Fluid Conductivity of In-Service Turbine and Hydraulic Oils
by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

Static discharge has become a more common problem in turbine and hydraulic fluid circulating systems. Modern hydraulic fluids and turbine oils have become increasingly less conductive, thus more susceptible to electrostatic discharge (ESD). The susceptibility for a turbine and hydraulic fluid electrostatic discharge is generally monitored by the fluid's conductivity.

Electric conductivity measures a fluid's electrostatic chargeability and whether a static discharge is potential. It usually is expressed in picosiemens per meter (pS/m). Static discharge is more prevalent in Group II lubricant base fluids with conductivity less than 35 pS/m.

Basically, the time for static charge to dissipate is inversely related to conductivity. The potential for electrostatic spark charging increases with decreased conductivity. Electrical conductivity is an important parameter in determining the potential for a turbine and hydraulic fluid to build up an electrical charge, causing electrochemical erosion (surface pitting). The build-up of the charge to levels that cause the discharge (sparks) and the associated damage is influenced by the conductivity of the fluid. The charge carried by oils with a high conductivity allows an accumulated charge to dissipate as it passes around the system, thus generally remaining at a level where electrostatic discharge (ESD) is not experienced.

In addition to causing electrochemical erosion (surface pitting) by electrostatic spark discharge other problems occur resulting in accelerated oil aging, damage to sensors and filter elements, and failures in control systems. In addition, subsonic combustion propagation (deflagration) may occur in return lines or in the reservoir. These discharges cause an extreme thermal fluid load due to hot spots, which accelerates varnish formation and oxidation. Electrostatic spark discharge due to low conductive fluids is a key contributor to the formation of vanish precursors in circulating systems. This is a key monitoring parameter for controlling the formation of varnish film and deposits. Good electrical conductivity helps improve deposit control.

Industrial lube oils formulated with highly refined oils with a low concentration of additives generally have low conductivities. This is characteristic of today's turbine and hydraulic fluids formulated with Group II base oil and using low or no concentrations of metallic anti-wear additives. While these oils may have lower conductivity, the lower conductivity means that the charge generated is more likely to accumulate and discharge destructively.

Where maintenance processes have improved to reduce used oil degradation products, impurities, oxidation, contaminants, and wear, this has resulted in fluids operating with a lower conductivity. Oil purity requirements have increased, which has led in turn to higher filtration rates, which is a source of charge generation. Lower impurities and higher filtration rates both contribute to lower fluid conductivity in a circulating system.

It's noted that increased temperatures and contaminant levels will increase electrical charge dissipation, but allowing this to reduce electrostatic spark discharge is counterproductive. Routine fluid monitoring and exploring some system refinements can help alleviate destructive electrical spark discharge.

• Installation of a conductive mesh downstream of the filter exit
• Increase charge dissipation time by increasing reservoir size
• Adding piping length between charge generators in the system
• Increasing the filter capacity size where practical
• Use an antistatic additive if available
• Grounding pipes and hoses, especially during fluid transfer
• Replace piping that is too small

Test methods ASTM D4308 or ASTM D2624 are generally used to monitor conductivity in turbine or hydraulic oil circulating systems. Some premium turbine lubricating oils are formulated to have good electrical conductivity to help reduce surface pitting, varnish formation and fluid degradation buildup on system components.

Monitoring conductivity as part of a routine oil analysis in a circulating system also provides information on the overall increase in degradation products and contaminants in the fluid. As the general level of degradation products and contaminants increase over time, the conductivity will correspondingly increase. This information can be correlated with other test parameters for product integrity and service life.

For further information feel free to contact the ALS Tribology laboratory in your area or contact us at www.alsglobal.com/Contact-Us.