Operator Generic Fundamentals Components - Sensors and Detectors 1

Operator Generic Fundamentals Components - Sensors and Detectors 1
K1.01 through K1.16 are covered in this chapter. K1.17 – K1.20 are covered in – Sensors and Detectors – 2. Operator Generic Fundamentals Components - Sensors and Detectors 1

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 that affect their accuracy and reliability. Describe the operation of pressure detectors and conditions that affect their accuracy and reliability. Describe the operation of level detectors and conditions that affect their accuracy and reliability. Describe the operation of flow detectors and conditions that affect their accuracy and reliability. Describe the operation of position detectors and conditions that affect their accuracy and reliability. Intro

Temperature Detectors
TLO 1 – Describe the operation of temperature detectors and conditions that affect their accuracy and reliability. 1.1 State the three basic functions of temperature detectors. 1.2 Describe the construction of a basic RTD including: Component arrangement Materials used 1.3 Describe how RTD resistance varies for temperature changes. 1.4 State the purpose of basic temperature instrument detection and control system blocks: RTD Bridge circuit DC-AC converter Amplifier Balancing motor/mechanical linkage 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
1.5 Describe bridge circuit compensation for changes in ambient temperature and environmental conditions that can affect temperature detection instrumentation. 1.6 Describe the effect on temperature indication(s) for the following circuit faults: Short circuit Open circuit 1.7 Describe alternate methods of determining temperature when the normal sensing devices are inoperable. 1.8 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) 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 a material Examples of temperature detectors Bulb thermometer, bimetallic strip RTD, thermocouple (explained in detail later) No related KA objectives ELO 1.1

Filled System Thermometer
Can provide local and remote indication Consists of a sensing element (a bulb containing gas or liquid) and an indicator scale As temperature changes, Gas or liquid pressure changes Can act on a spiral bourdon tube for more displacement Motion can drive a Pointer or 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
Figure: Bimetallic Strip 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 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. If heated, the bimetallic bends to adapt to the increased length of the metal with the greater temperature coefficient of expansion ELO 1.1

Resistance Temperature Detector Construction
ELO 1.2 – Describe the construction of a basic RTD including the following: component arrangement and 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

Elements are usually: Long, spring-like wires Surrounded by insulator Enclosed in metal (inconel) sheath Inconel normally used because of inherent corrosion resistance Figure: Internal Construction of a Typical RTD ELO 1.2

RTD inserted into protective measuring well Change in temperature causes platinum wire to heat or cool Resistance change measured by precision resistance measuring device Figure: RTD Protective Well and Terminal Head ELO 1.2 11

Temperature Resistance Relationship – Resistance Temperature Detector
ELO 1.3 – Describe how RTD resistance varies for temperature changes. Normally constructed of platinum, copper, or nickel Linear resistance- temperature characteristics Temperature increases, resistance increases High coefficient of resistance Ability to withstand repeated temperature cycles Figure: Resistance vs. Temperature Graph 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

Temperature Resistance Relationship – Resistance Temperature Detector
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 is C. Correct answer in C. Resistance of RTD increases 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. Basic bridge circuit components Three known resistances, R1, R2, and R3 (variable) Unknown variable resistor Rx RTD Source of voltage Sensitive ammeter No Related KAs for ELO, however, understanding of a bridge circuit helps understand failure modes of RTDs. Bridge circuit used with RTD element to obtain accurate measurements of temperature 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
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 As resistance of RTD changes voltages at points X and Y change Millivolt meter detects change in voltage Calibrated for temperature 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 Figure: Unbalanced Bridge Circuit ELO 1.4

Balanced Bridge Circuit
Similar to unbalanced, except: Slidewire resistor used to balance arms of bridge As RTD resistance changes Resistance of slide wire adjusted until galvanometer indicates zero Value of slide resistance determines temperature of RTD 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 Figure: Balanced Bridge Circuit ELO 1.4

Typical Temperature Detection Circuit
RTD measures temperature Detector is felt as resistance to bridge network Bridge network converts resistance to DC voltage signal Electronic instrument converts DC to AC AC voltage amplified to drive bi-directional motor Bi-directional motor positions slider balance circuit resistance provide temperature Block diagram represents balanced bridge temperature detection circuit modified to eliminate galvanometer Figure: Basic Temperature Detection Circuit 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 Variations in ambient temperature directly affect Resistance of components in bridge circuit Resistance of thermocouple reference junction Explained later If temperature surrounding wiring increases Resistance increases Indication increases No related KAs ELO 1.5

Humidity Humidity 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 Presence of ambient humidity affects most electrical equipment ELO 1.5

Knowledge Check 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 is D. Correct answer is D. NRC Bank Question – P6905 Analysis: The ability of a metal to conduct is dependent on its composition and temperature. As temperature rises, the ability of the metal to conduct electricity becomes somewhat diminished. An RTD employs this relationship to measure temperature. In order to calculate temperature based on resistance, the detector must be calibrated to this relationship. A true linear relationship for a metal such as platinum makes this calibration simple; cheaper metals such as nickel or copper aren’t quite as linear, but are still used due to relative cost. It was shown on Slide 12, that as temperature is decreased the resistance of the copper wire will also decrease. This decrease in resistance is equivalent to a similar decrease in the indication. Therefore, choice “D” is correct. ELO 1.5

RTD Circuit Failures ELO 1.6 – Describe the effect on temperature indication(s) for the following circuit faults: Short circuit, Open circuit. If RTD in either unbalanced or balanced bridge circuit becomes open: Resistance becomes infinite Temperature will indicate high temperature (fail high) If RTD becomes shorted: Resistance becomes zero Temperature will indicate a low temperature (fail low) 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 – NRC Bank
If shorting occurs within a resistance temperature detector, the associated indication will fail... low. high. as is. to midscale. Correct answer is A. Correct answer in A. NRC Bank Question – P312 Analysis: The ability of a metal to conduct is dependent on its composition and temperature. As temperature rises, the ability of the metal to conduct electricity becomes somewhat diminished. An RTD employs this relationship to measure temperature. Because a high temperature is indicative of a high resistance, a short will produce an output which is failed low (low resistance). 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

Alternate Temperature Indications
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

Thermocouples ELO 1.8 – Describe the construction and operation of a thermocouple. A thermocouple converts thermal energy into electrical energy Constructed of two dissimilar metal wires joined together at one end (junction) Other end of each wire is connected to a measuring instrument K1.13 Theory and operation of T/C, RTD, thermostats. 2.6, 2.8 Amount of current produced depends on: Temperature difference between measurement and reference junction Characteristics of two metals used Characteristics of attached circuit Figure: Simple Thermocouple Circuit ELO 1.8

Thermocouple Construction
Leads encased in a rigid metal sheath Measuring junction at bottom of thermocouple housing 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 Reference junction calibrated to “normal” environment temperature Figure: Simple Thermocouple Circuit Mathematical representation: Measuring Junction – Reference Junction = Voltmeter Voltmeter + Reference Calibration = Indication Assume: Measuring Junction = 400°F; Reference Junction Calibration = 100°F Therefore, 400°F - 100°F = 300°F; 300°F + 100°F = 400°F ELO 1.8

Thermocouple Failures and Disadvantages
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 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 Thermocouple is less accurate than a resistance temperature detector 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 is D. Correct answer in D. NRC Bank Question – P3011 Analysis: The output (or measured) voltage produced by a thermocouple is proportional to the temperature of the measuring (hot) junction compared to the reference (cold) junction. When the cold junction temperature is decreased by 15ºF, a larger differential temperature exists, therefore a larger output signal. Recall: Indication = Measuring – Reference + Calibrated (Initial conditions were, 410 – = 410) Final Conditions: 400 – = 425. Therefore, “D” is correct 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 is C. Correct answer in C. NRC Bank Question – P213 Analysis: WRONG. A thermocouple doesn’t normally fail HIGH. Even on a SHORT, a thermocouple fails LOW. B. WRONG. Even though an open circuit will cause a thermocouple to fail in the “low” direction, it doesn’t necessarily mean to ZERO (which is what I am assuming this choice references). C. CORRECT. If the junction between the dissimilar metals is interrupted by an open circuit, no path for current flow exists, and thus the temperature indication will fail low. Keep in mind that a thermocouple also employs a reference junction. When this junction is calibrated to some temperature above 0°F, depending on where the “open” exists, the thermocouple would fail to the reference junction calibrated temperature (which is still in the “low” direction). D. WRONG. Temperature indications can never fail AS IS. ELO 1.8

Pressure Detectors TLO 2 – Explain the operation of pressure detectors and conditions that affect their accuracy and reliability. 2.1 State the three functions of pressure measuring instrumentation. 2.2 Describe the theory and operation of the following differential pressure detectors: Bellows Diaphragm Bourdon tube Strain Gauge 2.3 Describe the factors that affect accuracy and instrumentation of differential pressure detectors, including their failure modes. Pressure Detector KA’s: 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 NOTE: Even though forced balance and variable capacitance types are included in K1.11, they are not currently tested. Also, strain gauge types are NOT listed in K1.11, however, there is a bank question on this type. 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 local and/or remote Alarm audible and/or visual Control Such that equipment is started or stopped as needed No related NRC KA ELO 2.1

Pressure Detector Theory and Operation
ELO 2.2 – Describe the theory and operation of the following differential pressure detectors: Bellows, Diaphragm, Bourdon tube, Strain gauge. Normally 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 Regardless of type of pressure detector, all operate on the concept: D/P = High pressure – Low pressure High pressure side is the sensing pressure (variable) Low pressure side is the reference pressure (atmospheric) Related KA - K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) ELO 2.2

Bellows Detector 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 Figure: Basic Metallic Bellows ELO 2.2

Bellows Detector Bellows One-piece Collapsible Seamless
Deep folds formed from very thin-walled tubing Figure: Basic Metallic Bellows ELO 2.2

Bellows Detector 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 2.2

Bellows 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 is B. Correct answer in B. ELO 2.2

Diaphragm Detector Used in low pressure applications
High pressure side – sensing pressure Low pressure side – some reference pressure (atmospheric) Diaphragm material types Metallic or non-metallic When system pressure changes Causes axial deflection of diaphragm Corrugated designs provide additional strength and sensitivity ELO 2.2

Bourdon Tube Detector Thin-walled tube
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 Pressure increase causes tube to “straighten out” less curvature causes meter deflection Figure: Bourdon Tube Detector Construction ELO 2.2

Bourdon Tube Detector Knowledge Check
If the pressure sensed by a bourdon tube increases, the curvature of the detector will ____________ because the greater force is being applied to the ____________ curve of the detector. increase; outer increase; inner decrease; outer decrease; inner Correct answer is C. Correct answer in C. NRC Bank Question – P413 Analysis: When system pressure is applied to a Bourdon tube, a force is applied to both the inner and outer walls of the tube, which is the product of pressure and area: P = F/A, or, F=PA The outer curve of the bourdon tube detector has a higher surface area because it has a larger radius from the center of the tube. Therefore, when system pressure is increased, a larger force is applied to the outer wall of the tube, which results in decreased curvature of the tube. ELO 2.2

Strain Gauge Sometimes used for RCS pressure instruments
Increase in pressure at inlet of bellows causes bellows to expand Expansion of bellows moves flexible beam 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 2.2

Strain Gauge Value of pressure found by measuring change in resistance of wire grid 𝑅=𝐾 𝐿 𝐴 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 Figure: Strain Gauge ELO 2.2

Strain Gauge Wire grid is distorted by elastic deformation of pressure change For an increase in pressure Length increases Cross-sectional area decreases Resistance increases Change in resistance used as variable resistance in bridge circuit that provides an electrical signal for indication of pressure Figure: Strain Gauge ELO 2.2

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 2.2

Strain Gauge Knowledge Check
Semiconductor strain gages are often used in transmitters for... reactor coolant pressure instruments. reactor coolant temperature instruments. control rod position instruments. steam generator level instruments. Correct answer is A. Correct answer in A. NRC Bank Question – P310 Analysis: Semiconductor strain gages measure the amount of deformation per unit length when a tensile stress is applied. Recall from – Brittle Fracture and Vessel Thermal Stress, that strain (e) equals change in length divided by original length. This “strain” or movement in the wire changes the resistance. As pressure is increased, a strain is produced on the reactor coolant system boundary (piping, vessels, etc) such that it undergoes mild plastic deformation. Recall when a wire gets longer (or its cross-sectional area gets smaller) its resistance gets larger. This change in resistance (and current) is related to the change in pressure in the system where it is being used. ELO 2.2

Pressure Detection Circuitry
Sensing Element (various types just discussed) Senses pressure of monitored system Converts pressure to a mechanical signal Supplies mechanical signal to transducer No related KA’s to the circuitry. Figure: Pressure Detection Circuit Block Diagram ELO 2.2

Transducer Converts mechanical signal to electrical signal that is proportional to system pressure Various types include: Slidewire, inductance-type, differential transformer, and variable capacitance If mechanical signal from sensing element is used directly, a transducer is not required For example, bourdon tube local pressure gage Brief overview of transducer types on next few slides. Figure: Pressure Detection Circuit Block Diagram ELO 2.2

Pressure Transducer Types - Slidewire
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 Figure: Slidewire Resistance Type Transducer ELO 2.2

Pressure Transducer Types - Inductance
The inductance-type transducer consists of the following three parts: Coil Movable magnetic core Pressure-sensing element The inductance of a coil is higher when the core is in it. In the above case, movement of the center tap against spring pressure raises the inductance of on coil and reduces the inductance of the other, off-balancing a bridge circuit. Figure: Inductance Type Transducer ELO 2.2

Pressure Transducer Types - 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 Changes coupling from Primary coil to Secondary coil (output) Since the motion is linear and the difference in the two coils is sensed, this is a Linear Variable Differential Transformer as used to sense Main Turbine valve position, etc. Figure: Differential Transformer ELO 2.2

Pressure Transducer Types - Capacitance
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 2.2

More common term - Transmitter Will amplify and/or transmit signal to pressure indicator Electrical signal generated by detection circuitry is proportional to system pressure Once the signal is transduced into an operable type of signal, the signal must be “transmitted” to the indication. Figure: Pressure Detection Circuit Block Diagram ELO 2.2

Pressure Indication Provides indication of system pressure May either be read locally or at remote location Figure: Pressure Detection Circuit Block Diagram ELO 2.2

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 is D. Correct answer in D. ELO 2.2

Pressure Detector Environmental Effects
ELO 2.3 – Describe the factors that affect accuracy and instrumentation of differential pressure detectors, including their failure modes. Ambient Pressure Pressure instruments are usually sensitive to variations in atmospheric pressure (reference pressure) If reference pressure increases, indication decreases by same amount Ambient Temperature Ambient temperature affects resistance of components in circuitry Slightly impacts material properties of metals used Temperature increase can cause metal to become more flexible Temperature compensation may be used, if necessary K1.11 Effects of operating environment (pressure, temperature) K1.12 Modes of failure ELO 2.3

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 2.3

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 2.3

Detector Failure or Overranging Pressure instruments are designed and selected to withstand pressure above normal design pressure However, sudden overpressurizations causing over-range conditions could straighten bourdon tubes or weaken bellows spring If sensing element of detector is stretched or stressed, indications may be erroneously high If sensing element has a leak or rupture Indication will fail low 0# – if calibrated to psig 15# – if calibrated to psia ELO 2.3

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. NRC Bank Question – P510 Analysis: For a typical bellows-type D/P detector, the fluid on the high pressure (RCS) connection exerts a force against the moveable wall. This force is opposed by both the low pressure fluid and the force of the spring, which will increase as it is compressed until all forces cancel out. The linear deflection is then measured and converted into a differential pressure. A spring which has weakened will have to compress further in order to provide the same amount of counterforce against the high pressure fluid. Additional compression of the spring will result in further axial deflection, and thus a higher indicated D/P (pressure). ELO 2.3

Knowledge Check – NRC Bank A cooling water system pressure detector uses a bourdon tube as the sensing element. Which one of the following explains how the indicated system pressure will be affected if a local steam leak raises the temperature of the bourdon tube by 50°F? (Assume the cooling water system pressure does not change.) Indicated pressure will decrease because the bourdon tube will become more flexible. Indicated pressure will increase because the bourdon tube will become more flexible. Indicated pressure will decrease because the bourdon tube internal pressure will increase. Indicated pressure will increase because the bourdon tube internal pressure will increase. Correct answer is B. Correct answer is B. NRC Bank Question – P7503 Analysis: A 50ºF increase in building temperature will slightly alter the material properties of the Bourdon tube. Unlike pressure, building temperature changes will not have a significant impact on Bourdon tube operation. Even though the impact on the bourdon tube by this temperature change is insignificant, the only correct answer is that the temperature rise will cause the tube to become more flexible causing it to move more for a given pressure difference. This will cause indicated pressure to increase. This makes Choice “B” the correct answer. ELO 2.3

Level Detectors TLO 3 – Explain the operation of level detectors and conditions that affect their accuracy and reliability. 3.1 Describe the three functions for using remote level indicators. 3.2 Describe the operation of the following types of level instrumentation: Gauge glass Magnetic bond Conductivity probe Differential pressure (D/P) 3.3 Describe density compensation in level detection systems to include: Why needed How accomplished 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 TLO 3

3.4 State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: Differential pressure (D/P) transmitter Amplifier Indication 3.5 Describe the environmental conditions which can affect the accuracy and reliability of level detection instrumentation. 3.6 State the various failure modes of level detection instrumentation. 3.7 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 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

Operation of Level Detectors
ELO 3.2 – Describe the operation of the following types of level instrumentation: gauge glass, magnetic bond, conductivity probe, and 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 Different types of D/P level detectors Wet-Reference type used for PZR and SG Related KA - K1.06 Temperature/pressure compensation requirements Only the D/P Level Detector type is tested by the NRC. 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 (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 Figure: 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

Three basic types of D/P level detectors (NRC uses a 4th for testing - #3) Open Reference (#2) Open Reference with loop seal (#3) Basically like #2, but with less D/P Dry Reference (#4) Wet reference (#1) D/P = HP – LP (all 4 tanks) Tanks 2, 3, 4 HP is tank If D/P increases, indicated level increases Tank 1 (wet reference) HP is reference leg If D/P increases, indicated level decreases Detector DP #3 is not real useful and is probably only there to give them 4 tanks to discuss. DP #3 is basically the same as DP #2, but with a lower DP. The only confusing question concept on DP Detectors #1 and #3 are when they have a less than full level in the reference leg. In these tanks, the mass would be constant so if the temperature increased, the pressure felt by the reduction in density and expansion of the water would result in NO change in DP, therefore, NO change in indicated level. For detector #1 when used in the SG or PZR, there is a condensing pot that keeps the level full. ELO 3.2

Only Tanks 2 and 3 affected by atmospheric pressure changes Tanks 1 and 4 gas pressure cancels out Sensed on both sides Tank #1 D/P (HP – LP) Pref level + Pgas – (Ptank level + Pgas) Tank #2 D/P (HP – LP) (Ptank level + Pgas) – Patm Tank #3 D/P (HP – LP) (Ptank level + Pgas) – (Patm + Ploop seal) Tank #4 D/P (HP – LP) (Ptank level + Pgas) – Pgas ELO 3.2

Knowledge Check Refer to the drawing of a differential pressure (D/P) level detection system (see figure below) for a pressurizer at normal operating temperature and pressure. The level detector has just been calibrated. The high pressure side of the detector is connected to the __________; and if the equalizing valve is opened, the indicated pressurizer level will be __________ than the actual level. condensing pot; lower condensing pot; higher pressurizer; lower pressurizer; higher Correct answer is B. Correct answer is B. NRC Bank Question – P5204 Analysis: Because a pressurizer contains a two-phase fluid, a wet reference D/P cell must be used. Remember that for a wet reference only, the high pressure tap is on the reference leg, while the low pressure tap is on the tank side. The high pressure tap is on the reference leg because this water is much cooler (hence significantly more dense) than the water on the variable leg, and thus a higher pressure will be developed on the reference leg (condensing pot). This results in indicated level being inversely proportional to differential pressure. If the equalizing valve is opened, this will result in a minimum differential pressure, thus a higher indicated level. ELO 3.2

Density Compensation In Level Detection
ELO 3.3 – Describe density compensation in level detection systems to include: why needed and 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= 𝑉𝑜𝑙𝑢𝑚𝑒 𝑀𝑎𝑠𝑠 Specific volume is the reciprocal of density: 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑉𝑜𝑙𝑢𝑚𝑒= 1 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 ELO 3.3

Vessel with Water at Saturated Boiling Conditions
Condensing pot at top of reference leg Condenses 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 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
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

Calibrated vs Actual Temperature (Tank)
Consider a Wet Reference D/P Level Detector If tank temperature increases (above calibrated value) Density decreases, LP pressure decreases, D/P increases, indicated level decreases In other words, if Tcal(tank) < Tact(tank), then, Lind < Lact Consider the other three D/P Level Detector types Density decreases, LP pressure decreases, D/P decreases, indicated level decreases You can see this thumbrule works for ALL four tank types If the reference leg temperature increases, the opposite effect occurs If Tcal(ref) < Tact(ref), then, Lind > Lact Wet reference detector is normally assumed to be a constant level detector. 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 is D. Correct answer in 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: Differential pressure (D/P) transmitter, Amplifier, Indication Diaphragm with HP and LP inputs on opposite sides As D/P changes, diaphragm moves Transducer changes mechanical motion into electrical signal Signal is amplified for level indication at remote location System provides alarms on high and low level via relays and may provide control functions Valve repositioning Pump tripping 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 Signal is amplified 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 Figure: Differential Pressure Level Detection Circuit 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 wet reference leg devices (PZR and SG) Saturated systems Ambient Temperature Effects on reference leg of wet reference D/P level detector Humidity Can cause short circuits, grounds, and corrosion which may damage components K1.08 Effects of operating environment (pressure and temperature) Fluid Density Tank Density If steam pressure decreases, temperature of tank decreases Tank density increases, LP side pressure increases D/P decreases, indicated level increases Reference Leg Density If reference leg density decreases Ambient Temperature Effects on reference leg of wet reference D/P level detector Can affect resistance of components in instrumentation circuitry, affecting calibration of electric/electronic equipment Effects of temperature changes reduced with density compensation 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 Refer to the drawing of a water storage tank with a differential pressure (D/P) level detector (see figure below). The level instrument has just been calibrated to read actual tank water level. If the reference leg subsequently experiences high ambient temperature, indicated level will... equal the actual level. read less than the actual level. read greater than the actual level. drift above and below the actual level. Correct answer is C. Correct answer in C. NRC Bank Question – P11 Analysis: This tank employs a wet reference D/P. Remember that for a wet reference only, the high pressure tap is on the reference leg, while the low pressure tap is on the tank side. The high pressure tap is on the reference leg because this water is much cooler (hence significantly more dense) than the water on the variable leg, and thus a higher pressure will be developed on the reference leg (condensing pot). A rise in reference leg temperature will result in less dense water in the reference leg. This lowers the HP side, causing D/P to decrease. Because indicated level is inversely proportional to differential pressure on a wet reference leg, If D/P decreases, indicated level will read higher than actual tank level (which has not changed at all). 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 decreases, indicated level will decrease If D/P increases, indicated level will increase Exception is wet reference leg level detection, which has the opposite effect If D/P decreases, indicated level will increase If D/P increases, indicated level will decrease Related KAs: 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

Level Detection Failure Modes
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 is D. Correct answer in D. NRC Bank Question – P2609 Analysis: Recall, on a wet reference leg D/P level detector, for indicated level to be greater than actual level, D/P would need to DECREASE. WRONG. External pressure has no impact on a wet reference D/P level detector. WRONG. If SG pressure increases, temperature increases, and density decreases. This causes LP pressure to decrease, causing D/P to INCREASE. We are looking for a decrease in D/P. WRONG. If actual level increased by 6 inches, indicated level would also increase by 6 inches (Indicated equals Actual). CORRECT. If reference leg temperature increases, density decreases, causing HP pressure to decrease. This results in a DECREASE in D/P (which is what we are looking for). ELO 3.6

Detector Transients ELO 3.7 – Analyze detector installation and applications to determine the effects of transients on level indication. Wet reference D/P transients have big impacts Changes in SG or PZR pressure Impacts temperature and density of tank Reference leg flashing Occurs on lowering of SG or PZR pressure Temperature change in tank temperatures impact ALL tank types Dry Reference Leg types also impacted by condensation in dry leg Pressure of dry (LP) side increases, D/P decreases Indicated level decreases Related KA - K1.03 Describe how pressure and level sensing instruments work Some W Pressurizer reference legs are “sealed” with a diaphragm just under the condensing pot to prevent flashing, esp. from Hydrogen in the PZR coming out of solution. ELO 3.7

Detector Transients – Wet Reference Leg
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

Loss of Reference Leg Pressure Reference leg pressure can be lost or reduced by Temperature increases Leaks or reference leg flashing Open or leaking equalizer valves Results in indicated level being higher than true level Loss of Variable Leg Pressure Variable leg pressure can be lost or reduced by Leaks Open or leaking vent valves Results in indicated level being lower than true level ELO 3.7

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

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

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 water storage tank with a differential pressure (D/P) level detection system (see figure). The level detector has just been calibrated. How will the indicated level be affected if condensation partially fills the normally dry reference leg? Indicated level will not be affected. Indicated level will be lower than actual level. Indicated level will be higher than actual level. Indicated level may be higher or lower than actual level depending on the pressure in the upper volume of the tank. Correct answer is B. Correct answer in B. NRC Bank Question – P7602 Analysis: This tank employs a dry reference D/P cell. Remember that for a dry reference, the high pressure tap is on the tank and the low pressure tap is on the reference leg. The only thing that changes is the pressure of the dry reference leg. If water was to condense in the dry reference leg, the pressure of the low pressure side (ref leg) would increase. This would cause the D/P to decrease since the HP side (tank) didn’t change. This causes D/P to lower, causing indicated level to also lower (indicated less than actual). This makes choice “B” correct. ELO 3.7

Flow Detectors Overview
TLO 4 – Describe the operation of flow detectors and conditions which effect their accuracy and reliability. 4.1 Describe the theory of operation of a basic head flow meter. 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 TLO 4

4.3 Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. 4.4 State the typical failure modes for head flow meters including the effects of vapor on a flow instrument. 4.5 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 Differential pressure converted to flow measurement Types include: Orifice, flow venturi, pitot tube, flow nozzle, elbow flow meter 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. High pressure and low pressure across device measured (D/P) D/P measured by manometer or D/P detector Restriction causes an increase in fluid velocity and decrease in pressure Recall Bernoulli’s equation Drop in flow energy (pressure), increase in kinetic energy (velocity) Volumetric flow rate remains unchanged Same amount of fluid passes through per unit time as does upstream of the restriction D/P is proportional to the square of volumetric flow rate ELO 4.1

Head Flow Meter Theory of Operation
Where: D/P = differential pressure caused by restriction = volumetric flow rate Also, K = flow constant for the meter ELO 4.1

Head flow meter actually measures volumetric flow rate Mass flow rate is required for certain flow systems Calculated by knowing Temperature/pressure Recall Figure: Head Flow Instrument Temperature and pressure affect density of fluid and, therefore, mass of fluid flowing past a certain point 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: 𝑚 =𝐾𝐴 𝐷/𝑃(𝑃) 𝑚 =𝐾𝐴 𝐷/𝑃 𝑇 Where: 𝑚 = mass flow rate A = area D/P = differential pressure P = pressure T = temperature K = flow coefficient 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... directly proportional to the differential pressure (D/P) squared. inversely proportional to the D/P squared. directly proportional to the square root of the D/P. inversely proportional to the square root of the D/P. Correct answer is C. Correct answer in C. NRC Bank Question – P9 Analysis: Differential pressure is measured across the venturi. Volumetric flow rate is proportional to the square root of the differential pressure. If the knowledge item is difficult to remember, this relationship can be derived using Bernoulli’s equation given on the equation sheet (change in flow energy and kinetic energy). 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, and pitot tube. There are several designs of flow meters that work on the theory that flow is proportional to the square root of the D/P Related KA - K1.05 Explain the operation of a flow D/P cell type flow detector ELO 4.2

Orifice Plates Fluid forced to converge through small hole
Velocity and pressure change Point of maximum convergence Called vena contracta 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 Disadvantages Cause high permanent pressure drop Outlet pressure will be 60% to 80% of inlet pressure Subject to clogging or erosion Clogging – high D/P, higher indicated flow Erosion – low D/P, lower indicated flow Figure: Orifice Plate Types Shouldn’t 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 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 Recovery cone allows for recovery of pressure Total pressure loss is only 10-25% Lowest pressure drop of any head flow meters Major disadvantages of venturi tube: High initial costs for installation Difficulty in installation and inspection Figure: Venturi Tube ELO 4.2

Flow Nozzle Used for high-velocity flow
Smooth contoured flow restriction High permanent pressure loss similar to the orifice Figure: Flow Nozzle ELO 4.2

Elbow Meter Small D/P allows for high accuracy D/P a function of:
centripetal force throws the fluid to the "outside of the curve", increasing pressure there Difference in surface area creates Low-pressure on the inner pipe wall High-pressure on the outer pipe wall Can measure flow in either direction Some elbow meters have three inner taps redundancy Figure: Elbow Meter ELO 4.2

Pitot Tube Pitot tube actually measures fluid velocity instead of fluid flow rate High pressure side = static pressure+dynamic pressure (velocity) Low pressure side = static pressure D/P is dynamic pressure (velocity) Must be calibrated for each specific application No standardization Can be used even when fluid is not enclosed in pipe or duct 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) Figure: Pitot Tube 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 A Point B Point C Point D Correct answer is A. Correct answer in A. NRC Bank Question – P807 Analysis: The highest pressure in a venture tube is upstream of the device. Normally the upstream tap is placed approximately a distance upstream of the convergence equal to ½ the diameter of the pipe. For example, if the pipe is a 6” pipe, the tap is 3” upstream of where it starts to converge. This minimizes any fluctuations of the upstream side by the turbulence caused by the converging pipe. ELO 4.2

Steam Flow - Density Compensation
ELO 4.3 – Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. Recall – steam flow detectors provide volumetric flow rate Pressure changes affect density of steam Flow measurement of steam systems requires compensation for density Gasses are compressible, while liquids are not When pressure decreases, so does density Less mass per unit of volume (density) Change in density takes place between high- and low-pressure taps Density compensation converts volumetric flow rate to mass flow rate Related KA - K1.02 Temperature/density compensation requirements ELO 4.3

Density Compensation The following equations are used to describe the fundamental relationship for volumetric flow and mass flow 𝑚 = 𝑉 𝜌 Where: 𝑉 = volumetric flow K = constant relating to the ratio of pipe to orifice D/P = differential pressure ρ = density 𝑚 = mass flow ELO 4.3

Steam Mass Flow Detection System
D/P measured by detector Square root extractor converts D/P to volumetric flow rate Temperature (at some plant types) and pressure used as density input Converts to mass flow rate Required for main steam flow control system(s) Figure: Simple Mass Flow Detection ELO 4.3

Density Compensation Example 𝑚 Act = 𝑚 Ind, or, 𝜌 𝑉 Act = 𝜌 𝑉 Ind If steam pressure increases (with constant volumetric flow rate) ↑𝜌↔ 𝑉 Act=↑ 𝑚 Act (actual mass flow rate increases) With density compensation ↑𝜌↔ 𝑉 Ind = ↑ 𝑚 Ind (indicated mass flow rate also increases) Without density compensation All 𝑚 Ind sees is 𝑉 Ind, therefore, since ↔ 𝑉 Ind ↑ 𝑚 Act = ↔ 𝑚 Ind Actual > Indicated ELO 4.3

Density Compensation Failure Example 𝜌 𝑉 Act = 𝜌 𝑉 Ind If density compensation fails high ↔𝑚 Act (no change in steam pressure or volumetric flow rate) However, ↑𝜌 𝑉 Ind, therefore, ↑𝑚 Ind If density compensation fails low Opposite effect (indicated flow decreases) ELO 4.3

Density Compensation Knowledge Check
A main steam flow rate measuring instrument uses a steam pressure input to produce main steam mass flow rate indication. Assuming steam volumetric flow rate does not change, a steam pressure decrease will cause indicated steam mass flow rate to... increase, because the density of the steam has increased. decrease, because the density of the steam has decreased. remain the same, because steam pressure does not affect the mass flow rate of steam. remain the same, because the steam pressure input compensates for changes in steam pressure. Correct answer is B. Correct answer is B. NRC Bank Question – P2505 Analysis: NOTE: This question is basically discussing how a steam flow detection is supposed to work! It is important to note that for saturated steam, changes in temperature and pressure will impact the mass flow rate of the steam. For example, if the pressure of the steam were increased, this would increase the density of the steam. However, the differential pressure measured by the venturi would not change, so there would be no indicated change in mass flow rate of the steam. Therefore, steam pressure (which is proportional to density) is also measured and used to density compensate the signal to provide accurate mass flow rate. If the steam pressure decreases, this will result in a lower density, therefore lower flow rate indication (with a constant volumetric flow rate). ELO 4.3

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 is A. Correct answer is A. NRC Bank Question – P1212 (modified order) Analysis: It is important to note that for saturated steam, changes in temperature and pressure will impact the mass flow rate of the steam. For example, if the pressure of the steam were increased, this would increase the density of the steam. However, the differential pressure measured by the venturi would not change, so there would be no indicated change in mass flow rate of the steam. Therefore, steam pressure (which is proportional to density) is also measured and used to density compensate the signal to provide accurate mass flow rate. If the steam pressure input fails low, this will result in a lower perceived density, therefore lower flow rate indication. Remember, as goes density compensation signal, so goes indication! ELO 4.3

Flow Meter Failure Modes
ELO 4.4 – State the typical failure modes for head flow meters including the effects of vapor on a flow instrument. Leakage is a common problem with head flow meters Erosion or blockage of orifice plates Vapors/gases in liquid systems Loss of density compensation Related KA - K1.04 Modes of failure Table on next slide ELO 4.4

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 indicated mass flow rate will be lower. Related KA - K1.04 Modes of failure ELO 4.4

Condition Indication Discussion 5. Steam pressure input fails low Indicated flow less than actual Apparent density has decreased, less mass is sensed passing the flow detector. 6. Steam pressure input fails 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 4.4

Knowledge Check If the orifice in a differential pressure (D/P) flow sensor erodes such that the orifice opening becomes larger, indicated flow rate will __________ due to a __________ D/P across the orifice. (Assume actual flow rate remains the same.) increase; larger increase; smaller decrease; larger decrease; smaller Correct answer is D. Correct answer is D. NRC Bank Question – P1205 Analysis: Erosion of the orifice will result in a larger cross-sectional area, therefore producing a smaller pressure drop (D/P). A D/P cell measures high pressure minus low pressure and calculates a flow rate. Flow is proportional to the square root of differential pressure. Thus, anything that reduces differential pressure across the orifice will cause indicated volumetric flow rate to decrease. ELO 4.4

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; larger increase; smaller decrease; larger decrease; smaller Correct answer is A. Correct answer in A. NRC Bank Question – P2107 Analysis: The pipe elbow can be used to measure differential pressure (which is proportional to flow). As the water flows through the pipe elbow/bend, a low pressure area is created on instrument line B. This is because the velocity of the flow tends to flow directly toward instrument line A, creating a higher pressure. A D/P cell measures high pressure minus low pressure and calculates a flow rate. Flow is proportional to the square root of differential pressure. If instrument line B (low pressure) develops a leak, D/P will increase, causing an increase in indicated flow. ELO 4.4

Environmental Effects On Flow Detection
ELO 4.5 – 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 4.5

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 4.5

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 4.5

Position Detectors Overview
TLO 5 – Describe the operation of position detectors and conditions which effect their accuracy and reliability. 5.1 Describe the following switch position indicators to include basic construction and theory of operation: Limit switches Reed switches Coil stacks 5.2 Describe the following variable output position indicators to include basic construction and theory of operation: Potentiometer Linear variable differential transformers (LVDT) TLO 5

5.3 Describe the environmental conditions that can affect the accuracy and reliability of position indication equipment. 5.4 Describe the failure modes for the following position detectors: Reed switch Limit switch Potentiometer LVDT TLO 5

Switch Position Indicators
ELO 5.1 – Describe the following switch position indicators to include basic construction and theory of operation: limit switches, reed switches, and coil stacks. Mechanical limit switches provide valve open and shut indications Reed switches can provide intermediate valve position(s) Reed switches or Coil stacks also used for control rod position 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 Electric circuit contacts usually provide for dual indication Any position other than full open or full closed Switches shown are mounted directly to the valve stem and are the most reliable. Some Motor-Operated valves have gear and cam driven limit switches mounted in the gear box, but these will indicate incorrectly if the gears slip or strip completely. Figure: Limit 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 positions can be measured 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 Switch - Valve Position ELO 5.1

Reed Switch Used for position indication of control rods
Permanent magnet installed on control rod drive shaft attracts moveable contact arm of each reed switch as drive passes by Closes contacts as rod withdrawn (S1, then S2, etc.) Shorts out resistors Current increases as rod is withdrawn Figure: Reed Switch – Control Rod Position ELO 5.1

Coil Stacks Coils wired in sets on outside of control rod drive housing with rod on bottom coil impedance is balanced all detector sets send a 0.0V signal As rod drive shaft passes through Coil A, impedance increases Current decreases in ammeter A As rod withdrawal continues Current in ammeter A stays the same Current in ammeter B decreases Figure: Coil Stacks NRC tests this simple concept: Reed switches – as rod is withdrawn, resistors are shorted out INCREASING current flow. Coil stacks – as rod is withdrawn, inductance (resistance) increases, DECREASING current flow. ELO 5.1

Limit Switches Knowledge Check
Reed switches are being used in an electrical measuring circuit to monitor the position of a control rod in a reactor. The reed switches are mounted in a column above the reactor vessel such that the control rod drive shaft passes by the reed switches as the control rod is withdrawn. Which one of the following describes the action that causes the electrical output of the measuring circuit to change as the control rod is withdrawn? An AC coil on the control rod drive shaft induces a voltage into each reed switch as the drive shaft passes by. A metal tab on the control rod drive shaft mechanically closes each reed switch as the drive shaft passes by. The primary and secondary coils of each reed switch attain maximum magnetic coupling as the drive shaft passes by. A permanent magnet on the control rod drive shaft attracts the movable contact arm of each reed switch as the drive shaft passes by. Correct answer is D. Correct answer order is D. NRC Bank Question – P2911 Analysis: Reed switches are commonly used to provide control rod position indication. As the control rod is withdrawn, permanent magnets in the control rod cause the reeds (made out of ferrous material) to move toward the rod and makes up a switch. The electrical switches/contacts are then arranged in a voltage divider network to change resistance/current in a circuit to determine control rod position. ELO 5.1

Potentiometers and LVDTs
ELO 5.2 – Describe the following variable output position indicators to include basic construction and theory of operation: Potentiometer, Linear Variable Differential Transformers (LVDT). Several applications require something other than full OPEN or full CLOSED indication Turbine governor control valves, for example 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 changes, changing current flow Amount of current is proportional to valve position Figure: Potentiometer ELO 5.2

Linear Variable Differential Transformers
Similar to potentiometer, but no physical connection Valve position indication provided by: Coupling primary to secondary windings of a transformer Secondary voltage ranges from -10 vdc to + 10 vdc Full closed to full open Valve position shown in Percent open Figure: Linear Variable Differential Transformer ELO 5.2

Variable Output Detectors
Knowledge Check Which one of the following devices is commonly used to provide remote indication of valve position on an analog meter in units of "percent of full open"? Limit switch Reed switch Linear variable differential transformer Resistance temperature detector Correct answer is C. Correct answer order is C. NRC Bank Question – P1313 Analysis: The key words in the stem are “percent of full open “. Linear variable differential transformers (LVDT’s) operate on the principal that as valve stem position changes, a moveable core will change the magnetic coupling between the primary and secondary windings. It is important to note that the two secondary windings are wired such that the electrical currents oppose each other. If these currents were additive, there would be no way to determine valve position in mid-stroke (i.e. as the stem moves up and the moveable core uncouples part of the lower secondary winding, it in turn couples part of the upper secondary winding that was not previously coupled). The output ranges from -10 volts (Full closed) to +10 volts (full open). If the primary winding power supply was to lose power, the secondary output indication would go to 50% open (0 volts). ELO 5.2

Environmental Effects On Position Detection
ELO 5.3 – 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, and therefore, indications Effects are reduced by design of circuitry and by maintaining position indication instrumentation in proper environment ELO 5.3

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 5.3

Environmental Effects
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 is D. Correct answer order is D. ELO 5.3

Position Indication Failure Mode
ELO 5.4 – Describe the failure modes for the following position detectors: Reed switch, Limit switch, Potentiometer, 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 (currently no bank questions on this KA) ELO 5.4

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 5.4

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 5.4

Linear Variable Differential Transformer Failures
LVDTs are extremely reliable Failures limited to rare electrical faults or loss of power An open primary winding will cause indication to fail to zero volts Normally corresponds to mid-position Failure of either secondary winding cause output to indicate either full open or full closed regardless of actual valve position OE: Calvert Cliffs had an actual failure of an LVDT (governor control valve) while at power back around 2001. The plant was getting ready to do a downpower and the control room staff noticed one of the governor control valves was indicating 50%. They sent an aux operator to go to the turbine building and investigate the valve position. The operator came back stating that the valve was FULL OPEN. Once the downpower was commenced, (at a certain power level) the plant shed a few hundred megawatts. An investigation found the power supply amphenol to the primary winding was disconnected (probably vibrated off). This caused the indication to fail to 50%. Since the plant had been at 100% power, EHC was calling for the valve to open. Since it was already FULL OPEN there was no impact on plant power. Once the downpower required the governor control valve to be less than 50% EHC kept sending a signal to close the valve (since indication was failed at 50%). This caused a rapid downpower and load shed. Figure: Linear Variable Differential Transformer ELO 5.4

NRC KA to ELO Tie KA # KA Statement RO SRO ELO K1.01
Characteristics of venturis and orifices 2.2 2.4 4.1, 4.2 K1.02 Temperature/density compensation requirements 2.7 2.9 4.3 K1.03 Effects of gas or steam on liquid flow rate indications (erroneous reading) 4.4 K1.04 Modes of failure K1.05 Explain the operation of a flow D/P cell type flow detector 2.6 2.8 K1.06 Temperature/pressure compensation requirements 2.5 K1.07 Theory and operation of level detectors 3.2 K1.08 Effects of operating environment (pressure and temperature) 3.1 3.5 K1.09 3.0 3.6 K1.10 Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows, forced balance, and variable capacitance) 2.3 K1.11 Effects of operating environment (pressure, temperature) K1.12 K1.13 Theory and operation of T/C, RTD, thermostats 1.2, 1.8 K1.14 Failure modes of T/C and RTD 1.5, 1.6 K1.15 Failure modes of reed switches, LVDT, limit switches, and potentiometers 5.4 K1.16 Applications of reed switches, magnets, LVDT, potentiometers, and limit switches 5.1, 5.2

Operator Generic Fundamentals Components - Sensors and Detectors 1

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Operator Generic Fundamentals Components - Sensors and Detectors 1

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