Process Control and Instrumentation Flashcards
6 cards from real Plant Operator Selection System Test practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Process Control and Instrumentation flashcards as text
A thermocouple measures temperature by:
Answer: Generating a small voltage at the junction of two dissimilar metals
Thermocouples produce a millivolt signal (Seebeck effect) at the junction of two different metals that varies predictably with temperature.
In a PID controller, the 'I' (integral) term is used to:
Answer: Eliminate steady-state offset by accumulating past error over time
The integral term sums past errors over time and applies a correction that eliminates the steady-state offset left by proportional control alone.
A control valve with an air-to-open (ATO) fail action will:
Answer: Close fully when instrument air pressure is lost
Air-to-open (also called fail-closed) valves close when air supply is lost, which is the safe failure mode for many process streams.
What does a differential pressure (DP) transmitter measure when installed across an orifice plate?
Answer: Flow rate based on the pressure drop across the restriction
Flow through an orifice creates a pressure drop proportional to the square of the flow rate, which the DP transmitter converts to a flow signal.
On a Process and Instrumentation Diagram (P&ID), a dashed line connecting an instrument to a control element typically represents:
Answer: An electrical or pneumatic signal
Dashed lines on P&IDs represent signal lines (electrical, pneumatic, or digital) that transmit information between instruments and controllers.
A level transmitter in a tank reads higher than the actual level. The most likely cause is:
Answer: A plugged or blocked impulse line on the low-pressure side
A blocked low-pressure impulse line traps pressure, reducing the differential and making the instrument report a falsely high level.
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Sources & references
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