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Practical Process Control System Questions & Answers – 7

There is a problem somewhere in this liquid flow control system. The controller is in automatic mode, with a setpoint of 65%, yet the flow indicator and the flow controller both register 0.3%: (nearly) zero flow. A P&ID of the loop appears here:

liquid flow control system

Explain how you would begin troubleshooting this system, and what possible faults could account for the controller not being able to maintain liquid flow at setpoint.

More Question

  1. Explain how you could divide this control system into distinct areas or zones which you may then begin to refer to when “dividing and conquering” the problem.

Answer :

One possible fault has to do with the control valve: perhaps something has happened to make it fail closed (loss of air supply, signal, etc.). Other possible problems include the following:

  • Pump not running (no source of fluid power to motivate flow)
  • Very poor controller tuning
  • Wrong controller action
  • Valve failed closed (loss of air supply, signal, etc.)
  • Transmitter failed, showing no flow when in fact there is

A good “first test” for troubleshooting the loop is to check the controller output: is it trying to open up the valve?

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