Views: 3472 Author: Site Editor Publish Time: 2020-06-25 Origin: Site
In the chemical installations of refineries, the mechanical seals of motorized equipment account for approximately 80% -85% of the total dynamic seals. Among the pump failures, the failure of mechanical seals accounts for about half of the total maintenance work. It can be seen that the position of the mechanical seal in the maintenance of the pump seals. Because the media delivered by the pump are very different and the process conditions are different, it is important to understand these conditions for the selection and maintenance of the machine seal. Today we will mainly introduces the working characteristics and precautions of mechanical seals under high and low temperature.
Under normal circumstances, the temperature of the medium exceeds 120 ℃ is considered to be a high temperature seal. The main problems of mechanical seals at this time are:
1) As the temperature of the end face of the friction pair increases, the liquid film between the seal end faces is vaporized, the friction coefficient increases, the wear increases, the temperature increases, and the seal ring is thermally deformed or cracked and fails.
2) The material of the auxiliary sealing ring in the mechanical seal is generally rubber or polytetrafluoroethylene. Due to long-term exposure to high temperature, it is easy to age and decompose, resulting in seal failure.
3) The elastic components of mechanical seals are prone to fatigue and creep at high temperatures, making the seals ineffective.
4) High temperature will aggravate the corrosion of metal materials and shorten the service life of the mechanical seal.
In order to ensure the normal operation of the mechanical seal in a high-temperature environment, the following measures can be taken.
1) Add a cooling and flushing device to the machine seal.
2) Choose high temperature resistant materials, and choose different materials according to different working temperatures. The upper temperature limit of nitrile rubber is 80℃, the upper temperature limit of silicone rubber and fluorine rubber are 200℃, and he upper temperature limit of polytetrafluoroethylene is 250℃. In addition, non-metallic friction pairs in mechanical seals are mostly made of graphite impregnated materials, so different types of impregnated graphite should also be selected according to different operating temperatures. Generally, the temperature range of impregnated babbittite graphite is less than 150℃, and the temperature range of resin impregnated graphite is 170 ~ 200℃. Graphite impregnated with copper, aluminum and lead can be used under working conditions of less than 400℃. The moving ring assembly should use materials with similar expansion coefficients as much as possible to prevent the loose connection between the moving ring and the moving ring seat at high temperatures.
3) Select metal elastomer bellow seals. Metal elastomer bellow seals have been used in high-temperature seals in recent years and have achieved good results. In this type of mechanical seals, the metal elastomer bellow seals replace the ordinary spring seals, eliminating the auxiliary seal ring of the moving ring. There is no need to overcome the friction and wear with the rotating shaft when the moving ring is compensated, so it is generally used at high temperatures, and it can achieve good results.
In oil refining and chemical equipment, the medium transported at low temperature generally has the characteristics of small gap, poor lubricity, and easy vaporization, and the medium absorbs a lot of heat during vaporization, causing the surrounding temperature to drop sharply and the condensation of water vapor in the air, which will cause ice formation on the end face and its surroundings, so that the end face will not fit tightly and cause leakage. At the same time, due to poor lubricity, more heat is generated by the friction. If it cannot be transferred in time, it will inevitably aggravate the vaporization at the end face, causing a vicious cycle and accelerating the early failure of the sealing surface.
For low-temperature sealing, in addition to selecting the appropriate auxiliary seal material, we should also pay attention to the following points.
1) Improve the sealing structure and choose the material of the friction pair correctly. Practice has proved that opening a circular arc groove on the surface of the static ring can form a hydrodynamic seal, increasing the bearing capacity of the fluid film, improving the lubrication condition of the end face, and effectively extending the working life of the mechanical seal. In terms of end-face matching, the tungsten carbide-M106K graphite is generally superior to other materials because of its good lubricity, low friction coefficient, and good thermal conductivity.
2) Select the appropriate end pressure. Seals at low temperature are prone to vaporization and icing due to poor lubricity of the end faces, so under the premise of ensuring reliable sealing, the specific pressure of the end face can be appropriately reduced. The general specific pressure value is recommended to be 0.29-0.59MPa.
3) Take corresponding measures in the process. Such as reducing the water content of the medium as much as possible, keeping the pressure of the sealed cavity relatively stable, equipping the machine with a flushing and cooling device as far as possible. Before the equipment is started, properly preheat the mechanical seal.
Pumps are very widely used in our lives and industries, and there are many pump usage scenarios, which may be high pressure, low pressure, corrosiveness, etc. Therefore, pump seals are very important. Understanding characteristics and treatment methods of mechanical seals in different scenarios can greatly improve the quality of our pump seals and increase the service life of the pump.