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External conditions affect pump seal

Through practical application and theoretical analysis of pump mechanical seal, the actual mechanical seal sealing effect not only related to their own performance and mechanical seal, and the conditions provided by the conditions in other parts, and sealing auxiliary system has an important relationship, Therefore, in the design of the pump unit products, provide a favorable external conditions for the use of the mechanical seal.

      
Mechanical seal pump products very widely, and with the improvement of the level of product technology and energy saving requirements, will be more extensive application prospects of the mechanical seal. Sealing effect of the mechanical seal will directly affect the operation of the machine, especially in the petrochemical field, due to the presence of flammable, explosive, volatile, toxic and other media, mechanical seal leak will seriously affect the production of normal, serious 's will also be a major security incidents. Analysis of the quality of cause of the malfunction is often used to find the cause mechanical seal itself aspects, such as: mechanical seal selection is appropriate, material selection is correct, the sealing surface than the pressure is correct, the choice of the friction pair is reasonable. But rarely in the mechanical seal of the external conditions to find out the reasons, for example: whether the conditions created by the pump to the mechanical seal is appropriate, auxiliary system configuration is appropriate, and these reasons is often very important. The authors analyzed from the point of view of the conditions of the external pump mechanical seal affect the sealing effect of several factors, and should take reasonable measures.

      
Principles and requirements of a mechanical seal

      
The mechanical seal is the relative motion of the end face of the ring by a pair of mutually abutting form a slight axial clearance from the sealing effect, this device is called the mechanical seal.

      
The mechanical seal is usually composed by a snap ring, stationary ring, pressed components and sealing elements. The end face of the ring and stationary ring is composed of a pair of friction ring by pressure of the liquid seal chamber face pressed to the stationary ring side, two ring-side surface of a layer of extremely specific pressure and maintain appropriate a thin liquid film to achieve the purpose of sealing. Pressed the components generate pressure, the pump also maintained the state does not run, the end face fit, guarantee no leakage sealing medium, and to prevent contaminants from entering the seal faces. The sealing element from the sealing ring and the axis of the gap B, the role of the clearance C of the stationary ring and the gland, and the vibration of the pump at the same time, the impact dampening effect.

      
Mechanical seal in the actual operation is not an isolated member, which is combined together with the other components of the pump run, through its basic principle can be seen, the normal operation of the mechanical seal is conditional, such as: Pump channeling can not be too large, otherwise the friction pair end face can not form the normal requirements than the pressure; mechanical seal at the pump shaft can not have too much deflection uneven pressure than would otherwise end face and so on. Only to meet the external conditions like this, coupled with good mechanical seal own performance in order to achieve the desired sealing effect.

      
2 external conditions affect analysis

      
The 2.1 pump shaft axial channeling large

      
The sealing surface of the mechanical seal to have a certain specific pressure, so as to play a role in sealing, which requires the mechanical seal the spring to have a certain amount of compression to the sealing end face of a thrust, spins up the sealing surface sealing the required ratio pressure. In order to guarantee a specific pressure, mechanical seal requirements of the pump shaft can not have too much channeling, and generally to ensure that less than 0.5 mm. But in the actual design which, due to improper design, often shaft produces a great amount of channeling is very detrimental to the use of the mechanical seal. This phenomenon often appears in the multi-stage centrifugal pumps, especially in the the pump startup process, channeling than larger.
Balance disc automatically change work balance disc and balance ring between the axial gap b, thereby changing the pressure on both sides around the balancing disc to produce an axial force in the opposite direction the biasing force to balance the axial force. Due to the fluctuations in the rotor ran the inertia and transient pump conditions, the operation of the rotor is not stationary at a certain axial equilibrium position. The balance disc always in the shifting of the state of the left and right. The balance disc normal work the axial channeling amount only 0105 ~ 011 mm, to meet the mechanical seal requirements allow the the axial channeling amount of 015 mm, but the balance disc the pump is started, stop, drastic changes in the working conditions axial channeling amount of possible greatly exceeds the mechanical seal allowed axial channeling amount of.

      
Pump after a long-running, balance disc and balance ring friction and wear, gap b increases, mechanical seal axial channeling amount of increasing. The suction side of the sealing surface of the pressing force due to the axial force increases, the sealing surface wear is intensified, until the sealing surface is damaged, the loss of sealing effect. The discharge side of the mechanical seal, with the balance the wear of the disc, the axial direction of the rotor member channeling greater than the sealing requirements of the amount of axial channeling, the pressing force of the sealing surface is reduced to reach the sealing requirements, and ultimately to the pump on both sides All of the mechanical seal to lose sealing effect.

      
2.2 axial force is too large
      
The mechanical seal during use are not able to withstand the axial forces, axial force, the influence of the mechanical seal is a serious.
Sometimes unreasonable pump axial force balance mechanism design and manufacture, installation, use of reason, causing axial force is not balanced. Mechanical seal to withstand an axial force, the operation when the temperature of the gland to high for the medium of the polypropylene type, at high temperatures will be melted, so pump starts soon after the loss of the sealing effect, and the pump is stationary seal faces appear intermittent spray leakage phenomenon.

      
2.3 shaft deflection is too large
      
Mechanical seals, also known as seal, movable seal is a rotary axial direction of the touch, it is in the fluid medium and the role of the elastic element, the seal faces of the two perpendicular to the axis line closely bonded, relative rotation, so as to achieve The sealing effect, thus requiring force uniform between the two seals. However, due to the the pump product design unreasonable, the operation of the pump shaft, mechanical seal installed at larger deflection discontinuity, between the sealing surface of the cause poor sealing effect.

      
2.4 no auxiliary flushing system or auxiliary flushing system set unreasonable
      
Secondary flushing system of the mechanical seal is very important, and it can effectively protect the sealing surface, and serves to cool, lubricate and carry away debris, and other effects. Sometimes designers no reasonable auxiliary flushing system is configured to reach the sealing effect; sometimes designers auxiliary system design, but due to impurities in the washing fluid, the flow of irrigation fluid pressure is not enough, rinse mouth position unreasonable design reasons , also reach the sealing effect.

      
2.5 vibration is too large
      
The mechanical seals vibration is too large, and ultimately result in the loss of the sealing effect. The large mechanical seal vibration because more often than not the reasons of the mechanical seal, the other parts of the pump generates the source of the vibration, shaft design unreasonable processing, bearing accuracy is not enough, the coupling of parallelism, radial force and other reasons.

      
2.6 reasons for pump cavitation
      
Bad because of the the device system operating unreasonable and cavitation performance of the pump inlet, the high speed of the pump, in the entrance of the pump the local cavitation, cavitation occurs, there will be bubbles in the water, it will, on the one hand, the impact of mechanical sealing surface The outer surface, its surface appears damaged; the other hand, causes the movement ring anastomosis surface flow membrane containing bubbles, can not form a stable flow of film, resulting in a dry friction of the surface of the static ring anastomosis, so that damage to the mechanical seal device.

      
2.7 machining accuracy is not enough
      
Machining accuracy is not enough, there are many reasons, and some mechanical seals itself machining accuracy is not enough, the reasons for this is easy to attract attention, but also easy to find.
      
But sometimes it is not enough, the machining accuracy of the other parts of the pump and in this regard it is not easy to attract attention. For example: shaft, bushings, pump seal cavity to increase the accuracy is not enough reason. The existence of these reasons the sealing effect of the mechanical seal is very unfavorable.

      
3 measures to be taken

      
3.1 to eliminate shaft channeling large measures
      
Rational design of the axial force of the balancing device, and to eliminate the axial amount of channeling. In order to meet this requirement, multistage centrifugal pump, ideal design: a balance disc and axial thrust bearing, balanced by balance disc axial force by the axial thrust bearing on the shaft axial limit; another balancing drum and the axial thrust bearing, to balance out the majority of the axial force by the balance drum, the remaining of the axial force by the thrust bearing bears the axial thrust bearing on the shaft axis limit. The second key to the program is reasonably designed balance drum, so that can really balance out most of the axial force. For single-stage pump, pump products, designed to take some measures to ensure that within the scope of the pump shaft channeling the required amount of mechanical seal.

      
3.2 measures to eliminate axial force is too large
      
Reasonable design axial force balance mechanism, so really fully balance out the axial force to create a favorable condition for mechanical seal. Important for applications in the field of power plant, petroleum, chemical and other products, before the product is shipped, you must do Channel test to detect and identify and solve problems. Some important pump rotor design of an axial force measuring ring at any time to monitor the size of the axial force, the problems to solve.

      
3.3 to eliminate the pump shaft deflection is too large measures
      
This phenomenon is mostly horizontal multistage centrifugal pump designed to take the following measures:
      
(1) to reduce the distance between both ends of the bearing. Pump impeller of the series is not too much, in the case of higher pump head requirements, try to improve each stage impeller head, reducing the number of stages.
      
(2) increasing the diameter of the shaft. In the design of the shaft diameter, do not simply consider only the size of the transmitted power, mechanical seals, shaft deflection, starting methods and the inertial load, radial force and other factors to consider. Many designers do not fully understand it.
      
(3) improve the level of the pump shaft material.
      
(4) after the completion of the pump shaft design, shaft deflection to check the test calculation.

      
3.4 auxiliary flushing system
      
Under the conditions allowed, try to design auxiliary flushing system.

      
The flushing pressure General requirements higher than the seal chamber pressure of 0107 to 011 MPa, if the transmission medium is easily vaporized, the vaporization pressure should be higher than the 01 175 to 012 MPa. Pressure in the sealed chamber to be calculated based on the structure of each pump type, system pressure, and other factors. Shaft seal chamber pressure is high or when pressure is almost close to the maximum limit of the seal can also be sealed chamber cited the liquid to a low pressure area, so the the shaft seal liquid flow in order to take away the frictional heat.
      
Arranged according to the operating conditions of each pump, piping and accessories. Such as the cooler, the orifice plate, filters, valves, flow indicators, pressure gauge, temperature and the like. Actually the reliability and life of the seal is largely dependent on the seal auxiliary system configuration.

      
3.5 measures to eliminate the pump inlet cavitation
      
(1) to improve the level of pump cavitation performance to meet the requirements of the cavitation performance of the on-site device.
      
(2) field test device requirements match the performance levels and pump cavitation.
      
(3) on-site installation and operating conditions create favorable conditions for adjustment to give the pump.

      
3.6 eliminate pump vibration measures
      
(1) pump in the design process to fully analyze the sources of vibration to eliminate vibration source.
      
(2) the pump manufacturing assembly process, in strict accordance with the standards and procedures to perform, to eliminate vibration source.
      
(3) auxiliary equipment such as pumps, motors, base, site piping when installed in the field, to strictly eliminate vibration source.
      
(4) on-site production, operation, maintenance, adjustment, strictly, to eliminate the source of vibration.

      
3.7 strictly enforce design standards

      
Pump design and the design of the mechanical seal products to the implementation of relevant domestic and international standards, in the product design process, the designer should seriously implement the standard, a deep understanding of the standard the specific meaning of each piece of content, and the implementation of the requirements of the standard content to product design process. So far, there are a lot of designers also did not understand the actual meaning of the standard, not strictly to the implementation of the new standards, but blindly copy the old drawings and old experience of the designer photo sets. This approach to improve our level of product technology and access to international markets is very unfavorable. Improve standardization understanding, is the urgent need to address the problems in machinery industry designers.

      
4 Conclusion

      
Pump mechanical seal design, not only to consider the impact factors of the mechanical seal, but also to consider the various factors affecting the external mechanical seal.

      
In the actual work, pay attention to the following questions:
      
(1) in pump product design process to fully consider the effect of the use of the mechanical seal to other parts of the pump and other equipment on site to create a favorable external conditions for the mechanical seal.
      
(2) increasing the understanding of the important role of the mechanical seal auxiliary system, is equipped with a perfect mechanical seal support systems, as much as possible in order to improve the sealing effect.
      
(3) of important pump mechanical seal, to increase protection measures, improve the quality of the seal, reducing the quality of the seal accident.
      
(4) analysis of the cause of the accident of the mechanical seal of quality to give full consideration to the other parts of the pump mechanical seal running, to take measures to continuously improve the effect of mechanical seals.

 

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Hits:  UpdateTime:2012-09-12 13:36:25  【Printing】  【Close