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Through the practical application and theoretical analysis of the mechanical seal of the pump, it is proposed that the actual sealing effect of the mechanical seal is not only related to the performance of the mechanical seal itself, but also has an important relationship with the conditions provided by other parts and the conditions provided by the seal assist system. Therefore, when designing pump unit products, it is necessary to provide a good external condition for the use of mechanical seals.
At present, mechanical seals are widely used in pump products, and with the improvement of product technology and energy conservation requirements, the application prospects of mechanical seals will be more extensive. The sealing effect of the mechanical seal will directly affect the operation of the whole machine, especially in the petrochemical industry. Due to the existence of flammable, explosive, volatile, highly toxic medium, the mechanical seal leaks, which will seriously affect the normal production, serious There will also be major security incidents. When analyzing the causes of quality failures, people tend to find reasons for mechanical seals themselves, such as whether the selection of mechanical seals is appropriate, whether the material selection is correct, whether the specific pressure of the sealing surface is correct, and whether the selection of friction pairs is reasonable. Rarely find the cause of the external conditions of the mechanical seal, such as: whether the conditions created by the pump for the mechanical seal are appropriate, and whether the configuration of the auxiliary system is appropriate, and the reasons for these are often very important. The author analyzes several factors affecting the sealing effect and reasonable measures to be taken from the perspective of the external conditions of the pump mechanical seal.
1 Principle and requirements of mechanical seal
The mechanical seal is sealed by a small axial gap formed by the abutting faces of a pair of relatively moving rings. This device is called a mechanical seal.
Mechanical seals usually consist of a moving ring, a stationary ring, a pressing element and a sealing element. The end faces of the moving ring and the stationary ring constitute a pair of friction pairs, and the moving ring is pressed against the end face of the stationary ring by the pressure of the liquid in the sealing chamber, and an appropriate specific pressure is generated on the end faces of the two rings and a layer is maintained. A thin liquid film for sealing purposes. The pressing element generates pressure, which keeps the end of the pump in the non-operating state, ensures that the sealing medium does not leak, and prevents impurities from entering the sealing end face. The sealing element acts to seal the gap B between the moving ring and the shaft, and the gap C between the stationary ring and the gland, and at the same time buffers the vibration and impact of the pump.
The mechanical seal is not an isolated component in actual operation. It is combined with other components of the pump. At the same time, it can be seen from the basic principle that the normal operation of the mechanical seal is conditional, for example: the pump shaft The amount of enthalpy should not be too large, otherwise the friction end face cannot form the normal required specific pressure; the pump shaft at the mechanical seal should not have too much deflection, otherwise the end face specific pressure will be uneven. Only when such external conditions are met, coupled with good mechanical seal performance, can achieve the desired sealing effect.
2 Analysis of the causes of external conditions
2.1 The axial load of the pump shaft is large
The sealing surface of the mechanical seal must have a certain specific pressure, so as to play a sealing role, which requires the mechanical seal spring to have a certain amount of compression, give the seal end face a thrust, and rotate it to make the sealing surface require the seal ratio required. Pressure. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft should not have too much enthalpy, and it is generally guaranteed to be within 0.5 mm. However, in the actual design, due to the unreasonable design, the pump shaft often produces a large amount of enthalpy, which is very unfavorable for the use of mechanical seals. This phenomenon often occurs in multi-stage centrifugal pumps, especially during pump start-up, where the amount of helium is relatively large. When the balance disc works, the axial gap b between the balance disc and the balance ring is automatically changed, thereby changing the pressure difference between the front and rear sides of the balance disc, and generating a force opposite to the axial force direction to balance the axial force. Due to the inertia of the rotor turbulence and fluctuations in transient pump conditions, the operating rotor does not rest at an axial equilibrium position. The balance disc is always in a state of being tilted left and right. The axial displacement of the balance disc during normal operation is only 0105 ~ 011 mm, which meets the requirement of 015 mm for the mechanical seal. However, the axial disc volume of the balance disc during pump start, stop, and working conditions may change. It greatly exceeds the axial amount allowed by the mechanical seal.
After the pump runs for a long time, the balance disc and the balance ring frictionally wear, and as the gap b increases, the axial volume of the mechanical seal increases continuously. Due to the axial force, the pressing force of the sealing surface on the suction side increases, and the wear of the sealing surface is intensified until the sealing surface is damaged and the sealing effect is lost. The mechanical seal on the discharge side, with the wear of the balance disc, the axial amount of the rotor part is greater than the axial amount required for the seal, the pressing force of the sealing surface is reduced, the sealing requirement is not achieved, and finally the sides of the pump are The mechanical seals all lose their sealing effect.
2.2 The axial force is too large. The mechanical seal cannot withstand the axial force during use. If there is axial force, the effect on the mechanical seal is serious.
Sometimes the axial force is not balanced due to the unreasonable design of the axial force balance mechanism of the pump and the manufacturing, installation, and use reasons. The mechanical seal is subjected to an axial force, and the temperature of the sealing gland will be high during operation. For the polypropylene medium, it will be melted at high temperature, so the sealing effect will be lost soon after the pump is started, and the sealing end face will appear when the pump is stationary. Intermittent leaks.
2.3 The deflection of the pump shaft is too large. The mechanical seal, also known as the end face seal, is a rotary axial contact dynamic seal. Under the action of the fluid medium and the elastic element, the two sealing end faces perpendicular to the axis line are closely attached. It is combined and rotated to achieve a sealing effect, so it is required to be evenly stressed between the two seals. However, due to the unreasonable design of the pump product, when the pump shaft is running, the deflection generated at the mechanical seal installation is large, so that the force between the sealing surfaces is uneven, resulting in poor sealing effect.
2.4 It is very important to have an auxiliary flushing system without auxiliary flushing system or auxiliary flushing system. It can effectively protect the sealing surface and cool, lubricate and wash away debris. Sometimes the designer does not properly configure the auxiliary flushing system to achieve the sealing effect. Sometimes, although the designer designs the auxiliary system, the flow rate and pressure of the flushing fluid are insufficient due to impurities in the flushing liquid, and the design of the flushing port is unreasonable. Also, the sealing effect is not achieved.
2.5 Vibration is too large The mechanical seal vibration is too large, eventually leading to the loss of sealing effect. However, the reason for the large vibration of the mechanical seal is often not the cause of the mechanical seal itself. Other parts of the pump are the source of vibration, such as unreasonable pump shaft design, processing reasons, insufficient bearing precision, and poor parallelism of the coupling. Radial force and other reasons.
2.6 Causes of pump cavitation Due to unreasonable operation of the system and the poor cavitation performance of the pump inlet and the high speed of the pump, local cavitation occurs at the inlet of the pump. After cavitation occurs, there will be bubbles in the water. It will impact the outer surface of the mechanical sealing surface and cause damage to its surface. On the other hand, the flow film of the matching surface of the static and dynamic ring also contains air bubbles, which cannot form a stable flowing film, resulting in dry friction of the matching surface of the static and dynamic ring. Damage the mechanical seal.
2.7 The machining accuracy is not enough. The machining accuracy is not enough. There are many reasons. Some of them have insufficient machining precision of the mechanical seal itself. The reason for this is easy to attract people's attention and easy to find.
However, sometimes the processing precision of other parts of the pump is not enough, and the reason for this is not easy to attract people's attention. For example, the pump shaft, the bushing, the pump body, and the sealing cavity have insufficient precision. The presence of these causes is very detrimental to the sealing effect of the mechanical seal.
3 Measures to be taken
3.1 Measures to eliminate the large amount of pump shaft åˆç† Reasonably design the axial force balance device to eliminate the axial enthalpy. In order to meet this requirement, for the multi-stage centrifugal pump, there are two ideal designs: one is the balance disc plus the axial thrust bearing, the balance shaft balances the axial force, and the axial thrust bearing carries the pump shaft. The axial limit; the other is the balance drum plus the axial thrust bearing, the balance shaft balances most of the axial force, the remaining axial force is carried by the thrust bearing, and the axial thrust bearing carries the shaft to the pump shaft To the limit. The key to the second option is to rationally design the balance drum so that it can truly balance most of the axial force. For other single-stage pumps, medium-open pumps, etc., some measures are taken during design to ensure that the pump shaft is within the required range of mechanical seals.
3.2 Measures to eliminate the large axial force The axial force balance mechanism is reasonably designed so that it can truly balance the axial force and create a good condition for the mechanical seal. For some important products used in power plants, petroleum, chemical and other fields, before the products leave the factory, it is necessary to test and detect problems and solve problems. Some important pumps can be designed with an axial force-measuring ring on the rotor to monitor the axial force at any time and find problems in time.
3.3 Measures to eliminate excessive deflection of the pump shaft This phenomenon is mostly found in horizontal multistage centrifugal pumps. The following measures are taken during design:
(1) Reduce the distance between the bearings at both ends. The number of stages of the pump impeller should not be too much. When the total head lift requirement is high, the lift of each stage of the impeller should be increased as much as possible to reduce the number of stages.
(2) Increase the diameter of the pump shaft. When designing the diameter of the pump shaft, do not simply consider the amount of power transmitted, but consider mechanical seals, shaft deflection, starting methods, and related inertial loads, radial forces, and other factors. Many designers are not fully aware of this.
(3) Improve the grade of the pump shaft material.
(4) After the pump shaft design is completed, the deflection of the pump shaft should be checked and calculated.
3.4 Adding an auxiliary flushing system Whenever possible, design an auxiliary flushing system.
The flushing pressure is generally required to be higher than the sealing chamber pressure of 0107 to 011 MPa. If the conveying medium is easily vaporized, it should be higher than the vaporization pressure of 01175 to 012 MPa. The pressure of the sealed chamber is calculated according to the structural type of each pump, system pressure and other factors. When the shaft seal chamber pressure is high or the pressure is close to the highest limit of the seal use, the liquid can be drawn from the seal chamber to the low pressure region, so that the shaft seal liquid flows to take away the friction heat.
Properly configure piping and accessories according to the operating conditions of each pump. Such as coolers, orifices, filters, valves, flow indicators, pressure gauges, temperatures, etc. In fact, the reliability and longevity of the seal depend to a large extent on the configuration of the seal assist system.
3.5 Measures to eliminate cavitation at the inlet of the pump
(1) Improve the cavitation performance level of the pump to meet the cavitation performance requirements of the field device.
(2) The requirements of the field test device should match the pump cavitation performance level.
(3) On-site installation and condition adjustment should create favorable conditions for the pump.
3.6 Measures to eliminate pump vibration
(1) During the design of the pump product, the source of the vibration should be fully analyzed to eliminate the vibration source.
(2) During the manufacturing and assembly process of the pump product, strictly follow the standards and operating procedures to eliminate the vibration source.
(3) When the auxiliary equipment such as pump, motor, base, and field pipeline is installed on site, it must be strictly controlled to eliminate the vibration source.
(4) When on-site production, operation, maintenance, and adjustment, strictly control and eliminate vibration sources.
3.7 Strict implementation of design standards
The design of the pump product and the design of the mechanical seal product must implement relevant domestic and international standards. In the design process of the product, the designer should conscientiously implement the standard, deeply understand the specific meaning of each content of the standard, and implement the requirements of the standard content to the product. During the design process. So far, many designers have not understood the actual meaning of the standard, and have not strictly enforced the new standards, but blindly copy the experience of the old drawings and the old designers. This practice is very unfavorable for improving the technical level of our products and entering the international market. Raising the awareness of standardization is an urgent problem that designers in the mechanical industry need to solve.
4 Conclusion
When designing the mechanical seal of the pump, it is necessary to consider not only the influencing factors of the mechanical seal itself, but also various influencing factors outside the mechanical seal.
Pay attention to the following issues in actual work:
(1) In the design process of the pump product, the influence of other parts of the pump and other equipment on the surface on the mechanical seal effect should be fully considered to create a good external condition for the mechanical seal.
(2) Increase the understanding of the important role of the mechanical seal assist system, as far as possible with a complete mechanical seal assist system to improve the sealing effect.
(3) For mechanical seals of important pump products, it is necessary to increase protective measures, improve sealing quality, and reduce seal quality accidents.
(4) When analyzing the cause of the quality accident of the mechanical seal, it is necessary to fully consider the influence of other parts of the pump on the mechanical seal operation, and take measures to continuously improve the effect of the mechanical seal.
At present, mechanical seals are widely used in pump products, and with the improvement of product technology and energy conservation requirements, the application prospects of mechanical seals will be more extensive. The sealing effect of the mechanical seal will directly affect the operation of the whole machine, especially in the petrochemical industry. Due to the existence of flammable, explosive, volatile, highly toxic medium, the mechanical seal leaks, which will seriously affect the normal production, serious There will also be major security incidents. When analyzing the causes of quality failures, people tend to find reasons for mechanical seals themselves, such as whether the selection of mechanical seals is appropriate, whether the material selection is correct, whether the specific pressure of the sealing surface is correct, and whether the selection of friction pairs is reasonable. Rarely find the cause of the external conditions of the mechanical seal, such as: whether the conditions created by the pump for the mechanical seal are appropriate, and whether the configuration of the auxiliary system is appropriate, and the reasons for these are often very important. The author analyzes several factors affecting the sealing effect and reasonable measures to be taken from the perspective of the external conditions of the pump mechanical seal.
1 Principle and requirements of mechanical seal
The mechanical seal is sealed by a small axial gap formed by the abutting faces of a pair of relatively moving rings. This device is called a mechanical seal.
Mechanical seals usually consist of a moving ring, a stationary ring, a pressing element and a sealing element. The end faces of the moving ring and the stationary ring constitute a pair of friction pairs, and the moving ring is pressed against the end face of the stationary ring by the pressure of the liquid in the sealing chamber, and an appropriate specific pressure is generated on the end faces of the two rings and a layer is maintained. A thin liquid film for sealing purposes. The pressing element generates pressure, which keeps the end of the pump in the non-operating state, ensures that the sealing medium does not leak, and prevents impurities from entering the sealing end face. The sealing element acts to seal the gap B between the moving ring and the shaft, and the gap C between the stationary ring and the gland, and at the same time buffers the vibration and impact of the pump.
The mechanical seal is not an isolated component in actual operation. It is combined with other components of the pump. At the same time, it can be seen from the basic principle that the normal operation of the mechanical seal is conditional, for example: the pump shaft The amount of enthalpy should not be too large, otherwise the friction end face cannot form the normal required specific pressure; the pump shaft at the mechanical seal should not have too much deflection, otherwise the end face specific pressure will be uneven. Only when such external conditions are met, coupled with good mechanical seal performance, can achieve the desired sealing effect.
2 Analysis of the causes of external conditions
2.1 The axial load of the pump shaft is large
The sealing surface of the mechanical seal must have a certain specific pressure, so as to play a sealing role, which requires the mechanical seal spring to have a certain amount of compression, give the seal end face a thrust, and rotate it to make the sealing surface require the seal ratio required. Pressure. In order to ensure this specific pressure, the mechanical seal requires that the pump shaft should not have too much enthalpy, and it is generally guaranteed to be within 0.5 mm. However, in the actual design, due to the unreasonable design, the pump shaft often produces a large amount of enthalpy, which is very unfavorable for the use of mechanical seals. This phenomenon often occurs in multi-stage centrifugal pumps, especially during pump start-up, where the amount of helium is relatively large. When the balance disc works, the axial gap b between the balance disc and the balance ring is automatically changed, thereby changing the pressure difference between the front and rear sides of the balance disc, and generating a force opposite to the axial force direction to balance the axial force. Due to the inertia of the rotor turbulence and fluctuations in transient pump conditions, the operating rotor does not rest at an axial equilibrium position. The balance disc is always in a state of being tilted left and right. The axial displacement of the balance disc during normal operation is only 0105 ~ 011 mm, which meets the requirement of 015 mm for the mechanical seal. However, the axial disc volume of the balance disc during pump start, stop, and working conditions may change. It greatly exceeds the axial amount allowed by the mechanical seal.
After the pump runs for a long time, the balance disc and the balance ring frictionally wear, and as the gap b increases, the axial volume of the mechanical seal increases continuously. Due to the axial force, the pressing force of the sealing surface on the suction side increases, and the wear of the sealing surface is intensified until the sealing surface is damaged and the sealing effect is lost. The mechanical seal on the discharge side, with the wear of the balance disc, the axial amount of the rotor part is greater than the axial amount required for the seal, the pressing force of the sealing surface is reduced, the sealing requirement is not achieved, and finally the sides of the pump are The mechanical seals all lose their sealing effect.
2.2 The axial force is too large. The mechanical seal cannot withstand the axial force during use. If there is axial force, the effect on the mechanical seal is serious.
Sometimes the axial force is not balanced due to the unreasonable design of the axial force balance mechanism of the pump and the manufacturing, installation, and use reasons. The mechanical seal is subjected to an axial force, and the temperature of the sealing gland will be high during operation. For the polypropylene medium, it will be melted at high temperature, so the sealing effect will be lost soon after the pump is started, and the sealing end face will appear when the pump is stationary. Intermittent leaks.
2.3 The deflection of the pump shaft is too large. The mechanical seal, also known as the end face seal, is a rotary axial contact dynamic seal. Under the action of the fluid medium and the elastic element, the two sealing end faces perpendicular to the axis line are closely attached. It is combined and rotated to achieve a sealing effect, so it is required to be evenly stressed between the two seals. However, due to the unreasonable design of the pump product, when the pump shaft is running, the deflection generated at the mechanical seal installation is large, so that the force between the sealing surfaces is uneven, resulting in poor sealing effect.
2.4 It is very important to have an auxiliary flushing system without auxiliary flushing system or auxiliary flushing system. It can effectively protect the sealing surface and cool, lubricate and wash away debris. Sometimes the designer does not properly configure the auxiliary flushing system to achieve the sealing effect. Sometimes, although the designer designs the auxiliary system, the flow rate and pressure of the flushing fluid are insufficient due to impurities in the flushing liquid, and the design of the flushing port is unreasonable. Also, the sealing effect is not achieved.
2.5 Vibration is too large The mechanical seal vibration is too large, eventually leading to the loss of sealing effect. However, the reason for the large vibration of the mechanical seal is often not the cause of the mechanical seal itself. Other parts of the pump are the source of vibration, such as unreasonable pump shaft design, processing reasons, insufficient bearing precision, and poor parallelism of the coupling. Radial force and other reasons.
2.6 Causes of pump cavitation Due to unreasonable operation of the system and the poor cavitation performance of the pump inlet and the high speed of the pump, local cavitation occurs at the inlet of the pump. After cavitation occurs, there will be bubbles in the water. It will impact the outer surface of the mechanical sealing surface and cause damage to its surface. On the other hand, the flow film of the matching surface of the static and dynamic ring also contains air bubbles, which cannot form a stable flowing film, resulting in dry friction of the matching surface of the static and dynamic ring. Damage the mechanical seal.
2.7 The machining accuracy is not enough. The machining accuracy is not enough. There are many reasons. Some of them have insufficient machining precision of the mechanical seal itself. The reason for this is easy to attract people's attention and easy to find.
However, sometimes the processing precision of other parts of the pump is not enough, and the reason for this is not easy to attract people's attention. For example, the pump shaft, the bushing, the pump body, and the sealing cavity have insufficient precision. The presence of these causes is very detrimental to the sealing effect of the mechanical seal.
3 Measures to be taken
3.1 Measures to eliminate the large amount of pump shaft åˆç† Reasonably design the axial force balance device to eliminate the axial enthalpy. In order to meet this requirement, for the multi-stage centrifugal pump, there are two ideal designs: one is the balance disc plus the axial thrust bearing, the balance shaft balances the axial force, and the axial thrust bearing carries the pump shaft. The axial limit; the other is the balance drum plus the axial thrust bearing, the balance shaft balances most of the axial force, the remaining axial force is carried by the thrust bearing, and the axial thrust bearing carries the shaft to the pump shaft To the limit. The key to the second option is to rationally design the balance drum so that it can truly balance most of the axial force. For other single-stage pumps, medium-open pumps, etc., some measures are taken during design to ensure that the pump shaft is within the required range of mechanical seals.
3.2 Measures to eliminate the large axial force The axial force balance mechanism is reasonably designed so that it can truly balance the axial force and create a good condition for the mechanical seal. For some important products used in power plants, petroleum, chemical and other fields, before the products leave the factory, it is necessary to test and detect problems and solve problems. Some important pumps can be designed with an axial force-measuring ring on the rotor to monitor the axial force at any time and find problems in time.
3.3 Measures to eliminate excessive deflection of the pump shaft This phenomenon is mostly found in horizontal multistage centrifugal pumps. The following measures are taken during design:
(1) Reduce the distance between the bearings at both ends. The number of stages of the pump impeller should not be too much. When the total head lift requirement is high, the lift of each stage of the impeller should be increased as much as possible to reduce the number of stages.
(2) Increase the diameter of the pump shaft. When designing the diameter of the pump shaft, do not simply consider the amount of power transmitted, but consider mechanical seals, shaft deflection, starting methods, and related inertial loads, radial forces, and other factors. Many designers are not fully aware of this.
(3) Improve the grade of the pump shaft material.
(4) After the pump shaft design is completed, the deflection of the pump shaft should be checked and calculated.
3.4 Adding an auxiliary flushing system Whenever possible, design an auxiliary flushing system.
The flushing pressure is generally required to be higher than the sealing chamber pressure of 0107 to 011 MPa. If the conveying medium is easily vaporized, it should be higher than the vaporization pressure of 01175 to 012 MPa. The pressure of the sealed chamber is calculated according to the structural type of each pump, system pressure and other factors. When the shaft seal chamber pressure is high or the pressure is close to the highest limit of the seal use, the liquid can be drawn from the seal chamber to the low pressure region, so that the shaft seal liquid flows to take away the friction heat.
Properly configure piping and accessories according to the operating conditions of each pump. Such as coolers, orifices, filters, valves, flow indicators, pressure gauges, temperatures, etc. In fact, the reliability and longevity of the seal depend to a large extent on the configuration of the seal assist system.
3.5 Measures to eliminate cavitation at the inlet of the pump
(1) Improve the cavitation performance level of the pump to meet the cavitation performance requirements of the field device.
(2) The requirements of the field test device should match the pump cavitation performance level.
(3) On-site installation and condition adjustment should create favorable conditions for the pump.
3.6 Measures to eliminate pump vibration
(1) During the design of the pump product, the source of the vibration should be fully analyzed to eliminate the vibration source.
(2) During the manufacturing and assembly process of the pump product, strictly follow the standards and operating procedures to eliminate the vibration source.
(3) When the auxiliary equipment such as pump, motor, base, and field pipeline is installed on site, it must be strictly controlled to eliminate the vibration source.
(4) When on-site production, operation, maintenance, and adjustment, strictly control and eliminate vibration sources.
3.7 Strict implementation of design standards
The design of the pump product and the design of the mechanical seal product must implement relevant domestic and international standards. In the design process of the product, the designer should conscientiously implement the standard, deeply understand the specific meaning of each content of the standard, and implement the requirements of the standard content to the product. During the design process. So far, many designers have not understood the actual meaning of the standard, and have not strictly enforced the new standards, but blindly copy the experience of the old drawings and the old designers. This practice is very unfavorable for improving the technical level of our products and entering the international market. Raising the awareness of standardization is an urgent problem that designers in the mechanical industry need to solve.
4 Conclusion
When designing the mechanical seal of the pump, it is necessary to consider not only the influencing factors of the mechanical seal itself, but also various influencing factors outside the mechanical seal.
Pay attention to the following issues in actual work:
(1) In the design process of the pump product, the influence of other parts of the pump and other equipment on the surface on the mechanical seal effect should be fully considered to create a good external condition for the mechanical seal.
(2) Increase the understanding of the important role of the mechanical seal assist system, as far as possible with a complete mechanical seal assist system to improve the sealing effect.
(3) For mechanical seals of important pump products, it is necessary to increase protective measures, improve sealing quality, and reduce seal quality accidents.
(4) When analyzing the cause of the quality accident of the mechanical seal, it is necessary to fully consider the influence of other parts of the pump on the mechanical seal operation, and take measures to continuously improve the effect of the mechanical seal.
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