Session: 25-02 Annular Seals 2

Paper Number: 80073

80073 - Measured Leakage and Rotordynamic Force Coefficients for Two Liquid Annular Seal Configurations: Smooth-Rotor/Grooved-Stator Versus Grooved-Rotor/Smooth-Stator 

This 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 seal’s 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 C_r = 203 µm. The seals have 15 circumferential grooves with groove and land lengths equal to 1.52 mm. Test variable ranges include: (a) shaft speeds (ω) ranging from 2 to 8 krpm (shaft surface speed ~ 43 m/s), and (b) pressure differences (ΔP) 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% more than the SR/GS seal. For both seals, the direct stiffnesses (K_XX, K_YY) have low magnitudes that drop with increasing ω; in some cases, they turn negative at 6 krpm. The GR/SS seal produces cross-coupled stiffness coefficients (K_XY, K_YX) that are ~2.5 times larger than those for the SR/GS seal; hence, it is significantly more destabilizing. Under the same conditions, the SR/GS seal has direct damping and added mass coefficients that are ~ 20% larger than those for the GR/SS seal.  Instability issues are likely to arise with either seal geometry because negative K_XX and K_YY drop a pump’s critical speed, aggravating the well-known destabilizing coefficients K_XY and K_YX.

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. To enhance the operation stability, an effective swirl brake that could drop the inlet preswirl ratio upstream of a seal would be helpful for the GR/SS seal out to 4 krpm and for the SR/GS seal out to 6 krpm.  Stated differently, swirl brakes (alone) would not be notably effective for ω greater than 4 krpm for the GR/SS seal or ω greater than 6 krpm for the SR/GS seal.

Presenting Author: Dara W. Childs Texas A&M University

Presenting Author Biography: Dr. Dara W. Childs, P.E. was Director of the Turbomachinery Laboratory from 1984 to 2018 and held the Leland T. Jordan Chair in Mechanical Engineering at Texas A&M University. He received B.S. and M.S. degrees (Civil Engineering, 1961, 1962) from Oklahoma State University, and his Ph.D. (Engineering Mechanics, 1968) from the University of Texas. He was named ASME Fellow Member in 1990 and received ASMEs Henry R. Worthington Medal in 1991. Dr. Childs’ expertise is in dynamics and vibrations, with an emphasis in rotordynamics. He has conducted research and engineering projects for NASA, DOD, and private firms. Current research includes high-pressure testing honeycomb and hole-pattern gas damper seals; testing high-pressure laminar oil seals; force measurements in magnetic bearings using fiber-optic strain gauges. Dr. Childs has authored numerous reviewed publications related to rotordynamics and vibrations, and the book, Turbomachinery Rotordynamics. He is presently completing a new dynamics book entitled, Dynamics in Engineering Practice.

Authors:

Dara W. Childs Texas A&M University
Jing Yang Texas A&M University
Luis San Andres Texas A&M University
Jose M. Torres Energy Recovery, Inc.
J. Alex Moreland Lynntech, Inc.