How to deal with mechanical seal leakage?

Mechanical seals are generally built-in, and the cause of seal leakage is often determined based on experience, on-site observation, and instrument measurement and analysis. First, find out the impact of seal damage on sealing performance, and then carefully check the wear marks of the seal ring, transmission parts, loading elastic elements, auxiliary seal rings, anti-rotation mechanisms, fastening screws, etc. The base, sleeve, seal chamber, sealing system, and other accessories should also be fully inspected.

• Long-term wear

Mechanical wear will destroy the normal matching relationship of the seal pair. When there is a certain amount of wear on the end face of the centrifugal water pump, the seal will produce axial displacement and radial swing every time the transmission shaft rotates one circle. The movement and wear conditions can be judged based on the wear marks, and the cause of the seal leakage can also be determined. For example, if the wear marks of the seal pair are uniform and all parts match well, it means that the coaxiality of the transmission parts is good. At this time, the leakage of the seal surface may not be caused by the seal itself. If the leakage continues, it means that the leakage does not occur between the two ends, but may occur in other parts, such as static seals. For example, if the seal leaks at the beginning of use, no wear marks can be observed on the friction end face. It may be that the rotating ring does not rotate or slips relative to the stationary ring. The reason may be that the anti-rotation pin is loose or broken, or the base aperture is smaller than the outer diameter of the seal. This is caused by improper installation of the centrifugal water pump.

• Overheating damage

Overheating will not only cause deformation and wear of the sealing pair, but may also cause thermal cracks and blistering. Generally, centrifugal water pumps will have radial cracks on the surface of the sealing ring under excessive thermal stress, which is called thermal cracks. Thermal cracks will occur under short-term mechanical load or thermal load. For example, due to thermal cracks such as dry friction and cooling system interruption, the wear of the sealing ring will be aggravated and the leakage will increase rapidly. For balanced seals, the sealing ring will even separate. In order to avoid the occurrence of thermal cracks, the mechanical and physical properties of the material must be mastered, and the possibility of thermal cracks must be considered during design, and operating conditions must be given. Factors such as poor medium lubricity, overload, high operating temperature, high linear speed, improper combination of matching materials, or the combined effect of the above factors will generate excessive friction heat. If the friction heat cannot be dissipated in time, thermal cracks will occur, causing centrifugal water leakage.

• Chemical corrosion

When the seal is in contact with the corrosive medium, surface corrosion will occur, and even serious corrosion spots will be produced on the surface, forming pitting. The intermediate corrosion produced on the metal boundary will penetrate into the metal, further damage and cause fracture. Corrosion has a great impact on the performance of the seal. Since the seal is smaller and more precise than the main machine parts, it is usually necessary to select materials that are more corrosion-resistant than the main machine. Experience shows that pressure, temperature and sliding speed will accelerate corrosion. The corrosion rate of the seal increases exponentially with the increase of temperature. When dealing with highly corrosive fluids, since the double-sided seal has fewer parts in contact with the process fluid, the impact of corrosion on the seal can be minimized. This is also the most important principle for selecting sealing structures under highly corrosive conditions.

• Failure of centrifugal water pump seals

The failure of auxiliary seals mostly causes the failure of mechanical seals. One of the main reasons why centrifugal water pump mechanical seals cannot work properly due to leakage is also caused by O-ring failure. The failure of O-rings includes aging, permanent deformation, expansion deformation, distortion, extrusion damage, etc. Therefore, when selecting O-rings, the safe use temperature of synthetic rubber should be considered. It is best to use rubber O-rings with larger cross-sections, appropriately increase the hardness, adopt a grooved assembly structure, and reasonably select materials through flooding tests. If necessary, composite materials such as rubber-coated polytetraoxyethylene seals should be used.

The reason why mechanical seals can play a sealing role is that there is an end face membrane between the end faces of the mechanical seal, and the shape of the end-face membrane determines the sealing effect. Due to different working conditions, the end face membrane between the end faces of the mechanical seal has three forms: liquid phase, gas-liquid mixed phase and gas phase. The reason why the end face membrane has different forms is that when the sealed liquid is close to saturation, boiling or flashing will occur, that is, phase change will occur. Once the end face membrane undergoes a phase change, the back pressure of the film will change greatly, resulting in unstable end face fitting. Therefore, the key to solving the problem is how to ensure that the end face membrane of the mechanical seal is a stable film. Obviously, to ensure that the end face membrane is a stable film, it is only necessary to ensure that the end face membrane does not undergo a phase change or the phase change does not destroy the end face fitting.

Based on the above analysis, the design and shape of the mechanical seal must be combined with the actual production, and friction pair materials and static seal ring materials with reliable sealing and long service life should be selected as much as possible. For example, the inner end of the double-end mechanical seal is mainly in contact with the medium; the outer end is not in contact with the medium. The main materials used are materials with good sealing reliability, such as WC rubber rings. There is no need to pursue the consistency of the material selection of the inner and outer ends. This can reduce costs and increase sealing reliability. The processing quality of mechanical seal parts must be strictly controlled. The instruction manual and maintenance procedures must specify the technical requirements for the installation and use of centrifugal water pump mechanical seals so that workers can follow the rules.

After the general pump mechanical seal is installed, static and dynamic tests must be carried out to confirm that the mechanical seal is installed correctly. When leakage is found, it is convenient for timely maintenance. In addition, leakage may occur suddenly during normal operation. At this time, a comprehensive analysis can be carried out according to the situation to determine the real cause of the mechanical seal leakage for easy resolution.