Correct choice of mechanical seal

For more than 100 years, people have used a variety of sealing materials such as fillers to reduce the leakage of liquid from the pump housing along the main shaft. Although the liquid pump used in the modern technological process is still widely used the oldest sealing design-stuffing box, because of its low starting cost, and it is familiar to the factory staff. However, due to environmental issues, the use of packing seals has gradually become unacceptable, especially for the more common and corrosive liquids in modern technological processes. Therefore, in practical applications, more and more mechanical seals are used instead of packing seals.
The basic element of sealing The mechanical seal uses the principle of friction between two planes to achieve the purpose of sealing. The rotating sealing surface is installed on the main shaft of the liquid pump, and the fixed sealing surface is installed in the sealing gland. Since one sealing surface is moving and the other sealing surface is stationary, this type of seal is called dynamic seal. The seal between the rotating surface and the stationary surface is the most critical factor determining the sealing performance. The basic mechanical seal, of which there are 4 leakage paths that need to be sealed: 1. The path between the sealing surfaces; 2. The path between the rotating surface and the spindle ; 3. The passage between the fixed surface and the gland; 4. The passage between the gland and the stuffing box. The latter two leakage paths generally use static seals because there is no relative movement between the two parts. This part of the seal is usually called the tertiary seal, and its sealing material is a gasket or an O-ring that is compatible with the process liquid.

In older seal designs, the secondary seal under the rotating surface leaves a certain gap, which can move back and forth on the main shaft, which is prone to wear and premature failure. However, in the newer seal design, the secondary seal is in a static state, so the problem of wear and corrosion on the spindle can be avoided. In the normal operation of the liquid pump, the pressure between the rotating surface and the stationary surface is kept in a sealed state due to the pressure generated by the liquid in the stuffing box. When starting and stopping, the pressure of the stuffing box is maintained by the pressure generated by the spring ( It can even be replaced by spring pressure). Most mechanical seal designs use softer materials to make the rotating surface, so that it rotates and rubs on the harder stationary surface. For many years, the most common combination is to use carbon material as a rotating surface to make it run on a stationary ceramic surface.

This type of material is still commonly used, but the static surface is made of stainless steel or harder materials, such as tungsten carbide or silicon carbide. No matter what material is used, a liquid film must be maintained between the contact surfaces to provide lubrication. However, in the stuffing box, the combination of spring load and liquid pressure can make the sealing surface have a good sealing effect. However, if the sealing pressure is too high, it will affect the formation of a liquid film between the contact surfaces, leading to increased heat and premature wear. If the sealing pressure is too low, the gap between the contact surfaces will increase, which will easily cause liquid leakage. Sealing manufacturers are constantly striving to improve the flatness of the contact surface. They use special polishing plates for grinding. Then, use a grating plate with a monochromatic light source to detect it.

From this point of view, it is very important to handle these sealing contact surfaces carefully and strictly follow the installation instructions to ensure that the sealing surfaces are properly protected and in place. The flexibility of the seal chooses the axial and radial movement of the spindle to maintain a certain degree of flexibility with the spring to ensure the seal between the contact surfaces. However, only a certain degree of flexibility can be provided. The mechanical condition of the liquid pump and its length-to-diameter ratio (a measure of the ratio of the diameter of the main shaft to its extension length, the lower the ratio, the better) play an important role in the reliability of the seal. The flexibility of the seal is generally guaranteed by a large main spring and a series of small springs or bellows sealing devices. The traditional seal design used in the chemical industry, whose sealing pressure is applied to the rotating surface, is called a rotary seal because the spring or bellows seal device rotates with the main shaft. Relatively novel design, the spring or corrugated sealing device is installed on the stationary surface. In the current mechanical seal, the above two sealing methods have very common applications, so that it has a certain degree of flexibility for installation. Many mechanical seals designed in the early days use a single large spring arranged around the main shaft, which can provide a strong sealing force for the sealing surface during the start of the liquid pump. The function of the seal relies on the rotation of the spindle to tighten the spring coil. The seal designed in the later period adopts a series of smaller springs arranged around the main shaft to produce a relatively uniform load pressure on the sealing surface. Since smaller springs can be installed in advance, most of these seals are completely isolated from the liquid being pumped.

Using multiple smaller springs can produce a relatively even load pressure on the sealing surface, so it has greater sensitivity to blockage. For many corrosive applications, the most common design is to use metal bellows seals. The bellows is welded by a series of metal discs to form a corrugated sealing device to prevent leakage. The use of this device can make the sealing pressure between the sealing surfaces more uniform, and there is no need to add a secondary seal on the sealing surface, so naturally there will be no corrosion and wear. Figure 3 The metal bellows sealing device is generally used for more corrosive liquids. Generally speaking, although the main sealing pressure depends on the pressure of the stuffing box itself, the spring and bellows can compensate for the liquid pump during the start and stop process. The pressure of the main shaft movement is insufficient, so that the sealing surface always maintains a certain sealing pressure. Corrosion and wear problems are caused by various reasons, such as bearing tolerances, shaft end clearance, vibration and spindle deviation, which will cause the main shaft of the hydraulic pump to produce radial and axial movement.

In addition, since it is very difficult to keep the contact surfaces absolutely parallel, it is normal for the internal movement of the mechanical seal itself. This type of movement is often caused by equipment and installation tolerances, thermal expansion, pipeline stress, or improper spindle debugging. In order to keep the sealing surfaces always in cooperation with each other, the spring acts as a constant adjustment between the mechanical seal and the moving main shaft. When an elastomer seal is used between the rotating surface and the spindle, the elastic body will move back and forth on the spindle. This repeated friction action will erode the anti-corrosion material on the spindle, lose the oxide film protection layer of the spindle, and eventually form abrasion grooves on the friction surface of the spindle, causing liquid to leak from the grooves and increasing the amount of necessary maintenance work Or even replace the spindle. To solve this problem, a replaceable shaft sleeve is generally installed in the stuffing box. However, the only permanent solution to the problem of corrosion and wear is to remove the internal dynamic seal. At present, most major seal manufacturers produce non-corrosive wear seals to prevent corrosion and wear of liquid pump parts. The balance of balanced and unbalanced seals has a great influence on the sealing pressure of the contact surface. This sealing pressure depends on the effective section of the seal itself and the pressure in the stuffing box. The cross section on the opposite side of the rotating surface of the unbalanced seal is completely exposed to the pressure range of the stuffing box. This situation will generate a high sealing pressure between the sealing surfaces, which will increase the working temperature and accelerate the wear rate. Under high temperature working conditions or when the liquid is corrosive and abrasive, the service life of the mechanical seal will be greatly reduced. Balancing the mechanical seal can reduce the sealing pressure and extend the service life of the seal.

Generally, a stepped spindle and shaft sleeve are used to reduce the effective cross-section of the rotating surface to achieve the above purpose. However, do not adjust the net sealing pressure to a level close to zero, because the result of this may cause unstable working conditions between the sealing surfaces, and may blow off the seal due to sudden changes. The answer to these sealing problems may be to use unbalanced seals. For some services, unbalanced seals may achieve better results. For example, in certain application areas, the safety issues caused by liquid leakage may be more emphasized than the service life of the seal. In this case, the selection of the seal may be understood as a higher sealing pressure. Similarly, when choosing a seal with a colder liquid, the increase in operating temperature may be insignificant. Regardless of the consideration, when the pressure of the stuffing box exceeds 50psi, it is generally recommended to use a balanced seal. The most common method of inner and outer seal is to install the seal inside the stuffing box. However, this approach requires the wet end of the liquid pump to be disassembled during seal maintenance. The main advantage is that the sealed environment is easy to control. The installation method of the outer seal is to reverse the direction of the static sealing surface, and the rotating unit on the main shaft is located outside the gland of the stuffing box.