There are various types and models of mechanical seals used in pumps, but there are mainly five leakage points:
(l) Sealing between shaft sleeve and shaft;
(2) Sealing between the moving ring and the shaft sleeve;
(3) Sealing between dynamic and static rings;
(4) Sealing between the stationary ring and the stationary ring seat;
(5) The sealing between the sealing end cap and the pump body.
Generally speaking, leaks between shafts with extended shaft sleeves and between sealed end caps and pump bodies are relatively easy to detect and solve, but require careful observation, especially when the working medium is liquefied gas or high-pressure, toxic and harmful gases, which are relatively difficult. The remaining leaks are difficult to distinguish and judge intuitively, and it is necessary to observe, analyze, and judge the leakage symptoms based on long-term management and maintenance practices in order to draw correct conclusions.
1, Analysis and Judgment of Leakage Causes
1. Leakage during installation and static testing. After the installation and commissioning of mechanical seals, a static test is generally conducted to observe the leakage rate. If the leakage is small, it is mostly due to problems with the dynamic or static sealing rings; When the leakage is large, it indicates that there is a problem between the dynamic and static ring friction pairs. On the basis of preliminary observation of the leakage amount and determination of the leakage location, manual turning observation is carried out. If there is no significant change in the leakage amount, there is a problem with the static and dynamic sealing rings; If there is a significant change in leakage during turning, it can be concluded that there is a problem with the dynamic and static ring friction pairs; If the leaked medium is sprayed along the axial direction, there are mostly problems with the dynamic ring seal. If the leaked medium is sprayed around or leaked from the water cooling hole, it is mostly due to the failure of the static ring seal. In addition, leakage channels can also exist simultaneously, but there is generally a distinction between primary and secondary. As long as one observes carefully and is familiar with the structure, they can definitely make a correct judgment.
2. Leakage during trial operation. After static testing, the centrifugal force generated by high-speed rotation during operation of the pump's mechanical seal will suppress the leakage of the medium. Therefore, during the trial operation, mechanical seal leakage is basically caused by the damage of the dynamic and static ring friction pairs after excluding the failure of the shaft to shaft and end cover seals. The main factors causing the failure of friction pair seals are:
(l) During operation, abnormal phenomena such as evacuation, cavitation, and pressure build-up cause significant axial forces, resulting in separation of the contact surfaces between the dynamic and static rings;
(2) Excessive compression during the installation of mechanical seals leads to severe wear and abrasion of the friction pair end face;
(3) The sealing ring of the moving ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring;
(4) The static ring seal is too loose, and when the dynamic ring floats axially, the static ring separates from the static ring seat;
(5) There is granular material in the working medium, which enters the friction pair during operation and detects the sealing end faces of the dynamic and static rings;
(6) Incorrect design selection, low sealing end face pressure ratio, or high cold shrinkage of sealing material. The above phenomenon often occurs during trial operation, and sometimes it can be eliminated by adjusting the static ring seat appropriately, but most of them require disassembly and replacement of the seal.
3. Sudden leakage during normal operation. Sudden leakage of centrifugal pumps during operation is mostly caused by normal wear and tear or reaching the end of their service life, while the majority are caused by significant changes in operating conditions or improper operation and maintenance.
(1) Evacuation, cavitation, or prolonged pressure buildup can lead to seal failure;
(2) The actual output of the pump is relatively small, causing a large amount of medium to circulate inside the pump and accumulate heat, resulting in medium gasification and seal failure;
(3) Excessive reflux caused sediment to rise at the bottom of the suction tube side container (tower, kettle, tank, pool), damaging the seal;
(4) For long-term shutdowns, there is no manual turning when restarting, and the sealing surface of the friction pair is torn due to adhesion;
(5) An increase in corrosive, polymeric, and adhesive substances in the medium;
(6) Rapid changes in environmental temperature;
(7) Frequent changes or adjustments in working conditions;
(8) Sudden power outage or malfunction shutdown, etc. If a centrifugal pump suddenly leaks during normal operation and cannot be detected in a timely manner, it often leads to major accidents or losses, which should be taken seriously and effective measures should be taken.
2, Several misconceptions in the maintenance of mechanical seals for pumps
1.The greater the compression of the spring, the better the sealing effect. In fact, excessive compression of the spring can cause rapid wear and instant burning of the friction pair; Excessive compression causes the spring to lose its ability to adjust the end face of the moving ring, resulting in seal failure.
2. The tighter the sealing diagram of the moving ring, the better. Actually, tightening the dynamic ring seal is harmful and not beneficial. One is to intensify the wear of the sealing ring and shaft sleeve, leading to premature leakage; Secondly, it increases the resistance to axial adjustment and movement of the moving ring, making it difficult to make timely adjustments when working conditions change frequently; Thirdly, the spring is prone to damage due to excessive fatigue; The fourth is to deform the sealing ring of the moving ring, which affects the sealing effect.
3. The tighter the static ring seal, the better. The static ring seal is basically in a static state, and a relatively tight seal will have better sealing effect, but being too tight is also harmful. One is to cause excessive deformation of the static ring seal, which affects the sealing effect; The second reason is that the material of the static ring is mostly graphite, which is generally brittle and prone to cracking under excessive stress; Thirdly, installation and disassembly are difficult, and the static ring is easily damaged.
4. The tighter the impeller lock nut, the better. In mechanical seal leakage, leakage between the shaft sleeve and the shaft (inter shaft leakage) is quite common. It is generally believed that inter shaft leakage is caused by the impeller locking nut not being tightened. In fact, there are many factors that can lead to inter shaft leakage, such as the failure and displacement of the inter shaft gasket, impurities in the inter shaft, significant positional errors at the fit between the shaft and the shaft sleeve, damage to the contact surface, gaps between various components on the shaft, and excessively long screw threads on the shaft head, all of which can cause inter shaft leakage. Excessive locking of the locking nut will only lead to premature failure of the shaft pad. On the contrary, moderately locking the locking nut can maintain a certain degree of compression elasticity of the shaft pad, and the locking nut will automatically lock in a timely manner during operation, ensuring that the shaft pad is always in a good sealing state.
5. The new is better than the old. Relatively speaking, the effect of using a new mechanical seal is better than the old one, but the quality or material selection of the new mechanical seal is not appropriate, and larger dimensional errors can affect the sealing effect; In polymeric and permeable media, it is better not to replace the static ring if there is no excessive wear. Because the stationary ring remains in a stationary state for a long time in the stationary ring seat, the polymer and impurities deposit together, providing a good sealing effect.
6. Disassembling and repairing is better than not dismantling at all. Once a mechanical seal leakage occurs, it is urgent to dismantle and repair it. In fact, sometimes the seal is not damaged, and only adjusting the working conditions or appropriately adjusting the seal can eliminate the leakage. This not only avoids waste but also verifies one's fault diagnosis ability, accumulates maintenance experience, and improves the quality of maintenance.
