Choose the mechanical seal correctly to prevent th

Correct selection of mechanical seals to prevent liquid pump leakage

for more than 100 years, people have used a variety of sealing materials such as packing - reducing noise, vibration and harshness (NVH) and friction consumption, so that the outward leakage of liquid from the liquid pump shell along the main shaft is continuously reduced. Although the liquid pump used in the modern process flow is still widely used, the oldest sealing design stuffing box is still widely used, because its starting cost is low, and it is familiar to the factory staff

however, due to environmental problems, the method of packing sealing has gradually become unacceptable, 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

basic elements of sealing

mechanical seal uses the principle of two planes rubbing 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. Because one sealing surface is moving and the other sealing surface is stationary, this kind of seal is called dynamic seal

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

the basic mechanical seal is shown in Figure 1, in which there are four leakage paths that need to be sealed:

1 Passage between sealing surfaces

2. The passage between the rotating surface and the main shaft

3. The passage between the fixed surface and the gland

4. Passage between gland and stuffing box

the latter two leakage paths generally adopt static sealing, 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 located 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 wear and corrosion problems on the spindle can be avoided

in 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 (even can be replaced by the pressure of the spring)

most mechanical seals are designed with soft materials to make the rotating surface rotate and rub 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 stationary surface of ceramics. Such materials are still widely 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 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, and 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 handle these sealing contact surfaces carefully and strictly follow the installation instructions to ensure that the sealing surfaces are properly protected and in place

flexibility of seal selection

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, 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 sealing 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 seal pressure is applied to the rotating surface, is called rotary seal, because the spring or corrugated seal device rotates with the spindle. The relatively novel design, its 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 a 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 function of sealing depends on the rotation of the spindle to tighten the spring coil

the seal designed in the later stage (see Figure 2) adopts a series of smaller springs, which are arranged around the shaft, which accounts for about 1/10 of the annual output of various plastic products in China, and 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

Figure 2 uses several smaller springs, which can produce relatively average load pressure on the sealing surface,

so it is more sensitive to blockage

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 sealing device to prevent leakage (see Figure 3). Using 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

Figure 3 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 starting and stopping 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 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 improper equipment and installation tolerances, thermal expansion, pipeline stress, or spindle commissioning

in order to keep the 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 eventually form a wear groove on the friction surface of the main shaft, causing liquid leakage from the groove, and increasing the maintenance work of the universal experimental machine in the case of the necessary single space structure, or even replacing 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 corrosion and 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 seals has a great impact 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 a high sealing pressure between the sealing faces, thus increasing the working temperature and accelerating the wear speed. 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, because the result of this may cause instability between the sealing surfaces and may blow the seal open due to sudden changes

the answer to these sealing problems may be to adopt 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 a preference for higher sealing pressure. Similarly, when choosing a cooler liquid seal, the increase in operating temperature may be negligible

for whatever reason, balanced seals are generally recommended when the pressure of the stuffing box exceeds 50psi

inner seal and outer seal

the most common way 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, and its main advantage is that the environmental sanitation of the seal 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 spindle is located outside the gland of the stuffing box. The external seal mainly has the following five advantages:

1 Easy installation

2. The cost is relatively low

3. Continuous monitoring and cleaning

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

5. Because its position is close to the bearing, it has less difficult influence on the deviation of the main shaft

its main disadvantage is that centrifugal force will throw solid particles from the bottom of the seal to the contact surface of the seal. Therefore, this kind of seal is mainly suitable for clean and abrasive free liquids

in recent years, the split 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 disassemble the liquid pump every time. This kind 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 to only replace the seal without further investigation of the root cause of the failure

cartridge seal

cartridge seal is a sealing device that integrates all functions, including all sealing elements, glands and shaft sleeves (see Figure 4). 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

Figure 4 has a cartridge type sealing device with all functions. Using this sealing can simplify the installation procedure,

at the same time, it can protect all internal components from accidental damage

almost all kinds of cartridge seals can be purchased in the market, so it can reduce the use