Recently we have faced with a shaft and bearings failure of one gas-turbine generator. The failure limited to the area under jurnal bearings.The machine specialists have the idea that the failure is wire wool(black scab) failure.I want to know more about this kind of failure, the cause and how to prevent it. masoud

Mahyar Journal bearing failure of gas turbines - can result due to typical contaminants in oil filtration system viz., dirt, water, sludge. Dirt comes from dust from surrounding atmosphere, rust and wear products from seal leaks. Water from : condensation, seal leaks. cooler leaks and gland leaks Sludge: from oxidation of oil due to high temperature, accumulation of fine particles and silt forming at the space between journal races. In hydrodynamic bearings there is a possibility for high frictional loss and temperature rise with large concentration of small particles. This can lead to situations of wire wool or black scab to damage journal bearing life drastically. For this filtration failure should precede to allow fine particles to accumulate in the space between bearing races. For failure of shaft, it could be an effect due to magnetic shaft current problem from moisture entry. Shaft current problem comes if inadvertent welding had been done i.e., welding jobs done to nearby pipeline or structures (close to gas turbine machinery) without proper ground or earthing arrangement to casing of gas turbines during shut down time. Please check entry of moisture in bearing housing. Shaft current and bearing failure in gas turbine machinery is a known identified cause. De-magentization of the rotor and casing is the best solution for shaft current problem and consequent magnetization of rotor and bearing assemblies. For journal bearing problem, i.e., dirt, water, sludge entry - look at the oil filtration system carefully and also oil cooler and seal/ gland leaks. Hope this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India May 5, 2008 E-mail: nishi@vsnl.com >Recently we have faced with a shaft and bearings failure of >one gas-turbine generator. >The failure limited to the area under jurnal bearings.The >machine specialists have the idea that the failure is wire >wool(black scab) failure.I want to know more about this kind >of failure, the cause and how to prevent it. >masoud