Electromagnetic Shaft Current Problems A User's Viewpoint

PaperPg. 191-197.This article covers a user’s experience, viewpoint and resolution, with destructive stray shaft currents on a centrifugal split-case design compressor. The compressor is driven by a 33,000 HP 13-stage condensing turbine. Shaft currents have been noted by many rotating equipment users for years and had been mostly associated with wet turbine steam causing a static current. A common type grounding brush, in continuous contact with the shaft, was used to bleed off all excess currents to ground. These brushes generally proved successful where “static” conditions were present. However, at times severe currents were generated causing quick bearing deterioration. The brushes simply could not carry the current away. Approximately ten years ago, a theory that stray currents could also be generated by high magnetic levels and fields within the equipment itself evolved. Currents generated in this manner were then termed “electromagnetic” in nature. Electromagnetic currents are definitely more destructive than static currents. The tell-tale signs are distractive from static currents as metal can actually be removed from the shaft, definite erosion (frosting) patterns are set up and frosting can and does appear at different locations throughout the machine during the magnetic destructive process. These stray currents have been present on this particular compressor since plant start-up in September of 1971. Necessary outages, as a direct result of bearing failure, have occurred as frequently as one month and as infrequently as 1 ½ years. Many paths have been followed in the attempt to resolve the bearing failures prior to implementing the electromagnetic current history. Solutions ranged from installation and modification of current drain brushes, to a total radial bearing redesign by the original equipment manufacturer (OEM). The following represents Northern Petrochemical Company’s (NPC) complete history and thrust into the electromagnetic theory and a final resolve