Requirement for higher turbine performance and lower costs has resulted in smaller lubrication and control fluid systems operating at higher cleanliness levels, thus requiring higher fluid flux and finer filtration levels. Triboelectric Charge (TEC) generation by fine filters and its eventual discharge is considered to be a source of lubricating fluid breakdown and the resulting formation of resinous byproducts. These fluid breakdown byproducts form deposits or ‘varnish’ on the metallic surfaces causing servo-valve stiction along with other adverse consequences.
TEC generation occurs in fluid systems as a result of friction between the fluid and mainly the filtration media fibers. The friction causes charge separation between the filtration medium and the fluid, with the fluid oppositely charged to the filtration medium. The magnitude of charge generated depends on many interrelated factors, including the nature of the filter material and the fluid, fluid velocity, viscosity, conductivity and the contact area. The electrostatic discharge manifests itself in several ways, the most easily noticeable way being an audible clicking noise as the discharge of the accumulated electrostatic charge causes sparking internally within the system. In addition to lubricant degradation and varnish formation, the electrical charge can travel downstream with the fluid causing damage to system components, including the filter. There have been reports of heat exchanger damage, located downstream of the filters, due to the electrostatic charging of the fluid. The use of metal support meshes or other methods to dissipate the electrostatic charge to prevent its build-up in the filter cartridge may prevent the discharge within the filter assembly and the resulting filter damage, but it does not prevent the charging of the fluid, and the migration of the charge downstream causing damage to the fluid and the downstream components. In 2005 Pall Corporation introduced filter cartridges that mitigate charge generation and dissipate the small charge generated at its inception, thus offering the advantage of eliminating filter damage entirely, and significantly reducing the migration of the charge into the fluid and its associated damage to the fluid and the resulting varnish formation. This paper discusses the experience with the electrostatic charge dissipating filtration for the gas turbine lubrication application.