Mechanical seal, also known as end face seal, is a device used to solve the sealing between rotating shaft and engine body. It is a device to prevent fluid leakage, which is composed of at least one pair of fluid pressure at the end face perpendicular to the rotating axis and the elastic force (or magnetic force) of the compensation mechanism, and the auxiliary seal, which is kept in contact with each other and slides relatively. It is commonly used in rotary fluid machinery such as pumps, compressors, reaction agitators, and also used for sealing gear boxes, ship stern shafts, and so on. Therefore, mechanical seal is a universal shaft seal device.
There are two types: contact type and non-contact type. At present, most of them use contact type mechanical seal, while non-contact type mechanical seal is developing and has attracted people's attention. Mechanical seal is a non functional part of the machine, but it plays a great role in reducing leakage and stabilizing production.
Working principle of mechanical seal
The working principle of mechanical seal is to use two sealing surfaces that are mutually bonded, flat and perpendicular to the rotation axis, and rotate relatively. It relies on the pressure of elastic elements (such as springs or bellows) and sealing media to generate appropriate compression force on the contact end of the moving ring rotating with the shaft and the static ring not rotating with the shaft, so that the two contact end faces closely fit, and an extremely thin liquid film is maintained between the end faces, so as to achieve the purpose of sealing.
Function of mechanical seal
1、 Improve machine efficiency and reduce energy consumption.
1. Reduce the internal leakage, external leakage and penetration of the machine, and improve the volumetric efficiency of the machine. For example, Daqing Refinery changed the impeller mouth ring of coker 5FDJ5 × 2 centrifugal pump to mechanical seal, which improved the pump efficiency by 3.6% and 5.6%.
2. Reduce friction loss and improve mechanical efficiency of the machine. For example, changing the double end seal to single end seal, changing the unbalanced type to balanced type, and reducing the seal at the closed end of the double support bottom reducing pump (the so-called chopping pump) can reduce the friction loss and improve the mechanical efficiency.
3. Change the sealing method to improve the efficiency of the machine or unit. For example, the canned motor pump is replaced by a mechanical seal pump to improve the efficiency of the motor; Adopt magnetic drive pump to improve machine efficiency.
4. Change the auxiliary system to reduce energy consumption and improve unit efficiency. For example, hot oil pump self flushing is changed to small impeller circulating flushing; Double face seal is replaced by single face seal to save energy consumption of auxiliary oil sealing system.
2、 Save raw materials. For example, the recovery of process fluid can reduce or eliminate the loss of power steam and process fluid, and reduce the loss of sealing oil.
3、 Improve machine reliability. For example, the leakage and life of the shaft seal determine the reliability of the shaft seal and the machine.
4、 Safety and environmental protection. According to the investigation report of a factory in Japan, 32 of the 786 accidents were caused by leakage (42%). In addition, process fluid leakage will also cause environmental pollution, including air pollution, water pollution and workshop environmental pollution.
Technical essentials of mechanical seal installation and use
1. The radial runout of the equipment shaft shall be ≤ 0.04mm, and the axial runout shall not be greater than 0.1mm;
2. The sealing part of the equipment shall be kept clean during installation, and the sealing parts shall be cleaned. The sealing end face shall be intact to prevent impurities and dust from entering the sealing part;
3. It is strictly prohibited to hit or knock during installation to avoid damaging the friction pair of mechanical seal and sealing failure;
4. During installation, a layer of clean mechanical oil shall be coated on the surface in contact with the seal to facilitate smooth installation;
5. When installing the stationary ring gland, the tightening screws must be evenly stressed to ensure the perpendicularity between the end face of the stationary ring and the axis line;
6. After installation, push the moving ring by hand to make it move flexibly on the shaft with certain elasticity;
7. After installation, rotate the shaft by hand, and the shaft should have no sense of weight;
8. The equipment must be filled with medium before operation to prevent seal failure due to dry friction.
