For more than 100 years, people have used a variety of sealing materials such as packing to reduce the amount of liquid leaking from the pump housing along the main shaft. Although the liquid pumps used in the modern process flow, the oldest sealing design, the stuffing box, is still widely used because of its low initial cost and familiarity with the factory staff. However, due to environmental problems, the method of using packing seals has gradually become unacceptable, especially for the common and highly corrosive liquids in modern technological processes. Therefore, in practical applications, more and more mechanical seals are used instead of packing seals. The seal between the rotating surface and the stationary surface is the most critical factor in determining the sealing performance. The mechanical seal on the basis of which has 4 leakage paths that need to be sealed: 1) The passage between the sealing surfaces; 2) The path between the rotating surface and the main shaft; 3) The passage between the fixed surface and the gland; 4) The passage between the gland and the stuffing box. The latter two leak paths are generally statically sealed since there is no relative motion between the two parts. This part of the seal is usually called a three-time seal, and its sealing material is a gasket or an O-ring that is compatible with the process liquid. In newer seal designs, the secondary seal is at rest, thus avoiding galling corrosion problems on the main shaft. During the normal operation of the liquid pump, the pressure between the rotating surface and the stationary surface is kept in a sealed state by the pressure generated by the liquid in the stuffing box. When starting and stopping, the pressure of the stuffing box is maintained by the pressure generated by the spring. Most mechanical seal designs use a softer material for the rotating face, causing it to rotate and rub against a harder, stationary face. For many years, the most common combination has been to utilize a carbon material as the rotating surface running on a ceramic stationary surface. From this point of view, it is very important that these sealing contact surfaces are handled with care and that installation instructions are strictly followed to ensure that the sealing surfaces are properly protected and properly seated. The flexibility of sealing selection The axial and radial movement of the main shaft needs to maintain a certain degree of 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 pump and its length-to-diameter ratio (a measure of the ratio of the diameter of the main shaft to its extension, the lower the better) play an important role in the reliability of the seal. Seal flexibility is typically provided by a large main spring and a series of smaller springs or bellows seals. Traditional seal designs used in the chemical industry, where the sealing pressure is applied to a rotating face, are called rotary seals because the spring or bellows seal rotates with the shaft. Newer designs have springs or bellows seals mounted on stationary surfaces. In the current mechanical seal, the above two sealing methods are very commonly used, so there is a certain flexibility for installation. Many mechanical seals of earlier designs used a single large spring arranged around the main shaft to provide strong sealing force to the seal faces during pump start-up. The function of the seal relies on the rotation of the spindle to tighten the spring coil. The seal designed in the later period adopts a series of smaller springs arranged around the main shaft, which can generate a relatively uniform load pressure on the sealing surface. Most of these seals are completely isolated from the liquid being pumped because the smaller spring can be pre-installed. Using multiple smaller springs can generate a more even load pressure on the sealing surface, so it has greater sensitivity to clogging. For many highly corrosive applications, the most common design is a metal bellows seal. The bellows are welded from a series of metal discs to form a bellows seal that prevents leakage. The adoption of this device can make the sealing pressure between the sealing surfaces more uniform, and there is no need to add secondary seals on the sealing surfaces, so naturally no corrosion and wear will occur. In order to ensure that the sealing surfaces always cooperate with each other, the spring plays a role of constant adjustment between the mechanical seal and the moving main shaft. When an elastomeric seal is used between the rotating surface and the spindle, the elastomer moves back and forth on the spindle. This repeated friction action will wear away the anti-corrosion material on the main shaft, lose the oxide film protection layer of the main shaft, and eventually form wear grooves on the friction surface of the main shaft, causing liquid to leak from the grooves and increasing the necessary maintenance workload Or even replace the spindle. In order to solve this problem, a replaceable shaft sleeve is generally installed in the stuffing box. However, the only permanent solution to the corrosion wear problem is to remove the dynamic seal inside it. Most major seal manufacturers now produce non-corrosive wear seals to prevent corrosion and wear of pump parts. Balanced and unbalanced seal The balance of the mechanical seal has a great influence on the sealing pressure of the contact surface. This sealing pressure depends on the effective cross section of the seal itself and the pressure inside the stuffing box. The section on the opposite side of the rotating surface of the unbalanced seal is completely exposed to the pressure range of the stuffing box, which will cause a high sealing pressure between the sealing surfaces, which will increase the working temperature and accelerate the wear rate. Under high temperature working conditions or when the liquid is highly corrosive and abrasive, the service life of the mechanical seal will be greatly reduced. Balancing the mechanical seal can reduce the sealing pressure and prolong the service life of the seal. The outer seal mainly has the following five advantages: 1) Easy to install; 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) Due to its location close to the bearing, it has less influence on the difficulty of the main shaft deviation. Its main disadvantage is that centrifugal force will throw solid particles from under the seal to the contact surface of the seal. Therefore, this type of seal is mainly suitable for clean and non-abrasive liquids. In recent years, split seals have become another important additional feature in outer seals. The separate seal is a complete assembly, installed between the stuffing box and the bearing sleeve. With this design, it is not necessary to disassemble the liquid pump every time the seal needs to be replaced. Such seals are being developed in conjunction with other design criteria discussed. Since this design allows for easy seal replacement, it is important to resist the temptation to simply replace the seal without further investigation into the root cause of the failure. Cartridge seal The cartridge seal is an all-in-one sealing device, which includes all sealing elements, glands and bushings. 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. It also means less time for seal repair and replacement. It has a cartridge-type sealing device with all functions, which can simplify the installation procedure and protect the internal components from accidental damage. Almost all types of cartridge seals are commercially available, thus reducing the risk factor in use and also saving maintenance time inherent in the use of conventional seals. Double seal and liquid barrier device adopts the seal of double sealing surface instead of single seal to have a higher degree of leak-proof characteristics. Most of these double seals are used in highly volatile, toxic, carcinogenic, dangerous and poorly lubricated liquids. There are generally three design types of 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 dual seal is a back-to-back mounted seal with rotating sealing faces aligned in opposite directions. It often requires a barrier liquid, whose pressure should be higher than the pressure of the stuffing box, about 20psi, so as to ensure that the internal seal can always be lubricated by the barrier liquid, and also ensure that the sealing surface reaches a certain sealing pressure. In the face-to-face type seal with a relatively complex structure, the rotating sealing surfaces are arranged face-to-face, and they often act in the opposite direction of the same static sealing surface. These seals are available in both high pressure and low pressure barrier fluid systems. The rotating faces of this type of seal are installed face-to-face, and a high-pressure or low-pressure barrier liquid system is used. The third design type of seal is arranged in a series, that is, both rotating sealing faces are away from 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-reducing joint working devices. All types of double seals require barrier fluid systems. They are generally external closed-loop systems, and the liquid used inside is generally different, but must match the liquid in the process. The system includes a reservoir located as close to the seal as possible. The design of these systems can vary widely. Some systems use a pump ring in the seal, while others use the principle of the thermosiphon effect. The liquid in the liquid storage tank often adopts the method of auxiliary heating or auxiliary cooling. In addition, an alarm device can be added to remind the staff to replace the liquid in time. Depending on the nature of the liquid being sealed, barrier fluid systems can operate at pressures below or above the stuffing box. In order to achieve the goal of zero leakage during the entire driving operation, the sealing industry has also developed a gas barrier seal, which uses an inert gas such as nitrogen to replace the liquid barrier system. In the double seal with gas barrier, if the inside of the sealing surface fails, only the inert gas will leak, and the liquid inside will not leak, thus will not cause environmental pollution. Regardless of whether liquid or gas is used, the barrier system must be suitable for the specific sealing application, and once any failure occurs inside the sealing surface, the alarm should be recognized immediately so that appropriate measures can be taken in time. Environmental control In many application fields, where the seal is installed, it is necessary to consider the reliable operation and control environment of the seal, so the following points must be paid attention to: 1) The seal should be installed on a high-strength spindle with minimum deviation. Although the industry standard specifies a maximum deviation of 0.002 inches for the sealing surface, the requirements for the spindle can be even higher. 2) The seal should be installed in a large-diameter seal chamber, which can 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 inside the stuffing box within the working parameter range of the sealing material. 5) Keep the liquid in the stuffing box clean.  

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