At present, the application of mechanical seal in pump products is very extensive. With the improvement of product technology and the requirement of saving energy, the application prospect of mechanical seal will be more extensive. Mechanical seal sealing effect will directly affect the operation of the whole machine, especially in the field of petrochemical industry, due to the presence of flammable, explosive, volatile, highly toxic and other media, mechanical seal leakage, will seriously affect the normal production, serious There will also be a major security accident. When analyzing the causes of quality failures, people are often used to find out the reasons for the mechanical seal itself, such as the selection of the mechanical seal, the material selection, the specific pressure of the sealing surface, the selection of the friction pair and so on. Rather than looking at the external conditions of a mechanical seal, it is often important to look for reasons such as the proper condition of the pump for the mechanical seal and the suitability of the auxiliary system. The author analyzes several factors that affect the sealing effect and the reasonable measures to be taken from the perspective of the external conditions of the pump mechanical seal.
1 mechanical seal principle and requirements
The mechanical seal is sealed by the tiny axial gap created by the mutual abutment of the end faces A of a pair of oppositely moving rings (one stationary and the other rotating with the shaft, see Fig. 1). Such a device is called a mechanical seal.
Mechanical seal is usually composed of moving ring, static ring, pressing element and sealing element. Wherein the moving ring and the stationary ring end face constitute a pair of friction pair, the moving ring by the liquid pressure in the sealing chamber so that its end face pressed against the stationary ring end face, and in the two ring end face to produce appropriate pressure and maintain a layer of pole Thin liquid film to achieve the purpose of sealing. Compression components produce pressure, the pump can not operate in the state, but also to maintain the end face fit, to ensure that the sealing medium is not leakage, and to prevent impurities into the sealing face. The sealing element plays a role of sealing the clearance B between the moving ring and the shaft, the clearance C between the static ring and the pressing cover, and simultaneously acts as a buffer for the vibration and impact of the pump. Mechanical seal in actual operation is not an isolated part, it is combined with other components of the pump running at the same time through its basic principle can be seen that the normal operation of the mechanical seal is conditional, for example: the pump shaft The amount of channeling can not be too large, otherwise the friction end face can not form the normal requirements of the specific pressure; mechanical seal at the pump shaft can not have too much deflection, or face than the pressure will be uneven and so on. Only to meet such external conditions, coupled with its own good mechanical seal performance, in order to achieve the desired sealing effect.
2 external conditions affect the analysis
2.1 axis of the axial channeling large
Mechanical seal sealing surface to have a certain pressure, so as to play the role of sealing, which requires the mechanical seal spring to have a certain amount of compression to the sealing end of a thrust, rotating the sealing surface to produce the required ratio of sealing Pressure In order to ensure this a specific pressure, the mechanical seal requires the shaft can not have too much channeling amount, generally to ensure that within 0.5 mm. However, in the actual design, due to unreasonable design, the pump shaft often produce a large amount of channeling, the use of mechanical seals is very unfavorable. This phenomenon often appears in the multistage centrifugal pump, especially in the pump start-up process, the channeling amount is relatively large.
Figure 2 balance plate approach to balance the working principle of axial force. The balance disc automatically changes the axial clearance b between the balance disc and the balance ring when working, so as to change the pressure difference between the front and the back of the balance disc to generate a force opposite to the axial force to balance the axial force. Due to the inertia of the rotor's motion and the transient pump conditions, the running rotor will not rest at an axial equilibrium position. Balance plate is always in the state of movement around. Balance plate in normal operation of the axial channeling amount of only 0105 ~ 011 mm, to meet the mechanical seal to allow the axial channeling amount of 015 mm requirements, but the balance plate in the pump start, stop, the case of sudden change in the amount of axial channeling may Greatly exceeded the allowable mechanical seal axial channeling amount.
Pump after a long run, the balance disc and the balance ring friction and wear, clearance b with the increase of mechanical seal axial channeling increased. As a result of the axial force, the suction side of the sealing surface of the pressing force increases, the sealing surface wear increased, until the sealing surface damage, loss of sealing effect. Spit out the side of the mechanical seal, with the wear of the balance plate, the axial component of the rotor is greater than the axial channeling required by the seal, the sealing surface of the pressing force decreases, not to seal requirements, and ultimately the pump on both sides of Mechanical seal all lose seal.
2.2 axial force is too large
Mechanical seal in the course of the process can not afford the axial force, if the presence of axial force, the impact on the mechanical seal is serious. Sometimes due to the pump axial force balance mechanism design unreasonable and manufacturing, installation, use and other reasons, resulting in axial force has not been balanced out. The mechanical seal withstands an axial force and the seal gland temperature will be high during operation. For polypropylene media, it will melt at high temperatures, so the pump will lose seal soon after it is started and when the pump is at a standstill, the seal faces will appear Intermittent jet leakage phenomenon.
2.3 deflection of the pump shaft is too large
Mechanical seal, also known as the end seal, is a rotary axial contact type dynamic seal, which is in the fluid medium and the elastic element under the action of two perpendicular to the axis of the seal face close fit, relative rotation, so as to achieve Sealing effect, thus requiring the seal between the force to be uniform. However, due to the unreasonable design of the pump product, when the pump shaft is operating, the deflection generated at the installation location of the mechanical seal is large, so that the stress between the sealing surfaces is not uniform, which results in poor sealing effect.
2.4 No auxiliary flushing system or auxiliary flushing system set unreasonable
Mechanical seal auxiliary flushing system is very important, it can effectively protect the sealing surface, play a cooling, lubrication, washed away debris and so on. Sometimes the designer does not configure the auxiliary flushing system reasonably and can not achieve the sealing effect. Sometimes, although the designer designs the auxiliary system, due to the impurities in the flushing fluid, the flow of the flushing fluid, the insufficient pressure and the irrational design of the flushing port, , Also failed to seal the same effect.
2.5 vibration is too large
Mechanical seal vibration is too large, eventually leading to loss of sealing effect. However, the reasons for the large mechanical seal vibration is often not the cause of the mechanical seal itself, the pump is the root cause of other parts of the vibration, such as the pump shaft design is not reasonable, the reasons for processing, bearing accuracy is not enough, Radial force and other reasons.
2.6 pump cavitation causes
Due to the unreasonable operation of the device system and the poor cavitation performance of the pump inlet, the pump speed is high and local cavitation occurs at the inlet of the pump. After the cavitation occurs, there will be bubbles in the water which will impact the mechanical sealing surface On the other hand, the moving film of the anastomosis surface of the static and dynamic rings also contains air bubbles and can not form a stable flow film, resulting in the dry friction of the anastomosis surface of the static and dynamic rings and damaging the mechanical sealing device.
2.7 machining accuracy is not enough
Machining accuracy is not enough, for many reasons, some mechanical seal itself is not enough precision, the reasons for this easy to attract the attention of people, but also easy to find. But sometimes the other parts of the pump processing accuracy is not enough, this reason, not easy to arouse people's attention. For example: pump shaft, bushings, pump body, seal the accuracy of the cavity is not enough and other reasons. The existence of these reasons is very detrimental to the sealing effect of the mechanical seal.
3 steps to be taken
3.1 Elimination of the large amount of pump shaft channeling measures
Reasonable design of axial force balance device to eliminate axial channeling. In order to meet this requirement, for the multistage centrifugal pump, there are two ideal design solutions: one is the balance disc plus axial thrust bearing, balanced disc balance axial force by the axial thrust bearing on the shaft Axial limit; the other is the balance drum plus axial thrust bearings, balanced by the balance of the drum most of the axial force, the remaining axial force by the thrust bearing, while the axial thrust bearing shaft on the pump To the limit. The key to the second solution is to rationalize the balancing drum so that it can truly balance most of the axial forces. For other single-stage pumps, pumps and other products in the design to take some measures to ensure that the amount of shaft slip in the mechanical seal within the required range.
3.2 eliminate the axial force is too large measures
Reasonable design of axial force balance mechanism, so that it can truly fully balance off the axial force, to create a good mechanical seal conditions. For some power plants, petroleum, chemical and other fields of application of important products, before the product is shipped, we must do bench testing to detect and identify problems and solve problems. Some important pumps can design an axial force ring on the rotor to monitor the size of the axial force at any time and find out the problems solved in time.
3.3 Elimination of pump shaft deflection is too large measures
Most of this phenomenon exists in horizontal multistage centrifugal pumps, in the design to take the following measures:
(1) reduce the distance between the bearings on both ends. Pump impeller of the series not too much, in the pump head higher requirements, try to improve the head of each impeller, reduce the number of stages.
(2) increase the diameter of the pump shaft. When designing the pump shaft diameter, do not simply consider only the magnitude of the transmitted power, but also consider the mechanical seal, shaft deflection, starting method, and related inertia load, radial force and other factors. Many designers do not fully recognize this.
(3) to improve the level of the pump shaft material.
(4) After the design of the pump shaft is completed, the deflection of the pump shaft shall be checked and calculated.
3.4 increase secondary flushing system
When possible, try to design a secondary flushing system. Flushing pressure is generally higher than the sealed chamber pressure 0107 ~ 011 MPa, if the transmission medium is easy to vaporize, it should be higher than the vaporization pressure 01175 ~ 012 MPa. Seal chamber pressure according to each type of pump structure, system pressure and other factors to calculate. When the pressure in the shaft seal chamber is very high or the pressure is close to the highest limit of the seal, the seal chamber may also draw liquid to the low pressure zone to flow the shaft seal fluid to remove the frictional heat. The recommended flushing amount is shown in Table 1.
Depending on the operating conditions of each pump, piping and accessories are properly configured. Such as coolers, orifice plates, filters, valves, flow indicators, pressure gauges, temperature and more. In fact the reliability and life of the seal depend largely on the configuration of the seal assist system.
3.5 to eliminate pump cavitation measures
(1) to improve the pump cavitation performance level to meet the cavitation performance requirements of field devices.
(2) The requirements of the field test device should be matched with the pump cavitation performance level.
(3) Site installation and condition adjustment to create favorable conditions for the pump.
3.6 eliminate pump vibration measures
(1) pump products in the design process, we must fully analyze the source of vibration in order to eliminate the vibration source.
(2) pump product manufacturing and assembly process, in strict accordance with the standards and operating procedures to implement, eliminating the vibration source.
(3) Pumps, motors, bases, on-site piping and other ancillary equipment installed at the scene, we must strictly control and eliminate the vibration source.
(4) On-site production, operation, maintenance, adjustment, strict control, eliminate vibration source.
3.7 strict implementation of design standards
Pump product design and mechanical seal product design to implement the relevant domestic and international standards in the product design process, designers should seriously implement the standards, a profound understanding of the specific meaning of each of the content of the standard, the implementation of the standard content requirements to the product Design process. So far, many designers have not yet understood the true meaning of the standard, not strictly to implement the new standard, but blindly copy the experience of the old photo and the old designer. This approach is very unfavorable to improving the technological level of our products and entering the international market. To raise awareness of standardization is a pressing issue urgently needed by the mechanical industry designers.
4 Conclusion
When designing a pump mechanical seal, consider not only the factors that affect the mechanical seal itself, but also the various external influences on the mechanical seal. In the actual work should pay attention to the following questions:
(1) In the pump product design process to fully consider the other parts of the pump and other equipment on the site the effect of the use of mechanical seals, mechanical seals to create a good external conditions.
(2) to increase awareness of the important role of mechanical seal auxiliary system, as far as possible with a complete mechanical seal auxiliary system to improve the sealing effect.
(3) For mechanical seals of important pump products, it is necessary to increase protective measures, improve the quality of seals and reduce the quality of sealing accidents.
(4) When analyzing the cause of quality accident of mechanical seal, we should fully consider the influence of other parts of pump on mechanical seal operation and take measures to continuously improve the effect of mechanical seal.
references
ã€1】 Mou Jigang 1 Propane Pump Design and Research 1 Pump Technology, 1999 (1): 9 ~ 131
[2] Shenyang Pump Institute 1 vane pump design manual 1 Beijing: Mechanical Industry Press, 19731
ã€3】 How to improve the performance of petrochemical pump mechanical seal and life expectancy 1 Petrochemical Equipment Technology, 1994 (6): 23 ~ 261
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