The importance of control valve selection

Regulating valve is an actuator in an automatic control system, and its application quality directly reflects the system's regulating quality. As a terminal component in process control, people have a newer understanding of its importance compared to the past. In addition to the quality of the product itself and whether the user correctly installs, uses, and maintains the application of the control valve, correct calculation and selection are very important. Due to errors in the calculation and selection, the system starts and stops, and some even cannot be put into use. Therefore, users and system designers should understand the importance of valves in the field and pay enough attention to the selection of regulating valves.

The principle of hydraulic machinery selection of control valve

One we choose the appropriate structure and material according to the process conditions.

Second, we select the flow characteristics of the control valve according to the characteristics of the process object.

Third, select the appropriate control valve diameter size according to the process operating parameters.

Fourth, according to the requirements of the process, select the necessary auxiliary devices.

Fifth, choose the implementing agency reasonably. The response speed of the actuator should be able to meet the process

Our requirements for the control stroke time: The selected control valve actuator should be able to meet the requirements of the valve stroke and process for the level of leakage. In some occasions, such as the selection of pressure control valves (including vent valves), the actual possible pressure difference should be considered for appropriate amplification, that is, the actuator is required to provide a greater force. Otherwise, when there is an abnormal situation in the process, the actual pressure difference before and after the control valve is relatively large, and the danger of not being able to be closed or opened may occur.

Accessories for hydraulic and mechanical control valves

In the production process, the control system puts forward a variety of special requirements for the valve. Therefore, the control valve must be equipped with various accessory devices (referred to as accessories) to meet the needs of the production process. Accessories for control valves include:

1. The valve positioner is used to improve the working characteristics of the control valve's regulating performance and achieve correct positioning.

2. The valve position switch shows the working position of the upper and lower limits of the valve.

3. Pneumatic retaining valve When the air source of the control valve fails, keep the valve at the opening position before the air source fails.

4. The solenoid valve realizes the electromagnetic switching of the gas circuit to ensure that the valve is in the desired safe opening position when the power supply fails.

5. Handwheel mechanism When the controller of the control system fails, the valve can be switched to manual mode.

6. The pneumatic relay accelerates the action of the actuator and reduces the transmission time.

7. The air filter pressure reducer is used to purify the air source and adjust the air pressure.

8. The gas storage tank ensures that when the gas source fails, the springless cylinder piston actuator can move the control valve to the fail-safe position. Its size depends on the size of the cylinder, the requirements of the valve action time and the working conditions of the valve.

In short, the role of accessories is to make the function of the control valve more complete, more reasonable and more complete.

3. Maintenance of control valve

The control valve has the characteristics of simple structure and reliable action, but because it directly contacts the process medium, its performance directly affects the quality of the system and environmental pollution, so the control valve must be maintained and regularly overhauled, especially for the harsh and important conditions In occasions, more attention should be paid to maintenance work. Key inspection sites:

1. The inner wall of the valve

For the control valve used in high pressure difference and corrosive medium occasions, the inner wall of the valve body and the diaphragm of the diaphragm valve are often impacted and corroded by the medium. The pressure and corrosion resistance must be checked.

2. Valve seat

When the control valve is working, due to the infiltration of the medium, the threaded inner surface for fixing the valve seat is easily corroded and the valve seat becomes loose. Pay attention to it during inspection. For valves that work under high pressure difference, check whether the sealing surface of the valve seat is damaged.

3. Spool

The spool is a movable part during adjustment work. It is most severely eroded and corroded by the medium. It is necessary to carefully check whether the various parts of the spool are corroded and worn during maintenance, especially under the condition of high pressure difference. (Due to cavitation), attention should be paid. The valve core should be replaced when it is seriously damaged. In addition, attention should be paid to whether the valve stem has similar phenomena, or the connection with the valve core is loose.

4. Diaphragm "O" ring and other gaskets. The diaphragm and "O"-shaped gasket in the control valve should be inspected for aging and cracks.

5, sealing packing

Attention should be paid to whether the PTFE packing and sealing lubricating grease are aging, and whether the mating surface is damaged, they should be replaced when necessary.