Today’s power generation market requests high efficiency for steam turbines. One key factor in fulfilling these demands is the increase of exhaust area: efficiency is increased by reducing exhaust losses and lower costs are reached by a lower number of flows. The 54″ last stage blade for a steam turbine has been developed with the application of modern design features. It was designed with integral cover, mid-span tie-boss connection, and straight fir-tree dovetail. Blades are continuously coupled by the blade untwist due to the centrifugal force, so vibration control and increased structural damping are provided. This paper describes in detail the design characteristics used in the development of the blade including non-linear structural analysis methods. The blade airfoil was optimized from point of view of minimization of its centrifugal force. The blade was well tuned in order to have natural frequencies safely away from possible excitation. Because of connection members, the number of the resonant vibration modes can be reduced by virtue of the vibration characteristics of the circumferentially continuous blades. Coupled rotor-blade frequencies were also analysed. Extensive material tests were performed to check titanium properties, which showed lower erosion and low cycle fatigue properties. New procedures were developed to improve titanium characteristics, so the material behaviour is acceptable for last stage blade construction. To validate the results, full scale rotational vibration test in vacuum chamber was carried out and suitable dynamic properties as well as mechanical integrity of the blade were confirmed.
After 10 years of operation of a steam turbine with large output power there was an accident during the turbine run-up. One of rotating blade fell off. All 6 LP rotors (two machines) were checked and many cracks on the L-1 blades were found. Due to economic reasons, blades with an identical geometry were manufactured quickly and a new material was used. A better material was chosen in terms of yield limit. The egalization of rotor grooves was performed because of manufacturing accuracy. Tip-timing measurement was installed on two L-1 stages to monitor and protect the blades. After one year of smooth operation new inspections were made. Surprisingly, it was found that the blades made of the new material had comparatively more cracks than the original blades. A new investigation has been started. This article describes measurements including rotor torsional excitation, blade tip-timing measurements, modal analysis and material tests. A computational analysis is presented in Part 2. Application of both approaches revealed what hypotheses should be rejected and, on the other hand, which of them should be analyzed in a deeper way. Consequently, the unstalled flutter has been identified as the most probable cause of blade cracks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.