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Practical Process Control System Questions & Answers – 6

This water filter level control system uses an ultrasonic level transmitter to sense the level of water in the filter, and a controller to drive a motor-actuated valve introducing raw water to be filtered:

water filter level control system

Assuming a direct-acting level transmitter (increasing filter level = increasing signal), and a signal-toopen control valve (increasing controller output signal = wider open valve), determine whether the level controller needs to be configured for direct-action or reverse-action, and explain your reasoning. Annotate the diagram with “+” and “−” symbols next to the PV and SP controller inputs to show more explicitly the relationships between the controller inputs and output.

Next, determine the response of the controller to the following situations. In other words, determine what the controller’s output signal will do when this water level control system is affected in the following ways:

  • A sudden increase in effluent flow rate (clean water demand)
  • Level transmitter fails high (indicating 100% full water level)
  • Control valve actuator fails, driving valve fully open (ignoring controller signal)

More Questions :

  1. Re-draw the diagram for this water filter level control system, replacing the controller (circle) with an op-amp symbol (triangle), determining the “+” and “−” input assignments on the opamp for PV and SP.
  2. Explain why level control is important in a water filter such as this.
  3. What do the “H” and “L” symbols near the LIR represent?

Answer :

This controller needs to be reverse-acting:

water filter level control system - 1

This re-drawing of the control system uses an opamp symbol in place of the ISA-standard circle used to represent a loop controller:

water filter level control system - 2

  • A sudden increase in effluent flow rate (clean water demand): controller output increases
  • Level transmitter fails high (indicating 100% full water level): controller output decreases
  • Control valve actuator fails, driving valve fully open (ignoring controller signal): controller output decreases

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