Process quantities (level, flow, pressure etc) in plant need to be maintained at target value and a difference from these values could result in a dangerous condition arising or loss production.
To maintain these process quantities condition over an extended period of time without any operator involvement. Automatic control system is usually installed and used.
The element of these control system (loop) are:
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Transmitter: Process variable is measured by the primary element of the transmitter and the measured signal is converted to electric 4 to 20 mA signal.
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Controller: Controller receive a signal from the transmitter and compares this value with the set point and computes the output signal to achieved the set point.
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Final Control Element: Correction device in the control loop. Usually a valve