Labyrinth Seal and Pocket Damper Seal High Pressure Rotordynamic Test Data

Abstract

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.

Orbit Decomposition Method for Rotordynamic Coefficients Identification of Annular Seals

“…To further validate the orbit decomposition method, its prediction results are compared with the experimental data tested on the high-pressure seal test rig owned by GE Oil & Gas [17]. The test object is the labyrinth seal.…”

“…Five whirl frequencies are measured with a 10,000 rpm running speed. The detailed test conditions can be found in [17].…”

“…With this method, the obtained rotordynamic coefficients are frequency independent [13,14]. The latest research results show that many annular seals possess frequency-dependent rotordynamic coefficients, including the labyrinth seal [16,17], pocket damper seal [17], hole-pattern seal [18,19], etc. Interaction between the acoustic waves in a circumferential annulus and the rotor excitation frequency is the cause of the frequency dependency [20].…”

Experimental Investigation on Rotordynamic Characteristics and Rotor System Stability of a Novel Negative Dislocated Seal

“…Relative to the labyrinth seal, the stator surface of this seal has larger roughness and bigger damping coefficient, which contribute to reduce circumferential flow velocity in the seal, decrease seal cross stiffness, and improve rotor system stability. At present, existing damping seals include honeycomb seal [14,15], hole-pattern seal [16], and pocket damper seal [17].…”

Numerical comparisons of rotordynamic characteristics for three types of labyrinth gas seals with inlet preswirl

“…The above experimental and numerical results of rotordynamic coefficients for the labyrinth seal are all assumed to be frequency-independent. However, the latest test results of Ertas et al 22 and Vannini et al 23 have demonstrated that the labyrinth seal possesses strongly frequency-dependent rotordynamic coefficients, especially for the direct stiffness and effective damping coefficients. The numerical results presented by Thorat and Childs 24 show frequency dependence of stiffness and damping coefficients of labyrinth seals when rotor speed approaching to Mach 1.…”