Theory And Application Of Leakage Predication For Mechanical Seals

Mechanical seals are widely used in the process industry, especially in the petroleum and chemical industry. The fundamental performances of mechanical seals include the frictional characteristic and the sealing behavior. The frictional characteristic of the end faces of mechanical seals should be improved to increase their service life on the premise that the leakage rate must be smaller than the maximum allowable one. There exists the potential failure risk for rotaries in which mechanical seals are used to seal working medium because the service life of mechanical seals is influenced by many factors. That the leakage rate and the service life of mechanical seals are very difficult to be predicted accurately results in the unnecessary replacement of mechanical seals or the unacceptable leakage rate. The leakage of mechanical seals often causes serious accidents, such as fire hazard, explosion, intoxication, environment contamination and so on. Many researches on the characteristics of mechanical seals were carried out and some leakage models were built, which perfected the theory of mechanical seals to some extent and played a very important role in engineering practice. But most of the early researches were based on the assumptions that the profile of the end faces of the sealing rings and the frictional conditions were invariant. In the early built models, the effect of the surface topography change of the end faces on the leakage rate was neglected, and only the initial roughness and waviness of the end faces were considered. The relationship between the leakage rate and the working time was not taken into consideration. Actually, both the profile of the end faces of the sealing rings and the gap composed by the two end faces were changed continually during the life period of mechanical seals. Therefore, a precise calculation of the leakage rate using the present models is very difficult.Based on the fractal geometry theory, the surface topography of the end faces of the sealing rings and its change during the frictional wear of mechanical seals were characterized by the fractal parameters with independent dimension. The prediction method of the leakage rate of mechanical seals was investigated and discussed in this thesis. The research work includes mainly:study on the fundamental theory of mechanical seals, research on the surface topography of the end faces of the sealing rings, building of the leakage rate prediction model, establishment of the accelerated test method for frictional wear, experimental investigation on the frictional wear characteristic and the sealing behavior, research on the practical application, and so on.The equation which represents the relationship between the leakage rate and the gap composed by the two end faces was derived based on the Navier-Stokes equation. The effect of the surface topography of the end faces on sealing behavior and friction characteristic of mechanical seals was analyzed. The investigation indicates that the surface topography of the end faces and its changes have significant influence on sealing behavior and friction characteristic of mechanical seals in operation. Accurate characterization of the surface topography and its change of the end faces is the key technology for predicting the leakage rate of mechanical seals.The fractal characteristic of the end faces of the sealing rings was analyzed by using the Weierstrass-Mandetbrot function. Some calculating methods for the fractal dimension, such as the footage method, the latitude enumeration method, the power spectrum method, the variate-diffrence method, the structure function method, the contour mean-squared method, the covariance method and the wavelet transformation approach, were discussed. It was found that the structure function method has better accuracy and determinacy than others. The profile of the end faces was measured by AF-LI profile testing instrument. It was found that the profile of the end faces possesses fractal characteristic according to the log-log relationship between the structure function and the measuring scale.The contact of the end faces of the rotating ring and the stationary ring was simplified to be the contact of a rough surface with an ideal rigid smooth surface according to the Majumdar-Bhushan fractal model. The void on the circumferential cross section at the outer radius of the end faces was regarded as the cross section of leakage path, which was characterized by fractal parameters. The time-correlation leakage prediction model of mechanical seals was built. The effect of the change of the surface topography of the end faces on the gap composed by the two end faces during the course of the frictional wear of the two sealing rings was analyzed. The relationship among the leakage rate of mechanical seals, the pressure of the sealed medium, the viscosity of the sealed medium, the pressure on the end faces, the performance of the material of the sealing rings, the change of the surface topography of the end faces, and the working time was discussed.The accelerated test method of the frictional wear was put forward based on the similarity theory. The time-correlation equation of the fractal dimension of the end faces was obtained from the experimental results of frictional wear of five groups of rotating rings and stationary rings by HDM-2 type friction wear testing machine. The wear amount is large at the early stage of the frictional wear experiment because the fractal dimension is small, the end faces are rough, the contact area of the two end faces is small and the contact stress is large. The fractal dimension of the end faces gradually increases and reaches the maximum value, and the contact area increases but the contact stress and the wear amount decrease when the frictional wear goes on. Since the wear amount becomes small, the fractal dimension slowly decreases and almost remains constant for a long time. However, when the fractal dimension decreases to a certain value, it will decrease rapidly because the end faces are so rough that the abrasion is aggravated.The sealing characteristics of GY-70 mechanical seal were investigated on the developed mechanical seal testing machine. The experimental leakage rate was large at the beginning running. However, the leakage rate was decreased with the increase of the testing time after the mechanical seal running-in. Then, the sealing surface laid in the steady friction state for a long time and the leakage rate almost kept constant. Under the same surface unit load, the leakage rate increases as the medium pressure. The change of the measured leakage rate with the testing time is similar to that of the theoretically predicting results. The former is smaller than the latter, and both of them possess the same order of magnitude. The research results indicate that the fractal model put forward in this thesis can be used to predict relatively accurately the leakage rate of mechanical seals at any operating time under certain working conditions.Fault tree analysis method was applied to investigate the causes of the leakage failure of mechanical seals. It is pointed out that the low-grade surface topography is the main factor causing the leakage of mechanical seals under the normal working conditions. The measured leakage rate and the service life of the mechanical seal in a pump used in a petrochemical company verify the correctness of the fractal model for predicting the leakage rate of mechanical seals.The theoretical and applied researches on the prediction of the leakage rate of mechanical seals help to find out the influence of the surface topography of the end faces of the sealing rings on the leakage rate during the operating time, the change regularity of the surface topography of the end faces, and the relationship between the leakage rate and the working time. With the help of the leakage rate prediction model, the leakage rate of mechanical seals can be obtained instantaneously, and then the decision whether mechanical seals need to be maintained or not can be made. Therefore, the safe, reliable and economical operation of mechanical seals can be realized. The research results in this thesis are valuable to the design, manufacture, use and maintenance of mechanical seals.