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Page 1: Fire resistant hydraulic fluids

Fire Safety Journal, 5 (1983) 115 - 122 115

Fire Resistant Hydraulic Fluids

J. B. R O M A N S

Hughes Associates, Inc., Kensington, Maryland (U.S.A.)

R. C. LITTLE

Dept. of the Navy, Naval Research Laboratory, Washington, DC 20375 (U.S.A.)

(Received in revised form September 10, 1982)

SUMMARY

In an attempt to select an appropriate fire-resistant hydraulic fluid for use in Naval applications, two approaches were used: (1) development o f a shear-resistan t, drag-reducing colloid dispersion to determine if it had anti- misting properties, and (2) use o f a water-in- oil emulsion loaded to high levels o f water. While the dispersion o f lithium phenylstearate colloid had demonstrated high drag reducing activity in previous work and would therefore presumably exhibit anti-misting characteristics, it failed to produce the desired fire-suppressive effect. The water-in-oil emulsion, on the other hand, showed promising fire-resistant prop- erties in a flammability testing device.

1. INTRODUCTION

It is common knowledge that many mate- rials not dangerously flammable in bulk may be ignited explosively if they are finely divided. If the material is in liquid form, the situation becomes more critical, for fine mist can readily form if liquid under high pressure is suddenly released into the atmosphere through a small orifice or crack. Such a state may develop in hydraulic fluid systems because of suddenly stressed components aboard ships or aircraft brought on by exposure to combat, accident, vibration or other causes. If the hydraulic fluid is an oil, the presence of any source of ignition may result in an explosion. It is well known that MIL-H-5606 hydraulic fluids are hazardous in this respect and even the less flammable MIL-H-83282 fluids may possibly be ignited if they are released into the atmo- sphere in the atomized state.

The relationship between the drag reducing properties of jet aircraft fuel containing certain polymers and misting characteristics has been established by Hoy t et al. [ 1 ]. Polyisobutylene, a drag reducing agent, has also been used successfully in the laboratory as an anti-misting fire suppressive agent for both JP-5 fuel and a marine diesel fuel [2]. It has also been estab- lished [3] that it is possible to reduce the extent of the formation of a "fire ball" by a burning jet aircraft fuel by dissolving certain high molecular weight chemical polymers in the fuel. Earlier work at this Laboratory [4] indicated that polyisobutylene also had a fire- suppressive effect on 2190-TEP hydraulic oil. The polymer suppresses the formation of mists by increasing the size of the fuel drop- lets. However, the chemical polymers are very sensitive to shearing forces and are degraded by the action of pumps, screens, valves, etc. (such as are found in aircraft hydraulic and fuel systems) [ 5]. This is not a serious problem when the chemical polymers are used in jet aircraft fuels, for the anti-misting character- istics must be nullified before the fuel can be utilized by the engines [6]. Thus, these type polymers are acceptable for single pass systems, such as an aircraft fuel system, but not for recirculating type hydraulic systems.

Earlier reports [5, 7] have shown that "association" colloid type materials such as the organo-metallic oil soluble polar com- pounds used in drag reduction studies are relatively shear resistant, especially when compared with the chemical polymer poly- isobutylene [5]. It has been suggested [5, 7] that the shear resistance of these polar mate- rials arises from the ability of the molecules to restructure themselves through associative processes. Work with the May aerosol gener- ator [8] has demonstrated that the association

0379-7112/83/$3.00 © Elsevier Sequoia/Printed in The Netherlands

Page 2: Fire resistant hydraulic fluids

116

colloids increase the average drop size for oils containing them. Although this behavior is considered critical in the development of anti-misting, fire-resistant fluids, final evalua- tion of the organo-metallic compounds rests on their behavior in flammability tests. Such tests of two oils of widely differing viscosities and containing lithium phenylstearate have been included in this study.

A number of non-hydrocarbon materials have been developed as fire resistant fluids [9]. These include an organophosphate, silicate and phosphate esters, silicones and halocarbons, especially those containing fluor- ine. Many of these fluids will require modifica- tion of the systems in which they would be used in order to assure compatibil i ty with the elastomers and metals involved. The problem of toxicity, particularly with the phosphates and the halocarbons and their reaction pro- ducts, is of concern.

Water-containing materials, as emulsions or solutions, have been developed as fire-resistant or "nonf lammable" fluids. Experiences with fire in the hydraulic systems of military equip- ment during and after World War II led to the development of the "Hydro lubes" [10]. These were based on ethylene glycol-water solutions containing additives for viscosity index improvement, wear prevention, and corrosion inhibition. The Hydrolubes found limited applications, in part because of pro- blems arising from wear prevention and corrosion inhibition, particularly with certain metals.

Experience with the glycol-water hydraulic fluids suggests that the absence of an oil phase in the fluids seriously limited their ability to compete with petroleum oils. However, it had been determined [10] that at least 40% by volume of water is required to prevent the propagation of flames in liquid spray in the atmosphere. This suggests that a stable, pump- able emulsion of water and oil might provide both the fire resistance desired and the lubri- cating qualities of petroleum-type hydraulic oils. Work in this field does no t appear to be very extensive. However, recent developments [ 11] in "nonf lammable" water-in-oil emul- sions have shown promise for use in hydraulic systems of mining machinery. The flamma- bility of one of these fluids containing 41% water has been investigated with promising results.

2. EXPERIMENTAL

Materials The fluids studied were petroleum type

2190-TEP hydraulic oil (MIL-L-17331F Ships 1973); experimental fluids consisting of petro- leum type aircraft turbine engine oil designated 1005 (MIL-O-6081B (ASG) and AM.4) con- taining lithium phenylstearate as a thickening agent in nominal concentrations of 0.25~, 0.5% and 1% compounded by a method used previously [ 5] ; a fluid made from equal parts of 2190-TEP hydraulic oil and 1005 oil con- taining 0.5% lithium phenylstearate; and a water-in-oil emulsion, Mobil NRL 1262. supplied by Mobil Research and Development Corp, Paulsboro, N.J. The significant compara- ble properties of 2190-TEP hydraulic oil, 1005 aircraft oil, and Mobil XRL 1262 hydraulic fluid are given in Table 1. The chemical and physical properties of the fluids containing lithium phenylstearate were not determined pending indication of the usefulness of the stearate as an anti-misting agent.

Apparatus A spinning disk atomizer of the type used

by Mannheimer [12] as a model to correlate results of laboratory studies of the misting flammability of jet aircraft fuels, particularly with those of large-scale fuel spillage tests, was selected for evaluating the mist flamma- bility of the hydraulic fluids. Because of the higher viscosities of hydraulic fluids as com- pared with jet aircraft fuels, it: was necessary to operate the disk at higher-rotational speeds and to enlarge the holes in the disk through which the fluids were dispersed. The disk was made of brass, 4 1/4 in. in diameter and 1/2 in. thick. It contained a cavity in the center, 1 in. dia. and 1/4 in. deep. Four radial holes, 0.149 in. dia. and spaced 90 ° apart, were drilled from the rim of the disk into the cavity. In operation, fluid delivered to the cavity while the disk was spinning was dis- pensed by centrifugal force through the holes.

The disk was mounted on the vertically oriented shaft of a drive motor, as shown in the schematic diagram in Fig. 1. Either a GAST air motor, (10 000 rpm, max.} or an AMETEK electric motor (22 000 rpm, max.) was used. The spinning disk atomizer with the electric motor drive is shown in Fig. 2. The speed of the spinning disk was measured by a

Page 3: Fire resistant hydraulic fluids

TABLE 1

Comparison of properties of fluids studied

117

Properties 2190-TEP Oil (a) 1005 Oil (b) Fluid (c)

Flash point 400 °F, min. 1 225 °F, min. 2 -- Viscosity 82- 110 cs at 37.8 °C (100 °F) 3 5 cs, min. at 37.8 °C (100 °F)4 110 cs at 40 °C Pour point --6.7 °C, max. (20 °F) s -- --35 °C Neutralization 0.30, max. 6 0.100, max. 7 pH 8.1 No. or pH Copper corrosion Slight tarnish permitted 8 Slight brown stain permitted 9 pass 1°

1. ASTM Method D 92. 2. Federal VV-L-791, Method 1103.5. 3. ASTM Method, D 445. 4. Federal VV-L-791, Method 305.2. 5. ASTM Method D 97. 6. ASTM Method D 974. 7. Federal VV-L-791, Method 5105.3. 8. ASTM Method, D 130. 9. Federal VV-L-791, Method 5303.3.

10. ASTM Method D 130-6.

(a) MILI-L-17331F (Ships) 1973 requirements. (b) MIL-O-6081B (ASG) 1953 requirements. (c) Data supplied by manufacturer.

_

~ C GENERATOR

~ ~ 1 : : SCREW DRIVE GEARMOTOR

'SPEED' I REDUCER

FLUID SUPPLY LINE ~7 REI-~YS

TACHOMETER DOWNI ° I READOUT

PROPANE SUPPLY

OPTICAL TACHOMETER PICKUP ~ SPINNING DISK......~ // PROPANEBuRNER

Fig. 1. Schematic diagram of NRL flammability apparatus.

P I O N E E R DT-36 photoe lec t r ic t achometer . The source o f ignit ion was a p ropane burner loca ted 8 in. f rom the disk and at the same level. A camera was used to record the flam- mabi l i ty characterist ics o f the fluids. The hydraul ic fluid was delivered to the spinning disk by means o f a low shear, positive displace- m e n t p u m p or syringe consist ing o f a vertically m o u n t e d cyl inder con ta in ing a p is ton driven at a slow, variable rate. A schemat ic o f the a r rangement o f the syringe and accessories is inc luded in Fig. 1.

P r o c e d u re

The hydraul ic fluids were delivered to the spinning disk at e i ther 400 ml /min or 850 ml/ min. As the fluid en tered the cavity in the disk, the disk speed of 10 000 rpm was reduced (12 - 13% at 400 ml /min delivery and 21 - 23% at the 850 ml /min level) because o f the cen- trifugal force required to disperse the fluid. Most o f the tests were made with no fur ther ad jus tment o f disk speed. However , it had been f o u n d tha t h igher disk speeds tended to increase f lammabi l i ty o f fluids, p resumably

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118

Fig. 2. The NRL spinning disk atomizer.

because of the format ion of smaller drops at higher speeds. This was observed earlier [4] with the May aerosol generator. Therefore, in order to increase the severity of the test, the flammabilities of both the 2190-TEP hydraulic oil and the Mobil XRL 1262 fluid were also determined while the speed of the disk was maintained at or near 10 000 rpm.

Any propagation of yel low flame from the blue propane flame was considered evidence of ignition of the fluid under test. An arc of flame was established which sometimes com- pletely encircled the spinning disk. The degree of encirclement at a given disk speed and fluid delivery rate was a qualitative indication of the relative flammabili ty of the fluid. The flame could be extinguished by stopping the syringe drive motor . If there was no propaga- tion of flame from the propane flame, the fluid under test was considered to be fire re- sistant.

3. RESULTS AND DISCUSSION

The results of the flammability studies are summarized in Table 2.

The flammability of 2190-TEP hydraulic oil in mist form was readily demonstra ted in the flammability apparatus. At a fluid delivery rate of 400 ml/min, and an initial disk speed of 8900 rpm with the air mo to r drive, ignition occurred and a 90 ° - 120 ° arc of flame was es- tablished. When the fluid delivery rate was increased to 850 ml/min, the arc of flame in- creased to about 180 ° . However, if the disk speed was maintained at a nominal 10 000 rpm during the run, the flame completely en- circled the spinning disk (Fig. 3).

The 1005 oil containing 1% lithium phenyl- stearate was too viscous to be uniformly dis- persed by the spinning disk. However, any material reaching the propane flame burned sporadically. Reducing the lithium phenyl- stearate content to 0.5% and 0.25% resulted in mixtures that were well dispersed by the spinning disk, but they burned in the flamma- bility apparatus much like a neat JP-5 jet fuel. In order to determine the effectiveness of the phenylstearate polymer in the 2190-TEP oil and to increase the viscosity of the 1005 oil solutions, a mixture consisting of equal vol- umes of 0.5% lithium phenylstearate in 1005 oil and 2190-TEP hydraulic oil was prepared.

Page 5: Fire resistant hydraulic fluids

TABLE 2

Flammability of fluids studied

119

Fluid Fluid delivery Disk speed rate (ml/min) (10 000 rpm + 22%

400 850 Initial Maintained

Ignition Circular flame projection

2190-TEP x x (8900) Yes x x Yes x x Yes

1% lithium phenylstearate x x Yes in 1005 oil

0.25% lithium phenylstearate x x Yes in 1005 oil x x Yes

0.5% lithium phenylstearate x x Yes in 1005 oil

0.5% lithium phenylstearate x x Yes in 1005 oil + 2190-TEP

Mobil XRL 1262 x x No x x No

90 ° - 120 ° 180 ° 360 °

120 °

360 ° 360 °

360 °

360 °

m

Fig. 3. Typical behavior of 2190-TEP hydraulic oil in the flammability apparatus.

Us ing t h e s a m e o p e r a t i n g c o n d i t i o n s as b e f o r e ,

t h e c o m p o s i t e m i x t u r e i g n i t e d r ead i l y in t h e

f l a m m a b i l i t y a p p a r a t u s a n d f o r m e d an i n t e n s e

f l a m e w h i c h c o m p l e t e l y e n c i r c l e d t h e s p i n n i n g disk.

T h e f l a m m a b i l i t y a p p a r a t u s f lu id d e l i v e r y

s y s t e m was p u r g e d o f t h e p r e v i o u s l y u s e d

m a t e r i a l a n d r e f i l l ed w i t h t he w a t e r - i n - o i l

e m u l s i o n , M o b i l X R L 1 2 6 2 . T h e f l a m m a -

b i l i t y t e s t s w e r e c o n d u c t e d u n d e r t h e c o n d i -

Page 6: Fire resistant hydraulic fluids

120

Fig. 4. Mobil XRL 1262 fluids fails to ignite in the flammability apparatus.

tions found to be conducive to the ignition and flame propagation of 2190-TEP oil by maintaining a nominal 10 000 rpm disk speed during delivery of the fluid. Some luminous scintillation effect was observed in the propane flame, but no ignition occurred during repeated trials at a fluid delivery rate of 400 ml/min. Neither did ignition occur when the delivery rate was increased to 850 ml/min, but as can be seen in Fig. 4, the flammabili ty apparatus was engulfed in a dense fog of a tomized emul- sion. Such a condit ion with a flammable fluid would be expected to enhance the chances of ignition, but the Mobil XRL 1262 did no t burn.

The vulnerability of 2190-TEP hydraulic oil in a tomized form to ignition was well de- monst ra ted by the flammabili ty apparatus. A similar condi t ion could arise as the result of a minute crack or break in a high pressure hy- draulic system containing this material. To prevent this, either an anti-misting agent with shear resistant qualities suitable for use with petroleum-oil- type hydraulic fluids will be

required, or an inherently fire-resistant fluid is needed.

Despite evidence that a dispersion of lithium phenylstearate in an oil results in the format ion of larger drops in the misting state, the effect appears to be too small to signif- icantly reduce the flammability of 1005 air- craft oil or 2190-TEP hydraulic oil. This is unfor tuna te when the shear resistant prop- erties of the association colloids, so essential to long life hydraulic oils, are considered. More work will be required to formulate a shear resistant antimisting additive for petro- leum-type hydraulic oils.

The absence of ignition of the water-in-oil emulsion, Mobil XRL 1262 fluid, despite the fact that it contains over 50% organic mate- rial, indicates its promise as a fire-resistant hydraulic fluid. It is seen als~ from Table il that the per t inent physical and chemical prop- erties are similar to those required for 2190- TEP oil. The pH of the emulsion is oil the slightly alkaline side, a condit ion well known to inhibit the corrosion of steel. The pour

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point of - -35 °C exceeds that required of 2190-TEP oil, despite the 41% water content. Finally, the Mobil fluid as well as similar fluids [11] have been subjected to hydraulic pump tests to demonstrate anti-wear properties. These tests would also be indicative of the shear resistant properties of these fluids.

The effect of water in water-oi l emulsion systems appears curious with respect to its effect on combust ion characteristics. Dispersed water in oil in the range 2 - 10 percen t , pro- motes combust ion of many fuel types [13]. It essentially acts as an anti-knock additive boosting the octane number of the fuel. Such water may be ultrasonically dispersed imme- diately before combust ion takes place or an emulsion may be prepared and stored for some time before use by employing small amounts of an emulsifier (surfactant) as an emulsion stabilizer. Water u n d e r these condi - t ions apparently acts in the following fashion [ 14]: as the emulsion droplet undergoes com- bustion, heat is transferred from the surround- ing diffusion flame to the drop. Sufficient superheating may occur so that the micro- droplets of water contained within the oil droplet explosively vaporize causing the oil droplet to become highly fragmented and dispersed in the air phase. However, when surfactant is added in amounts well above those required to stabilize the emulsion, the water may become solubilized or microemul- sions may possibly form. In this case the water does n o t p r o m o t e improved combust ion since it is apparently more intimately mixed with the combined oil and surfactant phases and less free to explosively superheat. It has been found, for example, that microemulsions of water in diesel fuel at the 10% level (stabi- lized by 6% surfactant) gave fire-resistant properties in mist flammability experiments [151.

The present emulsion tested in this report, containing 41% dispersed water and 5% sur- factant, would therefore be expected to be fire-resistant a priori. The mechanism by which flames are propagated through fuel mists, however, are unknown at present. For example, the propagation speed and structure of the detonation of gas-spray mixtures will be affected by drop heterogeneity [16]. The interaction of physical and chemical processes in the heterogeneous burning region also render definitive mapping of the propagating

flame front somewhat obscure [ 17]. Moreover, the mechanism by which dispersed water within mis t s of emulsions acts to inhibit or prevent combust ion would perhaps be even somewhat less understood. Clearly though, the heat involved in the combust ion process is sufficient to greatly overwhelm the latent heat of vaporization of dispersed water -- even at the 41% level (about 10 000 calories per g vs. 539 calories per g). That is, the latent heat of vaporization of the water itself is no t sufficient to inhibit combust ion and cannot, of itself, explain the effect. However, vaporiza- tion of the 41% water-in-oil emulsion droplet would produce a vaporized mixture containing 7% oil and 93% water by volume in the vapor form (excluding the oxidizing atmosphere). The high concentration of water molecules (approximately 14 molecules per oil molecule, assuming a molecular weight of 350 for the oil) probably acts to a large extent as an inert diluent, reducing the oxygen concentration to below that required for propagation of the flame. Early work of the Bureau of Mines [18] and later work at NRL [19 - 21] have shown that hydrocarbon flames do no t pro- pagate at oxygen concentrations below 12 - 14% (by volume).

4. CONCLUSIONS AND R E C O M M E N D A T I O N S

The flammability problem known to exist with petroleum-type hydraulic oils in finely divided mist form has been demonstrated by the behavior of 2190-TEP hydraulic oil in the NRL flammability apparatus. It has been found that the association colloid, lithium phenylstearate, has no value as an anti-misting agent for use with petroleum oils. It is rec- ommended that the search for other materials be continued.

It has been found that the water-in-oil emul- sion, Mobil XRL 1262, does not ignite in the NRL flammability apparatus. It shows promise as a potential substitute for petroleum oil type hydraulic oils now in use. It is recom- mended that additional studies of this and similar fluids be made to establish their suit- ability as hydraulic fluids for use in submarine hydraulic systems and for other Navy applica- tions.

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ACKNOWLEDGEMENTS

The authors thank the Mobil Research and Development Corporation for supplying the water-in-oil emulsion. They further express their gratitude to Drs. Carhart and Hazlett of the Combustion and Fuels Branch for useful suggestions and constructive criticisms. The authors also acknowledge the support of the Navy Sea Systems Command.

REFERENCES

1 J. W. Hoyt, J. J. Taylor and R. L. Altman, J. Rheol., 24 (5) (1980) 685.

2 Unpublished NRL data. 3 R. F. Salmon, Federal Aviation Administration

Rep. No. FAA-CT-81-11, February, 1981. 4 R. C. Little and J. B. Romans, NRL Lett. Rep.

6180-693: RCL: ara 22 Sept., 1980. 5 C. M. Henderson and R. C. Little, Lubr. Eng., 30

(1974) 458. 6 A. Fiorentino, R. DeSaro and T. Franz, Federal

Aviation Admin. Rep. No. FAA-CT.58, June, 1981.

7 H. R. Baker, R. N. Bolster, P. B. Leach and R. C. Little, Ind. Eng. Chem., Prod. Res. Dev., 9 (1970) 541.

8 H. W. Carhart and R. C. Little, NRL Lett. Rep. 6180-993: RCL: ara 17Nov., 1980.

9 NMAB Committee on Fire Resistant Hydraulic Fluids, Publ. NMAB-345, National Academy of Sciences, Washington, D.C., 1979.

10 J. E. Brophy, V. G. Fitzsimmons, J. G. O'Rear, T. R. Price and W. A. Zisman, Ind. Eng. Chem., 43 (1951) 884.

11 F. C. Kohout, E. N. Ladov and D. A. Law, U.S. Bureau of Mines Rep., BuMines OFR 19-81, March, 1979.

12 R. J. Mannheimer, Federal Aviation Admin. Rep.~ FAA-RD-79-62, 1979; Rep. FAA-CT-81-I53, 1981.

13 Symp. Use o f Water-in-Fuel Emulsions in Com~ bustion Processes, Transportation System Center, Cambridge, MA, April 20, 1977, unpublished.

14 J. Dooher, R. Genberg, R. Lippman, S. Moon, T. Morrone and D. Wright, Mech. Eng., 98 (11) (1976) 36.

15 W. D. Weatherford, Jr., G. E. Fodor, D. W. Naegeli, E. C. Owens and B. R. Wright, Rep. No. AD A083610, December 1979. (Available from the Defense Documentation Center~ Cameron Station, Alexandria, Va., 22314.

16 F. W. Williams, Combustion Theory, Addison- Wesley, Reading, Mass., 1965, pp. 274 - 285.

17 H. Carhart, personal discussions. Note : H. Carhart is a recognized authority in the field of combus~ tion and fuels.

18 H. F. Coward and G. W. Jones, U.S. Bur. Mines Bull. 503, 1952.

19 H. W. Carhart and G. F. Fielding, Syrup. An Appraisal o f Halogenated Fire Extinguishing Agents, April 11 - 12, 1972, National Academy of Sciences, Washington, DC, 1972, p. 239.

20 H. W. Carhart and R. G. Gann, Fire Suppression in Submarines, Report o f NRL Progress, May, 1974.

21 R. G. Gann, J. P. Stone, P. A. Tatem, F. W. Williams and H. W. Carhart, Combust Sci. Technol., 18 (1970) 155.