Causes of leakage of twin-screw pump mechanical seal
1. Mechanical seal performance issues
Problems with mechanical seal performance are mainly reflected in the following aspects:
1) Caused by improper selection of friction pair materials.
For example, the material used in the friction pair has poor wear resistance, a large friction coefficient, and the end face-specific pressure level is too high, etc., which will lead to a reduction in mechanical seal performance and shorten the service life of the mechanical seal.
2) Caused by an auxiliary sealing ring.
If the interference value of the auxiliary sealing ring of the moving ring in the equipment is too large, the friction between the sealing ring and the shaft will increase. The elastic force cannot overcome the resistance to prevent the axial sliding of the moving ring, and the compensation position cannot be obtained, thus causing the two rings to slide. The sealing properties are weakened, causing leaks.
3) Spring failure leads to leakage.
The device assembly spring is protected by an auxiliary seal and does not normally come into contact with the conveyed medium. However, if the spring sealing ring loses its sealing performance, or the leakage hole is clogged, causing the input liquid to cross-flow, the spring will come into contact with the conveyed material, causing the spring to fail, and ultimately leading to the failure of the mechanical seal.
2. Improper installation problem
During the installation process, due to the deformation or unevenness of the sealing end face, the flatness of the end face was not checked before installation, or the sealing face was damaged due to various reasons during the process; the dynamic and static rings were not concentric or had wrong dimensions, and damage occurred during the installation process. Or there is no compression; there is foreign matter on the seal end face; the compression amount between the mechanical seal adjustment pad and the spring matches, causing the spring to be unable to respond to the axial floating amount of the ring, and the two seal end faces are not in tight contact. During the installation process, due to the excessive compression of the mechanical seal and the high specific pressure on the end face, the temperature of the friction pair began to gradually rise and dry friction occurred, which in turn caused serious wear on the end face of the friction pair.
3. Corrosion of various components
1) Carbide is corroded
When cemented carbide is used in crude oil with high-temperature and strong alkali desulfurizers, the bonding phase metal is very susceptible to corrosion. The strength of the hard phase skeleton will be greatly weakened, and grain spalling will occur under the influence of mechanical force. Cemented carbide is prone to stress corrosion cracking, with cracks forming in weak areas and gradually developing in-depth, eventually leading to rupture.
2) The graphite ring is corroded
When the temperature of the end face of the equipment is too high, higher than 180°C, the impregnated resin will affect the graphite ring, resulting in a decrease in its wear resistance; due to improper selection of the impregnated resin, a chemical reaction will occur, resulting in a decrease in wear resistance; The resin impregnation depth is not enough, and when the impregnation layer is worn away, its wear resistance is reduced. In addition, the graphite ring will also be oxidized. The temperature of 350-400°C may cause the graphite ring to react chemically with oxygen to form carbon dioxide gas, causing the end surface to become rough and even damaged.
Countermeasures for twin-screw pump mechanical seal leakage
1. Mechanical seal selection
When improving the selection of mechanical seals, attention should be paid to the following points: seal working parameters, medium properties, installation space and working conditions. First, select the appropriate friction pair material based on the properties, pressure, temperature and other parameters of the substance that the pump needs to transport. Second, during the selection process, we should focus on selecting auxiliary sealing rings, and select acid-resistant, corrosion-resistant sealing rings based on the materials being transported.
2. Standard installation of mechanical seals
During the installation process of the mechanical seal, it should be noted that the spring compression amount must be strictly by relevant regulations, and must not exceed the standard or fail to meet the standard. If the compression amount exceeds the standard, the end face-specific pressure will increase, which will increase the friction heat and cause the sealing surface to appear. Thermal deformation accelerates the wear of the end face.”
If the amount of compression is too small, the end face pressure of the dynamic and static rings will be insufficient, making sealing impossible. The interference between the auxiliary seal and the shaft should be kept moderate. Under the condition of fully ensuring the radial seal, after the moving ring is assembled, it can move freely on the shaft. The sealing end faces of the static ring and the moving ring should be in contact with the auxiliary sealing ring. The surface should be considered and designed strictly per the level 7 accuracy of GB/T1184.
3. Choose appropriate material components
Appropriate material components must be selected for different conveyed materials. Taking crude oil as an example, the friction pair under crude oil with high sulfur content is suitable to choose the combination of silicon carbide and silicon nitride; the friction pair under crude oil with strong alkaline desulfurized is suitable to choose hard alloy steel with strong corrosion resistance; the friction pair under crude oil containing strong corrosive substances is suitable to choose The original friction pair is suitable for use of hard-on-hard materials. Choose to use an external sealing structure to minimize the contact between the original seal and the seal. The sealing metal surface that needs protection is plated with other protective types of metal to isolate contact with crude oil to prevent corrosion.
In general, the twin-screw pump is a kind of precision equipment that has strict requirements on the equipment environment. Its mechanical seal has high-level requirements for mechanical design, mechanical processing, mechanical assembly and other aspects. There are many reasons for twin-screw pump mechanical seal leakage, including mechanical seal performance, improper installation, corrosion of various components, etc. Only by adopting different response strategies for different reasons can the problem of twin-screw pump mechanical seal leakage be effectively solved. Extend the service life of twin-screw pumps.