Journal of Mechanical Engineering (Jun 2020)
Design Improvement of Stuffing Box Seals of Centrifugal Pump Shafts, Based on the Study of the Sealing Mechanism Physical Model
Abstract
Stuffing box seals are the most common type of pump rotor seals because they are adjustable and periodically restorable assemblies during operation. Based on the study of physical processes, a sealing mechanism model of the stuffing box seal is formed as a combination of two successive hydraulic resistances: a pre-switch resistance, which is similar to a slotted choke, and a contact seal, where the shaft is directly sealed. The area where the packing contacts the shaft is the sum of the microregions where contact pressures occur. The system of labyrinth channels through which leakage occurs is physically closest to the filtration of fluid through a porous body layer. A method is proposed for calculating the stress state of the packing by solving the hydroelasticity problem. Obtained are expressions for calculating the gap and sealed pressure distribution over the radial stuffing box seal as well as leakage through the seal. Radial and angular displacements of the shaft axis with respect to the axis of the stuffing box are taken into account, leading to the occurrence of additional contact packing pressures on the shaft and areas of weak contact of the packing with the shaft, which leads to increase in leakages. The desire to limit them encourages maintenance personnel to increase the axial compression of the packing, which leads to an even greater increase in local contact pressure. Proposed are stuffing box designs with a radially movable, self-aligning packing set relative to the shaft, which provide the equalization of contact pressure and increase in service life. Obtained are expressions for finding the minimum values of the parallel and angular misalignments, at which a stuffing box under the action of the centering force and moment starts tracking the shaft radial and angular displacements. Radial mobility prevents the areas of separation of the packing from the shaft and the formation of contact spots with increased pressure.
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