Key Techniques for the Impregnation Process of Graphite Material

Graphite materials for opaque treatment is the core of the process of making graphite pressure vessel. Graphite materials use different impregnation processes, physical performance is different. Graphite in the manufacture of pressure components, must follow the rules, at the same time, several
details are controlled.
graphite material
Graphite is a kind of non-metallic brittle material, but because of its unique advantages in thermal conductivity and corrosion resistance, it occupies a place in the field of pressure vessels. Especially in the field of heat exchanger equipment is the most widely used.
Artificial graphite is a kind of porous material. The properties of its internal pores, such as opening, closing, connecting and cracking, restrict its application in the field of pressure vessels. As a pressure vessel material, impregnating agent is used to treat the impermeability of the vessel to eliminate the degree of through-hole and meet the technological requirements. This is also the basis for making pressure vessels made of graphite materials, which is different from other materials for making pressure vessels.
Therefore, the impermeability treatment of graphite material, so that it will not leak because of the pressure, temperature and corrosion of the medium, is the core process control points of graphite pressure vessel manufacturing, is one of the most critical process links of graphite pressure vessel manufacturing.
The physical properties of graphite materials are different by using different impregnation processes. Therefore, according to the requirements of TSG 21-2016 “Safety Technical Supervision Regulations for Fixed Pressure Vessels” and GB/T 21432-2008 “Graphite Pressure Vessels”, it is necessary to evaluate the impregnation process to verify the correctness of the impregnation process. The manufacturing process of each graphite compression element must be conducted under the guidance of qualified impregnation process qualification.
As mentioned above, the manufacturing process of each graphite compression element should be conducted under the guidance of a qualified impregnation process qualification, which is a prerequisite and must be complied with the rules for the manufacture of graphite pressure vessels.
In the concrete production practice, it is very important to control the key points of graphite impregnation and ensure the quality of graphite impregnation.
Quality control of impregnant
The quality of impregnant is the most important part of the quality management of impregnation. It is necessary to measure and verify the viscosity of the impregnated resin before each impregnation and evaluate the impurities in the resin. It should also be noted that adequate inspection of the impregnated graphite surface and the impregnated tooling surface is also essential. The graphite powder on the surface of graphite parts and the residual resin layer on the tool for impregnation will be mixed with resin during the impregnation process, thus affecting the quality of resin. Therefore, the cleaning of graphite powder and impurities on the surface of graphite parts, the cleaning of resin layer on the surface of tools for impregnation, and the installation of filters on the resin pipeline should also be taken as important links in the quality control of impregnation process.
Storage of graphite parts before and after impregnation and protection during processing
The graphite parts to be impregnated should not be polluted by oil and water, otherwise it will affect the normal impregnation of impregnating agent, resulting in local impregnation quality defects of graphite parts, thus affecting the overall impregnation effect.
Because of the existence of oil products, contaminated graphite parts inevitably exist, which make them stick to greasy dirt and impurities. Once contacted with the graphite parts, the oil stains, like impregnating agent, immerse into the graphite pores and occupy the impregnating position, forming an obstacle to the successful impregnation of the impregnating agent. After entering the thermal polymerization process, volatilization will occur at temperature, so that the oil-soaked pores appear again, the formation of impregnation failure, and leakage. Therefore, it is necessary to do a good job in the daily maintenance of equipment, to prevent oil leakage, dropping phenomenon, and prevent graphite parts from sticking and staining oil.
Similarly, once water is immersed in graphite, it has the same hazards as oil immersion.
Graphite parts should be stored at a fixed point, away from oil, water and other fluids, to avoid storing in a humid environment, and do the necessary protection to ensure that not contaminated by oil and water. Once it is contaminated by oil and water, the corresponding drying process must be carried out before impregnation can be carried out.
Impregnation process and thermal curing process control
Impregnation process and thermal curing process control
Compressed air is usually used for pressurization. Compressed air contains water and oil, which mix with resin, long-term accumulation will affect the quality of resin, splashing to the surface of graphite will affect the quality of impregnation. Therefore, the necessary oil-water separation of compressed air, oil and water control measures, in the impregnation and thermal curing equipment structure design to take the necessary protection, but also one of the details of the impregnation control process need attention.
Quality test of graphite before and after impregnation
Graphite is a kind of heterogeneous material. Neither raw graphite nor impregnated impermeable graphite can detect its defects by nondestructive testing.
For graphite raw materials, even if the same batch of graphite materials, only through visual inspection, or sample testing, can not avoid the storage of material defects. In the specific operation, the same batch of materials had appeared before, and no apparent defects were found on the appearance of the surface, the same impregnation process was used in the same furnace operation, but serious leakage occurred in the unit. It has to be said that this is an important issue affecting equipment manufacturing. Graphite materials with the same specifications and the same machining process are used to determine whether there are serious defects. I think we can try to solve the problem by measuring its bulk density. Graphite parts of the same batch can be found to be of different quality by means of unit weighing. Graphite parts which differ greatly from other graphite parts should be treated as materials with serious defects. At the same time, through the weighing measurement of the unit, the corresponding data can be accumulated to guide the inspection of raw materials, so as to improve the overall quality control of raw materials.
Similarly, the weight gain rate of graphite parts after each impregnation is measured by weighing method, which is also a means to verify the impregnation process, find out the impregnation quality problems in time, and control the technological discipline.
Weighing of graphite materials should be controlled as an important daily process control point. The data measured by this method should be treated as reference data for guiding raw material selection, raw material inspection, and quality control of impregnation. The long-term collection and analysis of the data is very important for the process management of graphite pressure vessels.
As mentioned above, the key technical points of graphite impregnation process are in addition to the above. In practical operation, there are still many details affecting the quality control of the impregnation process.
Graphite impregnation process is a complex process, once into the impregnation operation, any defects produced during the process can not be completely repaired by technology. Therefore, on the basis of strictly abiding by the process control requirements determined by the impregnation process evaluation, the control of key technologies in each link can ensure the quality control of graphite pressure vessel and ensure the safe use.
The promulgation and implementation of TSG 21-2016 Safety Technical Supervision Regulations for Fixed Pressure Vessels put forward new technical requirements for graphite pressure vessels. In view of these requirements, it is the responsibility and obligation of quality control personnel to study the process control of each link of graphite pressure vessel manufacture and start with the detail control in order to further improve the safety performance of graphite pressure vessel.
Author: SHUI Dan-ping, YU Qing-dong, ZHANG Hai, HAO Yi, SHAO Qian