Sensors and Detectors – Process Measurement Flashcards

Introduction to Process Measurement

• The complex nuclear process requires the measurement of numerous parameters to ensure efficient, reliable production of low-cost electrical power while maintaining public health and safety.

• Constant monitoring is necessary to provide a safe, comfortable atmosphere for plant workers and to limit environmental emissions.

• The most frequently monitored parameters are temperature, pressure, level, and flow.

• Physical process measurement involves three primary components:

  • Detector: A device whose physical characteristics change in response to changes in a process parameter.

  • Transducer: A device that converts the detector's physical change into a usable signal format, typically electrical (voltage, current) or pneumatic (air signal).

  • Transmitter: A housing that often contains both the detector and the transducer to provide signals for indication, control, and protection.

Temperature Measurement Principles

• Temperature measurement is an inferred measurement based on the effects horizontal heat changes have on the properties of other substances.

• Common principles of operation for temperature instruments include:

  • Expansion of Solids or Liquids: Volumetric expansion when heated can be converted to a temperature indication.

  • Fluid Pressure: Increasing the temperature of a fluid in a closed container increases its pressure.

  • Electrical Resistance: The resistance of certain metals changes predictably with temperature.

  • Termoelectric Effect: Applying heat to the bonded junction of two dissimilar metals generates a measurable voltage.

Liquid-In-Glass Thermometer

• Utilizes the volumetric expansion and contraction of a liquid (usually mercury or alcohol) within a glass envelope connected to a capillary tube.

• As the bulb is heated, the liquid rises in the capillary. The scale is typically etched on or mounted behind the tube.

• Operating range is generally $-328^{\circ}\text{F}$ to $1,150^{\circ}\text{F}$, depending on the liquid used.

Filled-System Thermometer

• Designed for remote indication or recording, often at distances up to $400\text{ feet}$.

• Consists of a sensing bulb containing gas or fluid connected via capillary tubing to a receiving element, usually a Bourdon tube.

• Pressure changes in the bulb due to temperature are transmitted to the Bourdon tube, which moves a pointer, pen, or actuates a control switch (thermostat).

• Operating range is approximately $-400^{\circ}\text{F}$ to $1,000^{\circ}\text{F}$.

Bimetallic Strip Thermometer

• Constructed by fastening strips of two metals with different coefficients of thermal expansion.

• Coefficient of Linear Expansion: The quantity describing how much an object increases in length per $1^{\circ}\text{C}$ rise in temperature.

  • Aluminum: $25 \times 10^{-6}\,\text{cm/}^{\circ}\text{C}$

  • Copper: $17 \times 10^{-6}\,\text{cm/}^{\circ}\text{C}$

  • Iron: $11 \times 10^{-6}\,\text{cm/}^{\circ}\text{C}$

  • Quartz: $0.4 \times 10^{-6}\,\text{cm/}^{\circ}\text{C}$

  • Pyrex Glass: $3 \times 10^{-6}\,\text{cm/}^{\circ}\text{C}$

• Differential expansion causes the strip to bend. Many models use a spiral-wound element with one fixed end and a pointer attached to the other.

• Operating range is $-200^{\circ}\text{F}$ to $1,000^{\circ}\text{F}$.

Optical Pyrometer

• A non-contact, usually hand-held device that measures radiant energy and brightness of a surface.

• Brightness varies with temperature and is displayed on a scale.

• Requires a clear line of sight and is typically used for temperatures exceeding $1,300^{\circ}\text{F}$.

Thermocouples

• Consist of two dissimilar metal wires joined at one end, known as the sensing junction (or hot junction).

• When the sensing junction temperature increases, a voltage (Electromotive Force or EMF) develops at the reference junction (or cold junction).

• The voltage magnitude is a direct function of the temperature difference between the two junctions.

• Requirements for Practical Use:

  • Melting points must exceed the temperature being measured.

  • Voltage must be large enough to measure and vary linearly with temperature.

  • Metals must resist corrosion/oxidation and be economical.

• Common Materials: Platinum, Iron, Constantan (copper/nickel), Chromel (chrome/nickel), and Alumel (aluminum/nickel).

• Types and Ranges:

  • Iron-Constantan (Type J): $-210^{\circ}\text{C}$ to $760^{\circ}\text{C}$ ($-350^{\circ}\text{F}$ to $1,400^{\circ}\text{F}$).

  • Copper-Constantan (Type T): $-270^{\circ}\text{C}$ to $370^{\circ}\text{C}$ ($-455^{\circ}\text{F}$ to $700^{\circ}\text{F}$).

  • Chromel-Alumel (Type K): $-270^{\circ}\text{C}$ to $1,260^{\circ}\text{C}$ ($-455^{\circ}\text{F}$ to $2,000^{\circ}\text{F}$).

  • Chromel-Constantan (Type E): $-270^{\circ}\text{C}$ to $870^{\circ}\text{C}$ ($-455^{\circ}\text{F}$ to $1,600^{\circ}\text{F}$).

• Reference Junction (Cold Junction): To ensure accuracy, the reference junction is typically sealed in a thermostatically controlled aluminum block to maintain constant temperature. If the reference junction temperature increases, the EMF difference decreases, causing indicated temperature to fail low.

• Advantages: Fast response (thin wires) and generates its own signal (no external power supply required).

• Disadvantages: Nonlinear voltage signal (requires compensation), prone to reference junction temperature errors, and generally less accurate than RTDs.

Resistance Temperature Detectors (RTD)

• Operate on the principle that electrical resistance in a metal changes proportionally with temperature.

• Material Selection Factors: Pure metal availability, ability to draw into fine wire, response time, and linearity of resistance change.

• Platinum: The international standard for temperature measurement due to its linear and stable resistance-to-temperature relationship.

• Temperature Coefficient of Resistance: The fractional change in resistance ($\text{ohms per ohm}$) per degree of temperature change.

  • Platinum: $0.00217\,^{\circ}\text{F}^{-1}$

  • Nickel: $0.0037\,^{\circ}\text{F}^{-1}$

  • Tungsten: $0.0027\,^{\circ}\text{F}^{-1}$

  • Aluminum: $0.0025\,^{\circ}\text{F}^{-1}$

  • Copper: $0.0024\,^{\circ}\text{F}^{-1}$

• Construction: Fine wire element coiled around a mica form, annealed to remove stress, then housed in a protective sheath or thermowell. Springs ensure good thermal contact.

• Wheatstone Bridge Circuit: Uses a DC power supply to compare the resistance of the RTD ($R_X$) against a known resistance ($R_S$). Temperature variations change $R_X$, creating a voltage differential displayed as temperature.

Comparison of Temperature Sensors

• Advantages of RTDs over Thermocouples:

  • Better suited for small temperature bands.

  • Use applied voltage, requiring less signal-boosting equipment.

  • No reference junction required.

  • More tolerant to electrical noise.

  • Higher sensitivity and accuracy.

• Advantages of Thermocouples over RTDs:

  • More rugged and suitable for large temperature bands.

  • Faster response time due to thinner wires.

  • Generally less expensive.

  • Self-powered (no external supply required).

Temperature Instrument Failure Modes

• Mechanical Failures:

  • Filled-System: Leaks in tubing cause the instrument to fail low.

  • Bimetallic Strip: A break between dissimilar metal connections causes the instrument to fail low.

• Electrical Failures (Thermocouple):

  • Open Circuit/Wire Break: Instrument typically fails low.

  • Break at Sensing Junction: Indication fails to the reference junction temperature.

  • Short Circuit in External Leads: Voltage goes to zero, causing indication to fail low.

  • Short Circuit to External Location: Sensed temperature will be inaccurate, typically lower than actual.

  • Reference Junction Temperature Increase: Indicated temperature decreases below actual.

• Electrical Failures (RTD):

  • Open Circuit: Wheatstone bridge sees infinite resistance, causing indication to fail to a maximum (fail high).

  • Short Circuit: Bridge sees low resistance, causing indication to fail lower than actual (fail low).

Pressure Measurement Devices

Pressure elements sense pressure changes and convert them to mechanical motion or electrical signals.

Mercury Barometer

• Invented by Evangelista Torricelli in 1643.

• An inverted glass tube is placed in a dish of mercury. Atmospheric pressure on the mercury in the dish forces liquid up the tube.

• Standard atmospheric pressure supports a mercury column $29.92\,\text{inches}$ high (approx. $30\,\text{inches}$).

Manometers

• U-Tube Manometer: A "U" shaped tube filled with water or mercury. Differential pressure between the two ends causes the fluid to rise in the opposite tube.

• Well-Type Manometer: Features a large fluid reservoir at one end for increased sensitivity to pressure changes.

• Inclined Tube (Draft Gauge): A variation of the well-type where the column is positioned almost horizontally, lengthening scale graduations for extremely accurate readings.

Bourdon Tubes

• C-Type: A flattened, thin-walled tube bent into a $270^{\circ}$ to $300^{\circ}$ arc. Internal pressure tends to restore its round cross-section, straightening the tube. The tip movement, typically $1/4$ to $3/8\,\text{inches}$, is amplified by a gear train.

• Materials: Phosphor bronze, alloy steel, stainless steel, and beryllium copper.

• Spiral Bourdon Tube: Wound in a spiral to magnify tip movement (three to four times longer than C-type).

• Helical Bourdon Tube: Coils are arranged directly over each other. Compact and efficient at transforming pressure to torque.

Bellows and Diaphragms

• Bellows: A collapsible, seamless metallic unit with deep folds. More sensitive to low pressures (range $0.5\,\text{psig}$ to $75\,\text{psig}$). Usually opposed by a spring to maintain linearity; zero-adjustment changes spring tension.

• Diaphragm: A metal disc built into a housing. Under pressure, it exhibits an "S"-shaped deflection curve. Corrugated diaphragms produce four times the deflection of flat ones and provide better linearity.

Differential Pressure (D/P) Cell

• Measures the difference between high-pressure (H.P.) and low-pressure (L.P.) inputs.

• Commonly uses a bellows or diaphragm connected to a spring. If H.P. increases or L.P. decreases, the bellows compresses, increasing the output signal.

• Equalizing Valve: Connects the L.P. and H.P. sides to prevent over-ranging (deforming) the element when placing it in or out of service. An open or leaking equalizing valve results in a minimum D/P signal.

Pressure Instrument Failure and Effects

• Environmental Effects:

  • Reference Pressure: If the reference side is vented to containment, increases in containment pressure will decrease the sensed D/P and output signal.

  • Radiation: Extremely high radiation levels embrittle sensing metals and damage electronic circuits.

• Over-ranging: Bourdon tubes generally survive up to $35\%$ beyond the upper range. Past this, permanent deformation occurs, and the indication will remain higher than original after the transient.

• Ruptures/Leaks:

  • Bourdon, bellows, or diaphragm ruptures result in lower-than-actual readings, typically falling to $0\,\text{psig}$.

  • Sensing line breaks cause low readings as the element senses only atmospheric pressure.

  • D/P cells: A leak in the reference side causes indicated pressure to increase; a leak in the variable side causes indicated pressure to decrease.

Level Measurement Applications

Direct vs. Inferred Methods

• Direct: Measurement of distance from liquid level to a datum line (e.g., Dipstick, Gage Glass, Float).

• Inferred: Level implied from characteristics like hydrostatic head, buoyancy, or electrical properties (e.g., D/P cell, Bubbler).

D/P Cell Level Configurations

• Open Vessel: Only the H.P. connection is used; L.P. is vented to atmosphere. Indicates level based on hydrostatic head pressure.

• Dry Reference Leg: Used for closed/pressurized tanks. L.P. is connected to a reference leg containing only gas/vapor. Drains prevent liquid accumulation. Gas pressure cancels out on both sides, leaving only hydrostatic head.

• Wet Reference Leg: Used for condensable fluids (steam). The reference leg is filled with liquid (constant head). This hydrostatic pressure often equals the maximum detectable tank level.

  • At maximum tank level, $D/P = 0$.

  • As tank level decreases, $D/P$ increases (inverse relationship).

Density Compensation and Environmental Effects

• Reference Leg Density: If the reference leg is cooler than the tank water, it will be denser, exerting more force. This causes indicated level to be lower than actual.

• Reference Leg Flashing: During rapid depressurization below saturation pressure, reference leg water may flash to steam. The reduced height of water in the leg decreases sensed D/P, causing indicated level to fail high.

• Ambient Temperature: Increased building temperature decreases reference leg density, leading to lower sensed D/P and higher-than-actual level indication.

Failure Modes Summary for Wet Reference Leg (H.P. Connection)

• Variable Leg Break: High sensed D/P leads to low level indication.

• Reference Leg Break: Low sensed D/P leads to high level indication.

• Ruptured D/P Diaphragm: Pressure equalizes (minimum D/P), leading to maximum level indication.

Other Level Instruments

• Electric Probes: Use the liquid's ability to conduct electricity. A DC voltage is supplied; when liquid contacts electrodes, current flows to ground.

• Pressure Gage with Diaphragm Box: Uses an air-filled box with a flexible diaphragm. Rising liquid level increases air pressure transmitted to the gage.

• Bubbler Device: Regulated air is supplied to a dip tube placed $2-3\,\text{inches}$ from the container bottom. The air pressure needed to produce bubbles ($1-2\,\text{per second}$) equals the static head of the liquid.

Flow Measurement Theory

• Flowrate: Fluid quantity passing a point per unit time (e.g., $\text{lbm/hr}$ for steam mass flow, $\text{gpm}$ for volumetric liquid flow).

• Total Flow: Total quantity of material passed over a specified interval.

Differential Pressure Detectors (Head Flow Meters)

• Operate on the principle that placing a restriction in flow converts pressure head (energy) into velocity head.

• The volumetric flow rate ($\dot{V}$) remains constant, but pressure drops proportional to the square of flow:     $D/P \propto (\dot{V})^2$

• Flow Equation:     $\dot{V} = K\sqrt{D/P}$

• Square Root Extractors: Circuits that convert the $\Delta P$ signal into a linear flow signal using the relationship:     $\frac{\dot{V}_2}{\dot{V}_1} = \frac{\sqrt{D/P_2}}{\sqrt{D/P_1}}$

Flow Detecting Devices

• Orifice Plate: A thin metal plate with a sharp hole. Inexpensive but has a large unrecoverable pressure loss ($60\%$ to $80\%$) and is susceptible to erosion.

  • Vena Contracta: Point downstream from the orifice with maximum velocity and minimum pressure.

  • Drain/vent holes are drilled to prevent particle buildup or gas puddles.

• Venturi Tube: Consists of a converging inlet, a cylindrical throat (where L.P. is measured), and a diverging recovery cone. Most accurate flow detector with low pressure loss ($10\%$ to $25\%$).

• Flow Nozzle: Streamlined design suitable for high-velocity flows and fluids with small percentages of solids. Pressure loss comparable to orifice plates.

• Elbow Meter: Measures pressure difference between inner and outer pipe radii at a bend. repeatable and accurate at high flow rates.

• Gentilli Tube: Uses 5 pairs of tubes (pointing with/against flow) to provide a common $\Delta P$ header.

Density Compensation in Flow

• Mass flow rate ($\dot{m}$) is calculated as:     $\dot{m} = \rho\dot{V}$

• In steam flow, static pressure and temperature signals are used to compensate for density changes. If the pressure signal fails low, indicated flow decreases.

Position Detectors

• Potentiometers: Uses a slide wire to provide a $1-5\,\text{VDC}$ feedback signal proportional to valve stroke. Loss of power or open circuit causes indication to fail off-scale low ($0\,\text{VDC}$).

• Limit Switches: Mechanically closed electrical paths for remote indication or interlocking. Vulnerable to mechanical binding.

• Reed Switches: Flexible ferrous strips (reeds) activated by a permanent magnet (on a valve stem or control rod). More reliable than limit switches.

• Linear Variable Differential Transformer (LVDT): Uses a primary and two secondary coils with a movable core driven by the valve stem. Extremely accurate; open circuits cause the signal to show a mid-position (zero output).

• Magnetic Coil Stack: Used for control rod position; the rod drive shaft concentrates magnetic flux, changing counter-EMF in the coils.

• Torque Switches: Control the force applied to motor-operated valves. They can stop the motor if the valve binds before reaching full travel.

Signal Transmission and Transducers

Electronic Transducers

• Strain Gauge: Resistance increases as the conductor's length increases or cross-sectional area decreases.

• Capacitive: Measures change in capacitance from plates moving closer/further apart.

• Piezoelectric Crystals: Deforming a crystal produces an electric potential. Used for high-frequency measurements like shock or vibration (Loose Parts Monitors).

Signal Transmission

• P to E Converters: Convert pneumatic signals ($3-15\,\text{psig}$) to DC current ($4-20\,\text{mA}$ or $10-50\,\text{mA}$).

• Live Zero: A low-limit signal of $4\,\text{mA}$ that allows operators to distinguish between a zero reading and a failed/broken circuit (which would drop to $0\,\text{mA}$).

Pneumatic Systems

• Flapper-Nozzle Unit: Converts mechanical motion ($0.006\,\text{inches}$) into a $3-15\,\text{psig}$ air signal. Simple but sluggish.

• Relay Valve: Acts as a mechanical amplifier, using a diaphragm and ball valve to achieve faster response distances.

• Proportional Feedback Device: Adds a feedback bellows to dampen the signal, smoothing and stabilizing the output.

Loop Seals

• A "U" shaped water leg separating regions of different pressure while allowing drainage.

• Example 1: Sinks prevent sewage gas backflow.

• Example 2: Draining steam jet air ejector condensers ($3\,\text{psia}$) to the main condenser ($1\,\text{psia}$). A difference of $2\,\text{psia}$ requires a loop seal > 4.62\,\text{feet} ($1\,\text{psia} = 2.31\,\text{feet}$).

• Example 3: Overflow between UST ($28\,\text{in Hg Vac}$) and CST ($15\,\text{psia}$) requires a loop seal > 32.34\,\text{ft}.

Operating Experience (OE)

Oconee 3 Pressurizer Level Decrease (April 2009)

• While in Mode 5 (cold shutdown), pressurizer level dropped from $108\,\text{inches}$ to $77\,\text{inches}$ at a rate of up to $8\,\text{inches/minute}$ during the removal of a relief valve.

• Cause: Inadequate venting resulted in a slight vacuum; levels equalized when the safety valve was removed.

SOER 97-1: Potential loss of HPI and Charging Capability

• Common Reference Leg Failures: Detectors sharing a single reference leg face common cause failure. A leak in the common leg at Oconee 3 led to indicated normal levels in the LDST when the tank was actually empty, causing high-pressure injection (HPI) pump damage.

• Gas Intrusion Mechanisms:

  • Gas intrusion through existing piping.

  • Stripping of dissolved gases in recirculation lines.

  • Back-leakage through vent systems or check valves.

  • Failure of nitrogen-charged pulsation dampeners.

• Operational Impacts: Gas binding in HPI/Charging pumps increases fuel damage probability during small-break LOCAs.

• Maintenance: Over-tightening compression fittings and inadequate lubrication of motor-operated valves contribute to these failures.