Leakage in the mechanical seal of the pump

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Principles and requirements of mechanical seals // A mechanical seal is a sealing device in which two sealing elements come into contact with each other on their smooth and straight surfaces perpendicular to the axis, while rotating relative to one another. It typically consists of components such as a stationary ring, a rotating ring, a spring loading mechanism (including a push ring and a spring box), as well as auxiliary sealing rings (rotating ring sealing ring, stationary ring sealing ring). The anti-rotation pin inside the anti-rotation sleeve is fixed to the outer pressure cover to prevent the stationary ring from rotating. The end faces of the rotating ring and the stationary ring form a friction pair; the pressure of the liquid inside the seal chamber forces the end face of the rotating ring to press against the end face of the stationary ring, creating an appropriate pressure difference between the two end faces and maintaining a very thin layer of liquid film to achieve sealing. From a structural perspective, a mechanical seal transforms what is otherwise an axially oriented seal that is prone to leakage into an end-face seal that is less likely to leak. In actual operation, a mechanical seal is not a standalone component but functions as part of a larger assembly of components. Moreover, based on its basic principles, it can be seen that for proper operation, a mechanical seal must meet the following conditions: (1) The shaft movement of the pump should not be excessive ； (2) The deflection of the pump shaft at the mechanical seal should be low. Only by meeting such external conditions, along with the excellent performance of the mechanical seal itself, can an ideal sealing effect be achieved. // External factors affecting mechanical seals used in pumps // (I) Axial movement of the pump shaft. The sealing surfaces of a mechanical seal need to have a certain specific pressure (0.4–0.6 MPa) in order to achieve sealing; the degree of tightness between these surfaces can be adjusted using springs. To maintain this specific pressure, it is required that the pump shaft not experience excessive play (≤0.5mm), but in actual production, the pump shaft often experiences significant play. This is highly detrimental to the use of mechanical seals. In my factory, this issue manifests itself during pump startup: due to the presence of unbalanced axial forces, the entire shaft moves towards the suction inlet, causing the mechanical seal to lose its sealing function. Common solutions include creating balance holes in the impeller and installing balance tubes on the pump body. (II) Excessive deflection of the pump shaft: When the shaft bends, the linear displacement of the centroid of its cross-section along a direction perpendicular to the axis is referred to as deflection. Mechanical seals require even force distribution between their two sealing surfaces. However, due to the unreasonable design of the pump, excessive deflection occurs at the location where the mechanical seal is installed, resulting in uneven stress on the seal and thus loss of its sealing effect. This phenomenon mostly occurs in horizontal multi-stage centrifugal pumps. (III) Absence of an auxiliary flushing system or an inadequately designed one. The auxiliary flushing system for mechanical seals is extremely important; it can effectively protect the sealing surface by providing cooling, lubrication, and helping to remove debris. Sometimes, the designers create designs that are not proper, resulting in a failure to achieve a seal ； Some have been equipped with auxiliary systems, but due to impurities in the flushing fluid, insufficient flow and pressure of the fluid, and an improperly designed location of the flushing ports, they still fail to achieve the desired effect. Common measures include: 1. Where conditions permit, try to design an auxiliary flushing system. The flushing pressure is generally required to be 0.107~0.11 MPa higher than the pressure in the sealing chamber; the pressure in the sealing chamber is calculated based on factors such as the structural design of each pump and the system pressure. When the pressure in the shaft seal chamber is very high, or nearly reaches the maximum limit for which that seal can be used, it is also possible to direct liquid from the seal chamber to a low-pressure area, allowing the shaft seal fluid to flow and carry away the friction heat. 2. Configure the pipelines and accessories appropriately according to the operating conditions of each pump. Such as coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature, etc. In fact, the reliability and lifespan of seals depend to a large extent on the configuration of the seal assistance systems. Moreover, factors such as insufficient machining precision of the pump shaft, shaft sleeves, pump body, and seal chamber also have a negative impact on the sealing performance of mechanical seals; strict control is necessary in these areas. (IV) Excessive vibration and mitigation measures: Excessive vibration in mechanical seals is often not due to issues with the seal itself, but rather to reasons such as improper design of the pump shaft, insufficient machining precision, excessive parallelism of the backup wheels, or high radial forces. Measures to reduce pump vibration include ensuring strict quality control during the installation of auxiliary equipment such as pumps, motors, bases, and pipelines on-site, in order to eliminate sources of vibration. (5) Causes of pump cavitation and improvement measures: The inlet area of the centrifugal pump impeller is the part where the pressure inside the pump is lowest. When the pressure here equals or is lower than the saturated vapor pressure of the liquid being transported at the operating temperature, the liquid boils and vaporizes, resulting in the formation of numerous bubbles. At the same time, the bubbles that were originally dissolved in the liquid will also precipitate. As the liquid flows into the higher-pressure areas inside the impeller, these vapor bubbles re-condense. During the condensation process, due to the rapid reduction in volume, the liquid surrounding it rushes into the entire condensation space at high speed, causing shock vibrations and noise inside the pump. Under the continuous impact of liquid particles at high pressure and high frequency, the metal surface gradually deteriorates due to fatigue; this type of damage is known as erosion. Active gases such as oxygen, which are also dissolved in liquids, cause corrosion of metals as well. Due to the combined effect of chemical corrosion and mechanical erosion, the rate of metal degradation is accelerated, thereby damaging the impeller; this is known as cavitation damage. The impact phenomenon resulting from the vaporization and condensation of liquid is called cavitation. The measures to eliminate cavitation at the pump inlet are as follows: 1. Improve the pump’s cavitation resistance to meet the requirements of the installation site. 2. The pump should be operated at a level below the allowable suction height to ensure the safe and reliable operation of the entire system. 3. Impellers are manufactured from cavitation-resistant materials such as 2Cr13, rare earth alloy cast iron, and high-nickel-chromium alloy materials, which possess a much better resistance to cavitation than ordinary cast iron. // Conclusion // When analyzing leaks in mechanical seals for pumps, it is necessary to consider not only the internal factors of the mechanical seal but also its external factors. In practical work, the following points should be taken into consideration: 1. Use pumps manufactured by reputable manufacturers, as such pumps are produced in full compliance with design standards; they do not use inferior materials, and come with a specified warranty period. 2. Maintenance workers should gain a better understanding of the important role of the mechanical seal auxiliary systems, and strive to equip themselves with complete such systems in order to improve the sealing performance. 3. When analyzing the causes of mechanical seal leakage, it is necessary to fully consider the impact of other pump components on the operation of the mechanical seal, and take measures to continuously improve its performance.