Process Flow and Instrumentation Flashcards

What is the primary function of a PLC (Programmable Logic Controller) in an industrial process?

Answer: To monitor inputs and execute programmed logic to control outputs in an automated process

A PLC reads digital and analog input signals, executes a stored control program (ladder logic or other language), and activates output devices based on programmed conditions. It is the central controller for most industrial automation.

In process control, what is the function of a PID controller?

Answer: Uses proportional, integral, and derivative terms to minimize the error between a setpoint and measured process variable

A PID controller continuously calculates error (setpoint - process variable) and applies three corrections: Proportional (immediate response to current error), Integral (eliminates steady-state offset), and Derivative (dampens response based on rate of change).

What type of sensor is a thermocouple, and on what principle does it operate?

Answer: A temperature sensor that generates a voltage proportional to temperature based on the Seebeck effect

A thermocouple consists of two dissimilar metals joined at one end. The Seebeck effect produces a small voltage at the junction proportional to the temperature difference between the measuring junction and reference junction.

What is the difference between a 4-20mA signal and a 0-10V signal in instrumentation?

Answer: 4-20mA is current-based and immune to voltage drop over long distances; 0-10V is voltage-based and can drop on long runs

4-20mA current loops maintain signal strength over long distances because current is the same throughout a series circuit regardless of wire resistance. Voltage signals lose amplitude over long cables due to resistance. The 4mA live zero also allows wire break detection.

On a P&ID, what does the tag designation 'FCV-101' mean?

Answer: Flow Control Valve 101 — a control valve that regulates flow in loop 101

P&ID tag codes follow ISA S5.1 standards: first letter = measured variable (F = Flow), subsequent letters = function (CV = Control Valve), and number = loop identification. FCV-101 = Flow Control Valve in instrument loop 101.

What is the purpose of an I/P (current-to-pressure) transducer in a pneumatic control loop?

Answer: Converts the 4-20mA electronic control signal to a 3-15 psi pneumatic signal to operate a control valve actuator

An I/P transducer converts the electronic control signal (4-20 mA) from the controller into a proportional pneumatic signal (typically 3-15 psi). This pneumatic signal positions the diaphragm actuator on a pneumatic control valve.

What does 'fail-safe' mean for a control valve, and what are the two common failure modes?

Answer: The valve moves to a predetermined safe position on loss of signal/air; fail-open (FO) or fail-closed (FC)

Fail-safe means a control valve moves to a predetermined safe position if the control signal or instrument air is lost. Fail-open (air-to-close) springs open; fail-closed (air-to-open) springs closed. The safe position depends on the process safety requirement.

In a flow measurement system, a differential pressure (DP) transmitter across an orifice plate measures what, and how is flow calculated?

Answer: Measures pressure drop across the orifice; flow is proportional to the square root of the differential pressure

An orifice plate creates a pressure drop proportional to the square of flow rate (Bernoulli's principle). The DP transmitter measures this pressure difference, and the square root extraction gives flow: Q ∝ √ΔP.

What is a 'control loop' in process control, and what are its four main components?

Answer: A closed feedback system: sensor (measurement), transmitter, controller, and final control element (valve/actuator)

A control loop measures a process variable (sensor/transmitter), compares it to a setpoint (controller), calculates an output, and adjusts a final control element (valve, heater, VFD) to maintain the setpoint. Feedback makes it 'closed loop'.

What is the purpose of a sight glass (level gauge) on a vessel or tank?

Answer: To provide a visual indication of the liquid level inside the vessel

A sight glass (level gauge) is a transparent or reflex glass mounted on the side of a vessel, connected to the vessel above and below the expected liquid level. It provides a direct visual reading of the liquid level without instruments.

In industrial safety systems, what does SIL (Safety Integrity Level) describe?

Answer: The required reliability level of a Safety Instrumented System (SIS) to reduce risk to an acceptable level

SIL (IEC 61508/61511) defines the required risk reduction capability of a Safety Instrumented System. SIL 1-4 rates systems from least to most reliable, with SIL 4 requiring failure probability < 0.0001 per demand.

What is the function of a pressure regulator in a process or utility system?

Answer: To reduce and maintain a constant downstream pressure regardless of upstream pressure fluctuations or flow demand

A pressure regulator (reducing valve) automatically reduces inlet pressure to a desired, stable outlet pressure. It continuously adjusts its position to maintain the setpoint as inlet pressure or downstream flow demand varies.

What does RTD stand for, and how does it measure temperature?

Answer: Resistance Temperature Detector; measures temperature by detecting the change in electrical resistance of a metal element (typically platinum)

An RTD uses a platinum (Pt100, Pt1000), nickel, or copper element whose resistance increases predictably with temperature. Compared to thermocouples, RTDs are more accurate and stable but require an excitation current.

What is a safety relief valve (SRV), and when does it open?

Answer: A pressure protection device that opens automatically when system pressure exceeds the set pressure to prevent vessel overpressure

A safety relief valve is a spring-loaded automatic valve installed on pressurized vessels and piping. It opens when process pressure exceeds its preset value, venting fluid to relieve overpressure, and reseats when pressure returns to normal.

In a process plant, what does an 'interlock' system do?

Answer: Automatically takes protective action (stops equipment, closes valves) when process variables reach unsafe conditions

Process interlocks (trips) automatically protect equipment and personnel by shutting down equipment or taking corrective actions when process variables (temperature, pressure, level, flow) reach hazardous limits, without human intervention.

What is the purpose of instrument air in a process plant?

Answer: Dry, oil-free compressed air used to operate pneumatic instruments, control valves, and actuators

Instrument air is carefully conditioned: compressed, filtered, dried (dew point typically -40°F), and free of oil. Moisture or contamination in instrument air causes valve actuators, positioners, and control instruments to malfunction.

What is the difference between analog and digital control signals in industrial instrumentation?

Answer: Analog signals vary continuously (e.g., 4-20mA); digital signals are discrete on/off or coded values (e.g., 0V/24VDC, fieldbus)

Analog signals represent continuous ranges of values (4-20 mA, 0-10V). Digital signals are discrete (on/off) or encoded numerical data (HART, Profibus, Foundation Fieldbus). Modern plants use both: analog for control and digital for diagnostics.

In process control, what is 'cascade control'?

Answer: The output of a primary (master) controller becomes the setpoint of a secondary (slave) controller, improving control of the primary variable

Cascade control uses two feedback loops: the outer (primary/master) loop controls the main process variable; its output sets the setpoint for the inner (secondary/slave) loop. The slave responds quickly to disturbances before they affect the master variable.

What does 'span' mean in the context of an instrument's calibration range?

Answer: The difference between the upper range value (URL) and lower range value (LRL) of the instrument

Span = URL (Upper Range Limit) - LRL (Lower Range Limit). For example, a pressure transmitter calibrated 0-100 psi has a span of 100 psi. A 25-125 psi range also has a 100 psi span but a different zero (LRL).

What is a 'deadband' in a control system or switch?

Answer: A range around the setpoint within which no control action is taken, preventing hunting or chattering

Deadband (also called differential gap or hysteresis) is the range of input values around the setpoint that produces no output change. It prevents rapid cycling (hunting) of a controller or switch from small fluctuations near the setpoint.