for the pneumatic control valve, the situation is more complex, so we mainly discuss the three break method of pneumatic control valve. Generally speaking, when we choose the pneumatic diaphragm control valve, we should first determine whether to open or close the valve. This is to select the protection position when the control valve is cut off. If the process requires that the valve be opened when the valve is cut off, we should choose the normally open (air closed) control valve, otherwise, we should choose the normally closed (air open) control valve. This is only a superficial scheme. If the process requires the three break protection of gas cut-off, power cut-off and signal cut-off, then the control valve needs to be equipped with some accessories to form a protection system to realize the control requirements. These accessories mainly include holding valve, solenoid valve, gas tank, etc. The following is the single acting pneumatic diaphragm control valve and double acting pneumatic control valve of the two schemes.

 

Pneumatic diaphragm control valve scheme (control valve with electric pneumatic valve positioner)

 

   the scheme is mainly composed of pneumatic control valve, electro pneumatic valve positioner, power loss (signal) comparator, single electric control electromagnetic directional valve, pneumatic holding valve, valve position signal returner, etc. Its working principle is as follows:

 

   1. Cut off the air source: when the air source of the control system fails (loses air), the pneumatic valve will automatically close, lock the output signal pressure of the positioner in the membrane chamber of the pneumatic control valve, balance the output signal pressure with the reaction force generated by the control valve spring, and keep the valve position of the pneumatic control valve in the fault position. The valve should be set to start slightly below the minimum value of the air supply.

 

   2. Cut off the power supply: when the power supply of the control system fails (power loss), the output voltage of the single electric control solenoid directional valve controlled by the power loss (signal) comparator disappears, the single electric control solenoid directional valve loses power, the slide valve in the single electric control solenoid directional valve slides under the action of the return spring, the solenoid valve reverses, the membrane chamber pressure of the pneumatic holding valve is drained, the pneumatic holding valve is closed, and the output voltage of the positioner is closed The output signal pressure is locked in the diaphragm chamber of the pneumatic control valve, the output signal pressure is balanced with the reaction force generated by the control valve spring, and the valve position of the pneumatic control valve is kept in the fault position.

 

   3. Off signal: when the signal of the control system fails (loss of signal), after the loss of power (signal) comparator detects it, the voltage signal of the single electronic control solenoid directional valve is off, the single electronic control solenoid directional valve loses power, the slide valve in the single electronic control solenoid directional valve slides under the action of the return spring, the solenoid valve reverses, the membrane chamber pressure of the pneumatic holding valve is emptied, the pneumatic holding valve is closed, and the valve is set The output signal pressure of the positioner is locked in the diaphragm chamber of the pneumatic control valve, the output signal pressure is balanced with the reaction force generated by the control valve spring, and the valve position of the pneumatic control valve is kept in the fault position.

 

   position feedback signal is given by valve position signal return device.

 

   advantages of this scheme: when the "three break" protection is started, the system reacts quickly and acts quickly. The overall cost is relatively low.

 

   disadvantages of this scheme: the solenoid valve is charged for a long time, which affects its service life. With more accessories, installation and debugging are more complex. Valve position feedback needs to be equipped with valve position signal return device, which is more complex when equipped with hand wheel.

 

   II. Scheme of double acting pneumatic control valve (control valve equipped with electric pneumatic valve positioner)

 

   the scheme is mainly composed of control valve, pneumatic directional valve, positioner, self-locking valve, one-way valve, pressure reducing valve, gas storage tank, etc. Its working principle is as follows:

 

   when the air source of the control system fails (air loss), the self-locking valve (its action mode is opposite to that of the holding valve) opens automatically, cancels the control air source of the air control directional valve, the slide valve of the air control directional valve resets under the action of the spring, one of the two air control directional valves exhausts, the other enters, the check valve closes, and the air source is supplied to the valve from the air source stored in the air storage tank, so as to realize the air supply Now the valve is fully closed or fully opened. The conversion of full close or full open can be realized by adjusting the connection mode of pneumatic directional valve.

 

   if the valve is to be kept in position, the pneumatic valve should be added and the pipeline connection should be changed. The self-locking valve should be used to control the valve directly, and the pneumatic directional valve, one-way valve and air tank should be canceled.

 

   if the valve can operate several times when the gas source is cut off, the following scheme can be adopted.

 

   the scheme is composed of gas storage tank, one-way valve, locking valve, stop valve, etc. Its working principle is as follows:

 

   when the air source fails (air loss), the one-way valve closes and the lock valve loses air. Under the action of the spring, the slide valve of the lock valve resets and the air circuit reverses. The air source pipeline of the system is disconnected and the air tank pipeline is connected. The air tank supplies air to the valve. The valve acts several times to realize the purpose of continuous control. Due to the limited capacity of the gas tank, and the pressure of the gas source in the gas tank decreases with the action of the valve, the gas tank can not be used for a long time to supply gas to the valve. The capacity of the gas tank used in this scheme should be larger than that of the general protective gas tank. In this scheme, when the gas source is cut off, the number of valve actions is related to the capacity of the gas storage tank.

 

   there is another way to keep the position of pneumatic diaphragm control valve for reference: the positioner and the actuator are connected in series with a holding valve and a two position three-way solenoid valve. The holding valve is used to keep the position when the gas is cut off, and the solenoid valve is used to keep the position when the signal is cut off. However, the solenoid valve must be interlocked with the positioner (set in the control program), that is, when the positioner has a signal, the solenoid valve must be locked When there is power, once the positioner loses its signal, the solenoid valve must be powered off immediately.