Research Papers: Gas Turbines: Structures and Dynamics

Rotordynamic Computational and Experimental Characterization of a Convergent Honeycomb Seal Tested With Negative Preswirl, High Pressure With Static Eccentricity and Angular Misalignment

[+] Author and Article Information
Giuseppe Vannini

GE Oil&Gas,
Via F. Matteucci, 2,
Florence 50127, Italy
e-mail: Giuseppe.Vannini@ge.com

Carlo Mazzali

Martin Linges Vei 33,
Fornebu 1364, Norway
e-mail: cmaz@statoil.com

Harald Underbakke

Forusbeen 50,
Stavanger 4035, Norway
e-mail: hun@statoil.com

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 20, 2016; final manuscript received August 31, 2016; published online December 1, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(5), 052502 (Dec 01, 2016) (10 pages) Paper No: GTP-16-1248; doi: 10.1115/1.4034965 History: Received June 20, 2016; Revised August 31, 2016

Hole-pattern or honeycomb seals have been commonly used for many years in the Oil & Gas industry as damper seals for turbomachinery. The main motivation has been to introduce additional damping to improve the shaft rotordynamic stability operating under high-pressure conditions. Experience has shown that the dynamic and even static characteristics of those seals are very sensitive to the operating clearance profile as well as the installation tolerances. Rotordynamic stability is related not only to the seal effective damping but to the effective stiffness as well. In fact, for this kind of seal, the effective stiffness can be high enough to alter the rotor system's natural frequency. The seal stiffness is strictly related to the tapering contour: if the clearance profile changes from divergent to convergent, the effective stiffness may change from a strong negative to a strong positive magnitude, thus avoiding the rotor natural frequency drop as it is detrimental for the stability. Unfortunately, the effective damping is reduced at the same time but this effect can be mitigated using proper devices to keep the preswirl low or even negative (e.g., swirl brakes and shunt holes). This paper presents the results from an extended test campaign performed in a high-speed rotor test rig equipped with active magnetic bearings (AMBs) working under high pressure (14 krpm, 200 bar gas inlet pressure), with the aim to validate the rotordynamic characteristics of a negative preswirl, convergent honeycomb seal and demonstrate its ability to effectively act as a gas bearing as well as a seal. The test plan included variations of inlet pressure, differential pressure (given the same inlet pressure), as well as rotational speed in order to fully validate the seal behavior. This kind of test was performed in a “dynamic mode” that is to say exciting the spinning test rotor through a pair of AMBs along linear orbits. Additionally, the impact of the seal to rotor static eccentricity and the seal to rotor angular misalignment were both experimentally investigated and compared to relevant computational fluid dynamics (CFD) simulations. This kind of test was performed in a “static mode,” that is to say imposing through the AMBs the required eccentricity/angular misalignment and then measuring the forces needed to keep the rotor in the original position. Dynamic mode test was also performed in order to check the impact of the seal static eccentricity on its dynamic behavior. Finally, the test results were compared with predictions from a state of the art bulk-flow code in order to check the predictability level for future design applications.

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Fig. 1

Constant clearance seal profile with eccentric rotor

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Fig. 2

Convergent clearance seal profile with eccentric rotor

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Fig. 3

Schematic of convergent seal in tilted configuration

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Fig. 4

Seal pressure field—50% eccentricity

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Fig. 5

Seal pressure drop versus seal length

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Fig. 6

Effective stiffness for different seal preswirls

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Fig. 7

Effective damping for different seal preswirls

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Fig. 8

Effective stiffness versus shaft tilt

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Fig. 9

Effective damping versus shaft tilt

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Fig. 10

Preswirl and pressure trends during a typical experiment

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Fig. 11

Instrumented honeycomb seal

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Fig. 12

Seal inlet–outlet clearance measurements

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Fig. 13

Experimental centered versus eccentric dynamic coefficients

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Fig. 14

Effective stiffness: test versus predictions

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Fig. 15

Effective damping: test versus predictions

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Fig. 16

Conceptual scheme of static mode test

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Fig. 17

Experimental seal reaction forces due to eccentricity and tilt




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