Ion Exchange/Vacuum Dehydration Treatment of Phosphate Ester Fluid

Phosphate ester fluids, commonly used in electrohydraulic control (EHC) systems of steam turbines, are susceptible to hydrolytic breakdown in the presence of water and heat and therefore require specialized treatment. Preventive maintenance of phosphate ester fluids used in EHC systems of steam turbines requires particulate contamination, water, acid number (AN) and resistivity be kept within OEM specifications.(1,2)

The hydrolytic breakdown of the phosphate ester fluid results in the creation of acidic compounds. To overcome this, conditioning agents which maintain AN within acceptable limits are often recommended. Two such conditioning agents, fuller’s earth and activated alumina, are commonly employed to scavenge acidic compounds formed in the fluid.

While these acid scavengers are effective in removing acids when acid levels are not too high, neither the fuller’s earth nor activated alumina return the fluid to within OEM specifications once certain AN values are exceeded.(3) Consequently, no significant increase in fluid service life is achieved.

Once the fluid fails to meet specification, operators often bleed and feed the reservoir, replacing roughly fifty percent of the fluid. This is only a temporary solution because the main cause of fluid degradation has not been solved.

While this treatment method may bring the fluid within OEM specifications, the fluid usually returns to a poor state within a short time. In fact, because the rate of hydrolysis increases in an acidic environment, the added charge degrades faster than if the entire volume was initially replaced.

Figure 1

A point can be reached where the bleed and feed strategy does not work. In these instances, a full system flush is required. This costly treatment also fails to address the reasons why the fluid failed, and the fluid will likely degrade again.

Figure 2

Fuller’s earth and activated alumina are commonly used to remove acids from the fluid. Both conditioning agents contain extractable metals that combine with hydrolysis breakdown products to form soaps or salts. These soaps or salts may be present as particles or as a gelatinous material.

The formation of these by-products can reduce filter service life, clog last chance filters in the servovalves, and diminish air release properties of the fluid, which promotes dieseling.

Dieseling is an adiabatic compression of air bubbles in the fluid that results in increased rate of oxidation and hence, increased acidity and in the formation of carbonaceous fines.(3)

A number of utility companies have inquired about the nature of contaminants found on the last chance filters, high-pressure system filters and in the fluid itself. Typical examples of these contaminants are shown in Figures 1 and 2.

The nature of this contaminant, either in the form of a black substance or an insoluble yellowish gelatinous material, has been investigated by Pall Corporation using proton induced X-ray emission (PIXE), X-ray emission spectroscopy (XES) and fourier transform infrared spectroscopy (FTIR).

The results of their investigations are summarized in Table 1. In each case, the fluid was degraded through hydrolysis and exposed to fuller’s earth or activated alumina.

As expected, the presence of phosphorous, calcium, magnesium, aluminum and/or sodium was confirmed in each sample. The calcium/magnesium and aluminum/sodium are the extractable metals found in fuller’s earth and activated alumina, respectively.

The results of the investigations have shown the drawbacks inherent with the use of conditioning agents that contain extractable metals. In this regard, ion exchange resins looked particularly attractive because they offer a choice of functional groups that can be used to selectively target fluid parameters.(4)

Vacuum dehydration is essential for fluid service life extension because wetted resins are employed in this ion exchange treatment. Ion exchange/vacuum dehydration (IX/VD) reduces the water contamination, which is not addressed by fuller’s earth or activated alumina.(5) In addition, by also addressing the acid and particulate contamination, the IX/VD effectively restores the fluid to within OEM specifications for AN, water, resistivity and particulate.(6-11)

There are several IX/VD units used at utilities across the United States. In each of these applications, the operating parameters and the customer concerns were very different. Also, in some cases fuller’s earth and in others activated alumina, were used as acid scavengers.

As these conditioning agents were removed, the IX/VD technology was applied in a kidney loop configuration on the main reservoir. At each facility where this technology has been applied, the fluid has returned to OEM specifications within several months of continuous operation. Three case studies of different turbine OEM designs are detailed in Table 2.

The results from these installations illustrate the effectiveness of IX/VD treatment in restoring EHC fluid to within OEM specifications. In at least two cases, the fluid most likely could not have been restored with conventional media and treatment methods.

By addressing both the hydrolytic breakdown and the associated symptoms, the IX/VD treatment successfully arrested the rate of hydrolysis and significantly extended fluid service life. IX/VD treatment thus contributed to reduced operating and disposal costs, making it the most cost-effective solution for phosphate ester conditioning.

References
1. Marino, M.P. and Placek, D.G. (1994). “Phosphate Esters.” CRC Handbook of Lubrication and Tribology. CRC Press Inc.: Boca Raton, FL.

2. Staley, C. and McGuigan, B. (1976). “The European Use of Phosphate Esters in Steam and Gas Turbines.” Lubr. Eng., 33 (10).

3. Duchowski, J.K., Collins, K.G., Sutton, D.I., and Sinclair, B.S. (2001). “Ion Exchange/Vacuum Dehydration Treatment: an Improved Approach for Conditioning and Reclamation of Phosphate Ester Hydraulic Fluids.”

4. Phillips, W.D. and Sutton, D.I. (1996). “Improved Maintenance and Life Extension of Phosphate Esters Using Ion Exchange Treatment.” 10th International Colloquium on Tribology. Pall Corporation: Publication No. GSD2654/RPM/1M/0396. Esslingen, Germany.

5. Duchowski, J.K. (1996). “The Use of a Hydraulic Purifier for Conditioning of Phosphate Ester Hydraulic Fluids.” Lubr. Eng., 52 (11).

6. (1996). “Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration,” ASTM D664-95. Annual Book of ASTM Standards. ASTM: Philadelphia, PA.

7. (1998). “Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration,” ASTM D974-97. Annual Book of ASTM Standards. ASTM: Philadelphia, PA.

8. (1992). “Standard Test Method for Determination of Water in Liquid Petroleum Products by Karl Fischer Reagent,” ASTM D1744-92. Annual Book of ASTM Standards. ASTM: Philadelphia, PA.

9. (1994). “Standard Test Methods for DC Resistance or Conductance of Insulating Materials,” ASTM D257-93. Annual Book of ASTM Standards. ASTM: Philadelphia, PA.

10. (1987). “Hydraulic Fluid Power - Fluids - Methods for Coding Level of Contamination by Solid Particles.” International Standard 4406, 1st Ed., Geneva, Switzerland.

11. (1977). “Hydraulic Fluid Power - Calibration of Liquid Automatic Particle-Count Instruments - Method Using Air Cleaner Fine Test Dust Contaminant.” International Standard 4402. 1st Ed. Geneva, Switzerland.