Instrumentation & Process Control Flashcards
7 cards from real CCST practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Instrumentation & Process Control flashcards as text
A control valve with 'fail-open' (air-to-close) action is specified for a cooling water service. What is the safety rationale?
Answer: Cooling water must continue flowing on loss of instrument air to prevent overheating
A fail-open valve ensures cooling water continues flowing during an instrument air failure, preventing overheating or runaway reaction in the protected equipment.
Which pH sensor reference electrode type is most commonly used in industrial process applications?
Answer: Silver/silver chloride (Ag/AgCl) electrode
The silver/silver chloride (Ag/AgCl) reference electrode is the most widely used industrial pH reference because of its stability, low toxicity, and ease of maintenance.
What is the key advantage of a magnetic flowmeter (magmeter) over a differential pressure-based flowmeter?
Answer: It has no obstruction in the flow path, resulting in very low pressure drop
A magmeter has no moving parts or flow restrictions in the pipe bore, producing negligible pressure drop and eliminating wear associated with traditional DP devices.
On a process and instrumentation diagram (P&ID), a dashed line connecting two instrument bubbles most commonly indicates:
Answer: An electrical or signal connection (as opposed to a process line)
In ISA S5.1 P&ID symbology, a dashed line between instruments represents an electrical or binary signal connection between devices.
A pressure transmitter is reading high in a liquid service with condensate in the high-side impulse line but a dry low-side line. What corrective action is taken during calibration?
Answer: Apply zero elevation to shift the measurement downward
Liquid in the high-side impulse line adds a static head that biases the reading high; zero elevation (negative zero shift) corrects for this additional pressure.
What does the term 'rangeability' mean for a control valve?
Answer: The ratio of the maximum controllable flow to the minimum controllable flow
Rangeability describes the ratio of maximum to minimum controllable flow rates, indicating how wide a flow range the valve can accurately control.
In a feedforward control scheme, the controller output is adjusted based on:
Answer: Measured disturbance variables before they affect the process variable
Feedforward control measures disturbances entering the process and applies a correction to the manipulated variable before the process variable is affected, unlike feedback which reacts after deviation.