Repair of mechanical seals in industrial pumps

Thread Content

Repair of mechanical seals in industrial pumps. Key points: There are many different types and models of mechanical seals used in pumps, but the main leakage points are five: (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. Repair of mechanical seals in industrial pumps. There are a wide variety of mechanical seals used in pumps, with different models, but there are mainly five areas where leaks can occur: (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. Generally, leaks between the shafts where the shaft sleeves protrude, as well as between the sealing end caps and the pump body, are relatively easy to detect and resolve; however, careful observation is required, especially when the working medium is a liquefied gas or a high-pressure, toxic, or harmful gas, in which case it becomes more difficult. The remaining leaks are difficult to identify and determine intuitively; it is necessary to observe, analyze, and evaluate the symptoms of these leaks based on long-term management and maintenance practices in order to arrive at correct conclusions. 1. Leakage during static installation testing. After the mechanical seal is installed and adjusted, a static test is generally carried out to observe the leakage rate. If the leakage amount is small, it is usually due to a problem with the seal rings of the rotating or stationary ring; when the leakage amount is large, it indicates a problem in the friction pair between the rotating and stationary rings. Based on an initial assessment of the leakage amount and identification of the leakage location, manual rotation of the shaft is carried out for further inspection. If there is no significant change in the leakage amount, then there is a problem with the static or dynamic seal rings; if the leakage amount changes significantly during rotation, it indicates a problem with the friction pair between the dynamic and static rings. If the leaking fluid sprays in an axial direction, it is most likely that there is a problem with the dynamic seal ring, while if the fluid sprays in all directions or leaks out from the water cooling holes, it is usually due to a failure of the static seal ring. In addition, leakage paths can also exist simultaneously, but there is usually a primary and secondary distinction; with careful observation and a good understanding of the structure, it is definitely possible to make the correct judgment. 2. Leaks that occur during the trial operation of the mechanical pump. After undergoing a static test, the mechanical seal used in pumps prevents leakage of the medium due to the centrifugal force generated by high-speed rotation during operation. Therefore, during commissioning, mechanical seal leakage, after ruling out failures in the shaft-to-shaft and end-cap seals, is basically caused by damage to the friction pairs of the rotating and stationary rings. The main factors that cause failure in the sealing of friction pairs are: (1) During operation, abnormal conditions such as evacuation, cavitation, and pressure buildup generate significant axial forces, causing the contact surfaces of the moving and stationary rings to separate; (2) An excessive compression amount when installing the mechanical seal leads to severe wear and damage on the end faces of the friction pairs; (3) The sealing ring of the moving ring is too tight, preventing the spring from adjusting the axial movement of the moving ring; (4) The sealing ring of the stationary ring is too loose, resulting in the stationary ring coming loose from its seat when the moving ring moves axially; (5) Presence of particulate matter in the working medium, which enters the friction pair during operation and damages the end faces of the moving and stationary rings; (6) Errors in design and selection, such as an insufficient specific pressure on the sealing end faces or a high degree of thermal contraction of the sealing material. The aforementioned phenomenon occurs frequently during trial operations; sometimes it can be eliminated by making appropriate adjustments to the static ring seat, but in most cases it is necessary to disassemble and reassemble the components and replace the mechanical seal. 3. Sudden leakage of the mechanical pump during normal operation. Sudden leakage from a centrifugal pump during operation is occasionally due to normal wear or the reaching of its service life, but in most cases it is caused by significant changes in operating conditions or improper operation and maintenance. (1) Vacuum, cavitation, or prolonged pressure buildup can lead to the failure of the mechanical seal; (2) The actual output volume of the pump is too low, causing a large amount of fluid to circulate within the pump, heat to accumulate, and the fluid to vaporize, which in turn results in the failure of the mechanical seal; (3) An excessive amount of backflow causes the sediment at the bottom of the container (tower, reactor, tank, pool) on the suction side to be displaced, thereby damaging the mechanical seal; (4) After a long period of shutdown, if the pump is restarted without manual rotation, the friction surfaces may get stuck together and damage the seal; (5) An increase in corrosive, polymeric, or gelling substances in the fluid; (6) Sudden changes in ambient temperature; (7) Frequent changes or adjustments in operating conditions; (8) Sudden power outages or unexpected shutdowns. If a centrifugal pump starts leaking suddenly during normal operation, and this is not detected in time, it can often lead to serious accidents or losses; therefore, it is necessary to pay close attention and take effective measures. 4. The greater the spring compression, the better the sealing effect. This is not the case. Excessive spring compression can lead to rapid wear of the friction pairs, resulting in instant burnout; excessive compression also deprives the spring of its ability to adjust the end surface of the moving ring, thereby causing the mechanical seal to fail. 5. The tighter the dynamic ring seal, the better. In fact, an overly tight dynamic ring seal is more harmful than beneficial. First, it exacerbates the wear between the sealing ring and the shaft sleeve, leading to premature leakage. Second, it increases the resistance to axial adjustment and movement of the moving ring, making it impossible to make adjustments in a timely manner when operating conditions change frequently. Third, the spring is prone to excessive fatigue and damage. Fourth, it causes the sealing ring of the moving ring to deform, thereby affecting the sealing performance. 6. The tighter the static ring seal, the better. The static ring seal is essentially at rest, which results in a better sealing effect; however, being too tight is also harmful. First, excessive deformation of the stationary ring seal can affect its sealing performance. Second, the material used for the stationary ring is often graphite, which is relatively brittle, and excessive stress can easily cause it to break. Third, installation and removal are difficult, and this can easily lead to damage to the sealing ring of the stationary ring. 7. The tighter the impeller lock nut, the better. Among mechanical seal leaks, leakage between the shaft sleeve and the shaft (inter-shaft leakage) is relatively common. It is generally believed that shaft leakage is caused by the impeller lock nut not being tightened properly. In fact, there are many factors that can lead to shaft leakage, such as the failure of the shaft seals, misalignment, the presence of contaminants inside the shaft gap, significant geometric errors at the interface between the shaft and its sleeve, damage to the contact surfaces, gaps between various components on the shaft, and excessively long thread lengths on the shaft ends – all of these can cause shaft leakage. Excessive tightening of the lock nut will only cause the shaft gasket to fail prematurely; on the other hand, moderate tightening of the lock nut ensures that the gasket maintains a certain degree of compressive elasticity. During operation, the lock nut will tighten automatically at the right time, keeping the shafts in a proper sealed state. 8. The new is better than the old. Relatively speaking, new mechanical seals perform better than old ones. However, if the quality or material selection of the new mechanical seals is inappropriate, significant dimensional errors can affect their sealing performance. In polymeric and permeable media, it is advisable not to replace the static ring unless it is severely worn out. Since the stationary ring remains stationary in its seat for an extended period of time, polymers and impurities accumulate together, thereby providing an effective sealing effect. 9. It’s better to repair it than not to do anything at all. Once a leakage occurs in the mechanical seal, there is a tendency to rush into repairs. In fact, sometimes the mechanical seal is not damaged at all; the leakage can be eliminated simply by adjusting the operating conditions or making appropriate adjustments to the mechanical seal itself. Installation and use of mechanical seals for pumps Installation and use of mechanical seals for pumps Mechanical seals are widely used in various types of pumps. A mechanical seal is a high-precision sealing device that has certain requirements regarding installation and operating conditions. 1. General principles for the installation and use of mechanical seals: (1) Understand the conditions of the equipment, including the rotation speed and shaft diameter of its rotating shaft ； Comprehensive and balanced consideration must be given to the manufacturing precision of the equipment and the dimensions of its sealing chamber, as well as the equipment’s own service life and its role in the production process. (2) Estimate the medium pressure. The pressure in the pump’s sealing chamber is generally not the pump’s outlet pressure, but rather lower than it. (3) Find out the situation of the sealing medium. To determine the state of the sealing medium – whether it is gaseous or liquid – as well as whether the medium contains particles and what the condition of those particles is ； Understand the properties and temperature of the medium in order to select the appropriate type and implement necessary cooling, flushing, and lubrication measures. 2. Technical requirements for the installation and use of mechanical seals
(1) Radial runout tolerance of the shaft or shaft sleeve where the mechanical seal is to be installed: 0.04 mm for Ф10–50 mm; 0.06 mm for diameters greater than Ф50–120 mm. The surface roughness of the shaft or sleeve shall meet the technical requirements, with the outer diameter tolerance being h6. (2) The axial play of the drive shaft shall not exceed 0.2 mm. (3) Runout tolerance of the end face of the sealing chamber relative to the shaft surface: For shafts or sleeve outer diameters, the runout tolerance is 0.04 mm for diameters ranging from Ф10 to 50 mm, and 0.06 mm for diameters greater than Ф50 to 120 mm. (4) The end faces of the shaft or sleeve, as well as those of the sealing chamber, must have chamfers. It is 3×10° when the shaft diameter ≤ Φ30mm, and 4×10° when the shaft diameter > Φ30mm. (5) When the temperature of the conveyed medium is too high, too low, or when it contains impurity particles, as well as flammable, explosive, or toxic substances, corresponding measures such as sealing, flushing, cooling, and filtering should be taken. (6) For certain mechanical seals that rely on springs for operation, attention must be paid to the winding direction of the springs; that is, the axis should rotate in such a way as to tighten the springs over time, otherwise the seal will fail. The choice of spring winding direction is determined by the following method: when looking from the stationary ring to the moving ring, if the axis rotates clockwise, a right-handed spring should be used; otherwise, a left-handed spring should be chosen. 3. Installation method of mechanical seals: Mechanical seal components require high precision both in terms of manufacturing and installation. If installed improperly, it will affect the lifespan and sealing performance of the seal; in severe cases, this can lead to rapid failure of the seal. (1) Pre-installation preparations and precautions ① Check whether the model and specifications of the mechanical seal to be installed are correct, and whether any parts are missing. ②Check whether all components of the mechanical seal are damaged, especially to see if the sealing surfaces of the rotating and stationary rings are scratched. If any damage is found, it is necessary to repair them or replace them with new parts. All sealing components must be cleaned thoroughly with gasoline or kerosene to ensure that their surfaces are free of dust and foreign particles. ③Check whether there are burrs, grooves, etc. on the surface of the shaft or sleeve, as well as on the inner wall of the sealing chamber and the inner surface of the sealing end cover. If burrs or grooves are found, they should be smoothed and polished, and cleaned with gasoline or kerosene to keep the surface clean; no dust or debris is allowed to remain on it. ④Do not wipe the surfaces of the rotating ring and the stationary ring with dirty cloths or gauze; instead, use clean and soft gauze, absorbent cotton, or similar materials for cleaning. ⑤Cleanliness should be maintained during the assembly process to ensure that the sealing surfaces of the moving and stationary rings are not scratched or damaged. To facilitate installation, oil should be applied to the surface of the shaft or sleeve, as well as to the surfaces where the gland meets the seal rings, in order to avoid dry friction at the moment of startup. (2) Assembly sequence ① Assembly of the stationary parts of the mechanical seal: a. Insert the anti-rotation pin into the corresponding hole in the seal end cover ； b. Put the stationary ring seal ring onto the stationary ring, then install the stationary ring into the seal end cover. Make sure that the anti-rotation pin enters the groove on the stationary ring. When installing the gland, be careful not to let the stationary ring touch the shaft. The bolts should be tightened in several stages evenly. ②Assembly of the rotating components of the mechanical seal: Assemble the rotating components of the mechanical seal onto the shaft one by one in sequence. If there is a shaft sleeve, the rotating components of the mechanical seal must be assembled onto the shaft sleeve from the outside, and then the shaft sleeve with these rotating components mounted on it should be fitted onto the shaft. ③The end cover is mounted on the seal body and tightened evenly with screws. ④Check whether the rotation test is easy; if it is difficult to rotate or requires effort, then verify whether the assembly dimensions are correct. 4. Fault handling for mechanical seal operation (1) Leakage from the start ① Check whether the assembly quality meets the technical requirements and whether the spring compression is within the specified range. ②Check whether the sealing surfaces of the rotating and stationary rings are damaged. ③Check whether the sealing end face is aligned properly. (2) If the leakage rate suddenly increases during operation, the machine should be stopped for inspection. ① Check the wear condition of the sealing surfaces of the rotating ring and the stationary ring, as well as any changes in their surface quality; if they are damaged, they should be repaired or replaced with new parts. ②Check whether the installation positions of the auxiliary sealing rings on the rotating ring and the stationary ring are correct (for example, the lip of the V-ring should face the pressure side) and whether they are damaged. If the installation is incorrect, reinstall it; if it is damaged, replace the component. ③Check whether there are any solid impurities mixed in the sealing chamber, and whether the drive seat is filled with impurities that could affect the axial movement of the moving ring and the compensation provided by the spring. ④Check whether the set screw is loose and whether it affects the proper operation of the mechanical seal. ⑤Check whether the screws securing the end cover are loose, which may cause the sealing end cover to tilt. ⑥Check whether the axial movement and radial vibration of the pump exceed the technical requirements for use. ⑦In the case of installing a shaft sleeve, check whether the seal between the shaft sleeve and the shaft is damaged and whether its position is correct. ⑧Check whether there is seal fluid circulation inside the seal chamber, and whether the mechanical seal is in a dry-friction state.