Field Experiences of Gas Turbines Vibrations-A Review and Case Studies

Abstract

Blade failures represent the highest percentage of failures in gas turbines. This paper presents some typical examples of blade related failures. A literature review of common types of blade faults and research on detection methods are presented. Some methods are however less feasible under practical operating conditions in the plant. Three case studies of gas turbines vibrations are presented. The first case reports on stator blades and labyrinth glands rubs resulting in recurring shaft seizure during the coast down after load removal. Comparison of vibration spectrum undertaken a day before failure with prior data showed increase in specific blade passing frequencies (BPF) with increased side bands at intervals of the synchronous rpm. These increases were from 13 to 28 times above standard deviations of baseline datum. The stator blades rub was suspected to originate from a distorted casing. Another case relates experiences of a cracked shaft which resulted in severe rubbing during a run up. The unit experienced steady increase in vibration levels on the compressor non-drive end bearing several months prior to the incident. A full rub occurred as the unit passed through the second critical, with further development of a thermal bent shaft aggravating the problem with instantaneous severe vibration excursion. An approximate 200° phase shift was also noted. A 100 mm longitudinal crack on the shaft was found together with signs of severe rubs on the compressor blades. The third case involved time varying vibrations where vibration amplitudes and phase angles increased over a time period which dropped off after each time period; and repeated in regular cycles. This was due to oil leaks carbonization at the glands resulting in a rub. The paper concludes with a brief discussion on issues relating to relatively poor detection of blade faults in the plant.