Hydraulic oils used in steam turbine electro-hydraulic control systems (EHC) are required to be fire resistant. All but one OEM require phosphate ester fluids to be used in these applications because they are self-extinguishing and are inherently safer than other fluid chemistries. These fluids are not your typical hydraulic oil and require a different mindset to monitor and maintain in an acceptable condition. However, plants that properly maintain these fluids should expect years of excellent performance. These fluids may also be considered as Fill-for-Life fluids, meaning that they have the potential to last the life your steam turbine.
Understanding how these fluids fail is critical. Each of these failure modes has a proven solution if the problem is detected in time.
1. Water – Phosphate ester fluids will decompose in the presence of elevated moisture, creating acids and alcohols. The alcohols can further oxidize, creating varnish. It is suggested to maintain water levels lower than 500ppm for optimum performance. Luckily, this can be easily accomplished with an inexpensive, OEM-approved technology called StealthEHC.
2. Gels & Metallic Soap Deposits – Some acid absorption media such as Fuller’s Earth and activated alumina can react with phosphate ester degradation products creating gelatinous deposits that impair valve performance. If you are using these acid absorption technologies, monitoring the total dissolved metals in the fluid through metal spectroscopy is important. Or, you can move to the Endure IX technology which doesn’t leave behind metals or fine contaminants that act as reactionary bodies.
3. Oxidation – Phosphate esters, like every other type of hydraulic oil chemistry, degrade in the presence of oxidation and heat. Monitoring the fluid’s acidic constituents and varnish potential via the ASTM D7843 MPC test (Membrane Patch Colorimetry) are ways to ensure that oxidation will not spiral out of control.
4. Low resistivity causing electro-kinetic etching – phosphate ester fluids with low resistivity have the potential to cause valve erosion. Contaminants and fluid degradation will cause the resistivity to drop. Most ion exchange media have an adverse impact on resistivity. Endure IX can restore resistivity values to the levels of a new fluid.
5. Micro-dieseling or extreme thermal degradation – Extremely high temperatures cause phosphate ester fluids to form dark, sub-micron particles. Over time, these particles cause valve silting and accelerated filter change-out. Understanding the root cause of the fluid turning black is important, so that corrective actions can be taken. It is often possible to eliminate the cause of thermal degradation in the fluid. In cases where it is not practical to make the required engineering changes, permanent Band-Aids may be installed, such as electrostatic oil cleaners.
Do you have performance problems with your EHC fluid? If so, contact us today so that we can properly diagnose and solve the issue. Ideally, you will never have to change your phosphate ester fluid again, while experiencing optimum valve performance.