Abstract: Through practical application and theoretical analysis of mechanical seals for pumps, this paper proposes that the actual sealing effect of a mechanical seal is not only related to its own performance, but also significantly influenced by the conditions provided by other components and the sealing auxiliary system. Therefore, when designing pump units, it is essential to provide favorable external conditions for the use of mechanical seals. Currently, mechanical seals are widely used in pump products, and with the improvement of product technology and the requirement for energy conservation, their application prospects will become even broader. The sealing effect of a mechanical seal directly affects the operation of the entire machine, especially in the petrochemical industry. Due to the presence of flammable, explosive, volatile, and highly toxic media, leakage from the mechanical seal will seriously affect normal production and may even lead to major safety accidents. When analyzing the causes of quality failures, people often focus on the mechanical seal itself, such as whether the selection of the mechanical seal is appropriate, whether the material selection is correct, whether the specific pressure of the sealing surface is correct, and whether the selection of the friction pair is reasonable. However, they rarely look for causes in the external conditions of the mechanical seal, such as whether the conditions created by the pump for the mechanical seal are suitable and whether the configuration of the auxiliary system is appropriate. These aspects are often crucial. This article analyzes several factors affecting the sealing effect of mechanical seals for pumps from the perspective of external conditions and proposes reasonable measures. 1. Principle and Requirements of Mechanical Seals A mechanical seal relies on the tiny axial gap formed by the contact of the end faces A of a pair of relatively moving rings (one fixed, the other rotating with the shaft, see Figure 1) to achieve a sealing effect. This type of device is called a mechanical seal. A mechanical seal typically consists of a rotating ring, a stationary ring, a clamping element, and a sealing element. The end faces of the rotating and stationary rings form a friction pair. The pressure of the liquid in the sealing chamber causes the end face of the rotating ring to press against the end face of the stationary ring, generating an appropriate specific pressure on the two ring end faces and maintaining a very thin liquid film to achieve the sealing purpose. The clamping element generates pressure, ensuring that the end faces remain in contact even when the pump is not running, preventing leakage of the sealing medium and preventing impurities from entering the sealing end face. The sealing element seals the gap B between the rotating ring and the shaft, and the gap C between the stationary ring and the gland, while also buffering the pump's vibration and impact. Mechanical seals are not isolated components in actual operation; they operate in conjunction with other pump parts. Their basic principle reveals that normal operation is conditional. For example, the pump shaft axial movement cannot be too large, otherwise the friction pair end faces cannot form the required specific pressure; the pump shaft at the mechanical seal cannot have too much deflection, otherwise the end face specific pressure will be uneven, etc. Only by meeting such external conditions, coupled with good mechanical seal performance, can the ideal sealing effect be achieved. 2. Analysis of the Causes of External Conditions 2.1 Large Axial Movement of the Pump Shaft The sealing surface of a mechanical seal needs a certain specific pressure to achieve a sealing effect. This requires the mechanical seal spring to have a certain compression, providing a thrust to the sealing end face, and rotating it to generate the required specific pressure for sealing. To ensure this specific pressure, the mechanical seal requires the pump shaft to have a small axial movement, generally within 0.5 mm. However, in actual design, due to unreasonable design, the pump shaft often exhibits large axial movement, which is very detrimental to the use of the mechanical seal. This phenomenon often occurs in multi-stage centrifugal pumps, especially during pump startup, where the axial movement is relatively large. Figure 2 illustrates the working principle of the balancing disc method for balancing axial forces. During operation, the balancing disc automatically changes the axial clearance *b* between the disc and the balance ring, thereby altering the pressure difference between the front and rear sides of the disc and generating a force opposite to the axial force to balance it. Due to the inertia of rotor movement and fluctuations in transient pump operating conditions, the rotating rotor will not remain stationary at a specific axial equilibrium position. The balancing disc is always in a state of lateral movement. Under normal operating conditions, the axial movement of the balancing disc is only 0.105–0.11 mm, meeting the allowable axial movement requirement of 0.15 mm for mechanical seals. However, during pump startup, shutdown, or drastic changes in operating conditions, the axial movement of the balancing disc may significantly exceed the allowable axial movement of the mechanical seal. After prolonged pump operation, friction and wear occur between the balancing disc and the balance ring, increasing the clearance *b* and continuously increasing the axial movement of the mechanical seal. Due to the axial force, the clamping force on the suction side sealing surface increases, accelerating wear until the sealing surface is damaged and loses its sealing function. 2.2 Excessive Axial Force: Mechanical seals cannot withstand axial forces during operation. The presence of axial force significantly impacts the seal. Sometimes, due to improper design of the pump's axial force balancing mechanism, manufacturing, installation, or usage issues, the axial force is not balanced. When a mechanical seal bears an axial force, the sealing gland temperature will be too high during operation. For polypropylene media, this temperature can cause melting, leading to a rapid loss of sealing effect after pump startup. When the pump is stationary, intermittent leakage occurs at the sealing end face. 2.3 Excessive Pump Shaft Deflection: Mechanical seals, also known as end face seals, are a type of rotating axial contact dynamic seal. Under the action of the fluid medium and elastic elements, two sealing end faces perpendicular to the shaft centerline are tightly fitted and rotate relative to each other to achieve a sealing effect. Therefore, uniform force is required between the two seals. However, due to unreasonable pump design, the pump shaft experiences significant deflection at the mechanical seal installation point during operation, leading to uneven stress between the sealing surfaces and poor sealing performance. 2.4 Lack of an auxiliary flushing system or an improperly designed auxiliary flushing system: The auxiliary flushing system for mechanical seals is crucial, effectively protecting the sealing surfaces and providing cooling, lubrication, and flushing away debris. Sometimes, designers fail to properly configure the auxiliary flushing system, resulting in inadequate sealing. Even when an auxiliary system is designed, impurities in the flushing fluid, insufficient flow or pressure, or improper flushing port placement can also prevent proper sealing. 2.5 Excessive vibration: Excessive vibration of the mechanical seal ultimately leads to loss of sealing performance. However, excessive vibration is often not caused by the mechanical seal itself; other pump components are the root cause of vibration, such as unreasonable pump shaft design, machining issues, insufficient bearing precision, poor coupling parallelism, and excessive radial force. 2.6 Causes of Pump Cavitation Due to improper operation of the system, poor cavitation performance at the pump inlet, and excessively high pump speed, localized cavitation occurs at the pump inlet. After cavitation occurs, air bubbles are generated in the water. These bubbles impact the outer surface of the mechanical seal, causing damage. Furthermore, they contribute to the presence of air bubbles in the flow film at the mating surface of the dynamic and static rings, preventing the formation of a stable flow film and causing dry friction at the mating surface of the dynamic and static rings, ultimately damaging the mechanical seal. 2.7 Insufficient Machining Accuracy Insufficient machining accuracy can have many causes. Sometimes it's due to insufficient machining accuracy of the mechanical seal itself, which is easily noticeable and easy to identify. However, sometimes it's due to insufficient machining accuracy of other pump components, which is less likely to be noticed. For example, insufficient enlargement accuracy of the pump shaft, bushing, pump body, and sealing cavity. These causes are very detrimental to the sealing effect of the mechanical seal. 3. Measures to be Taken 3.1 Measures to Eliminate Large Pump Shaft Movement A reasonable axial force balancing device should be designed to eliminate axial movement. To meet this requirement, there are two ideal design schemes for multistage centrifugal pumps: one is a balance disc with an axial thrust bearing, where the balance disc balances the axial force and the axial thrust bearing provides axial restraint for the pump shaft; the other is a balance drum with an axial thrust bearing, where the balance drum balances most of the axial force, and the thrust bearing bears the remaining axial force while also providing axial restraint for the pump shaft. The key to the second scheme is the rational design of the balance drum to ensure that it can truly balance most of the axial force. For other single-stage pumps, split-case pumps, etc., measures should be taken during the design to ensure that the pump shaft axial displacement is within the range required by the mechanical seal. 3.2 Measures to eliminate excessive axial force: The axial force balancing mechanism should be rationally designed to truly and fully balance the axial force, creating favorable conditions for the mechanical seal. For important products used in power plants, petroleum, chemical and other fields, each unit must be tested and inspected before leaving the factory to identify and resolve problems. Some important pumps can be equipped with an axial force measuring ring on the rotor to monitor the magnitude of the axial force at any time and resolve problems promptly. 3.3 Measures to eliminate excessive pump shaft deflection This phenomenon mostly exists in horizontal multistage centrifugal pumps. The following measures should be taken during the design: (1) Reduce the distance between the bearings at both ends. The number of pump impeller stages should not be too many. When the total pump head requirement is high, try to increase the head of each impeller stage and reduce the number of stages. (2) Increase the diameter of the pump shaft. When designing the pump shaft diameter, do not simply consider the size of the transmitted power, but consider factors such as mechanical seal, shaft deflection, starting method and related inertial load, radial force, etc. Many designers do not fully realize this. (3) Improve the grade of pump shaft material. (4) After the pump shaft design is completed, the pump shaft deflection should be checked and calculated. 3.4 Add an auxiliary flushing system When conditions permit, try to design an auxiliary flushing system. The flushing pressure is generally required to be 0.107 to 0.11 MPa higher than the sealing cavity pressure. If the conveyed medium is easily vaporized, it should be 0.1175 to 0.12 MPa higher than the vaporization pressure. The sealing chamber pressure should be calculated based on factors such as the structure and system pressure of each pump. When the shaft seal chamber pressure is very high or the pressure is close to the maximum limit of the seal, liquid can be drawn from the sealing chamber to the low-pressure area to allow the shaft seal liquid to flow and carry away the frictional heat. The recommended flushing volume is shown in Table 1. According to the operating conditions of each pump, the pipeline and accessories should be configured reasonably, such as coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature gauges, etc. In fact, the reliability and life of the seal depend to a large extent on the configuration of the sealing auxiliary system. 3.5 Measures to eliminate pump inlet cavitation (1) Improve the cavitation performance level of the pump to meet the cavitation performance requirements of the field device. (2) The requirements of the field test device should match the cavitation performance level of the pump. (3) Field installation and operating condition adjustment should create favorable conditions for the pump. 3.6 Measures to eliminate pump vibration (1) During the design process of pump products, the source of vibration should be fully analyzed to eliminate the vibration source. (2) During the manufacturing and assembly process of pump products, the standards and operating procedures should be strictly followed to eliminate the vibration source. (3) When installing auxiliary equipment such as pumps, motors, bases, and field pipelines on site, strict control must be exercised to eliminate vibration sources. (4) During on-site production, operation, maintenance, and adjustment, strict control must be exercised to eliminate vibration sources. 3.7 Strictly implement design standards The design of pump products and mechanical seal products must comply with relevant domestic and international standards. During the product design process, designers should conscientiously implement the standards, deeply understand the specific meaning of each clause of the standard, and implement the requirements of the standard in the product design process. To date, many designers have not understood the actual meaning of the standards and have not strictly implemented the new standards. Instead, they blindly copy old drawings and the experience of old designers. This approach is very detrimental to improving the technical level of our products and entering the international market. Improving the understanding of standardization is an urgent problem that designers in the machinery industry need to solve. 4. Conclusion When designing mechanical seals for pumps, not only should the influencing factors of the mechanical seal itself be considered, but also various influencing factors outside the mechanical seal. In practical work, the following issues should be noted: (1) During the design process of pump products, the influence of other pump components and other equipment on the mechanical seal should be fully considered to create a good external condition for the mechanical seal. (2) Increase awareness of the important role of mechanical seal auxiliary systems and equip them with complete mechanical seal auxiliary systems as much as possible to improve the sealing effect. (3) For mechanical seals of important pump products, protective measures should be added to improve sealing quality and reduce sealing quality accidents. (4) When analyzing the causes of mechanical seal quality accidents, the influence of other pump components on the operation of the mechanical seal should be fully considered, and measures should be taken to continuously improve the mechanical seal effect.