An Analysis Of The Vibration Characteristics And Service Life Of Shrouded Blades For The Wetness Removal Stage Of A Marine Steam Turbine

By way of theoretical analysis and tests a study is conduced of the vibrationcharacteristics and fatigue failure of shrouded blades in the wetness removal stageof a marine steam turbine.Blade safety and reliability constitutes an important guarantee for the safeoperation of a steam turbine. Blade vibrations and the resulting fatigue damage are amajor cause leading to the failure of the blades of a rotating machine. The wetnessremoval stage fitted with shrouded blades operates under rigorous conditions of highrotating speeds, changing loads and wet steam, etc. Furthermore, the structure of theshrouded blades is markedly different from that of traditional ones. The systematicanalysis of its characteristics, such as strength, vibration and fatigue failure, etc is ofmajor realistic significance.A nonlinear damping model has been used during the present study and a bladestructure integral dynamics equation set up for the blade three-dimensional entityunit with collision damping. An investigation was undertaken of the mechanismbased on the collision vibration-abatement of flat-plate blades. The vibrationcharacteristics of two or three blades were calculated and analyzed under theconditions of different clearances, different vibration-exciting force amplitudes andfrequencies. The natural frequencies of the shrouded blades were calculated with apenalty function method being introduced. An analysis was carried out of theinfluence of such factors as the shroud structure and mass of the shrouded blades,blade shroud clearance and the longitudinal groove of the blade body, etc. Theresults of the analysis indicate that the collision of blades exhibits nonlinearvibration features with the appearance of distinct frequency spectrum components.The blade with collision conditions can suppress doing-work by external forces. Theability to do work by external forces will decrease with the decrease in blade shroudclearance. The blade shroud clearance exercises a great influence on the bladedynamic stress, showing in this connection a distinct regularity. When theabove-mentioned clearance is constant, the corresponding maximum dynamic stresswill not change with an increase in a vibration-exciting force. The longitudinalgroove of the blade body has a relatively small influence on blade intrinsiccharacteristics. However, there appears a phenomenon of stress concentration at thegroove location. A contrast study was also conducted of damage mechanics theory andaccumulative damage theory. In addition, by applying a full coupled damagemechanics theory an analysis was performed of the fatigue life and damage of steamturbine blades. The accumulative damage theory has overestimated blade servicelife during its service life prediction. Meanwhile, a multi-axial complex stress statehas accelerated the damage evolution of blade material, thus shortening the fatiguelife of blades. A continuous damage accumulation model can more accurately reflectthe evolving process of nonlinear damage accumulation. The use of the damagemechanics method can result in a more precise description of the damage andservice life of structural members and more realistically reflect the damagedevelopment process of materials. Compared with the now universally adoptedaccumulative damage theory, the above-mentioned method is more rational both inrespect of damage theory model or analytical method. The experimental research has mainly been focused on the blade vibrationabatement mechanism and vibration characteristics. By utilizing a test rig andmeasurement system being set up an experimental study was conducted of themechanism of the flat-plate blade vibration abatement and the dynamiccharacteristics of the shrouded blades. The results of the study indicate that thecollision condition can lead to the emergence of distinct frequency spectrumcomponents in blade vibrations. The collision condition can also suppress the doingof work by external forces. The whole ring of shrouded blades installed withclearances has the natural frequency characteristics of a single blade. A decrease inblade shroud clearance can effectively reduce vibration stresses. The results of the present study have laid a solid foundation for the design ofshrouded blades and the analysis of their safety and reliability for the wetnessremoval stage of a marine steam turbine. Some beneficial methods for furtherresearch are also provided.