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Correct Selection of Mechanical Seal Is the Key to Preventing Leakage of Liquid Pump
Date: 2022-01-04

Correct Selection of Mechanical Seal Is the Key to Preventing Leakage of Liquid Pump

Author: Admin  Source: Original Article  Article Hits:


     For more than 100 years, a variety of sealing materials such as fillers have been used 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 elements 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 a dynamic seal.

    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 the sealing material is a gasket or an O-ring that is compatible with the process liquid.

    In the older seal design, the secondary seal under the rotating surface has a certain gap, 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 problem of wear and corrosion on the main shaft can be avoided.

    In the normal operation of the liquid pump, the pressure generated by the liquid in the stuffing box between the rotating surface and the stationary surface keeps it in a sealed state. 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).

    The design of most mechanical seals uses 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 stationary surface is made of stainless steel or harder materials, such as tungsten carbide or silicon carbide.

    No matter what material is used, in short, 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.

    Seal manufacturers are constantly striving to improve the flatness of the contact surface, and 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, these sealing contact surfaces must be handled carefully and the installation instructions must be strictly followed to ensure that the sealing surfaces are properly protected and properly seated. This is very important.

Flexible choice of seals

    The axial and radial movement of the main shaft needs to maintain a certain 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 ensured by a large main spring and a series of small springs or bellows sealing devices.

    The traditional seal design used in the chemical industry has a sealing pressure applied to the rotating surface. This kind of seal 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 seals, the above two sealing methods have very common applications, so that they have a certain 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.

    For many corrosive applications, the most common design is to use a metal bellows sealing device. The bellows is welded by a series of metal discs to form a corrugated sealing device to prevent leakage (see Figure 3). 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.

    Generally speaking, although the main sealing pressure depends on the pressure of the stuffing box itself, the spring and bellows can compensate for the lack of pressure due to the movement of the main shaft during the start and stop of the liquid pump, so that the sealing surface always maintains a certain sealing pressure.

Corrosion and wear problems

    For many reasons, such as bearing tolerance, shaft end clearance, vibration and spindle deviation, the spindle of the liquid pump will produce radial and axial movement. 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. Such movement is often caused by equipment and installation tolerances, thermal expansion, pipeline stress or improper spindle commissioning.

    In order to keep mutual cooperation between the sealing surfaces, 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 finally form a wear groove on the friction surface of the main shaft, resulting in liquid leakage from the groove, increase the necessary maintenance workload or even replace the main shaft. In order to solve this problem, a replaceable shaft sleeve is generally installed in the stuffing box.

    However, the only permanent solution to the problem of corrosive wear is to remove its internal dynamic seal. Now, most major seal manufacturers produce non corrosive wear seals to prevent corrosive wear of liquid pump parts.

Balanced and unbalanced seals

    The balance of mechanical seal has a great influence on the sealing pressure of contact surface. This sealing pressure depends on the effective section of the seal itself and the pressure in the packing box.

    The 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. This situation will produce a high sealing pressure between the sealing surfaces, which will increase the working temperature and accelerate the wear speed. The service life of mechanical seal will be greatly reduced under high temperature working conditions or when the liquid has great corrosivity and friction.

    Balancing the mechanical seal can reduce the sealing pressure and prolong the service life of the seal. Generally, the spindle and shaft sleeve with steps are used to reduce the effective section of the rotating surface. However, do not adjust the net sealing pressure to a level close to zero, as the result may cause unstable working conditions between the 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 seal may also be understood as a preference for higher sealing pressure. Similarly, the increase in operating temperature may be insignificant when using a cooler liquid seal.

    Regardless of considerations, balanced seals are generally recommended when the pressure of the stuffing box exceeds 50 psi.

Inner seal and outer seal

    The most common method 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. The outer seal has the following five major benefits:

   1. Easy to install;

   2. The cost is relatively low;

   3. It can be continuously monitored and cleaned;

   4. Suitable for small stuffing boxes that cannot be sealed inside;

   5. Because its position is close to the bearing, it has less influence on the difficulty of spindle deviation.

    The main disadvantage is that the centrifugal force throws solid particles from the bottom of the seal to the contact surface of the seal. Therefore, this type of seal is mainly suitable for liquids that are clean and do not contain abrasives.

    In recent years, separate seals have become another important additional feature in outer seals. The separate seal is a complete assembly, which is installed between the stuffing box and the bearing sleeve. With this design, when the seal needs to be replaced, there is no need to disassemble the pump every time. This type of seal is being gradually developed in conjunction 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 into the root cause of the failure.

    Cartridge type seal is a kind of sealing device that integrates all functions, including all sealing elements, glands and shaft sleeves (see Figure 4). Because this type of seal does not require any strict installation measures, the installation procedure is greatly simplified, and the sealing surface and the sealing elastomer are well protected to prevent accidental damage. These advantages also mean that seal repair and replacement time can be reduced.

    Almost all kinds of cartridge type seals are available on the market, so the risk factors in use can be reduced, and the maintenance time inherent in the use of ordinary seals can also be saved.

Double seal and liquid barrier device

    The use of double sealing surface seals instead of a single seal has a higher degree of leakage resistance. This type of double seal is mostly used in liquids with strong volatility, toxic, carcinogenic, dangerous and poor lubricity.

    There are generally three design types for double seals, each of which requires a liquid barrier system between the double sealing surfaces to prevent leakage of liquid or gas. The more commonly used low-cost double seal is a back-to-back seal with rotating sealing surfaces arranged in opposite directions. It often requires a barrier liquid whose pressure should be higher than the stuffing box pressure, about 20 psi, so that the internal seal can always be lubricated by the barrier liquid, and it can also ensure that the sealing surface reaches a certain sealing pressure.

    In a face-to-face type seal with a more complex structure, the rotating sealing surfaces are arranged in a face-to-face manner (see Figure 5), and they often act in opposite directions to the same static sealing surface. This type of seal can use either a high-pressure barrier liquid system or a low-pressure barrier liquid system.

    The seal of the third design type adopts a serial arrangement, that is, both rotating sealing surfaces leave the impeller and are arranged in the same direction. The barrier liquid pressure of this seal is generally lower than the pressure of the liquid pump. In fact, it is equivalent to two sealed, two-stage pressure reduction joint working devices.

    All types of double sealing devices require a liquid barrier system. They generally belong to an external closed-loop system, and the liquid used inside is generally different, but it must match the liquid in the process. The system includes a reservoir, which should be located as close as possible to the seal.

    The design of these systems varies widely. Some systems use a pump ring in the seal, while other systems use the principle of thermosiphon effect. The liquid in the liquid storage tank often adopts auxiliary heating or auxiliary cooling methods. In addition, an alarm device can be added to remind the staff to replace the liquid in time.

    According to the nature of the liquid to be sealed, the liquid barrier system can work at a pressure lower or higher than the stuffing box.

    In order to achieve the goal of zero leakage during the entire driving operation, the sealing industry has also developed gas barrier seals, which use inert gases such as nitrogen to replace liquid barrier systems. In the double seal with gas barrier, if the inside of the sealing surface fails, only inert gas will be leaked, and the liquid inside will not leak, so that it will not cause environmental pollution.

    Regardless of whether liquid or gas is used, the barrier system must be suitable for the specific sealing use, and should be able to immediately identify and alarm once any failure occurs inside the sealing surface, so that appropriate measures can be taken in time.

Environmental control

    In many application fields, where the seal is installed, the reliable operation and control environment of the seal need to be considered. Therefore, the following points must be paid attention to:

   1. The seal should be installed on a high-strength main shaft with minimal deviation. Although the industry standard specifies a maximum deviation of 0.002in for the sealing surface, the requirements for 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 hydraulic pump manufacturers can provide this kind of product.

   3. Control the internal pressure of the stuffing box to avoid reaching the flash point.

   4. Keep the temperature in the stuffing box within the working parameters of the sealing material.

   5. Keep the liquid in the stuffing box clean.