Signal transmission issues with pneumatic control valves

A positioner is installed on the pneumatic control valve; the DCS outputs an analog signal based on changes in speed to control the positioner, which in turn controls the control valve

The pneumatic valve positioner is used in conjunction with pneumatic diaphragm actuators, and it is a key component of the pneumatic control valve assembly. A pneumatic analog signal of 0.02>0.10 MPa is output from the pneumatic remote control panel; this signal is then processed by a pneumatic valve positioner, which generates a pneumatic actuation signal to drive the actuator into action. This mechanism is used to control the travel of the pneumatic control valve. Through feedback on the valve’s position, the control signal output by the pneumatic remote control panel corresponds proportionally to the valve’s travel, thereby ensuring accurate positioning. Working principle: The pneumatic valve positioner operates on the principle of torque balance. When the signal pressure P1 acting on the diaphragm 2 increases, it causes the main lever 3 to rotate around its pivot point, bringing the nozzle baffle 9 closer to the nozzle. The back pressure at the nozzle is amplified by the one-way amplifier 8, which in turn increases the pressure in the actuator’s diaphragm chamber, causing the valve stem to move downward. This causes the feedback rod to rotate around the pivot, and the feedback cam also rotates in a counterclockwise direction. The roller enables the secondary lever 4 to rotate around the pivot, thereby stretching the feedback spring. When the force exerted by the spring on the primary lever 3 is in torque balance with the force applied to the bellows by the signal pressure, the instrument reaches a state of equilibrium. The valve position of the actuator is maintained at a certain opening; a specific signal pressure corresponds to a specific valve opening degree. Solenoid valves can be driven by contactors; of course, the contactor must be connected in series with the relay’s contacts. Since the load-carrying capacity of most DCS modules is not sufficient to drive solenoid valves, a relay is used as an intermediary. The power supply for the relay can come from the DCS module (provided that the module has enough power capacity), or else a 24V power source can be used, but it must be connected to the DCS’s DO terminals. When it is necessary to shut off the solenoid valve, the DCS sends an output signal, which causes the relay coil to become energized and its contacts to close. This in turn causes the contactor to become energized, thereby activating the solenoid valve. It’s that simple

DcS does not collect the on-site opening value; the locator is equipped with a tube