DYNAMIC EVALUATION OF HIGH TEMPERATURE HYDRAULIC FLUIDS V ERN H0
R
. J.
P K I N S , Midwest Research Institute, Kanias City 10, Ma.
B E N Z I N G , WriEht-Patterson Air Force Base, Ohio
A simulated hydraulic circuit has been developed to evaluate potential high temperature hydraulic fluids to
700' F.
Information on fluid degradation, corrosion, lacquering, and sludging tendencies can b e obtained from long-term tests in this circuit. Results are given for six fluids pumped a t 3000 p.s.i.g., and 550" F. for 100 hours in this circuit: 50-50blend of phenyl methyl silicone and an ester of trimethylolpropane; diphenyldi-n-dodecylsilane; chlorinated phenyl methyl silicone; bis(phenoxyphenoxy)benzene; deep dewaxed mineral oil with TCP, PANA, and DC 200; and phenyl methyl silicone. All fluids except the siliconeester blend warrant testing in mock-ups of hydraulic circuits for advanced flight vehicle systems.
rapid research and development progress in the hydraulic fluid and hydraulic systems areas for aircraft and aerospace systems during the last 15 years has greatly complicated the evaluation process for new materials. Through World War 11, hydraulic systems in military aircraft were mainly limited to about 165' F. maximum temperatures with such fluids as the petroleum-based type materials. Since that time, hydraulic systems have progressed to 400' F. top temperature in operating vehicles using the disiloxane-type fluids. Contemplated temperatures for systems now being designed and constructed go as high as 1000" F. .4bout 5 years ago, the .4ir Force initiated a program to develop a dynamic evaluation procedure for the study of materials in a circulating system which could carry fluid evaluation beyond the normal physical and chemical tests of such properties as viscosity. thermal stability, and corrosion. In the past, this dynamic type evaluation had mainly been performed using actual aircraft pumps already available for systems use. However, the advancing temperatures caused development problems for the hydraulic pump industry as well as the fluid manufacturers and there was no one aircraft pump suitable for the temperature range desired. This new dynamic test was then to fit between the chemical bench tests and the system mock-up programs, and provide information valuable to each area. From the results of this dynamic evaluation, fluids may be selected for further evaluation in aircraft and aerospace system mock-up circuits. These results are also useful in the development of new fluids and the further development of evaluated fluids. The fluids were selected for the dynamic evaluation work described primarily on the basis of their thermal stability. These fluids are a t various stages of development and include both base stock and formulated fluids. This paper briefly outlines the test method developed and discusses in detail the results of tests on various experimental fluids a t 550" F. A more complete description of the equipment has been published ( 3 ) . HE
Test Equipment
This dynamic test system is referred to as the pump stand. Its purpose is to evaluate fluids subjected to high shear rates a t pressures up to 3000 p.s.i.g. and at temperatures up to 700' F. This stand simulates a high temperature hydraulic fluid circuit. Its basic function is to obtain the following information on the fluid being evaluated: fluid degradation; corrosion tendency of the fluid in relation to materials used in the construction of advanced flight vehicles; lacquering tendency, as it may affect close-fitting parts such as servo valves; and sludging tendency as a function of time. T h e stand consists of a pump, hydraulic circuit, oven, and associated instrumentation. The schematic of the pump. oven, and fluid circuit is shoivn in Figure 1.
NITROGEN SUPPLY
GLASS
@ @
INDICATOR THERMOCOUPLE PRESSURE TAP
Figure 1.
ENCLOSURE
EXCHANGER
Schematic of pump stand fluid circuit VOL. 2
NO. 1
MARCH 1963
71
WATER COOLED
24
- S E A L L E A K A G E PORT
Figure 2.
Seal housing assembly, pump stand
'The fluid in this hydraulic circuit is pumped by a threepiston pump, hlanton Gaulin ,Model 500 HP-KL6-3PA. modified to operate a t high temperatures. Unlike conventional hydraulic pumps. the pistons d o not mate with other metal surfaces. Since wear characteristics were not a n objective of the program. this feature is highly desirable. Figure 2 shows that the piston mates with a Graphitar sleeve-type primary seal and a secondary seal consisting of six Teflon 1. rings, located in a water-cooled section of the seal housing. T h e space betxveen the primary and secondary seal is vented to the inlet of the pump, so that the 1. rings d o not have to seal against a high pressure. The pump is equipped with a mechanical relief valve xvhich connects the discharge line directly to the inlet of the pump in the event of a n overpressure condition. Fluid at 3000 p.s.i.g. is delivered to the lacquer indicator and the throttling valve. -4 small quantity of fluid passes through the lacquer indicator to the low pressure side of the circuit. T h e function of the indicator is to obtain information on the tendency of a fluid to form lacquer deposits which would inhibit free motion of close-fitting parts such as those found in a servo valve. I t consists of a 0.75-inch-diameter piston and cylinder having a 0,0002-to 0.0004-inch diametrical clearance. The piston has three lands: 0.312, 0.50. and 0.625
Table 1.
inch long. separated by 0.50-inch-long relieved sections. .4 static piston is used in this unit because it furnishes a more sensitive indication of lacquer formation than could be obtained from a moving piston. This indicator is made of 440C stainless steel; however? a lacquer indicator of 31 6 stainless steel was used in the first three tests described here. T h e piston has been hardened to Kockwell C 50 and the cylinder to C 40. Most of the fluid delivered by the pump passes through a n air-operated spline plug-type throttling valve, Hammel-Dah1 Part S o . 7505 ES. The pressure across the valve drops from 3000 p.s.i.g. to between 20 and 500 p.s.i.g. T h e major portion of this 20 to 500 p.s.i.g. may be attributed to the pressure drop across the filter and the reservoir pressure. .4fter leaving the throttling valve the fluid passes through an orifice-type flo\v transducer, past a normally closed bypass line connected to the reservoir and a dead-end line section which contains the corrosion specimens, and then goes to the filter. Circular washer-type corrosion specimens indicate the corrosion tendencies of the fluid on various materials of construction. These specimens are positioned in a dead-end section of the circuit. which is felt, with the long residence time of the fluid. to be the most severe on both the specimens and fluid. A 33-micron filter furnishes a means of checking the tendency of a fluid to form sludge. This Air Maze filter contains 12 disks with 5.53 sq. inches of filter area per disk. T h e fluid floivs from the filter back to the pump through a single-tube heat exchanger. A fluid reservoir is attached to the circuit just ahead of the pump to supply make-up fluid. T h e fluid in the reservoir is not a part of the circulating fluid. The reservoir can be pressurized to keep the entire circuit at a positive pressure. T h e oven, which is capable of operating at controlled temperatures to 800' F.. encloses most of the hydraulic circuit. The pump block and the remainder of the circuit. which carries hot fluid, is insulated. T h e oven is equipped with a C o t inerting system to prevent fires which could result from spontaneous ignition of an)- fluid escaping inside the oven. Instrumentation measures and records fluid flow, oven air temperature, fluid temperatures on both sides of the throttling valve. pump discharge pressure. and pressures before and after the filter. Test Method
T h e procedure consists of a cleaning cycle and a n evaluation cycle. At certain times during the evaluation cycle. fluid sam-
Summary of 550' F. Pump Stand Tests
Fluid -Vurnbers '1ILO
.MLO
MLO
MLO
56-843.
59-692'
60-29$
5 22 44 20.900
5.1 44 20,000
2.4 27 9,700
TrJt Data
iQ-9K'
MLO 57-6376
Av. pumping rate. 5.p m. yo initial charge at test end Total shear cycles
4 19 45 16.800
3 71 54 14,800
9 80
40
74
140
165
Filtar A P . p.s.i. Initial Final Neut. No., mg. KOH!g. N e w fluid Final sample Flash point, F. New fluid Final sample Fire point. F. New fluid Final sample Viscosity. cs.. at 100' F. New fluid Final sample Viscosity, cs.. at 210' F New fluid Final sample Lacquer indicator force, lb.
0.12 3.25
0.13 0.20
0.08h 0.15
4 122 0.06 0.06
Q F-258f 3.9 42 15.500
0.06 0.06
I&EC
-, in accordance with Federal Testing Method 5308.4 (1).The corrosion specimens are weighed and then the lacquer indicator and corrosion specimens are installed. Te\v filter disks are installed in the filter housing. Finally. the circuit is charged Ivith neir fluid for the next experiment. During rhe 100-hour run a t the test temperature and 3000 p.s.i.g.. the fluid normally passes through the circuit from 10.000 to 20,000 times. Various experimental conditions are monitored. including pump discharge pressure, pressure before and after the filter: oven temperature, fluid temperarures before and after the throttling valve. and fluid flow rate. '1-0 obtain information on the fluid performance, the following fluid properties are determined on 100-ml. samples with2-. 4-: 6-. 8->lo-, 2 5 , 50-, 75-, dra\vn from the circuit after 0-> and 100-hour operation a t test conditions: flash and fire points (Cleveland open cup), viscosity at 100' and 210' F., the formation of insolubles, and neutralization number. The flash and fire points. viscosities, and neutralization numbers are determined in accordance with ASTM specification numbers ( I ) D 92-57, D 445-61, and D 664-58, respectively. Ai parr of the last four fluid samples taken during a test are checked for insolubles by centrifuging for 30 minutes ar 2.500 r.p.m. on a 20-cm. mean radius arm. Flash and fire points are determined first, so that any light fractions produced as a result of this high remperature test \rill not be lost through handliiig the fluid sample. Infrared spectral traces are run on new fluid and the last fluid sample to see if a molecular change can be detected. The pressure drop across the filter provides an indicarion of sludge formation and is observed as the test progressrs. After the 100-hour run has been completed, the force required to move the lacquer indicator piston is measured and i t is inspected for lacquer deposits. The corrosiveness of a test fluid toward various materials is determined by comparing the weights of the corrosion specimens before and after each test. These materials are: aluminum (2024-T4), tool steel ( M l ) , chrome-molybdenum steel (4140)> stainless steels (302) and (440), titanium (RC 130 B), and beryllium copper (QQ-(2-530).
't
ob
20
60
40
80
I
I
7I
,-PUMPING
6
00 I 8
20 i
40
RATE
60
z
80 o
100 F 160
I
6t -PUMPING RATE
60
r P R E S S U R E DROP
Test Results
Results for a series of tests performed a t 550" F. are presented in Table I. Test data on the fluids are shown in Table 11, while corrosion data for all runs are presented in Table 111. Table 11. compares oxidation corrosion data from bench and pump tests. O n several occasions, tests were interrupted for varying lengths of time. On resuming the test, the pumping rate (Figure 3) had usually increased a small amount. Hoivever: after the last test interruption shown in Figure 3, A ? the pumping rate decreased, probably because of increased internal
a
/ ACROSS F I L T E R
I
0
40 3
v, OI
ob
20
40
60
80
P U M P I N G TIME ( H R S )
Figure 3. Effect of pumping time a t 550" F. on pumping rate a n d pressure d r o p across filter MLO 5 9 - 9 8 MLO 56-843 C. MLO 60-294 D. QF-258
A.
B.
VOL. 2
NO. 1
MARCH 1963
73
~
Table II. Test Fluid Data 5 5 0 0 F., 3000 p.s.i.g., 100 hours
.WLO 5948
Visc.? cs., 1 0 0 / 2 1 0 ° F. 62.13/13.52 50.83/11.61 49,3711 1 . 4 4 48.22/11.28 49.26/11.48 48.42/11.44 50.31 /11 . 6 7 52.30/12.24 53. 76/11 . 7 7 33.38/ . . . 34.30/8.42
Hour New 0 2 4 6 8 10 25 50 75 100
Flasho/'re p t . , F. 515/550
Visc., cs., 100/210° F. 37.24/6.06 36.81/6.02 35.94/6.01 35.88/6.00 35.94/6.00 35.86/6.01 35.96/6.09 36.53/6.23 36.77/6.07 36.82/6.11 36.61/6.08
KOH/g. 0.12
. . .
540/550 515/555 515/545 495/540 495/550 500/545 450/505 390/440 300/430
0:435 0.62 0.69 0.77 0.82 1.56 2.36 2.75 3.25
MLO 59-692
New
360.5/12.93 343.9/12.96 349 9/12 96 348 7/12 96 353 4/12 93 347.5/12.96 349.9/13,03 347.5/13.03
0 2 4 6 8 10 25 25a 30 50 50a 75 100 a
iWL0 56-843
MLO 57-637 Neut. No mg.
,Veut. N o . mg. KOHIg. 0.14 0.13 0.15 0.15 0.18 0.18 0.18 0.18 0.20 0.20 0.20
Flast/fre p t . , F. 495/575 515/565 520/570 515/560 515/560 515/555 510/550 515)560 5151550 515/560 520/560
Visc., cs.. 7 0 0 / 2 1 0 a F. 58.16/18.62 48.22/15.43 46.23/14.70 45.21 /14, 50 45.91/14.73 45.81/14.64 47.48/15 09 48 01/15 19 47 59/15 00 46 86/14 80 48 01/15 29
MLO 6 0 - 2 9 4
598/681 576/677 606/670 595/679 591/671 578/674 573/671 578/675
, . .
...
0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 ...
355.8ji3.13
56ijin
341 . 6 j i 3 . 0 3 341,6112.93
563j675 581/677
0,06 ... 0.06 0.06
, . .
.kfeasurement made nearly 2 years later than run.
b
14 0 8 / 3 . 2 1 14 05/3 15 13 75/3 12 13 65/3 12 13 69/3 12 1 3 , 79/3.12 13 . 7 9 / 3 . 1 3 13.69/3.12 13.72/3.12b 1 3 88/3 12 13 79/3 12
388/435 403/435 415/436 380/422 386/424 393/424 400/421 398j423 412/427b 384/424 376/419
13.88j3.13 13.79/3.12
370/412 381/412
A'eut. ~VO., nig. KOH/g. 0.08 0.08 0.08 0.08 0.08 0 08 0 08 0 08 0 15 0.15 0.15
p t . , F. Flasi/Jire 306a/756 550/725 4601720 505/740 485/710 470/705 520/725 500/700 520/715 505/705 510/710 QF-258
0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 ... 0.06 0.06
59.10/20.07 53.66/18.62 53.71/18.62 54,50/18.79 54.29/18.69 54.91/18.88 55,121'18.98 54.50118 . 5 2
628/779 613/775 612/773 6041775 609 1769 6121775 614/778 5961 775
< O . 05
605'/769 606/773h 605/768 609/771
J
/
20/
VISCOSITY AT 210'F
- I O 2
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I-
W 3
\ / IO - I
z
I
a W
300I-
200 *
20
Figure 5.
,
40 60 PUMPING TIME ( H R S )
100
80
Effect of pumping time at
550" F.
MLO 59-98 A. 6.
Effect on viscosity and neutralization number Effect on flash point and fire point
70,
6o
I
I
3.0 ? Y
50 i\VISCOSITY AT IOOOF
-2 4 0 05
2ol, 1
-s
- 2.0 s'
I
z
I E
5
FLUID , VISCOSITY AT 2 1 0 ° F
IO
NEUT NO
IO
a
W 3 I -
z
_ _ - - - - - _ _ _ _ _ -_ -------- _, - - _ - - 0 ,
~
OO
2
20
40
60
I00
80
I
700,
B
FIRE PT.
500 F L A S H PT.
W
300 I-
zool 100
20
0;
Figure 6. A. B.
40 60 PUMPING T I M E (HRS)
80
J
100
Effect of pumping time a t 550" F. MLO 57-637
Effect on viscosity and neutralization number Effect on Rash point and fire poin'
VOL. 2
NO. 1
M A R C H 1963
75
' O F N E w
FLUID,,,,rVISCOSITI
AT l O O * F
2.0 0
z
2
NEW FLUID VISCOSITY AT 21OoF
20
t
IO -
2
___________L_ - - - - ----- -------__ NEUT. NO.
W
z
800 40-
-
lool
2001
ob
m
t
20
Figure A. 8.
C.
A
N E W FLUID
60,
9.
40 60 PUMPING T I M E ( H R S )
80
Effect of pumping time a t MLO 59-692
550"
J
$ 30-
100
II
>
N E W FLUID
\VISCOSITY
AT 210°F
201-
~
F. IO -
Effect on pumping rate and pressure drop across filter Effect on viscosity and neutralization number Effect on flash point and fire point
shearing was detected by the infrared spectral trace method. 'Table I1 and Figure 10 shoiv that the neutralization number is l o ~ vand constant and that the viscosity, flash point, and fire point remain nearly constant with pumping time. ._ Table I11 shows that all corrosion specimens lost weight during this test. The largest loss was experienced by aluminum and the least b>- Type 302 stainless steel. Neither changed color. The beryllium copper specimen was very dark and slightly pitted. S o other specimens changed in texture. 'l'he 14-1 tool steel. the 4140 chrome-molybdenum steel, the 440 stainless steel. and the titanium were slightly darkened. Test 6, QF-258, phenyl methyl silicone. Figure 3: D ,shows that the pressure drop across the filter was 8 p.s.i. a t the start of the run and increased uniformly to 11 p.s.i. during the first 50 hours. The tcst was halted for one day for circuit repairs. \Vhen it \vas resumed. the pressure drop across the filter was 20 p.s.i. and increased fairly uniformly to 86 p.s.i. during the last
OO 900
20
800;&EW
FLU'D
E- 700-/NEW
40
FLUID
80
60
100
B FIRE P T
6 0 d h
t E
w a I
FLASH P T
$2 200-
IOOI
Figure 11.
Effect of pumping time a t
550" F.
QF-258 A. 8.
Effect on viscosity Effect on flash point and fire point
VOL. 2
NO. 1
MARCH 1963
77
Table IV.
Oxidative and Corrosive Tendencies in Pump and Bench Test Fluid ' u m b e r s .ML 0 .MLO .MLO .MLO 57-637 56-843 59-692 60-294 ~~
&MLO 59-98
QF-253
Viscosity, Cs., 100/210° F. 0 - C test" Initial Final Pump test Initial Final
61,6113.6 14,9/5,0
37.316.1 38.8/6.3
56.3/18.3 64.1119.7
361.2113.1 376,0113.4
13.813.1 14.613.2
39.4/20.9 59,?/20.3
62.1/13.5 34.3/8.4
31.2/6.0 36.6/6,1
58,1/18.6 48.0/15.3
360,5!12,9 341,642.9
14,1/3.2 13.8/3.1
59 1 20 1 55.3 18.8
Neutralization No., Mg. KOH/ Gram 0 - C testa Initial Final Pump test Initial Final
0.05 0.04
0.13 0.16
0.10 0.15
0.01 0.03
0.0: 0.04
0.02 0 02
0.12 3.25
0.13 0.20
0.08 0.15
0.06 0.06
0.06 0.06
0.05 0.05
Fluid Appearance Dark, no Dark, no sludge sludge Dark brown Slight darkening Pump test Brown a Bench oxidation-corrosion test f o r 700 hr., n . 2 nlmosfihere. 550' F., Mil-L-9236B method. 0 - C testa
Dark, no deposits Dark yellow
Clear
50 hours of this 100-hour test. S o sludge was found in the filter housing, but a very thin compact layer of dark residue was found on each filter disk. The 31 pounds required to move the lacquer indicator piston in its cylinder indicates that this fluid may have a tendency to form lacquer deposits when pumped for a long time at 550" F. However, in a later test a t 700' F., no tendency toward lacquer formation was evident; two small spots on the piston were the only residue found in the lacquer indicator. No insolubles were detected by centrifuging the last four fluid samples. New fluid was colorless; the 0-hour sample was slightly yellow but no further color change was evident in the succeeding samples. No molecular change was detected by the infrared spectral trace method. Table I1 and Figure 11 show that viscosity, flash point, and fire point all drop a little during warm-up, then remain fairly constant throughout the 100-hour test. The small increase in these properties a t the 50-hour point is thought to be the result of a small addition of new fluid which was fed to the circuit when the run was halted for one day. The neutralization number for new fluid and fluid samples was basic and less than 0.05 mg. of KOH per gram. All corrosion specimens lost weight during this test, although none had a change in surface texture (Table 111). The M-I tool steel lost the most weight and t h e titanium RC 130 B the least. Only the beryllium copper specimen had a color change (darkened slightly). Conclusions
A system (pump stand) has been developed in which the performance of potential high temperature hydraulic fluids can be determined a t temperatures to 700" F. It furnishes a
78
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
Dark
Clear
Translucent
Slight yellow
means of selecting the more promising fluids for further testing in a mock-up for an advanced flight vehicle s p m wirh actual system components. All of the six fluids except MLO 59-98 warrant testing a t temperatures to 550' F. in mock-ups for advanced flight systems. I t would also be desirable to determine their performance in the pump stand at higher temperatures, perhaps 700°F. The results of the pump stand tests along with lubricity data from other tests could be used to set up a preferred order of testing in a system mock-up. The agreement between the bench oxidation corrosion-type thermal test and the pump test appears to be very good, as shown in Table IV. This is particularly true when one takes into consideration mechanical shear of the polymeric materials. Only M L O 59-98 does not line up well. Acknowledgment
The authors are indebted to members of Materials Central, Aeronautical Systems Division, for their encouragement and assistance. literature Cited (1) Am. SOC.Testing Materials, Philadelphia, Pa., ASTM Standards 1961, Part 7, October 1961. (2) Federal Test Method Standard 791, December 15. 1955, Method 5308.4, "Corrosiveness and Oxidation Stability of Light
Oils." (3) Hopkins. Vernice, St. John, A. D., Wilson. Donnell, '.Lubrica-
tion Behavior and Chemical Degradation Characteristics of Experimental High Temperature Fluids and Lubricants," Wright Air Development Division, WADD TR 60-855, Part I1 (January 1962). RECEIVED for review September 7 , 1962 ACCEPTED December 26. 1962 Work supported by the Air Research and Development Command USAF, under Contracts AF 33(616)-5202 and AF 33(616)-6854
