Brief introduction to the selection principle and

The selection principle and maintenance method of control valve brief introduction

a, the importance of control valve selection

the control valve is the actuator in the automatic control system, and its application quality is directly reflected in the regulation quality of the system. As a terminal component of the biodegradation of products in process control, people have a newer understanding of its importance than in the past. The regulating valve should be used for the grand opening of the international plastics and Rubber Exhibition (k exhibition) in Dusseldorf, Germany, 2016 on October 19, 2016. In addition to the quality of the product itself and whether the user has correctly installed, used and maintained, it is very important to correctly calculate and select the type. Due to the error of calculation and type selection, the system starts and stops, and some even cannot be put into use. Therefore, users and system designers should recognize the importance of valves on site, and pay enough attention to the type selection of regulating valves

b, the principle of control valve selection

1. Select the appropriate structural form and material according to the process conditions

2. Select the flow characteristics of the control valve according to the characteristics of the process object

3. Select the appropriate diameter and size of the control valve according to the process operation parameters

4. Select the required auxiliary devices according to the requirements of the process

5. Select the actuator reasonably. The response speed of the actuator shall meet the requirements of the process

on the control stroke time: the selected control valve actuator shall meet the requirements of the valve stroke and the process on the leakage level. In some cases, if the pressure control valve (including vent valve) is selected, the actual possible differential pressure should be appropriately amplified, that is, the actuator is required to provide a large force. Otherwise, the actual pressure difference between the front and back of the control valve may be large, and there may be a danger that it will not close or open when there are abnormal conditions in the process

II. Accessories of control valve

in the production process, the control system puts forward various special requirements for the valve. Therefore, the control valve must be equipped with various accessories (hereinafter referred to as accessories) to meet the needs of the production process. The accessories of the control valve include:

1. The valve positioner is used to improve the working characteristics of the regulation performance of the control valve to achieve correct positioning

2. The valve position switch displays the working position of the upper and lower limit of the valve

3. 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 in case of power failure

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

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 regulate the air pressure

8. The air reservoir ensures that when the air source fails, the spring free cylinder piston actuator can move the control valve to the fault safe position. Its size depends on the size of the cylinder, the requirements for the action time of the reversing door caused by the valve that the reversing switch does not reverse, and the working conditions of the valve

in a word, the function of accessories is to make the function of control valve more perfect, reasonable and complete

III. maintenance of control valve

control valve has the characteristics of simple structure and reliable action. However, because it is directly in contact with process media, its performance directly affects the system quality and environmental pollution. Therefore, the control valve must be maintained and overhauled regularly, especially in bad and important occasions, the maintenance work should be paid more attention to. Key inspection parts

1. Inner wall of valve body

for the control valve used in the occasions of high pressure difference and corrosive medium, the inner wall of valve body and the diaphragm of diaphragm valve are often impacted and corroded by the medium, so the pressure resistance and corrosion resistance must be inspected

2. Valve seat

when the control valve is working, due to the infiltration of medium, the inner surface of the thread used to fix the valve seat is easy to be corroded, which makes the valve seat loose. Attention should be paid during inspection. For valves working under high pressure difference, check whether the sealing surface of the valve seat is damaged

3. Valve element

valve element is a movable part during adjustment work, which is most seriously scoured and corroded by the medium. During maintenance, we should carefully check whether all parts of the valve element are corroded and worn. Treat every customer as our closest friend or family member, especially under the condition of high pressure difference, the wear of the valve element is more serious (due to cavitation), which should be paid attention to. Replace the valve core when it is seriously damaged. In addition, pay attention to whether the valve rod has similar phenomena, or the connection with the valve core is loose

4. Diaphragm O-rings and other gaskets. Check whether the diaphragm and O-ring gasket in the control valve are aging and cracked

5. Sealing packing

pay attention to whether the PTFE packing and sealing lubricating grease are aging and whether the mating surface is damaged. It should be replaced if necessary