Resistivity for Fyrquel EHC fluid

From Bently Tribology Services web site
EHC Fluid- Guidelines for Condition Monitoring
The purpose of the electro hydraulic control system is to provide motive force active through the servo actuators, which position the steam turbine valves in response to electrical commands from an electronic controller. The fluid used in an EHC system has typically been a fire retardant synthetic tri-aryl phosphate ester, the most common brand names being:

Akzo Nobel Fyrquel EHC
Solutia Skydrol

The condition of the EHC fluid is critical to the operation of the EHC system. The following are suggested test and limits for the condition of the fluid.

Limits (Max)
Iron ppm 15
Lead ppm 15
Copper ppm 15
Tin ppm 15
Aluminum ppm 25
Nickel ppm 15
Silicon ppm 30
Sodium ppm 500
Magnesium ppm 50
Calcium ppm 50
Phosphorus ppm No upper limit established
Viscosity, 40 °C cSt 37.8-46.2
TAN, mg KOH/g 0.3 max.
Chlorine Content 150 ppm max.
Resistivity, G ohm/cm 4 min
Particle Count ISO 14/10 – 16/13 Max.
Water Content 1500 – 2000 ppm Max.

Notes:

Aluminum can increase when using Activated Alumina filters as a substitute for Fuller Earth filters. Phosphorus will always be high as the fluid is a phosphate ester. Increases in Magnesium/calcium can indicate acidity problems, as well as possible mineral oil contamination. High chlorine is indicative of servo valve corrosion.

There is a paper from Reliant Energy/CleanOil formally EPT on dealing with low resistivity EHC fluids
http://www.phosphateester.com/uploads/reliant_article.pdf

When resistivity drops below 4 (some would say 5 or 6) the fluid creates allows Electrokinetic wear or etching on the fine control parts of servo valves and actuators. Please keep your resistivity up.
We use Azko Nobel Fyrquel EHC with a Selexsorb cleanup loop. Our Chemistry of the main EHC resivior tank has been good. With resistivity around 25.

The minimum resisitivity set by both GE and Westinghouse is 5 Gohm.cm. This level is based on problems years ago with electrokinetic wear of the servovalve spools. It was an issue in control systems for steam turbines and also for the different type of phosphate ester fluid being used on air planes. What they found was that by keeping the chlorine content and the resistivity within the limits there were not problems. There can be other factors as well including the type of servovalve but generally if you stay above 5 you should be okay. INPO recommends 20. With a properly maintained fluid using either fullers earth or Selexsorb this should not be a problem. Keeping the resistivity high with IX or its variations can be more difficult and activated alumina does not appear to help either. However, AA is still used in some lower pressure systems that do not have servovalves.

We had some issues with our EHC system and hooked up a Electro static cleaner -- has anyone done this and noticed a drop in resistivity and a increase in acid number?

An electrostatic cleaner will not help the acid number nor the resistivity so if you are using this instead of fullers earth or Selexsorb or other suitable purification media, then the acid number rise and the resistivity fall. Normally just following the OEM recommendations will be enough in most cases. If not find the root cause.

Okay, this old but I noticed a mistake in an earlier post. Fyrquel EHC is a triarly phosphate ester but Skydrol is not. Skydrol is a trialkyl phosphate ester. This is important because the material compatibility issues are different. With Skydrol you generally have to use EPR but with the ICL Fyrquel EHCs and the similar Chemtura Reolube Turbofluids you can use EPR but also fluorocarbons like Viton.