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What are the various positioning solutions for regulating valve failures? The role of regulating valves in process control is well known; in many control processes, it is necessary for these valves to remain in a certain position in the event of a failure, so as to prevent accidents in the process. This requires that regulating valves be designed with fail-safe measures involving three types of disconnection: disconnection of air supply, disconnection of power supply, and disconnection of signals. For electric control valves, it is relatively simple: when the signal is interrupted, they can remain in one of the positions of fully open, fully closed, or in their current position, depending on the settings of the control module. In the event of a power loss, they naturally stay in the fault position; however, electric actuators equipped with a reset mechanism can also move the valve to either the fully open or fully closed position. The situation is more complex for pneumatic control valves, so we will mainly discuss the three cut-off position retention methods for such valves. Generally, when selecting a pneumatic diaphragm control valve, we first need to decide whether it should operate in the open or closed position when there is no air supply – this determines the protective position of the valve in such conditions. If the process requirements dictate that the valve should remain open when there is no air supply, then an normally open (air-closed) control valve is chosen; otherwise, a normally closed (air-open) control valve is selected. This is only a preliminary plan; if the process requirements call for triple protection against loss of air, power, and signal, then the control valve needs to be equipped with certain accessories to form a protection system in order to meet the control requirements. These accessories mainly include position control valves, solenoid valves, air tanks, etc. The following are the two position-holding solutions for single-acting pneumatic diaphragm control valves and double-acting pneumatic control valves. I. Pneumatic diaphragm control valve solution (control valve equipped with an electro-pneumatic valve positioner) This solution mainly consists of a pneumatic control valve, an electro-pneumatic valve positioner, a power loss (signal) comparator, a single-electrode solenoid directional valve, a pneumatic hold-back valve, and a valve position signal returner. Its working principle is as follows: 1. Cut-off of air supply: When there is a failure in the air supply for the control system (loss of air pressure), the pneumatic hold-down valve closes automatically, locking the output signal pressure of the positioner within the diaphragm chamber of the pneumatic control valve. The output signal pressure then balances with the counterforce generated by the spring of the control valve, causing the valve position to remain at the faulty position. This pressure holding valve should be set to activate at a value slightly below the minimum of the air supply. 2. Power cutoff: When there is a power failure in the control system, the power-loss signal comparator causes the output voltage of the single-electrovalve solenoid directional control valve to disappear. As a result, the solenoid directional control valve loses power; the spool inside it slides under the action of the return spring, causing the solenoid valve to change direction. This leads to the evacuation of the pressure in the diaphragm chamber of the pneumatic holding valve, which then closes. The output signal pressure is thus locked within the diaphragm chamber of the pneumatic control valve, where it balances out the counterforce generated by the spring of the control valve. As a consequence, the position of the pneumatic control valve remains at the fault position. 3. Signal interruption: When there is a fault in the control system signal (loss of signal), the power-loss detector detects this and cuts off the voltage signal to the single-electrovalve solenoid directional control valve. As a result, the solenoid valve loses power; the spool inside it slides under the action of the return spring, causing the valve to change direction. This action empties the pressure in the diaphragm chamber of the pneumatic holding valve, which then closes. The output signal pressure is thus locked within the diaphragm chamber of the pneumatic control valve, where it balances out the counterforce generated by the valve’s spring. As a consequence, the position of the pneumatic control valve remains at the faulty position. The position feedback signal is provided by the valve position signal returner. Advantages of this scheme: When the \"three-break\" protection is activated, the system responds quickly and acts promptly. The overall cost is relatively low. Disadvantage of this solution: The solenoid valve remains energized for an extended period, which affects its service life. It comes with many accessories, making installation and debugging more complex; a valve position signal returner is required for valve position feedback, and things become even more complicated when a handwheel is used. II. Double-acting pneumatic control valve solution (control valve equipped with electric-pneumatic valve actuator) This solution mainly consists of a control valve, a pneumatically controlled directional valve, an actuator, a self-locking valve, a check valve, a pressure reducing valve, and an air reservoir. Its working principle is as follows: When there is a failure in the air supply for the control system (loss of air pressure), the self-locking valve (which operates in the opposite manner to a hold-in valve) opens automatically, thereby cutting off the control air supply to the air-controlled directional valve. The spool of this valve returns to its original position under the action of springs; one of the two air-controlled directional valves releases air while the other takes in air, and the check valve closes. Air is supplied to the valve from the air stored in the air tank, thus enabling the valve to be fully closed or fully opened. Switching between fully closed and fully open positions can be achieved by adjusting the connection method of the pneumatically controlled directional valve. To achieve valve positioning, a pneumatic positioning valve should be installed and the piping connections adjusted; the positioning valve can be controlled directly by a self-locking valve, eliminating the need for a pneumatically controlled directional valve, check valve, and air reservoir. To ensure that the valve operates several times in the event of a gas supply interruption, the following approach can be adopted. This system consists of a gas storage tank, check valves, lock valves, stop valves, etc. Its working principle is as follows: When there is a failure in the air supply (loss of air pressure), the check valve closes. The locking valve also loses air pressure, and its spool returns to its original position under the action of a spring. This causes the air flow direction to change, thereby disconnecting the system’s air supply line and connecting it to the air reservoir line. Air is then supplied from the reservoir to the valve, ensuring that it can operate several times and thus enabling continuous control. Due to the limited capacity of the air storage tank, and the fact that the pressure of the air within it keeps dropping as the valve operates, it is not possible to use the air storage tank to supply air to the valve over a long period of time. The capacity of the air storage tank used in this system should be larger than that of a conventional air storage tank for protection purposes. In this system, the number of times the valve operates when the air supply is cut off depends on the capacity of the air storage tank. Another option that can be considered for the position-holding mechanism of pneumatic diaphragm control valves is to connect a position-holding valve and a two-position three-way solenoid valve in series between the actuator and the positioner. The position-holding valve is used to maintain the position when there is no air supply, while the solenoid valve takes over this function when the signal is lost. However, the solenoid valve must be interlocked with the positioner (as specified in the control program); that is, as long as the positioner receives a signal, the solenoid valve must be powered, and once the positioner loses its signal, the solenoid valve must immediately lose power.