Functions of valves and factors to consider when selecting them

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The selection of valves is the result of empirical considerations that take into account operational, safety, and economic factors for a comprehensive balance. Before selecting a valve, the following initial conditions must be specified: 1. Physical properties (1) State of the material a. The state of gaseous materials includes relevant physical property data, whether it is a pure gas or a mixture, whether there are liquid droplets or solid particles present, and whether there are components that tend to condense. 　　b. The material state of liquid materials includes: (1) data on physical properties, such as whether the pure component or mixture contains volatile components or dissolved gases (which can precipitate and form a two-phase flow when pressure decreases), whether it contains solid suspensions, as well as the viscosity, freezing point, or pour point of the liquid. 　　(2) Other properties ； These include properties such as corrosivity, toxicity, solubility in the materials used in valve construction, and whether it is flammable or explosive. These properties sometimes affect not only the material but also give rise to special structural requirements or the need to upgrade the pipe grade. 　　2. Operating conditions during operation 　　(1) The temperature and pressure under normal operating conditions, in addition to the conditions during startup, shutdown, or regeneration. 　　a. The pump outlet valve should take into account the pump’s maximum shut-off pressure, etc. 　　b. When the regeneration temperature of the system is much higher than normal while the pressure decreases, for this type of system, the combined effect of temperature and pressure must be taken into account. 　　c. Degree of operational continuity: that is, the frequency at which the valve is opened and closed, which also affects the requirements regarding wear resistance; in systems where the valve is opened and closed frequently, it should be considered whether to install a dual valve. 　　(2) Allowable pressure drop of the system a. If the allowable pressure drop is low, or it is not low but flow regulation is not required, then valve types with a low pressure drop such as gate valves and straight-through ball valves should be selected. 　　b. If flow regulation is required, a valve type with good regulating performance and a certain pressure drop should be selected (the proportion of the pressure drop to the total pressure drop in the pipeline is related to the sensitivity of regulation). 　　(3) Environment in which the valve is located: In outdoor areas with cold climates, especially when dealing with chemical substances, the valve body should generally not be made of cast iron, but rather of cast steel (or stainless steel). 　　3. Valve functions 　　(1) Shutting off: Almost all valves have the function of shutting off flow. Gate valves and ball valves can be used when cutting off flow alone without the need for flow regulation; for rapid cutting off, plug valves, ball valves, butterfly valves, etc., are more suitable. A stop valve can both regulate flow and shut it off. Butterfly valves can also be suitable for regulating large flow rates. 　　(2) Changing the flow direction: By using a two-way (L-shaped channel) or three-way (T-shaped channel) ball valve or plug, it is possible to quickly change the direction of the material flow. Since one valve can serve the function of multiple straight-through valves, this simplifies operations, ensures accurate switching, and reduces the space required. 　　(3) Regulation: Globe valves and plunger valves can meet general flow regulation needs, while needle valves can be used for fine adjustments of very small flow rates ； For stable regulation (of pressure and flow) over a wide range of flow rates, a throttle valve is the appropriate choice. 　　(4) Check valve: A check valve can be used to prevent the backflow of material. 　　(5) For different production processes, valves with additional functions can be selected, such as valves with jackets, drain ports, and bypasses, as well as valves with air injection ports to prevent the deposition of solid particles. 　　4. Power for operating valves 　　For the vast majority of valves that are operated locally, hand wheels are used; for those that need to be operated from a certain distance, sprockets or extended rods can be employed. Some large-diameter valves are equipped with motors from the design stage due to their high starting torque. Appropriate grade of explosion-proof motors should be used in the explosion-proof area. Remote control valves: The power sources used include pneumatic, hydraulic, electric, etc. Among these, electric types can be further divided into solenoid valves and valves driven by motors. It should be chosen based on needs and the available energy source.