Mechanical pump shaft seals: Dynamic stability and dynamic sealing considerations

The majority of power reactor designs utilize pumps to circulate the primary coolant past the fuel to transfer heat away and provide the thermal power to generate electricity. Mechanical shaft seals are critical nuclear components used to prevent leakage between a rotating pump shaft and the stationary housing for the pump. Success is usually achieved by finding the right balance of providing sufficient leakage to lubricate the seal, without overwhelming the collection system. Mechanical pump seals have two lapped-flat faces in close proximity, one rotating and the other stationary, across which a small amount of leakage (lubricant) is driven by the pressure being sealed (hydrostatic seals) or leakage is driven by the shaft rotation (hydrodynamic seals). This paper will discuss design aspects associated with hydrostatic seals. The gap between these seal faces can change over operation due to intermittent rubbing from loss of film uniformity as a result of: shaft tilt, eccentricity, misalignment, elastomeric seal friction, and vibration. This intermittent rubbing can cause a loss of the initial convergence set during manufacturing, which can cause unstable dynamic response, such as hard rubbing (unlubricated seal face operation). Additionally, this paper will discuss considerations related to the design of dynamic elastomeric seals, which observe the same shaft operating challenges, but also need to move along the shaft without affecting the lubricating film.