Pozovnyi O. Influence of multi-annular seals on tightness and vibration reliability of centrifugal pumps

The thesis is devoted to the development of calculation methods and improvement of the geometry of multi-annular seals of centrifugal pumps. Scientific substantiation and elaboration of the method of determining static and dynamic forces characteristics and refined calculation of leakage in multi-clearance seals allow to improve existing structures and increase energy efficiency while ensuring a low level of vibration of centrifugal pump rotors. Based on the analysis of literature sources, the possibility of increasing the total efficiency of centrifugal pumps by reducing the volume losses on the front and interstage annular seals, with large values of pressure drops and rotational speeds. This is possible through the use of multi-clearance seals. Sequentially placed throttles connected by chambers creates the design of such seals, the coefficient of hydraulic losses is approximately equal to the sum of the coefficients of loss of all throttle. Accordingly, leakage through seals is reduced. From the literature, it is clear that in such seals, as in conventional single-clearance, not only limited losses but also due to high values of pressure drops radial forces that affect the dynamic characteristics of the rotors of centrifugal pumps, i.e. they perform hydrodynamic support and can both to stabilize the rotor and to cause the loss of its dynamic stability. The flow equation of a viscous incompressible fluid (Reynolds equation) determines the pressure distribution in a short annular channel with corresponding boundary conditions. Analytical dependences for calculating radial forces arising in the ring chokes of two-annular and three-annular seals due to the radial displacement of the shaft and the skew of the axes of the rotor and stator are obtained. The influence of the conicity of the slits on the force coefficients of the multi-annular seals is estimated. The forces due to the axial pressure drop and the displacement flow are considered: dissipative force and circulating force , which can lead to loss of stability, accompanied by self-oscillations of the rotor with a large amplitude. Inertial forces (gyroscopic and inertial forces), due to their relatively small values, are not taken into account. Analytical dependences for the determination of amplitude and phase of forced radial oscillations of a shaft and the definition of its stability condition are received. The free oscillations of the rotor in the multi-annular seal are also considered, and the natural frequencies of the rotor-seal system are obtained, taking into account damping depending on the rotational frequency at different values of the sealing pressure. Comparing the values of dynamic coefficients and leakage of two- and three-annular seals with a one-clearance seal under the condition of using sufficiently large chambers demonstrates the significant advantages of multi-annular seals. For example, a three-clearance seal with the same radial clearance has an average of 48.5% higher dynamic coefficients and 41.2% fewer leakages. A similar two-clearance seal has 15% greater direct stiffness and 9.1 % more direct damping, and 20.6% fewer leaks. To conduct experimental research on the basis of the problem laboratory of hermomechanics and vibrodiagnostics of the Department of Computer Mechanics named after Volodymyr Martsynkovskyy, the modernization of the existing experimental installation for the study of single-well seals was performed. The unit provides the supply of sealing pressure from 0 to 1 MPa, provided sufficient suppression of its pulsations, with the value of leaks up to 1.2 l/s and shaft speed - 8000 rpm. Experimental studies of three-annular seals of two design variants were performed: with the same radial clearance on each hole and twice the radial clearance on the second hole with two axial sizes of connecting chambers - 1 and 3 mm. To determine the influence of radial and tangential forces, separate studies with both non-rotating and rotating shafts were conducted. In experimental studies with a non-rotating shaft, the pressure distribution was measured along the length of the first clearance and in the connecting chamber at its outlet in two opposite radial positions of the shaft - in the place with the minimum and maximum radial clearance; and total leaks from the seal depending on the radial displacement of the shaft in the range of 0.04-0.16 mm (eccentricity) at different values of the sealing pressure of 1.25; 2.5; 5; 7.5, 10 atm.