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.