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Research Papers: Gas Turbines: Structures and Dynamics

Hybrid Gas Bearings With Controlled Supply Pressure to Eliminate Rotor Vibrations While Crossing System Critical Speeds

[+] Author and Article Information
Luis San Andrés

Mechanical Engineering Department, Texas A&M University, College Station, TX 77843-3123lsanandres@mengr.tamu.edu

Keun Ryu

Mechanical Engineering Department, Texas A&M University, College Station, TX 77843-3123keun@tamu.edu

J. Eng. Gas Turbines Power 130(6), 062505 (Aug 28, 2008) (10 pages) doi:10.1115/1.2966391 History: Received March 28, 2008; Revised April 01, 2008; Published August 28, 2008

Microturbomachinery implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and wear quickly during transient rub events. Flexure pivot tilting pad bearings offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability; and when modified for hydrostatic pressurization, demonstrate superior rotordynamic performance over other bearing types. External pressurization stiffens gas bearings thus increasing system critical speeds, albeit reducing system damping. Most importantly, measurements demonstrate that external pressurization is not needed for rotor supercritical speed operation. In practice, the supply pressure could be shut off at high rotor speeds with substantial gains in efficiency. This paper introduces a simple strategy, employing an inexpensive air pressure regulator to control the supply pressure into the hybrid bearings, to reduce or even eliminate high amplitudes of rotor motion while crossing the system critical speeds. Rotor speed coast-down tests with the pressure controller demonstrate the effectiveness of the proposed approach. A simple on-off supply pressure control, i.e., a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed. A rotordynamic analysis integrating predicted bearing force coefficients forwards critical speeds in agreement with the test results. Predicted rotor responses for the controlled supply conditions show an excellent correlation with measured data. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of flexure pivot hybrid gas bearings.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Layout of gas bearing test rig and air supply system with controller

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Figure 2

Dimensions of flexure pivot pad hydrostatic gas bearing (units: mm)

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Figure 3

Controlled supply pressure versus rotor speed for operating conditions #2–#8

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Figure 4

Increase of controlled supply pressure over selected time span. Operating conditions #2 and #3.

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Figure 5

Determination of speed range for control of supply pressure. Right bearing horizontal direction (RH). Operating condition #1 (constant pressure supply, rotor speed coast down).

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Figure 6

Estimation of shaft speed change rate based on recorded coast-down rotor speed versus time. 2.36 bar feed pressure. Operating condition #1 (constant pressure supply, rotor speed coast down).

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Figure 7

Rotor synchronous response versus shaft speed. Right bearing horizontal direction (RH). Operating conditions #2 and #3. Manual pressure supply setting.

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Figure 8

Rotor synchronous response versus shaft speed. Right bearing horizontal direction (RH). Operating condition #4. step-wise manual pressure supply setting.

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Figure 9

Rotor synchronous response versus shaft speed. Right bearing horizontal direction (RH). Operating conditions #5 and #7. Controller activated system.

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Figure 10

Rotor synchronous response versus shaft speed. Right bearing horizontal direction (RH). Operating conditions #6 and #8. Controller activated system.

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Figure 11

Comparison of predicted and measured imbalance response of test rotor for increasing supply pressures. Right bearing horizontal direction (RH).

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Figure 12

Comparison of predicted and measured imbalance responses. Measurement for operating condition #5 and prediction for one-step increase in supply pressure (2.36 bar) at 14.2 krpm. Right bearing horizontal direction (RH).

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Figure 13

Comparison of predicted and measured imbalance responses. Measurement for operating condition #4 and prediction for four-step increase in supply pressure (2.36 bar) from 13.9 krpm to 13.0 krpm. Right bearing horizontal direction (RH).

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Figure 14

Comparison of predicted and measured imbalance responses. Measurement for operating condition #6 and prediction for ramp increase in supply pressure (2.36 bar) from 14.2 krpm to 13.0 krpm. Right bearing horizontal direction (RH).

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Figure 15

Predicted gas bearing static journal eccentricity for step, multiple step-wise, and ramp increase in supply pressure. Static load W=4 N.

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Figure 16

Predicted gas bearing attitude angle for step, multiple step-wise, and ramp increase in supply pressure. Static load W=4 N.

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Figure 17

Comparison of gas bearing direct stiffnesses for step, multiple step-wise, and ramp increase in supply pressure. Right bearing horizontal direction.

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Figure 18

Comparison of gas bearing direct damping coefficients for step, multiple step-wise, and ramp increase in supply pressure. Right bearing horizontal direction.

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