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I. Principles and requirements for mechanical seals in high-temperature chemical pumps: A mechanical seal for chemical pumps 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 a stationary ring, a rotating ring, a spring loading mechanism (including a thrust ring and a spring box), as well as auxiliary sealing rings (the rotating ring seal and the stationary ring seal). A rotation prevention pin located inside the rotation prevention 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, thereby 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 would otherwise be 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 – it operates in combination with other parts. This can also be understood from its basic principles. For the proper operation of a mechanical seal, it is necessary that the shaft movement of the pump be minimal; the deflection of the pump shaft at the location of the mechanical seal should also be low. Only by meeting such external conditions, along with the excellent inherent performance of the mechanical seal itself, can an ideal sealing effect be achieved. II. External factors affecting the mechanical seal of high-temperature resistant chemical pumps 1. When the shaft bends, the linear displacement of the centroid of its cross-section along a line perpendicular to the axis is known as deflection; mechanical seals require even force distribution between the 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 efficiency. This phenomenon mostly occurs in horizontal multi-stage centrifugal pumps. 2. Axial play of the pump shaft: The sealing surfaces of the 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 two 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. 3. Causes of pump cavitation and improvement measures The inlet area of the centrifugal pump impeller is 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 that results from the vaporization and condensation of liquids is called cavitation. The measures to eliminate cavitation at the pump inlet are as follows: a. Improve the pump’s cavitation resistance to meet the requirements of the installation site. b. The pump should be operated at a suction level below the allowable limit to ensure the safe and reliable operation of the entire system. c. The impeller is manufactured from materials resistant to cavitation, such as 2Cr13, rare-earth alloy cast iron, and high-nickel-chromium alloy materials, which offer much better resistance to cavitation than ordinary cast iron. 4. Excessive vibration and measures to eliminate it: Excessive vibration in mechanical seals is often not due to issues with the seals themselves, but rather to problems such as an unsuitable 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 the pump, motor, base, and pipelines on-site, in order to eliminate the sources of vibration. 5. Absence of an auxiliary flushing system or an inadequately designed auxiliary flushing system. The auxiliary flushing system for mechanical seals is extremely important; it helps to protect the sealing surface by providing cooling, lubrication, and by removing debris. Sometimes, the designers create designs that are not appropriate, resulting in a failure to achieve proper sealing. In other cases, although auxiliary systems are designed, factors such as impurities in the flushing fluid, insufficient flow or pressure of the flushing fluid, and improper placement of the flushing ports also prevent the desired effect from being achieved. Common measures include: a. 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 is close to the limit at which the seal can function, 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 frictional heat. b. 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 the seal depend to a large extent on the configuration of the seal assistance system. Additionally, factors such as insufficient precision in the machining of the pump shaft, shaft sleeves, pump body, and seal chamber also have a negative impact on the sealing performance of the mechanical seal; strict control is necessary in these areas.