Process Control & Instrumentation Flashcards
7 cards from real CIRO 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 & Instrumentation flashcards as text
What is the primary purpose of a hot gas defrost solenoid valve in an instrumented industrial refrigeration system?
Answer: To admit hot discharge gas into the evaporator coil to melt accumulated frost during defrost cycles
The hot gas defrost solenoid valve opens during scheduled defrost cycles to route hot, high-pressure discharge gas into the evaporator, supplying heat energy to melt frost accumulation from the coil.
A capacitance-type level sensor installed on a high-pressure receiver in an ammonia plant measures liquid level based on:
Answer: The change in electrical capacitance between the sensor probe and the vessel wall as liquid level changes
Capacitance level sensors detect the difference in dielectric constant between liquid ammonia and ammonia vapor; as liquid level changes, the capacitance between probe and vessel wall changes proportionally.
When using an electronic data logger to trend compressor discharge temperature over time, a gradual upward drift could indicate:
Answer: Fouled or scaled condenser tubes reducing heat rejection efficiency
Gradual rise in discharge temperature over time often indicates reduced condenser efficiency from fouling or scaling, which raises condensing pressure and compressor work, increasing discharge temperature.
An oil pressure differential switch on a screw compressor has a 3-second time-delay relay before tripping. What is the purpose of this time delay?
Answer: To allow the compressor to fully build speed before oil pressure is monitored, preventing nuisance trips during startup
During compressor startup, oil pressure takes a brief moment to build; the time delay relay prevents nuisance safety trips during the normal startup transient before steady-state oil pressure is established.
In industrial refrigeration control systems, what does the term 'dead band' refer to in a two-position (on/off) controller?
Answer: A range of values around the setpoint within which the controller takes no action, preventing rapid cycling
Dead band is a control range centered on the setpoint where the controller neither turns the system on nor off, preventing short-cycling (rapid switching) that would damage compressors and other equipment.
Which alarm condition should trigger an immediate plant evacuation and activation of the emergency response plan in an ammonia refrigeration facility?
Answer: High ammonia concentration alarm exceeding 300 ppm in an occupied area
An ammonia concentration exceeding 300 ppm in an occupied area represents an IDLH (Immediately Dangerous to Life and Health) threshold requiring immediate evacuation and emergency response per OSHA PSM and RMP requirements.
A technician notices that the 4-20 mA output of a suction pressure transmitter reads 3.8 mA with the system running. This most likely indicates:
Answer: A failed or open-circuit transmitter, since valid 4-20 mA signals cannot fall below 4 mA during normal operation
A reading below 4 mA in a 4-20 mA loop typically indicates a transmitter fault, wiring open-circuit, or power supply problem, since 4 mA represents the zero/minimum measurement value and signals below that indicate an error condition.