Among threaded connections, flange connections and welded end connections, the first two are the most commonly used. Valves with threaded connections are mainly those with a nominal diameter of less than 50mm. If the diameter size is too large, the installation and sealing of the connection part will be very difficult. Flanged valves are relatively convenient to install and disassemble, but they are heavier and more expensive than threaded valves. Therefore, they are suitable for the connection of pipelines of various diameters and pressures. Welding connection is suitable for more severe conditions and is more reliable than flange connection. However, the disassembly and reinstallation of valves connected by welding are rather difficult, so their application is limited to places where they can operate reliably for a long time or where the operating conditions are harsh and the temperature is relatively high.

Confirm the working conditions of the valve: the nature of the applicable medium, working pressure, working temperature and control mode, etc. When choosing the materials for the valve body, internal components and sealing surfaces, in addition to considering the physical properties (temperature, pressure) and chemical properties (corrosiveness) of the working medium, the cleanliness level of the medium (whether there are solid particles) should also be controlled. Besides, relevant regulations of the state and the user department should be referred to.

The correct and reasonable selection of valve materials can achieve the most economical service life and the best performance of the valves. The selection sequence of valve body materials is: cast iron - carbon steel - stainless steel, and the selection sequence of sealing ring materials is: rubber - copper - alloy steel -F4. The correct selection of valve types is predicated on the designer's thorough control over the entire production process and operating conditions. When choosing valve types, designers should first control the structural characteristics and performance of each type of valve.