How to deal with the leakage of mechanical seal
1、 Main causes of mechanical of centrifugal water pump seal leakage
The mechanical seal is generally of built-in type, and it is often necessary to determine the cause of seal leakage according to experience, on-site observation and instrument measurement analysis. First, find out the impact of damaged seals on sealing performance, and then carefully check the wear marks of sealing rings, transmission parts, loading elastic elements, auxiliary sealing rings, anti rotation mechanisms, fastening screws, etc. in turn. The accessories, such as base, shaft sleeve, sealing chamber and sealing system, shall also be comprehensively inspected.
1 Long term wear
Mechanical wear will cause damage to the normal matching relationship of the seal pair. When there is a certain amount of wear on the end face of centrifugal water pump, axial displacement and radial swing shall be made for the seal element every time the transmission shaft rotates. The movement and wear can be judged according to the wear trace, and the cause of seal leakage can also be determined. For example, the wear trace of the sealing pair is even, and the parts are well matched, which means that the transmission part has good coaxiality. At this time, the leakage of the seal face may not be caused by the seal itself. If the leakage is constant, it means that the leakage does not occur between the two ends, but may occur on other parts, such as the static seal. For another example, the seal leaks at the beginning of use, and no wear trace on the friction end face can be observed. It may be that the rotating ring does not rotate or slip relative to the stationary ring, and the reason may be that the anti rotation pin is loose or broken, or the hole diameter of the base is smaller than the outer diameter of the seal, which is caused by improper installation of centrifugal water pump.
2 Overheating damage
Overheating not only causes deformation and wear of the sealing pair, but also may cause hot cracks and blisters. Generally, under the action of excessive thermal stress of centrifugal water pump, radial cracks appear on the surface of the seal ring, which is called thermal cracking. Under the action of mechanical load or thermal load for a short time, thermal cracking will occur. For example, due to dry friction, cooling system interruption and other thermal cracking, seal ring wear will intensify and leakage will increase rapidly. For balanced seals, even the seal rings are separated. In order to avoid hot cracking, it is necessary to master the mechanical and physical properties of materials, take into account the possibility of hot cracking during design, and give operating conditions. Factors such as poor medium lubricity, overload, high operating temperature, high linear velocity, improper combination of mating materials, or the combination of the above factors can generate excessive friction heat. If the friction heat cannot be distributed in time, thermal cracks will occur, causing leakage of centrifugal water pump.
3 Chemical corrosion
As the seal contacts with corrosive medium, it will produce surface corrosion, and even produce severe corrosion spots on the surface to form pitting corrosion. The intergranular corrosion generated on the grain boundary of metal will penetrate into the interior of the metal and further damage to cause fracture. The performance of corrosion has a great influence. Because the sealing element is smaller than the parts of the host and more precise, it is usually necessary to select materials that are more resistant to corrosion than the host. Experience shows that pressure, temperature and sliding speed can accelerate corrosion. The corrosion rate of seals increases exponentially with temperature. When dealing with strong corrosive fluid, double face seal can minimize the impact of corrosion on the seals, because it has less parts in contact with the process fluid. This is also the most important principle for selecting sealing structure under strong corrosion conditions.
4 Failure of sealing parts of centrifugal water pump
The failure of mechanical seal parts is mostly caused by the failure of auxiliary seal ring. One of the main reasons why mechanical seal of centrifugal water pump cannot work normally due to leakage is also caused by the failure of O-ring. The failure of O-ring includes aging, permanent deformation, swelling deformation, distortion and extrusion damage. Therefore, the safe use temperature of synthetic rubber shall be considered when selecting O-ring. Rubber O-ring with large section shall be selected as far as possible, and the hardness shall be appropriately improved. The grooved assembly structure shall be adopted. The materials shall be selected reasonably through immersion test. If necessary, composite materials shall be selected, such as rubber coated PTFE sealing ring.
2、 Countermeasures against mechanical seal leakage of centrifugal water pump
The reason why the mechanical seal can play a sealing role is that there is an end facial mask between the end faces of the mechanical seal, and the shape of the end facial mask determines the sealing effect. Due to different working conditions, there are three types of end facial mask between the faces of mechanical seals: liquid phase, gas-liquid mixed phase and gas equal. The reason why the end facial mask has different forms is that when the sealed liquid is close to saturation, boiling or flashing occurs, that is, phase change occurs. Once the phase change of the end facial mask occurs, the back pressure of the film will change greatly, which will lead to unstable end fitting. So the key to solve the problem is how to ensure that the end facial mask of the mechanical seal is a stable film. Obviously, to ensure that the end facial mask is a stable film, it is only necessary to ensure that the end facial mask does not undergo phase change or that the phase change does not destroy the end fitting.
Based on the analysis of the above reasons, the mechanical seal design and modeling must be combined with the actual production situation, and the friction pair materials and static seal ring materials with reliable sealing and long service life shall be selected as far as possible. For example, the inner end of the double end mechanical seal is mainly used to contact the medium; The outer end is not in contact with the medium, and the material with good sealing reliability is mainly selected, such as WC rubber ring. It is not necessary to pursue the consistency of the material selection of the inner and outer ends, which can reduce the cost and increase the sealing reliability. The processing quality of mechanical seal parts must be strict. The instructions and maintenance procedures must specifically specify the technical requirements for the installation and use of mechanical seals of centrifugal water pump, so that workers can follow the rules.
Generally, the mechanical seal for the pump shall undergo static and dynamic tests after installation to confirm that the mechanical seal is installed correctly. When leakage is found, it is convenient for timely maintenance. In addition, leakage may also occur suddenly during normal operation. At this time, comprehensive analysis can be carried out according to the situation to determine the real cause of mechanical seal leakage for easy solution.
