Mechanical seal uses the principle that two planes rub against each other 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. Since one sealing surface is moving while the other is stationary, this type of seal is called dynamic seal.

Fig. 1 The seal between the rotating surface and the stationary surface is the most critical factor to determine the sealing performance.

The mechanical seal of the foundation is shown in Figure 1, of which 4 leakage paths 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 stuffing 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 wear and corrosion problems on the main shaft can be avoided.

During 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. During the start and stop, the pressure of the stuffing box is maintained by the pressure generated by the spring (even can be replaced by the pressure of the spring).

Most mechanical seals are designed to use soft materials to make rotating surfaces, so that they can rotate and friction on a 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. This kind of material is still widely used, but the static surface is made of stainless steel or harder materials, such as tungsten carbide or silicon carbide.

No matter what materials are used, in short, a layer of liquid film must be kept between the contact surfaces to play the role of lubrication. However, in the stuffing box, the combination of spring load and liquid pressure can play a good sealing role between sealing faces. 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.

The seal manufacturer is making continuous efforts to improve the flatness of the contact surface, and they use special polishing plates for grinding. Then, the grating plate with 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 correctly positioned.

Flexible selection of seals

The axial and radial movements of the spindle need to maintain a certain flexibility with the spring to ensure the sealing between the contact surfaces. However, only a certain degree of flexibility can be provided. 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 seals.

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 corrugated seal device rotates with the main shaft. The spring or corrugated sealing device of the novel design is installed on the static surface. On the current mechanical seal, the two sealing methods mentioned above are very common, which has certain flexibility for installation.

Many mechanical seals designed in the early stage use 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 sealing function relies on the rotation of the main shaft to tension the spring coil.

Figure 2 uses several smaller springs, which can produce a relatively average load pressure on the sealing surface, so it is more sensitive to blockage.

The later designed seal (see Figure 2) uses a series of smaller springs 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.

Fig. 3 Metal bellows sealing device is generally used for highly corrosive liquids.

For many corrosive application fields, the most common design is to use metal bellows sealing device. The bellows is welded by a series of metal discs to form a corrugated seal device to prevent leakage (see Figure 3). Using this device can make the sealing pressure between sealing surfaces more uniform, and there is no need to add a secondary seal on the sealing surface, so that no corrosion and wear will occur naturally.

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 problems

Due to various reasons, such as bearing tolerance, shaft end clearance, vibration and spindle deviation, the spindle of the hydraulic 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. This kind of movement is often caused by improper adjustment of equipment and installation tolerance, thermal expansion, pipeline stress or main shaft.

In order to keep the sealing surfaces matching each other all the time, the spring plays a constant regulating 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, causing liquid leakage from the groove, and increasing the necessary maintenance workload or even replacing the main shaft. 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 the dynamic seal inside. At present, most major seal manufacturers produce non corrosive wear type 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 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 non-equilibrium seal is completely exposed to the pressure range of the stuffing box, which will generate high sealing pressure between the sealing faces, 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 is highly corrosive and abrasive.

Balancing the mechanical seal can reduce the seal pressure and extend the service life of the seal. Generally, the above purpose can be achieved by using the main shaft and shaft sleeve with steps to reduce the effective section of the rotating surface. However, never adjust the net sealing pressure to a level close to zero, because the result may cause 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 seals. For some services, unbalanced seals may achieve better results. For example, in some applications, the safety problems caused by liquid leakage may be emphasized more than the service life of seals. In this case, the choice of seal may also be understood as a preference for a higher seal pressure. Similarly, the increase in operating temperature may be negligible when choosing a cooler liquid seal.

Regardless of the consideration, balanced seal is generally recommended when the pressure of stuffing box exceeds 50psi.

Inner seal and outer seal

The most common practice 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 environmental sanitation of the seal is easy to control.

The external seal is installed by reversing the direction of the static sealing surface, and the rotating unit on the main shaft is located outside the stuffing box gland. The outer seal has the following five main benefits:

1. Easy installation;

2. The cost is relatively low;

3. It can be continuously monitored and cleaned;

4. Suitable for very 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 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 applicable to 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 stuffing box and the bearing sleeve. With this design, when the seal needs to be replaced, it is not necessary to remove the liquid pump every time. This type of seal is being gradually developed in combination with other design standards. Since this design is easy to replace the seal, it is very important to resist the temptation of only replacing the seal, without further investigation of the root cause of the failure.

Cartridge seal

Figure 4 Cartridge type sealing device with all functions

This seal simplifies the installation process and protects the internal components from accidental damage.

Cartridge seal is a sealing device that integrates all functions, and its interior includes all sealing elements, gland and shaft sleeve (see Figure 4). Since this type 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 maintenance and replacement time of seals can be reduced.

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 using ordinary seals can also be saved.

Double seal and liquid barrier device

The use of double sealing surfaces instead of a single seal has a higher degree of leak proof characteristics. This kind of double seal is mostly applied to the liquid with strong volatility, toxicity, cancer susceptibility, danger and poor lubrication.

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

Figure 5 The rotating surface of this type of seal is installed face to face, and the high-pressure or low-pressure barrier liquid system is used.

In the face to face type seal with complex structure, the rotating seal faces are arranged face to face (see Figure 5), and they often act in the opposite direction of the same static seal face. 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 serial arrangement, that is, the two 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 depressurization joint working devices.

All types of double sealing devices need to block the liquid system. They are generally external closed-loop systems, and the liquid used inside them is generally different, but must match the liquid in the process flow. The system consists of a reservoir located as close to the seal as possible.

The design of these systems varies widely. Some systems use a pump ring in the seal, while others use the principle of thermosyphon effect. The liquid in the liquid storage tank is often heated or cooled by auxiliary means. In addition, an alarm device can be added to remind the staff to change the liquid in time.

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

In order to achieve the goal of zero leakage in the whole drive operation process, the sealing industry has also developed gas barrier seals, which use inert gases such as nitrogen to replace the liquid barrier system. In the double seal with gas barrier, if the internal fault occurs on the sealing surface, only inert gas will be leaked, and the internal liquid will not leak, so as not to cause environmental pollution.

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

environmental control

In many application fields, where seals are installed, the reliable operation and control environment of seals should be taken into account, so the following points must be noted:

1. The seal shall be installed on the high strength main shaft and keep the minimum deviation. Although the maximum deviation of the sealing surface specified in the industrial standard is 0.002in, the requirements for the main shaft can be even higher.

2. The seal shall be installed in the large diameter seal chamber to improve the reliability of the seal. Almost all liquid pump manufacturers can provide such products.

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

4. Keep the temperature in the packing box within the operating parameters of the sealing material.

5. Keep the liquid in the stuffing box clean.