Electrostatic Spark Discharge (ESD) is a phenomenon that occurs when oil generates sufficient static electricity to form sparks. This form of oil degradation has been studied extensively in many systems, including large frame gas turbines. Although most power plants are now aware of this form of turbine oil degradation, there is insufficient clarity on how critical of an issue ESD is.
Many people assume that ESD is caused by triboelectric charging – the reason why you can generate sparks with your fingertips in dry weather when wearing polyester clothing. Research by Dr. Sasaki however has shown that ESD is not caused by the friction of oil passing over another surface but by “internal friction” of the oil (Helmholtz Double Layer). Fast flowing, dry oil passing through small areas is at greatest risk for generating sparks. Often these sparks are generated in the lube oil filters or servo-valve last chance filters. Sometimes, the discharge of energy is so great that we can actually hear popping sounds emanating from the oil housing. This is especially prevalent when the lube oil is not yet up to operating temperature and has a higher viscosity. In most cases, ESD occurs without any audible noise because the size of the electric-arc is very small.
One reason that ESD has become such a high profile issue today is due to changes in turbine oil formulations. The use of more highly refined basestocks in most of today’s turbine oils creates a more conducive environment for ESD. Research by Dr. Duchowski has shown that oils are especially susceptible to ESD if their conductivity is <400pS/m. Group I turbine oils have conductivity values over this critical limit while most Group II and III turbine oils have conductivity values far below. Some new turbine oil formulations have conductivity values of less than 10pS/m, making it quite easy for ESD to occur.
There’s a good chance that you have sparks in your turbine oil. How serious is this? In some cases, this will lead to localized deposits in last chance filters with a black, thermo-plastic appearance. This can lead to significant impairment of the valve. In other cases, the sparks will create large holes in the main lube oil filters essentially turning your filters into screens.
The good news is that there is a cure for ESD. Utilizing specially engineered filters or making engineering changes to the system can eliminate ESD. Although ESD is not considered the primary reason for turbine oil failure and varnish formation, eliminating this degradation mechanism from your turbine oil system helps ensure long-lasting, higher performing, cleaner fluid.