Sanitary Diaphragm Valve Selection of valve body material
When selecting the material for the diaphragm valve body, the right material is very important. Plastic and stainless steel are usually selected. Materials that meet the application process and material characteristics are very important to ensure the operation and production of the factory. Due to the chemical and physical properties of plastics, they are often used in highly chemically corrosive processes. For example, ion exchange is the exchange of ions between chemical reagents in ion exchangers, such as hydrochloric acid, sodium hydroxide, etc. and various ions in the solution. However, when the temperature and pressure are increased or sterilized at high temperature, the mechanical properties of plastics will be unstable. When the temperature and pressure combination exceeds the respective characteristic curves, stainless steel lined plastic or all stainless steel materials should be used.
When producing ultrapure water and ultrapure steam, various alloys such as stainless steel are usually used due to the need for high temperature and stability. Depending on the content of the alloy components, the material will have different properties. In addition to good corrosion resistance, there are other excellent properties, such as good thermal conductivity and electrical conductivity, high strength and hardness. Tolerance in high and low temperature environments, etc. Casting, forging or whole steel processing of diaphragm valve body materials can be used. Each material has its advantages and disadvantages according to its production process. Can be heated quickly. For example, this not only saves time when sterilizing a plant, but also reduces the amount of pure steam used. Due to the relatively loose crystal structure of cast materials, there are serious deficiencies in the internal surface polishing and alloy composition transfer process. For example, surface damage cannot be detected in advance during mechanical polishing. In order to avoid surface damage caused by this material itself, some manufacturers no longer perform high-level surface polishing on cast diaphragm valve bodies.
Due to different production processes, forged diaphragm valve bodies have a tighter crystal structure, which can provide better and higher polishing levels and prevent the transfer of alloy components. Generally, all surface polishing has the same requirement: the smooth surface needs a * passivation layer to prevent corrosion and adhesion of particles or bacteria. DIN11866 and ASME BPE specify various hygiene levels and surface polishing levels based on roughness values. The range of roughness values ​​is between 0.25um~1.6um or 15~30uin (0.38~0.76um). ASME BPE further specifies acceptable surface polishing standards. Different products have different polishing values. For example, the surface must not have scratches, cavities, inclusions, depressions, and must guarantee a smooth, shiny surface. The average polish value is the only measure of the surface finish. When considering the definition in more detail: the arithmetic mean of the sum of the ordinate values ​​within the sampling length. This average, although it provides a reference point for the roughness, does not show whether individual very deep areas exist or whether the height and depth are evenly distributed.
In order to correctly measure the surface polish value, it is important to understand the surface preparation process method. For example, to achieve a good polish value, mechanical polishing is performed first and then electrolytic polishing. In this case, electrolytic polishing is used to remove wear marks and retained abrasives, for example to flatten the height and depth differences mentioned above. The choice of material to be removed depends on the initial state of the diaphragm valve body. According to the BN94 standard, polishing to 30um is no longer required. This is because this will produce over-polishing due to excessive polishing time, resulting in the appearance of "orange skin". Once the shiny metal surface appears, it can be passivated to protect the surface and avoid contact with corrosive or rough products. Passivation can be carried out using oxidants or using oxygen from the air. New passivation processes are recommended to maintain corrosion and roughness protection after a reasonable period of time.