Chemical valve overview _ selection _ material selection instructions

Chemical valve overview:
Chemical valves are an important accessory for fluid control on industrial pipelines. Facing the various industrial conditions and the wide variety of valve conditions, you should choose the right valve for the pipeline system. You should understand the performance of the valve. The steps and basis for selecting the valve should follow the principle of selecting valves for petroleum and chemical industries. Chemical valves are not only suitable for a wide range of applications, but also have a large amount of use. Of course, chemical valves are more demanding than ordinary valves. The valves used in chemical valves are generally easy to corrode, simple chlor-alkali industry, to large petrochemical enterprises, there are high temperature, high pressure, corrosion resistance, wear resistance, temperature and pressure difference. For this kind of dangerous valve, it should be strictly in accordance with chemical standards during the selection and use.
Chemical valve selection principle:
In the chemical industry, valves with straight-flow valves are generally used, and their flow resistance is small. Valves for cut-off and open media are usually selected. Valves with easy to adjust flow are used as control flow. Plug valves and ball valves are more suitable for commutating shunt. The sliding of the closure member along the sealing surface with a wiping action is most suitable for media with suspended particles. Common chemical valves include ball valves, gate valves, globe valves, safety valves, plug valves, check valves, etc. The chemical medium mainly contains chemical substances, including acid and alkali corrosion medium. The chemical valve material mainly dominates 304L and 316. Ordinary medium selects 304 as the leading material, and various chemical substances combine the corrosion fluid with alloy steel or fluorine-lined valve. .
The role of chemical valve types:
Open and close type: cut off or communicate the flow of fluid in the pipe; adjust type: adjust the flow and flow rate in the pipe;
Throttle type: causes a large pressure drop after passing the fluid through the valve;
Other types: a. Automatic opening and closing b. Maintain a certain pressure c. Block steam drainage.
Precautions before using chemical valves:
1There are defects such as blisters and cracks on the inner and outer surfaces of the valve;
2 Whether the valve seat and the valve body are firmly connected, whether the valve core and the valve seat are in agreement, and whether the sealing surface has defects;
3 Whether the connection between the valve stem and the valve core is flexible and reliable, whether the valve stem is bent or not, and whether the thread is damaged;
Corrosion has always been one of the most headaches of chemical equipment. If it is slightly careless, it will damage the equipment and cause accidents and even disasters. According to relevant statistics, about 60% of the destruction of chemical equipment is caused by corrosion. Therefore, in the selection of chemical valves, we must first pay attention to the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a “universal material” and it is dangerous to hold stainless steel valves regardless of the medium and environmental conditions.
The main points of common chemical valve media selection:
Sulfuric acid: As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. Sulfuric acid with different concentrations and temperatures has a great difference in corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and temperature less than 80 °C, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. Not suitable for pump valve materials; ordinary stainless steel such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) also has limited use of sulfuric acid media. Therefore, the pump valve for conveying sulfuric acid is usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (20 alloy). Fluoroplastics have better sulfuric acid resistance, and the use of fluorine-lined valves is a more economical option.
Hydrochloric acid: Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon iron can only be used for hydrochloric acid at 50 ° C and 30%. Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber valves and plastic valves (such as polypropylene, fluoroplastic, etc.) are the best choice for transporting hydrochloric acid.
Nitric acid: Most metals are quickly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material. It has good corrosion resistance to all concentrations of nitric acid at normal temperature. It is worth mentioning that it contains molybdenum-containing stainless steel (such as 316). 316L) The corrosion resistance to nitric acid is not only not superior to ordinary stainless steel (such as 304, 321), and sometimes even worse. For high temperature nitric acid, titanium and titanium alloy materials are usually used.
Acetic acid: It is one of the most corrosive substances in organic acids. Ordinary steel is severely corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid resistant material. Molybdenum-containing 316 stainless steel can also be used for high temperature and dilute acetic acid. steam. For high temperature, high concentration acetic acid or other corrosive media, high alloy stainless steel valves or fluoroplastic valves are available.