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Research Papers: Gas Turbines: Structures and Dynamics

Labyrinth Seal and Pocket Damper Seal High Pressure Rotordynamic Test Data

[+] Author and Article Information
Giuseppe Vannini

e-mail: giuseppe.vannini@ge.com

Stefano Cioncolini

e-mail: stefano.cioncolini@ge.com

Giuseppe Del Vescovo

e-mail: giuseppe.delvescovo@ge.com

Massimiliano Rovini

e-mail: massimiliano.rovini@ge.com

GE Oil & Gas,
Via F. Matteucci, 2,
Florence 50127, Italy

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 7, 2013; final manuscript received August 4, 2013; published online October 25, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 022501 (Oct 25, 2013) (9 pages) Paper No: GTP-13-1238; doi: 10.1115/1.4025360 History: Received July 07, 2013; Revised August 04, 2013

The current centrifugal compressor design for the oil & gas market is more and more challenging, and the presence of many competitors is pushing technology towards both a casing size reduction and a rotational speed increase. The first point is leading to an increase in the number of wheels per rotor (to do the same service), and the second point is forcing to cross two or even three rotor modes (hence a higher control of rotor damping is necessary). The two points together are leading to increase the rotor “flexibility ratio” (defined as the ratio between the maximum continuous speed and the first critical speed at infinite support stiffness according to API standard, and finally the rotordynamic stability is very much challenged. The centrifugal compressor's rotordynamic stability is strongly related to the internal seals' dynamic behavior, and for this reason, the authors' company decided several years ago to develop internally a high pressure seal test rig to measure internal seals stiffness and damping. The rig is now in operation, and in a previous paper the authors described its main capabilities, the applied identification procedures, and the preliminary test results captured for a long labyrinth seal (smooth rotor, straight toothed stator) tested up to 200 bar. This paper is intended to show more data for the same long Laby with special focus on some peculiar test as negative preswirl test, single frequency versus multifrequency test, offset versus centered seal test. The negative preswirl test shows a drastic change in the effective damping (from destabilizing to stabilizing) and provides a support in favor of the selection of swirl reversal devices at seals upstream. The multifrequency excitation test approach (based on the concurrent presence of several frequencies not multiples at each other) is compared with a single frequency excitation providing similar results and thus confirming the soundness of the multiple effects linear superimposition assumption. The effect of a static offset (simulating the real position of a rotor inside an annular seal) is also investigated proving that the relevant impact is negligible within the range of eccentricity explored (10% of seal clearance). Moreover, a pocket damper seal (PDS) with the same nominal diameter, clearance, and effective length has been tested (up to 300 bar) and compared with the Laby. As expected, the PDS shows both a higher effective stiffness and damping at the same test conditions, so the promising results already collected in a previous test campaign which was performed on a smaller scale and lower pressure test rig were mostly confirmed with the only exception for the effective damping crossover frequency which was lower than expected.

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Figures

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Fig. 2

3D section of casing with internal bundle

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Fig. 4

Pocket damper seal

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Fig. 5

Direct stiffness coefficients: (+) versus (−) preswirl

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Fig. 6

Cross-coupled stiffness coefficients: (+) versus (−) preswirl

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Fig. 7

Direct damping coefficients: (+) versus (−) preswirl

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Fig. 8

Effective damping coefficients: (+) versus (−) preswirl

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Fig. 9

WFR: (+) versus (−) preswirl

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Fig. 10

Predictions versus measurements

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Fig. 11

Direct stiffness: multi versus single frequency

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Fig. 12

Cross coupled stiffness: multi versus single frequency

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Fig. 13

Direct damping: multi versus single frequency

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Fig. 14

Cross coupled damping: multi versus single frequency

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Fig. 15

Overall orbit versus emergency bearing clearance

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Fig. 16

Cross coupled stiffness: offset versus centered

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Fig. 17

Direct damping: offset versus centered

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Fig. 18

Direct stiffness: laby versus PDS

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Fig. 19

Cross coupled stiffness: laby versus PDS

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Fig. 20

Direct damping: laby versus PDS

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Fig. 21

Effective damping: laby versus PDS

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Fig. 22

Leakage data: laby versus PDS

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