CCST Control System Design & Architecture 4 — Questions and Answers
Question 1: When performing a hazard and operability study (HAZOP), what do the guide words 'MORE OF' and 'LESS OF' primarily evaluate?
- Equipment reliability and maintainability
- Deviations in process parameter quantities such as flow, pressure, or temperature (Correct answer)
- Communication errors between control system nodes
- Instrumentation calibration drift
Correct answer: Deviations in process parameter quantities such as flow, pressure, or temperature
HAZOP guide words like 'MORE OF' and 'LESS OF' prompt the team to consider quantitative deviations in process parameters and their potential hazardous consequences.
Question 2: What is the function of a 'split-range' control strategy?
- Splitting the controller output signal to operate two or more final control elements across different portions of the output range (Correct answer)
- Dividing a large control loop into two independent feedback loops
- Using two controllers to share responsibility for one process variable
- Switching between manual and automatic modes based on process conditions
Correct answer: Splitting the controller output signal to operate two or more final control elements across different portions of the output range
Split-range control sends portions of a single controller output to multiple actuators (e.g., 0–50% opens a cooling valve; 50–100% opens a heating valve) to cover the full control range.
Question 3: According to ISA-5.1, what shape symbol represents a controller function that is located in the primary instrument panel (board-mounted, accessible to the operator)?
- A circle with no additional markings
- A circle with a single horizontal line through the middle (Correct answer)
- A circle with a double horizontal line (dashed) through the middle
- A square surrounding a circle
Correct answer: A circle with a single horizontal line through the middle
ISA-5.1 uses a circle with a single horizontal line to denote instruments mounted in the main control panel (accessible to the operator), while a dashed line indicates a DCS/shared display function.
Question 4: In a DCS environment, what is 'controller redundancy' and what is its primary benefit?
- Having two operators monitor the same loop
- Duplicating the controller hardware so a backup takes over automatically if the primary fails (Correct answer)
- Using two independent sensors to validate each process measurement
- Running two separate control algorithms and averaging their outputs
Correct answer: Duplicating the controller hardware so a backup takes over automatically if the primary fails
Controller redundancy provides a hot-standby backup controller that assumes control seamlessly if the primary controller fails, minimizing process downtime.
Question 5: A process has a very long dead time relative to its time constant. Which control strategy is MOST effective for handling this characteristic?
- Proportional-only control to avoid integral windup
- Smith predictor or dead-time compensator added to a PID controller (Correct answer)
- Derivative-heavy tuning to anticipate future error
- Decreasing controller gain to improve stability margins
Correct answer: Smith predictor or dead-time compensator added to a PID controller
The Smith predictor uses a process model to compensate for dead time, effectively allowing the controller to act on predicted future process output rather than delayed feedback.
Question 6: What is the significance of the 'minimum detectable signal' (MDS) specification for an analog input card in a DCS?
- The maximum input voltage the card can accept without damage
- The smallest change in input signal that the card can reliably distinguish from noise (Correct answer)
- The time required for the card to scan all connected inputs
- The number of engineering units the card can display
Correct answer: The smallest change in input signal that the card can reliably distinguish from noise
MDS (also called resolution) defines the smallest meaningful input change the card can detect, which determines measurement precision and affects control performance.
Question 7: In the context of ISA-88 batch control, what is a 'recipe parameter'?
- A fixed equipment setting that never changes between batches
- A variable value (e.g., temperature setpoint or ingredient quantity) that can differ between batch runs (Correct answer)
- The name assigned to a unit procedure
- A network address used by the batch server
Correct answer: A variable value (e.g., temperature setpoint or ingredient quantity) that can differ between batch runs
Recipe parameters are configurable values within a master recipe that operators can adjust for each batch run, such as reaction temperature, hold time, or ingredient amounts.
CCST Control System Design & Architecture 4
When performing a hazard and operability study (HAZOP), what do the guide words 'MORE OF' and 'LESS OF' primarily evaluate?