Measured Leakage and Rotordynamic Force Coefficients for Two Liquid Annular Seal Configurations: Smooth-Rotor/Grooved-Stator Versus Grooved-Rotor/Smooth-Stator
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AbstractThis paper reports and compares the experimental results of leakage and dynamic force coefficients for two liquid annular pressure seals, one having a smooth-rotor/circumferentially-grooved stator (SR/GS), the other one with a circumferentially-grooved rotor/smooth-stator (GR/SS). Differing only in the grooves location, the GR/SS seals geometry and operating conditions are representative of those in electrical submersible pumps (ESPs) used for oil recovery. Supplied with an ISO VG2 oil at 46 C, both seals have the same diameter D = 102 mm, length-to-diameter ratio L/D = 0.5, and nominal land clearance Cr = 0.203 mm. The seals have 15 circumferential grooves with groove and land lengths equal to 1.52 mm. Test variable ranges include: (a) shaft speeds () ranging from 2 to 8 krpm (shaft surface speed 43 m/s), and (b) pressure differences (P) from 2 to 8 bar. Upstream of the test seals, three separate pre-rotation rings generate a range of inlet circumferential velocities (entrance swirl). Under all conditions, the GR/SS seal leaks about 10% less than the SR/GS seal. For both seals, the direct stiffnesses (KXX, KYY) have low magnitudes that drop with increasing ; in some cases, they turn negative at 6 krpm. The GR/SS seal produces cross-coupled stiffnesses (KXY, KYX) that are 1.5 times larger than those for the SR/GS seal. Under the same conditions, the SR/GS seal is more stabilizing as its direct damping and added mass coefficients are 20% larger than those for the GR/SS seal. Instability issues are likely to arise with either seal geometry because negative KXX and KYY drop a pump critical speed, aggravating the well-known destabilizing coefficients KXY and KYX. The whirl frequency ratio (WFR) combines the effects of the cross-coupled stiffness, direct damping and cross-coupled mass terms, thus providing a good basis for comparing two seals stability characteristics. Overall, the WFR magnitudes for the GR/SS seal are about three times higher than those for the SR/GS seal. Note that, irrespective of the inlet swirl condition, the GR/SS approaches a WFR 0.50 for operating shaft speeds greater than 4 krpm. At the lowest shaft speed (2 krpm), the WFR >> 0.5 for the low inlet pre-swirl ring, whereas WFR < 0.8 for the medium and high pre-swirl rings. For the SR/GS seal, the WFR 0.2 at the highest shaft speed of 6 krpm, and not affected by the inlet pre-swirl condition. On the other hand, at the lowest shaft speed, the WFR ranges from 0.5 to 0.75 for the SR/GS seal with medium and high pre-swirl rings. At this speed, the WFR = 0 with the low inlet pre-swirl ring. Hence, to enhance the operational stability, an effective swirl brake that could drop the inlet pre-swirl ratio upstream of a seal is helpful for the GR/SS seal out to 4 krpm and for the SR/GS seal out to 6 krpm.
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Volume 8A: Structures and Dynamics Aerodynamics Excitation and Damping; Bearing and Seal Dynamics; Emerging Methods in Engineering Design, Analysis, and Additive Manufacturing; Fatigue, Fracture, and Life Prediction
Volume 8A: Structures and Dynamics Aerodynamics Excitation and Damping; Bearing and Seal Dynamics; Emerging Methods in Engineering Design, Analysis, and Additive Manufacturing; Fatigue, Fracture, and Life Prediction
author list (cited authors)
Childs, D. W., Yang, J., San Andrs, L., Torres Rueda, J. M., & Moreland, J. A.
