Operator Generic Fundamentals Components - Sensors and Detectors 1

Operator Generic Fundamentals Components - Sensors and Detectors 1

Sensors and Detectors Introduction
Operator and automatic actions rely on accurate information provided by sensors and detectors Operators use sensors and detectors to: Monitor key parameters that can affect plant operation and public safety Analyze the parameters for trends and abnormal conditions Sensors, detectors, and their associated circuitry measure and indicate different kinds of parameters Temperature, pressure, level, flow, position, radiation, and reactor power level Related KAs are from: Sensors and Detectors (CFR 41.7) K1.01 Characteristics of venturis and orifices 2.2* 2.4 K1.02 Temperature/density compensation requirements K1.03 Effects of gas or steam on liquid flow rate indications (erroneous reading) K1.04 Modes of failure K1.05 Explain the operation of a flow D/P cell type flow detector K1.06 Temperature/pressure compensation requirements K1.07 Theory and operation of level detectors K1.08 †Effects of operating environment (pressure and temperature) K1.09 Modes of failure K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) K1.11 Effects of operating environment (pressure, temperature) K1.12 Modes of failure K1.13 Theory and operation of T/C, RTD, thermostats K1.14 Failure modes of T/C and RTD K1.15 Failure lodes of reed switches, LVDT, limit switches, and potentiometers K1.16 Applications of reed switches, magnets, LVDT,potentiometers, and limit switches Intro

Terminal Objectives At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80 percent on the following Terminal Learning Objectives (TLOs): Describe the operation of temperature detectors and conditions which effect their accuracy and reliability. Describe the operation of pressure detectors and conditions which effect their accuracy and reliability. Describe the operation of level detectors and conditions which effect their accuracy and reliability. Describe the operation of flow detectors and conditions which effect their accuracy and reliability. Describe the operation of position detectors and conditions which effect their accuracy and reliability. Intro

Temperature Detectors Overview
TLO 1 – Describe the operation of temperature detectors and conditions which effect their accuracy and reliability. The hotness or coldness of a piece of a material depends upon the molecular activity Kinetic energy measures the molecular activity Temperature is a measure of the kinetic energy Temperature detectors provide an important indication of the condition of equipment and material An operator uses temperature-monitoring data to prevent equipment problems Can result from temperatures that are either too high or too low Whether attempting to determine the temperature of the surrounding air, the temperature of coolant in a car’s engine, or the temperature of components of an industrial facility, it is necessary to have some means of measuring the kinetic energy of the material. Most temperature measuring devices use the energy of the material or system they are monitoring to raise (or lower) the kinetic energy of the device in order to provide an indication of temperature. TLO 1

Enabling Learning Objectives for TLO 1
State the three basic functions of temperature detectors. Describe the construction of a basic RTD including: Component arrangement Materials used Describe how RTD resistance varies for temperature changes. State the purpose of basic temperature instrument detection and control system blocks: RTD Bridge circuit DC-AC converter Amplifier Balancing motor/mechanical linkage TLO 1

Describe bridge circuit compensation for changes in ambient temperature and environmental conditions which can affect temperature detection instrumentation. Describe the effect on temperature indication(s) for the following circuit faults: Short circuit Open circuit Describe alternate methods of determining temperature when the normal sensing devices are inoperable. Describe the construction and operation of a thermocouple. TLO 1

Functions of Temperature Detectors
ELO 1.1 – State the three basic functions of temperature detectors. The three basic functions of temperature detectors are: Indication Alarm Control (in the form of both protective interlocks and automatic trip functions) No related KA objectives ELO 1.1

Temperature Monitoring
Temperature is a measure of molecular activity Kinetic energy is a measure of the activity of atoms which make up molecules of any material Temperature ⇒ measure of kinetic energy of material in question ELO 1.1

Filled System Thermometer
Can provide local and remote indication Record of temperature away from system Consists of a sensing element (a bulb containing gas or liquid) and an indicator scale As temperature changes, Gas or liquid pressure changes Acts on a receiving element (spiral bourdon tube) Motion can drive a pointer, an indicator, or actuate a switch for control response Filled system thermometers are available to detect temperatures ranging from approximately -400oF to 1,000oF, depending on the filling medium used in the detector bulb. These types of detectors can detect temperature from distances of up to 400 feet. Figure: Filled System Thermometer ELO 1.1

Bimetallic Strip Thermometer
A simple, rugged device for monitoring temperature Two strips of metal fastened together throughout their length One end fixed, the other free to move Two different coefficients of thermal expansion Two metals will both always be at the same temperature If heated, the bimetallic bends to adapt to the increased length of the metal with the greater temperature coefficient of expansion Figure: Bimetallic Strip Figure: Bimetallic Strip Thermometer Often, the bimetallic element is wound into a spiral with one end fixed. A pointer attached to the free end of the element will rotate with temperature changes to provide temperate indication as shown in figure below. The general range of operation for bimetallic strip thermometers is from -200oF to 1,000oF. ELO 1.1

Functions of Temperature Detectors
Knowledge Check Temperature detection is used to provide the following: (select all that apply) Interlocks Indications Alarms Automatic trips Correct answer in A, B, C, and D. Correct answers are A, B, C, and D. ELO 1.1

Resistance Temperature Detector Construction
ELO 1.2 – Describe the construction of a basic RTD including the following: a. Component arrangement b. Materials used RTDs act like electrical transducers Convert changes in temperature to changes in voltage Usually constructed of pure metal or alloy Increase resistance as temperature increases Decrease resistance as temperature decreases Related KAs: K1.13 Theory and operation of T/C, RTD, thermostats ELO 1.2

Metals best suited for RTD use are as follows: Pure Uniform quality Stable within a given range of temperature Able to give reproducible resistance-temperature readings Capable of being drawn into fine wire ELO 1.2

Figure: Internal Construction of a Typical RTD Elements are usually: Long, spring-like wires Surrounded by insulator Enclosed in metal (inconel) sheath Inconel normally used because of inherent corrosion resistance Change in temperature causes platinum wire to heat or cool Resistance change measured by precision resistance measuring device ELO 1.2

Figure: RTD Protective Well and Terminal Head ELO 1.2 15

Knowledge Check A resistance temperature detector operates on the principle that the change in electrical __________ of a metal is ________ proportional to its change in temperature. conductivity; directly current; directly resistance; indirectly resistance; directly Correct answer in D. resistance; directly Correct answer is D. ELO 1.2

Temperature Resistance Relationship – Resistance Temperature Detector
ELO 1.3 – Describe how RTD resistance varies for temperature changes. Figure: Resistance vs. Temperature Graph Some metals have a linear coefficient of resistivity Change in resistance as temperature changes RTDs operate on the principle that the change in electrical resistance of a metal is directly proportional to its change in temperature KA K1.13 Theory and operation of T/C, RTD, thermostats The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature. ELO 1.3

Normally constructed of platinum, copper, or nickel Linear resistance- temperature characteristics High coefficient of resistance Ability to withstand repeated temperature cycles Figure: Resistance vs. Temperature Graph ELO 1.3

Knowledge Check What happens to the resistance of a resistance temperature detector (RTD) when the temperature of the substance it is measuring increases? Resistance of the RTD decreases and then increases. Resistance of the RTD decreases. Resistance of the RTD increases. Resistance of the RTD remains the same. Correct answer in C. Resistance of RTD increases Correct answer is C. ELO 1.3

RTD Temperature Detection Circuit
ELO 1.4 – State the purpose of basic temperature instrument detection and control system blocks: RTD, bridge circuit, DC-AC converter amplifier, and balancing motor/mechanical linkage. Bridge circuit used with RTD element to obtain accurate measurements of temperature Basic bridge circuit components Three known resistances, R1, R2, and R3 (variable) Unknown variable resistor Rx Source of voltage Sensitive ammeter No Related KAs for ELO Figure: Typical Bridge Circuit ELO 1.4

RTD Bridge Circuit Ratio arms of bridge: R1 and R2
Standard arm: R3 (variable resistor) Adjusted to match unknown resistor (Rx) Sensing ammeter visually displays current that is flowing through bridge circuit Figure: Typical Bridge Circuit ELO 1.4

Unbalanced RTD Bridge Circuit
Uses millivolt meter calibrated in units of temperature that correspond to RTD resistance Battery connected to two opposite points of bridge circuit Millivolt meter connected to two remaining points Rheostat regulates bridge current Regulated current is divided between two branches One branch has fixed resistor Rx and range resistor R1, Other branch with RTD and range resistor R2 Figure: Unbalanced Bridge Circuit ELO 1.4

Unbalanced RTD Bridge Circuit
As electrical resistance of RTD changes, voltage at points X and Y changes Millivolt meter detects change in voltage caused by unequal division of current in two branches Figure: Unbalanced Bridge Circuit ELO 1.4

Balanced Bridge Circuit
Uses galvanometer to compare RTD resistance with that of fixed resistor Galvanometer uses pointer that deflects on either side of zero when resistance of arms not equal Slidewire resistor used to balance arms of bridge Circuit balanced whenever value of slidewire resistance is such that no current flows through galvanometer Figure: Balanced Bridge Circuit ELO 1.4

Balanced Bridge Circuit
Resistance of slide wire adjusted until galvanometer indicates zero Value of slide resistance used to determine temperature of system being monitored For each temperature change, there is new value of resistance Slider must be moved to new position to balance circuit Figure: Balanced Bridge Circuit ELO 1.4

Typical Temperature Detection Circuit
Block diagram represents balanced bridge temperature detection circuit modified to eliminate galvanometer RTD measures temperature Detector is felt as resistance to bridge network Bridge network converts resistance to DC voltage signal Figure: Basic Temperature Detection Circuit ELO 1.4

Typical Temperature Detection Circuit
Electronic instrument converts DC voltage of potentiometer or bridge to AC voltage AC voltage amplified to higher (usable) voltage to drive bi-directional motor Bi-directional motor positions slider on slidewire to balance circuit resistance Figure: Basic Temperature Detection Circuit ELO 1.4

RTD Temperature Detection Circuit
Knowledge Check Typical temperature bridge circuits use low voltage (millivolt) signals. How does this low voltage drive a remote meter indication? The signal is amplified, which raises the voltage. The signal is converted from AC to DC, which raises the voltage. The signal is amplified, which lowers the voltage. The signal is converted from DC to AC, which raises the voltage. Correct answer in A. Correct answer is A. ELO 1.4

Environmental Effects On Temperature Detection
ELO 1.5 – Describe bridge circuit compensation for changes in ambient temperature and environmental conditions which can affect temperature detection instrumentation. Ambient Temperature Ambient temperature variations will affect accuracy and reliability of temperature detection instrumentation Variations in ambient temperature directly affect Resistance of components in bridge circuit Resistance of thermocouple reference junction ELO 1.5

Humidity Presence of ambient humidity affects most electrical equipment Causes moisture to collect on the equipment Leads to short circuits, grounds, and corrosion Could damage components The proper use of HVAC equipment controls humidity ELO 1.5

Knowledge Check To compensate for ambient temperature change, both three and four wire resistance temperature detector circuits use the same amount of lead wire in both branches of the bridge circuit because... the change in resistance will be felt on neither branch. the change in resistance is not an important factor in temperature measurement. the change in resistance will be felt on both branches equally. the change in resistance is important only when calibrating temperature circuits. Correct answer in C. Correct answer is C. ELO 1.5

RTD Circuit Failures ELO 1.6 – Describe the effect on temperature indication(s) for the following circuit faults: a. Short circuit b. Open circuit If RTD in either unbalanced or balanced bridge circuit becomes open: Resistance infinite (open) Temperature indicating meter will indicate very high temperature If RTD becomes shorted: Resistance zero (short) Temperature indicating meter will indicate a very low temperature Related KA - K1.14 Failure modes of T/C and RTD If bidirectional motor used to balance bridge (slidewire) open may cause indication to “spin” in high temperature direction without stopping. ELO 1.6

RTD Circuit Failures Knowledge Check
Consider the circuit below. What would the meter read if the lead between Y and the resistance temperature detector developed an open circuit? 300° 600° 0° Dependent on thermocouple temperature Correct answer in B. Correct answer is B. ELO 1.6

RTD Circuit Failures Knowledge Check – NRC Bank
If shorting occurs within a resistance temperature detector, the associated indication will fail... low. high. as is. to midscale. Correct answer in A. Correct answer is A. ELO 1.6

Alternate Temperature Indications
ELO 1.7 – Describe alternate methods of determining temperature when the normal sensing devices are inoperable. Installed spare/dual-element RTDs Dual-element RTD has two sensing elements If operating element becomes faulty, second element may be used Contact pyrometer (portable thermocouple) or optical pyrometer If detector itself is still functional, connect an external bridge circuit to detector Temperature obtained by comparing resistance readings to detector calibration curves Related KA - K1.13 Theory and operation of T/C, RTD, thermostats ELO 1.7

Knowledge Check In the circuit below, a dual element resistance temperature detector (RTD) indicates temperature. If the RTD develops an internal open circuit (bridge circuit remains intact), temperature indication could be obtained by… connecting a spare RTD into the circuit. doing nothing, the existing circuit will still measure temperature with an open circuit. direct resistance measurements. surface resistor. Correct answer in A. Correct answer is A. ELO 1.7

Knowledge Check – NRC Bank A simple two-wire resistance temperature detector (RTD) is being used to measure the temperature of a water system. Copper extension wires run from the RTD to a temperature instrument 40 feet away. If the temperature of the extension wires decreases, the electrical resistance of the extension wires will __________ and the temperature indication will __________, unless temperature compensation is provided. increase; increase increase; decrease decrease; increase decrease; decrease Correct answer in C. Correct answer is C. ELO 1.7

Thermocouples ELO 1.8 – Describe the construction and operation of a thermocouple. A thermocouple converts thermal energy into electrical energy Amount of current produced depends on: Temperature difference between measurement and reference junction Characteristics of two metals used Characteristics of attached circuit 1.8 Describe the construction and operation of a thermocouple. ELO 1.8

Thermocouple Construction
Thermocouple is a sensitive and highly accurate temperature measuring device Constructed of two dissimilar metal wires joined together at one end (junction) Other end of each wire is connected to a measuring instrument Figure: Simple Thermocouple Circuit ELO 1.8

Leads of thermocouple are encased in a rigid metal sheath Measuring junction at bottom of thermocouple housing Figure: Internal Construction of a Typical Thermocouple ELO 1.8

Magnesium oxide surrounds thermocouple wires Prevents vibration that could damage fine wires Enhances heat transfer between measuring junction and medium surrounding thermocouple Figure: Internal Construction of a Typical Thermocouple ELO 1.8

Thermocouple Operation
Heating measuring junction of thermocouple produces a voltage Temperature indicated equals hot junction voltage minus cold junction voltage Figure: Simple Thermocouple Circuit ELO 1.8

Thermocouple Failures and Disadvantages
If break occurs in wire and there is no current flow, the device fails low If break or open occurs in the detector, the indicated temperature fails to the reference junction temperature Change in reference junction temperature causes a change in indication If the temperature at the reference junction were to decrease, the indicated temperature would increase If the temperature at the reference junction were to increase, the indicated temperature would decrease Reference junction temperature should be controlled Thermocouple is less accurate than a resistance temperature detector ELO 1.8

Thermocouples Knowledge Check – NRC Bank
A resistance temperature detector (RTD) and a thermocouple (TC) are commonly used sensors for temperature measurement. If a temperature display fails, which of the sensors, if any, has a property that can be measured manually and converted to a temperature value with the aid of conversion tables. TC only RTD only Both TC and RTD Neither TC nor RTD Correct answer in C. Correct answer is C. ELO 1.8

Thermocouples Knowledge Check – NRC Bank
Refer to the drawing of a simple thermocouple circuit below. A thermocouple temperature indication is initially 410°F with the reference (cold) junction at 125°F. An ambient temperature decrease lowers the reference junction temperature to 110°F, while the measuring junction temperature remains constant. Without temperature compensation for the reference junction, the new thermocouple temperature indication will be... 380°F. 395°F. 410°F. 425°F. Correct answer in D. Correct answer is D. ELO 1.8

Thermocouples Knowledge Check – NRC Bank Question
An open circuit in a thermocouple detector causes the affected temperature indication to fail... high. low. to reference junction temperature. as is. Correct answer in C. Correct answer is C. ELO 1.8

Temperature Detectors
Knowledge Check – NRC Bank Question Which one of the following is a characteristic of a resistance temperature detector but not a thermocouple? Sensing element is made from a single metal or alloy Requires a reference junction for accurate temperature measurement Extension leads made from relatively expensive metals or alloys are required for accurate temperature measurement Temperature measurement relies on a sensor material property that varies directly with the change in the measured temperature Correct answer in A. Correct answer is A. ELO 1.8

TLO 1 Summary Give students approximately 20 minutes to complete the puzzle. Review key with class after all students have completed. TLO 1

TLO 1 Summary Temperature detector uses are as follows: Indication
Alarm functions Control functions RTD operates on the principle that change in electrical resistance of a metal is directly proportional to its change in temperature. As temperature increases, resistance increases As temperature decreases, resistance decreases Open circuit in temperature instrument is indicated by a very high temperature Short circuit in a temperature instrument is indicated by a very low temperature Review the enabling objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary If a temperature detector becomes inoperative:
A spare detector may be used (if dual element installed) A contact or optical pyrometer can be used. A thermocouple consists of two dissimilar wires joined at one end encased in a metal sheath The other end of each wire connects to a meter or measuring circuit The heating measuring junction produces a voltage greater than voltage across the reference junction Less accurate than the RTD Open circuit in detector is indicated by temperature failing to the reference junction temperature Review the enabling objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary TLO 1 – Explain the operation of temperature detectors and conditions which effect their accuracy and reliability. State the three basic functions of temperature detectors. Describe the construction of a basic RTD including the component arrangement and the materials used. Describe how RTD resistance varies for temperature changes. State the purpose of the following basic temperature instrument detection and control system blocks: RTD, bridge circuit, DC-AC converter, amplifier, and balancing motor/mechanical linkage. Describe bridge circuit compensation for changes in ambient temperature and environmental conditions which can affect temperature detection instrumentation. Describe the effect on temperature indication(s) for the following circuit faults: short circuit and open circuit. Describe alternate methods of determining temperature when the normal sensing devices are inoperable. Describe the construction and operation of a thermocouple. Review the enabling objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

Pressure Detectors Overview
TLO 2 – Explain the operation of pressure detectors and conditions which effect their accuracy and reliability. Pressure measurements control many processes It is important to understand how pressure changes affect pressure gauges Gauge pressure: The pressure indicated by a pressure detector The pressure of the system less the atmospheric pressure. The equation below expresses this relationship: 𝑃 𝑔𝑎𝑢𝑔𝑒 = 𝑃 𝑠𝑦𝑠𝑡𝑒𝑚 − 𝑃 𝑎𝑡𝑚 TLO 2

State the three functions of pressure measuring instrumentation. Describe the theory and operation of the following pressure detectors: Bellows Diaphragms Bourdon tubes Variable capacitance Describe how a bellows-type pressure detector produces an output signal including: Method of detection Method of signal generation TLO 2

Describe how a bourdon tube-type pressure detector produces an output signal including: Method of detection Method of signal generation Describe how a strain gauge pressure transducer produces an output signal including: TLO 2 - Explain the operation of Pressure detectors and conditions which effect their accuracy and reliability. TLO 2

Describe how the following pressure transducers develop a signal proportional to pressure changes: Slidewire Inductance-type transducer Differential transformer Capacitance-type transducer State the purpose of typical pressure detection device blocks used on basic block diagram: Sensing element Transducer Pressure detection circuitry Pressure indication TLO 2 - Explain the operation of Pressure detectors and conditions which effect their accuracy and reliability. TLO 2

Describe the environmental conditions which can affect the accuracy and reliability of pressure detection instrumentation. Describe alternate methods of determining pressure when the normal pressure sensing devices are inoperable. TLO 2

Pressure Detector Functions
ELO 2.1 – State the three functions of pressure measuring instrumentation. Pressure detectors are used to provide three basic functions: Indication Alarm Control Display may be local or remote May have audible and/or visual alarms May have control functions Such that equipment is started or stopped as needed No related NRC KA ELO 2.1

Knowledge Check Pressure detectors provide the following: (select all that apply) Indications Automatic trips Interlocks Alarms Correct answer in A, B, C, and D. Correct answers are A, B, C, and D. ELO 2.1

Pressure Detector Theory
ELO 2.2 – Describe the theory and operation of the following pressure detectors: a. Bellows b. Diaphragms c. Bourdon tubes d. Variable capacitance Normally a device connected so that system pressure is exerted on one side while atmospheric pressure exerted on the other The pressure difference produces movement This movement is directly proportional to the pressure differential change Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) ELO 2.2

Pressure Detector Theory
Example In the detector shown, the metallic bellows expands and contracts as the system pressure (connected to the high-pressure tap) changes Movement is a result of the actual differential pressure between the system pressure and atmospheric pressure Bellows connect to a rod that moves a pointer connected to a linkage Pointer deflects across a scale, allowing the operator to read pressure directly Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) Figure: Basic Metallic Bellows ELO 2.2

Knowledge Check In the pressure detector shown below, the high-pressure tap is connected to the system. As system pressure increases... the bellows will collapse. the bellows will expand. the bellows will remain as is. the linkage will expand. Correct answer in A. Correct answer is A. ELO 2.2

Bellows-Type Detectors
ELO 2.3 – Explain how a bellows-type pressure detector produces an output signal including: a. Method of detection b. Method of signal generation Sensitive to low pressures Most accurate when measuring pressures from 0.5 to 75 psig When used in conjunction with a heavy range spring, some bellows can be used to measure pressures of over 1,000 psig Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) ELO 2.3

Bellows Construction Bellows One-piece Collapsible Seamless
Deep folds formed from very thin-walled tubing Figure: Basic Metallic Bellows ELO 2.3

Bellows Operation System pressure applied to area surrounding the bellows Bellows will expand or contract Moving end of the bellows is connected to a mechanical linkage assembly As bellows and linkage assembly move, either: Electrical signal is generated Direct pressure indication Figure: Basic Metallic Bellows ELO 2.3

Bellows-Type Detectors
Knowledge Check A bellows pressure transmitter with its low-pressure side vented to containment atmosphere measures reactor coolant system (RCS) pressure. A decrease in the associated pressure indication could be caused by either a containment pressure ____________ or an RCS pressure ____________. decrease; increase increase; decrease decrease; decrease increase; increase Correct answer in B. Correct answer is B. ELO 2.3

Bourdon Tube-Type Detectors
ELO 2.4 – Explain how a bourdon tube-type pressure detector produces an output signal including: a. Method of detection b. Method of signal generation Thin-walled tube that is flattened diametrically on opposite sides Pressure applied to the inside of the tube causes tube to straighten slightly Tip of the tube used to position a pointer or to develop an equivalent electrical signal Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) Figure: Bourdon Tube Detector Construction ELO 2.4

Bourdon Tube-Type Detectors
Knowledge Check If the pressure sensed by a bourdon tube increases, the curvature (amount of curve) of the detector will ____________ because the greater force is being applied to the ____________ curve of the detector. increase; outer decrease; outer increase; inner decrease; inner Correct answer in B. Correct answer is B. ELO 2.4

Strain Gauge ELO 2.5 – Explain how a strain gauge pressure transducer produces an output signal including: a. Method of detection b. Method of signal generation Strain gauge measures external force (pressure) applied to fine wire Fine wire usually arranged in form of grid Pressure change causes resistance change due to distortion of wire Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) Figure: Strain Gauge ELO 2.5

Strain Gauge Value of pressure found by measuring change in resistance of wire grid R = resistance of wire grid in ohms K = resistivity constant for particular type of wire grid L = length of wire grid A = cross sectional area of wire grid 𝑅=𝐾 𝐿 𝐴 Figure: Strain Gauge ELO 2.5

Strain Gauge Wire grid is distorted by elastic deformation:
Length increases Cross-sectional area decreases Changes cause increase in resistance of wire Change in resistance used as variable resistance in bridge circuit that provides an electrical signal for indication of pressure Figure: Strain Gauge ELO 2.5

Strain Gauge Increase in pressure at inlet of bellows causes bellows to expand Expansion of bellows moves flexible beam to which strain gauge has been attached Movement of beam causes resistance of strain gauge to change Temperature compensating gauge compensates for heat produced by current flowing through fine wire of strain gauge Figure: Strain Gauge Pressure Transducer ELO 2.5

Strain Gauge When change in resistance in strain gauge causes an unbalanced condition Error signal enters amplifier Actuates balancing motor Moves slider along slidewire, restoring bridge to balanced condition Slider’s position is noted on scale marked in units of pressure Figure: Strain Gauge Used in a Bridge Circuit ELO 2.5

Strain Gauge Knowledge Check
Semiconductor strain gages are often used in transmitters for... control rod position instruments. reactor coolant pressure instruments. reactor coolant temperature instruments. steam generator level instruments. Correct answer in B. Correct answer is B. ELO 2.5

Pressure Transducers ELO 2.6 – Describe how the following pressure transducers develop a signal proportional to pressure changes: a. Slidewire b. Inductance-type transducer c. Differential transformer d. Capacitance-type transducer Some resistance-type transducers combine a bellows or a bourdon tube with a variable resistor Expansion and contraction causes the attached slider to move along the slidewire, increasing or decreasing the resistance Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) Figure: Slidewire Resistance Type Transducer ELO 2.6

Inductance Type The inductance-type transducer consists of the following three parts: Coil Movable magnetic core Pressure-sensing element Figure: Inductance Type Transducer ELO 2.6

Differential Transformer
Differential transformer pressure transducer utilizes two coils wound on a single tube Magnitude and direction of the output depends on distance core is displaced from its center position Figure: Differential Transformer ELO 2.6

Variable Capacitive Type
Two flexible conductive plates and a dielectric Dielectric is the fluid whose pressure is being measured As pressure increases, Flexible conductive plates move farther apart Changes capacitance of transducer Figure: Variable Capacitive Type Transducer ELO 2.6

Pressure Transducers Knowledge Check
A type of pressure sensor that is constructed of two conductive plates separated by a dielectric substance is a _______________ pressure detector. bellows-type bourdon-type capacitive-type inductance-type Correct answer in C. Correct answer is C. ELO 2.6

Pressure Detection Circuitry
ELO 2.7 – State the purpose of typical pressure detection device blocks used on basic block diagram: a. Sensing element b. Transducer c. Pressure detection circuitry d. Pressure indication No Related KA Figure: Pressure Detection Circuit Block Diagram ELO 2.7

Sensing Element Senses pressure of monitored system
Figure: Pressure Detection Circuit Block Diagram Senses pressure of monitored system Converts pressure to a mechanical signal Supplies mechanical signal to transducer ELO 2.7

Transducer Figure: Pressure Detection Circuit Block Diagram Converts mechanical signal to electrical signal that is proportional to system pressure If mechanical signal from sensing element is used directly, a transducer is not required ELO 2.7

Detector Circuit Figure: Pressure Detection Circuit Block Diagram Will amplify and/or transmit signal to pressure indicator Electrical signal generated by detection circuitry is proportional to system pressure ELO 2.7

Pressure Indicator Provides indication of system pressure
Figure: Pressure Detection Circuit Block Diagram Provides indication of system pressure May either be read locally or at remote location ELO 2.7

Pressure Detection Circuitry
Knowledge Check In a typical pressure detection circuit, the __________ senses the pressure of the monitored system and converts the pressure to a mechanical signal. pressure indicator transducer slidewire sensing element Correct answer in D. Correct answer is D. ELO 2.7

Environmental Effects On Pressure Detection
ELO 2.8 – Describe the environmental conditions which can affect the accuracy and reliability of pressure detection instrumentation. Ambient Pressure Pressure instruments are sensitive to variations in atmospheric pressure surrounding detector Especially apparent when detector is located within an enclosed space Ambient Temperature Ambient temperature affects resistance of components in circuitry Effects are reduced by design of circuitry and by maintaining proper environment K1.11 Effects of operating environment (pressure, temperature) ELO 2.8

Humidity Presence of humidity will affect electrical equipment, especially electronic components High humidity causes moisture to collect on equipment Moisture can cause short circuits, grounds, and corrosion, which effect component performance Effects due to humidity are controlled by maintaining equipment in proper environment ELO 2.8

Penetrating Radiation Radiation levels can affect detector reliability Extremely high radiation environments permanently embrittle the metal in detectors Changes characteristics and elasticity of sensing mechanisms, introducing errors High radiation levels can also affect the sensitive electronic circuits housed in detectors ELO 2.8

Knowledge Check A pressure-sensing element located inside a primary containment will be subject to which of the following environmental effects during a steam leak inside containment? (Select all that apply.) Humidity Atmospheric pressure Temperature Alpha radiation Correct answer in A, B, and C. Correct answers are A, B, and C. ELO 2.8

Alternate Pressure Detection
ELO 2.9 – Describe alternate methods of determining pressure when the normal pressure sensing devices are inoperable. Spare detector elements may be utilized if installed If spare detectors are not installed: Pressure may be read at an independent local mechanical gauge, if available Precision pressure gauge (Heise gauge) may be installed in system at convenient point If detector is functional, obtain pressure readings by measuring voltage or current values and comparing this reading with calibration curves Related KA - K1.12 Modes of failure ELO 2.9

Detector Failure Pressure instruments are designed and selected to withstand pressure above normal design pressure However, sudden overpressurizations causing over-range conditions could straighten bourdon tubes and bellows If sensing element of detector is stretched or stressed, indications may be erroneously high If sensing element has a leak or rupture, instrument will fail with a low indication ELO 2.9

Knowledge Check – NRC Bank Refer to the drawing of a bellows‑type differential pressure (D/P) detector. The spring in this detector (shown in a compressed state) has weakened from long‑term use. If the actual D/P is constant, how will indicated D/P respond as the spring weakens? Increase, because the spring will expand more Decrease, because the spring will expand more Increase, because the spring will compress more Decrease, because the spring will compress more Correct answer is C. Correct answer is C. ELO 2.9

Detector Failures Knowledge Check – NRC Bank
Refer to the drawing of a bellows‑type differential pressure (D/P) detector. A bellows-type pressure detector with its low-pressure side vented to containment atmosphere is being used to measure reactor vessel pressure. A decrease in the associated pressure indication will be caused by either a containment pressure __________ or a __________. increase; ruptured bellows increase; broken spring decrease; ruptured bellows decrease; broken spring Correct answer is A. Correct answer is A. ELO 2.9

TLO 2 Summary Pressure detector basic functions: Indication Alarm
Control In a bellows-type detector: System pressure is applied to external area surrounding a bellows As pressure changes, the bellows and linkage assembly move and cause production of an electrical signal or movement of a gauge pointer In a bourdon tube-type detector: System pressure is applied to inside of a slightly flattened arc- shaped tube Pressure increases tube tends to restore to its original round cross- section causes tube to straighten Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary Operation of strain gauge:
Measures pressure applied to a fine wire usually arranged in form of a grid Pressure change causes a resistance change due to distortion of wire grid Often used in transmitters for reactor coolant pressure instruments Slidewire pressure transducer operation: Consists of a bellows or a bourdon tube with a variable resistor Expansion or contraction of bellows causes attached slider to move along slidewire, increasing or decreasing the resistance, and thereby indicating an increase or decrease in pressure Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary Inductance-type pressure transducer operation:
Consists of a coil, movable magnetic core, and pressure-sensing element Sensing element and magnetic core are tied together, and, as pressure varies, the element and the core move inside the coil An AC voltage acts on the coil, and, as the core moves, the inductance of the coil changes The current through the coil will increase as the inductance decreases Differential transformer pressure transducer operation: Utilizes two coils wound on a single tube Primary coil winds around the center of the tube; secondary coil splits, with one half wound around each end of the tube Each end winds in the opposite direction, which causes the induced voltages to oppose one another A core, positioned by a pressure element, is movable within the tube The magnitude and direction of the output depends on the amount the core moves from its center position Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary Capacitive-type transducer operation:
Consists of two flexible conductive plates and a dielectric As pressure increases, the flexible conductive plates will move farther apart, changing the capacitance of the transducer This change in capacitance is measurable and is proportional to the change in pressure If a pressure instrument fails: A spare detector element may be utilized if installed Pressure may be read at an independent local mechanical gauge A precision pressure gauge may be installed in the system If the detector is functional, it may be possible to obtain pressure readings by measuring voltage or current values across the detector leads and comparing this reading with calibration curves Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary TLO 2 – Explain the operation of pressure detectors and conditions which effect their accuracy and reliability. State the three functions of pressure measuring instrumentation. Describe the theory and operation of the following pressure detectors: Bellows Diaphragms Bourdon tubes Variable capacitance Describe how a bellows-type pressure detector produces an output signal including: Method of detection Method of signal generation Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary Describe how a bourdon tube-type pressure detector produces an output signal including: Method of detection Method of signal generation Describe how a strain gauge pressure transducer produces an output signal including: Describe how the following pressure transducers develop a signal proportional to pressure changes: Slidewire Inductance-type transducer Differential transformer Capacitance-type transducer Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary State the purpose of typical pressure detection device blocks used on basic block diagram: Sensing element Transducer Pressure detection circuitry Pressure indication Describe the environmental conditions which can affect the accuracy and reliability of pressure detection instrumentation. Describe alternate methods of determining pressure when the normal pressure sensing devices are inoperable. Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

Level Detectors Overview
TLO 3 – Explain the operation of level detectors and conditions which effect their accuracy and reliability. Level detectors provide operators with both local and remote indication of level Remote-level indication is necessary to provide transmittal of vital tank and vessel-level information to a central location, such as the control room Without accurate level indication: Tanks could overflow and result in spills of hazardous materials Tank levels could fall to a low level where equipment damage will result TLO 3

Describe the three functions for using remote level indicators. Describe the operation of the following types of level instrumentation: Gauge glass Magnetic bond Conductivity probe Differential pressure (D/P) Describe density compensation in level detection systems to include: Why needed How accomplished TLO 3

State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: Differential pressure (D/P) transmitter Amplifier Indication Describe the environmental conditions which can affect the accuracy and reliability of level detection instrumentation. State the various failure modes of level detection instrumentation. Analyze detector installation and applications to determine the effects of transients on level indication. TLO 3

Level Detector Functions
ELO 3.1 – Describe the three functions for using remote level indicators. Level detectors are used to provide the following basic functions: Indication Alarm Control ELO 3.1

Level Detector Functions
Liquid level measuring devices are classified into the following groups: Direct method Dipstick in car which measures height of oil in oil pan Inferred method Pressure gauge at bottom of tank which measures hydrostatic head pressure from height of liquid ELO 3.1

Level Detectors Knowledge Check
Level detection provides the following: (select all that apply) Interlocks Alarms Automatic trips Indications Correct answer in A, B, C, and D. Correct answers are A, B, C, and D. ELO 3.1

Operation of Level Detectors
ELO 3.2 – Describe the operation of the following types of level instrumentation: a. Gauge glass b. Magnetic bond c. Conductivity probe d. Differential pressure (D/P) Multiple methods of monitoring and detecting level in plant systems and components Each type of level detector has advantages and disadvantages Related KA - K1.06 Temperature/pressure compensation requirements ELO 3.2

Gauge Glass Transparent tube attached to bottom and top of tank
Top connection not needed in tank open to atmosphere Height of liquid in tube will be equal to height of water in tank Figure: Gauge Glass ELO 3.2

Gauge Glass Figure: Gauge Glass (a) shows a gauge glass used for vessels where liquid is at ambient temperature and pressure conditions (b) shows a gauge glass used for vessels where liquid is at elevated pressure or partial vacuum ELO 3.2

Gauge Glass For higher temperatures and pressures, gauge glass has metal body and heavy glass or quartz used Glass section usually flat to provide strength and safety One side of glass section is prism-shaped Glass molded such that one side has 90°angles which run lengthwise Light rays strike outer surface of glass at 90° angle Figure: Reflex Gauge Glass ELO 3.2

Magnetic Bond Level Detector
Developed to overcome problems of cages and stuffing boxes Magnetic bond mechanism consists of magnetic float which rises and falls with changes in level Figure: Magnetic Bond Level Detector ELO 3.2

Conductivity Probe Level Detector
Consists of: One or more level detectors Operating relay Controller When liquid makes contact with any of electrodes, electric current will flow between electrode and ground Figure: Conductivity Probe Level Detection System ELO 3.2

Differential Pressure Level Detectors
D/P detector connected to bottom of tank being monitored Higher pressure, caused by fluid in tank, is compared to lower reference pressure (usually atmospheric) Comparison takes place in D/P detector Figure: Open Tank Differential Pressure Detector ELO 3.2

Knowledge Check A calibrated differential pressure (D/P) level detector measures the level in a vented tank inside the auxiliary building, shown in the figure below. If building pressure increases with no change in temperature, the associated level indication will... decrease, then increase and stabilize at the actual level. increase and stabilize above the actual level. increase and stabilize below the actual level. remain at the actual level. Correct answer in D. Correct answer is D. ELO 3.2

Density Compensation In Level Detection
ELO 3.3 – Describe density compensation in level detection systems to include: Why needed How accomplished Density compensation considers hydrostatic pressure added by vapor needs Considered when vapor with significant density exists above liquid in tank or vessel Ensures accurate transmitter output K1.06 Temperature/pressure compensation requirements ELO 3.3

Specific Volume Specific volume is the standard unit used when working with vapors and steam that have low values of density For applications that involve water and steam, specific volume can be found using "Saturated Steam Tables” Specific volume is volume per unit mass: Specific volume is the reciprocal of density: 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑉𝑜𝑙𝑢𝑚𝑒= 1 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑉𝑜𝑙𝑢𝑚𝑒= 𝑉𝑜𝑙𝑢𝑚𝑒 𝑀𝑎𝑠𝑠 ELO 3.3

Vessel with Water at Saturated Boiling Conditions
Condensing pot at top of reference leg Condense steam Maintains reference leg filled Effect of steam vapor pressure is cancelled at D/P transmitter Pressure is equally applied to both LP and HP sides of transmitter Figure: Effects of Fluid Density ELO 3.3

Vessel with Water at Saturated Boiling Conditions
Differential pressure seen by transmitter is due only to hydrostatic head pressure 𝐻𝑦𝑑𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐻𝑒𝑎𝑑 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =𝐷𝑒𝑛𝑠𝑖𝑡𝑦×𝐻𝑒𝑖𝑔ℎ𝑡 Figure: Effects of Fluid Density ELO 3.3

Reference Leg Temperature Considerations
When level to be measured is in pressurized tank at elevated temperatures, a number of additional consequences must be considered Temperature of fluid in tank increases, density of fluid decreases As density decreases, fluid expands, occupying more volume Even though density is less, mass of fluid in tank is same ELO 3.3

Reference Leg Temperature Considerations
As fluid in tank is heated and cooled, density of fluid in tank changes Reference leg density remains relatively constant Density of fluid in reference leg is dependent upon ambient temperature Relatively constant and independent of tank temperature Causes indicated level to remain constant If fluid in tank changes temperature (and density) compensation must be provided to have accurate indication Problem is encountered when measuring steam generator water levels ELO 3.3

Compensating for Reference Leg Temperature Changes
Calibration charts allow manual correction Changes in reference leg density can be accounted for during instrument alignments and calibrations Density compensation may be accomplished through electronic circuitry Compensate for density changes automatically Compensate for density by having operators manually adjust inputs to level detection circuitry ELO 3.3

Steam Generator Level Density Compensation
D/P detector measures actual differential pressure Separate pressure detector measures pressure of saturated steam Pressure signal used to correct differential pressure for density Since saturation pressure is proportional to saturation temperature Electronic circuit uses pressure signal to compensate for difference in density between reference leg water and steam generator fluid Figure: Steam Generator Level Detection System ELO 3.3

Knowledge Check Refer to the drawing of a pressurizer differential pressure (D/P) level detection system below. With the nuclear power plant at normal operating conditions, a pressurizer level D/P instrument that had been calibrated while the plant was in a cold condition would indicate _________ than actual level because of a ___________ D/P sensed by the D/P detector at normal operating conditions. higher, smaller lower, smaller higher, larger lower, larger Correct answer in D. Correct answer is D. ELO 3.3

Level Detection Circuitry
ELO 3.4 – State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: a. Differential pressure (D/P) transmitter b. Amplifier c. Indication Typical differential pressure detector consists of: D/P transmitter (transducer) Amplifier Level indication Figure: Differential Pressure Level Detection Circuit ELO 3.4

Figure: Differential Pressure Level Detection Circuit D/P transmitter consists of diaphragm with HP and LP inputs on opposite sides As D/P changes, diaphragm moves Transducer changes mechanical motion into electrical signal ELO 3.4

Figure: Differential Pressure Level Detection Circuit Electrical signal generated by transducer is amplified and passed on to level indicator for level indication at remote location System provides alarms on high and low level via relays May also provide control functions and protective features: Valve repositioning Pump tripping ELO 3.4

Knowledge Check Place the following components in order starting with level sensing to output signal. Alarm Transducer Amplifier Bourdon tube Correct answer in D, B, C, then A. Correct answer is D, B, C, then A. ELO 3.4

Environmental Effects On Level
ELO 3.5 – Describe the environmental conditions which can affect the accuracy and reliability of level detection instrumentation. Fluid Density Primarily affects level sensing instruments which use wet reference leg Possible for reference leg temperature (density) to be different from temperature of fluid whose level is to be measured ELO 3.5

Environmental Effects On Level Detection
An example of this is level detection instrumentation for a boiler steam drum Water in reference leg is at lower temperature than water in steam drum Water in reference leg is more dense and must be compensated for to ensure indicated steam drum level is accurate ELO 3.5

Ambient Temperature Variations in ambient temperature can directly affect resistance of components in instrumentation circuitry, affecting calibration of electric/electronic equipment Effects of temperature variations are reduced by design of circuitry and by maintaining level detection instrumentation in proper environment ELO 3.5

Humidity High humidity causes moisture to collect on equipment Can cause short circuits, grounds, and corrosion which may damage components Effects due to humidity are controlled by maintaining equipment in proper environment ELO 3.5

Environmental Effects On Level - Fluid Density
Knowledge Check Consider the level indicator for a steam generator depicted. A steam leak has occurred and the temperature of the area around the reference leg is increasing. What effect would this have on the indicated level? Indicate higher than actual because resistance of the D/P cell components is increasing Indicate higher that actual because reference leg density is decreasing No effect Indicate lower than actual because reference leg density is increasing Correct answer in B Correct answer is B. ELO 3.5

Level Detection Failure Modes
ELO 3.6 – State the various failure modes of level detection instrumentation. Failure mode depends on high-pressure and low-pressure connection setup For most level detectors: If D/P goes down, indicated level will also go down If D/P increases, indicated level will increase Exception is wet reference leg level detection, which has the opposite effect Related KAs: K1.08 Effects of operating environment (pressure and temperature) K1.09 Modes of failure ELO 3.6

Failures- Wet Reference Leg
Reference leg is connected to the high-pressure side of the D/P cell, causing the opposite reaction A break in variable leg or low-pressure side would cause low-level indication A break on high-pressure (reference leg) side results in a lower D/P and higher level than indicated ELO 3.6

Knowledge Check The level indication for a reference leg differential pressure (D/P) level instrument will fail low because of... a break on the reference leg. closing the equalizing valve in the D/P cell. the reference leg flashing to steam. a break on the variable leg. Correct answer in D. Correct answer is D. ELO 3.6

Knowledge Check – NRC Bank Refer to the drawing of a steam generator (SG) differential pressure (D/P) level detection system. The SG is at normal operating temperature and pressure with accurate level indication. Which one of the following events will result in a SG level indication that is greater than actual level? The external pressure surrounding the D/P detector increases by 2 psi. SG pressure increases by 50 psi with no change in actual water level. Actual SG level increases by 6 inches. The temperature of the reference leg increases by 20˚F. Correct answer in D. Correct answer is D. ELO 3.6

Detector Transients ELO 3.7 – Analyze detector installation and applications to determine the effects of transients on level indication. Determine effect that transient has on the force exerted by either reference leg or variable leg. Transients include: Loss of reference leg - reference leg force decreases; indicated level higher than actual level Loss of variable leg - variable leg force decreases; indicated level lower than actual level Equalization - similar to losing reference leg force; indicated level being higher than actual level Related KA - K1.03 Describe how pressure and level sensing instruments work ELO 3.7

Detector Transients Some tanks closed to prevent vapors or steam from escaping, or to pressurize contents of the tank When measuring level in a tank that is/can be pressurized by vapor pressure from the liquid, both high-pressure and low-pressure sides of the D/P transmitter must connect as shown in figure below Figure: Closed Tank Dry Reference Leg Level Detector ELO 3.7

Detector Transients Dry Reference Leg Level Detector
High-pressure connects to tank at or below the lower range value measured Low-pressure side connects to a reference leg that is connected at or above the upper range value to be measured Reference leg must stay dry so that there is no liquid head pressure on the low-pressure side of the transmitter Figure: Closed Tank Dry Reference Leg Level Detector ELO 3.7

Detector Transients Wet Reference Leg Level Detector
Liquid in reference leg applies a hydrostatic head to high-pressure side of the transmitter value of this level is constant as long as reference leg is full If pressure remains constant, any change in D/P is due to a change on low-pressure side of transmitter Figure: Closed Tank Wet Reference Leg Differential Pressure Detector ELO 3.7

Detector Transients Loss of Reference Leg Force
Reference leg force can be lost or reduced by Temperature increases Leaks Open or leaking equalizer valves Results in indicated level being higher than true level Loss of Variable Leg Force Variable leg force can be lost or reduced by Open or leaking vent valves Results in indicated level being lower than true level ELO 3.7

Detector Transients Equalization
Occurs when the equalization valve is either open or leaking Similar to losing the reference leg force Results in the indicated level being higher than actual ELO 3.7

Detector Transients Example
Refer to the figure for a pressurizer at normal operating temperature and pressure. Calibration at normal operation temperature and pressure High-pressure side connects to the reference leg If equalizing valve is opened, indicated pressurizer level will be greater than the actual level Results in a minimum D/P and a maximum indicated level Figure: Steam Generator Level Detector ELO 3.7

Detector Transients Example
Now consider a transient where reference leg temperature decreases Results in higher density of reference leg fluid Force exerted on reference leg side of D/P detector is a result of the height of the fluid and the density If density increases, resultant force will increase, resulting in a higher differential pressure and lower indicated level than the true fluid level Figure: Steam Generator Level Detector ELO 3.7

Detector Transients Knowledge Check
Refer to the drawing of a differential pressure (D/P) level detection system below for a pressurizer at normal operating temperature and pressure. Assume that the level detector was just calibrated. The low-pressure side of the detector is connected to the __________; if a leak develops on the variable leg, the indicated pressurizer level will be ___________ than the true level. condensing pot; higher pressurizer; higher condensing; lower pressurizer; lower Correct answer in D. Correct answer is D. ELO 3.7

TLO 3 Summary Major reasons for utilizing remote level indication:
May be necessary to take level measurements at locations far from main facility Level to be controlled may be a long distance from the point of control Measured level may be in an unsafe/hazardous area Gauge glass A transparent tube is attached to bottom and top (top connection not needed in a tank open to atmosphere) of tank that is monitored Liquid height in tube will be equal to height of liquid in tank Magnetic bond level detector Consists of a magnetic float that rises and falls with changes in level Float travels outside a nonmagnetic tube which houses an inner magnet connected to a level indicator When float rises and falls, outer magnet will attract inner magnet, causing inner magnet to follow level within vessel and actuate level indicator Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary Conductivity probe
Consists of one or more level detectors, an operating relay, and a controller When liquid makes contact with any electrodes, electric current will flow between electrode and ground Current energizes a relay which causes relay contacts to open or close depending on state of the process involved Relay will actuate alarm, pump, control valve, or combination of the three Differential pressure (D/P) detector Uses a pressure detector connected to bottom of monitored tank This pressure comparison takes place in the D/P detector Density Compensation If a vapor with significant density exists above liquid, vapor hydrostatic pressure may need to be added to liquid hydrostatic pressure to obtain accurate transmitter output The three options for density compensation are electronic circuitry, pressure detector manual input, and instrument calibration. Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary Environmental effects on level detection:
Density of the fluid Ambient temperature changes Humidity Basic block diagram of a differential pressure level instrument: A D/P transmitter that consists of a diaphragm with HP and LP inputs on opposite sides As differential pressure changes, diaphragm will move Transducer changes this mechanical motion into electrical signal Amplifier amplifies electrical signal generated by the transducer and sends it to level indicator Level indicator displays level indication at a remote location Failure mode of an indirect level detector depends on how HP and LP D/P cell connections are made Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary Now that you have completed this objective, you should be able to do the following: Describe the operation of level detectors and conditions which effect their accuracy and reliability. Describe the three functions for using remote level indicators. Describe the operation of the following types of level instrumentation: Gauge glass Magnetic bond Conductivity probe Differential pressure (D/P) Describe density compensation in level detection systems to include: Why needed How accomplished Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: Differential pressure (D/P) transmitter Amplifier Indication Describe the environmental conditions which can affect the accuracy and reliability of level detection instrumentation. State the various failure modes of level detection instrumentation. Analyze detector applications to determine the effects of the installation and transients on the indication. Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

Flow Detectors Overview
TLO 4 – Describe the operation of flow detectors and conditions which effect their accuracy and reliability. Flow measurement is an important process measurement in operating a facility’s fluid systems. Flow measurement is necessary for efficient and economic operation of these fluid systems. Flow detecting instruments and circuitry (like temperature, pressure, and level detection instruments) are designed and configured to provide either local or remote indication and can be used to control process parameters and provide alarm functions. To control plant systems, an operator must determine mass flow rates through various processes. Flow measurements provide important data that operators use in their plant process adjustments. Flow rate is critical when determining heat transfer rates and total power through heat balance. TLO 4

Describe the theory of operation of a basic head flow meter. Describe the basic construction of the following types of head flow detectors: Orifice plates Venturi tube Dall flow tube Flow nozzle Elbow meter Pitot tube State the typical failure modes for head flow meters including the effects of vapor on a flow instrument. Describe the necessity for density and/or temperature compensation to include: Why it may be required Compensation methods TLO 4

Describe the following types of mechanical flow detectors, including the basic construction and theory of operation. Rotameter Nutating disk Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. State the purpose of typical flow detection device blocks used on a simple block diagram: Differential pressure (ΔP) transmitter Extractor Indicator Describe the environmental conditions which can affect the accuracy and reliability of flow sensing instrumentation. TLO 4

Head Flow Meters ELO 4.1 – Explain the theory of operation of a basic head flow meter. Head flow meters operate on principle of placing restriction in line to cause differential pressure head Differential pressure, which is caused by head, is measured and converted to flow measurement Related KA - K1.05 Explain the operation of a flow D/P cell type flow detector ELO 4.1

Head Flow Meter Construction
Two elements in head flow meter are: Primary element - restriction in line Secondary element - differential pressure measuring device Figure: Head Flow Instrument Figure shows basic operating characteristics of a head flow meter. ELO 4.1

Head Flow Meter Theory of Operation
Flow path restriction (orifice) results in differential pressure being felt across orifice Pressure differential is measured by mercury manometer or differential pressure detector Using measurement, flow rate is determined from known physical laws Restriction causes an increase in fluid velocity and decrease in pressure Volumetric flow rate remains unchanged Same amount of fluid passes through per unit time as does upstream of the restriction Change is fluid pressure is proportional to the square of volumetric flow rate ELO 4.1

Where: D/P = differential pressure caused by restriction = volumetric flow rate To find the volumetric flow rate, the following equation is used based on the relationship between pressure and volumetric flow: K = flow constant for the meter ELO 4.1

Head flow meter actually measures volumetric flow rate Mass flow rate is easily calculated or computed from volumetric flow rate by knowing or sensing temperature and/or pressure Figure: Head Flow Instrument ELO 4.1

Temperature and pressure affect density of fluid and, therefore, mass of fluid flowing past a certain point If volumetric flow rate signal is compensated for changes in temperature and/or pressure, true mass flow rate signal can be obtained In thermodynamics, it is described that: Temperature and density are inversely proportional Pressure and density are directly proportional ELO 4.1

To show the relationship between temperature or pressure, the mass flow rate equation is often written as follows: Where: = mass flow rate A = area D/P = differential pressure P = pressure T = temperature K = flow coefficient 𝑚 =𝐾𝐴 𝐷/𝑃(𝑃) 𝑚 =𝐾𝐴 𝐷/𝑃 𝑇 ELO 4.1

Head Flow Meter Knowledge Check
Flow detectors (such as an orifice, flow nozzle, and venturi tube) measure flow rate using the principle that flow rate is... inversely proportional to the differential pressure squared. inversely proportional to the square root of the differential pressure. directly proportional to the square root of the differential pressure. directly proportional to the differential pressure squared. Correct answer in C. Correct answer is C. ELO 4.1

Flow Meter Construction
ELO 4.2 – Describe the basic construction of the following types of head flow detectors: Orifice plates Venturi tube Dall flow tube Flow nozzle Elbow meter Pitot tube Related KA - K1.05 Explain the operation of a flow D/P cell type flow detector There are several designs of flow meters that work on the theory that flow is proportional to the square root of the D/P. ELO 4.2

Orifice Plates Fluid forced to converge through small hole
Velocity and pressure change Point of maximum convergence, downstream of orifice is the vena contracta point Beyond vena contracta, fluid expands; velocity and pressure change back close to original Measuring D/P between normal pipe section and vena contracta to find flow Figure: Orifice Plate ELO 4.2

Orifice Plates Three kinds of orifice plates used are: Concentric
Eccentric Segmental Figure: Orifice Plate Types ELO 4.2

Orifice Plates Most common of three types
Orifice is equidistant (concentric) to inside diameter of pipe Flow through sharp-edged orifice plate is characterized by a change in velocity Circular section of segmental orifice is concentric with pipe Segmental portion of orifice eliminates damming of foreign materials on upstream side when in horizontal pipe Depending on fluid, segmental section placed on top/bottom of horizontal pipe to increase accuracy of measurement Eccentric orifice plates shift edge of orifice to inside pipe wall Design also prevents upstream damming and is used in same way as segmental orifice plate ELO 4.2

Disadvantages of Orifice Plates
Cause high permanent pressure drop Outlet pressure will be 60% to 80% of inlet pressure Subject to erosion, which will eventually cause inaccuracies in measured differential pressure Cannot be used for fluids that may have gases or vapors in solution ELO 4.2

Venturi Tube Most accurate flow-sensing element when properly calibrated Converging conical inlet Cylindrical throat Diverging recovery cone No projections into fluid No sharp corners No sudden changes in contour Figure: Venturi Tube ELO 4.2

Venturi Tube Inlet section decreases area of fluid stream
Figure: Venturi Tube Inlet section decreases area of fluid stream Velocity increase Pressure decrease Low pressure measured in center of cylindrical throat Pressure will be at its lowest value Neither pressure nor velocity is changing ELO 4.2

Venturi Tube Recovery cone allows for recovery of pressure
Total pressure loss is only 10-25% Lowest pressure drop of any head flow meters High pressure is measured upstream of entrance cone Major disadvantages of venturi tube: High initial costs for installation Difficulty in installation and inspection ELO 4.2

Flow Nozzle Used for high-velocity flow
Smooth contoured flow restriction High permanent pressure loss similar to the orifice Does not allow for collection of particulate as an orifice does Figure: Flow Nozzle ELO 4.2

Elbow Meter D/P developed as fluid on inner radius does not have as far to travel as fluid next to outer radius Difference in surface area creates Low-pressure area on the inner pipe wall Higher-pressure area on the outer pipe wall Pressure difference is proportional to the volumetric flow rate squared ELO 4.2

Elbow Meter Small pressure difference created by elbow meter allows high accuracy Accurate at high flow rates D/P instrument used is more costly than some other head flow meters Simple design Can measure flow in either direction Figure: Elbow Meter ELO 4.2

Pitot Tube Another primary flow element used to produce differential pressure for flow detection In simplest form, consists of a tube with an opening at end Opening at end faces flowing fluid Velocity of fluid at opening of tube decreases to zero Provides for HP input to differential pressure detector Pressure tap provides LP input Figure: Pitot Tube ELO 4.2

Pitot Tube Pitot tube actually measures fluid velocity instead of fluid flow rate Volumetric flow rate can be obtained using the following equation: 𝑉 =𝐾𝐴𝑣 Where: 𝑉 = volumetric flow rate A = area of flow cross-section v = velocity of flowing fluid K = flow coefficient (normally about 0.8) Must be calibrated for each specific application No standardization Can be used even when fluid is not enclosed in pipe or duct ELO 4.2

Flow Meter Construction
Knowledge Check Refer to the drawing of a venturi flow element below with direction of fluid flow indicated by the arrow. Where should the high-pressure tap of a differential pressure flow detector be connected? Point D Point B Point C Point A Correct answer in D. “Point A” Correct answer is D. ELO 4.2

Flow Meter Failure Modes
ELO 4.3 – State the typical failure modes for head flow meters including the effects of vapor on a flow instrument. Head flow meters are reliable for long-term continuous operation Leakage of differential pressure cell connections is one of the most common problems with head flow meters Related KA - K1.04 Modes of failure ELO 4.3

Condition Indication Discussion 1. Leak on high-pressure connection Indicated flow less than actual Leak on the high-pressure tap would result in a lower D/P, which corresponds to lower indicated flow. 2. Leak on low-pressure connection Indicated flow more than actual Leak on the low-pressure tap would result in a higher D/P, which corresponds to higher indicated flow. 3. Orifice plate erosion The orifice size will increase due to the erosion. This results in a lower D/P for the same flows. 4. Loss of density compensation input Density compensation adjusts the indication to take into account the effect of pressure change on the gas being measured. Without density compensation, the D/P will be less. Related KA - K1.04 Modes of failure ELO 4.3

Link to Module Summary Flow Meter Failure Modes Condition Indication Discussion 5. Steam pressure input fail low Indicated flow less than actual Apparent density has decreased, less mass is sensed passing the flow detector. 6. Steam pressure input fail high Indicated flow more than actual Apparent density has increased, more mass is sensed passing the flow detector. 7. Vapor in a liquid Erratic, unstable flow indication As vapor goes through the measuring device, the difference in pressure is dependent on the density of the fluid. Gas has much less density that liquid and therefore the D/P will change rapidly as the vapor goes through the detector. Related KA - K1.04 Modes of failure ELO 4.3

Knowledge Check The most probable cause for fluctuating indication from a liquid flow rate differential pressure detector is... unequal temperature gradients in the liquid. gas or steam being trapped in the liquid. vortexing of the liquid passing through the flow device. the valve on the high-pressure sensing line being partially closed. Correct answer in B. Correct answer is B. ELO 4.3

Knowledge Check Refer to the drawing below of a pipe elbow used for flow measurement in a cooling water system. A differential pressure (D/P) flow detector connects to instrument lines A and B. If instrument line B develops a leak, indicated flow rate will ____________ due to a ___________ measured D/P. increase; smaller decrease; larger increase; larger decrease; smaller Correct answer in C. Correct answer is C. ELO 4.3

Density Compensation ELO 4.4 – Describe the necessity for density and/or temperature compensation to include: a. Why it may be required b. Compensation methods Flow measurement of gasses and fluid/vapor mixtures requires compensation for density Density of compressible fluids changes with changing environment Steam through a head flow meter pressure change is not as large as when a liquid is pumped through Pressure will react like liquid with velocity increasing and pressure decreasing Related KA - K1.02 Temperature/density compensation requirements ELO 4.4

Density Compensation When pressure decreases, so does density
Less mass per unit of volume (density) velocity also increases due to the change in density and not just the change in area Change in density takes place between high- and low-pressure taps Density compensation converts volumetric flow rate to mass flow rate ELO 4.4

Density Compensation Knowledge Check
If the steam pressure input to a density-compensated steam flow instrument fails low, the indicated flow rate will... decrease because the density input has decreased. decrease because the density input has increased. increase because the density input has increased. increase because the density input has decreased. Correct answer in A. Correct answer is A. ELO 4.4

Area Flow Meter ELO 4.5 – Describe the following types of mechanical flow detectors, including the basic construction and theory of operation: a. Rotameter b. Nutating disk Area flow meters rely on head causing flow through meter to be relatively constant such that rate of flow is directly proportional to metering area Variation in area is produced by rise and fall of floating element Flow meter must be mounted so that floating element moves vertically and friction is minimal No NRC Related KA ELO 4.5

Rotameter Area flow meter named for rotating float used as indicating element Consists of metal float and conical glass tube Glass tube constructed such that diameter increases with height Space between float and tube allows for flow past float Figure: Rotameter ELO 4.5

Rotameter When there is no fluid passing through rotameter
Float rests at bottom of tube As fluid enters tube Higher density of float will cause float to remain on bottom As flow increases in tube Pressure drop increases When pressure drop is sufficient, float will rise to indicate amount of flow Figure: Rotameter ELO 4.5

Rotameter Float should stay at constant position at constant flow rate
With a smooth float, fluctuations appear even when flow is constant By using float with slanted slots cut in head, float maintains constant position with respect to flow rate This type of flow meter is usually used to measure low flow rates Figure: Rotameter ELO 4.5

Displacement Meter In a displacement flow meter, all of the fluid passes through meter in almost completely isolated quantities Number of quantities is counted Indicated in terms of volume or weight units by a register ELO 4.5

Nutating Disk (Wobble Plate Meter)
Most common type of displacement flow meter Normally used for water service Such as raw water supply and evaporator feed Movable element is circular disk attached to central ball A shaft is fastened to ball and held in an inclined position by cam or roller Disk is mounted in chamber which has spherical side walls and conical top and bottom surfaces Figure: Nutating Disk ELO 4.5

Fluid enters opening in spherical wall on one side of partition and leaves through other side As fluid flows through chamber, disk wobbles, or executes “nutating” motion Volume of fluid required to make disc complete one revolution is known Total flow through nutating disc can be calculated by multiplying number of disc rotations by known volume of fluid Figure: Nutating Disk ELO 4.5

Figure: Nutating Disk Cutaway ELO 4.5

Ultrasonic Flow Equipment
Use Doppler frequency shift of ultrasonic signals reflected from discontinuities in fluid stream to obtain flow measurements Discontinuities can be Suspended solids Bubbles Interfaces generated by turbulent eddies in flow stream ELO 4.5

Ultrasonic Flow Equipment
Sensor mounted on outside of pipe Ultrasonic beam from piezoelectric crystal transmitted through pipe wall into fluid at angle to flow stream Signals reflected off flow disturbances Detected by second piezoelectric crystal located in same sensor Transmitted and reflected signals are compared in electrical circuit Corresponding frequency shift proportional to flow velocity Figure: Ultrasonic Flow Detector ELO 4.5

Area Flow Meter Knowledge Check
What type of flow meter is depicted in the cross-section below? Analog Ultrasonic Nutating disk Rotameter Correct answer in C. Correct answer is C. ELO 4.5

Steam Flow Detection - Steam Flow Nozzle
ELO 4.6 – Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. Flow nozzle requires less straight run piping than orifice plate Pressure drop is about same for both nozzle and orifice plate Related KA - K1.02 Temperature/density compensation requirements Figure: Steam Flow Nozzle ELO 4.6

Density Compensation 𝑚 = 𝑉 𝜌
Steam changes in pressure and temperature greatly affect density The following equations are used to describe the fundamental relationship for volumetric flow and mass flow 𝑉 =𝐾 ℎ𝑒𝑎𝑑𝑙𝑜𝑠𝑠 𝜌 ℎ 𝑚 = 𝑉 𝜌 Where: 𝑉 = volumetric flow K = constant relating to the ratio of pipe to orifice h = differential pressure ρ = density 𝑚 = mass flow ELO 4.6

Density Compensation It is possible to substitute for density in the relationship Where: ρ = density p = upstream pressure m = molecular weight of the gas θ = absolute temperature R = gas constant 𝜌= 𝑝𝑚 𝑅𝜃 ELO 4.6

Density Compensation By substituting for density, equation values are used by electronic circuit to calculate density automatically Steam temperature is relatively constant in most steam systems Upstream pressure is only variable in equation that changes as system operates If other variables are hardwired, measuring system pressure is all that is required for circuit to calculate fluid’s density ELO 4.6

Mass Flow Detection System
Temperature and pressure values can be used to electronically compensate flow for changes in density Mass flow detection system can be constructed where measurements of temperature and pressure are made with commonly used instruments Figure: Simple Mass Flow Detection ELO 4.6

Gas Flow Computer For precise measurement of gas flow (steam) at varying pressures and temperatures, it is necessary to determine density Density is pressure and temperature dependent Density then used to calculate actual flow ELO 4.6

Gas Flow Computer Essential to measure flow with changing pressure or temperature Designed to accept input signals from Commonly used differential pressure detectors Density or pressure plus temperature sensors Provides an output which is proportional to actual flow rate Accuracy of better than +0.1% at flow rates of 10% to 100% Figure: Gas Flow Computer ELO 4.6

Steam Flow Detection - Steam Flow Nozzle
Knowledge Check Because steam behaves like a gas, changes in __________ and temperature greatly affect its _________. pressure; density pressure; mass density; pressure mass; pressure Correct answer in A. Correct answer is A. ELO 4.6

Flow Detection Circuitry
ELO 4.7 – State the purpose of typical flow detection device blocks used on a simple block diagram: a. Differential pressure (ΔP) transmitter b. Extractor c. Indicator In basic flow detection circuitry, primary elements provide input to secondary element which provides for indications, alarms, and controls D/P transmitter operation is dependent on pressure difference across orifice, venturi, or flow tube Differential pressure is used to position mechanical device such as bellows Figure: Differential Flow Detection Circuitry ELO 4.7

Bellows act against spring pressure to reposition core of differential transformer Transformer’s output voltage on each of two secondary windings varies with change in flow Flow rate is proportional to square root of differential pressure Extractor used to electronically calculate square root of differential pressure and provide an output proportional to system flow Figure: Differential Flow Detection Circuitry ELO 4.7

Constants are determined by selection of the appropriate electronic components Extractor output is amplified and sent to indicator Indicator provides either local or remote indication of system flow Figure: Differential Flow Detection Circuitry ELO 4.7

Knowledge Check Place the following components in order starting with flow sensing to output signal. Alarm Transducer Square root extractor Correct answer order is D, B, C, A. Correct answer order is D, B, C, then A. ELO 4.7

Environmental Effects On Flow Detection
ELO 4.8 – Describe the environmental conditions which can affect the accuracy and reliability of flow sensing instrumentation. Fluid Density Effect of density is most important when flow sensing instrumentation is measuring gas flows, such as steam Density of gas directly affected by temperature and pressure Any changes in either of these parameters has direct effect on measured flow No Related KA ELO 4.8

Ambient Temperature Ambient temperature variations will affect accuracy and reliability of flow sensing instrumentation Directly affect resistance of components in instrumentation circuitry Affects calibration of electric/electronic equipment Effects reduced by design of circuitry and by maintaining flow sensing instrumentation in proper environment ELO 4.8

Humidity Humidity will affect most electrical equipment, especially electronic equipment High humidity causes moisture to collect on equipment Can cause short circuits, grounds, and corrosion, which, in turn, may damage components Effects due to humidity are controlled by maintaining equipment in proper environment ELO 4.8

TLO 4 Summary Head flow meters
Operate on principle of placing restriction in the line to cause pressure drop Differential pressure caused by the head restriction is measured and converted to a flow measurement Orifice plates Flat plates 16–63 mm (1/16 to 1/4 in.) thick, mounted between pair of flanges Installed in a straight run of smooth pipe to avoid disturbance of flow patterns due to fittings and valves Venturi tube Converging conical inlet, a cylindrical throat, and a diverging recovery cone No projections into fluid, no sharp corners, no sudden contour changes Consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube (inlet shoulder), followed by a converging inlet cone and a diverging exit cone A slot gap between the two cones separates them Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary Pitot tube Consists of a tube with an opening at the end; the opening in the end is positioned so that it faces the flowing fluid Rotameter Consists of a metal float and a conical glass tube Tube diameter increases with height High density float will remain on the bottom of tube with no flow Space between the float and the tube allows for flow past the float As flow increases, the pressure drop increases; when the pressure drop is sufficient, the float rises to indicate the amount of flow Nutating Disc Circular disk attached to a central ball Shaft protrudes from ball and a cam or roller holds shaft in inclined position Fluid enters an opening in the spherical wall on one side of partition and leaves through the other side As fluid flows through chamber, disk wobbles, executing a nutating motion Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary Hot-Wire Anemometer
Electrically heated, fine platinum wire immersed in flow Wire cools as flow is increased Measure either change in wire resistance or heating current to determine flow Electromagnetic Flowmeter Magnetic field established around system pipe Electromotive force induced in fluid as it flows through magnetic field Electromotive force measured with electrodes and proportional to flow rate Ultrasonic flow equipment Uses Doppler frequency shift of ultrasonic signals reflected off discontinuities in fluid Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary Density Compensation
Changes in temperature and pressure greatly affect indicated steam flow By measuring temperature and pressure, a computerized system can electronically compensate steam or gas flow indication for changes in fluid density Environmental Effects on Flow Detection: Density of the fluid Ambient temperature Humidity The purpose of each block of a typical differential pressure flow detection circuit: D/P transmitter uses differential pressure to provide an output proportional to the flow Extractor electronically calculates the square root of the differential pressure and provides an output proportional to system flow Indicator provides either a local or a remote indication of system flow Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary Now that you have completed this objective, you should be able to do the following: Describe the operation of flow detectors and conditions which effect their accuracy and reliability. Describe the theory of operation of a basic head flow meter. Describe the basic construction of the following types of head flow detectors: Orifice plates Venturi tube Dall flow tube Flow nozzle Elbow meter Pitot tube Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary State the typical failure modes for head flow meters, including the effects of vapor on a flow instrument. Describe the necessity for density and/or temperature compensation to include: Why it may be required Compensation methods Describe the following types of mechanical flow detectors, including the basic construction and theory of operation: Rotameter Nutating disk Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

TLO 4 Summary Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. State the purpose of typical flow detection device blocks used on a simple block diagram: Differential pressure (ΔP) transmitter Extractor Indicator Describe the environmental conditions which can affect the accuracy and reliability of flow sensing instrumentation. Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 4

Position Detectors Overview
TLO 5 – Describe the operation of position detectors and conditions which effect their accuracy and reliability. Position indicating instrumentation is used in industrial facilities to provide remote equipment indication such as for the indication of open or shut valves. There are several types of position detection devices. They include switches which are on-off type devices and variable output devices. Remote indication is necessary for monitoring vital components located within inaccessible or remote areas, or to obtain indication data from a piece of equipment where there is a personnel safety concern. TLO 5

Describe the following switch position indicators to include basic construction and theory of operation: Limit switches Reed switches Describe the following variable output position indicators to include basic construction and theory of operation: Potentiometer Linear variable differential transformers (LVDT) State the purpose of the following components: Detection device Indicator and control circuits TLO 5

Describe the environmental conditions that can affect the accuracy and reliability of position indication equipment. Describe the failure modes for the following position detectors: Reed switch Limit switch Potentiometer LVDT TLO 5

Limit Switches ELO 5.1 – Describe the following switch position indicators to include basic construction and theory of operation: a. Limit switches b. Reed switches Mechanical limit switches and reed switches provide valve open and shut indications Can also be used to determine physical position of equipment Related KA - K1.16 Applications of reed switches, magnets, LVDT, potentiometers, and limit switches ELO 5.1

Limit Switches A limit switch is a mechanical device used to determine physical position of equipment Limit switch gives on/off output that corresponds to valve position Figure: Limit Switches ELO 5.1

Limit Switches Many limit switches are push-button variety
When valve extension comes in contact with limit switch, switch depresses to complete, or turn on, electrical circuit As valve extension moves away from limit switches, spring pressure opens switch, turning off circuit ELO 5.1

Reed Switches Reed switches are more reliable than limit switches due to their simplified construction Constructed of flexible ferrous strips (reeds) Placed near intended travel of a valve stem Figure: Reed Switches ELO 5.1

Reed Switches Extension used with reed switch is permanent magnet
As magnet approaches, the reed switch closes When magnet moves away, reed switch opens On/off indicator is similar to mechanical limit switch Incremental position can be measured during valve travel by using large number of reed switches Figure: Reed Switches ELO 5.1

Reed Switch Used for position indication in the control rod drive monitor for rod position A permanent magnet on control rod drive shaft attracts moveable contact arm of each reed switch as drive passes by ELO 5.1

Limit Switches Knowledge Check
What is the most common type of sensor used to provide remote position indication of a valve that is normally either fully open or fully closed? Linear variable differential transformer Limit switch Reed switch Servo transmitter Correct answer order is B. Correct answer is B. ELO 5.1

Variable Output Detectors
ELO 5.2 – Describe the following variable output position indicators to include basic construction and theory of operation: a. Potentiometer b. Linear variable differential transformers (LVDT) Variable output position indicators are often used when an accurate and reliable position indication for a particular piece of equipment (valve) is required Related KA - K1.16 Applications of reed switches, magnets, LVDT, potentiometers, and limit switches ELO 5.2

Potentiometer Provides an accurate indication of position throughout travel of valve Extension is physically attached to variable resistor As extension moves up or down, resistance of attached circuit changes, changing amount of current flow in circuit Amount of current is proportional to valve position Figure: Potentiometer ELO 5.2

Linear Variable Differential Transformers
Device which provides accurate position indication throughout range of valve travel Unlike potentiometer position indicator, no physical connection to valve extension required Valve extension is made of metal suitable for acting as movable core of transformer Figure: Linear Variable Differential Transformer ELO 5.2

Linear Variable Differential Transformers
Moving extension between primary and secondary windings of transformer causes inductance between two windings to vary Varies output voltage proportional to position of valve extension If open and shut position is all that is desired, two small secondary coils could be utilized at each end of extension’s travel Figure: Linear Variable Differential Transformer ELO 5.2

Variable Output Detectors
Knowledge Check List two types of position indicators that can be used to indicate the position of a valve throughout its entire range of travel (not just open and shut). Potentiometer Reed switch Transducer Linear variable differential transformer Correct answer order is A and D. Correct answers are A and D. ELO 5.2

Position Detection Circuits
ELO 5.3 – State the purpose of the following components: a. Detection device b. Indicator and control circuits Valve position circuitry provides indication and control functions Functions can be provided locally, or from a remote area such as a control room or remotely located control panel ELO 5.3

Position Indication Circuitry
Position detection devices provide a method to determine position of a valve Four types of position indicators: Limit switches Reed switches Potentiometer valve position indicators LVDTs Indicating and control circuitry provides for remote indication of valve position and/or various control functions Position indications vary from simple indications, such as a light, to meter indications showing exact position ELO 5.3

Position Indication Circuitry
Reed and limit switches act as on/off indicators to provide open and closed indications and control functions Reed switches can also be used to provide coarse, incremental position indication Potentiometer and LVDT position indicators provide accurate indication of valve position throughout travel In some applications, LVDTs can be used to indicate open and closed positions when small secondary windings are used at either end of valve stem stroke ELO 5.3

Position Detector Circuits
Knowledge Check A condensate pump requires opening the suction valve prior to starting. What type of position device would best serve this purpose? Linear variable differential transformers Potentiometer Inductance/Capacitance Limit switch Correct answer order is D. Correct answer is D. ELO 5.3

Environmental Effects On Position Detection
ELO 5.4 – Describe the environmental conditions that can affect the accuracy and reliability of position indication equipment. Ambient Temperature Variations in ambient temperature can directly affect resistance of components in instrumentation circuitry, and, therefore, affect calibration of electric/electronic equipment Effects of temperature variations are reduced by design of circuitry and by maintaining position indication instrumentation in proper environment ELO 5.4

Environmental Effects On Position Detection
Humidity High humidity causes moisture to collect on equipment Moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components Effects due to humidity are controlled by maintaining equipment in proper environment ELO 5.4

Position Detector Circuits
Knowledge Check Variations in ______________ can directly affect the ____________ of components in the instrumentation circuitry. ambient temperature; voltage voltage; resistance voltage; temperature ambient temperature; resistance Correct answer order is D. Correct answer is D. ELO 5.4

Position Indication Failure Mode
ELO 5.5 – Describe the failure modes for the following position detectors: a. Reed switch b. Limit switch c. Potentiometer d. Linear variable differential transformers Limit switch failures are normally mechanical in nature If proper indication or control function is not achieved, limit switch is probably faulty In case of failure, local position indication should be used to verify equipment position Related KA - K1.15 Failure modes of reed switches, LVDT, limit switches, and potentiometers ELO 5.5

Reed Switch Failures Failures normally limited to reed switch which is stuck open or stuck shut If reed switch is stuck shut, indication (open or closed) will be continuously illuminated If reed switch stuck open, position indication for that switch remains extinguished regardless of valve position Two clicks reveals entire slide. ELO 5.5

Potentiometer Failures
Normally electrical in nature Electrical short or open will cause indication to fail at one extreme or other Increase or decrease in potentiometer resistance leads to erratic valve position indication Two clicks will reveal entire slide content. Figure: Potentiometer ELO 5.5

Linear Variable Differential Transformer Failures
LVDTs are extremely reliable Failures limited to rare electrical faults which cause erratic or erroneous indications An open primary winding will cause indication to fail to some predetermined value equal to zero differential voltage Normally corresponds to mid-stroke of valve Failure of either secondary winding will cause output to indicate either full open or full closed, regardless of actual valve position Three clicks reveals entire slide. Figure: Linear Variable Differential Transformer ELO 5.5

Position Indication Failure Mode
Knowledge Check In the figure below, there are four sets of reed switches. One set is for full closed, another for full open, and the other sets are for intermediate positions. If each reed switch set completes a circuit to indicating lights, what would be the indication if the bottom set of reed switches failed closed as the valve goes from closed to open? The intermediate lights would remain lit in addition to the full open light as the valve travels full stroke. The full open light would remain lit in addition to the intermediate and full open light as the valve travels full stroke. No effect, all indicating lights would work as designed. The full closed light would remain lit in addition to the intermediate and full open light as the valve travels full stroke. Correct answer order is D. Correct answer is D. ELO 5.5

TLO 5 Summary Limit switch
Mechanical device used to determine physical position of valves An extension on a valve shaft mechanically trips the switch as it moves from open to shut or shut to open Gives on/off output, which corresponds to valve position Reed switches Consist of flexible ferrous strips affixed to stationary point adjacent to the intended travel of the valve stem The extension used is a permanent magnet As the magnet approaches the reed switch, the switch shuts; when the magnet moves away, the reed switch opens Potentiometer valve position indicators Use an extension with an attached variable resistor Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 5

TLO 5 Summary LVDT Uses extension shaft of valve as a movable core of a transformer Moving the extension between the primary and secondary windings of a transformer causes inductance between two windings to vary, thereby varying output voltage proportional to position of valve extension The accuracy and reliability of position indication instrumentation can be affected by: Ambient temperature Humidity Indicating and control circuitry provides for remote indication of valve or component position and/or various control functions Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 5

TLO 5 Summary Now that you have completed this objective, you should be able to do the following: Describe the operation of position detectors and conditions which effect their accuracy and reliability. Describe the following switch position indicators to include basic construction and theory of operation: Limit switches Reed switches Describe the following variable output position indicators to include basic construction and theory of operation: Potentiometer Linear variable differential transformers (LVDT) State the purpose of the following components: Detection device Indicator and control circuits Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 5

TLO 5 Summary Describe the environmental conditions that can affect the accuracy and reliability of position indication equipment. Describe the failure modes for the following position detectors: Reed switch Limit switch Potentiometer LVDT Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 5

Sensors and Detectors 1 Module Summary
Temperature detectors are used for: indication, alarm functions, and control functions. RTD operates on principle that a change in electrical resistance of a metal is directly proportional to it’s change in temperature. If RTD becomes inoperative: Use spare element if installed Substitute pyrometer for temporary indication Thermocouples consist of two dissimilar metals joined at one end, encased in metal sheath. Other end connects to measuring circuit Measuring junction voltage compared to reference junction voltage Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Pressure detectors are used for: indication, alarm functions, and control functions. Bellows type detector uses linkage assembly to produce electrical signal or gauge movement Bourdon type detector uses flattened arc shape tube which tends to move based on pressure Strain gauge uses a fine wire arranged in grid, pressure changes cause resistance change on wire grid Slidewire transducer uses bellows or bourdon tube with variable resister, slidewire movement causes resistance change Inductance type transducer uses coil, movable magnetic core and pressure sensing element, pressure causes element and core to move inside coil, changes in coil conductance changes current flow Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Level detectors - reasons for using remote level indication are: remote level indication may be located far from main facility, level to be controlled is long distance, and measured level may be located in unsafe/hazardous area Gauge glass, magnetic bond, conductivity, and D/P detector Density compensation can be performed by: electronic circuitry, manual input, or instrument calibration Failure mode is dependent on arrangement of high and low pressure taps Flow is important process measurement in system operation Head flow meters Operate on principle of placing a restriction in the line to cause a pressure drop Head restriction causes a differential pressure; sensors measure differential pressure and convert it to a flow measurement Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Flow is important process measurement in system operation Orifice plates Flat plates 1/16 to 1/4 inch thick, mounted between a pair of flanges Installed in straight run or smooth pipe to avoid disturbance of flow patterns Venturi tube Comprised of a converging conical inlet, a cylindrical throat, and a diverging recovery cone; no projections into fluid, no sharp corners, and no sudden changes in contour Dall flow tube Consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube (inlet shoulder), followed by a converging inlet cone and a diverging exit cone (slot gap separates them) Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Flow is important process measurement in system operation. Pitot tube Consists of a tube with an opening at the end; opening in the end is positioned so that it faces flowing fluid Rotameter Consists of metal float and conical glass tube, high-density float will remain on the bottom of tube with no flow As flow increases, pressure drop increases; when pressure drop is sufficient, float rises to indicate amount of flow Nutating disc Circular disk attached to central ball; shaft protrudes from the ball and a cam or roller holds shaft in inclined position As fluid flows through the chamber, disk wobbles, executing a nutating motion Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Flow is important process measurement in system operation. Density compensation Changes in temperature and pressure greatly affect indicated steam flow. By measuring temperature and pressure, a computerized system can electronically compensate steam or gas flow indication for changes in fluid density. Failure modes - head flow meters are fairly reliable, most common leakage is at D/P cell connections (link to failure table) Position detection devices (see TLO 5 summary) Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Now that you have completed these TLOs, you should be able to do the following: Describe the operation of temperature detectors and conditions which effect their accuracy and reliability. Describe the operation of pressure detectors and conditions which effect their accuracy and reliability. Describe the operation of level detectors and conditions which effect their accuracy and reliability. Describe the operation of flow detectors and conditions which effect their accuracy and reliability. Describe the operation of position detectors and conditions which effect their accuracy and reliability. Review the terminal objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Operator Generic Fundamentals Components - Sensors and Detectors 1

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Components - Sensors and Detectors 1"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Operator Generic Fundamentals Components - Sensors and Detectors 1

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Components - Sensors and Detectors 1"— Presentation transcript:

Similar presentations

About project

Feedback