Mechanical seal maintenance plan under different working conditions

In the pump device, the mechanical seal of the motorized equipment accounts for about 80%-85% of the total number of dynamic seals. In the pump failure, the failure of the seal accounts for about half of the entire maintenance workload. It can be seen that the position occupied by the seal in the maintenance of the pump. Since the medium and process conditions conveyed by the pump are very different, it is very important to understand these conditions for the selection and maintenance of the mechanical seal. Combined with years of experience in the selection and maintenance of mechanical seals, the working characteristics and precautions of seals under several special working conditions are introduced.

1. Seals under high temperature

Under normal circumstances, the temperature of the medium exceeds 120°C, which is considered a high-temperature seal.

At this time, the main problems of the seals are:

1) Due to the temperature rise of the end faces of the friction pair, the liquid film between the sealing end faces is vaporized, the friction coefficient increases accordingly, the wear is intensified, the temperature rise is increased, and the sealing ring is thermally deformed or thermally cracked and fails;

(2) The auxiliary sealing ring material in the seal is generally rubber or polytetrafluoroethylene, which is easy to age and decompose due to prolonged exposure to high temperature, resulting in seal failure;

(3) The elastic element of the seal is prone to fatigue and creep at high temperature, making the seal invalid;

(4) High temperature will aggravate the corrosion of metal materials and shorten the service life of mechanical seals.

In order to ensure the normal operation of the seal in high temperature environment, the following measures can be taken.

1. Add a cooling flushing device to the mechanical seal.

2. Choose materials with high temperature resistance, and choose different materials according to different working temperatures. The upper temperature limit of nitrile rubber is 80°C, that of silicone rubber and fluororubber is 200°C, and that of polytetrafluoroethylene is 250°C. In addition, the non-metallic friction pairs in the seals are mostly made of graphite impregnated materials, so different types of impregnated graphite should be selected according to different working temperatures. Generally, the applicable temperature range of Babbitt alloy impregnated graphite is less than 150°C, the applicable temperature range of resin impregnated graphite is 170~200°C, and the graphite impregnated with copper, aluminum and lead can be used under the working condition of less than 400°C. The moving ring components should use materials with similar expansion coefficients as much as possible to prevent the connection between the moving ring and the moving ring seat from loosening under high temperature.

3. Select seals for metal bellows.

For example, reduce the water content of the medium as much as possible to keep the pressure of the sealing chamber relatively stable; equip with flushing and cooling devices as much as possible; properly preheat the mechanical seal before starting the equipment, etc.

3. Seals under 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, due to the high linear velocity of the friction end surface, the heat generated by friction is also large. At this time, the liquid film on the end surface is not easy to form, 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 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 should be adopted. It can also strengthen the cooling and lubrication of the end faces of the friction pair. In addition, friction pair materials with high [PV] value can also be selected to reduce the contact width of the end face, or a controlled membrane mechanical seal can be used. In recent years, a type of 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 dynamic and static ring, the frictional line speed can be reduced by half, and a good effect has been achieved.

4. Seals under high pressure

When the working pressure exceeds 4-5MPa, it belongs to the high-pressure sealing range. Under high pressure conditions, due to the high specific pressure value of the sealing end face, the end face heats up, thereby destroying the formation of the liquid film and causing abnormal wear. High pressure may also deform and break the friction pair. High-pressure seals generally adopt a balanced structure so that the specific pressure of the end face is in a suitable range.

5. Seals in corrosive environments

Some seals are in a corrosive environment. At this time, we should not only pay attention to the corrosion resistance of each part in the machine seal, but also prevent the possible galvanic coupling phenomenon when they are combined. For seals in corrosive environments, attention should be paid to the use of corrosion-resistant materials when selecting them. Additionally, externally mounted seals may be used. For those with strong corrosion and easy to cause accidents, double-end seals can be used to protect them by introducing spacer fluid.

6. Seals under granular media

Some mechanical seals may work in a medium containing solid particles. When solid particles enter the friction end face, it is prone to severe wear and tear, resulting in rapid failure of the seal end face. In addition, solid particles may prevent the moving ring sealing ring from moving in the axial direction, so that the moving ring cannot float, lose compensation, and cause the mechanical seal to fail. A more effective way to do this is to add an external flushing device. Since the pressure of the flushing fluid introduced into the sealed cavity is higher than the pressure of the medium in the sealed cavity, it can effectively prevent the granular medium from entering the sealed cavity. In addition, double-end seals can be used to add sealing liquid in the middle of the sealing cavity. Externally mounted seals are also available.

At present, mechanical seals have become a common form of sealing in the motorized equipment of oil refining and chemical plants, and with the continuous improvement of its design and manufacturing level and the continuous development of new materials, its application range will be further expanded.