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 antimisting properties, and (2) use o f a water-inoil 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 properties in a flammability testing device.

1. INTRODUCTION

It is c o m m o n knowledge that many materials n o t 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 atmosphere in the atomized state. 0379-7112/83/$3.00

The relationship between the drag reducing properties of jet aircraft fuel containing certain polymers and misting characteristics has been established by H o y 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 established [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 firesuppressive effect on 2190-TEP hydraulic oil. The polymer suppresses the formation of mists by increasing the size of the fuel droplets. 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 characteristics 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, b u t 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 compounds used in drag reduction studies are relatively shear resistant, especially when compared with the chemical polymer polyisobutylene [5]. It has been suggested [5, 7] that the shear resistance of these polar materials arises from the ability of the molecules to restructure themselves through associative processes. Work with the May aerosol generator [8] has demonstrated that the association © Elsevier Sequoia/Printed in The Netherlands

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 evaluation of the organo-metallic c o m p o u n d s rests on their behavior in flammability tests. Such tests of t w o 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 fluorine. Many of these fluids will require modification of the systems in which they would be used in order to assure compatibility with the elastomers and metals involved. The problem of toxicity, particularly with the phosphates and the halocarbons and their reaction products, is of concern. Water-containing materials, as emulsions or solutions, have been developed as fire-resistant or " n o n f l a m m a b l e " fluids. Experiences with fire in the hydraulic systems of military equipment during and after World War II led to the development of the " H y d r o l u b e s " [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 problems 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 c o m p e t e 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, pumpable emulsion of water and oil might provide both the fire resistance desired and the lubricating qualities of petroleum-type hydraulic oils. Work in this field does n o t appear to be very extensive. However, recent developments [ 11] in " n o n f l a m m a b l e " water-in-oil emulsions have shown promise for use in hydraulic systems of mining machinery. The flammability 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 petroleum type aircraft turbine engine oil designated 1005 (MIL-O-6081B (ASG) and AM.4) containing lithium phenylstearate as a thickening agent in nominal concentrations of 0.25~, 0.5% and 1% c o m p o u n d e d by a method used previously [ 5] ; a fluid made from equal parts of 2190-TEP hydraulic oil and 1005 oil containing 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 comparable 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 flammability of the hydraulic fluids. Because of the higher viscosities of hydraulic fluids as compared 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 dispensed by centrifugal force through the holes. The disk was m o u n t e d 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 m o t o r (22 000 rpm, max.) was used. The spinning disk atomizer with the electric m o t o r drive is shown in Fig. 2. The speed of the spinning disk was measured by a

117 TABLE 1 Comparison of properties of fluids studied Properties

2190-TEP Oil (a)

1005 Oil (b)

Flash point Viscosity Pour point Neutralization No. or pH Copper corrosion

400 °F, min. 1 82- 110 cs at 37.8 °C (100 °F) 3 --6.7 °C, max. (20 °F) s 0.30, max. 6

225 °F, min. 2 -5 cs, min. at 37.8 °C (100 °F)4 110 cs at 40 °C ---35 °C 0.100, max. 7 pH 8.1

Slight tarnish permitted 8

Slight brown stain permitted 9

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

ASTM Method D 92. Federal VV-L-791, Method ASTM Method, D 445. Federal VV-L-791, Method ASTM Method D 97. ASTM Method D 974. Federal VV-L-791, Method ASTM Method, D 130. Federal VV-L-791, Method ASTM Method D 130-6.

Fluid (c)

pass 1°

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

1103.5. 305.2. 5105.3. 5303.3.

_

~ C

GENERATOR ~~1::

GEARMOTOR I

SCREWDRIVE

OPTICALTACHOMETERPICKUP~ SPINNINGDISK......~ //

PROPANEBuRNER

'SPEED'

REDUCER

~7 REI-~YS DOWNI ° I

FLUIDSUPPLYLINE

TACHOMETER READOUT

PROPANESUPPLY Fig. 1. Schematic diagram of NRL flammability apparatus.

P I O N E E R DT-36 photoelectric tachometer. The s o u r c e o f ignition was a p r o p a n e b u r n e r l o c a t e d 8 in. f r o m the disk and at the same level. A c a m e r a was used t o r e c o r d the flamm a b i l i t y characteristics o f the fluids. The h y d r a u l i c fluid was delivered t o the spinning disk by m e a n s o f a l o w shear, positive displacem e n t p u m p or syringe consisting o f a vertically m o u n t e d c y l i n d e r c o n t a i n i n g a p i s t o n driven at a slow, variable rate. A s c h e m a t i c o f the a r r a n g e m e n t o f the syringe and accessories is i n c l u d e d in Fig. 1.

P r o c e d u re

The h y d r a u l i c fluids were delivered to the spinning disk at e i t h e r 4 0 0 m l / m i n or 8 5 0 ml/ min. As the fluid e n t e r e d the cavity in the disk, the disk speed o f 10 0 0 0 r p m was r e d u c e d (12 - 13% at 4 0 0 m l / m i n delivery and 21 - 23% at the 8 5 0 m l / m i n level) because o f the centrifugal force r e q u i r e d t o disperse the fluid. Most o f the tests were m a d e with n o f u r t h e r a d j u s t m e n t o f disk speed. However, it h a d been f o u n d t h a t h i g h e r disk speeds t e n d e d t o increase f l a m m a b i l i t y o f fluids, p r e s u m a b l y

118

Fig. 2. The NRL spinning disk atomizer. because o f the f o r m a t i o n of smaller drops at higher speeds. This was observed earlier [4] with the May aerosol generator. Therefore, in order to increase the severity o f the test, the flammabilities o f b ot h the 2190-TEP hydraulic oil and the Mobil X R L 1262 fluid were also d e t e r m i n e d while the speed of the disk was maintained at or near 10 000 rpm. Any propagation of yellow flame from the blue propane flame was considered evidence of ignition of the fluid under test. An arc of flame was established which sometimes completely encircled the spinning disk. The degree of encirclement at a given disk speed and fluid delivery rate was a qualitative indication of the relative flammability of the fluid. The flame could be extinguished by stopping the syringe drive motor. If there was no propagation o f flame from the pr opane flame, the fluid u n d er test was considered to be fire resistant.

3. RESULTS AND DISCUSSION The results o f the flammability studies are summarized in Table 2.

The flammability of 2190-TEP hydraulic oil in mist form was readily d e m o n s t r a t e d in the flammability apparatus. At a fluid delivery rate of 400 ml/min, and an initial disk speed of 8900 rpm with the air m o t o r drive, ignition occurred and a 90 ° - 120 ° arc of flame was established. When the fluid delivery rate was increased to 850 ml/min, the arc of flame increased to about 180 ° . However, if the disk speed was maintained at a nominal 10 000 rpm during the run, the flame completely encircled the spinning disk (Fig. 3). The 1005 oil containing 1% lithium phenylstearate was t o o viscous to be uniformly dispersed by the spinning disk. However, any material reaching the propane flame burned sporadically. Reducing the lithium phenylstearate c o n t e n t to 0.5% and 0.25% resulted in mixtures t hat were well dispersed by the spinning disk, but they burned in the flammability apparatus much like a neat JP-5 jet fuel. In order to determine the effectiveness of the phenylstearate p o l y m e r in the 2190-TEP oil and to increase the viscosity of the 1005 oil solutions, a m i xt ure consisting of equal volumes of 0.5% lithium phenylstearate in 1005 oil and 2190-TEP hydraulic oil was prepared.

119 TABLE 2 Flammability of fluids studied Fluid

2190-TEP

Fluid delivery rate (ml/min)

Disk speed (10 000 rpm + 22%

400

Initial

850

x x x

Ignition

Circular flame projection

Yes Yes Yes

90 ° - 120 ° 180 ° 360 °

x

Yes

120 °

x x

Yes Yes

360 ° 360 °

Maintained

x (8900) x x

1% lithium phenylstearate in 1005 oil

x

0.25% lithium phenylstearate in 1005 oil

x

0.5% lithium phenylstearate in 1005 oil

x

x

Yes

360 °

0.5% lithium phenylstearate in 1005 oil + 2190-TEP

x

x

Yes

360 °

x

Mobil XRL 1262

x x

x x

No No

m

Fig. 3. Typical behavior of 2190-TEP hydraulic oil in the flammability apparatus. U s i n g 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 e a d 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 flame which completely encircled the spinning disk.

The flammability apparatus fluid delivery s y s t e m w as 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 material and refilled with the water-in-oil emulsion, Mobil XRL 1262. The flammability tests were conducted under the condi-

120

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

tions f o u n d to be conducive to the ignition and flame propagation of 2190-TEP oil by maintaining a nominal 10 000 rpm disk speed during delivery o f the fluid. Some luminous scintillation e f f e c t 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, b u t as can be seen in Fig. 4, the flammability apparatus was engulfed in a dense fog of a t om i zed emulsion. Such a condition with a flammable fluid would be e x p e c t e d to enhance the chances of ignition, b u t the Mobil X R L 1262 did n o t burn. The vulnerability of 2190-TEP hydraulic oil in ato mized form to ignition was well dem o n s t r a t e d by the flammability apparatus. A similar co n d itio n could arise as the result of a minute crack or break in a high pressure hydraulic 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 f o r m a t i o n o f larger drops in the misting state, the e f f e c t appears to be t oo small to significantly reduce the flammability of 1005 aircraft oil or 2190-TEP hydraulic oil. This is u n f o r t u n a t e when the shear resistant properties 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 petroleum-type hydraulic oils. The absence of ignition of the water-in-oil emulsion, Mobil X RL 1262 fluid, despite the fact that it contains over 50% organic material, indicates its promise as a fire-resistant hydraulic fluid. It is seen als~ from Table il that the pert i nent physical and chemical properties are similar to those required for 2190TEP oil. The pH of the emulsion is oil the slightly alkaline side, a condition well known to inhibit the corrosion of steel. The pour

121

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-oil emulsion systems appears curious with respect to its effect on combustion characteristics. Dispersed water in oil in the range 2 - 10 p e r c e n t , promotes combustion 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 immediately before combustion 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 c o n d i tions apparently acts in the following fashion [ 14]: as the emulsion droplet undergoes combustion, heat is transferred from the surrounding diffusion flame to the drop. Sufficient superheating may occur so that the microdroplets 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 microemulsions may possibly form. In this case the water d o e s n o t p r o m o t e improved combustion 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 (stabilized 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% surfactant, would therefore be expected to be fire-resistant a priori. The mechanism by which flames are propagated through fuel mists, however, are u n k n o w n 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 m i s t s of emulsions acts to inhibit or prevent combustion would perhaps be even somewhat less understood. Clearly though, the heat involved in the combustion 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 n o t sufficient to inhibit combustion and cannot, of itself, explain the effect. However, vaporization 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 n o t propagate at oxygen concentrations below 12 14% (by volume).

4. C O N C L U S I O N S A N D 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 reco m m e n d e d that the search for other materials be continued. It has been found that the water-in-oil emulsion, Mobil X R L 1262, does n o t ignite in the NRL flammability apparatus. It shows promise as a potential substitute for petroleum oil type hydraulic oils n o w in use. It is recommended that additional studies of this and similar fluids be made to establish their suitability as hydraulic fluids for use in submarine hydraulic systems and for other Navy applications.

122 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, N R L 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, N R L 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, AddisonWesley, 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 N R L 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.