CCST Control System Design & Architecture 5 — Questions and Answers
Question 1: When designing a control system for a high-vibration environment, which signal transmission method is LEAST susceptible to mechanical vibration-induced errors?
- 4–20 mA current loop (Correct answer)
- 0–10 V DC voltage signal
- Pneumatic 3–15 psi signal
- Thermocouple millivolt signal
Correct answer: 4–20 mA current loop
4–20 mA current loops are immune to resistive voltage drops from long cable runs and are far less affected by vibration-induced connector resistance changes than low-level voltage or millivolt signals.
Question 2: A control engineer is designing a gas compressor surge control system. What is the primary purpose of surge control?
- Maximizing compressor throughput at all times
- Preventing flow reversal through the compressor that causes damaging oscillations (Correct answer)
- Regulating discharge temperature within limits
- Balancing load between parallel compressor trains
Correct answer: Preventing flow reversal through the compressor that causes damaging oscillations
Surge control keeps compressor operation to the right of the surge line by recycling flow when needed, preventing destructive flow reversal and mechanical damage.
Question 3: In IEC 61511 (functional safety for the process industry), what is a 'protection layer' in the context of a Layer of Protection Analysis (LOPA)?
- A physical barrier such as a blast wall or dike
- An independent safeguard that can prevent a hazardous event without relying on other protection layers (Correct answer)
- A redundant sensor used to validate a process measurement
- A network firewall separating the safety system from the DCS
Correct answer: An independent safeguard that can prevent a hazardous event without relying on other protection layers
A protection layer (or independent protection layer, IPL) in LOPA is an independent safeguard — such as a BPCS, SIS, or relief valve — that reduces risk without depending on the failure of other layers.
Question 4: What design practice helps prevent 'alarm flood' during a process upset in an advanced alarm management system?
- Setting all alarms to the same priority level
- Implementing alarm suppression, shelving, and state-based alarming to present only actionable alarms (Correct answer)
- Increasing the deadband on all analog alarms to the maximum allowable value
- Removing alarms that have not triggered in the past year
Correct answer: Implementing alarm suppression, shelving, and state-based alarming to present only actionable alarms
Techniques such as alarm suppression, shelving, and state-based alarming rationalize alarm presentation so operators receive only relevant, actionable alerts during upsets, in compliance with EEMUA 191 / ISA-18.2.
Question 5: A control valve is specified with an equal-percentage inherent flow characteristic. What does this characteristic describe?
- Flow increases linearly with valve travel
- Equal increments of valve travel produce equal percentage changes in flow (Correct answer)
- Flow is constant regardless of pressure drop
- The valve passes equal flow in both directions
Correct answer: Equal increments of valve travel produce equal percentage changes in flow
Equal-percentage valves produce the same percentage change in Cv (flow coefficient) for each equal increment of stem travel, providing better rangeability for applications with variable pressure drop.
Question 6: In a SCADA system architecture, what is the role of the Remote Terminal Unit (RTU)?
- To serve as the central database for all process data
- To collect field data and execute basic control at remote unmanned sites, communicating to the master station (Correct answer)
- To provide the human-machine interface for local operators
- To manage cybersecurity policies across the control network
Correct answer: To collect field data and execute basic control at remote unmanned sites, communicating to the master station
An RTU acquires I/O from field devices at a remote site, may execute local control logic, and communicates process data to and from the SCADA master station over wide-area communication links.
Question 7: Which design approach is used in 'model predictive control' (MPC) that distinguishes it from conventional PID control?
- MPC uses a fixed proportional gain regardless of process conditions
- MPC uses a dynamic process model to predict future outputs and optimize manipulated variables over a receding time horizon (Correct answer)
- MPC eliminates the need for any feedback measurement
- MPC applies control action only when the error exceeds a threshold
Correct answer: MPC uses a dynamic process model to predict future outputs and optimize manipulated variables over a receding time horizon
MPC solves an optimization problem at each control interval using a dynamic process model to predict future behavior and select manipulated variable moves that minimize a cost function over a future prediction horizon.
CCST Control System Design & Architecture 5
When designing a control system for a high-vibration environment, which signal transmission method is LEAST susceptible to mechanical vibration-induced errors?