Causes of leakage in mechanical seals of chemical process pumps and solutions

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The sealing of chemical pumps plays an important role in the operation of the entire equipment; leaks in the sealing system of chemical pumps can severely affect the proper functioning of the equipment. Chemical pumps often handle corrosive or toxic and volatile media; therefore, their sealing performance is one of the criteria for evaluating the quality of chemical pumps. The main types of shaft sealing devices used in pumps in our country include packing seals, labyrinth seals, mechanical seals, oil seal seals, gasket seals, threaded seals, and dynamic seals. Below is an introduction to the analysis of leakage causes and corresponding solutions for mechanical seals in chemical process pumps. The principle of mechanical seals for chemical pumps: A mechanical seal, also known as an end-face seal, is a type of shaft sealing device used in rotating machinery. It consists of at least one pair of end faces that are perpendicular to the axis of rotation; these end faces remain in close contact with each other and slide relative to one another under the action of liquid pressure, the elastic force of a compensating mechanism, and the assistance of auxiliary seals, thereby preventing fluid leakage. Its main function is to change the axially sealed design, which is prone to leakage, into an end-sealed design that is less likely to leak. It is widely used for sealing the rotating shafts of pumps, kettles, compressors, and similar equipment. A mechanical seal typically consists of a rotating ring, a stationary ring, a compression element, and a sealing element. Among them, the rotating ring rotates together with the pump shaft; the rotating ring and the stationary ring fit closely together to form a sealing surface that prevents leakage of the medium. The rotating ring is pressed against the end face of the stationary ring by the pressure of the liquid in the sealing chamber; this creates an appropriate specific pressure on the end faces of both rings and maintains a very thin layer of liquid film, thereby achieving sealing. The compression element generates pressure, which allows the pump to maintain tight contact between its end surfaces even when it is not in operation; this ensures that the sealing medium does not leak out and prevents contaminants from entering the sealed end surfaces. The sealing dynamic ring and the shaft form a gap in the sealing element, while there is a gap between the sealing static ring and the gland; moreover, the elastic elements act as buffers against vibrations and shocks in the pump. In actual operation, mechanical seals function in combination with other components of the pump; their proper operation is greatly influenced by both their own performance and external conditions. Problems with mechanical seals in chemical pumps The causes of failure in the mechanical seals of chemical pumps are varied; they are related not only to the internal components of the mechanical seals but also affected by external factors. Under normal circumstances, the main reasons for the failure of mechanical seals in chemical pumps are mechanical damage ； In rotating equipment where mechanical seals are used, the seal faces often suffer from wear, thermal cracking, deformation, and damage during operation. Over time, springs may also become loose, break, or corrode. The auxiliary sealing ring can also develop cracks, twisting and deformation, as well as ruptures. Reasons for leakage in the mechanical seals of chemical process pumps and methods for dealing with it: 1. Leakage occurs during the static testing phase. Static testing is typically used to determine the level of leakage in mechanically assembled equipment. There are mainly four aspects to consider when assessing leakage levels: first, the situation where the leakage from the mechanical equipment is low. When the leakage volume is relatively small, it can be determined that there are sealing issues with either the stationary or rotating ring of the chemical pump ； The second is the situation of large leakage amounts. A large mechanical leakage indicates an abnormality in the friction pair between the moving and stationary rings of the equipment. By determining the leakage amount and location of mechanical equipment, once the fault location is identified, manual turning of the equipment is used to assess the condition of the dynamic and static sealing rings ； Third, if there are no significant changes in the leakage, it can be determined that the problem lies with the dynamic and static sealing rings ； Fourth, faults are determined by the location of the medium leakage. When the fault is a failure of the static ring seal, it manifests as the medium leaking out from the cold water holes or spraying in all directions. Furthermore, leaks may occur simultaneously or in a primary/secondary manner; the correct location of the fault can only be determined through careful observation based on an understanding of the structure. 2. Leakage occurs during the commissioning phase. Damage to the friction pairs between the rotating and stationary rings during commissioning is the main cause of leakage in mechanical seals. There are five primary reasons for the failure of these friction pair seals: first, improper sealing of the rotating and stationary rings, which leads to axial movement of the rotating ring and subsequent separation of the stationary ring ； Secondly, during operation, cavitation, pressure buildup, and evacuation can generate strong axial forces, leading to separation between the moving and stationary ring contact surfaces ； Third, improper design leads to increased cold shrinkage of the sealing material and a decrease in the specific end-face pressure ； Fourth, particulate matter enters during the operation of the machinery, and under the effect of friction, this causes damage to the sealing surfaces of the moving and stationary rings ； Fifth, improper control of sealing during mechanical installation leads to severe scarring and wear on the surfaces of the friction pairs. When a leakage fault occurs during the commissioning phase, the leakage can be eliminated by adjusting the static pressure seat and replacing the damaged materials or components. 3. Leakage occurs during normal operation. There are mainly two reasons for leakage in chemical pumps during normal operation: one is that the components of the chemical pump have worn out severely and have reached the end of their useful life ； Secondly, leaks in chemical pumps occur during normal operation due to improper operation caused by human factors, significant changes in operating conditions, or inadequate maintenance of the chemical pumps. For example, erosion, excessive pressure holding time, and evacuation can cause damage to the seal leading to leakage ； Due to the low output of the chemical pump, an excessive amount of fluid circulates within the pump, which leads to heat accumulation and vaporization of the fluid; this in turn causes the seal to fail and results in leakage ； The sediment at the bottom of the tube-side container was drawn up in large quantities due to excessive backflow, resulting in damage to the mechanical seal ； For chemical pumps that have not been in use for a long time, the lack of manual rotation during restart, along with excessive viscosity, causes damage to the sealing surfaces of the friction pairs, resulting in leaks ； A large amount of cohesive substances, corrosive substances, and polymeric substances are present in the medium of chemical process pumps ； Factors such as frequent adjustments or changes in operating conditions to adapt to environmental temperature variations can cause chemical pumps to leak during normal use. 4. Vibration and heating of mechanical seals: During the rotation of the equipment, the mating surfaces of the stationary and rotating rings become rough. The gap between these rings and the seal chamber is too small; vibrations occur due to collisions caused by wobbling. Sometimes, vibration and heating of mechanical seals can be caused by poor corrosion and temperature resistance of the sealing surfaces, insufficient cooling, or the presence of particulate impurities trapped in the surfaces during installation. 5. Leakage of the medium through the mechanical seal: Leakage can occur during hydrostatic testing. Carelessness during the installation of the mechanical seal can lead to damage, deformation, or impairment of the sealing surfaces; these surfaces may also contain particulate impurities. Additionally, loose setting screws, inadequate compression of the gland, or insufficient precision of the machine or equipment can result in the sealing surfaces not fitting together properly, all of which can cause leakage of the medium. If it is a leakage at the sleeve, it is because the sealing ring of the sleeve was not compressed properly during installation, the degree of compression was insufficient, or the ring is damaged. 6. Periodic or intermittent leakage: Periodic vibration of the rotor assembly in mechanical seals, as well as excessive axial movement, can both cause leakage. 7. Frequent leakage of mechanical seals: There are many reasons for the frequent leakage of mechanical seals. Firstly, frequent leaks caused by defects in the sealing end face ； The second aspect is the frequent leaks caused by the auxiliary sealing ring ； Thirdly, there is leakage caused by spring defects. In other aspects, these include leaks caused by rotor vibration, as well as leaks resulting from poor quality or loosening of coupling, fastening, and thrust components ； 8. Excessive vibration of mechanical seals: Excessive vibration of mechanical seals ultimately leads to a loss of their sealing effectiveness. However, the reason for excessive vibration in mechanical seals is often not solely due to the seals themselves; other components of the pump can also be the source of such vibration. Maintenance measures to improve the mechanical seal performance of chemical pumps. As important equipment in chemical production, the operating condition of chemical pumps has a significant impact on the smooth progress of the entire chemical production process. Therefore, the mechanical seals of chemical process pumps require very high standards in terms of design, manufacturing, and assembly quality. In the field where mechanical seals are used, every effort should be made to meet both the technical requirements of those seals and the operating conditions. The mechanical seal of the pump also requires a high level of skill from the maintenance personnel. To resolve the mechanical sealing issues associated with pumps, the following measures can be taken. 1. Selecting the appropriate material: When choosing the material for chemical pumps, it is advisable to opt for materials with a higher sulfur content. For the friction pairs, silicon carbide or silicon nitride should be used, as this helps to improve the mechanical sealing performance of the chemical pumps. Silicon carbide materials have a relatively low density, and they possess advantages such as high strength, good wear resistance, corrosion resistance, excellent thermal insulation, and high-temperature tolerance; therefore, they can be used in the manufacture of friction pairs. To prevent chemical pumps from being corroded by the medium, a metal protective layer must be formed on their surface; this layer effectively isolates the mechanical seal metal from the medium inside the pump, thereby preventing corrosion. Alloy steel is steel to which elements other than iron and carbon are added. It possesses high strength, high toughness, corrosion resistance, heat resistance, and wear resistance, but its production process is complex and its cost is high. Therefore, when selecting mechanical seal materials for chemical process pumps, it is necessary to take into full account the properties of the medium inside the pump as well as its actual operating environment, in order to make a targeted choice that ensures the selected materials are technically advanced and economically reasonable. 2. Operate strictly in accordance with the specifications. To prevent operational errors from adversely affecting the mechanical sealing performance of chemical pumps, it is necessary to minimize the rate of operational errors among production workers. During the operation of the twin-screw pump, operators should strive to maintain a steady operation to avoid any adverse effects on the pump. At the same time, it is necessary to monitor the pump’s operating conditions in real time, especially its vibration level; accurate measurements should be taken using vibration detectors, so as to enable scientific and effective control of the pump’s vibrations. 3. Fully consider vibration: Vibration has a very severe impact on the mechanical sealing performance of chemical pumps. Therefore, during maintenance, it is necessary to conduct a scientific and rational analysis of the vibration levels of these pumps, and to implement targeted improvement measures based on the actual operating conditions of the pumps. This helps to minimize the vibration generated during operation, thereby preventing adverse effects of vibration on the mechanical seals of the chemical pumps. To minimize the adverse effects of excessive pump shaft deflection on the mechanical sealing performance of chemical pumps, effective measures should be taken to reduce the distance between the bearings at both ends. Meanwhile, the diameter of the pump shaft can be increased appropriately to reduce the negative impact of vibrations on the chemical pump. 4. Install the mechanical seal strictly in accordance with requirements. After the maintenance of chemical pumps is completed, they must be properly installed. The quality of this installation is of great practical significance for ensuring the sealing performance of chemical pumps. Therefore, during the installation process, it is necessary to strictly adhere to the provisions of **relevant standards and specifications**. The expansion and contraction of the spring must be carefully controlled to ensure that the moving ring can move freely along the shaft. Additionally, the surface roughness of the contact areas between the stationary ring, the moving ring, and the auxiliary seal ring must not exceed 5 μm. The dimensional tolerances for the inner or outer diameters should be H8. The axial runout tolerance of the mechanical seal component must not exceed 0.06 mm, and its axial movement amount must not exceed 0.3 mm; meanwhile, when installing the seal, the surface roughness must not exceed 3.2 μm. Issues to be noted when maintaining mechanical seals for chemical pumps: 1. Spring compression amount. For the springs used in chemical pumps, it is necessary to control their compression amount in a scientific and reasonable manner. When the compression amount is excessive, it will increase the friction between the frictional surfaces, thereby **increasing frictional losses. As these losses continue to rise, the mechanical seal of the chemical pump may fail; in severe cases, the frictional surfaces might even become damaged or burned out, posing a serious threat to the safety of chemical production.** ； When the spring stretches too much, it may fail to provide effective adjustment of the end face of the moving ring, which in turn leads to the failure of the pump’s mechanical seal and results in leakage in the chemical pump. 2. Rotating ring seal: The tension of the rotating ring seal in chemical pumps must be appropriate. When it is too tight, on one hand, it not only causes excessive friction between the shaft sleeve and the seal, thereby **increasing frictional losses and shortening the service life of the mechanical seal of the chemical pump**, but it also increases the resistance to axial adjustment and movement of the rotating ring. During operation of the chemical pump, when its speed increases, it becomes impossible to make timely adjustments ； On the other hand, excessive tightness can also cause excessive fatigue in the springs of chemical pumps; springs that are under overloaded conditions for a long time will have a **reduced service life, which in turn leads to seal failure. 3. Static ring seal: The static ring seal of a pump is generally at rest; therefore, increasing its tension appropriately can improve the sealing performance to some extent. However, it should not be overlooked that an overly tight static ring seal can have adverse effects on the mechanical seal of the chemical pump as well. Firstly, the static ring seal, which is under excessive stress for a long time, undergoes a certain degree of plastic deformation, which in turn has an adverse effect on its sealing performance ； Secondly, most static ring seals are made of graphite. Due to the properties of this material, they are brittle and prone to breaking; they can easily crack when subjected to excessive stress. Moreover, if the static ring seal is too tight, it can make installation and removal difficult, which in turn increases the likelihood of damage to the seal. 4. It’s not true that the impeller lock nut should be tightened as much as possible. Among the causes of leakage in the mechanical seals of chemical pumps, leakage between the shaft sleeve and the shaft accounts for a significant proportion; therefore, sufficient attention should be paid to this aspect. Normally, inter-shaft leakage is mainly caused by the failure to tighten the impeller lock nut, but the reasons for leakage are not limited to this; for example, failed inter-shaft gaskets, misalignment, and significant geometric errors at the interface between the shafts and sleeves can also cause leakage. Therefore, over-tightening the locknut will only cause premature failure of the inter-shaft washer. Conversely, moderately tightening the locknut ensures that the inter-shaft washer always retains a certain degree of compressive elasticity. During operation, the locknut will automatically tighten at the appropriate time, thereby maintaining a proper seal between the shafts at all times. 5. Selection of new mechanical seals: In addressing leaks in the mechanical seals of chemical pumps, it is not sufficient to simply replace the mechanical seal units; rather, a targeted selection must be made based on the actual conditions of the chemical pump. For example, in the case of chemical pumps used to transport permeable media, if there is no excessive wear on the stationary ring of the pump, there is no need to replace it. This is because the stationary ring remains stationary for long periods of time, allowing impurities and polymers to deposit on it; this, in turn, improves the effectiveness of the mechanical seal. 6. It’s better to carry out repairs than to do nothing. When dealing with the failure of the mechanical seal in a chemical pump, it’s not appropriate to start by repairing it right away; rather, a thorough analysis of the situation is required before deciding whether repairs are necessary. For some chemical pumps that experience a certain degree of leakage but whose mechanical seals have not failed and are still in good condition, there is no need to carry out repair work on them. The leakage issue can be resolved by adjusting the operating conditions of the pump or making appropriate adjustments to the mechanical seals. This not only improves the efficiency of maintaining chemical pumps, **saves time on repairs, and allows chemical manufacturing companies to resume production more quickly, but it also reduces maintenance costs significantly, bringing substantial economic benefits to such companies. In summary, chemical pumps are important equipment in the production of chemical enterprises, and their proper operation holds great practical significance for such enterprises. However, in actual production processes, chemical pumps operate in very harsh environments. Chemical pumps that are exposed to such conditions for extended periods experience varying degrees of failure in their mechanical seals, which in turn leads to leaks in the pumps. To take scientific and reasonable measures to address leaks in chemical pumps, it is necessary to conduct a systematic and comprehensive analysis of their actual conditions, so as to implement targeted maintenance actions that ensure the safe and stable operation of these pumps.