On the job, you are sent to troubleshoot a brand-new control system, consisting of a pneumatic liquid level transmitter connected to a pneumatic controller, which in turn drives a pneumatic control valve. The process vessel, piping, control valve, controller, and level transmitter are all brand-new: they even sport a fresh coat of paint.
According to the unit operator, this level control system has never worked. As she shows you, the liquid level inside the vessel is so low that the level gauge (LG) registers empty, yet the controller is commanding the valve 100% open, which of course continues to drain the vessel and prevent any liquid level from accumulating.
Being versed in process control theory, you decide to check how the controller is configured. Looking inside the controller case, you notice the controller is set for direct action: an increasing PV results in an increasing output signal (MV), which will move the air-to-close valve more toward the “closed” state.
Realizing how to fix the problem, you reach inside the controller and move a lever that switches it into reverse action mode.
Explain why this fixes the problem.
More Questions :
- Explain the significance of the “newness” of this process. How would your assumptions differ if you saw this process vessel was old and rusted instead of shiny-new?
- How do you suppose the controller got to be mis-configured in the first place?
- What would have to be different in this control system to permit a direct-acting controller instead of a reverse-acting controller?
- Suppose you did not discover the controller’s action set for direct action. If the controller had been left in manual mode instead of automatic mode, could this account for the problems exhibited by this system?
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