Which Instrument is Faulty? Transmitter, Recorder, or Controller

In this process, steam is introduced into “stripping” vessel C-7 to help remove volatile sulfur compounds from “sour” water.

The temperature of the stripped gases exiting the tower’s top is controlled by a pneumatic temperature control loop. Unfortunately, this loop seems to have a problem.

Which instrument is faulty?

The temperature indicating recorder TIR-21 registers 304 degrees Fahrenheit, while the temperature indicating controller TIC-21 registers 285 degrees Fahrenheit. The calibrated range of TT-21 is 100 to 350 degrees Fahrenheit.

A technician connects a test gauge to the pneumatic signal line and reads a pressure of 12.8 PSI:

Which instrument is faulty: the transmitter, the recorder, or the controller, or is it impossible to tell from what little information is given here?

Explanation

We know the indicating controller (TIC-21) must be miscalibrated because the pneumatic signal pressure of 12.8 PSI agrees with the recorder’s indication of 304 degrees F.

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Credits: Tony R. Kuphaldt