ISA CAP Foundational Concepts & Principles 3 — Questions and Answers
Question 1: What distinguishes a 'cascade control' system from a simple feedback loop?
- Cascade uses two setpoints operating independently on the same variable
- An outer (primary) controller sets the setpoint of an inner (secondary) controller (Correct answer)
- Cascade control eliminates the need for derivative action
- The secondary loop measures a disturbance rather than a process variable
Correct answer: An outer (primary) controller sets the setpoint of an inner (secondary) controller
In cascade control, the primary controller's output becomes the setpoint for the secondary controller, allowing the inner loop to reject disturbances faster than the outer loop could alone.
Question 2: Which of the following is the correct hierarchy in a Purdue Reference Model (ISA-95) from bottom to top?
- Field devices → Control → MES → ERP (Correct answer)
- ERP → MES → Control → Field devices
- Control → Field devices → MES → ERP
- Field devices → MES → Control → ERP
Correct answer: Field devices → Control → MES → ERP
The Purdue model layers go from Level 0 (field devices/sensors) through Level 1 (control), Level 2 (supervisory), Level 3 (MES/operations), to Level 4 (ERP/business planning).
Question 3: In the context of ISA-88 batch control, what is a 'recipe'?
- The physical equipment module that executes a process step
- The complete set of information needed to produce a specific batch of product (Correct answer)
- A SCADA screen template for operator interaction
- A validated sequence of safety interlocks
Correct answer: The complete set of information needed to produce a specific batch of product
Per ISA-88, a recipe contains the procedure, formula, equipment requirements, and other information required to manufacture one batch of a particular product.
Question 4: A pressure transmitter has an output of 4–20 mA with a calibrated range of 0–100 psi. What current corresponds to a process pressure of 50 psi?
- 8 mA
- 10 mA
- 12 mA (Correct answer)
- 14 mA
Correct answer: 12 mA
50 psi is 50% of the 100 psi span, and 50% of the 16 mA output span (4–20 mA) added to the live zero gives 4 + 8 = 12 mA.
Question 5: Which control mode is most appropriate for a process with a very slow response time and no need to eliminate steady-state offset quickly?
- Derivative-only control
- Proportional-only control (Correct answer)
- Pure integral control
- PID with high derivative gain
Correct answer: Proportional-only control
Proportional-only control is suitable for slow processes where a small residual offset is acceptable and where integral action could cause excessive overshoot or instability.
Question 6: What does the term 'turndown ratio' mean for a flow measurement device?
- The ratio of maximum to minimum measurable flow within the device's specified accuracy (Correct answer)
- The pressure drop across the device at maximum flow
- The time required to respond to a step change in flow
- The angle at which the device must be installed
Correct answer: The ratio of maximum to minimum measurable flow within the device's specified accuracy
Turndown ratio (also called rangeability) is the ratio of the maximum flow rate to the minimum flow rate that the device can measure within its stated accuracy limits.
Question 7: In control valve sizing, the variable Cv represents:
- The velocity of flow through the valve seat at rated conditions
- The flow coefficient indicating how many gallons per minute of water flow through the valve at 1 psi differential (Correct answer)
- The pressure recovery factor of the valve body
- The ratio of actual flow to theoretical maximum flow
Correct answer: The flow coefficient indicating how many gallons per minute of water flow through the valve at 1 psi differential
Cv is the flow coefficient defined as the number of US gallons per minute of 60°F water that will flow through the valve with a 1 psi pressure drop across it.
ISA CAP Foundational Concepts & Principles 3
What distinguishes a 'cascade control' system from a simple feedback loop?