A. Convert the valve controller pneumatic output signal into a mechanical force to position the valve.
B. Convert the valve controller pneumatic output signal into an electrical output to position the valve.
C. Compare the valve controller pneumatic output signal to the valve position, and adjust the valve actuator air supply pressure to position the valve.
D. Compare the valve controller pneumatic output signal to the setpoint error, and adjust the valve actuator air supply pressure to position the valve.
Show Answer
Answer: C
Share your understanding of this question with the correct explanation.
In a typical pneumatic valve control system, a valve positioner is used to compare the valve controller’s pneumatic output signal to the actual position of the valve. It then adjusts the air supply pressure to the valve actuator as needed to position the valve accordingly.
Here’s a bit more information on each of the options:
Option A: This option is partially correct as a valve positioner does convert a control signal into a mechanical force. However, it also performs the additional task of comparing the valve’s actual position with the control signal and adjusting as necessary, which makes this option incomplete.
Option B: This is incorrect because in a typical pneumatic valve control system, a valve positioner doesn’t convert a pneumatic signal into an electrical output. The communication is typically pneumatic, not electrical.
Option C: This is the correct answer. The valve positioner compares the control signal (which represents the desired valve position) with the valve’s actual position. If there’s a difference, the positioner changes the supply pressure to the valve actuator to move the valve to the desired position.
Option D: This is incorrect. A valve positioner doesn’t compare the control signal to the setpoint error. The job of comparing the process variable to the setpoint and calculating an error is typically performed by a process controller, not a valve positioner. The positioner only compares the controller’s output signal (a result of that error calculation) to the actual valve position.
