Little knowledge about mechanical seals
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1. Working principle of mechanical seals: A mechanical seal is a type of shaft sealing device that prevents leakage by relying on a pair or several pairs of end faces that slide relative to each other perpendicular to the axis; these end faces remain in close contact under the action of fluid pressure and the elastic force (or magnetic force) of a compensation mechanism, with additional sealing elements being used to enhance this effect. 2. Selection of common materials for mechanical seals – Fresh water ； Room temperature ； (Moving) 9Cr18, 1Cr13 – surfacing with cobalt-chromium-tungsten, cast iron ； (Silent) Resin-impregnated graphite, bronze, phenolic plastic. River water (with sediment) ； Room temperature ； (Moving) Tungsten carbide, (Stationary) Tungsten carbide – seawater ； Room temperature ； (Moving) Tungsten carbide, 1Cr13 surfacing cobalt-chromium-tungsten, cast iron ； (Silent) Resin-impregnated graphite, tungsten carbide, cermets ； Supercritical water 100 degrees ； (Moving) Tungsten carbide, 1Cr13 surfacing cobalt-chromium-tungsten, cast iron ； (Silent) Resin-impregnated graphite, tungsten carbide, cermets ； Gasoline, lubricating oil, liquid hydrocarbons ； Room temperature ； (Moving) Tungsten carbide, 1Cr13 surfacing cobalt-chromium-tungsten, cast iron ； (Silent) Resin-impregnated or tin-antimony alloy graphite, phenolic plastic. Gasoline, lubricating oil, liquid hydrocarbons ； 100 degrees ； (Moving) Tungsten carbide, 1Cr13 surfacing cobalt-chromium-tungsten ； (Silent) Immersed in bronze or resin graphite. Gasoline, lubricating oil, liquid hydrocarbons ； With particles ； (Moving) Tungsten carbide ； (Silent) Tungsten carbide. 3. Types and uses of sealing materials Sealing materials must meet the requirements of the sealing function. Due to the different sealed media and the varying operating conditions of the equipment, the sealing materials are required to have different levels of adaptability. The general requirements for sealing materials are: 1) The material should have good density to prevent leakage of the medium ； 2) It has appropriate mechanical strength and hardness ； 3) Good compressibility and resilience, with minimal permanent deformation ； 4) Does not soften or decompose at high temperatures, and does not harden or crack at low temperatures ； 5) It has excellent corrosion resistance, enabling long-term operation in media such as acids, alkalis, and oils. Its volume and hardness change little, and it does not adhere to metal surfaces ； 6) Low coefficient of friction and good wear resistance ； 7) Possesses flexibility to fit with the sealing surface ； 8) Good aging resistance, durable over time ； 9) It is easy to process and manufacture, inexpensive, and the materials are readily available. Rubber is the most commonly used sealing material. In addition to rubber, materials suitable for use as sealing materials include graphite, polytetrafluoroethylene, and various sealants. 4. Key technical points for the installation and use of mechanical seals 1. The radial runout of the equipment’s rotating shaft should be ≤0.04 millimeters, and the axial displacement must not exceed 0.1 millimeters ； 2. The sealing parts of the equipment must be kept clean during installation; the sealing components should be cleaned, and the sealing surfaces must remain intact to prevent impurities and dust from entering those areas ； 3. During installation, it is strictly prohibited to strike or hit the device, to avoid damaging the mechanical seal friction pair and resulting in a loss of sealing performance ； 4. During installation, a layer of clean machine oil should be applied to the surfaces in contact with the seal to ensure smooth installation ； 5. When installing the static ring gland, the screws must be tightened evenly to ensure that the end face of the static ring is perpendicular to the axis line ； 6. After installation, pushing the moving ring by hand allows it to move freely on the shaft while exhibiting a certain degree of elasticity ； 7. After installation, turn the shaft by hand; it should not feel heavier or lighter in any way ； 8. The equipment must be filled with the medium before operation to prevent seal failure due to dry friction ； 9. For crystalline and granular media that are prone to caking, when the medium temperature exceeds 80°C, appropriate flushing, filtering, and cooling measures should be taken. Refer to the relevant standards for mechanical seals regarding various auxiliary devices. 10. During installation, a layer of clean machine oil should be applied to the surfaces in contact with the seal. Special attention must be paid to the choice of machine oil, as it varies depending on the material of the auxiliary seals; this is to prevent the O-ring from expanding due to exposure to oil or from aging more rapidly, which could lead to premature failure of the seal. 5. What are the three sealing points of a mechanical shaft seal, and what is the sealing principle behind these three points? The sealing between the rotating ring and the stationary ring is achieved through elastic elements (such as springs or bellows) and the pressure of the sealing fluid; these factors generate an appropriate compressive force (specific pressure) at the contact surface (end face) between the rotating and stationary rings, thereby ensuring that the two smooth, flat end faces stay in close contact ； A very thin layer of liquid film is maintained between the end faces to achieve sealing. This film possesses dynamic and static pressures, and it serves to balance the pressures as well as lubricate the end surfaces. The two end faces must be highly smooth and flat in order to achieve perfect fit between them and ensure uniform pressure, which is essential for relative rotary sealing. ??? 6. Types of mechanical seal technologies: New technologies for mechanical seals that utilize new materials and manufacturing processes are advancing rapidly, and the following are such new mechanical seal technologies. The grooved sealing technology for sealing surfaces has seen the creation of various flow channels on the sealing surfaces of mechanical seals in recent years, in order to generate hydrostatic and dynamic pressure effects; this technology is still evolving continuously. In the past, zero-leakage sealing technology held the view that both contact and non-contact mechanical seals could not achieve zero leakage (or no leakage). Israel has utilized grooved sealing technology to introduce a new concept of zero-leakage, contactless mechanical face sealing, which has been applied in lubrication oil pumps for nuclear power plants. Dry-running gas seal technology is a type of seal that applies slotted seal technology to gas sealing. The upstream pumping sealing technology utilizes flow channels on the sealing surface to pump a small amount of leaked fluid from the downstream back to the upstream. The structural features of the aforementioned types of seals are as follows: they utilize shallow grooves, with both the film thickness and the depth of the flow channels being in the micron range; in addition, lubrication grooves are used, and radial sealing dams together with circumferential sealing weirs constitute the sealing and supporting components. It can also be said that slotted sealing is a combination of flat sealing and slotted bearings. Its advantages are low leakage (even no leakage), large membrane thickness, elimination of contact friction, and low power consumption and heat generation. Thermohydraulic dynamic pressure sealing technology utilizes sealing surfaces with deep grooves of various shapes to induce local thermal deformation, thereby creating a hydrodynamic wedge effect. This type of seal with hydrodynamic load-bearing capacity is known as a thermohydrodynamic wedge seal. Bellows sealing technology can be divided into mechanical sealing technologies for formed metal bellows and welded metal bellows. Multi-face seal technology is divided into double sealing, intermediate ring sealing, and multi-seal technology. There are also parallel surface sealing technology, monitoring sealing technology, combined sealing technology, and others. 7. Mechanical seal flushing scheme and characteristics: The purpose of flushing is to prevent the accumulation of impurities, avoid the formation of air bubbles, and maintain and improve lubrication; when the temperature of the flushing fluid is low, it also serves a cooling function. The main methods of flushing are as follows: 1. Internal flushing 1. Positive flushing (1) Characteristics: It makes use of the sealed medium in the main working unit, which is introduced into the sealing chamber through pipelines at the pump’s outlet end. (2) Application: Used for cleaning fluids; p1 is slightly higher than p_in. When the temperature is high or there are impurities, coolers, filters, etc. can be installed in the pipeline. 2. Backwashing (1) Characteristics: It makes use of the sealed medium in the main operating unit; this medium is introduced into the sealing chamber from the pump’s outlet, and after backwashing it flows back to the pump’s inlet through the pipelines. (2) Application: Used for cleaning fluids, with p inlet
Dry gas seals are also a type of mechanical seal.
1 Principle and requirements of mechanical seals: A mechanical seal functions by relying on the tiny axial gap that is created between the end faces A of a pair of rings in relative motion – one of which is fixed while the other rotates together with the shaft, as shown in Figure 1. This type of device is known as a mechanical seal. A mechanical seal typically consists of a rotating ring, a stationary ring, a compression element, and a sealing element. The end faces of the rotating ring and the stationary ring form a friction pair; the pressure of the liquid in the sealing chamber forces the end face of the rotating ring to press against the end face of the stationary ring, thereby creating an appropriate specific pressure between the two end faces and maintaining a very thin layer of liquid film to achieve sealing. The compression element generates pressure, which allows the pump to maintain tight contact between its end faces 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 faces. The sealing element serves to seal the gap B between the rotating ring and the shaft, as well as the gap C between the stationary ring and the gland; it also acts as a buffer against vibrations and shocks in the pump. In actual operation, a mechanical seal is not a standalone component; it operates in conjunction with other parts of the pump. Moreover, its basic principles show that the proper functioning of a mechanical seal depends on certain conditions – for example, the shaft play of the pump must not be too large, otherwise the contact surfaces between the frictioning elements cannot achieve the required pressure ratio ； The pump shaft at the mechanical seal should not have excessive deflection; otherwise, the surface pressure will be uneven, among other problems. Only by meeting external conditions like these, along with the excellent performance of a good mechanical seal itself, can an ideal sealing effect be achieved. 2 Analysis of the reasons affecting external conditions 2.1 Excessive axial movement of the pump shaft: The sealing surfaces of a mechanical seal need to have a certain specific pressure in order to function as seals. This requires that the springs in the mechanical seal have a certain degree of compression, thereby applying a force to the sealing surfaces and creating the specific pressure necessary for sealing when the shaft rotates. To maintain this specific pressure ratio, mechanical seals require that the pump shaft not have excessive play, which should generally be kept within 0.5 mm. However, in actual design, due to unreasonable design, the pump shaft often experiences significant axial movement, which is highly detrimental to the use of mechanical seals. This phenomenon often occurs in multi-stage centrifugal pumps, especially during the pump startup phase when the amount of shaft movement is relatively large. Figure 2 shows the working principle of the balance disc method for balancing axial forces. When the balance disk is in operation, it automatically changes the axial gap b between the balance disk and the balance ring, thereby altering the pressure difference on the front and rear sides of the balance disk. This creates a force that acts in the opposite direction to the axial force, thus balancing it out. Due to the inertia of rotor runout and the fluctuations in transient pump operating conditions, the rotating rotor does not remain at a fixed axial equilibrium position. The balance disc is constantly moving back and forth. During normal operation, the axial displacement of the balance disc is only 0105 to 011 mm, which meets the requirement of 015 mm for allowable axial displacement in mechanical seals; however, during pump startup, shutdown, or when there are sudden changes in operating conditions, the axial displacement of the balance disc may **exceed the allowable axial displacement specified for mechanical seals. After prolonged operation of the pump, frictional wear occurs between the balance disk and the balance ring, causing the gap b to increase; as a result, the axial displacement of the mechanical seal keeps rising. Due to the axial force, the compressing force on the sealing surface on the suction side increases, leading to accelerated wear of the sealing surface until it is damaged and loses its sealing function. In the mechanical seal on the discharge side, as the balance disk wears out, the axial movement of the rotor components exceeds the allowable level specified for sealing purposes; as a result, the compression force on the sealing surfaces decreases, failing to meet the sealing requirements. This ultimately leads to the loss of sealing function in both mechanical seals on either side of the pump. 2.2 Excessive axial force: Mechanical seals are not capable of withstanding axial forces during operation; the presence of such forces has a severe impact on them. Sometimes, due to an unreasonable design of the pump’s axial force balancing mechanism, as well as issues related to manufacturing, installation, and operation, the axial force fails to be balanced. Mechanical seals are subjected to an axial force, and the temperature of the seal gland rises during operation. For media made of polypropylene, this material melts at high temperatures; as a result, the sealing effect is lost shortly after the pump starts operating. When the pump is stationary, intermittent leakage occurs at the seal surfaces. 2.3 Excessive deflection of the pump shaft: Mechanical seals, also known as face seals, are types of rotary axial contact dynamic seals. They function by allowing two sealing surfaces perpendicular to the axis line to press tightly against each other and rotate relative to one another, thereby achieving a sealing effect; therefore, it is necessary for even force to be applied between these two sealing surfaces. However, due to the unreasonable design of the pump, significant deflection occurs at the location where the mechanical seal is installed during the operation of the pump shaft, resulting in uneven stress distribution between the sealing surfaces and thus poor sealing performance. 2.4 Absence of an auxiliary flushing system or improper setup of such a system: 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, designers fail to properly configure the auxiliary flushing system, resulting in insufficient sealing ； Sometimes, even though designers have created auxiliary systems, the sealing effect is not achieved due to impurities in the flushing fluid, insufficient flow or pressure of the flushing fluid, or an improperly designed location of the flushing ports. 2.5 Excessive vibration: Excessive vibration of the mechanical seal ultimately leads to a loss of sealing efficiency. However, the reason for excessive vibration in mechanical seals is often not due to the seals themselves; rather, other components of the pump are the source of such vibration, such as an improperly designed pump shaft, issues related to manufacturing, insufficient bearing precision, poor parallelism of the couplings, and high radial forces. 2.6 Causes of pump cavitation: Due to improper operation of the system, poor cavitation resistance at the pump inlet, and excessively high pump speed, local cavitation occurs at the pump inlet. After cavitation takes place, bubbles form in the water; these bubbles impact the outer surface of the mechanical seal, causing damage to that surface ； On the other hand, this causes bubbles to be present in the flow film at the interface between the stationary and rotating rings, preventing the formation of a stable flow film. As a result, dry friction occurs at this interface, leading to damage to the mechanical seal device. 2.7 Insufficient machining precision: There are many reasons for insufficient machining precision. Some of these relate to the inadequate machining precision of the mechanical seal itself; such causes are easy to notice and identify. But sometimes it is the insufficient machining precision of other components of the pump; this reason does not easily draw people’s attention. For example: reasons such as insufficient precision in the pump shaft, shaft sleeves, pump body, and sealing chamber. The presence of these factors is highly detrimental to the sealing performance of mechanical seals. 3 Measures to be Taken 3.1 Measures to Eliminate Excessive Axial Movement of the Pump Shaft Design a proper balance mechanism for axial forces in order to eliminate such axial movement. To meet this requirement, for multi-stage centrifugal pumps, there are two relatively ideal design approaches: one involves using a balance disk in combination with axial thrust bearings, where the balance disk is used to balance the axial forces, while the axial thrust bearings provide axial restraint for the pump shaft ； The other option is a balance drum combined with axial thrust bearings; the balance drum handles most of the axial force, while the remaining axial force is taken care of by the thrust bearings, which also provide axial positioning for the pump shaft. The key to the second approach is to properly design the balance drum so that it can effectively balance most of the axial forces. For other single-stage pumps, split-case pumps, and similar products, certain measures are taken during design to ensure that the shaft play remains within the range required by the mechanical seal. 3.2 Measures to eliminate excessive axial force: Design the axial force balancing mechanism properly so that it can effectively balance the axial force, thereby creating favorable conditions for the mechanical seal. For important products used in industries such as power plants, petroleum, and chemicals, it is necessary to conduct testing on each unit before they leave the factory, in order to identify problems and resolve them. For some important pumps, an axial force sensing ring can be designed on the rotor to monitor the magnitude of the axial force at any time, allowing issues to be addressed promptly once they are detected. 3.3 Measures to eliminate excessive deflection of the pump shaft This phenomenon is common in horizontal multi-stage centrifugal pumps; the following measures can be taken during design: (1) Reduce the distance between the bearings at both ends. The number of stages in the pump impeller should not be too large; when a high total head is required from the pump, it is advisable to increase the head per impeller stage and reduce the number of stages. (2) Increase the diameter of the pump shaft. When designing the pump shaft diameter, one should not consider solely the amount of power to be transmitted; rather, factors such as mechanical seals, shaft deflection, starting methods, and related inertial loads and radial forces must also be taken into account. Many designers do not fully realize this. (3) Upgrade the grade of the pump shaft material. (4) After the pump shaft design is completed, a check calculation for the deflection of the pump shaft must be carried out. 3.4 Adding an auxiliary flushing system: Where conditions permit, design an auxiliary flushing system as much as possible. The flushing pressure is generally required to be 0107 to 011 MPa higher than the pressure in the sealing chamber; if the medium being transported is volatile, it should be 01175 to 012 MPa higher than its vaporization pressure. The pressure in the sealed chamber must be 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 when it is close to the maximum limit for which that seal can operate, liquid can also be drawn from the seal chamber to a low-pressure area, allowing the shaft seal fluid to flow and carry away the friction heat. The recommended flushing volume is shown in Table 1. 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. 3.5 Measures to eliminate cavitation at the pump inlet (1) Improve the cavitation performance of the pump to meet the requirements of the on-site installation. (2) The requirements of the field test setup must be compatible with the pump’s cavitation performance level. (3) On-site installation and operating condition adjustment should create favorable conditions for the pump. 3.6 Measures to Eliminate Pump Vibration (1) During the design of pump products, it is necessary to thoroughly analyze the sources of vibration in order to eliminate them. (2) During the manufacturing and assembly of pump products, standards and operating procedures must be strictly followed to eliminate sources of vibration. (3) When installing auxiliary equipment such as pumps, motors, bases, and on-site pipelines on site, strict quality control must be applied to eliminate vibration sources. (4) During on-site production, operation, maintenance, and adjustment, strict controls must be applied to eliminate vibration sources. 3.7 Strictly adhere to design standards: When designing mechanical seals for pumps, it is necessary to take into account not only the factors affecting the mechanical seal itself but also various external factors as well. In practical work, the following points should be taken into consideration: (1) During the design of pump products, it is necessary to fully account for the impact of other components of the pump as well as other equipment on site on the performance of the mechanical seal, in order to create favorable external conditions for it. (2) Increase awareness of the important role of mechanical seal auxiliary systems, and equip them as fully as possible to improve the sealing performance. (3) For the mechanical seals of important pump products, additional protective measures should be implemented to improve seal quality and reduce seal-related quality failures. (4) When analyzing the causes of quality failures in mechanical seals, it is necessary to fully consider the impact of other components of the pump on the operation of the mechanical seal, and take measures to continuously improve its performance.