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 46C, both seals have the same diameter D=102mm, length-to-diameter ratio L/D=0.5, and nominal land clearance Cr=0.203mm. The seals have 15 circumferential grooves with grooves and land lengths equal to 1.52mm. Test variable ranges include (a) shaft speeds () ranging from 2 to 8 krpm (shaft surface speed43m/s), and (b) pressure differences (P) from 2 to 8bar. Upstream of the test seals, three separate prerotation 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 are20% 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 WFR0.50 for operating shaft speeds greater than 4 krpm. At the lowest shaft speed (2 krpm), the WFR0.5 for the low inlet preswirl ring, whereas WFR<0.8 for the medium and high preswirl rings. For the SR/GS seal, the WFR0.2 at the highest shaft speed of 6 krpm, and not affected by the inlet preswirl 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 preswirl rings. At this speed, the WFR=0 with the low inlet preswirl ring. Hence, to enhance the operational stability, an effective swirl brake that could drop the inlet preswirl 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.
