Measured Static and Rotordynamic Characteristics of a Smooth-Rotor/Grooved-Stator Liquid Annular Seal With ISO VG 2, VG 46, and VG 100 Oils

Plain (smooth) annular seals are commonly used in centrifugal pumps as were ring, interstage, or balance-piston seals and have clearnce-to-radius ratios around 0.003 to 0.005.  Henry Black was among the fitrst to study the reaction forces generated by annular seals and showed that these forces can strongly impact pump rotor stability and rotordynamics. As with journal bearings, annular seals develop reaction forces from the hydrodynamic effect, bt also develop a centering force from the Lomakin Effect.  Annular seals can emply circumferential grooves on either the rotor or stator.  There are very limited test data available for grooved seals.

Test result are presented to show how static and rotordynamic characteristics of a circumferentially-grooved-stator/smooth rotor annular seal are affected by changes in viscosity.  Tests were conducted with three test fluids: ISO VGT 2 (turbulent-flow regime), VG 46 (turbulent, transition, and laminar flow), and VG 100 (laminar flow).  Speed was varied over 2-8 krpm, DP across the seal 2.07-8.27 bars, and eccentricity ratios from 0.0 (centered) to 0.8. The seal had a radial clearance of 0.1905mm with 15 equally-spaced square grooves with a groove depth and width of 1.52 mm. The length to diameter ratio (L/D) is 0.5.  The geometry matched that of seals used in Electriucal Submersible Pumps (ESPs) for oil recovery. The test fluid is injected upstream of the seal with a circumferentail component in the direction of rotation to introduce (measured) preswirl at the seal inlet.

Increased viscosity produced the following results: (1) decreased leakage, (2) increased direct and effective damping, and (3) increased direct virtual mass coefficients.  Direct stiffness was almost always negative (statically destabilizing).  Cross-coupled stiffness coefficients were opposite in sign  and destabilizing for forward whirl.  For an ESP using these seals, increasing viscosity would create a static offset, possibly leading to rubbung and lower the pump rotor's natural frequency.  Increasing viscosity increases the seal's direct damping.  Hence, aside from dropping the pump's critical speeds, increasing viscosity is not expectewd to directly result in rotor instability.

The VG 46 test results were markedly different from those for the VG 2 and VG 100 cases.  Specifically, the VG 46 test results displayed much smaller destabilizing cross-coupled stffness coefficients.

Comparison smooth-seal test data were only avaialable for the VG 2 oil cases.  As expected, the grooves substantially reduces all rotordynamic coefficients and greatly reduces the dependence of rotordynamic coefficients on eccentricity ratio.

For the VG 2 oils cases, static and rotordynamic characteristics were poorly predicted by a turbulent-flow seal-analysis code.  For the VG 46 and VG 100 test cases, an available laminar-flow seal code predicted reasonable values for leakage flow rates, cross-coupled damping, and direct damping.  Both direct and cross-coupled stiffness were poorly predicted.