What is a Control Valve？

The control valve is composed of two main components: the valve body assembly and the actuator assembly (or actuator system), which are divided into four series: single-seat series control valves, double-seat series control valves, sleeve series control valves and self-operated series control valves. Variants of the four types of valves result in a wide variety of applicationable structures, each with its own specific applications, characteristics, advantages and disadvantages. While some control valves have a wider range of applications than others, not all control valves can be used to create the best solution for enhanced performance and reduced costs.

Type

There are many types of valve bodies of control valves, and the commonly used ones are straight-through single-seat, straight-through double-seat, angled, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve type, spherical, etc. When making a specific choice, the following considerations can be made:

1. It is mainly considered according to the selected flow characteristics and unbalanced forces.

2. When the fluid medium is a suspension containing a high concentration of abrasive particles, the internal material of the valve should be hard.

3. Due to the corrosive nature of the medium, try to choose a valve with a simple structure.

4. When the temperature and pressure of the medium are high and the change is large, the valve with little change in temperature and pressure should be selected for the material of the valve core and valve seat.

5. Flash evaporation and cavitation are produced only in liquid media. In the actual production process, flash evaporation and cavitation will form vibration and noise, shortening the service life of the valve, so when selecting the valve, the valve should be prevented from flashing and cavitation.

Features

1. There are various types of control valves, and their applicable occasions are different, therefore, the type of control valve should be reasonably selected according to the requirements of the process production process.

2. Pneumatic control valves are divided into two types: air opening and air closing. The air-open control valve closes in the event of a fault and the air-closed control valve opens in the event of a fault. Some auxiliary equipment can be used to form a position retention valve or make the control valve self-locking, that is, the control valve can maintain the valve opening before the failure when it fails.

3. The way of air opening and gas closing can be realized by the combination of positive and negative actuator types and positive and negative body valves, and can also be realized by valve positioner when using valve positioner.

4. All kinds of control valves have different structures and have their own characteristics. 

Selection

1. 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 an end-of-line element in process control, there is a newer awareness of its importance than in the past. In addition to the quality of the product itself, whether the user is correctly installed, used and maintained, it is very important to calculate and select the correct type of control valve application. Due to the error of calculation and selection, the system starts and stops, and some can not even be put into use, so users and system designers should understand the importance of the valve in the field, and must pay enough attention to the selection of the control valve.

1. The principle of control valve selection: according to the process conditions, select the appropriate structure and material; According to the characteristics of the process object, the flow characteristics of the control valve are selected; According to the process operation parameters, select the appropriate control valve caliber size; According to the requirements of the process, select the required auxiliary devices; Reasonable selection of actuator, actuator response speed should be able to meet the process.

Maintenance

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

1. The inner wall of the valve

For the control valve used in the case of high pressure difference and corrosive medium, the diaphragm of the valve body wall and diaphragm valve is often affected and corroded by the medium, and the pressure resistance and corrosion resistance must be checked.

2. a valve seat

When the control valve is working, due to the infiltration of the medium, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and loosens the valve seat, so attention should be paid to it during inspection. For valves working under high pressure differential, the sealing surface of the valve seat should also be checked to see if it is broken.

3. a valve core

The valve core is a movable part when adjusting the work, and is most seriously corroded and corroded by the medium, and the valve core should be carefully checked whether the parts of the valve core are corroded and worn during maintenance, especially in the case of high pressure difference, the wear of the valve core is more serious (due to cavitation), which should be paid attention to. When the valve core is seriously damaged, it should be replaced, and it should also be noted whether the valve stem has a similar phenomenon, or the connection with the valve core is loose.

4. a diaphragm

"O" rings and other gaskets. The diaphragm and "O" shaped gasket in the control valve should be inspected for aging and cracking.

5. Sealing packing

Attention should be paid to whether the PTFE packing and sealing grease are aged, and whether the mating surface is damaged, and should be replaced if necessary.