Joseph A. Tecza

Analysis of Fluid-Structure Interaction in a Steam Turbine Throttle Valve

Throttle valves in steam turbines often operate at very small lift positions during turbine startup. The large pressure differentials across these valves, combined with the very small openings at the valve seat, result in large pressure drops across these valves and high local steam Mach numbers. A steam turbine throttle valve operated under these conditions was found to be undergoing self-excited vibration. Stress and structural dynamic finite element analyses (FEA) were performed to identify the structural mode for the valve oscillations. A three-dimensional transient computational fluid dynamics (CFD) analysis of the valve revealed an unsteady fluid dynamics phenomenon in the pressure balancing arrangement that served as a forcing function for this vibration. Valve modifications were implemented as a result of these analyses. The improved valve has performed successfully, and the design modifications have been incorporated in other production valves.Copyright

Modeling and Evaluation of Centrifugal Compressor Performance Variations Using Probabilistic Analysis

This paper presents a methodology for analyzing the variation in compressor stage performance due to component dimensional variations for a range of flows, speeds, gas compositions and scale factors. Due to the large number of input parameters involved, a Design of Experiments (DOE) approach was used to develop key variables, and to develop response surface models of head coefficient and efficiency in terms of these key variables. These response surface-based performance models then are used for a probabilistic analysis of head coefficient and efficiency as functions of dimensional variations, for a range of compressor sizes. The variations in dimensions are expressed as probability distributions and evaluated using a Monte-Carlo integration technique. The techniques for developing the response surfaces and performing the probabilistic analysis are described, as are methods for evaluating both the effects of dimensional variation on performance and for evaluating how much dimensional variation can be tolerated before the variation exceeds established limits.© 2005 ASME

Statistical Analysis of Damper Seal Clearance Divergence and Impact Upon Rotordynamic Stability

Rotordynamic stability in a high-speed, high-pressure centrifugal compressor is achieved through introducing stabilizing forces and managing destabilizing forces. It becomes the original equipment manufacturer’s (OEM) responsibility to provide sufficient stabilizing influence to ensure acceptable operational behavior. A variety of mechanisms exist to provide stability: damper bearings, de-swirl elements on seals, shunt holes and damper seals are commonly provided by the compressor manufacturer to combat instability. The application of damper seals provides a significant increase in rotordynamic stability if designed properly. A previous case study (Camatti, et. al. [1]) presented the detrimental effect of damper seal clearance divergence and seal gas pre-swirl upon predicted stability. Accurate prediction of rotordynamic stability requires knowledge of the destabilizing influences, and evaluation of the variation possible in the stability promoting elements. In particular, the damper seal can be strongly influenced by the presence of divergence in the seal-to-rotor clearance and can result in unexpected at-load behavior. One approach to ensure stable operation is to perform a “worst case” evaluation. However, this often results in excessive compromise (large seal clearance and impact on aerodynamic performance) to “eliminate” the possibility of unstable operation. A different and new approach is to define an acceptable envelope of probability. This study presents a statistical evaluation of the impact of damper seal clearance divergence, along with other compressor design parameters, and provides a method for ensuring stable operation regardless of manufacturing or operational variation. The logarithmic decrement is evaluated as a function of compressor design values and a statistical response surface is created. From the response surface and defined variation in design values, a measure of probability for instability can be obtained. Allowable parameter variation can be restricted to ensure stable operation. Three case studies will be presented where the potential for instability exists and the risk quantified using this statistical technique. Additionally, in one of these cases, test results are presented to support the analytical work.Copyright

Dynamic Evaluation of a Three Point Mount Baseplate for a Motor Driven, Centrifugal Compressor Package

Maintaining acceptable alignment can be an issue for rotating machinery on off-shore floating platforms. Distortion arising from motion within the platform can change the alignment of the production machinery if that machinery is mounted rigidly to the platform. One way to eliminate the effects of such distortion is to attach the machinery on its own baseplate, which is connected to the platform using a three point mounting system. Anti-Vibration Mounts (AVMs) are used at each support location, and these devices are effective in isolating the baseplate and its machinery from the surrounding structure. The package discussed in this paper consists of a centrifugal compressor, a gear, and a 6.3 MW (8500 hp) variable speed motor (1000–1890 RPM) mounted on a baseplate. The equipment train has vibratory modes that could not be moved from the operating speed range. Some of these mode shapes show coupled motion between the motor and the baseplate. Test results were compared to the detailed Harmonic Response Finite Element Analysis (FEA) of the coupled structure, which predicts both baseplate resonance and motor rotor response amplitudes. Both analysis and testing demonstrate that these modes do not have a significant effect on package operation.Copyright