CCST Control System Design & Architecture 3 — Questions and Answers
Question 1: Which document type formally defines the measurement ranges, alarm setpoints, and control modes for every instrument in a plant?
- Process Flow Diagram (PFD)
- Instrument Index / Instrument Data Sheet (Correct answer)
- Piping and Instrumentation Diagram (P&ID)
- Cause and Effect Matrix
Correct answer: Instrument Index / Instrument Data Sheet
Instrument data sheets capture all engineering specifications for each tagged instrument, including range, calibration, alarm limits, and materials of construction.
Question 2: In control system design, what is 'control loop interaction' (coupling) and why is it a concern?
- Electrical noise coupling between two signal cables
- The unwanted influence one control loop has on another's controlled variable (Correct answer)
- Mechanical vibration transferred between actuators
- Communication latency between controllers on a network
Correct answer: The unwanted influence one control loop has on another's controlled variable
Loop interaction occurs in multi-variable processes where adjusting one manipulated variable also affects another loop's controlled variable, complicating tuning and stability.
Question 3: A safety instrumented system (SIS) is designed to achieve SIL 2. Approximately what probability of failure on demand (PFD) range does SIL 2 require?
- ≥10⁻¹ to <10⁰
- ≥10⁻³ to <10⁻² (Correct answer)
- ≥10⁻⁵ to <10⁻⁴
- ≥10⁻⁷ to <10⁻⁶
Correct answer: ≥10⁻³ to <10⁻²
SIL 2 corresponds to a PFD range of 10⁻³ to 10⁻² (average probability of failure on demand between 1 in 1,000 and 1 in 100) per IEC 61508/61511.
Question 4: What is the difference between 'fail-safe' and 'fail-secure' in control system design?
- Fail-safe moves to a safe process state; fail-secure maintains the last known state (Correct answer)
- Fail-safe and fail-secure are synonymous terms
- Fail-safe applies to pneumatic valves only; fail-secure applies to electric actuators
- Fail-secure de-energizes all outputs; fail-safe energizes all outputs
Correct answer: Fail-safe moves to a safe process state; fail-secure maintains the last known state
Fail-safe drives the system to a predetermined safe condition (e.g., valve opens or closes), while fail-secure prioritizes maintaining the secured/locked state to prevent unauthorized access or process upset.
Question 5: In a feedforward control strategy, the feedforward signal is derived from:
- The error between setpoint and process variable
- A measured disturbance variable that is expected to affect the process (Correct answer)
- The derivative of the process variable
- The output of a secondary (inner) control loop
Correct answer: A measured disturbance variable that is expected to affect the process
Feedforward control measures a disturbance before it affects the controlled variable and applies a corrective action predictively, rather than waiting for error to develop.
Question 6: Which network topology provides the highest redundancy for a plant-wide control network but requires the most cable?
- Star topology
- Ring topology
- Mesh topology (Correct answer)
- Bus topology
Correct answer: Mesh topology
A fully meshed topology connects every node to every other node, providing multiple redundant paths but requiring significantly more cable than star or ring topologies.
Question 7: A batch recipe calls for adding ingredient A only after the reactor temperature has reached 80°C. This sequential condition is best implemented using which design element?
- PID feedback controller with setpoint ramp
- Interlocking logic or sequential function chart (SFC) step transition (Correct answer)
- Ratio control between ingredient flows
- Override (select) control strategy
Correct answer: Interlocking logic or sequential function chart (SFC) step transition
Sequential function charts (SFCs) or interlock logic define transitions between steps, ensuring ingredient A addition only begins when the temperature condition is satisfied.
CCST Control System Design & Architecture 3
Which document type formally defines the measurement ranges, alarm setpoints, and control modes for every instrument in a plant?