Protecting Turbomachinery From Self-Excited Rotor Whirl

Abstract

Aerodynamic exciting forces have caused severe rotor whirl of axial compressors and turbines. One disturbing force investigated is due to circumferential variation of static pressure acting on the cylindrical surface of rotor, particularly within labyrinth seals. Another aerodynamic disturbing force is due to eccentricity of rotor causing circumferential variation of blade-tip clearance, and a corresponding variation of local efficiency and unbalanced torque. Seal deflection criteria and torque deflection criteria are presented as design guides for stable rotor systems. These criteria, the form of which comes from analysis of rotor dynamics, correlate design parameters of four examples of unstable rotor systems which exhibited whirl.

Numerical Estimation of Torsional Dynamic Coefficients of a Hydraulic Turbine

“…The later coefficients are added to the mechanical system, that is, (2). The polar inertia of the mechanical system is J P = 1.57 Nms 2 , the damping is C = 0 Nms, and the stiffness is K = 49000 Nm.…”

“…He suggested an analytical model of destabilising forces due to nonsymmetric clearance in steam turbines. Alford [2] developed a similar model for compressors, where the forces are obtained as a function of the change in efficiency due to increased eccentricity. Urlichs [3] carried out the first research in a test rig and suggested corrections to Thomas and Alford's models.…”

Differentiating Rotor Response Due to Radial Rubbing

“…This unsymmetry indicates that the rotor is potentially unstable as described by Black (1) and Alford (2). Depending on the strength of the tangential friction cross-coupling force, acting opposite to the spin direction, the rotor can go unstable very abruptly in a backward precession in some systems, while in others it may act as a deflection limiter.…”

Rotordynamic Stability Case Studies

“…Self-induced vibrations are caused by forces controlled by the motion itself as opposed to forced vibration which is a function of only time (Boeker and Sternlicht, 1956). Such whirling motion may be caused by external factors such as internal damping (Gunter, 1966), hydrodynamic forces in fixed geometry journal bearings (Reddi and Trumpler, 1962;Newkirk and Taylor, 1925), or oil seals (Kirk and Miller, 1979 (Alford, 1965) as well as centrifugal compressors (Wachel, 1982;Sood, 1979). Subsynchronous vibration in centrifugal compressors may occur in high gas pressures and gas densities applications beyond certain operating conditions and speeds.…”