|  +1-303-562-7814

Fact or Fiction EHC Fluid Resistivity

By Barry Sibul, Barry Sibul Company 3720 S. Ocean Blvd., Suit 108, Highland Beach, FL 33487 Phone 888-433-0003 FAX 877-789-4133.


EHC Fluid Resistivity
August, 2009

Since the mid 1960’s turbine hydraulic fluid systems have been using Fire Resistant Fluid (FRF) to control their turbine inlet valves.

It is a known “FACT” that low fluid resistivity adversely affects the internal components of servo valves within the hydraulic fluid system.

Root cause analysis studies conducted by numerous agencies have determined that acids, conductive water and ferrous particulates affect the resistivity of fire resistant hydraulic fluid.

OEM’s, fluid suppliers and EPRI all agree that EHC fluid resistivity should be maintained above 5.0 G ohm-cm.

Once the resistivity drops below this point “free streaming electron” damage can occur to the servos throughout the hydraulic fluid system resulting in “etching” of their components.

The result of etched components is increased flows past the servo spools leading to increased pump duty, and in some cases a failure of the servo valve torque motor due to damaged nozzles resulting in a possible tripping of the unit.

Etched Servo Spool

Etched Servo Spool

New Torque Motor

New Torque Motor

Servo Torque Motor Nozzles

Servo Torque Motor Nozzles

Since acids, conductive water and ferrous particulates all affect resistivity it is critical that all three of these variables be properly maintained.


Acids are controlled through the use of acid control filters. For Phosphate Ester fluid (the most widely used FRF) either Fullers Earth filters (attapulgus clay), Selexsorb filters (activated alumina) or Ion Exchange filters (synthetic resin) are used. It is a known “FACT” that ion exchange filters have a negative affect on fluid resistivity. Tests have shown that the use of Ion Exchange filters limits the maximum level of resistivity to less than 10 G ohm-cm. Even though this is above the recommended limit, there is less margin for error when using Ion Exchange filters to control fluid acidity.

Fullers Earth filters and Selexsorb filters can only control acids up to 0.25 mg KOH/g. Beyond this point the acids cannot be controlled. Ion Exchange filters can control fluid acidity to much higher levels and are therefore the desired filter to be used to remediate a system with acids levels greater than 0.25 mg KOH/g.

The “FACT or FICTION” of acid control filters will be presented in another topic.


Conductive water is the second cause of low resistivity in FRF. OEM’s recommend the water levels in the fluid should be maintained less than 0.2% (2,000 ppm). It is a “FACT” that the lower the water content in the fluid the better the fluid will perform. In most systems it is possible to maintain the water levels in the fluid less than 0.05% (500 ppm). The best way to do this is by keeping the water from getting into the fluid by using an air blanketing device similar to The Stealth ( Once the water gets mixed with phosphate ester fluid, a vacuum dehydrator will be needed to remove the entrained water from the fluid.


Ferrous particulates are the third contributor of lowering fluid resistivity. OEM’s and fluid suppliers state the particulate levels in fluid should be maintained at an ISO level of at least 15/12. The “FACT” is that the cleaner the fluid, the better it is for the overall system including the resistivity levels of the fluid. With the proper particulate filters in the system the fluid can easily be maintained at an ISO level of at least 14/11 or better. In order to achieve this level of cleanliness particulate filters of at least 3-5 micron absolute should be installed in the system.

A number of fluid suppliers have stated that resistivity is not a factor with their fluid. This is “FICTION”. It is not the fluid that causes the resistivity to drop but rather the acids, conductive water and ferrous particulates. Once the fluid is introduced into the hydraulic fluid system, it is subject to external contaminates (acids, conductive water and ferrous particulates) that ultimately can have the affect of lowering the fluid’s resistivity leading to servo damage or failure. It does not matter which fluid is used in the system. Maintaining the fluid’s resistivity greater than 5.0 G ohm-cm is critical in assuring long term reliable operation of your system.

Post a Comment