with metal deactivators (Figure 2). This problem inav aswine more iniportance in the future because of the increasing copper content of drum steels (presumably due to scrap ierycle). Intrinsic Factors. H Y D R O C ~ R B O NCOMPOSITION. One of the most important factors affecting stability is the over-all hydrocarbon composition of the fuel, which may be characterized by the type of process by which the fuel was produced-distillation (straight run), thermal cracking, or catalytic cracking. Many variables besides hydrocarbon composition are involved, but this is a useful method of comparison, T h e relative stability of the three categories, ae determined by hot room storage of JP-3 fuels, is shown in Figure 2, which summarizes work done by Shell Development Co. Storage stability decreases in the order, straight run, catalytically cracked, and thermally cracked. Kork reported by Schwartz and coworkers (8) on diesel fuels is in accord with this general rating of fuels. However, in spite of this over-all correlation between type of fuel and stability, there is little correlation between this classification and detailed hydrocarbon composition, as revealed by the fluorescent indicator absorption method (2) or bromine number. The amount and kind of minor components, such as sulfur and nitrogen compounds, are probably as important as hydrocarbon composition (Figure 2). This is borne out by the observation that the gum, both soluble and insoluble, formed during aging always contains more of these elements than the fuel from which i t was formed. Polysulfides have been found to have the most virulent effect, followed by higher aliphatic mercaptans, thiophenol, and lower aliphatic mcrcaptans. Other sulfur types were practically inert. I n this particular study nitrogen-containing compounds did not appear to have a large effect on stability, although a pyrrole or an aliphatic triamine had some detrimental effect. This is somewhat a t variance, in detail, with the results obtained by Thompsonband coworkers (10, 11), although there is agreement t h a t minor components do affect stability. It is hard to generalize with confidence on the effect of minor components, however, because of the undoubted interrelationship between the particular type and concentration of minor component and the type and concentration of hydrocarbons present. For instanccl, i t has been found that stability, in the same type of fuel, decreases with increasing end point, but it is not known whether this is due to the hydrocarbons or introduction of minor components. Effect on Fuel Properties. The most important consideration is the way in which stability affects the properties of the fuel and their relation to the use of the fuel. Qualified appraisals of the effect of storage and the build-up of gum and insolubles in jet furl on the important properties of the fuel can be made. FREEZISG POINT. Military specifications require that the freezing point, a rather critical property of jet fuels, not exceed - 60' C. The authors' experience has shown that in some cases significant changes in freezing point do occur on aging, but that there is no relation between the quantity of soluble and insoluble gum formed and the change in freezing point. For example, an increase in freezing point of 19" F. has been noted while only 10 mg. per deciliter of total gum was formed, while an increase of 90 mg. per deciliter in gum content without significant increase in freezing point has also been observed. DIELECTRIC CONSTANT.The dielectric constant is important because of the use of electrical gaging instruments that rely on the dielectric properties of the fuel for proper performance.
October 1954
Fuels-
Moderate increases in gum content (up to 45 mg. per deciliter) have failed to produce any significant change in the dielectric constant of the fuel, over a range of frequencies and water content. FILTERABILITY
One of the most important jet fuel properties is the way the fuel behaves in the fuel system. Study of the problem with a simple laboratory constant flow filter test a t ambient and at low temperatures has revealed a number of interesting results, as follows : 1. Soluble gun1 has littlc effect on filterability, either wet or dry, at ambient or a t low temperature. 2. Cooling below the freezing point of a fuel causes a large decrease in filterability, 3. Soluble water, per se, appears to have little effect on filterability. 4. The presence of insolubles in a fuel may decrease filterability, but there is no correlation. in general, between the amount of insolubles present and the effect on filterability. 5. With a fuel with poor filterability characteristics due to the presence of insolubles, even minor amounts of water will markedly increase filtration difficulties a t low temperatures 6. Detergent-type additives improve the filtration chaiacteristics of jet fuels.
The iriflucnce of ice on filterability has caused much confusion. The matter has recently been thoroughly investigated by Finn and Lifson (4))and the important variables defined so that a much better understanding now exists. For example, Figure 3 indicates the rapidity with which water is transferred from the fuel to cold air. Tank geometry, vapor space, vent size, and air temperatures all influence this rate. This relatively rapid loss of water is probably a major factor in the avoidance of filter plugging by ice. The investigators postulate that the normally slow rate of diffusion is accelerated by convective mixing and fuel evaporation. 0010 Jp-4REGULAR
0
F A S T COOL SLOW COOL
DOTTED CURVE FOR FAST COOLING OF FULL CAN (AVIATION GASOLINE)
3 n
0 004
ooozt----' E '3
0000
I
EOUlLlBRiUM WATER CONTENT AT -IO°F. AND iOO% RELATIVE HUMIDITY I I I I I I I 2 3 TIME, HOURS
J 4
Figure 3. Effect of Cooling Kate on Water Loss When a 50% Vapor Space Is above Fuel (4) Fuels saturated a t 7.5' F. and cooled t o - l o 3 F. Burning Quality. Available evidence indicates that the presence of moderate amounts of gum (up to 50 mg. per deciliter) has little effect on the burning qualities of jet fuels, at least in injection-type burners. It is possible that vaporizing-type burners might run into difficulties due to the accumulation of residue in the vaporizing tube. HIGH TEMPERATURE PROBLEM
Certain types of jet aircraft now in operation or projected are equipped with heat exchanger arrangements by means of which the fuel serves as a heat sink to cool lubricating oil accessories and, possibly, air frame members subject to aerodynamic heating. I n these systems the fuel is heated rapidly to temperatures in excess of 250" F. for residence times of as much as several minutes. Under these conditions, some fuels have been found to
1 3 D U S T R I A-L A N D E N G IN E E R 1 N
G C H E MI S T R Y
2169
if desired by adjusting hot oil temperature via thermoswitch IC and by changing the pumping rate with variable speed control J . The hot fuel passes into the filter holder, D , and hence t o the cooler, E, and back-pressure regulat'or, F . The fuel sample is prefiltered t,hrough relatively coarse paper
to eliminate extraneous contamination of the sample by dust,,
WATER COOLER
Figure 4. Jet Fuel Filter Clogging Test Regulated fuel pressure, 50 lb./sq. inch TC (fuel outlet temperature), 300' F. Residence t i m e i n exchanger, 5 sec. Filter rate, 3.2 gal/(sq. inch) (hr.)
produce sufficient insoluble matter to retard flow through fine orifices and filters, giving rise t o concern that the operation of flow control instruments and burner nozzles may be seriously impaired. Aircraft manufacturers are interested in determining the limitations on future jet aircraft designs t h a t would be imposed b y the formation of fuel insolubles a t high temperature or, conversely, the improvements t h a t could be made in current types of fuels to give design engineers greater leeway in this respect. ,-i14 3 3/4
O.D.
-
,/
ia
N.P.T.
>
5
- -
va 1
2 1/4
t 5/a
lint, etc., during handling. All specimens of the 2-micron paper employed in the present tests are checked for uniformity by a simple permeahilit,y measurement, n i t h filtered kerosine. (At a 700 mm. of mercury driving head, the average flow rate over a 6-minute period should be betn-een 100 and 120 cc. per minute.) About 5% rejection of specimens has been encountered. Jl'hen serious filter clogging occurs in t8hetest, frequent checks on fuel flow rate have been found necessary prior t o recording the preesure drop. Correlation with Full Scale System. The bench test was run with five JP-4 type fuels (Military Specification MIL-F5624S)on which evaluations had been made in a full scale mockup of a late fuel sj-stem. One of these samples was a so-called reference fuel, formulated to approximate a maximum production condition and ostensibly containing blending stocks not included in current JP-4 fuels. The other four samples were taken from commercial product,ion from refineries in various parts of the country (but not necessarily representRtive of all fuels of this type currently available). The laboratory test conditions approximated as nearly as possible the full scale conditions except for the unit flow t'hrough the filter; the rate was increased about fourfold t o shorten the test t'o a reasonable length. The Reynolds number in the bench test heat exchanger ]vas only 1000-i.e., streamline flo\T--ahich undoubt.edly increased the relative severit,y of the method. Figure i is a plot, of the pressure-time curves; Table I presents the comparative results ivhich demonstrate that the berich test rates the five fuels in the same order as the full scale procedure. This approximate correlat,ion was considered adequate t,o permit the use of the bench apparatus to study the effect of changes in test conditions and t o compare fuels of various compositions
1 100-MEm
TAP 1/4-20 THD.
SCREEN
27"
Figure 5 - Filter Holder A
T h e following investigation includes the design and operation of a bench scale apparatus, correlation x i t h a full scale mock-up of a late model engine fuel system, and a preliminary study of the relation of fuel characteristics t o performance in the test equipment. Apparatus and Procedure. T h e original conception of an apparatus to measure performance characteristics involved the follon-ing basic components-fuel reservoir, pump, heat exchanger, insolubles-sensitive element, and fuel disposal system. For economy of construction and convenience of laboratory operation, certain modifications of the prototype fuel system were desirable. 1. A constant fuel flow and residence time in the exchanger were arbitrarily chosen in preference t o a cyclic operation. 2. Pressure drop across a standard paper filter was used t o detect t h e formation of fuel insolubles. 3. T h e hot filtered fuel was cooled for possible recirculation. 4. Back pressure was held at 50 pounds per square inch, considerably lower than in actual practice.
Figure 4 is a schematic diagram of the apparatus. Details of the filter holder and heat exchanger are shown in Figures 5 and 6 . Fuel is pumped from the sample reservoir b y means of a constant speed metering pump, B, &-hose output is checked occasionally by direct volumetric measurement. Heat is supplied t o the exchanger, C , by pumping hot oil from an electrically heated reservoir, L, and the outgoing fuel temperature may be varied
2110
n
l 7 7
6 5
3
2
l L 6
Figure 6. Heat Exchanger 1. Galvanized pipe, 3/8 inch 2. Galvanized tee, 3/8 inch 3. Bushing, 3/8 X 1/4 inch Connector, 1/4 inch pipe X 1'4 inch tube a. Fitting bored 4/64 to 17/64 inch (graphite string used to effect seal) 6. Aluminum tubing, 1/4 inch outside diam.
4.
Bench Test Variables. PRECISIOS.il series of four runs was conducted on a stock sample of JP-4 fuel a t 300" F. (fuel outlet temperature) to establish the repeatability of the method. Figure 8 illustrates the spread of test values. The standard deviation of the pressure drop a t 2.5 hours !\-as 0.5 pound per square inch (average value 6.0 pounds per square inch). I n v i e v of the desirability of minimizing RECIRCULATIOK. sample size, a series of runs was made on a relat,ively poor fuel (JP-4fuel E) and a relatively good fuel (JP-4 fuel F ) t o determine whether recirculat,ion of a 1-gallon sample for a standard 2.5-hour period would give the same degree of filter plugging as a oncethrough test. Fuel outlet temperatures of 240°,300", and 350" F. were selected. The results in Table I1 indicate t'hat, in the caw of the poorer fuel, recirculation gives lower pressure values a t the lower temperatures b u t higher at 350" F. than the oncethrough test. I n general, however, i t would appear that t'he use
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 10
-Turbine AND FULLSCALE hfOCK-UP TABLEI. LABORATORY FUELSTABILITY TESTS
JP-4 Fuel
kC
D Reference a
Full Scale Test Time, h r . ,'iA lb./sq. inch 10 0 10 12 10 15 7 200 20a 2
AIRCR.4FT
Fuels-
5 SEC. RESIDENCE TIME 1-GALLON SAMPLZ
Bench Test Timeb, H r .
6t
4.5 3.3 1.3 0.3
Test terminated a t 20 Ib./sq. inch AI>. AP.
b Time t o reach 20 lb./sq. inch
TABLE11. EFFECTOF RECIRCULATION o s FILTERPLUGGING IN BENCH SCAL JET ~ FUELSTABILITY TEST
0.5
O_ R.
:
JP-4 Fuel E
F
240--
300-
Recirculated 17 2 6
One pass 50f 3 0
One pass 37 5 0
Recirculated 19 5 0
-350One pass 26 4 5
30
20
10
4
3
2
1
CIRCULATION TIME, HOURS
Figure 7 . Bench Scale Aircraft Fuel Stability Test
2.5
Figure 8. Bench Scale Aircraft Fuel Stability Test on JP-14 Fuel H
Recirdated 44 5 0
40
2.0
1.5
1.0
TEST D U R A n O S , HOURS
Pressure Drop, Lb./Sq. Inch, after 2.5 Hr. a t Fuel Outlet Temp.,
Four runs
or precipitate in particles too small (or of too unfavorable surface characteristics) to product plugging of the 2-micron filter. It might be expected that certain fuels could PREFILTRATION. contain preformed insoluble materials which pass the rather coarse (10-micron) filter paper used to remove incidental contaminants but are retained by the 2-micron paper employed in the test. Several cold samples were passed through the test equipment, but pressure drops in excess of 1 pound per square inch were not obtained. This substantiated the original supposition that the filter clogging substances formed on heating. At least one of the fuels tested (JP-4 fuel A) gave no plugging a t elevated temperatures but plugged the filter badly when cooled to room temperature after a heating cycle, indicating precipitation or flocculation of particles formed in the exchanger. Nature of Filter Deposits. I n general, the filter deposits obtained in the aircraft turbine fuel stability test bear the superficial appearance and solubility characteristics of the sludge or
of a recirculation procedure would not change the relative ratings of the two fuels. FUELOUTLETTENPERATURE. Fuel outlet temperature has a considerable effect on pressure, and the nature of the effect depends on whether a one-pass or recirculation procedure is employed (Table 11). This suggests that more than one process takes place-for example, thermal condensation and polymerization reactions, oxidation of trace constituents by dissolved air, and changes in size or surface characteristics of particles, Figure 9 shows the effect of fuel outlet temperature on pressure a t 2.5 hours for several JP-4 type fuels in the recirculation test. While temperature has little influence on results obtained with fuels F and H, the performance of fuel E becomes worse as temperature increases while fuel G produces the most plugging a t 300' F. I n the latter case it seems reasonable to conclude that either the reaction products become more soluble above 300" F.
a50
300
350
F U E L OUTLET TEMPERATURE.
'r
Figure 9. Effect of Fuel Outlet Temperature in Jet Fuel Stability Test
TABLE 111. PROPERTIES AKD ANALYSES OF AIRCRAFTTURBINE FUELS
-
ASThI Distn.,
Fuel
Initial b.p.
50%
149 140 138 128 153 128 144 138 146
372 289 376 128 278 286 278 294 345
A J I D
' F. E n d point
4ir Jet Gum a t 400' F - hlg./lOO Mi'. Original Ageda
470 3 6 0.19 502 6 21 0.22 431 7 13 ... 480 1 4 0.06 B 504 2 7 0.12 G 476 1 4 0.12 H 498 1 5 ... C 509 2 4 0.08 Reference 517 3 9 0.28 0 16 hr. at 100' C., 100 Ib./sq. inch oxygen. b Determined by silica gel adsorption.
October 1954
Sulfur
Content, W t . % Total RSH
Satd.
