Mechanical Seal Maintenance Program Under Special Working Conditions

In the pumping device, the mechanical seal of the mobile device accounts for about 80%-85% of the total number of dynamic seals. In the case of a pump failure, the mechanical seal failure accounts for about half of the total maintenance work. It can be seen that the mechanical seal occupies the position in the maintenance of the pump. Since the medium delivered by the pump varies widely and the process conditions are different, it is important to understand these conditions for the selection and maintenance of the machine seal. Combined with years of experience in the selection and maintenance of machine seals, the working characteristics and precautions of mechanical seals under several special working conditions are introduced.

 

 First, the mechanical seal at high temperature

Under normal circumstances, the temperature of the medium exceeds 120 ° C is considered to be a high temperature seal. At present, the main problems of the mechanical seal are: (1) due to the increase of the temperature of the end face of the friction pair, the liquid film is vaporized between the sealing end faces, the friction coefficient increases, the wear is intensified, the temperature rise is increased, and the seal ring is thermally deformed. Or thermal cracking and failure; (2) The auxiliary sealing ring material in mechanical seal is generally rubber or PTFE, which is easy to aging and decompose due to long time in high temperature environment, resulting in seal failure; (3) mechanical seal at high temperature The elastic element is prone to fatigue and creep, which causes the seal to fail; (4) The high temperature will aggravate the corrosion of the metal material and shorten the service life of the machine seal.

 To ensure that the mechanical seal works properly in a high temperature environment, the following measures can be taken.

  1. Add a cooling flush to the machine seal.

  2. Choose materials that are resistant to high temperatures and use different materials depending on the operating temperature.

The upper limit of the nitrile rubber temperature is 80 ° C, the silicone rubber and the fluororubber are 200 ° C, and the polytetrafluoroethylene is 250 ° C. In addition, the non-metallic friction pairs in the mechanical seal are mostly made of graphite impregnated materials, so different types of impregnated graphite should be selected according to different working temperatures. Generally, the temperature range of the immersed babbitt graphite is less than 150 ° C, and the temperature of the immersion resin graphite is 170 to 200 ° C. The graphite immersed in copper, aluminum and lead can be used under working conditions of less than 400 ° C. As far as possible, the moving ring assembly should use materials with similar expansion coefficients to prevent the connection between the moving ring and the moving ring seat from loosening at high temperatures.

  3. Select the 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 machine seal, the metal bellows replaces the spring in the ordinary machine seal, eliminating the dynamic ring auxiliary seal ring, and does not need to overcome the friction and wear of the rotating shaft when compensating the moving ring, so it is generally used at high temperatures. Can achieve good results.

  

 Second, the mechanical seal at low temperature

  In the refinery chemical plant, the medium transported at a low temperature generally has the characteristics of small creepage, poor lubricity, and easy gasification, and the medium absorbs a large amount of heat during gasification, causing the ambient temperature to drop sharply, causing condensation of moisture in the air. Leading to the end face and its surrounding ice, so that the end face is not tight enough to cause leakage. At the same time, due to poor lubricity, the heat generated by the end face friction is high. If it cannot be transferred in time, the gasification at the end face will be intensified, causing a vicious cycle and accelerating the early failure of the sealing surface. In addition, the low-temperature sealed auxiliary seal ring is hard to break off at low temperatures and is also prone to leakage.

  For low temperature sealing, in addition to selecting the appropriate auxiliary sealing material, pay attention to the following points.

  1. Improve the sealing structure and correctly select the friction pair material. Practice has proved that opening a circular arc groove 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 machine 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 face specific pressure. Low-temperature sealing is prone to gasification and icing due to poor lubricity of the end face. Therefore, the end face specific pressure can be appropriately reduced under the premise of ensuring reliable sealing. Generally, the mechanical seal ratio is recommended to be 0.29-0.59 MPa, and the lower limit can be taken for low temperature sealing. For the metal bellows mechanical seal, since 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. For example, the water content of the medium should be reduced as much as possible; the pressure in the sealed chamber should be relatively stable; the flushing and cooling device should be equipped as much as possible; before the equipment is started, the machine sealing part should be properly preheated.

 

  Third, the mechanical seal at high speed

  When the rotational speed exceeds 3000r/min or the linear velocity of the friction pair exceeds 25m/s, it can generally be regarded as a high-speed seal. Under high-speed conditions, the friction end face has a large linear velocity, and the heat generated by the friction is also large. At this time, the liquid film on the end face is not easily formed, and the wear is intensified. The high speed also easily causes the vibration of the rotating ring and is subjected to a large centrifugal force, which is disadvantageous for 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 minimized, and the spring static structure is adopted. It is also possible to enhance the cooling and lubrication of the end face of the friction pair. In addition, a friction material having a high [PV] value may be selected to reduce the contact width of the end face or a mechanical seal using a controlled film. In recent years, a mechanical seal called a differential ring has also been used in high speed equipment. Since a differential ring is added in the middle of the original moving ring, the speed of the friction line can be reduced by half, and a good effect is obtained.

 

 Fourth, the mechanical seal under high pressure

  The mechanical seal working pressure exceeds 4-5 MPa, which belongs to the high pressure sealing range. Under high pressure conditions, the sealing end face is excessively pressed, causing the end face to generate heat, thereby destroying the formation of the liquid film and causing abnormal wear. High pressure can also cause deformation and cracking of the friction pair. High-pressure seals generally use a balanced structure so that the end face specific pressure is in a suitable range. However, considering the reliability of the seal, the balance factor cannot be increased without limitation, and in some cases, although the balanced structure has been adopted, the end face specific pressure may still be large. (Public number: pump butler) It is difficult to meet the requirements of use only by the balanced structure. In this case, multi-end machine seal or controlled membrane mechanical seal can be used.

 

  Five, Mechanical seals in corrosive environments

  Some mechanical seals are in a corrosive environment, in which case not only the corrosion resistance of each part in the machine seal, but also the electrical coupling that may occur when they are combined. For mechanical seals in corrosive environments, care should be taken to use corrosion-resistant materials. In addition, an external mechanical seal can be used. For corrosion-prone and easy to cause safety accidents, double-face seals can be used, and isolation liquid is introduced to protect them.

 

  Sixth, the mechanical seal under the granular medium

  Some machine seals may work in media containing solid particles. When the solid particles enter the friction end face, they are prone to severe wear and tear, resulting in a quick failure of the seal face. In addition, the solid particles may make the moving ring seal unable to move in the axial direction, so that the moving ring cannot float, and the compensation is lost, resulting in failure of the machine seal. A more effective way to do this is to add an external flushing device. Since the pressure of the flushing liquid introduced into the sealed chamber is higher than the pressure of the medium in the sealed chamber, the particulate medium can be effectively prevented from entering the sealed chamber. In addition, a double mechanical seal can be used to seal the seal cavity. An external mechanical seal can also be used.

 

  At present, mechanical seals have become the most common form of seal in the mobile equipment of refinery and chemical plants, and its application range will be further expanded with the continuous improvement of its design and manufacturing level and the continuous development of new materials.


Post time: Sep-10-2019

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