The role of leak sealing compounds play in online leak sealing technology
The leak sealing compound is injected into the sealing cavity formed by the leak repair clamp and the outer surface of the leaking part, and it is in direct contact with the medium, and is the first defense line of the newly established sealing structure. Its performance directly relates to the scope of the technology, and also directly affects the service life of the new sealing structure. How to choose leak sealing compound correctly is the key to the implementation of online leak seal. The leak sealing compound used in online leak sealing technology is a kind of plastic material with certain elastic modulus under normal temperature, which is made of synthetic rubber, synthetic resin, graphite, plastic and other components, including curing agent, modifier, plasticizer, accelerator, stabilizer and filler. At present, there are nearly 40 varieties in domestic production, which are generally divided into two types: thermal curing sealant and non-thermal curing sealant.
Basic performance index:
1） Injection process performance: During the injection process, the leak sealing compounds should have good plasticity and fluidity, so that they can be injected into the sealing cavity and packed smoothly, and all kinds of leaking defects such as cracks, grooves, holes and so on can be filled. For the thermal curing sealant, it is a hard round rod solid at room temperature, and has no fluidity. However, after injection gun loads high pressure, with the increasing pressure, the sealant is heated by motion friction and shear effect. The extruded sealant will naturally rise temperature and enhance the plasticity. If the ambient temperature is increased by heating, the fluidity of sealant in the high pressure injection gun will be further enhanced, and the injection pressure will be reduced obviously. For non-thermal curing sealants, no matter at room temperature, or high temperature, even some varieties have good injection process performance at low temperature.
2） Operating temperature: The application temperature of the leak sealing compounds are determined by experiment and verified by practical application. For the sealant based on synthetic rubber and resin, the thermal decomposition temperature is the basis to determine its service temperature. For any leak sealing compound, the atoms of its constituent molecules are formed by chemical bonds with different bond energies. In order to separate the atoms of molecules with high bond energy, it is necessary to break the chemical bond with high energy and decompose them into atoms or smaller compound molecules. Therefore, when the sealant is heated and reaches its thermal decomposition temperature, the macromolecules will rapidly decompose into low molecules, and drift with the granular smoke, leaving only the residual material, which will lose the sealing effect. The upper limit of the actual use temperature of various leak sealing compounds is the upper limit of the use temperature after the thermal decomposition temperature is measured by the experimental method and reduced by about 50 ℃.
3） Curing time: Curing time is one of the important indexes of leak sealing compounds. Whether it is a thermal curing or non-thermal curing sealant, there is a process of transforming from plastic to elastic (or from fluid to solid). The time interval for completing this process is the curing time of the leak sealing compounds. The curing time of leak sealing compound is closely related to the temperature. The curing time of most leak sealing compounds is inverse to the temperature. The higher the temperature, the shorter the curing time. The leak sealing compounds shall not cure too fast or too slowly at injection temperature. If curing is too fast and curing time is short, it is easy to early curing. The sealant injected cannot form a continuous whole well, and it is easy to appear discontinuous interface, and reduce the reliability of sealing. For some non-thermal curing sealants, if the curing time is too short, there will be no sealant injected into the sealing cavity, it will be solidified in the material chamber or a part of the channel in the high pressure injection gun, so the sealing operation cannot be carried out. On the contrary, the curing time is too long. During the process of sealant injection, the uncured sealant will be squeezed out of the gap of the sealing cavity by strong injection pressure, and it is not easy to fill the whole sealing cavity, and the sealing effect is not good. At the same time, the curing time of sealant is also related to the dosage. The higher the dosage, the curing time is required to be prolonged correspondingly. At the injection temperature, the curing time is more than 15-50 minutes. The curing time of sealant is related to the variety of base material and the dosage of curing agent, accelerator and filler in the process formula. The appropriate curing time can be obtained by adjusting the dosage properly.
4） Medium resistance: The medium resistance refers to the newly established sealing structure. Within the specified period, the cured sealant will not lose the sealing performance due to the corrosion of the leakage medium. For a specific brand sealant, the medium resistance is limited, and it must be used correctly according to its operation instructions. The newly established sealing structure, the medium contacts with the sealant and the medium molecules enter the cross-linking structure network of sealant molecules. The macroscopic performance is the volume increase, which is called the swelling phenomenon of sealant in the medium. Some of the sealants that have not completed the curing process, whether they are thermal cured or non-thermal cured, will dissolve in oil and other chemical media. Especially when the medium is a good dissolving agent relative to the sealant, the sealant is partially dissolved, because the medium molecules as solvents enter the sealant, which makes the curly polymer loose, the cohesion is weakened, and the curing cannot play the role of sealing. Therefore, the dissolution and swelling of sealant is an important index of its medium resistance. The two indexes basically characterize the medium resistance of sealant. If you choose a sealant to ignore its media resistance, it will either fail to seal, or the newly established seal structure will have a short life.
5） Service life: The service life of sealant refers to the time interval from the establishment of new sealing structure to the next leakage of the dynamic seal. Different sealants, different media, different service life. Therefore, it is required to use the sealant in strict accordance with the instruction manual, the applicable medium indicated and the temperature conditions.
6） Pressure resistance: Sealant can only transmit pressure in the sealing cavity, and maintain enough sealing pressure to ensure the reliability of sealing. The pressure of the sealed fluid medium passes through the sealant, and it will eventually act on the special clamp. Even if we use this technology to deal with high pressure leaking medium with 10MPa or more, as long as the rigidity and strength of the clamp meet the requirements, the sealant selection is appropriate, and the success of dynamic sealing operation is absolutely guaranteed.