ISA CAP Control Systems & Instrumentation 3 — Questions and Answers
Question 1: In ISA S88 batch control, what is the role of a 'unit procedure'?
- An ordered set of unit operations that makes one type of product (Correct answer)
- A single equipment-level control action
- The highest level in the procedural control hierarchy
- A collection of equipment modules performing a function
Correct answer: An ordered set of unit operations that makes one type of product
A unit procedure is an ordered set of unit operations that directs a unit to make a batch of a specific product.
Question 2: What phenomenon occurs when integral action continues to accumulate during a period when the controller output is saturated?
- Derivative kick
- Integral windup (Correct answer)
- Process gain inversion
- Reset rate instability
Correct answer: Integral windup
Integral windup occurs when the integrator keeps accumulating error while the output is at its limit, causing sluggish response when the constraint is released.
Question 3: A differential pressure transmitter used for level measurement in a closed vessel requires:
- A single pressure tap at the vessel bottom only
- High-side connection at the bottom and low-side at the vapor space (Correct answer)
- Both taps at the same elevation with a reference column
- Only a static pressure measurement at the midpoint
Correct answer: High-side connection at the bottom and low-side at the vapor space
The high-side tap senses hydrostatic head plus vapor pressure, while the low-side tap cancels the vapor pressure, yielding a signal proportional only to liquid level.
Question 4: In FOUNDATION Fieldbus, what is the function of a 'function block'?
- A hardware module that provides power to field devices
- A software algorithm inside a device that performs a specific control or I/O task (Correct answer)
- A communication protocol layer for bus arbitration
- A diagnostic tool for detecting wiring faults
Correct answer: A software algorithm inside a device that performs a specific control or I/O task
Function blocks are standardized software algorithms (e.g., AI, PID, AO) that execute within fieldbus devices to implement control and I/O functions.
Question 5: Which of the following is the correct ISA symbology for a flow indicating controller on a P&ID?
- FT
- FIC (Correct answer)
- FC
- FE
Correct answer: FIC
FIC stands for Flow Indicating Controller, combining the measured variable (F), the indicating function (I), and the control function (C).
Question 6: What is the primary purpose of valve positioners in a control loop?
- To protect the actuator from excess supply pressure
- To ensure the valve stem position accurately corresponds to the controller output signal (Correct answer)
- To convert the process variable into a 4-20 mA signal
- To manually override the automatic control signal
Correct answer: To ensure the valve stem position accurately corresponds to the controller output signal
Positioners use feedback from the valve stem to force the actual position to match the commanded position, overcoming friction, hysteresis, and pressure imbalance.
Question 7: For an underdamped second-order process, which response characteristic increases as the damping ratio decreases below 1?
- Rise time increases and overshoot decreases
- Overshoot increases and oscillation frequency increases (Correct answer)
- Settling time decreases monotonically
- Gain margin improves significantly
Correct answer: Overshoot increases and oscillation frequency increases
As damping ratio decreases below 1, the system exhibits greater overshoot and higher-frequency oscillations in the transient response.
ISA CAP Control Systems & Instrumentation 3
In ISA S88 batch control, what is the role of a 'unit procedure'?