Process Control and Instrumentation

Process Variables and Instrumentation

• Four main process variables are crucial to understanding process control.
• Control loops can be open or closed, affecting control system performance.
• Pneumatic instruments use pressure signals.
• Electronic instruments use electrical signals.
• Digital instruments use binary code.
• Analog instruments use continuous signals.
• Electronic transmitters commonly employ 4-20mA output signals.
• Pneumatic transmitters use 3-15 psi signals.
• Process variable relationships influence system performance.
• Specific gravity relates to density and liquid properties (e.g., sink/float in water).

Control Loops

• Control loops consist of five parts:
  • Sensor: Measurers process variable
  • Transmitter: Converts signal to usable form
  • Controller: Determines set point
  • Actuator: Causes a desired change
  • Final element (e.g., valve): Makes physical changes

Instrument Signals

• Instrument signals include pneumatic, electronic, and digital formats.
• Loop error accumlates if adjustments are inadequate.
• Calibration is a key process that ensures accuracy.
• Different instrument signals involve milliamps, millivolts, binary codes, and ohms.

Pressure Calculations

• Calculating head pressure (P_head) requires understanding of relevant equations.
• Calculating liquid level involves relationships among pressure, height, and density.
• Converting units of pressure (e.g., psi to inches of water).
• Delta P transmitters measure differences in pressure.

Instrumentation Scaling

• Span, Upper Range Value (URV), and Lower Range Value (LRV) are fundamental concepts concerning instrument ranges.
• Pneumatic and electrical instrumentation both require understanding of scaling parameters.
• Cascade loops use two controllers to enhance control.

Controllers

• Controllers can be manual or automatic.
• Cascade loops feature automatic controllers.
• Process error (offset) arises from discrepancies between set point and process variable.
• Bumpless transfer ensures smooth transitions between control modes.
• Proportional band, integral action, and derivative action are essential controller tuning parameters.
• Split-range controllers manage two valves with different ranges.
• Direct and reverse acting controllers have different output and input actions.
• Ratio controllers manage mixing components according to a specific ratio.
• Controllers track setpoints automatically.

Control Valve Types

• Air to open valves allow process output when air supply is present.
• Air to close valves restrict output with air supply.
• Pressure units conversion (psig to psia and vice versa).

Pressure Measurement Tools

• Manometers measure pressure differences.
• Vacuum represents pressure below atmospheric pressure.
• Density and specific gravity are important when dealing with fluids.

Symbols & Diagrams

• P&IDs use standard symbols for equipment.
• Local and remote instruments are indicated.
• Symbols help identify valves, pumps, transducers, and DCS (computer control systems).
• Block flow diagrams and process flow diagrams illustrate processes.
• Legends explain the meaning of symbols in P&ID.

Heat Transfer

• Heat transfer mechanisms include conduction, convection, and radiation.
• BTU (British Thermal Unit) measures heat.
• Latent heat is associated with phase changes.
• Sensible heat involves temperature changes of single-phase matter.
• Absolute zero represents the absence of molecular motion.
• Temperature scales (Fahrenheit, Celsius, Kelvin) provide different units for measuring temperatures.

Level Measurement

• Level instruments measure liquid levels in tanks.
• Bubbler head pressure measurement doesn't touch the liquid.
• Displacer floats measure liquid levels.
• Tape gauges are used for solids measurement.
• Ultra-sonic devices measure level using time of flight.
• Sight glasses observe the level.
• Dip sticks determine the liquid level.
• Interface levels measure levels between different fluids.
• Differential pressure (DP) cells employ a DP transmitter for measuring liquid levels.
• Level measurement methods include direct and indirect approaches.

Flow Measurement

• Flow quantity measurement pertains to the movement of fluids (liquid or gas).
• Mass flow and volumetric flow are different ways to measure gas flow.
• Mass flow (steam) utilizes LDs measurements.
• Laminar flow is smooth.
• Turbulent flow is irregular.
• Rotameters are precision measuring instruments for low flows.
• Venturi tubes offer low pressure loss.
• Coriolis mass flow meters provide higher accuracy to consider temperature factors.
• Volume, rate, and time calculations involve correct unit conversions.

Control Valves

• Control valves regulate flow.
• Fail-open valves allow output when the supply is lost.
• Fail-closed valves stop output when the supply is lost.
• Control valves have various parts.

Fluid Mechanics

• Bernoulli's principle explains pressure and velocity relationships in fluids.
• Thermodynamic concepts are important to understand fluid behavior

Temperature Measurement

• Various temperature measurement instruments (e.g., thermocouples) exist.
• Thomas Seebeck developed the thermocouple.

Other Instruments

• Analyzers measure components in fluids.
• Load cells relate weight to other values.

This quiz focuses on the key concepts of process variables and their significance in process control systems. Learn about control loops, types of instruments, and the relationship between different process variables. Test your knowledge of pneumatic and electronic signal transmission used in industrial systems.