The best choice of mechanical seal

2022-08-08
  • Detail

Correct selection of mechanical seals

over the past 100 years, people have used a variety of sealing materials such as fillers to reduce the outward leakage of liquid from the liquid pump shell along the main shaft. Although the liquid pump used in the modern process flow still widely uses the oldest sealing design - packing box, because it has low starting cost and is familiar to the factory staff. However, due to the environmental problems, the packing sealing method has been gradually not accepted by people, especially for the more common and corrosive liquids in modern technological processes. Therefore, in practical applications, mechanical seals are more and more used to replace packing seals

the basic element of seal mechanical seal is to use 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, while the fixed sealing surface is installed in the sealing gland. Because one sealing surface is moving and the other sealing surface is stationary, this kind of seal is called dynamic seal. The seal between the rotating surface and the stationary surface is the mechanical seal which is the basis of the most critical factor determining the sealing performance. Among them, there are four leakage paths that need to be sealed: 1 Passage between sealing surfaces; 2. passage between rotating surface and main shaft; 3. passage between fixed surface and gland; 4. passage between gland and packing box. The latter two leakage paths are generally statically sealed because there is no relative movement between the two parts. This part of the seal is usually called tertiary seal, and its sealing material is gasket or O-ring suitable for the process fluid

in the older seal design, the secondary seal under the rotating surface has a certain clearance, which can move back and forth on the main shaft, so it is easy to cause wear and premature failure. However, in the newer seal design, the secondary seal is in a static state, so the wear and corrosion problems on the spindle can be avoided. During the normal operation of the liquid pump, the pressure between the rotating surface and the stationary surface caused by the liquid in the stuffing box keeps it in a sealed state. During startup and shutdown, the pressure of the stuffing box is maintained by the pressure generated by the spring (or even replaced by the pressure of the spring). Most mechanical seals are designed with softer materials to make the rotating surface, so that it rotates and rubs on the hard stationary surface. For many years, the most common combination is to use carbon material as the rotating surface to make it run on the ceramic stationary surface

these materials are still widely used, but the stationary surface is made of stainless steel or harder materials, such as tungsten carbide or silicon carbide. Domestic large aircraft projects have driven the development trend of high-end manufacturing industry. No matter what materials are used, in short, a layer of liquid film must be maintained between the contact surfaces to play the role of lubrication. However, in the stuffing box, the combination of spring load and liquid pressure can make the sealing surfaces play a good sealing role. However, if the sealing pressure is too high, it will affect the formation of liquid film between the contact surfaces, resulting in increased heat and premature wear. If the sealing pressure is too low, the gap between the contact surfaces will increase, which is easy to cause liquid leakage. Seal manufacturers are constantly trying to improve the flatness of the contact surface. They use special polishing plates for grinding. Then, the grating plate of monochromatic light source is used to detect it

from this point of view, it is very important to carefully handle these sealing contact surfaces and strictly follow the installation instructions to ensure that the sealing surfaces are properly protected and properly positioned. Flexibility of sealing axial and radial movements of the main shaft need to maintain a certain flexibility with the spring to ensure the sealing between the contact surfaces. However, it can only provide a certain degree of flexibility. The mechanical condition of the liquid pump and its length 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 corrugated sealing devices. The traditional seal design used in the chemical industry, whose sealing pressure is applied to the rotating surface, is called rotary seal because the spring or corrugated seal device rotates with the spindle. The spring or corrugated sealing device is installed on the stationary surface. In today's mechanical seals, the above two sealing methods are widely used, which has certain flexibility for installation. Many mechanical seals designed in the early stage used a single large spring arranged around the main shaft, which can provide a strong sealing force for the sealing surface during the starting process of the liquid pump. The function of the seal depends on the rotation of the spindle to tighten the spring coil. In the later design, a series of smaller springs are arranged around the main shaft, which can produce 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

several smaller springs are used to compare the average load pressure of the members of the board of directors and the chief commercial officer markussteilemann of Obstetrics think innovation of the sealing surface, so they are more sensitive to blockage. For many corrosive application fields, the most common design is to use metal bellows sealing device. The corrugated pipe is welded by a series of metal discs to form a corrugated sealing device to prevent leakage. With this device, the sealing pressure between the sealing surfaces can be more uniform, and there is no need to add a secondary seal on the sealing surface, so there will be no corrosion and wear. Fig. 3 the metal bellows sealing device is generally used for highly corrosive liquids. Generally speaking, although its main sealing pressure depends on the pressure of the stuffing box itself, the spring and bellows can compensate for the insufficient pressure of the main shaft movement during the startup and shutdown of the liquid pump, so that the sealing surface always maintains a certain sealing pressure. Corrosion and wear caused by various reasons, such as bearing tolerance, shaft end clearance, vibration and spindle deviation, will cause radial and axial movement of the spindle of the liquid pump

in addition, it is very difficult to keep the contact surfaces absolutely parallel, so it is normal for the internal movement of the mechanical seal itself. This movement is often caused by equipment and installation tolerances, thermal expansion, pipe stress, or improper spindle adjustment. In order to make the sealing surfaces always match each other, the spring plays a constant adjustment role between the mechanical seal and the moving spindle. When the artificial rubber seal is used between the rotating surface and the main shaft, the elastomer will move back and forth on the main shaft. This repeated friction action will abrade the anti-corrosion materials on the main shaft, lose the oxide film protective layer of the main shaft, and eventually form a wear groove on the friction surface of the main shaft, resulting in liquid leakage from the groove, and increase the necessary maintenance workload or even replace the main shaft. In order to solve this problem, a replaceable shaft sleeve is usually installed in the stuffing box. However, the only permanent solution to the problem of corrosive wear is to remove its internal dynamic seal. At present, most major seal manufacturers produce non corrosive wear seals to prevent corrosion wear of pump parts whose allowable stress is based on tensile strength. The balance of balanced and unbalanced mechanical seals has a great influence on the sealing pressure of the contact surface. This sealing pressure depends on the effective cross-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 non-equilibrium seal is completely exposed to the pressure range of the stuffing box, which will generate a high sealing pressure between the sealing surfaces, thus increasing the working temperature and accelerating the wear rate. The service life of mechanical seals will be greatly reduced under high-temperature working conditions or when the liquid has greater corrosion and friction. Balancing the mechanical seal can reduce the sealing pressure and prolong the service life of the seal

generally, the main shaft and shaft sleeve with steps are used to reduce the effective section of the rotating surface to achieve the above purpose. However, do not adjust the net sealing pressure to a level close to zero, as this may result in unstable working conditions between sealing surfaces and may blow the seal open due to sudden changes. The answer to these sealing problems may be to use unbalanced sealing. For some services, unbalanced sealing may achieve better results. For example, in some applications, the safety problems caused by liquid leakage may be more emphasized than the service life of the seal. In this case, the selection of seals may also be understood as the preference for higher sealing pressure. Similarly, the increase in operating temperature may be negligible when using a cooler liquid seal. Regardless of the considerations, balanced seals are generally recommended when the packing box pressure exceeds 50psi. The most common practice for inner and outer seals is to install the seal inside the stuffing box. However, this practice requires that the wet end of the liquid pump be removed during seal maintenance. Its main advantage is that the sealing environment is hygienic and 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

the external seal mainly has the following five advantages: 1 Easy installation; 2. relatively low cost; 3. continuous monitoring and cleaning; 4. suitable for small packing boxes that cannot be sealed internally; 5. because its position is close to the bearing, it has less difficult impact on the deviation of the main shaft. Its main disadvantage is that the centrifugal force will throw the solid particles from the bottom of the seal to the contact surface of the seal. Therefore, this type of seal is mainly suitable for clean and abrasive free liquids. In recent years, the separate seal has become another important additional feature of the outer seal. The split seal is a complete assembly, which is installed between the packing box and the bearing sleeve. With this design, it is not necessary to disassemble the liquid pump every time when the seal needs to be replaced. This kind of seal is being developed in combination with other design standards. Since this design is easy to replace the seal, it is very important to resist the temptation to only replace the seal without further investigation of the root cause of the failure. Cartridge seal cartridge seal is a kind of sealing device that integrates all functions, including all sealing elements, glands and shaft sleeves. Since this kind of seal does not require any strict installation measures, the installation procedure is greatly simplified, and the sealing surface and sealing elastomer are well protected to prevent accidental damage. These advantages also mean that the repair and replacement time of the seal can be reduced

cartridge type sealing device with all functions can simplify the installation procedure and protect internal components from accidental damage. Almost all kinds of cartridge seals can be purchased in the market, so the risk factors in use can be reduced, and the maintenance time inherent in the use of ordinary seals can be saved. The double sealing and liquid barrier device adopts the double sealing surface to replace the single seal, which has a higher degree of leak proof characteristics. This kind of double seal is mostly applied to liquids with strong volatility, toxicity, cancer, danger and poor lubricity

there are generally three settings for double sealing

Copyright © 2011 JIN SHI