Shaft Assembly in Aero-derivative Gas Turbine System
In a Valve-Shaft system, aerodynamic force induced torque acts on the disk causing dynamic valve motion. The response of the mechanical system to such a torque load is called "forced response". The aerodynamic torque or flow-induced torque in the butterfly valve is strongly dependent on: (1) disk geometry, (2) the disk opening position, (3) the operating pressure ratio, (4) the local piping configuration, and (5) material density. In this study, aerodynamic load and the geometry of the valve with various densities are modeled with the Patran. Dominant natural frequencies and modal of the valve-shaft are determined. Through a rigorous computational dynamic analysis. The natural frequency can then be checked with loading and the design to avoid the occurrence of the resonance.
MODELING OF VALVE-SHAFT SYSTEM
The disk and the associated shaft are modeled by solid finite elements. The dominant natural frequency and associated modal shapes of a given loading are obtained. A geometric model for the valve disk and shaft system has been constructed by using Patran as shown in Fig.1. The finite element mesh used contains 1,568 hexahedra elements and 2,034 nodes. For reference purpose, stainless steel is used first as the model material for the dynamic analysis of the disk and the shaft. The result of the first mode with 3-D solid finite element model also shown in Fig.1
EFFECT OF VARIOUS DISK DENSITY
To study the weight effect on dynamic response, the disk is modeled with various densities but with steel rigidity. The shaft is kept as steel material. The peak dynamic torque in the shaft will be reduced 5.6%, 9.7% and 8.9% with disk density 0.75r, 0.5r and 0.25r, respectively, comparing to peak torque with full steel density r as shown in Fig. 2. The low density disk will reduce the inertial effect and reduce the peak dynamic response. However, the reductions will be less effective while reducing disk density further.
Dynamic responses are also studied by modeling the disk-shaft with various composites disk (reduced the disk density) but with steel shaft. The peak dynamic torque for carbon/epoxy composite disk will be reduced by 9% comparing to that of steel case. System with composite disk with a stiffer shaft (as steel) seems a good candidate for improving dynamic response.
Fig.1 Disk-shaft system deformation in the First Mode with 3-D solid finite element model.
Fig 2. Comparison of aerodynamic torque response for various disk densities with steel shaft.
Dr. Tung-Pei Yu
University of Houston