【Sealing Knowledge】Why do mechanical seals leak?

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Introduction: As environmental regulations become stricter, environmental issues in the chemical industry have become increasingly prominent, prompting companies to pay close attention to sealing failures and leaks. Pump sealing issues are particularly important. Sealing plays a crucial role in the stable operation of the entire equipment and apparatus, as well as in ensuring the safe and stable progress of production. Especially in chemical enterprises, the function of sealing is particularly important. In chemical manufacturing plants, the fluids handled by pumps are often flammable, explosive, toxic, and harmful; if a leak occurs and is not detected in time, it can result in incalculable losses. Therefore, controlling leaks in mechanical seals is one of the key tasks in chemical plants. Mechanical seal structure: A mechanical seal, also known as an end-face seal, consists of a pair of end faces that are perpendicular to the axis of rotation. Under the action of fluid pressure and forces generated to counteract mechanical expansion, these end faces remain in contact with each other at the other end, thanks to the assistance of auxiliary seals, while sliding relative to one another, thereby preventing leakage of the medium. A mechanical seal consists of two components: a stationary ring and another component that moves in opposition to it, thereby minimizing the level of leakage at the seal. The common mechanical seal structure is shown in Figure 1 below. It is composed of components such as 1 stationary ring, 2 moving ring, 3 elastic element, 4 spring seat, 5 set screw, 6 rotating ring auxiliary seal ring, and 8 stationary ring auxiliary seal ring; the anti-rotation pin 7 is fixed to the gland 9 to prevent the stationary ring from rotating. Remove. Click here to add caption text for the image. Common leakage points in mechanical seals: There are various types of mechanical seals, but the most common leakage points include: (1) the seal between the shaft sleeve and the shaft; (2) the seal between the rotating ring and the shaft sleeve; (3) the seal between the rotating ring and the stationary ring; (4) the seal between the stationary ring and its seat; (5) the seal between the sealing end cover and the pump body. Careful observation, discrimination, and judgment are required to reach the correct conclusion. Analysis of the main causes of leakage: 1. Leakage during static testing – Conduct static testing to observe the amount of leakage, and make a preliminary judgment based on that amount. If there are issues with the moving or stationary ring seals, then the leakage amount will be low ； If there are issues with the friction between the rotating and stationary rings, the leakage rate will be high. On this basis, perform a manual turning test; if the leakage amount does not change much, then there is a problem with the seal ring ； If there are problems with the friction between the rotating and stationary rings, the leakage rate will change significantly ； If there is a problem with the rotating ring seal, the medium will be ejected in the axial direction ； If the sealing ring of the static ring fails, the medium will leak out from the water cooling holes or spray in all directions. Furthermore, different leakage points can exist simultaneously, but there is usually a primary and secondary one; with careful observation and a good understanding of the structure, it is entirely possible to make an accurate judgment. II. Leakage during testing: The high centrifugal force generated by the pump’s rapid rotation during testing helps to prevent leakage. Therefore, if there is a leak during operation, it is necessary to first rule out causes of failure in the shaft-to-shaft and end-cap seals; in most cases, this is due to damage to the friction pairs between the rotating and stationary rings. The main factors causing seal failure in friction pairs are as follows. (1) During operation, various abnormal phenomena such as pressure buildup, cavitation, and evacuation occur, resulting in excessive axial force; this causes the contact surfaces of the stationary and rotating rings to separate, leading to leakage ； (2) Excessive compression during the installation of the mechanical seal leads to severe damage of the friction surfaces, resulting in leakage ； (3) The dynamic ring seal ring is installed either too loosely or too tightly, preventing the end faces of the dynamic and static rings from maintaining a seal and thus causing leakage ； (4) Hard particles in the medium act as impurities that enter the friction pair, causing wear on the sealing surfaces of the moving and stationary rings and leading to leaks ； (5) The design selection was inappropriate; most of them require disassembly and reassembly as well as replacement of seals. III. Leaks during normal operation: Sudden leaks in mechanical seals while the equipment is in operation are rarely caused by normal wear or the end of the seal’s service life; instead, they are mostly due to sudden changes in operating conditions, incorrect operations, or inadequate inspection and maintenance. (1) Various abnormal operations that cause seal failure, such as vacuum formation and cavitation ； (2) Vaporization of the medium: if the actual output volume of the pump is low, a large amount of the medium circulates within the pump, leading to heat accumulation and subsequent seal failure ； (3) Excessive backflow can stir up the sediment at the bottom of the container, thereby damaging the seal ； (4) After a long period of shutdown, restarting without manually turning the shaft causes the stuck friction pairs to damage the sealing surfaces ； (5) Increase in corrosive and polymerizing substances in the medium ； (6) Unstable ambient temperature ； (7) Unstable production operation. During normal operation, if the mechanical seal of a centrifugal pump suddenly starts leaking toxic, harmful, and flammable substances, and the maintenance personnel fail to detect this anomaly and take action promptly, it can lead to irreversible damages such as fires, pollution, and casualties. Therefore, this issue must be taken seriously, and effective preventive measures must be implemented. Conclusion: In summary, the more common leakage locations and causes of leakage in the mechanical seals of chemical process pumps are outlined. Only by fully understanding the structure of mechanical seals and common leakage phenomena, as well as correctly analyzing the causes of various leaks, can appropriate measures be taken to address them and ensure safe production.