Assessment of fatigue damage in steam turbine shafting due to torsional vibrations

Abstract

We present a technique and results of fatigue damage evaluation of steam turbine shafting material under conditions of torque vibrations occurring at turbogenerator abnormal modes of operation (short circuit, random paralleling start, etc.). Combinations of loading parameters, which provide the shafting limiting conditions, are identified.

Torsional Vibrations and Damageability of Turboset Shaftings under Extraordinary Generator Loading*

“…In this case, a considerable dynamic load arises under extraordinary generator operation conditions [short circuits (SC), switching into a network with rough synchronization, quick reswitching after the cessation of the action of short circuit, etc.]. On short circuits, the generator rotor is acted upon by considerable electromagnetic torques for 0.01-0.265 s [3,4], which exceed the nominal moment by a factor of 8-10 in the most unfavorable cases.The effect of torsional vibrations of shafting, which arise under extraordinary turbogenerator operation conditions, on the strength of steam turbine rotors was considered in [5][6][7]. The investigations of transient processes were carried out numerically for the shafting of a 200 MW steam turbine.…”

“…In [5], a three-dimensional finite element model of shafting was employed without regard for the influence of the generator rotor in the case of loading with a short rectangular pulse, which exceeds the nominal moment by a factor of six. In [6,7], the fatigue damageability due to torsional vibrations was assessed numerically with the aid of a simplified discrete model of shafting with four localized masses, which had moments of inertia and were connected by elastic elements. The effect of the parameters and form of torque burst on the degree of damageability of the shafting material was studied.…”

“…The effect of torsional vibrations of shafting, which arise under extraordinary turbogenerator operation conditions, on the strength of steam turbine rotors was considered in [5][6][7]. The investigations of transient processes were carried out numerically for the shafting of a 200 MW steam turbine.…”

“…Transient torsional vibrations of turboset shaftings are considered for the case of twoand three-phase short circuit. In contrast to [5][6][7], the shafting sections are modeled by bars with piecewise constant cross section with distributed mass; the rotor disks and flanges are represented by localized masses and moments of inertia. The problem was solved for 325 and 220 MW turbosets as an example, schematics of whose shaftings are shown in Fig.…”

“…The first and second ones are the most typical among them, and the eigenfrequencies corresponding to them are close together (0.31 and 0.5). In the range up to 3w there are six eigenfrequencies and six torsional vibration modes; therefore, the use of four-mass models [5,6] in the calculation of such systems is unsuitable if it is taken into account that the main power of their response to the action in question is contained in this frequency range, as was shown in research. Moreover, discrete models do not reflect the wave nature of dynamic processes due to suddenly applied short-circuit torques.…”