Function characteristics and advantages of pneumatic valve

Pneumatic valve and control valve can not be separated. Only the combination of pneumatic valve and control valve can bring greater benefits and help to our life more effectively. Pneumatic valve is constantly common in our life, and the control valve is also constantly improving. So we will introduce the relevant knowledge of pneumatic valve and control valve to you.

1. Valve positioner

The valve positioner is the main accessory of the pneumatic actuator. It is used with the pneumatic actuator to improve the position accuracy of the valve, overcome the influence of stem friction and medium imbalance, so as to ensure the correct positioning of the valve according to the signal from the regulator.

2. Solenoid valve

When the system needs to realize program control or two position control, it needs to be equipped with solenoid valve. When selecting solenoid valve, in addition to AC and DC power supply, voltage and frequency, it is necessary to pay attention to the relationship between the solenoid valve and the function type of regulating valve, which can be equipped with "normally open" and "normally closed". If it is required to increase the capacity of the solenoid valve to shorten the operation time, two solenoid valves can be used in parallel or the solenoid valve can be used as a pilot valve and a large capacity pneumatic relay.

3. Pneumatic relay

It is a kind of power amplifier, which can send the air pressure signal to a far place and eliminate the lag caused by the extension of the signal pipeline. It is mainly used between the field transmitter and the regulating instrument in the central control room, or between the regulator and the field regulating valve. Another function is to amplify or reduce the signal.

4. Converter

The converter is divided into air electric converter and electric air converter. Its function is to realize the mutual conversion of certain relations between air and electric signals. It is mainly used to convert 4-20mA electric signal into 20-100pa air signal when the electric actuator is operated by electric signal. On the contrary, when the electric actuator is operated by air signal (relatively rare) or for centralized monitoring, 20-100kpa is used when connected with computer network The gas signal is converted into 4-20mA electric signal.

5. Valve position transmitter

When the regulating valve is far away from the control room, it is necessary to equip a valve position transmitter in order to know the switch position of the valve accurately without going to the site. The displacement of the valve opening mechanism is converted into electrical signal according to certain rules and sent to the control room. This signal can be a continuous signal reflecting any opening of the valve, or it can be considered as the reverse action of the valve positioner

6. Travel switch (return)

A device that reflects the two limit positions of the valve switch and sends out an indication signal at the same time. According to the signal, the control room can judge the switch status of the valve so as to take corresponding measures.

Pneumatic control valve action is divided into air open type and air close type. Air to open is when the air pressure on the membrane head increases, the valve acts in the direction of increasing opening, when the upper limit of input air pressure is reached, the valve is in the fully open state. Conversely, when the air pressure decreases, the valve acts in the closing direction, and when there is no air input, the valve is fully closed. So sometimes the air open valve is also called fail to close FC. The action direction of air to close is just opposite to that of air to close. When the air pressure increases, the valve acts in the closing direction; when the air pressure decreases or does not, the valve moves in the opening direction or fully open. So it is sometimes called fail to open fo. The pneumatic opening or closing of the pneumatic control valve is usually realized by the positive and negative action of the actuator and the different assembly methods of the valve structure.

The selection of gas opening and gas closing is based on the safety of process production. When the air source is cut off, is the regulating valve in the closed position safe or in the open position safe? For example, for the combustion control of a heating furnace, the regulating valve is installed on the fuel gas pipeline, and the fuel supply is controlled according to the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is safer to select the air opening valve, because once the air supply stops, it is more appropriate for the valve to be closed than the valve to be fully open. If the air source is interrupted and the fuel valve is fully opened, it will cause danger of over heating. Another example is a heat exchange equipment cooled by cooling water. The hot material is cooled by heat exchange with cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe. The temperature of the material after heat exchange is used to control the cooling water volume. When the air source is interrupted, the regulating valve should be in the open position, which is safer. It is better to select the air closed (FO) regulating valve.

Valve positioner

The valve positioner is the main accessory of the regulating valve, which is widely used with the pneumatic regulating valve. It receives the output signal of the regulator, and then controls the pneumatic regulating valve with its output signal. When the regulating valve acts, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the valve position status is transmitted to the upper system through the electrical signal.

According to its structure and working principle, valve positioner can be divided into pneumatic valve positioner, electro pneumatic valve positioner and intelligent valve positioner.

The valve positioner can increase the output power of the regulating valve, reduce the transmission lag of the regulating signal, accelerate the movement speed of the valve stem, improve the linearity of the valve, overcome the friction force of the valve stem and eliminate the influence of the unbalanced force, so as to ensure the correct positioning of the regulating valve.

Common actuators include pneumatic actuator and electric actuator, including direct stroke and angle stroke. It is used to open and close all kinds of cutting doors and air boards automatically and manually