Bellows refer to cylindrical thin-walled corrugated shells with multiple transverse corrugations. The function of the bellows on the control valve is only one - isolation, mainly divided into three types: one is to prevent the diffusion of the medium inside the valve to the outside, which is to prevent the flow of chemically active or toxic gases, volatile liquids, and valuable media such as chlorine gas, silicon powder, etc. inside the valve. Prevent them from leaking, causing pollution to the environment or loss of resources; Another way is to prevent external impurities and air from entering the valve, which is based on the purity requirements of the environment and medium in which the valve is located. For example, if there are many granular substances such as dust in the outside, as the valve moves, the valve stem will bring these substances into the valve or accumulate at the valve stem packing, which may easily cause pollution of the valve medium and valve locking; One is to protect the internal components of the valve and prevent the medium from causing erosion and corrosion to the valve stem.
There are two installation positions for stainless steel corrugated pipes. One is installed inside the valve body, and the corrugated pipe is made of polytetrafluoroethylene. The upper end is integrated with the fluorine lining of the upper valve cover, and the lower end is connected to the valve core, which plays a role in fixing the valve core and protecting the valve stem from corrosion and erosion; A type of bellows installed outside the valve body, mostly made of metal, with one end fixed on the upper part of the valve stem and the other end as a free end. There is also a protective layer around the bellows, which not only protects the bellows but also ensures that the medium inside the valve will not pollute the environment for a certain period of time when the bellows is damaged.
According to the material of corrugated pipes, they can be divided into two categories: plastic corrugated pipes and metal corrugated pipes. Plastic corrugated pipe, also known as polytetrafluoroethylene corrugated pipe, is made by heating and pressing tubular polytetrafluoroethylene and block polytetrafluoroethylene in a molten state. The production method of plastic corrugated pipes is simple, and in some cases, they can replace metal corrugated pipes to save costs. They are inexpensive and have outstanding characteristics such as stretchability. However, the application of plastic corrugated pipes is limited by three factors. One is the temperature of the environment in which the valve is located and the temperature of the medium. If the temperature is too high, it can easily cause aging and deformation of the corrugated pipe. Therefore, its long-term use temperature generally does not exceed 180 ° C, and there are no specific requirements for low temperature situations; Secondly, due to the pressure limitation inside the pipeline (generally, the working pressure of plastic corrugated pipes is not higher than 12 kilograms), if the pressure is too high, the valve lining material will be sucked out from the inner wall of the valve, causing valve blockage; Another factor is the chemical properties of the medium. Plastic corrugated pipes can be applied to highly corrosive media such as formic acid, hydrochloric acid, chloride, etc. However, when the medium is a medium with a similar molecular formula, similar phase dissolution occurs and cannot be used, such as silicon tetrachloride. The lifespan of plastic corrugated pipes is generally 5 years, and manufacturers that can produce them include RICHTER, Volkswagen, Samson, Fischer, etc. Plastic corrugated pipes have a wide range of applications in various industries, with a much larger usage than metal corrugated pipes.