SACA Process Control and Instrumentation 4 — Questions and Answers
Question 1: In a cascade control system, which controller receives the process variable from the field sensor?
- Primary (master) controller
- Secondary (slave) controller (Correct answer)
- Feed-forward controller
- Ratio controller
Correct answer: Secondary (slave) controller
In cascade control, the secondary (slave) controller measures the inner loop process variable and receives its setpoint from the primary controller.
Question 2: What is the primary purpose of a dead band setting in a process controller?
- Increase controller gain
- Prevent hunting around the setpoint (Correct answer)
- Speed up the integral action
- Eliminate derivative kick
Correct answer: Prevent hunting around the setpoint
Dead band creates a neutral zone around the setpoint where no corrective action is taken, preventing excessive valve cycling or 'hunting.'
Question 3: An orifice plate differential pressure transmitter reads zero even though flow is present. Which fault is MOST likely?
- High-side impulse line plugged
- Low-side impulse line plugged
- Both impulse lines are open and equalized (Correct answer)
- Transmitter span is set too high
Correct answer: Both impulse lines are open and equalized
If both impulse lines are open and equalized (e.g., isolation valves open and equalizer valve also open), the DP transmitter sees zero differential pressure.
Question 4: Which valve characteristic provides equal percentage of remaining flow change per equal increment of valve travel?
- Quick-opening
- Linear
- Equal percentage (Correct answer)
- Parabolic
Correct answer: Equal percentage
An equal percentage valve changes flow by a fixed percentage of its current flow rate for each unit of stem travel, giving better rangeability at low flows.
Question 5: A HART communicator is connected to a 4-20 mA loop. What is the carrier frequency of the digital HART signal superimposed on the analog signal?
- 50 Hz
- 1200 Hz (Correct answer)
- 9600 Hz
- 19200 Hz
Correct answer: 1200 Hz
HART uses Bell 202 FSK at 1200 baud (1200 Hz for mark, 2200 Hz for space) superimposed on the 4-20 mA signal.
Question 6: Which control mode is BEST suited for a process where offset cannot be tolerated but fast disturbance rejection is not critical?
- Proportional-only (P)
- Proportional-Integral (PI) (Correct answer)
- Proportional-Derivative (PD)
- On-off control
Correct answer: Proportional-Integral (PI)
PI control eliminates steady-state offset through integral action while providing proportional response, making it ideal when precise setpoint tracking matters more than speed.
Question 7: In ISA S5.1 instrumentation symbology, a circle with a horizontal line through the middle drawn on a P&ID represents an instrument located:
- In the field, accessible
- On the main control panel (front)
- Behind the main control panel (Correct answer)
- In an auxiliary cabinet
Correct answer: Behind the main control panel
Per ISA S5.1, a circle with a horizontal line through it indicates an instrument mounted behind (or on the back of) the main control panel, not accessible to operators.
SACA Process Control and Instrumentation 4
In a cascade control system, which controller receives the process variable from the field sensor?