The selection of valves is based on the rationality of operation, safety and economy, and the experience result of comprehensive balance comparison. The following original conditions must be proposed before selecting the valve:
Physical property
(1) Material status
- The material status of gas material includes relevant physical property data, pure gas or mixture, whether there are liquid drops or solid particles, and whether there are easily condensed components.
- The material state of liquid material includes: relevant physical property data, whether the pure component or mixture contains volatile component or dissolved gas (when the pressure is reduced, it can precipitate to form two-phase flow), whether it contains solid suspension, and the viscosity, freezing point or pour point of the liquid.
(2) Other properties:
Including corrosiveness, toxicity, solubility of valve structural materials, and whether it is flammable and explosive. Sometimes these properties not only affect the material, but also cause special structural requirements, or need to improve the pipe grade.
Operating conditions in operation
(1) According to the temperature and pressure under normal working conditions, it is also necessary to combine the working conditions during startup, shutdown or regeneration.
- The maximum closing pressure of the pump shall be considered for the pump outlet valve.
- When the regeneration temperature of the system is much higher than the normal temperature, but the pressure is reduced, the comprehensive influence of temperature and pressure should be considered for this type of system.
- Continuous degree of operation: that is, the frequency of valve opening and closing also affects the requirements for wear resistance. For systems with frequent opening and closing, it is necessary to consider whether to install double valves.
(2) Allowable pressure drop of the system
- When the allowable pressure drop of the system is small, or the allowable pressure drop is not small but flow regulation is not required, the valve type with small pressure drop shall be selected, such as gate valve, through ball valve, etc.
- If it is necessary to regulate the flow, the valve type with good regulation performance and certain pressure drop shall be selected (the proportion of pressure drop in the pressure drop of the whole pipeline is related to the sensitivity of regulation).
(3) Environment of valve: in cold area, especially for chemical materials, the valve body material is generally not cast iron but cast steel (or stainless steel).
Valve function
(1) Cut off
Almost all valves have a cut-off function. Gate valve and ball valve can be selected if they are only used for cut-off and do not need to adjust the flow. When it is required to cut off quickly, cock, ball valve and butterfly valve are more suitable. The stop valve can adjust the flow and cut off. Butterfly valve is also suitable for large flow regulation.
(2) Change flow direction:
Select two-way (L-shaped channel) or three-way (T-shaped channel) ball valves or cocks, which can quickly change the flow direction of materials. Because one valve plays the role of more than two through valves, it can simplify operation, make the switching accurate and reduce the space occupied.
(3) Regulation
The stop valve and plunger valve can meet the general flow regulation, and the needle valve can be used for micro regulation; the throttle valve is suitable for stable regulation (pressure and flow) in a large flow range.
(4) Check
The check valve can be used to prevent material backflow.
(5) Valves with additional functions can be selected for different production processes, such as valves with jacket, drain port and bypass, and valves with air blowing port to prevent solid particles from settling, etc.
Power for opening and closing valves: most of the locally operated valves use handwheel. For those with a certain distance from the operation, chain wheel or extension rod can be used. Some large-diameter valves have motors in the valve design due to the excessive starting torque. In the explosion-proof area, explosion-proof motors of corresponding grade shall be used.
Remote control valve: the power types adopted are pneumatic, hydraulic, electric, etc., among which electric can be divided into solenoid valve and motor-driven valve. It should be selected according to the needs and energy available.