Process Control and Instrumentation Flashcards
7 cards from real SACA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Process Control and Instrumentation flashcards as text
In a cascade control system, which controller receives the process variable from the field sensor?
Answer: Secondary (slave) controller
In cascade control, the secondary (slave) controller measures the inner loop process variable and receives its setpoint from the primary controller.
What is the primary purpose of a dead band setting in a process controller?
Answer: Prevent hunting around the setpoint
Dead band creates a neutral zone around the setpoint where no corrective action is taken, preventing excessive valve cycling or 'hunting.'
An orifice plate differential pressure transmitter reads zero even though flow is present. Which fault is MOST likely?
Answer: Both impulse lines are open and equalized
If both impulse lines are open and equalized (e.g., isolation valves open and equalizer valve also open), the DP transmitter sees zero differential pressure.
Which valve characteristic provides equal percentage of remaining flow change per equal increment of valve travel?
Answer: Equal percentage
An equal percentage valve changes flow by a fixed percentage of its current flow rate for each unit of stem travel, giving better rangeability at low flows.
A HART communicator is connected to a 4-20 mA loop. What is the carrier frequency of the digital HART signal superimposed on the analog signal?
Answer: 1200 Hz
HART uses Bell 202 FSK at 1200 baud (1200 Hz for mark, 2200 Hz for space) superimposed on the 4-20 mA signal.
Which control mode is BEST suited for a process where offset cannot be tolerated but fast disturbance rejection is not critical?
Answer: Proportional-Integral (PI)
PI control eliminates steady-state offset through integral action while providing proportional response, making it ideal when precise setpoint tracking matters more than speed.
In ISA S5.1 instrumentation symbology, a circle with a horizontal line through the middle drawn on a P&ID represents an instrument located:
Answer: Behind the main control panel
Per ISA S5.1, a circle with a horizontal line through it indicates an instrument mounted behind (or on the back of) the main control panel, not accessible to operators.