Prevent pump leakage - how to choose the correct mechanical seal

For more than 100 years, a variety of packing materials, such as packing, have been used to reduce the amount of liquid leaking out of the pump housing along the major axis. Despite the pumps used in modern processes, the oldest sealed design, the stuffing box, is still widely used because of its low initial cost and familiarity to the plant staff. However, due to the environmental problems, the method of using packing seal has gradually become unacceptable, especially for the more common and corrosive liquids in modern process. Therefore, in practical applications, the use of mechanical seals instead of packing more and more. The basic elements of the seal Mechanical seal is the use of friction between the two planes running principle, to seal the purpose. The rotary sealing surface is installed on the main shaft of the liquid pump, and the fixed sealing surface is installed in the sealing gland. Because one sealing surface is moving while the other sealing surface is stationary, this type of seal is called a dynamic seal. Figure 1 The sealing surface between the rotating surface and the stationary surface is the most critical factor that determines the sealing performance, of which 4 leak paths need to be sealed: 1. The passage between the sealing surfaces; 2. The passage between the rotating surface and the spindle; 3 . The passage between the fixing surface and the gland. 4. The passage between the gland and the stuffing box. The latter two leak paths generally use static seals because there is no relative movement between the two parts. This part of the seal are usually referred to as the tertiary seal, the sealing material is a gasket or O-ring seal compatible with the process liquid. In older seal designs, the secondary seal, located below the surface of revolution, left some clearance to move back and forth on the spindle, thus easily causing wear and premature failure. However, in newer seal designs, the secondary seal is at rest, thereby avoiding the problem of wear and corrosion on the spindle. During normal operation of the pump, the pressure between the rotating face and the stationary face due to the fluid in the stuffing box keeps it in a sealed condition, and the pressure in the stuffing box is maintained by the pressure generated by the spring during start up and shut down Can even be replaced by the pressure of the spring). Most mechanical seals are designed with a softer material to create a rotating surface that rotates friction on a harder stationary surface. Over the years, the most common combination is the use of carbon material as a rotating surface, so that the ceramic stationary surface running. These types of materials are still in common use today, but stationary faces are made of stainless steel or harder materials such as tungsten carbide or silicon carbide. Regardless of the material used, in any event, a liquid film must be maintained between the contact surfaces to provide lubrication. However, in packing boxes, the combination of spring load and fluid pressure provides a good seal between the sealing surfaces. However, the sealing pressure is too high, it will affect the formation of liquid film between the contact surface, leading to increased heat and premature wear. If the sealing pressure is too low, the gap between the contact surface increases, likely to cause liquid leakage. Seal manufacturers are constantly working to improve the flatness of the contact surfaces, and they use a special polishing plate for grinding. It is then detected using a monochromatic light grid plate. From this point of view, it is important that these sealing surfaces be handled with care and in strict accordance with the installation instructions to ensure that the sealing surfaces are adequately protected and properly seated. Sealing flexibility Selecting the spindle's axial and radial motions requires some flexibility with the spring to ensure a tight seal between the contact surfaces. However, it can only provide a degree of flexibility. The mechanical condition of the pump and its length-to-diameter ratio (a measure of the ratio of the diameter of a spindle to its extension, the lower the ratio, the better) have an important bearing on the reliability of the seal. Sealing flexibility is generally guaranteed by a large main spring and a series of small springs or bellows seals. The traditional seal design used in the chemical industry, the sealing pressure is applied to the rotating surface, this seal is called a rotary seal, because the spring or bellows seal with the spindle rotation. The more novel design, with its spring or bellows seal mounted on the stationary face. In the current mechanical seal, the two sealing methods have very common applications, so for the installation has some flexibility. Many of the early designs of mechanical seals were arranged around the spindle using a single large spring that provided a strong sealing force on the seal faces during the pump start-up. The sealing action relies on the rotation of the spindle to tension the spring roll. Late design of the seal using a series of smaller springs, arranged around the spindle, the sealing surface can produce a more uniform load pressure. Most of these types of seals are completely isolated from the fluid being pumped because smaller springs can be installed in advance. Figure 2 uses a number of smaller springs, which produce a more uniform load pressure on the sealing surface and are therefore more susceptible to blockage. For many corrosive applications, the most common design is the use of metal bellows seals. The bellows are welded from a series of metal discs to form a leak-proof bellows seal (see Figure 3). With this device, the sealing pressure between the sealing surfaces can be made even more uniform, and there is no need to increase the secondary sealing on the sealing surface, so naturally, no corrosion and abrasion occurs. In general, although the primary seal pressure depends on the pressure in the stuffing box itself, the spring and bellows compensate for the lack of pressure on the spindle during start-up and shut-down of the pump, keeping the seal face always tight. Corrosion Wear problems For many reasons, such as bearing tolerances, shaft clearances, vibrations, and spindle deviations, both radial and axial motions of the pump's main shaft can occur. In addition, since it is very difficult to keep the contact surfaces absolutely parallel, it is normal for the inside of the mechanical seal itself to move. Such movements are often caused by equipment and installation tolerances, thermal expansion, pipe stress, or improper spindle tuning. In order to always keep the sealing surface between each other, the spring has played a mechanical seal and the movement of the spindle between the constant regulation. When sealed between the rotating surface and the main shaft with elastomer, the elastomer moves back and forth on the main shaft. Such repeated frictional action will abrade the anti-corrosion material on the spindle and lose the protective coating of the oxide film on the spindle, eventually forming a wear groove on the friction surface of the spindle causing liquid to leak from the groove and adding the necessary maintenance effort Or even replace the spindle. In order to solve this problem, it is common to install a replaceable sleeve in the stuffing box. However, the only permanent solution to the problem of corrosion wear is to remove the dynamic seal inside it. Most major seal manufacturers now produce non-corrosive wear seals to prevent corrosive wear on pump components. The balance of unbalanced and unbalanced mechanical seal seals has a significant effect on the seal pressure at the contact surfaces. This sealing pressure depends on the effective cross-section of the seal itself and the pressure inside the packing box. Unbalanced seals are exposed to the opposite side of the rotating face from the pressure range of the stuffing box. This creates a high seal pressure between the sealing faces, resulting in higher operating temperatures and faster wear rates. Under high temperature working conditions or the liquid has greater corrosiveness and abrasion, the service life of the mechanical seal will be greatly reduced. Balancing the mechanical seal reduces the seal pressure and extends the service life of the seal. The general use of the step with the spindle and sleeve to reduce the effective cross-section rotating surface, you can achieve the above purpose. But do not adjust the net seal pressure to near zero because of this

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