On-Site Identification of Dynamic Annular Seal Forces in Turbo Machinery Using Active Magnetic Bearings: An Experimental Investigation

Abstract

Significant dynamic forces can be generated by annular seals in rotordynamics and can under certain conditions destabilize the system leading to a machine failure. Mathematical modeling of dynamic seal forces are still challenging, especially for multiphase fluids and for seals with complex geometries. This results in much uncertainty in the estimation of the dynamic seal forces which often leads to unexpected system behavior.This paper presents the results of a method suitable for on-site identification of uncertain dynamic annular seal forces in rotordynamic systems supported by Active Magnetic Bearings (AMB). An excitation current is applied through the AMBs to obtain perturbation forces and a system response, from which, the seal coefficients are extracted by utilizing optimization and a-priori information about the mathematical model structure and its known system dynamics. As a study case, the method is applied to a full-scale test-facility supported by two radial AMBs interacting with one annular center mounted test-seal. Specifically, the dynamic behavior of a smooth annular seal with high preswirl and large clearance (worn seal) is investigated in this study for different excitation frequencies and differential pressures across the seal. The seal coefficients are extracted and a global model on reduced state-space modal form are obtained using the identification process. The global model can be used to update the model based controller to improve the performance of the overall system. This could potentially be implemented in all rotordynamic systems supported by AMBs and sub- *

Design of Active Magnetic Bearing Controllers for Rotors Subjected to Gas Seal Forces

“…A method for identification of uncertain/unknown seal and electromechanical parameters, to obtain a precise global model of an AMB-rotor-seal test facility is presented in [30]. The test facility is described in [31].…”

“…This method could potentially be implemented in all rotor dynamic systems supported by AMBs and subjected to seal forces. This paper uses the rotor dynamic model with in-situ identified seal coefficients, presented in [30], to design model based controllers, for handling dynamic seal forces. The idea is to enhance the performance of the global system without increasing the direct stiffness or damping of the system.…”

“…3 Mathematical Modeling, Perturbed Plant and System Identification This section presents details about the mathematical system, the perturbed system representation and the results of system identification -all related to the models used for control synthesis. The modeling and identification of uncertain/unknown parameters of the test facility are described in detail in [30].…”

“…where K i are K s are constants. Initial estimates of K i and K s have been obtained using first principle methods and identified as shown in [30]. The dynamics of the electromechanical system, including the inductance of the coil and the amplifiers, is approximated using a second order model, denoted G act…”

“…The nominal model can be extended to include changes or uncertainties in the plant dynamics. This is utilized for identification of uncertain AMB parameters K i , K s as well as unknown seal parameters K, k, D, d, as described in [30]. In this paper, Section 4, the perturbed system representation is utilised for the design of robust controllers.…”