The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids.

Transcription

1 John K. Duchowski City of Industry CA The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids. Abstract: An improved procedure for controlling the acidity ofphosphate esters is described. In the presence of water phosphate esters undergo hydrolysis which increases the acidity of the jluid and eventually lea& to fluid degradation. Currently this problem is addressed through the use of acid adsorbing andor neutralizing agents such as Fuller's earth or activated alumina. Bese agents neutralize the acidic species formed in the hydrolysis reaction but do not remove water which is the cause of the problem. In contrast, the procedure described herein relies on water removal to prevent hydrolysk. Removal of water is accomplished with a spinning disc purifier which employs the vacuum dehydratiodmass transfer principle. In addition, the purlper is equipped with a 3 pm (,& rated) filter for particulate removal. Results of studies at East and West coast power plants operated by two major utilities indicate that by maintaining low water and particulate content in the fluid, the purifier effectively reduced the rate of phosphate ester hydrolysis. These data therefore suggest that the service lfe of both thejluid and the neutralizing agent can be extended beyond the present two year period observed on average in the power generation industry. INTRODUCTION Phosphate esters are used in industrial hydraulic and lubricating applications where the use of petroleum based fluids would constitute a fire hazard. Typical applications include aircraft hydraulic systems, electrohydraulic turbine control (EHC) systems in power plants and hydraulic furnace door controls in aluminum smelting plants'-'. Phosphate esters exhibit excellent lubricating characteristics and decompose thermally into phosphoric acid which acts as a fire retardant. However, these fluids absorb water much more readily than their petroleum counterparts: water concentrations may be as 0.10 percent or higher'". The presence of water is undesirable because the fluid can decompose into acidic components. The quantity which measures the extent of phosphate ester decomposition is the total acid number or TAN. The TAN of the fluid is determined either from a potentiometric titration (ASTM D-664 method') or from a color-indicator titration (ASTM D-974 methodg) and is expressed in milligrams of potassium hydroxide required to neutralize one gram of fluid (mg KOWg fluid). To reduce the rate of decomposition, adsorbing and/or neutralizing agents (Fuller's earth or activated alumina) have been employed24*'0*11. The latter is often available in the form of a proprietary activated ahmindzeolite composite. The use of these agents often presents several drawbacks of its own. Variability in composition of Fuller's earth leads to differences in activity; -1-

2 John K. Duchowski City of Industry CA presence of extractable metals may contribute to Contamination of the hydraulic system'. Although activated alumina exhibits a higher consistency and a more uniform activity, it also contains extractable metals and particulates with a wide particle size distribution". Both Fuller's earth and activated alumina require low flow rates to achieve optimal results (typically 1 gpm where a reservoir may contain 200 gallons). Finally, while both of these agents neutralize acidic species, their water removal capabilities vary. Consequently, the fluid continues to hydrolyze. The media eventually lose their acid neutralizing properties and a rapid increase in acidity follows. This behavior has been observed at several power plants where the TAN of the fluid was found to increase beyond the OEM recommended specifications'2* l3 after an average service life of about two years. In view of these limitations it would be desirable to design a better method which removes water from the phosphate ester to minimize the rate of hydrolysis in order to extend the service life of the fluid by preventing decomposition'3. The service life of the neutralizing agent should also be extended because there would be less acid present in the system to neutralize. This paper describes the results of studies performed at two power plants operated by a major East Coast and West Coast utilities. The aim of these studies was to determine the effectiveness of a hydraulic spinning disc purifier in controlling water and particulate contamination and hence the acidity level in an EHC system equipped with a phosphate ester fluid. At each location, the purifier was connected to the reservoir in a kidney loop configuration. The purifier start up and operation as well as the sampling protocol were adapted to local conditions. At the East Coast utility, the purifier was operated once a week for 24 hours at a time; samples were collected by the plant personnel and sent to the fluid manufacturer for analysis. The fluid was and continues to be conditioned with Fuller's earth. At the West Coast utility, within the limitations of plant operation, the purifier was operated in a nearly continuous mode. The samples were collected at monthly intervals and analyzed at a local laboratory. The fluid was conditioned with Fuller's earth until Nov. 6,1991 when Fuller's earth was replaced with proprietary activated alumindzeolite composite. The latter was employed in the course of the present study; a new cartridge was indalled on August 23, 1993 and remained in service throughout the test. At both plants then, the purifier was used as a complimentary conditioning device added on to an existing system. HYDROLYSIS OF PHOSPHATE ESTERS The fact that phosphate esters undergo hydrolysis in the presence of water is well Briefly, both alkyl and aryl esters of phosphoric acid react with water in a step wise manner until all the organic ester groups are replaced by protons. The reaction may proceed either &mu& the cleavage of the P-0 or C-0 bond but in both cases the final product is phosphoric acid. The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester IIydraulic Fluids. -2-

3 John K. Duchowski City of Industry CA An important aspect of this process is that the intermediate products, the mono and diesters where one or two organic groups have been replaced with a proton, are also strongly acidic. In general, the diesters are stronger acids than the mono esters which are in turn stronger than phosphoric acid itself. The strength of acidity appears to be dependent on the electron donating capability of the ester substituent. Additional complications arise from reactions which may occur between various products of the above hydrolysis reaction. For example, a condensation reaction involving two partial esters 15,16 will lead to a product of the type R,P,O, referred to as pyrophosphate. Furthermore, reactions with either mono or divalent metal ions which may leach out of the neutralizing media lead to formation of salts of the type (P0,)U' l2 which deposit on components such as valves, filters, etc., and interfere with the operation of the system. Because Fuller's earth and activated alumina adsorb only some of the water present in the fluid, hydrolysis and the associated reactions discussed above still proceed at an appreciable rate under normal operating conditions. For example, the phosphate ester fluid at a power plants described in this article exhibited water concentrations close to or well above the limits recommended by the OEM (see Figures 1 and 2). Examination of the TAN data from both systems shows that under these conditions, the rate of hydrolysis was sufficient to necessitate fluid replacement at about two to three year intervals.. HYDRAULIC SPINNING DISC PURIFIER The hydraulic spinning disc purifier (HSP) is a device which relies on the vacuum dehydration mass transfer principle for removal of water and entrained gases from hydraulic and lubricating fluids. Unlike conventional flash distillation dehydrators, the HSP uses a relatively low vacuum (61 1 mm Hg) and operates at ambient temperature. The water removal process is optimized by spreading the fluid onto a large surface area of the spinning disc. The less severe operating conditions of the HSP help to preserve the chemical integrity of the treated fluid. Some conventional flash distillation units in contrast use relatively high temperatures and power concentration per unit volume of the fluid which promote oxidation. The HSP is capable of removing up to 100% of free water and up to 80% of dissolved water under standard operating conditions. The final water concentration and the rate of water removal will be dependent on the nature and temperature of the fluid. In general, faster rates will be obseried at temperatures at or above 38'C. The EHC fluid in question was maintained at a temperature of 49 C without additional heating. SAMPLE COLLECTION ANI) FLUID ANALYSIS 250 ml of fluid were collected from the sampling port on the system reservoir into particle The Use of a nydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids. -3-

4 John K. Duchowski City of Industry CA Ph. (8 18) Fx. (8 18) free glass bottles or into pre cleaned sample bottles supplied by the fluid manufacturer. At the East Coast utility, the samples were collected as deemed necessary by the plant personnel and sent out to the fluid manufacturer for analysis. The same procedure was followed at the West Coast utility until August From that point on, the samples were collected at approximately monthly intervals and analyzed at a local laboratory. The fluid was analyzed for water concentration (Karl Fischer, ASTM D-1744 procedure"), total acid number (potentiometric method, ASTM D-664 procedure? and the particle count (automatic particle count, IS procedure18). The latter was performed on an automatic particle counter equipped with a laser sensor operating in light extinction mode (calibrated as per IS procedure"). Because of small but significant differences in automatic particle count procedures at two locations, particle count data only for the utility where the IS0 standards were strictly adhered to is reported. FLUID CONDITIONING WITH CONVENTIONAL TREATMENT East Coast Utility. The water and TAN data obtained from the study at the East Coast utility are displayed graphically in Figure 1 The data covers roughly five years during which time the EHC fluid was conditioned with Fuller's earth. Figure 1 shows that prior to the installation of the HSP, water concentration significantly exceeded the limits specified by the OEM'2 for a large percentage of the time period reported here. In addition, each rapid increase in water concentration was reflected in a corresponding increase in the TAN. These observations suggest that Fuller's earth may experience difficulties in stabilizing fluid conditions when exposed to high water concentrations andor moderate to high TAN levels. West Coast Utility. The corresponding data obtained for the West Coast utility are displayed graphically in Figure 2. At this utility, the fluid was conditioned with Fuller's earth until Nov. 6, 1991 at which time Fuller's earth was replaced with a conditioning system which employs a proprietary activated ahmindzeolite composite medium. Figure 2 shows that regardless of the conditioning agent employed, water concentration in the EHC fluid remained at a relatively stable level of ca percent (0.02 percent below the 0.10 percent threshold value recommended by tine OEM'') on average over a four year period. No significant changes in water concentration were observed following the deployment of the activated ahmindzeolite composite. The rapid rise in water concentration observed during the Nov Dec interval was most likely due to maintenance procedures performed on the EHC system at that time. Figure 2 shows that the total acid number value exceeded the OEM recommended value of 0.25 mg KOWg fluid on two separate occasions. Furthennore, the data show that a rapid rise in the total acid number usually occurred after about a year following the introduction of new fluid. These The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids. -4-

5 John K. Duchowski City of Industry CA observations suggest that when conventional fluid conditioning means are employed, the hydrolysis reaction still proceeds at a sufficiently rapid rate to necessitate replacement of a 200 gallon volume of fluid roughly at every two years. Because the fluid conditioning media are also replaced at the same time, the total volume of hazardous waste (some of which can be regenerated) is quite considerable and presents a significant cost to the utility both in terms of disposal and in terms of purchase of new materials. FLUID CONDITIONING WITH HYDRAULIC SPINNING DISC PURIFIER East Coast Utility. The water and TAN data obtained from the study at the East Coast utility are displayed graphically in Figure 1. Examination of these data shows that within one month of the HSP going on-line, water concentration stabilized at ca percent which is an order of magnitude below the limit specified by the OEM'2 and therefore represents a comfortable safety margin for the plant operators. The data also show that by maintaining water concentration at the reduced level, the TAN was also stabilized. No significant fluctuations in the TAN data have been recorded since the installation of the HSP. Similarly, within one month of the HSP going on-line, the TAN values have stabilized at around 0.02 mg KOWg fluid, well below the OEM recommended value of 0.20 mg KOWg fluidi2. West Coast Utility. The data obtained for the West Coast utility are displayed graphically in Figure 2. Like at the East Coast utility, the water concentration was reduced from 0.19 percent to 0.06 percent within one month from the installation of the HSP and then stabilized to an average value of about 0.05 percent. It should be noted that the anomalously high value of 0.12 percent recorded on Apr. 6, 1994 is included in the above average, which consequently represents a conservative estimate. Examination of the last five data points shows that the system is returning to a more stable condition. An average water concentration calculated for those last five points is 0.03 percent, more in keeping with the data obtained previously for the East Coast utility. The high value recorded on Apr. 6,1994 most likely resulted fiom maintenance procedures performed on the EHC system at that time. While the maintenance procedures were being carried out, the purifier was out of operation for approximately a week. Overall, the above data indicate that the purifier succeeded in lowering water concentration to well below the 0.10 percent limit specified by the OEM13 and considerably increased the safety margin observed under previous operating conditions. Figure 2 shows that a similar trend toward stability has been observed for the total acid number. Following the initial increase in the total acid number recorded on January 6, 1994, which most likely resulted from anomalously high water level recorded in December 1993, the total acid value stabilized around 0.07 mg KOWg fluid, ;.e., also well below the threshold value of 0.25 mg The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester IIydraulic Fluids. -5-

6 John K Duchowski City of Industry CA KOWg fluid specified by the OEMI3. These observations indicate that a 0.03 percent reduction in water concentration resulted in sufficiently slowing down the rate of hydrolysis to allow the fluid conditioning medium (activated alumina) to effectively neutralize any acidic species present in the fluid. At the present time, Le., nearly ten months following the start up of the purifier, the total acid number of the fluid remains stable and does not show any indications of taking an upward turn. Finally, Figure 3 reveals an additional benefit stemming from employment of the purifier for fluid conditioning purposes. Originally the unit was equipped with a p rated filter which reduced particulate population by nearly a factor of two for particles in almost all of the monitored size ranges - compare data from Aug. 23, 1993 (new fluid) and Jan. 6, Furthermore, a substantial improvement in control of particulate contamination was observed following the upgrade to a rated filter as is evidenced by a sharp drop in particle counts observed from Apr. 6, 1994 onwards. A sharp increase in particle counts recorded on July 14, 1994 is most likely due to maintenance procedures performed on the purifier on July 6, In general, the particle counts appeared to have stabilized at about 14/11 on the IS scale although this is most likely a conservative estimate due to the limitations associated with the bottle sampling technique. Overall, this represents a reduction by nearly a factor of four for particles in almost all of the monitored size ranges - for example, compare data from Aug. 23, 1993 (new fluid) and Jul. 6, CONCLUSION The results of the present study show that the hydraulic spinning disc purifier provided an effective control of both of water and particulate contamination leading to a significant decrease in the rate of phosphate ester hydrolysis. The present development of water and total acid number trends suggests that a substantial increase in semce life of the fluid (and the neutralizing agent) may be realized by taking advantage of the fluid conditioning capabilities of the purifier. Work to determine the optimal length of fluid and neutralizing agent replacement interval is presently in progress. A more stable EHC system would be expected to have numerous benefits for the plant operators, such as fewer servo valve fouling problems, less frequent routine maintenance procedures and fewer fluid and conditioning medium change outs. The latter would also be expected to have a positive environmental impact stemming from the reduced amount of hazardous waste. REFERENCES 1. Marino, M.P. and Placek, D.G., "Phosphate Esters," in CRC Handbook of Lubrication and Tribology, Booser, E.R., Ed., Vol. 111, , CRC Press, Inc., Boca Raton, (1994). The Use of a Ilydraulic Purifier for Conditioning of Phosphate Ester nydraulic Fluids. -6-

7 John K. Duchowski City of Industry CA Phillips, W.D., "The Conditioning of Phosphate Ester Fluids in Turbine Applications," Lubr. Eng., 39,766, (1983). 3. Marino, M.P., "Phosphate Ester Synthetic Lubricants," STLE Education Course, (May, 1993). 4. Wolfe, G.F. and Whitehead, A., "Experience with Phosphate Ester Fluids as Industrial Steam Turbine-Generator Lubricants," STLE Special Publication, SP-12, 67-74, Society of Tribologists and Lubrication Engineers, Park Ridge, Illinois (1983). 5. Stark, L.R., "Status of Fire-Resistant Turbine Lubricants," STLE SpecMl Publication, SP-12, 75-77, Society of Tribologists and Lubrication Engineers, Park Ridge, Illinois (1983). 6. Staley, C. and McGuigan, B., "The European Use of Phosphate Esters in Steam and Gas Turbines," STLE Special Publication, SP-12, 78-85, Society of Tribologists and Lubrication Engineers, Park Ridge, Illinois (1983). 7. Farmer, E.P., Jr., "Fire-Resistant Lubricants in Gas Turbines," STLE Special Publication, SP-12, 86-89, Society of Tribologists and Lubrication Engineers, Park Ridge, Illinois (1983). 8. "Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration," ASTM Designation D , Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, Pennsylvania (1990). 9. "Standard Test Method for Acid and Base Number by Color-Indicator Titration," ASTM Designation D , Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, Pennsylvania (1994). 10. Wilson, A.C.M., "Phosphate Ester Hydraulic Fluids: Their Electrical Conductivity and Earth Treatment Relative to Test and Service Performance," Perform. Tat. Hjrdraul. fluids, Pap. Int. Symp., , (1978). 11. Hunter, R.A., "Recovery and Reuse of Aryl Phosphate Ester Hydraulic Fluid," Proc. Id. Wuste Gnz, 34, , (1979). 12. "Steam Turbine-Generator EHC Fluid Specifications and Maintenance," General Electric Publication GEK 46357E, Rev. E, October "Care, Handling and Application of Control System Fluid," Westinghouse Publication I.L C, Rev. 5/22/ Corbridge, D.E.C., "Phosphorus- An Outline of its Chemistry, Biochemistry and Technology," 3rd Ed., , Elsevier Science Publishing Co. Inc., New York (1985). 15. Penczek, S., "Mechanism of Ionic Polymerization of Cyclic Esters of Phosphoric Acid," J. Polym. Sci., Pol'm. Symp., 67, , (1980). 16. Penczek, S., "Polymerization of Cyclic Esters of Phosphoric Acid," Pure & Appl. Chem., 48, , (1976). 17. "Standard Test Method for Detemination of Water in Liquid Petroleum Products by Karl Fischer Reagent," ASTM Designation D , Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, Pennsylvania (1992). 18. "Hydraulic Fluid Power - Fluids - Method for Coding Level of Contamination by Solid Particles," International Standards Organization, International Standard 4406, (1987). 19. "Hydraulic Fluid Power - Calibration of Liquid Automatic Particle-Count Instruments - Method Using Air Cleaner Fine Test Dust Contaminant," International Standards Organization, International Standard 4402, (1 977). The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids. -7-

8 Fig. 1 - Water and TAN Data for EHC Fluid in a Power Plant Operated by a Major East Coast Utility, -1)- Water Concentration (percent) +TAN (mg KOH/g fluid) I I 0.30 I #1 Water and TAN Limits 1 Fluid,-A,/ Change: Mar JuI NOV-89 9-Apr-91 d HSP Installed: 1 -MAY-92 & 21 -Aug-92 Sample Date

9 Fig.2 - Water and TAN Data for EHC Fluid in a Power Plant Operated by a Major West Coast Utility Water Concentration ([ercent) TAN (ma KOH/Q fluid) I Fluid Change: 25-FEB-88 OEM #2 TAN Limit n I '/ AVZeolRe replaces FE: 6-NOV-91 / I HSP Installed: 6-DEC OEM #I2 Water Limit Feb-88 b u i d Change: 19-FEB Jun Oct Mar-92 Sample Date Fluid Change: 23-AUG JuI-93

10 Fig. 3 - Automatic Particle Count Data (IS0 4406) for EHC Fluid in a Power Plant Operated by a Major West Coast Utility HSP Maintenance: 6-JUL-94 OEM #2 >5 pm Limit c 0 & P E a z 10 Fluid Change: 23-AUG-93 1 pm Filter Installed: 2-MAR Aug ct NOV Jan-94 7-Mar Apr Jun-94 4-Aug Sep-94 Sample Date