In the mechanical pump device, the mechanical seal of the motorized equipment accounts for about 80% -85% of the total dynamic seal. Among the pump failures, the mechanical seal failure 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. Since the media delivered by the machine 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. Combining years of experience in the selection and maintenance of mechanical seals, the working characteristics and precautions of mechanical seals under several special working conditions are introduced.
1. Mechanical seal at high temperature
Under normal circumstances, the temperature of the medium exceeds 120 ℃ is considered to be a high temperature seal. At this time, the main problems of the mechanical seal are: (1) Due to the increase in the temperature of the end face of the friction pair, the liquid film between the seal end faces is vaporized, the friction coefficient is increased, the wear is increased, the temperature rise is increased, and the seal ring is thermally deformed It may fail due to thermal cracking; (2) The material of the auxiliary sealing ring in the mechanical seal is generally rubber or polytetrafluoroethylene, which is prone to aging and decomposition in a high temperature environment for a long time, resulting in seal failure; (3) mechanical seal at high temperature The elastic components are prone to fatigue and creep, making the seal invalid; (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 cooling flushing device to 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 ° C, silicone rubber and fluorine rubber are 200 ° C, and polytetrafluoroethylene is 250 ° C. In addition, the 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 applicable temperature range of impregnated babbittite graphite is less than 150 ℃, and the applicable temperature range of resin impregnated graphite is 170 ~ 200 ℃. Graphite impregnated with copper, aluminum, and lead can be used under operating 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 bellows mechanical seal.
Metal bellows seals have been used in high-temperature seals in recent years and have achieved good results. In this type of mechanical seal, the metal bellows replace the spring in the ordinary mechanical seal, eliminating the auxiliary seal ring of the moving ring, and 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 Can achieve good results.
2. Mechanical seal at low temperature
In oil refining and chemical equipment, the medium transported at low temperature generally has the characteristics of small gap, poor lubricity, and easy gasification, and the medium absorbs a lot of heat during gasification, causing the surrounding temperature to drop sharply, causing the condensation of water vapor in the air. It causes ice formation on the end face and its surroundings, so that the end face is not tightly attached to cause leakage. At the same time, due to poor lubricity, the heat generated by the end face friction is more. If it cannot be transferred in time, it will inevitably aggravate the gasification at the end face, causing a vicious cycle and accelerating the early failure of the sealing surface. In addition, the low-temperature sealing auxiliary seal ring is hard to lose resilience at low temperature, which is also easy to cause leakage.
For low-temperature sealing, in addition to selecting the appropriate auxiliary seal material, also pay attention to the following points.
1. Improve the sealing structure and select the friction pair material correctly. Practice has proved that opening circular arc-shaped grooves on the surface of the static ring can form a hydrodynamic seal, increase the bearing capacity of the fluid film, improve the lubrication condition of the end face, and effectively extend the working life of the mechanical seal. In terms of end face matching, the tungsten carbide-M106K graphite pair (ie, dynamic and static ring materials) is generally superior to other materials because of its good lubricity, low friction coefficient, and good thermal conductivity.
2. Choose the appropriate end pressure. Low-temperature seals are prone to vaporization and icing due to poor lubricity of the end face. Therefore, under the premise of ensuring reliable sealing, the specific pressure of the end face can be appropriately reduced. The general mechanical seal specific pressure value is recommended to be 0.29-0.59MPa, the lower limit can be taken for low temperature seal. For the metal bellows mechanical seal, because the moving ring has no axial friction, the end face specific pressure can even be lower than the lower limit.
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; as far as possible equipped with a flushing and cooling device; before the equipment is started, properly preheat the mechanical seal site, etc.
3. Mechanical seal at high speed
When the rotation speed exceeds 3000r / min or the linear speed of the friction pair exceeds 25m / s, it can generally be regarded as a high-speed seal. Under high-speed conditions, due to the high linear velocity of the friction end face, the heat generated by the friction is also large. At this time, the liquid film on the end face is not easy to form, and the wear is increased. High speed is also easy to cause the vibration of the rotating ring, and is subjected to a large centrifugal force, which is not conducive to the formation and maintenance of the liquid film between the end faces. Therefore, in the case of high speed, in order to reduce the influence of centrifugal force and vibration, the rotating parts should be reduced as much as possible, and the spring static structure is adopted. It can also strengthen the cooling and lubrication of the end face of the friction pair. In addition, the friction pair material with high [PV] value can be selected to reduce the contact width of the end face, or a controlled membrane mechanical seal can be used. In recent years, a mechanical seal called differential ring type has also been used in high-speed equipment. Since a differential ring is added in the middle of the original dynamic and static ring, the linear speed of the friction pair can be reduced by half, and good results have been achieved.
4. Mechanical seal under high pressure
The working pressure of mechanical seal exceeds 4-5MPa, which belongs to high pressure sealing range. Under high-pressure conditions, the specific pressure value of the sealing end face is too large, which causes the end face to generate heat, thereby destroying the formation of the liquid film and causing abnormal wear. High pressure may also deform and rupture the friction pair. High-pressure seals generally adopt a balanced structure, so that the end surface specific pressure is in a suitable range. However, considering the reliability of the seal, the balance coefficient cannot be increased without limit, so in some cases, although a balanced structure has been adopted, the end surface specific pressure may still be large. (Public number: pump housekeeper) It is difficult to meet the requirements for use by the balanced structure alone. In this case, a multi-end mechanical seal or a controlled membrane mechanical seal can be used.
5. Mechanical seal in corrosive environment
Some mechanical seals are in a corrosive environment. At this time, not only the corrosion resistance of each part in the mechanical seal should be paid attention to, but also the electrical coupling phenomenon that may occur when they are combined together. When selecting mechanical seals in a corrosive environment, attention should be paid to the use of corrosion-resistant materials. In addition, an external mechanical seal can be used. If the corrosion is extremely strong and it is easy to cause a safety accident, a double-end face seal can be used to protect it by introducing isolation liquid.
6. Mechanical seal under granular media
Some mechanical seals may work in a medium containing solid particles. When the solid particles enter the friction end face, it is prone to violent wear and tear, resulting in the seal end face quickly failing. In addition, solid particles may make the moving ring seal ring unable to move in the axial direction, so that the moving ring cannot float, lose compensation, and cause the mechanical seal to fail. The more effective way is to add an external flushing device. Since the pressure of the rinsing liquid introduced into the sealed cavity is higher than the pressure of the medium in the sealed cavity, the particulate medium can be effectively prevented from entering the sealed cavity. In addition, a double-end mechanical seal can be used, and a sealing liquid is added in the middle of the sealing cavity. You can also use an external mechanical seal.
At present, mechanical seals have become one of the most common forms of sealing in motorized equipment in oil refining and chemical installations. Moreover, with the continuous improvement of their design and manufacturing level and the continuous development of new materials, their application scope will be further expanded.