Prevent pump leakage - correct choice of 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. Although modern technological process used in liquid pumps , still widely in use oldest seal design - packing boxes, because of its low initial cost, but also for the staff of the factory familiar. 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 seal between the rotating surface and the stationary surface is the most critical factor in determining the sealing performance. The basic mechanical seal is shown in Figure 1, of which 4 leak paths need to be sealed: 1. The path between the sealing faces; 2. Rotation Surface and the spindle between; 3. Fixed surface and the gland between the access; 4. Gland and packing box between the access. 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. The late-design seal (see Figure 2) uses a series of smaller springs arranged around the spindle to create a more uniform load pressure on the seal face. 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 more uniform, and there is no need to increase the secondary sealing on the sealing surface, so naturally, no corrosion and abrasion phenomenon will occur. Figure 3 Metal bellows sealing device is generally used for corrosive liquids In general, although the main sealing pressure depends on the pressure in the packing box itself, but the spring and bellows can compensate for the pump during start-up and shut down due to Spindle motion of the lack of pressure, so that the sealing surface always maintain a certain sealing pressure. 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. However, do not adjust the net seal pressure to near zero as the result of this may cause unstable working conditions between the seal faces and may blow away the seal due to sudden changes. The answer to these sealing problems may be with unbalanced seals, which for some services may be better with unbalanced seals. For example, in some applications, the safety issues associated with fluid leakage may be more emphasized than the service life of the seal. In this case, the choice of seal may be understood as more hope to choose a higher sealing pressure. Also, the increase in operating temperature may be negligible when using seals that use colder liquids. Regardless of the considerations, a balanced seal is generally recommended when the packing box pressure exceeds 50 psi. The most common practice for inner and outer seals is to mount the seal inside the stuffing box. However, this practice requires that the wet end of the liquid pump be removed during seal maintenance. The main benefit is that the sealed environment is easily controlled. The outer seal is installed by reversing the direction of the static seal face and the rotating unit on the spindle is located outside the gland packing. The outer seal has the following five main benefits: 1. Easy to install; 2. Relatively low cost; 3. Can be continuously monitored and cleaned; 4. Suitable for small stuffing boxes that can not be internally sealed; 5. Due to its proximity to the bearings , Less impact on the spindle deviation less. The main disadvantage is that centrifugal forces can throw solid particles from the underside of the seal toward the sealed contact surface. Therefore, this type of seal is mainly suitable for clean and abrasive-free liquids. In recent years, separate seals have become another important additional feature in the outer seal. The split seal is a complete assembly installed between the packing box and the bearing housing. This design eliminates the need to disassemble the pump every time the seal needs to be replaced. This type of seal to explore other design standards is being developed gradually. Because this design makes it easy to change seals, it is important to resist the temptation to simply replace the seals without further investigation of the source of the failure. Cartridge Seal Cartridge Seal is an all-in-one seal that includes all the sealing elements, glands and bushing inside (see Figure 4). Due to the fact that this type of seal does not require any rigorous installation, the installation procedure is greatly simplified and the sealing surfaces and sealing elastomer are well protected against accidental damage. These advantages also mean reduced seal repair and replacement time. Figure 4 With its own full-featured cartridge seal, this seal simplifies the setup process while protecting the internal components from accidental damage. Almost all cartridge seals are available on the market, thereby reducing the risk factors associated with their use and also saving on the time required to maintain them using conventional seals. Double seals and liquid barriers use a double seal instead of a single seal with a higher degree of leak-proofing. These double seals are mostly used in volatile, toxic, carcinogenic, dangerous and poor lubricity of the liquid. There are three general designs for double seals, each of which requires the use of a liquid barrier system between the double seal faces to prevent liquid or gas leaks. The more common low-cost dual seal is a back-to-back mounted seal with rotating sealing surfaces in opposite directions. It often requires a barrier liquid that is at a pressure higher than the pack pressure of about 20 psi so that the internal seal is always lubricated with a barrier liquid and also ensures that the seal faces achieve a certain seal pressure. In more complex face-to-face seals, the rotating seal faces are arranged in a face-to-face fashion (see Figure 5), which tend to act in the opposite direction of the same stationary seal face. This type of seal can be either a high pressure barrier liquid system or a low pressure barrier liquid system. Figure 5 This type of sealed rotating surface is mounted face-to-face with a high-pressure or low-pressure barrier liquid system. The third design type of seal is a serial arrangement where both rotating sealing surfaces leave the impeller toward the same Directional arrangement. This sealed barrier liquid pressure is generally lower than the pressure of the liquid pump. In fact, equivalent to two sealed, two step-down joint work device. All types of double seals require a barrier liquid system. They generally belong to the external closed-loop system, the liquid used inside them are generally different, but must match the liquid in the process. The system includes a reservoir that is located as close as possible to the seal. The design of these systems varies widely. Some systems employ a pump ring in the seal, while others use the principle of thermosyphon. Liquids in reservoirs often use auxiliary heating or auxiliary cooling methods. In addition, you can also add an alarm device to remind staff to promptly replace the liquid. Depending on the nature of the liquid to be sealed, the barrier fluid system can operate at a pressure below or above the pressure in the cartridge. In order to achieve the goal of zero leakage throughout the drive operation, the seal industry has also developed gas barrier seals that use a class of inert gas, such as nitrogen, to replace the liquid barrier system. In gas barrier dual seals, if the inside of the sealing face fails, the inert gas leaks and the liquid inside does not leak and will not cause environmental pollution. Whether using liquid or gas, the barrier system must be suitable for a particular seal and should be alerted immediately of any failure within the seal face so that appropriate action can be taken promptly. Environmental Control In many applications, where a seal is installed, reliable operation and control of the seal must be taken into account. Therefore, the following points must be observed: 1. The seal should be mounted on a high-strength spindle with minimum deviation. Although the industry standard specifies a maximum deviation of 0.002 in sealing surface, the requirement on the spindle can be even higher. 2. The seal should be installed in a large diameter sealed chamber, which can improve the reliability of the seal. Almost all liquid pump manufacturers of these products can provide. 3. Control packing box internal pressure to avoid reaching flash point. 4. Keep the temperature inside the packing box within the operating parameters of the sealing material. 5. Keep the packing box liquid clean. Originally PROCESS "Process Industry"

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