| A dry gas seal (DGS) has been widely used in petroleum, chemical, metallurgical, aeronautics and space and other important industries because of its advantages with energy conservation, environmental protection, long life, high reliability, etc. However, in the actual operation, when a centrifugal compressor or pump is driven by motors or steam turbines which turn reverse rotation, or when the process is operated improperly, a DGS with the conversional single grooves would fail to work normally and result in severe face wear, even in antiseizure faults. The reason for this is that such a DGS cannot provide enough openning force to form a full-thin gas film between the two faces to separate them. A bi-directional fir tree groove dry gas seal (F-DGS) is invented and presented by John Crane Inc. to meet the needs. Although such a F-DGS has been used in the fields successfully, the seal mechanisms and performance of a F-DGS remain unknown, and the theory of design of a F-DGS is unopen to the world. Therefore, research on performance and design methods of a F-DGS is needed to meet the documental needs of petroleun and chemical industries.Firstly, a general numerical analysis model is set up for both a F-DDS and a spiral-grooved dry gas seal based on the theory of gas lubrication, and the finite element method is used to solve the equations. CoMParisons of the numerical results with the test results published in the references have been proved the validity and reliability of the models and the calculation method. Secondly, the hydrodynamic effect of F-DGS is analyzed through it coMPare with spiral groove dry gas seal. The results show that, F-DGS has obvious hydrodynamic effect and could form stable and uniform gas film. The research which describe the influence rule of operating parameters to seal performance parameters (including open force, film stiffness, leakage and the ratio of film stiffness to leakage) at different groove types is done, to tell that high-speed and low-pressure are the most suitable conditions for F-DGS.And then the coMParions of surface hydrodynamic effect, sealing ability and stability between a F-DGS and a spiral groove dry gas seal are made. The results show that the stability and hydrodynamic effect of a F-DGS are inferior to that of a spiral groove dry gas seal, but a F-DGS has relative strong sealing ability and more even distribution of gas film pressure than a spiral groove dry gas seal. A F-DGS also can rotate in bi-direction.Thirdly, the sealing performances of a F-DGS are studied under different operating conditions and different geometrical parameters of fir-tree grooves, which mainly include opening force, gas film stiffness, leakage, and leakage to film stiffness ratio. The most suitable operating conditions corresponding to the different geometrical parameters are presented, and also the optimized values of fir-tree grooves under the given operating conditions.Last, the effect of radial raw number, the circumferential asymmetric construction, radial heteromorphic construction on sealing performance of a F-DGS are studied on the above basis. The results show that a F-DGS with two radial rows performs best, in which the width ratio of groove to sealing dam of each single groove is equal. The width ratio of groove to sealing dam of the down grooves is suggested to be decreased to some extent so as to improve the opening ability and stability of a F-DGS.The results of this paper will provide theoretical basis for the seal face design, how to choose appropriate F-DGS under different operating conditions and improve stability as well as reliability. This will be also in possession of theoretical meaning and engineering value to meet requirements of reliability, stability and bi-direction conditions of large shaft seal. |
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