Double-acting pneumatic control valve holding position

In industrial automation control, pneumatic control valves sometimes require fail-safe positioning. Below are two methods for fail-safe positioning of double-acting pneumatic control valves for your reference. 

   1. This scheme mainly consists of a pneumatic control valve, solenoid valve, positioner, fail-safe valve, check valve, diaphragm pump pressure reducing valve, and air storage tank. Its working principle is as follows: When the control system experiences an air source failure (loss of air), the fail-safe valve (whose action is opposite to that of a fail-safe valve) automatically opens, withdrawing the control air source from the solenoid valve. The spool valve of the solenoid valve resets under the action of the spring, with one of the two solenoid valves exhausting and the other admitting air. The check valve closes, and the air source is supplied to the valve from the air stored in the air storage tank, thus achieving full closure or full opening of the valve. The transition between full closure and full opening can be achieved by adjusting the connection method of the pneumatic directional control valve. To achieve valve positioning, install a pneumatic fail-safe valve and change the pipeline connection, using a self-locking valve to directly control the fail-safe valve, solenoid valve, check valve, and air storage tank.

    2. To ensure that the valve can operate multiple times when the gas source is cut off, the following scheme can be adopted for the regulating valve. This scheme consists of a gas storage tank, a check valve, a latch valve, a stop valve, etc. Its working principle is as follows: When the gas source fails (loses gas), the check valve closes, the latch valve loses gas, and the spool valve of the latch valve resets under the action of the spring. The gas path reverses, disconnecting the system's gas source pipeline and connecting the gas storage tank pipeline. The gas storage tank supplies gas to the valve to ensure that the valve can operate multiple times and achieve continuous control. Due to the limited capacity of the gas storage tank and the continuous decrease in gas source pressure in the tank as the valve operates, the gas storage tank cannot be used for long-term gas supply to the valve. The capacity of the gas storage tank used in this scheme should be larger than that of a general protective gas storage tank. In this scheme, the number of valve operations during gas source cut-off is related to the capacity of the gas storage tank.

    For the holding scheme of pneumatic diaphragm control valves, there is another option for reference: a pneumatic control valve is connected in series between the positioner and the actuator, with one holding valve and one two-position three-way solenoid valve each. When there is a loss of air supply, the holding valve is used for holding; when there is a loss of signal, the solenoid valve is used for holding. However, the solenoid valve must be interlocked with the positioner (set in the control program), meaning that when the positioner has a signal, the solenoid valve must be powered on, and when the positioner loses the signal, the solenoid valve must be immediately powered off.