Improved Phosphate Ester Condition Monitoring Using Membrane Patch Colorimetry

Abstract

Electrohydraulic control (EHC) systems are critical steam turbine components that use a series of high-precision servo valves to control steam flow and, ultimately, turbine output. Because of the high temperatures involved, fire-resistant triarylphosphate ester-based lubricants have become the control fluid of choice in EHC systems. Despite their critical nature, many EHC condition monitoring programs fail to detect the fine semisoluble and insoluble contaminants that are responsible for costly valve-related downtime and failures. Although it was originally developed for gas turbine lube oils, the membrane patch colorimetry (MPC) standard test method (ASTM D7843) was recently shown to be suitable for use in phosphate ester applications. The chemically distinct nature of phosphate esters and lube oils, however, requires that certain parameters within the ASTM D7843 method be reevaluated (solvent, patch material, sample preparation, etc.). Chief among these is the test output: MPC ΔE. In lube oils, ΔE highlights the presence of varnish; however, ΔE detects both varnish and thermolysis products in phosphate esters. The latter typically originate from microdieseling events that are more commonplace in EHC applications. The accurate differentiation of these breakdown products is imperative so that phosphate ester users can apply appropriate corrective actions. This requires that MPC a and b measurements be reported in addition to MPC ΔE. The mass of the contaminants isolated on membrane patches has also been shown to provide an effective means of quantifying varnish and other breakdown products present in in-service EHC fluids. By using the information provided by a, b, and patch weight, MPC analysis can be exploited to its full potential, allowing for the identification of application-specific problems such as varnishing and dieseling in addition to quantifying their severity. Once these previously hidden problems have been identified, effective maintenance practices can be developed to ensure steam turbine performance and reliability.