Torres, Jose Maria (2016-12). Static and Rotordynamic Characteristics of Liquid Annular Seals with a Circumferentially-Grooved Stator and Smooth Rotor using Three Levels of Circumferential Inlet-Fluid Rotation. Master's Thesis. Thesis uri icon

abstract

  • Original Equipment Manufacturers (OEMs) increase pump efficiency by reducing process fluid leakage from high-pressure stages into low-pressure ones. Smooth, liquid annular seals are used between pump stages to achieve this goal. In an effort to reduce leakage, OEMs sometimes machine circumferential grooves in the stators of annular liquid seals. Unfortunately, grooved seals do more than improve pump efficiency; they sometimes help degrade the system's (pump, motor) rotordynamics, causing adverse effects that overshadow its helpful qualities. The rotordynamic community recognizes that fluid rotating in the shaft direction, at the entrance of the seal, is a source instability. The relevant literature lacks test results showing how high levels of inlet-fluid rotation affect a grooved seal's performance, and how this effect changes as the shaft operates very close to the stator. The present study addresses this lack. Supplied with VG2 oil @ 46 ?C (115 ?F), the grooved seal used for this investigation has a length-to-diameter ratio L/D of 0.5, and a minimum radial clearance Cr of 203 ?m (8 mil). It features 15 circumferential grooves with a length Gl, and depth Gd of 1.52 mm (60 mils), which are equally-spaced by a land length of 1.52 mm (60 mils). The experimenter conducts tests at shaft angular speeds w of 2, 4, and 6 krpm, eccentricity ratios e0 of 0.00, 0.27, 0.53, and 0.80, and axial pressure drops ?P of 2.1, 4.1, 6.2, 8.3 bar (30, 60, 90, 120 PSI). Using 3 distinct inlet-fluid rotation inserts, the author induces increasing levels of circumferential fluid velocity at the seal's inlet. Pre-swirl ratio (PSR) and outlet swirl ratio (OSR) are defined as the ratio of circumferential velocity at the seal's inlet and outlet, respectively, to the rotor's tangential surface velocity. To assess the seal's static performance, the author measures leakage rate Q, eccentricity ratio e0, PSR, and OSR. To assess the seal's dynamic performance, the author measures stator-rotor relative displacement, stator acceleration, and dynamic excitations. The author uses the dynamic measurements to calculate the seal's rotordynamic coefficients and Whirl Frequency Ratio (WFR). Finally, the author calculates effective stiffness and damping coefficients to compare the grooved seal's rotordynamic performance to that of a smooth seal with the same Cr, L/D, and operating conditions. In regards to static performance, the grooved seal's leakage rate ranges from a low 15.64 LPM (4.13 GPM) at w = 6 krpm, and ?P = 2 bar (30 PSI), to a high 56.36 LPM (14.16 GPM) at w = 2 krpm, and ?P = 8 bar (120 PSI). When compared to the smooth seal, the grooved seal provides a 20% Q reduction at w = 2 krpm, and a 6% reduction at w = 6 krpm. Test results show all of the smooth seal's rotordynamic coefficients increase markedly for e0 > 0.50, while those of the grooved seal generally remain unchanged through the entire eccentricity range. In essence, the grooves eliminate the seal's dependency on eccentricity. Next, the grooved seal generally produces lower-magnitude cross-coupled stiffness and damping coefficient values than the smooth seal. Furthermore, the only positive effective stiffness values arise from the smooth seal operating at w = 2 krpm. The smooth seal consistently produces higher Keff than the grooved seal. Specifically, the smooth seal's effective stiffness is higher than that of the grooved seal by at least 30% at w = 6 krpm, across the ?P range, for e0 = 0.00. Also, the grooved seal's measured OSR is lower than that of the smooth seal by at least 10%, across the test matrix, suggesting that the grooves effectively slow down circumferential flow. For the grooved seal, the test program measures PSR values ranging from ?0 to 0.98, and OSR values bounded between 0.21 and 0.34. At w = 2 krpm, increasing PSR across its range reduces the grooved seal's direct stiffness and damping, drives its cross-coupled stiffness and damping away from zero, increases its whirl frequ

publication date

  • December 2016