High-pressure gas passing through labyrinth seals, shrouds, and impellers generates cross-coupled aerodynamic forces. These forces act perpendicular to rotor displacement, feeding energy into the system and destabilizing the rotor. 2. Modeling and Simulation Methodology
axes) at a bearing location, engineers can view the actual path of the shaft centerline. The shape of this orbit (circular, elliptical, banana-shaped, or figure-eight) provides an immediate visual signature of unbalance, misalignment, rubbing, or oil whirl.
Occurring in hydrodynamic journal bearings, oil whirl happens when the fluid film drives the journal to vibrate at approximately 42% to 48% of the running speed. If the machine's speed increases to twice its first natural frequency, oil whirl locks into the critical speed, becoming oil whip , an instability that can rapidly destroy a machine.
A 6-stage back-to-back centrifugal compressor operating in a gas reinjection plant experienced sudden, catastrophic high-amplitude vibrations whenever discharge pressure exceeded 350 bar. turbomachinery rotordynamics with case studies pdf
Deals with bending vibrations, critical for determining operating speeds.
Upon startup after a short shutdown (hot restart), a large steam turbine experienced severe synchronous (1X) vibration. Diagnosis: Because the turbine cooled unevenly, the rotor developed a thermal bow
Case Study 3: Thermal Mortensen Effect in a Subsea Multiphase Pump Modeling and Simulation Methodology axes) at a bearing
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High-pressure impellers and labyrinth seals generate tangential fluid forces perpendicular to rotor displacement. This cross-coupling acts as a negative damping mechanism, destabilizing the rotor. Torsional Rotordynamics
A modern high-speed compressor exhibited strong subsynchronous vibration at a frequency of about 0.5 times running speed, causing high bearing loads and eventual machine trips. The vibration increased as discharge pressure was raised. If the machine's speed increases to twice its
, the system is unstable; vibrations will grow exponentially. API 617 requires a minimum log dec (typically >0.1is greater than 0.1
Finite Element Method (FEM) vs. Transfer Matrix Method (TMM)
A crucial goal of rotordynamics is identifying —rotational speeds that coincide with the natural frequencies of the rotor system. Operating at or near these speeds causes resonance, resulting in high vibration amplitudes. Rotordynamic Stability