Ruian Chaoyang Standard Parts Co., Ltd. , https://www.cybzj.com
Research on the External Conditions Affecting the Mechanical Seal of Pump
According to the practical application and theoretical analysis of mechanical seal, it is proposed that the actual seal effect of mechanical seal is not only related to the performance of mechanical seal, but also to the conditions provided by other components and the conditions provided by seal auxiliary system. Therefore, in the design of pump units, the mechanical seal should be used to provide a good external conditions. 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. A mechanical seal principle and requirements Mechanical seal is by a pair of relative movement of the end face of the ring together form a small axial gap sealing effect, this 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 the other parts of the pump to run at the same time can be seen through its basic principles, 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 a sealing effect, which requires the mechanical seal of the spring to have a certain amount of compression to A thrust force is applied to the sealing end face, causing the sealing face to produce the required specific pressure for sealing. 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. 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 withstand the axial force, if there is 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 elastic elements under the action of two perpendicular to the axis of the seal face close Together, the relative rotation, so as to achieve the sealing effect, thus requiring two seals to force evenly. 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 cooling, lubrication, washed away debris and so on. Sometimes the designer does not reasonably configure the auxiliary flushing system 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 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 unreasonable system operation of the device and the pump inlet cavitation performance is not good, the pump speed is high, the local cavitation at the pump entrance, cavitation occurs, there will be bubbles in the water, on the one hand Will impact the mechanical seal surface of the outer surface, so that the surface damage; the other will make the dynamic and static ring surface of the film also contains bubbles in the flow, can not form a stable flow film, resulting in static and dynamic ring of the match surface dry friction, Damage to the mechanical seal. 2.7 Machining accuracy is not enough Machining accuracy is not enough for many reasons, some mechanical seal itself is not enough precision, this cause easily lead to people's attention, 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 measures to be taken 3.1 elimination of large measures of the amount of shaft channeling 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 to eliminate the axial force is too large The 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 to eliminate the pump shaft deflection is too large This phenomenon most of the horizontal horizontal multistage centrifugal pump in the design to take the following measures: (1) to reduce the distance between the bearings at both ends. Pump impeller of the series not too much, in the pump head higher requirements, try to improve the head of each impeller, reducing the number of stages. (2) & nbs