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Introduction to fluid film bearings and seals

Introduction to fluid film bearings and seals -...

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Introduction to bearings and seals. Dr. Luis San Andrés © 2006 1 Introduction to fluid film bearings and seals A turbomachinery is defined as a rotating structure where the load or the driver handles a process fluid from which power is extracted or delivered to. Examples of turbomachines include pumps and compressors, gas and steam turbines, turbo generators and turbo expanders, turbochargers, APU (auxiliary power units), etc. Most turbomachinery is supported on oil lubricated fluid film bearings, although modern advances and environmental restrictions are pushing towards the implementation of process fluid bearings and even gas bearing applications. Fluid film bearings are used due to their adequate load support, good damping characteristics and absence of wear if properly designed and operated. Turbomachines also include a number of other mechanical elements which provide stiffness and damping characteristics and affect the dynamics of the rotor-bearing system. Impeller seals, floating ring seals, thrust collars and balance pistons are a few of these elements. The adequate operation of a turbomachine is defined by its ability to tolerate normal (and even abnormal) vibrations levels without affecting significantly its overall performance (reliability and efficiency). The rotordynamics of turbomachinery encompasses the structural analysis of rotors (shafts and disks) and the design of fluid film bearings and seals that determine the best dynamic performance given the required operating conditions. This best performance is denoted by well-characterized natural frequencies (and critical speeds) with amplitudes of synchronous dynamic response within required standards and demonstrated absence of subsynchronous vibration instabilities. A rotordynamic analysis considers the interaction between the elastic and inertia properties of the rotor and the mechanical impedances from the fluid film bearing supports, oil seal rings, seals, etc. The most commonly recurring problems in rotordynamics are 1. Excessive steady state synchronous vibration levels. 2. Subharmonic rotor instabilities. Steady state vibration levels may be reduced by: a) Improving balancing. b) Modifying rotor-bearing systems: tune system critical speeds out of RPM operating range. c) Introducing damping to limit peak amplitudes at critical speeds that must be traversed. Subharmonic rotor instabilities may be avoided by: a) Raising the natural frequency of rotor system as much as possible. b) Eliminating the instability mechanism, i.e. change bearing design if oil whip is present. c) Introducing damping to raise onset speed above the operating speed range.
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Introduction to bearings and seals. Dr. Luis San Andrés © 2006 2 Rotordynamic instabilities have become more and more common as the speed and horsepower of turbomachinery have increased. These instabilities can sometimes be erratic, seemingly increasing vibration amplitudes for no apparent reason. A common denominator among many stability problems is that they tend to grow with time as the affected component(s) begins to wear or fatigue.
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