Second-moment Turbulence Model of Flow Through a Rotating Annular Seal

Abstract

Annular seals are widely used in industry to seal a fluid between a stationary and rotating member. They are commonly used in pumps in all types of industry to prevent recirculation inside the pump casing. Figure 1.1 below shows a typical multi-stage pump that has several examples of annular seals. Annular seals between the pump casing and the impeller are termed "wear rings" and those located directly on the pump shaft are simply described as "bushings". The amount of leakage that passes through the seals can greatly impact the performance of the pump. As the seals wear and open up they can cause so much recirculation that the pump overloads the driver. The need to reduce the amount of leakage requires that the clearance between the stationary and rotating members be kept at the absolute minimum. Since large pressure drops sometimes exist across seals 3.5 :MPa[500 psi], very highvelocities exist within the seals[2]. The small clearances seen in many seals (0.127 rom [0.005 in] for a 63.5 mm[2.5 in] diamseal) and large pressure drops may cause the seals to act as hydrodynamic bearings (see Figure 1.2). This can be both beneficial as well as detrimental to the operation of the pump. Additional "bearings" in the center of a long multi-stage pump as seen in Figure 1. 1 are a necessity to support the shaft due to the high tangential loads inflicted by the impellers. However, if these "bearings" have the wrong internal stiffness due to very high velocities (see Figure 1.2), the rotordynanlic stability of the pump can be greatly effected. The internal stiffness and dampening of a seal can be calculated if the pressure and velocity profiles are known. A representation of the linkage between the fluid velocities, pressures, and hydrodynamic characteristics is shown in (1) below [2,3,37].