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

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 (,& 2 200 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 l f e 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;