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How to choose the Pneumatic control valve air open and air close?

How to choose the pneumatic control valve air open and air close?

Pneumatic control valve air open and air closed is in the industrial control system is a vital link, the correct choice and use can guarantee the safety and normal operation of the system. Pneumatic control valve as a common control element in industrial production, mainly used to adjust the pressure, flow, level, temperature and other parameters of the fluid in the pipeline to meet the demand for accurate control of fluid parameters in the production process.

I. What is the pneumatic control valve air open and air close?

Air-open type (normally closed) is when the diaphragm head air pressure increases, the valve to increase the opening direction of action, when the input air pressure reaches the upper limit, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve acts in the direction of closing, and is fully closed when there is no air input. Usually we call the air-open type regulating valve as the fault closure type valve.

The air-close type (normally open type) operates in exactly the opposite direction to the air-open type.When the air pressure increases, the valve moves in the closing direction; when the air pressure decreases or disappears, the valve moves in the opening direction or until it is fully open.. Usually we call the air-off type regulating valve as the fault opening type valve.

For example, a heating furnace combustion control, control valves installed in the fuel gas pipeline, according to the temperature of the furnace chamber or the temperature of the heated material in the furnace outlet to control the supply of fuel. In this case, it is safer to use a gas-open valve, because once the gas supply is stopped, it is more appropriate to keep the valve closed than to keep it fully open. If the gas supply is interrupted, the fuel valve is fully open, will make the danger of overheating. Another example is a cooling water cooled heat transfer equipment, hot materials in the heat exchanger and cooling water for heat exchange is cooled, the control valve is installed in the cooling water pipe, with the temperature of the material after the heat transfer to control the cooling water, in the interruption of the gas supply, the control valve should be in the open position safer, it is preferable to choose the gas shut-off (i.e., FO) control valve.

II. How to choose to start the regulating valve air open and air close?

1. According to the characteristics of the medium to choose the air open and air close

In the selection of pneumatic control valve air open and air close, the first thing to consider the characteristics of the medium. For easy to crystallise, easy to solidify or solid particles of the media, generally recommend the use of air-open mode, to avoid the media blocking the valve or lead to the valve can not be closed properly. For some corrosive media or high-temperature and high-pressure media, you need to consider choosing a more corrosion-resistant and high-temperature pressure-resistant valve structure, and in the gas source control on the corresponding strengthening.

2. According to the process requirements of the selection of air open and air close

In addition to media characteristics, but also according to specific process requirements to select the gas open and gas shutdown. For example, for some of  the requirements of rapid response, high-frequency repeated open and close occasions, will generally choose the air open and close the gas time is short, high flexibility, in order to improve the system’s adjustment accuracy and response speed. For the need to maintain a specific state for a long time or a stable output process, you need to choose a relatively stable and reliable air open and air close to ensure the long-term stable operation of the system.

3. Selection of air open and air close according to safety requirements

In some of the higher security requirements of the occasion, such as chemical production, petrochemical and other fields, the pneumatic control valve gas open and gas shut-off mode also has strict requirements. Then you need to choose some with a double seal or safety valve device of the gas open and gas shut-off mode, in order to avoid the accidental situation lead to media leakage or valve failure, so as to ensure the safe operation of the production system.

III. In the selection and use of pneumatic control valve air open and air close, need to pay attention to the following points:

  1. Understand the characteristics of the medium, select the appropriate valve structure and material.
  2. According to the process requirements and the characteristics of the control system, select the appropriate air open and air closed.
  3. Ensure the stability and reliability of the gas source system, in order to avoid the valve can not be opened and closed properly due to gas source problems.
  4. Regularly maintain and repair the pneumatic control valve to ensure the normal service life and performance of the valve.

IV. Pneumatic fault position of the common way of expression

  1. Pneumatic valve device interlocking action, the valve position should be the following cases:

FC – loss of gas supply, the valve is closed position

FO – the loss of gas, the valve is in the open position

FL – loss of gas supply, the valve is in the moment position and has been maintained

FLC  Loss of gas supply, valve holds position but tends to close, valve in closed position (gas in cylinder is consumed)

FLO-loss of gas supply, the valve holds position but tends to open, the valve is in the open position (gas consumption in the cylinder is complete)

2. Regulating valves or switching valves involved in the device interlocking action case, the valve position should be the following cases:

FC  loss of gas supply or solenoid valve power failure, the valve is in the closed position;

FO  loss of gas source or loss of power to the solenoid valve, the valve is in the open position;

AFL/EFC-

(1) When the air source is lost, the solenoid valve does not lose power and the valve remains in position;

(2) Regardless of whether the air source is lost or the solenoid valve loses power, the valve is in the closed position;

AFL/EFO-

(1) When the air source is lost, the solenoid valve does not lose power and the valve remains in position;

(2) Regardless of whether the air source is lost or the solenoid valve loses power, the valve is in the open position

V. Summary

Through the above content in detail, I believe that we have a more in-depth understanding of how to choose the pneumatic control valve air open and air closed. Correct selection and use of gas open and gas shut-off mode, not only can improve the stability and safety of the system, but also can maximize the pneumatic control valve regulating function, to meet the needs of process production.

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