Plant Operator Selection System Test Process Control and Instrumentation 2 — Questions and Answers
Question 1: In process control, 'cascade control' means:
- Two controllers where the output of the primary sets the setpoint of the secondary (Correct answer)
- A controller that switches between manual and automatic modes
- Using three or more sensors for the same variable
- Bypassing the primary controller during upsets
Correct answer: Two controllers where the output of the primary sets the setpoint of the secondary
In cascade control, the primary (master) controller's output drives the setpoint of the secondary (slave) controller, improving disturbance rejection.
Question 2: A Safety Instrumented System (SIS) is designed to:
- Optimize process efficiency during normal operations
- Automatically bring the process to a safe state when a dangerous condition is detected (Correct answer)
- Replace the Basic Process Control System (BPCS)
- Monitor product quality only
Correct answer: Automatically bring the process to a safe state when a dangerous condition is detected
The SIS is an independent layer of protection that takes automatic action (shutdown, depressurization, etc.) when the BPCS fails to prevent a hazardous condition.
Question 3: Which instrument tag designation on a P&ID indicates a flow controller?
- FT
- FC (Correct answer)
- FI
- FS
Correct answer: FC
In ISA instrument tag notation, 'F' is the measured variable (flow) and 'C' is the function (controller), making FC a flow controller.
Question 4: Dead band in a control instrument refers to:
- The range over which input changes cause no output response (Correct answer)
- The maximum operating range of the instrument
- The time required for the instrument to warm up
- The signal range reserved for alarm functions
Correct answer: The range over which input changes cause no output response
Dead band is the input change range that produces no detectable change in output, often caused by mechanical friction or hysteresis in final control elements.
Question 5: When a process control loop is placed in 'manual' mode by the operator, it means:
- The controller automatically adjusts the output based on the setpoint
- The operator directly sets the controller output and the setpoint is ignored (Correct answer)
- The loop is bypassed entirely and the valve is de-energized
- Alarms for that loop are automatically suppressed
Correct answer: The operator directly sets the controller output and the setpoint is ignored
In manual mode, the operator fixes the controller output at a chosen value while the automatic setpoint-based calculation is suspended.
Question 6: A 4-20 mA current signal is used in process instrumentation primarily because:
- It is the only signal that can travel long distances
- It is less susceptible to electrical noise and a live zero detects broken wires (Correct answer)
- It requires no power supply at the transmitter
- It automatically compensates for temperature changes
Correct answer: It is less susceptible to electrical noise and a live zero detects broken wires
The 4-20 mA standard's live zero (4 mA = 0%) allows wire breaks (0 mA) to be distinguished from a valid zero-range reading, and current signals resist voltage noise pickup.
Plant Operator Selection System Test Process Control and Instrumentation 2
In process control, 'cascade control' means: