Comprehensive Study Notes: Instrumentation and Temperature Control and Temperature Measurement

Introduction to Instrumentation and Control (Module 1)

  • Module Objectives:

    • Introduce trainees to basic process control instruments.
    • Provide an understanding of technical terms used in the instrumentation field.
    • Introduce basic process variables.
    • Develop a basic understanding of how instruments function.

  • Introduction to Instrumentation Systems:

    • An instrumentation and control system provides accurate and adequate information regarding the process parameters of a system.
    • Purpose: Ensures safe, continuous, reliable, and economical plant operation.
    • Function: Eliminates guesswork or imagination during plant operation by operators.

  • Defining Instrumentation:

    • It is a branch of engineering dealing with the measurement and control of process variables.
    • It describes the application of instruments (systems or devices) to achieve specific objectives in measurement, control, or both.
    • Collective Term: Used for measuring instruments that indicate, measure, and record physical quantities.

  • Process Variables:

    • Physical quantities being measured include flow, temperature, level, distance, angle, speed, pH, force, humidity, and pressure.

  • Role of the Instrumentation Technician:

    • Tasks include testing, calibrating, installing, and inspecting manufacturing/process equipment and monitoring devices.

The Process and Control Systems

  • Nature of a Process:

    • Definition: The changing of raw materials into a finished product.
    • Transformation: Raw materials flowing through equipment are subjected to conditions that alter composition and chemical structure.
    • Requirement: Process conditions must be accurately controlled at all times.
    • Operation: Instruments perform the controlling; operators direct the instruments. Proper instrument use ensures products meet specifications.

  • Definition of a Control System:

    • A set of devices that manages, commands, directs, or regulates the behavior of other devices or systems to achieve desired results.
    • Functionality: Maintains required physical system conditions by adjusting several variables.
    • Composition: Interconnected components acting together to provide a required response.

  • The Control Loop:

    • Fundamental building block of industrial control systems.
    • Components: Consists of physical components and control functions needed to automatically adjust the value of a measured Process Variable (PV) to equal a desired Set-Point (SP).
    • Structure: Simple block diagrams consist of Input →\rightarrow Control System →\rightarrow Output.

  • Transfer Functions in Closed-Loop Systems:

    • Transfer functions relate the system output to the input.
    • Symbols:
      • R(s)R(s): Reference input/Set-point.
      • C(s)C(s): Controlled output.
      • E(s)E(s): Error signal.
      • G(s)G(s): Feedforward elements (gain).
      • H(s)H(s): Feedback elements.
    • Equation for Closed-Loop Transfer Function:         C(s)R(s)=G(s)1+G(s)H(s)\frac{C(s)}{R(s)} = \frac{G(s)}{1 + G(s)H(s)}
    • Significance: Relates closed-loop system dynamics to the dynamics of feedforward and feedback elements.

  • Importance of Control Systems:

    • Integral to manufacturing industries.
    • Enable automated industrial processes.
    • Allow for varied command inputs and control of parameter variations.
    • Provide disturbance rejection.

Common Instrumentation Terminology

  • Transducer: A device that converts one form of energy to another (e.g., electrical to pneumatic).
  • Transmission: Method of standardizing signals sent from various plant locations.
  • Transmitter: A device that takes a measurement and converts it into a standard signal.
  • Correcting Unit: Works on the controller's command (Final Control Element) to adjust the measured value and obtain a zero error signal (e.g., control valve).
  • Calibrate: Determining correct values on a scale by measurement or comparison with a standard.
  • Configure: Setting up a computer system/program for a specific application.
  • DCS (Distributed Control System): Dividing plant responsibility into areas, each managed by its own controller/processor, interconnected via communication buses.
  • Fieldbus: A digital, two-way, multi-drop communication link; serves as a Local Area Network (LAN) for advanced control and automation.
  • HART (Highway Addressable Remote Terminal): A communication protocol providing digital communication to microprocessor-based (smart) analog instruments.
  • Profibus: A communication protocol.
  • Range: The region between limits within which a quantity is measured; expressed by upper and lower range values.
  • Strain Gauge: Measures applied force using the change of electrical resistance in a wire under strain.
  • SCADA (Supervisory Control and Data Acquisition): A package used to monitor and control remote processes, including telemetry and servers.
  • UPS (Uninterruptible Power Supply): Keeps critical equipment running during power failures.
  • VFD/VSD (Variable Frequency/Speed Drive): Electronic equipment allowing electric motors to run at varying speeds.

Components of Control Loops

  • Four Essential Elements:

    1. Sensing Element (Sensor): Detects changes in the measured variable and provides a proportional output. It is usually in contact with the process (Primary Element).
      • Examples: Orifice plate, float switch, thermocouple, Bourdon tube.
    2. Measuring Element (Transmitter): Responds quantitatively to the variable to produce a signal for transmission. It sends the Process Variable (PV) or Measured Value (MV) to the controller and other devices like alarms and recorders.
    3. Controller Element (Decision Maker): The "brain" of the loop. It compares the measured value (MV) to the set-point (SP). If a difference (Error/Deviation) exists, it computes the corrective action.
      • Examples: Pneumatic/electronic controllers, digital computers.
    4. Final Control Element (FCE): The part that carries out the correction. It receives signals from the controller and physically adjusts the process variable.
      • Most important industrial FCE: Control Valve.

  • Control Cycle Routine:

    1. Collect/Observe: Check information via the measuring element.
    2. Compare: Compare actual values with the set-point.
    3. Correct: Adjust the FCE (Error Correction).
    4. Wait: Allow the process to respond.
    5. Repeat: Continue the cycle to correct persisting errors.

Types of Control Systems

  • Open-Loop Control System (Manual Control):

    • Definition: Control action is independent of the output; there is no feedback.
    • Mechanism: Input →\rightarrow Controller →\rightarrow Actuating Signal →\rightarrow Plant →\rightarrow Output.
    • Operator Role: Must observe, compare, and adjust manually.
    • Example: Traffic lights (on/off times based on a pre-set schedule, not actual traffic density).
    • Pros: Simple, economical, easy to maintain, stable.
    • Cons: Inaccurate, unreliable, requires constant operator presence, unable to handle fast-changing processes, prone to human error.

  • Closed-Loop Control System (Automatic Control):

    • Definition: Control action is dependent on the output; output is fed back to the input.
    • Mechanism: Error Detector →\rightarrow Controller →\rightarrow Plant →\rightarrow Feedback Elements →\rightarrow Back to Input.
    • Steady State: When the loop reaches equilibrium where the input and feedback signals are balanced.
    • Example: Thermostat heater, voltage stabilizer, missile launcher.
    • Pros: Accurate, more robust, reduces noise through feedback mechanisms.
    • Cons: Difficult to design, costlier, complex construction, potential for oscillatory response (instability).

Signal Types and Data Transfer

  • Nature of Signals: Electronic messages sent between devices.
  • Data Transfer: The movement of signals using electrical or pneumatic lines.
  • Three Classes of Signals:
    1. Binary: Two values only (On/Off, e.g., a thermostat).
    2. Analogue: Continuous range of values (e.g., a speedometer or most sensors/FCEs).
    3. Digital: A series of pulses (e.g., a digital clock or computer chip signals).
  • Conversion: Transducers are used if an instrument cannot understand the transmitter's signal type (e.g., converting analog sensing signals to digital for a computer).

Purposes of Final Control Elements in Oil & Gas

  • Continuous Process Control: Regulating Flow Rate, Pressure, Level, and Temperature.
  • Intermittent Process Control: Louvers/vanes regulating combustion air to furnaces.
  • Safety & Protection: Maintenance of minimum flow in pumps (spillback lines) or recycle lines in compressors to prevent surge/overheating.
  • Emergency Shut-Down (ESD):
    • ESD Valves: Isolate leaking equipment/flammable liquids.
    • Vent Valves: Depressurize equipment by venting gas to a flare.
    • Quick Release Isolation Valves: Fast isolation (e.g., marine loading arms).

Advanced Process Control Modes

  • On/Off Control:

    • Valve state: Either 0%0\% (fully closed) or 100%100\% (fully open).
    • Characteristics: Results in "mean level" control; impossible to maintain exact set-point due to differential requirements to prevent excessive cycling/wear.
    • Physics Application: Pressure at a tank bottom switch P1=ρgh1P_1 = \rho g h_1.

  • Feedback Control:

    • Correction occurs after a disturbance has affected the system (retrospective).
    • Does not require measurement of the disturbance itself.

  • Feedforward Control:

    • Primary disturbance is measured before it affects the output.
    • The manipulated variable is adjusted to minimize predicted deviations.
    • Example: Measuring outside temperature to adjust house heating before the interior cools down; cruise control predicting throttle needed based on road conditions.
    • Pros: Acts before the system is affected; effective for slow systems.
    • Cons: Requires measurement of the disturbance; requires deep process knowledge.

  • Continuous Control Modes:

    • Proportional (P): Output is adjusted proportionally to the magnitude of the error (e=SP−Me = SP - M).
    • Integral (I).
    • Derivative (D).

Temperature Measurement (Module 4)

  • Definitions:

    • Temperature: The heat intensity or degree of sensible heat in a body.
    • Heat: The amount of energy a body possesses; depends on temperature, mass, and composition.
    • Heat Flow: Occurs from higher to lower temperature based on a temperature gradient.

  • Measurement Accuracy and Errors:

    • Varying fluid temperatures in different parts of a vessel.
    • Radiation of heat from measuring devices to the environment.
    • Time Lag: The time required for heat to be conducted to the sensing element.

  • Temperature Scales:

    • Celsius (∘C^{\circ}\text{C}): Freezing point of water at 0∘C0^{\circ}\text{C}; Boiling point at 100∘C100^{\circ}\text{C} (at 101.325 kPa101.325\,kPa).
    • Kelvin (K): Absolute scale where 0 K=−273∘C0\,K = -273^{\circ}\text{C}.
    • Fahrenheit (∘F^{\circ}\text{F}): Freezing point at 32∘F32^{\circ}\text{F}; Boiling point at 212∘F212^{\circ}\text{F}.
    • Rankine (∘R^{\circ}\text{R}): Absolute imperial scale where ∘R=∘F+460^{\circ}\text{R} = ^{\circ}\text{F} + 460.

  • Conversion Formulas:

    • K=∘C+273{K} = {^{\circ}\text{C}} + 273
    • ∘F=95∘C+32{^{\circ}\text{F}} = \frac{9}{5} {^{\circ}\text{C}} + 32
    • ∘C=59(∘F−32){^{\circ}\text{C}} = \frac{5}{9} ({^{\circ}\text{F}} - 32)

  • Scale Relationship Examples:

    • Boiling Water: 672∘R672^{\circ}\text{R} | 212∘F212^{\circ}\text{F} | 100∘C100^{\circ}\text{C} | 373 K373\,K
    • Freezing Water: 492∘R492^{\circ}\text{R} | 32∘F32^{\circ}\text{F} | 0∘C0^{\circ}\text{C} | 273 K273\,K
    • Absolute Zero: 0∘R0^{\circ}\text{R} | −459∘F-459^{\circ}\text{F} | −273∘C-273^{\circ}\text{C} | 0 K0\,K

Common Temperature Measuring Devices

  • Glass-Stem Thermometers:

    • Principle: Thermal expansion/contraction of liquid (mercury or alcohol) in a glass capillary.
    • Range: Mercury fill typically covers −40∘C-40^{\circ}\text{C} to 400∘C400^{\circ}\text{C} (750∘F750^{\circ}\text{F}). Nitrogen pressurization can increase the limit to 550∘C550^{\circ}\text{C}.
    • Safety: Broken mercury thermometers require Occupational Health and Safety (OH&S) cleanup.

  • Thermowells:

    • Protective cups installed in pressure vessels and pipes.
    • Function: Protect against high pressure, corrosion, and erosion.
    • Enhanced Sensitivity: Oil is poured into the well to increase heat transfer efficiency; the thermometer tip should touch the bottom.

  • Bimetallic Thermometers:

    • Principle: Dissimilar metals expand at different rates when heated.
    • Construction: Strips of Invar (low expansion coefficient) and Brass (high expansion coefficient) fused together.
    • Amplification: Strip is wound into a spiral or helix to increase pointer motion.
    • Range: −50∘C-50^{\circ}\text{C} to +550∘C+550^{\circ}\text{C}.
    • Pros: Rugged, vibration resistant, easy to read.

  • Filled Thermal Elements:

    • Consists of a bulb, capillary tube, and pressure sensor (Bourdon tube/bellows).
    • Function: Temperature change causes fluid expansion, increasing internal pressure felt by the Bourdon tube.
    • Capillary length: Up to 75 m75\,m (246 ft246\,ft) for remote indication.
    • Classifications:
      • Class 1: Liquid-filled (other than mercury).
      • Class 2: Vapor-filled (partially liquid); has non-linear scale.
      • Class 3: Gas-filled (e.g., Nitrogen).
      • Class 4: Mercury-filled (compensated, seldom used).
      • Class 5: Mercury-filled (non-compensated).
    • Ambient Temperature Compensation:
      • Case Compensation: Bimetallic strip on the Bourdon tube acts in opposition to ambient changes.
      • Full Compensation: Uses a second compensating capillary and spiral alongside the measuring one to cancel out effects.

  • Thermocouples:

    • Principle: Two dissimilar metal wires joined at a "measuring junction" generate a voltage proportional to the temperature differential relative to the "reference junction."
    • Types and Ranges:
      • Type J (Iron/Constantan): 0∘C0^{\circ}\text{C} to 775∘C775^{\circ}\text{C}.
      • Type K (Chromel/Alumel): 300∘C300^{\circ}\text{C} to 1300∘C1300^{\circ}\text{C}.
      • Type S & R (Platinum-Rhodium/Platinum): 550∘C550^{\circ}\text{C} to 1500∘C1500^{\circ}\text{C}.
      • Type T (Copper/Constantan): −175∘C-175^{\circ}\text{C} to 400∘C400^{\circ}\text{C}.

  • Resistance Thermometers (RTD):

    • Principle: Electrical resistance of metals (Platinum, Copper, Nickel) increases with temperature.
    • Circuit: Platinum wires form part of a Wheatstone bridge; voltage imbalance indicates temperature.

  • Thermistors:

    • Made from oxides of cobalt, copper, iron, manganese, etc.
    • Negative Temperature Coefficient (NTC): Resistance decreases as temperature increases (typical fallback is −4%-4\% per ∘C^{\circ}\text{C} at 20∘C20^{\circ}\text{C}).
    • Positive Temperature Coefficient (PTC): Made from germanium or silicon; response is extremely fast (63%63\% of final temperature in 15 ms15\,ms).
    • Usage: Remote measurement (high resistance makes lead resistance negligible), motor protection.

  • Radiation Pyrometer:

    • Principle: Measurement without physical contact. Stefan-Boltzmann law application: heat radiation intensity increases to the fourth power of absolute temperature.
    • Mechanism: Pyrometer lens focuses energy onto a thermopile (series of thermocouples).
    • Range: 50∘C50^{\circ}\text{C} to 6000∘C6000^{\circ}\text{C}.
    • Application: Where temperatures exceed thermocouple limits or the atmosphere is detrimental to sensors.