Revision 1 December 2014 Sensors and Detectors Part 1 Instructor Guide Reviewed by: Cassandra Bitler 10/31/2014 Project Manager, OGF Date Approved by: Robert Coovert 10/31/2014 Manager, INPO Learning Development Date Approved by: Kevin Kowalik 10/31/2014 Chairperson, Industry OGF Working Group Date NOTE: Signature also satisfies approval of associated student guide and PowerPoint presentation. GENERAL DISTRIBUTION
GENERAL DISTRIBUTION: Copyright 2014 by the National Academy for Nuclear Training. Not for sale or for commercial use. This document may be used or reproduced by Academy members and participants. Not for public distribution, delivery to, or reproduction by any third party without the prior agreement of the Academy. All other rights reserved. NOTICE: This information was prepared in connection with work sponsored by the Institute of Nuclear Power Operations (INPO). Neither INPO, INPO members, INPO participants, nor any person acting on behalf of them (a) makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe on privately owned rights, or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this document. ii
Table of Contents INTRODUCTION... 1 TLO 1 TEMPERATURE DETECTORS... 2 Overview... 2 ELO 1.1 Temperature Detector Functions... 3 ELO 1.2 Resistance Temperature Detector Construction... 6 ELO 1.3 Temperature Resistance Relationship... 8 ELO 1.4 Temperature Detection Circuits... 9 ELO 1.5 Environmental Effects... 12 ELO 1.6 Circuit Faults... 13 ELO 1.7 Alternate Temperature Detection... 14 ELO 1.8 Thermocouples... 15 TLO 1 Summary... 19 TLO 2 PRESSURE DETECTORS... 20 Overview... 20 ELO 2.1 Pressure Detector Functions... 21 ELO 2.2 Pressure Detector Theory... 22 ELO 2.3 Bellows Detector... 24 ELO 2.4 Bourdon Tube... 25 ELO 2.5 Strain Gauge... 27 ELO 2.6 Pressure Transducers... 29 ELO 2.7 Pressure Detection Circuit... 32 ELO 2.8 Environmental Effects... 34 ELO 2.9 Alternate Pressure Detection... 36 TLO 2 Summary... 37 TLO 3 LEVEL DETECTORS... 40 Overview... 40 ELO 3.1 Level Detection Functions... 40 ELO 3.2 Operation of Level Detectors... 41 ELO 3.3 Density Compensation... 45 ELO 3.4 Level Detection Circuits... 49 ELO 3.5 Environmental Effects... 50 ELO 3.6 Failure Modes... 53 ELO 3.7 Detector Transients... 54 TLO 3 Summary... 58 TLO 4 FLOW DETECTORS... 60 Overview... 60 ELO 4.1 Flow Meter Theory of Operations... 61 ELO 4.2 Flow Meter Construction... 64 ELO 4.3 Failure Modes... 70 ELO 4.4 Density/Temperature Compensation... 73 ELO 4.5 Mechanical Flow Detectors... 74 ELO 4.6 Steam Flow Density Compensation... 77 ELO 4.7 Flow Detection Circuits... 81 ELO 4.8 Environmental Effects... 82 TLO 4 Summary... 83 TLO 5 POSITION DETECTORS... 85 Overview... 85 iii
ELO 5.1 Switch Type Detectors... 86 ELO 5.2 Variable Output Detectors... 88 ELO 5.3 Position Detector Circuits... 90 ELO 5.4 Environmental Effects... 91 ELO 5.5 Failure Modes... 93 TLO 5 Summary... 95 SENSORS AND DETECTORS PART 1 SUMMARY... 96 iv
Sensors and Detectors Part 1 Revision History Revision Date Version Number Purpose for Revision Performed By 10/31/2014 0 New Module OGF Team 12/11/2014 1 Added signature of OGF Working Group Chair OGF Team Duration Logistics 12 hours Ensure that the presentation space is properly equipped with the following: Projector Internet access, if needed Whiteboard or equivalent Space for notes, parking lot, mockups, or materials Sufficient space for all students Ensure that the following course materials are prepared and staged: All student materials Instructor materials Media, photos, and illustrations Props, lab equipment, or simulator time, as applicable Ensure that all students have fulfilled the course prerequisites, if applicable. Instructor preparation: Review the course material prior to beginning the class. Review the NRC exam bank, and as many new exams as are available prior to the class to ensure that you are prepared to address those items. Ensure that all students have access to the training material for selfstudy purposes. Introduction Proper operation of an industrial plant, such as a nuclear power generating station, requires the measurement of many plant parameters. Operator and automatic actions rely on accurate information provided by sensors and Rev 1 1
detectors installed within the plant systems for controlling plant parameters. Nuclear facility operators are required to monitor key parameters that can affect plant operation and public safety on a regular schedule and analyze those parameters for trends and abnormal conditions. Sensors, detectors, and their associated circuitry measure and indicate parameters including temperature, pressure, level, flow, position, radiation, and reactor power level. It is important to have an understanding of how these sensors and detectors measure plant parameters and how they are prone to failure. Recognizing the indications associated with failed sensors and detectors is an essential skill for plant operators. Familiarity with instrument failure modes will ensure proper interpretation of plant parameters during abnormal operating events, allowing operators to take appropriate mitigating actions. Logistics Use PowerPoint slides 1 3 and the instructor guide (IG) to introduce the Sensors and Detectors Part 1 module. 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 or higher on the following Terminal Learning Objectives (TLOs): 1. Describe the operation of temperature detectors and conditions that effect their accuracy and reliability. 2. Describe the operation of pressure detectors and conditions that effect their accuracy and reliability. 3. Describe the operation of level detectors and conditions that affect their accuracy and reliability. 4. Describe the operation of flow detectors and conditions that effect their accuracy and reliability. 5. Describe the operation of position detectors and conditions that affect their accuracy and reliability. TLO 1 Temperature Detectors Duration 2 hours 15 minutes Logistics Use PowerPoint slides 4 6 and IG to introduce TLO 1. Use the crossword puzzle to enhance objective review. Overview The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon the molecular activity of the material. Kinetic energy is a measure of the activity of the atoms that make up the molecules of any material. Temperature, therefore, is a measure of the average molecular kinetic energy of any material. Temperature detectors provide an important indication of the condition of equipment and material. An operator uses temperature-monitoring data to prevent equipment problems due to 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 2 Rev 1
they are monitoring to raise (or lower) the kinetic energy of the device in order to provide an indication of temperature. Objectives Upon completion of this lesson, you will be able to do the following: 1. State the three basic functions of temperature detectors. 2. Describe the construction of a basic resistance temperature detector (RTD), including: a. Component arrangement b. Materials used 3. Describe how RTD resistance varies for temperature changes. 4. State the purpose of basic temperature instrument detection and control system blocks: a. RTD b. Bridge circuit c. DC-AC converter d. Amplifier e. Balancing motor/mechanical linkage 5. Describe bridge circuit compensation for changes in ambient temperature and environmental conditions that can affect temperature detection instrumentation. 6. Describe the effect on temperature indication(s) for the following circuit faults: a. Short circuit b. Open circuit 7. Describe alternate methods of determining temperature when the normal sensing devices are inoperable. 8. Describe the construction and operation of a thermocouple. ELO 1.1 Temperature Detector Functions Introduction Although different facility design details require monitoring varying specific temperatures, temperature detectors usually provide the following three basic functions in industrial applications: Indications Alarms Control Duration 20 minutes Logistics Use PowerPoint slides 7 11 and IG to present ELO 1.1. Display of the monitored temperatures may be local or in a central location, such as a control room, and may have audible and/or visual alarms that trigger when specified preset limits are exceeded. The monitored temperatures may have control functions tied to them so that equipment is started or stopped to support a given temperature condition or so that a protective action occurs. An ordinary household thermometer is an example of a simple temperature detector. The mercury, or other liquid, in the bulb of the thermometer Rev 1 3
expands as heat increases its average molecular kinetic energy level. By measuring this expansion against a scale calibrated to indicate temperature, the temperature of the object in contact with the bulb can be determined. Temperature is an important parameter in many industrial processes and many types of instruments measure it. Filled System Thermometer A filled system thermometer is a type of temperature detector that can provide both local and remote indication and/or a record of temperature some distance from the point of measurement. The detector consists of a sensing element, which is a bulb containing gas or liquid, and an indicator scale, as shown in the figure below. Figure: Filled System Thermometer As the temperature surrounding the sensing bulb changes the pressure of the fill gas or liquid inside the bulb changes. This change in pressure acts on a receiving element (spiral bourdon tube) via capillary tubing connected to the bulb. The spiral tube responds to the changing pressure in the sensing bulb and produces motion that is proportional to the temperature of the sensing bulb. This motion can drive a pointer on an indicator, a pen on a recorder, or actuate a switch for control response (e.g. a thermostat). Filled system thermometers are available to detect temperatures ranging from approximately -400 F to 1,000 F, depending on the filling medium used in the detector bulb. These types of detectors can detect temperature from distances of up to 400 feet. Bimetallic Strip Thermometer A bimetallic strip thermometer is a simple, rugged device for monitoring temperature. The temperature-measuring element is comprised of two strips of metal that have different coefficients of thermal expansion, fastened together throughout their length, as shown in the figure below. One end is fixed and the other is free to move. Since the two strips of metal act as one, they will both always be at the same temperature. If heated, the bimetallic will bend to adapt to the increased length of the metal with the greater temperature coefficient of expansion. Conversely, if cooled, the 4 Rev 1
bimetallic strip will bend to adapt to the decreased length of the metal with the greater temperature coefficient of expansion. Figure: Bimetallic Strip Often, the thermometer is wound into a spiral-formed bimetallic element 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 -200 F to 1,000 F. Knowledge Check Figure: Bimetallic Strip Thermometer Temperature detection is used to provide the following: (select all that apply) A. Interlocks B. Indications C. Alarms D. Automatic trips Rev 1 5
Knowledge Check Which of the following is not a function of a temperature detector? A. Indication B. Control functions C. Alarm functions D. Amplification Duration 15 minutes Logistics Use PowerPoint slides 12 16 and the IG to present ELO 1.2. ELO 1.2 Resistance Temperature Detector Construction Introduction Resistance temperature detector (RTD) circuits act somewhat like electrical transducers, converting temperature changes to voltage changes through the measurement of changing resistance. The RTD itself is a pure metal or alloy that increases its resistance to electrical current flow as its temperature increases. Conversely, the RTD will decrease its resistance to electrical current flow as its temperature decreases. Resistance Temperature Detectors Construction The RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal. Therefore, the material used to fabricate the RTD element must be drawn into fine wire so that the element can be long, yet compactly constructed. The figure below shows the internal construction of an RTD. Figure: Internal Construction of a Typical RTD 6 Rev 1
The design shown in the figure has a platinum wire element surrounded by a porcelain insulator. The insulator prevents a short circuit between the wire and the metal sheath when an electric current is applied. The RTD sheath is normally comprised of Inconel, a nickel-iron-chromium alloy, because of its temperature response time and its inherent corrosion resistance. When placed in a liquid or gas medium, the Inconel sheath quickly reaches the temperature of the medium. The change in temperature will cause the platinum wire to heat or cool, resulting in a proportional change in resistance. A precision resistance-measuring device calibrated to give the proper temperature reading then measures this change in resistance. The figure below shows a cross-section view of a RTD protective well and terminal head. The well protects the RTD from damage by the gas or liquid measured by the RTD. Protecting wells are normally made of stainless steel, carbon steel, Inconel, or cast iron, and protect the RTD from temperatures up to 1,100 C. Knowledge Check Figure: RTD Protective Well and Terminal Head A resistance temperature detector operates on the principle that the change in electrical of a metal is proportional to its change in temperature. A. conductivity; directly B. conductivity; indirectly C. resistance; indirectly D. resistance; directly Rev 1 7
Duration 5 minutes Logistics Use PowerPoint slides 17 19 and the IG to present ELO 1.3. ELO 1.3 Temperature Resistance Relationship Introduction The resistance to electrical current flow (resistivity) of certain metals will change as temperature changes. Some of these metals exhibit a linear coefficient of resistivity (change in resistance) as temperature changes. This characteristic is the basis for the operation of RTD equipment. An RTD operates on the principle that the change in electrical resistance of a metal is directly proportional to its change in temperature. Temperature vs. Resistance The metals that are best suited for RTD sensors are pure, uniform in quality, stable within a given range of temperature, and able to give reproducible resistance-temperature readings. Only a few metals have the properties necessary for use in RTD elements. The figure below shows temperatureresistance graphs of three of the most commonly used metals. Figure: Resistance vs. Temperature Graph Platinum, copper, or nickel, typically comprise RTD elements. These metals are best suited for RTD applications because of their linear resistance-temperature characteristics, their high coefficient of resistance, and their ability to withstand repeated temperature cycles. The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature. Additionally, it is necessary to draw the material used to fabricate the RTD element must be drawn into a fine wire so that the element can be long and compactly constructed. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal. Knowledge Check A resistance temperature detector operates on the principle that the change in electrical resistance of... 8 Rev 1
Knowledge Check A. a metal is inversely proportional to its change in temperature. B. two dissimilar metals is inversely proportional to the temperature change measured at their junction. C. two dissimilar metals is directly proportional to the temperature change measured at their junction. D. a metal is directly proportional to its change in temperature. What happens to the resistance of a resistance temperature detector (RTD) when the temperature of the substance it is measuring increases? A. Resistance of the RTD decreases and then increases. B. Resistance of the RTD decreases. C. Resistance of the RTD increases. D. Resistance of the RTD remains the same. ELO 1.4 Temperature Detection Circuits Introduction A temperature detection circuit consists of components with specific functions to detect temperature changes and condition the signal so that it is in a readable form for operators to monitor or for control circuits to interpret. Each component is necessary for the temperature monitoring circuit because of the range and environmental conditions in which the system is required to function. Duration 30 minutes Logistics Use PowerPoint slides 20 28 and the IG to present ELO 1.4. Temperature Detection Circuits Bridge Circuit A bridge circuit is used with RTD elements to obtain accurate temperature measurements. A bridge circuit consists of three know resistances and one unknown variable resister, R X; a voltage source and a sensitive voltmeter. Rev 1 9
Figure: Typical Bridge Circuit R 1 and R 2 form the ratio arms of the bridge and R 3, the standard arm is a variable resister is adjusted to match the unknown resistance. Unbalanced Bridge Circuit An unbalance bridge circuit uses a millivolt meter calibrated in units of temperature that correspond to RTD resistance. (see figure below) A battery connects to two opposite points of the bridge circuit, while a millivolt meter connects to the two opposite points. A rheostat balances the bridge circuit, while regulated current divides between two branches. One branch consists of R x and R 1, while the other consists of the RTD and resister R 2. Balanced Bridge Circuit Figure: Unbalanced Bridge Circuit A balanced bridge circuit uses a galvanometer to compare RTD resistance to a fixed resister. (See figure below) The galvanometer pointer deflects to either side of zero when the resistance arms are not equal. The slidewire resister balances the arms of the bridge, such that no current will flow when the circuit is balanced. The resistance of the slidewire adjusts until the 10 Rev 1
galvanometer indicates zero, the value of the slidewire determines temperature of the monitored system. As temperature changes, there is a new value of resistance developed to balance the circuit. Figure: Balanced Bridge Circuit The figure below is a block diagram of a typical temperature detection circuit. This represents a balanced bridge temperature detection circuit modified to eliminate the galvanometer. Figure: Basic Temperature Detection Circuit The temperature measuring steps are as follows: The resistance temperature detector (RTD) reacts to the temperature. The detector reaction modifies resistance to the bridge network. The bridge network converts this resistance to a DC voltage signal. The DC-AC converter is an electronic instrument that converts the DC voltage of the potentiometer, or the bridge, to an AC voltage. Rev 1 11
An amplifier increases the AC voltage to a higher (usable) voltage that is used to drive a bi-directional balancing motor. The bi-directional balancing motor positions the slider on the slidewire to balance the circuit resistance. Knowledge Check Typical temperature bridge circuits use low voltage (millivolt) signals. How does this low voltage drive a remote meter indication? A. The signal is amplified, which raises the voltage. B. The signal is converted from AC to DC, which raises the voltage. C. The signal is amplified, which lowers the voltage. D. The signal is converted from DC to AC, which raises the voltage. Duration 5 minutes Logistics Use PowerPoint slides 29 31 and the IG to present ELO 1.5. ELO 1.5 Environmental Effects Introduction Resistance temperature circuits measure the resistance of a metal in a process, and correlate the measured resistance changes to temperature. These circuits operate at very low voltages (millivolt) and amperage (milliamp). At these very low voltages and currents, it is necessary to consider environmental effects on the circuit itself because ambient temperature and humidity changes affect the circuit's resistance. These changes can affect the circuit output signal and give a false indication of temperature; therefore, the circuitry design includes compensation features. Ambient Temperature Ambient temperature variations will affect the accuracy and reliability of temperature detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in a bridge circuit and the resistance of the reference junction for a thermocouple. In addition, ambient temperature variations can affect the calibration of electric/electronic equipment. Circuitry design and maintaining the temperature detection instrumentation in the proper environment will reduce the effects of temperature variations. Humidity The presence of ambient humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes 12 Rev 1
moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. Maintaining electronic equipment in the proper environment will control the detrimental effects of humidity. The proper use of heating, ventilation, and air conditioning equipment controls humidity in plant electrical equipment. Design Compensation Proper electronic circuitry design will compensate for ambient temperature changes in the equipment cabinet. It is also possible for the resistance of the detector leads to change due to a change in ambient temperature. To compensate for these ambient temperature changes, three and four wire RTD circuits are used. In this way, both branches of the bridge circuit use the same amount of lead wire, and both branches will feel a change in resistance, thus negating the effects of the change in ambient temperature. 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... A. the change in resistance will be felt on neither branch. B. the change in resistance is not an important factor in temperature measurement. C. the change in resistance will be felt on both branches. D. the change in resistance is important only when calibrating temperature circuits. ELO 1.6 Circuit Faults Introduction Electrical faults affect the indication because RTD circuits actually measure the changes in electrical circuit performance. Short circuits and open circuits are two electrical faults that can result in faulty indication. In a short circuit, the short diverts the signal, precluding a complete circuit; in an open circuit, the open halts the signal, also precluding a complete circuit. Duration 10 minutes Logistics Use PowerPoint slides 32 34 and the IG to present ELO 1.6. Circuit Fault In an RTD: If either an unbalanced or balanced bridge circuit becomes open, the resistance will be infinite, and the temperature-indicating meter will indicate a very high temperature. Rev 1 13
If there is a short circuit, resistance will be zero, and the temperatureindicating meter will indicate a very low temperature. 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? A. 300 B. 600 C. 0 D. Dependent on measured temperature Duration 10 minutes Logistics Use PowerPoint slides 35 37 and the IG to present ELO 1.7. ELO 1.7 Alternate Temperature Detection Introduction In the event that primary temperature sensing instruments become inoperative, several alternate methods may obtain temperature indications. Some methods use the temperature detection circuit even though there may be a failure within the circuit. Alternate Temperature Detection The design of the circuit, the nature of the circuit or detector failure and the components that remain functional will determine the viable alternate method of temperature indication. Some temperature detecting circuit applications utilize installed spare temperature detectors or dual-element RTD's. The dual-element RTD has two sensing elements, only one of which is normally connected. If the operating element becomes faulty, connect the second element to provide temperature indication. 14 Rev 1
If there is no installed spare, use a contact pyrometer (portable thermocouple) or an optical pyrometer to obtain temperature readings on those pieces of equipment or systems that are accessible. If the malfunction is in the circuitry and the detector itself is still functional, it may be possible to obtain temperatures by connecting an external bridge circuit to the detector. Record resistance readings and obtain a corresponding temperature from the detector calibration curves. 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 A. connecting a spare RTD into the circuit. B. doing nothing, the existing circuit will still measure temperature with an open circuit. C. direct resistance measurements. D. surface resistor. ELO 1.8 Thermocouples Introduction A thermocouple is a device that converts thermal energy into electrical energy. The thermocouple operates on the principle that when two dissimilar metals form two junctions at different temperatures, they produce a measurable voltage. Because of their construction, thermocouples are capable of measuring temperatures in much harsher environments than RTDs, but are not as accurate as the RTD. High-temperature applications Duration 10 minutes Logistics Use PowerPoint slides 38 47 and the IG to present ELO 1.8. Rev 1 15
often use thermocouples, and thermocouples often serve as a backup means of measuring temperature when other temperature detection methods fail. Thermocouples A thermocouple is comprised of two dissimilar metal wires joined together at one end (the measuring junction). When the other end of each wire connects to a measuring instrument (the reference junction), the thermocouple becomes a sensitive and highly accurate temperaturemeasuring device. Voltage produced across the reference junction, based on the temperature at the measuring (sensing) junction, is in the millivolt range. A meter connected across the reference junction measures voltage, which is proportional to temperature. The figure below shows an example of a simple thermocouple. Figure: Simple Thermocouple Circuit Several different combinations of materials may comprise thermocouples. The most important factor when selecting a pair of materials is the "thermoelectric difference" between the two materials. A higher thermoelectric difference between the two materials will result in better thermocouple performance. Thermocouples often use platinum as one of the paired materials; a combination of another material paired with platinum serves as the performance standard when evaluating other possible thermocouple materials. The figure below shows the internal construction of a typical thermocouple. A rigid metal sheath encases the leads of the thermocouple. The bottom of the thermocouple housing normally contains the measuring junction. Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage the fine wires and to enhance heat transfer between the measuring junction and the medium surrounding the thermocouple. 16 Rev 1
Figure: Internal Construction of a Typical Thermocouple The dissimilar thermocouple wires lead to a reference junction; a sealed aluminum block normally protects the reference junction, and the junction is temperature controlled. Changes in temperature at the reference junction will affect the temperature reading. If the temperature at the reference junction were to decrease, the indicated temperature would increase. Many thermocouple circuits include a reference junction panel to ensure that temperature changes away from the thermocouple-measuring junction do not affect thermocouple temperature indication. Thermocouple Operation Example Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current produced depends on the temperature difference between the measurement and reference junction, the characteristics of the two metals used, and the characteristics of the attached circuit. The figure below shows a basic thermocouple circuit. Figure: Simple Thermocouple Circuit Rev 1 17
Heating the measuring junction of the thermocouple produces a voltage that is greater than the voltage across the reference junction. The voltmeter measures the difference between the two voltages (in millivolts); the voltage is proportional to the difference in temperature. If temperature at the reference (cold) junction were to decrease, indicated temperature would increase and vice versa. For ease of operator use, some voltmeters are set up to read out directly in temperature through use of electronic circuitry. Other applications provide only the millivolt readout. In order to convert the millivolt reading to its corresponding temperature, the operator must refer to vendor-supplied thermocouple tables. The thermocouple manufacturer supplies these tables, and they list the specific temperature corresponding to a series of millivolt readings. Thermocouple Failures and Disadvantages Thermocouples generally fail in several common modes. If a break occurs in wire, and there is no current flow, the device normally fails low. If a break or open occurs in the detector, the indicated temperature fails to the reference junction temperature. A change in reference junction temperature causes an indication change. The indication is proportional to the signal difference between the measured temperature and the reference temperature; therefore, the reference junction temperature should be controlled or accounted for. By comparison, a thermocouple is less accurate than a resistance temperature detector. Knowledge Check NRC Bank What is the purpose of the reference junction panel that is provided with many thermocouple circuits? A. Ensures that electrical noise in the thermocouple extension wires does not affect thermocouple temperature indication B. Ensures that thermocouple output is amplified sufficiently for use by temperature indication devices C. Ensures that temperature changes away from the thermocouple measuring junction do not affect thermocouple temperature indication D. Ensures that different lengths of thermocouple extension wires do not affect thermocouple temperature indication 18 Rev 1
Knowledge Check An open circuit in a thermocouple detector causes the affected temperature indication to fail... A. as-is. B. high. C. low. D. to reference junction temperature. TLO 1 Summary Temperature detectors uses are as follows: Indication Alarm functions Control functions An 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. An open circuit in a temperature instrument is indicated by a very high temperature. A short circuit in a temperature instrument is indicated by a very low temperature. If a temperature detector becomes inoperative: A spare detector may be used (if installed). Substitute a contact or optical pyrometer for temporary use. Duration 30 minutes Logistics Use PowerPoint slides 49 51 and the IG to review TLO 1 material. Use directed and nondirected questions to students, check for understanding of ELO content, and review any material where student understanding of ELOs is inadequate. Allow 20 minutes to complete and review the crossword puzzle. 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 measuring junction produces a voltage greater than voltage across the reference junction. They are less accurate than the RTD. An open circuit in a detector is indicated by temperature failing to the reference junction temperature. Now that you have completed this lesson, you should be able to: 1. State the three basic functions of temperature detectors. 2. Describe the construction of a basic RTD, including: a. Component arrangement b. Materials used 3. Describe how RTD resistance varies for temperature changes. Rev 1 19
4. State the purpose of basic temperature instrument detection and control system blocks: a. RTD b. Bridge circuit c. DC-AC converter d. Amplifier e. Balancing motor/mechanical linkage 5. Describe bridge circuit compensation for changes in ambient temperature and environmental conditions that can affect temperature detection instrumentation. 6. Describe the effect on temperature indication(s) for the following circuit faults: a. Short circuit b. Open circuit 7. Describe alternate methods of determining temperature when the normal sensing devices are inoperable. 8. Describe the construction and operation of a thermocouple. TLO 2 Pressure Detectors Duration 45 minutes Logistics Use PowerPoint slides 52 56 and the IG to introduce TLO 2. Use NRC exam example questions to enhance review. Overview Pressure measurements control many processes. Gauge pressure (P gauge, psig) is the pressure felt by a pressure detector and equivalent to the pressure of the system less the atmospheric pressure (P atm, psia). The equation below expresses this relationship: Knowing this relationship is important to understanding how pressure changes can affect pressure gauges. Solving Problems with Pressure Caution Some pressure instruments or indicators read out in psia versus psig, so always ensure you check the detector values and convert where necessary. Objectives Upon completion of this lesson, you will be able to do the following: 1. State the three functions of pressure measuring instrumentation. 2. Describe the theory and operation of the following pressure detectors: a. Bellows b. Diaphragm c. Bourdon tube d. Variable capacitance 3. Describe how a bellows-type pressure detector produces an output signal including: 20 Rev 1
a. Method of detection b. Method of signal generation 4. Describe how a bourdon tube-type pressure detector produces an output signal including: a. Method of detection b. Method of signal generation 5. Describe how a strain gauge pressure transducer produces an output signal including: a. Method of detection b. Method of signal generation 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 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 8. Describe the environmental conditions that can affect the accuracy and reliability of pressure detection instrumentation. 9. Describe alternate methods of determining pressure when the normal pressure sensing devices are inoperable. ELO 2.1 Pressure Detector Functions Functions for Pressure Detectors Regardless of the pressures monitored (they vary slightly depending on the details of facility design), all pressure detectors provide up to the following three basic functions: Indication Alarm Control Duration 5 minutes Logistics Use PowerPoint slides 57 58 and the IG to present ELO 2.1. Display of monitored pressures may be local, or in a central location, such as a control room, and may have audible and/or visual alarms associated with them when specified preset limits are exceeded. These pressures may have control functions associated with them so that equipment is started or stopped to support a given pressure condition or so that a certain protective action occurs. Since a fluid system may operate at both saturation and subcooled conditions, accurate pressure indication must be available to maintain proper system parameters. Some pressure detectors have audible and visual alarms associated with them to alert an operator upon exceedence of specified preset limits. Some pressure detector applications provide inputs for protective features and control functions. Rev 1 21
Knowledge Check Pressure detectors provide the following: (select all that apply) A. Indications B. Automatic trips C. Interlocks D. Alarms Duration 10 minutes Logistics Use PowerPoint slides 59 61 and the IG to present ELO 2.2. ELO 2.2 Pressure Detector Theory Introduction Pressure detectors are devices that convert changes in pressure energy to physical movement that can change the characteristics of a circuit. The circuit develops signals proportional to the pressure and/or pressure changes that can provide indication, alarms, or control of a process. Pressure detection is accomplished by connecting a bellows, diaphragm, or bourdon tube device to a system so that system pressure is exerted on the inside of the device while the external surface of the device is exposed to atmospheric pressure. Pressure detection devices actually measure differential pressure between a system and atmospheric pressure. The device will respond to a difference in pressure across the internal to external boundary. The pressure difference produces movement of the bellows, diaphragm, or bourdon tube. This movement is directly proportional to the differential pressure change. The pressure detector converts the movement to an electrical signal or mechanical movement of an indicator proportional to the pressure change. Example In the bellows detector shown in the figure below, the metallic bellows expands and contracts as the system pressure (connected to the highpressure tap) changes. The movement is a result of the actual differential pressure between the system pressure and atmospheric pressure. The bellows connects to a rod that moves a pointer that is connected to a linkage. The pointer deflects across a scale allowing the operator to read pressure directly. 22 Rev 1
Figure: Basic Metallic Bellows Knowledge Check In the pressure detector below, the high-pressure tap is connected to the system. As system pressure increases A. the bellows will collapse. B. the bellows will expand. C. the bellows will remain as-is. D. the linkage will expand. Rev 1 23
Duration 10 minutes Logistics Use PowerPoint slides 62 65 and the IG to present ELO 2.3. ELO 2.3 Bellows Detector Introduction The need for a pressure-sensing element that is extremely sensitive to low pressures and provides power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressuresensing element. The metallic bellows is most accurate when measuring pressures from 0.5 psig to 75 psig. However, when used in conjunction with a heavy range spring, some bellows can measure pressures of over 1,000 psig. Bellows The figure below shows a basic metallic bellows pressure-sensing element. The bellows is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thin-walled tubing. The diameter of the bellows ranges from 0.5 inch to 12 inches, and may have as many as 24 folds. System pressure acts on the area surrounding the bellows. The pressure acts upon the moveable wall and as the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows connects to a mechanical linkage assembly. As the bellows and linkage assembly moves, the linkage provides a direct pressure indication, as shown in the figure below. Though not shown, movement of the bellows and linkage can alternately provide an electrical signal. Figure: Basic Metallic Bellows The flexibility of a metallic bellows is similar in character to that of a helical, coiled compression spring. Up to the elastic limit of the bellows, the relationship between changes in load and deflection is linear. However, this relationship exists only when the bellows is under compression. It is necessary to construct the bellows such that all of the travel occurs on the compression side of the point of equilibrium. A spring must always oppose the bellows, and the deflection characteristics will be the difference in the forces of the spring and bellows. 24 Rev 1
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. A. decrease; increase B. increase; decrease C. decrease; decrease D. increase; increase ELO 2.4 Bourdon Tube Introduction The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. It relies on the differential pressure between system pressure and atmospheric pressure. Atmospheric pressure could be a containment atmosphere in the case of primary system pressure detectors. Duration 5 minutes Logistics Use PowerPoint slides 66 67 and the IG to present ELO 2.4. Bourdon Tube-Type Detectors The bourdon tube consists of a thin-walled tube, partially flattened to a cross-sectional area elliptical in shape, having two long flat sides and two short round sides. The manufacturing process bends the tube lengthwise into an arc of a circle ranging from 270 to 300 degrees. The figure below shows basic parts of a Bourdon tube detector. Rev 1 25
Figure: Bourdon Tube Detector Construction There are many different bourdon tube designs for different applications, but all operate in the same manner. Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. Additionally, greater force acts on the outer radius of the tube due to its larger surface area. Atmospheric pressure opposes the tube expansion. With one end of the tube fixed in place, the other end of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can position a pointer or develop an equivalent electrical signal to indicate the value of the applied internal pressure. When pressure is removed from the tube, it tends to coil and return to its original shape. The spring action of the metal and atmospheric pressure overcomes the force on the inside of the tube. The normal distance of travel for the tip of the tube, depending on application, is approximately 1/4 inch to 3/8 inch. A series of gears translates and amplifies this small amount of tip movement, causing the indicator needle to rotate, moving the indicator needle (pointer) across the scale. Calibration of the scale on the gauge face of the detector allows the gauge to accurately indicate pressure based on tip movement. Changes in atmospheric pressure will affect the indication from a bourdon tube detector. Consider atmospheric pressure that acts on the outside of the bourdon tube. If that pressure were to change significantly, it would change the indicated pressure output of the detector. 26 Rev 1
For example, a bourdon tube pressure detector measuring system pressure in an isolated room in which the pressure rises rapidly due to an event such as a steam line rupture would indicate lower than actual system pressure. In this case, a 10-psi increase in atmospheric pressure (14.7 psi to 24.7 psi) acting inside the room would result in a 10-psig decrease in indicated system pressure. Note that under normal operating conditions, large pressure changes do not occur and small deviations in atmospheric pressure have a negligible effect on the output of a bourdon tube detector. 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. A. increase; outer B. decrease; outer C. increase; inner D. decrease; inner ELO 2.5 Strain Gauge Introduction A strain gauge measures the external force (pressure) applied to a fine wire. The fine wire, in an accordion pattern, forms a grid on a flexible backing, as shown in the figure below. The pressure change causes movement in the flexible backing, and a resistance change due to the distortion (shortening or lengthening) of the wire. Measuring the change in resistance of the wire grid will yield the change in pressure. Duration 20 minutes Logistics Use PowerPoint slides 68 73 and the IG to present ELO 2.5. Figure: Strain Gauge As the wire grid stretches by elastic deformation, its length increases, and its cross-sectional area decreases. These changes cause an increase in the resistance of the strain gauge wire. This change in resistance is the variable resistance in a bridge circuit that provides an electrical signal for indication of pressure. The figure below shows a common strain gauge application. Rev 1 27
Figure: Strain Gauge Pressure Transducer In the figure above, an increase in pressure at the inlet of the bellows causes the bellows to expand. The expansion of the bellows moves a flexible beam with an attached strain gauge. As the beam deflects, the resistance of the strain gauge changes. The temperature-compensating gauge compensates for the heat produced by current flowing through the fine wire of the strain gauge. Strain Gauge Example Strain gauges act as resistors in bridge circuits, as shown in figure below. Figure: Strain Gauge Used in a Bridge Circuit An exciter provides alternating current, replacing the battery and eliminating the need for a galvanometer. When a change in resistance in the strain gauge causes an unbalanced condition, an error signal enters the amplifier and actuates the balancing motor. The balancing motor moves the slider along the slidewire, restoring the bridge to a balanced condition. The slider indicates pressure on a scale marked in units of pressure. Strain gauges often monitor pressure in transmitters for reactor coolant pressure instruments. Knowledge Check Semiconductor strain gages are often used in transmitters 28 Rev 1
for... A. control rod position instruments. B. reactor coolant pressure instruments. C. reactor coolant temperature instruments. D. steam generator level instruments. ELO 2.6 Pressure Transducers Introduction A pressure transducer is comprised of pressure detectors joined to an electrical device. Transducers produce a change in resistance, inductance, or capacitance in order to produce a signal representative of pressure. Resistance Type Transducers Duration 15 minutes Logistics Use PowerPoint slides 74 78 and the IG to present ELO 2.6. Some resistance-type transducers combine a bellows or a bourdon tube with a variable resistor. As pressure changes, the bellows will either expand or contract. This expansion and contraction causes the attached slider to move along the slidewire, increasing or decreasing the resistance, and thereby indicating an increase or decrease in pressure. The figure below shows an example of a slidewire resistance transducer. Figure: Slidewire Resistance Type Transducer Inductance Type Transducers The inductance-type transducer consists of the following three parts: Coil Moveable magnetic core Pressure-sensing element The pressure-sensing element attaches to the magnetic core, and, as pressure varies, the element causes the core to move inside the coil. An AC voltage Rev 1 29
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. For increased sensitivity, designs use a coil separated into two coils by utilizing a center tap. As the core moves within the coils, the inductance of one coil will increase, while the inductance of the other will decrease. The figure below shows an example of an inductance type transducer. Figure: Inductance Type Transducer Differential Transformer Type Transducers A differential transformer pressure transducer is another type of inductance transducer. The differential transformer pressure transducer uses two coils wound on a single tube. The primary coil winds around the center of the tube. The 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 able to move within the tube. When the core is in the lower position, the lower half of the secondary coil provides the output. When the core is in the upper position, the upper half of the secondary coil provides the output. The magnitude and direction of the output depends on the amount the core has moved from its center position. When the core is in the mid-position, there is no secondary output. The figure below shows a cross-section of a differential transformer type transducer. 30 Rev 1
Figure: Differential Transformer Variable Capacitive-Type Transducers Variable capacitive-type transducers consist of two flexible conductive plates and a dielectric. In this case, the transducer measures the pressure of the dielectric fluid. As pressure increases, the flexible conductive plates (C 1, C 2 ) will move farther apart, changing the capacitance of the transducer. This change in capacitance is measurable and is proportional to the change in pressure. The figure below shows a cross-section of a variable capacitive-type transducer. Figure: Variable Capacitive Type Transducer The figure below shows a pressure instrument that can measure differential pressures (D/P). By using high and low-pressure detection taps, the total movement of the capacitor plate reflects differential pressure. Highpressure acts on one side of a moveable capacitor plate and low-pressure acts on the other side of the plate. Dielectric fluid surrounds the moveable capacitor plate. Diaphragms separate the system fluid from the dielectric fluid. The diaphragm transmits pressure changes to the dielectric fluid. As the moveable capacitor deflects, the distance between the fixed and Rev 1 31
moveable capacitors will change causing a capacitance change, and an electrical signal converts the change to a D/P indication. Knowledge Check Figure: Variable Capacitance D/P Cell A type of pressure sensor that is constructed of two conductive plates separated by a dielectric substance is a pressure detector. A. bellows-type B. bourdon-tube C. capacitive-type D. inductance-type Duration 10 minutes Logistics Use PowerPoint slides 79 84 and the IG to present ELO 2.7. ELO 2.7 Pressure Detection Circuit Introduction Pressure detection circuitry consists of the following four basic components: Sensing element Transducer Pressure detection circuitry Pressure indication In this section, you will learn the function of these components and how they work together to function as a pressure detection circuit. 32 Rev 1
Pressure Detection Circuitry The figure below shows a block diagram of a typical pressure detection circuit. The following paragraphs discuss each of the blocks in the typical pressure detection circuit. Sensing Element Figure: Pressure Detection Circuit Block Diagram The sensing element senses the pressure of the monitored system and converts the pressure to a mechanical signal. The sensing element supplies the mechanical signal to a transducer, as shown above. Transducer The transducer converts the mechanical signal to an electrical signal that is proportional to system pressure. If the mechanical signal from the sensing element reads pressure directly, a transducer is not required. Detector Circuitry The detector circuitry amplifies and/or transmits the signal to the pressure indicator. The electrical signal generated by the detection circuitry is proportional to system pressure. The exact operation of detector circuitry depends upon the type of transducer used. Pressure Indicator The pressure indicator provides remote indication of the measured pressure. Display of pressure may be local or at a remote location, depending on the application of the detector. Some applications use both a local and a remote indication. Knowledge Check In a typical pressure detection circuit, the senses the pressure of the monitored system and converts the pressure to a mechanical signal. Rev 1 33
A. pressure indicator B. transducer C. slidewire D. sensing element Duration 15 minutes Logistics Use PowerPoint slides 85 88 and the IG to present ELO 2.8. ELO 2.8 Environmental Effects Introduction Pressure detection circuits sense small changes in process pressure by directly measuring the difference in pressure of a process system compared to atmospheric. These circuits operate at very low voltages (millivolt) and amperage (milliamp). At these low voltages and currents, it is important to consider environmental effects on the circuit itself because temperature and humidity effects change the circuit resistance. These changes can modify the circuit output signal and give a false indication of pressure. Ambient Pressure Pressure instruments are sensitive to variations in the atmospheric pressure surrounding the detector. This is especially apparent when the detector is located within an enclosed space. Variations in the pressure surrounding the detector will cause the indicated pressure from the detector to change when there may not have been an actual pressure change at the detector. Pressure variations surrounding the detector will greatly reduce the accuracy of the pressure instrument; minimizing these variations when installing and maintaining these instruments will improve their accuracy. Ambient Temperature Ambient temperature variations will affect the accuracy and reliability of pressure detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. Proper circuitry design and maintaining the pressure detection instrumentation in the proper environment will reduce the effects of temperature variations. Humidity Humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. Maintaining electronic equipment in the proper environment controls the effects due to humidity. 34 Rev 1
Example Consider a typical pressurized water reactor, depicted below, that experiences a steam leak in the primary containment. As containment pressure rises, pressure sensors located inside the containment will feel the effects of the changing atmospheric pressure. The rise in atmospheric pressure will reduce the difference in pressure between the primary system and atmospheric. It is necessary to reduce the resultant pressure indication by the exact amount that the primary containment pressure increased to yield an accurate pressure rise. Pressure transducers located outside the containment will not feel this effect. Knowledge Check Figure: Typical Pressurized Water Reactor 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. A. Humidity B. Atmospheric pressure C. Temperature D. Alpha radiation Rev 1 35
Duration 5 minutes Logistics Use PowerPoint slides 89 92 and the IG to present ELO 2.9. ELO 2.9 Alternate Pressure Detection Introduction In the event that primary pressure sensing instruments become inoperative, there are alternate methods to obtain pressure indications. Some methods use the detection circuit even though there may be a failure within the circuit. Pressure Detector Failure If a pressure instrument fails, use spare detector elements, if installed. If there are no spare detectors installed, read the pressure with an independent local mechanical gauge, if available, or install a precision pressure gauge (Heise gauge, for example) in the system at a convenient point. 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. Pressure instruments include a safety factor above normal design pressure. However, sudden overpressurization causing over-range conditions could permanently straighten bourdon tubes and bellows, damaging the sensing element. If overpressurization stretches or stresses the sensing element beyond its design operating range, the indications may be erroneously high. If the sensing element has a leak or rupture, the instrument would fail with a low indication. Knowledge Check - NRC Bank Refer to the drawing of a bellows-type differential pressure (D/P) detector below. 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? 36 Rev 1
Knowledge Check A. Increase, because the spring will expand more. B. Decrease, because the spring will expand more. C. Increase, because the spring will compress more. D. Decrease, because the spring will compress more. If a bourdon-tube pressure detector is over-ranged sufficiently to permanently distort the bourdon tube, subsequent pressure measurement will be inaccurate because the of the detector tube will be inaccurate. A. change in the volume B. change in the length C. expansion of the cross-sectional area D. distance moved by the tip TLO 2 Summary Pressure detector basic functions are as follows: Indication Alarm Control In a bellows-type detector: System pressure acts on the 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 acts on the inside of a slightly flattened, arc-shaped tube. Pressure increases tend to restore the tube to its original round crosssection, causing the tube to straighten. Duration 30 minutes Logistics Use PowerPoint slides 93 99 and the IG to review TLO 2 material. Use directed and nondirected questions to students, check for understanding of ELO content, and review any material where student understanding of ELOs is inadequate. Use NRC exam example questions to enhance review. Operation of strain gauge: The operation of a strain gauge measures pressure applied to a fine wire, usually arranged in the form of a grid. A pressure change causes a resistance change due to distortion of the wire grid. Rev 1 37
These are often used in transmitters for reactor coolant pressure instruments. Slidewire pressure transducer operation: Operation consists of a bellows or a bourdon tube with a variable resistor. Expansion or contraction of bellows causes attached slider to move along the slidewire, increasing or decreasing the resistance, thereby indicating an increase or decrease in pressure. Inductance-type pressure transducer operation: Inductance-type pressure transducer operation consists of the following three parts: a coil, a movable magnetic core, and a pressure-sensing element. The sensing element and magnetic core are tied together; as pressure varies, the element and the core move inside the coil. An AC voltage acts on the coil; 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: This operation utilizes two coils wound on a single tube. The primary coil winds around the center of the tube; the 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. Capacitive-type transducer operation: The transducer consists of two flexible conductive plates with a dielectric separating them. 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. Pressure instrument failure: 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. 38 Rev 1
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. Now that you have completed this lesson, you should be able to do the following: 1. State the three functions of pressure measuring instrumentation. 2. Describe the theory and operation of the following pressure detectors: a. Bellows b. Diaphragm c. Bourdon tube d. Variable capacitance 3. Describe how a bellows-type pressure detector produces an output signal including: a. Method of detection b. Method of signal generation 4. Describe how a bourdon tube-type pressure detector produces an output signal including: a. Method of detection b. Method of signal generation 5. Describe how a strain gauge pressure transducer produces an output signal including: a. Method of detection b. Method of signal generation 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 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 8. Describe the environmental conditions that can affect the accuracy and reliability of pressure detection instrumentation. 9. Describe alternate methods of determining pressure when the normal pressure sensing devices are inoperable. Rev 1 39
Duration 1 hour 45 minutes Logistics Use PowerPoint slides 100 102 and the IG to introduce TLO 3. TLO 3 Level Detectors Overview Accurate indication of tank and other process-related vessel level is vital to the control of any industrial process. Without accurate level indication, tanks could overflow resulting in spills of hazardous materials or tank levels could fall to a low level where equipment damage will result. Level detectors provide operators with both local and remote indication of levels associated with the process at a particular facility. Remote level indication is necessary to provide transmittal of vital tank and vessel level information to a central location, such as the control room, where all level information associated with an industrial process can be coordinated and evaluated. Objectives Upon completion of this lesson, you will be able to do the following: 1. Describe the three functions for using remote level indicators. 2. Describe the operation of the following types of level instrumentation: a. Gauge glass b. Magnetic bond c. Conductivity probe d. D/P 3. Describe density compensation used in level detection systems, why systems need it, and how it is accomplished. 4. State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: a. D/P transmitter b. Amplifier c. Indication 5. Describe the environmental conditions that can affect the accuracy and reliability of level detection instrumentation. 6. State the various failure modes of level detection instrumentation. 7. Analyze detector installation and applications to determine the effects of transients on level indication. Duration 5 minutes Logistics Use PowerPoint slides 103 105 and the IG to present ELO 3.1. ELO 3.1 Level Detection Functions Introduction Although different facility designs require monitoring varying system and process levels, all level detectors provide one or more of the following basic functions: Indication Alarm Control 40 Rev 1
Liquid level measuring devices fall into the following two groups: Direct method Inferred method An example of the direct method is the dipstick in a car, which measures the height of the oil in the oil pan. An example of the inferred method is a pressure gauge at the bottom of a tank, which measures the hydrostatic head pressure from the height of the liquid. Level Detector Functions The following are the three major reasons for using remote level indication: It is possible to monitor and record level measurements at locations far from the main facility. Controlled level may be a long distance from the control room or control station. Measured level may be in an unsafe/hazardous area. Knowledge Check Level detection provides the following: (select all that apply) A. Interlocks B. Alarms C. Automatic trips D. Indications ELO 3.2 Operation of Level Detectors Introduction There are various ways to detect levels in tanks, steam generators, pressurizers, and other plant components. The system conditions determine the best level detector. For example, in a high-pressure and hightemperature application, a D/P cell that provides remote signals may be appropriate, while a simple gauge glass may work fine in a low-pressure tank vented to atmosphere. Each level detector has advantages and disadvantages and it is up to the designer to choose the appropriate detector for a specific application. Duration 10 minutes Logistics Use PowerPoint slides 106 113 and the IG to present ELO 3.2. Gauge Glass A very simple liquid level measuring device (direct method) in a vessel is the gauge glass. In the gauge glass device, a transparent tube is attached to the bottom and top (top connection is not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in the Rev 1 41
tube will be equal to the height of water in the tank. The figure below shows two possible applications of a gauge glass. Figure: Gauge Glass Figure (a) above shows a gauge glass used for vessels where the liquid is at ambient temperature and pressure conditions. Figure (b) shows a gauge glass used for vessels where the liquid is at an elevated pressure or a partial vacuum. Notice that gauge glasses in effect form a "U" tube manometer where the liquid seeks its own level due to the pressure of the liquid in the vessel. Gauge glasses made from tubular glass or plastic suffice for service up to 450 psig and 400 F. If measuring the level of a vessel at higher temperatures and pressures, a different type of gauge glass is required. The type of gauge glass used in these conditions has a body made of metal with a heavy glass or quartz section for visual observation of the liquid level. The glass section is usually flat to provide strength and safety. Another type of gauge glass is the reflex gauge glass where one side of the glass section is prism-shaped. The glass is flat on the outside, with molded 90-degree angles that run lengthwise (prisms) on the inside. Light rays strike the outer surface of the glass at a 90-degree angle. The light rays travel through the glass striking the inner side of the glass at a 45-degree angle. The light rays refract into the chamber, or reflect back to the outer surface of the glass, depending on the presence or absence of liquid in the chamber. The figure below shows a front view and a cross-section of a reflex gauge glass. 42 Rev 1
Figure: Reflex Gauge Glass When the liquid is at an intermediate level in the gauge glass, the light rays encounter an air-glass interface in one portion of the chamber and a waterglass interface in the other portion of the chamber. Where an air-glass interface exists, the light rays reflect back to the outer surface of the glass since the critical angle for light to pass from air to glass is 42 degrees. This causes the gauge glass to appear silvery-white. In the portion of the chamber with the water-glass interface, the light prisms refract into the chamber. No reflection of the light back to the outer surface of the gauge glass occurs because the critical angle for light to pass from glass to water is 62 degrees. This results in the glass appearing black, since it is possible to see through the water to the black-painted walls of the chamber. Magnetic Bond Level Detector The magnetic bond method of level detection overcomes the problems of cages and stuffing boxes. The magnetic bond mechanism consists of a magnetic float that rises and falls with changes in level. The float travels outside of a nonmagnetic tube, which houses an inner magnet connected to a level indicator. When the float rises and falls, the outer magnet will attract the inner magnet, causing the inner magnet to follow the level within the vessel. The figure below shows the basic elements of a magnetic bond level detector. Figure: Magnetic Bond Level Detector Rev 1 43
Conductivity Probe Level Detector A conductivity probe level detection system consists of one or more level detectors, an operating relay, and a controller. When the liquid makes contact with any of the electrodes, an electric current will flow between the electrode and ground. The current energizes a relay, which causes the relay contacts to open or close depending on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control valve, or a combination of the three. The figure below shows a typical system with three probes: a low-level probe, a high-level probe, and a high-level alarm probe. The system below would indicate a high level; however, the alarm would not yet be active. Figure: Conductivity Probe Level Detection System Open Tank Differential Pressure Level Detector The differential pressure (D/P) detector method of liquid level measurement uses a D/P detector connected to the bottom of the monitored tank. The fluid level in the tank creates a pressure (high), from which a lower reference pressure (usually atmospheric) is subtracted. This subtraction takes place in the D/P detector. The figure below illustrates a typical differential pressure detector attached to an open tank. Figure: Open Tank Differential Pressure Detector Knowledge Check A calibrated differential pressure (D/P) level detector 44 Rev 1
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... A. decrease, then increase and stabilize at the actual level. B. increase and stabilize above the actual level. C. decrease and stabilize below the actual level. D. remain at the actual level. ELO 3.3 Density Compensation Introduction If a vapor with a significant density exists above the liquid in a particular tank or vessel, the vapor adds hydrostatic pressure to the liquid surface. Accurate level transmitter output must account for the hydrostatic pressure added by the vapor. Duration 30 minutes Logistics Use PowerPoint slides 114 122 and the IG to present ELO 3.3. Specific Volume Specific volume equals volume per unit of mass, as shown in equation below. ( ) Specific volume is the reciprocal of density as shown in equation below. Rev 1 45
( ) Specific volume is the standard unit used when working with vapors and steam that have low density values. For applications that involve water and steam, specific volume values are in "Saturated Steam Tables," which list the specific volumes for water and saturated steam at different pressures and temperatures. Effects of Vapor Density on Level Detection The density of steam (or vapor) above the liquid level will have an effect on the weight of the steam or vapor bubble and the hydrostatic head pressure. As the density of the steam or vapor increases, the weight increases and causes an increase in hydrostatic head even though the actual level of the tank has not changed. The larger the steam bubble, the greater the change in hydrostatic head pressure. The figure below illustrates a vessel in which the water is at saturated boiling conditions. Figure: Effects of Fluid Density A condensing pot or chamber at the top of the reference leg condenses the steam and maintains the reference leg filled. Because steam vapor pressure acts equally on both the low and high sides of the transmitter, there is no effect of the steam vapor pressure at the D/P transmitter. The differential pressure seen by the transmitter is due only to hydrostatic head pressure, as shown in equation below. If the reference leg containing saturated water has a pressure drop below the saturation pressure, liquid could flash to steam. A condensing pot located in the reference leg condenses this steam. In order to enhance heat dissipation, the reference leg is located away from the vessel and is uninsulated so it readily gives up heat to atmosphere. This helps keep the liquid reference level steady by minimizing flashing. The flashing action may result in minor level indication fluctuations. 46 Rev 1
Reference Leg Temperature Considerations When measuring the level in a pressurized tank at elevated temperatures, a number of additional factors affect the measurements. As the temperature of the fluid in the tank increases, the density of the fluid decreases. As the fluid s density decreases, the fluid expands, occupying more volume. Even though the density is less, the mass of the fluid in the tank is the same. The issue is that as the fluid in the tank heats and cools, the density of the fluid changes, but the reference leg temperature and density remain relatively constant, which causes the indicated level to remain constant. The density of the fluid in the reference leg depends on the ambient temperature of the room in which the leg is located; therefore, it is relatively constant and independent of tank temperature. An accurate tank level indication requires some means of density compensation to account for fluid temperature changes, and therefore density. This is the problem encountered when measuring steam generator water levels. Compensating for Reference Leg Temperature Changes Calibration charts are available that allow manual level corrections for changes in level indication due to reference leg temperatures. It is possible to account for changes in reference leg density during instrument alignments and calibrations; however, this is not an actual method of density compensation. Electronic circuitry may perform density compensation. Some systems compensate for density changes automatically through the design of the level detection circuitry. Other applications compensate for density by having operators manually adjust inputs to the level detection circuitry as the affected vessel cools down and depressurizes, or heats up and pressurizes. Steam Generator Level Density Compensation The figure below illustrates a typical steam generator level detection arrangement. The D/P detector measures actual differential pressure. A separate pressure detector measures the pressure of the saturated steam. Since saturation pressure is proportional to saturation temperature, a pressure signal can correct the differential pressure for liquid density. An electronic circuit uses the pressure signal to compensate for the difference in density between the reference leg water and the steam generator fluid. As the saturation temperature and pressure increase, the density of the steam generator water decreases. The level instrument should now indicate a higher level, even though the actual D/P has not changed. The increase in pressure feeds into the level instrument to compensate for the change in the density of the liquid so that the level instrument will reflect the change in actual liquid level. Rev 1 47
Knowledge Check Knowledge Check Figure: Steam Generator Level Detection System Many steam generator water level instruments include a condensing chamber in the reference leg. The purpose of the condensing chamber is to... A. ensure the reference leg temperature remains close to the temperature of the variable leg. B. maintain a constant water level in the reference leg during normal operations. C. provide reference leg compensation for the steam generator pressure exerted on the variable leg. D. prevent reference leg flashing during a rapid depressurization of the steam generator. 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. 48 Rev 1
A. higher; smaller B. lower; smaller C. higher; larger D. lower; larger ELO 3.4 Level Detection Circuits Introduction A typical level detection circuit consists of a D/P detector, a transducer, an amplifier, and an indicator. These components together sense the D/P in a system and convert that signal into an electrical signal proportional to the D/P. The electrical signal then provides indication, alarm, or control. Duration 12 minutes Logistics Use PowerPoint slides 123 126 and the IG to present ELO 3.4. Level Detection Circuit The figure below illustrates a block diagram of a typical differential pressure detector. It consists of the following: D/P transmitter (transducer) Amplifier Level indication Figure: Differential Pressure Level Detection Circuit Rev 1 49
The D/P transmitter consists of a diaphragm with the high-pressure (HP) and low-pressure (LP) inputs on opposite sides. As the differential pressure changes, the diaphragm will move. The transducer changes this mechanical motion into an electrical signal. The electrical signal generated by the transducer is amplified, and passed on to the level indicator for display at a remote location. Using relays, this system provides alarms for high and low levels. It may also provide control functions such as repositioning a valve and protective features such as tripping a pump. Knowledge Check 4 A. Alarm Place the following components in order starting with level sensing to output signal. 2 B. Transducer 3 C. Amplifier 1 D. Bourdon tube Duration 15 minutes Logistics Use PowerPoint slides 127 131 and the IG to present ELO 3.5. ELO 3.5 Environmental Effects Introduction Level detection circuits sense small changes in levels by measuring the actual pressure difference between the height of the fluid and atmospheric pressure or a reference leg level. The conditions surrounding the reference legs and process can affect the properties of the fluid and thereby effect the indication. In addition, circuits operate at very low voltages (millivolt) and amperage (milliamp). At these low voltages and currents, temperature and humidity changes will affect the resistance in the circuit itself. These changes can affect the circuit output signal and result in a false level indication. Environmental Effects Fluid Density When measuring the level of a fluid, the fluid density can have a large effect on level detection instrumentation. Fluid density affects level sensing instruments that utilize either wet or dry reference legs. In a wet reference leg instrument, it is possible for the reference leg's fluid temperature to be different from the vessel fluid temperature where the level is measured. An example of this is the level detection instrumentation for a boiler steam drum. The water in the reference leg is at a lower temperature than the water in the steam drum. Therefore, the water in the reference leg is denser, and level indicators must adjust the reference leg level for the density difference to ensure the indicated steam drum level is accurate. 50 Rev 1
Ambient Temperature Ambient temperature variations will affect the accuracy and reliability of level detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and therefore can affect the calibration of electric/electronic equipment. Proper circuitry design and maintaining the level detection instrumentation in the proper environment reduces the effects of temperature variations. Ambient temperature will change the density of the water in the reference leg and will affect level indication. If the ambient temperature around a wet reference leg rises, the density of the reference leg liquid will decrease causing the reference leg to overflow into the tank. The mass in the reference leg will decrease and therefore the hydrostatic pressure in the reference leg will decrease, which will cause indicated level to increase when actual tank level has not changed. Calibrations of level transmitters use the ambient conditions where the transmitters will perform. Calibrations for some transmitters will use cold or shutdown conditions, while calibrations for others will use hot or normal operating conditions, to reflect density and pressure conditions that the instrument will see during its operation. This is necessary because instruments will not read correctly under conditions that differ from their calibration conditions. Humidity Humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. Maintaining the electrical equipment in the proper environment controls the effects due to humidity. Example Refer to the drawing of two tank differential pressure (D/P) level indicators (see figure below). A large water storage tank has two D/P level indicators installed. Calibration of Indicator No. 1 took place at 100 F water temperature and calibration of Indicator No. 2 took place at 200 F water temperature. Assuming both indicators are on scale, which one will indicate the higher level? Rev 1 51
Figure: Tank Differential Level Detectors The instrument calibrated at a higher temperature reflects the liquid at a higher expansion state, or lower density. In this open tank, the expansion will equal a high column of water pushing down on the detector. Indicator 2 will indicate a higher level at all water temperatures. Knowledge Check Consider the level indicator for a steam generator below. 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? A. Indicate higher than actual because resistance of the D/P cell components is increasing. 52 Rev 1
B. Indicate higher than actual because reference leg density is decreasing. C. No effect. D. Indicate lower than actual because reference leg density is increasing. ELO 3.6 Failure Modes Introduction Level detection systems are extremely reliable for long-term operation. The indirect level detector failure mode depends on the high-pressure and lowpressure connection setup. For most level detectors if the D/P decreases because of the malfunction, the indicated level will also decrease. Conversely, if D/P increases because of the malfunction, the indicated level will increase. This is true with the exception of the wet reference leg level detection. Duration 10 minutes Logistics Use PowerPoint slides 132 135 and the IG to present ELO 3.6. Failure Modes In the wet reference leg arrangement, the reference leg connects to the highpressure side of the D/P cell causing the opposite reaction. With the wet reference leg connected to the high-pressure detector, a break in the variable leg or low-pressure side will cause a low-level indication. If the break was on the high-pressure reference leg side, then a lower D/P results and the indicated level is higher than the true level. However, for D/P cell problems, the set up most be closely analyzed to determine the high and low-pressure sides of the detector to correctly answer the question. Knowledge Check The level indication for a reference leg differential pressure (D/P) level instrument will fail low because of... A. a break on the variable leg. B. closing the equalizing valve in the D/P cell. C. the reference leg flashing to steam. D. a break on the reference leg. Knowledge Check NRC Bank Refer to the drawing of a steam generator (SG) differential pressure (D/P) level detection system below. Rev 1 53
The SG is at normal operating temperature and pressure with accurate level indication. Which one of the following events will result in an SG level indication that is greater than actual level? A. The external pressure surrounding the D/P detector increases by 2 psi. B. SG pressure increases by 50 psi with no change in actual water level. C. Actual SG level increases by 6 inches. D. The temperature of the reference leg increases by 20 F. Duration 10 minutes Logistics Use PowerPoint slides 136 144 and the IG to present ELO 3.7. ELO 3.7 Detector Transients Introduction Open Tank Differential Pressure Level Detector The tank in the figure below is open to the atmosphere; it is necessary to use only the high-pressure (HP) connection on the D/P transmitter. The lowpressure (LP) side vents to the atmosphere; the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. The maximum detectable level for the D/P transmitter depends on the maximum height of liquid above the transmitter. The minimum detectable level depends on the tank height above the transmitter connection to the tank (usually close to the bottom). 54 Rev 1
Figure: Open Tank Differential Pressure Detector Closed Tank Dry Reference Leg Level Detector Not all tanks or vessels are open to the atmosphere. Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank. When measuring the level in a tank that is pressurized or that can become pressurized by vapor pressure from the liquid, both the high-pressure and low-pressure sides of the D/P transmitter must connect as shown in the figure below. Figure: Closed Tank Dry Reference Leg Level Detector Dry Reference Leg The high-pressure connection joins the tank at or below the lower range value measured. The low-pressure side connects to a "reference leg" that is connected at or above the upper range value to be measured. The gas or vapor pressure in the vessel pressurizes the reference leg and no liquid remains in the reference leg. The reference leg must stay dry so that there is no liquid head pressure on the low-pressure side of the transmitter. The hydrostatic head of the liquid and the gas or vapor pressure exerted on the liquid surface both act on the high-pressure side. The gas or vapor pressure acts equally on the low and high-pressure sides. Therefore, the output of the D/P transmitter is directly proportional to the hydrostatic head pressure, that is, the level in the tank. Wet Reference Leg Where the tank contains a condensable fluid, such as steam, a slightly different arrangement is used, shown in the figure below. Because ambient Rev 1 55
temperature surrounds the reference leg, the fluid vapor condenses in the leg. Filling the reference leg with the same liquid that occupies the tank prevents vapor condensation in the reference leg. The liquid in the reference leg applies a hydrostatic head to the high-pressure side of the transmitter, and the value of this level is constant as long as the reference leg is full. If this pressure remains constant, any change in D/P is due to a change on the low-pressure side of the transmitter. Figure: Closed Tank Wet Reference Leg Differential Pressure Detector The filled reference leg applies a hydrostatic pressure to the high-pressure side of the transmitter, which is equal to the maximum detectable level. The D/P transmitter receives equal pressure on the high and low-pressure sides when the liquid level is at its maximum; therefore, the differential pressure is zero. As the tank level decreases, the pressure applied to the low-pressure side decreases also, and the differential pressure increases. As a result, the differential pressure and the transmitter output are inversely proportional to the tank level. Guidelines Consider the basic designs of level detectors. A differential pressure level detector measures the difference in force exerted between a reference and a variable leg across a diaphragm. When there are factors in addition to the actual level changes that affect these force differences, it is necessary to account for these non-level related forces to obtain the true liquid level. When a transient acts on a differential pressure level detector, determine the effect that the transient has on the force exerted by either the reference leg or the variable leg. The direction and magnitude of the force change will determine the direction and magnitude of the indication mis-match. For most differential level detectors, a higher D/P results in a lower indicated level and a lower D/P (the closer to equal) results in a higher indicated level. Loss of Reference Leg Force Reference leg force can be lost or reduced by temperature increases, leaks or by open or leaking equalizer valves. When this occurs, the reference leg force decreases. When compared to the variable leg, the difference in 56 Rev 1
pressure decreases resulting in the indicated level being higher than the true level. Loss of Variable Leg Force Variable leg force can be lost or reduced by temperature increases, leaks or by open or leaking vent valves. When this occurs, the variable leg force decreases. When compared to the reference leg, the difference in pressure increases, resulting in the indicated level being lower than the true level. Equalization Equalization of a differential level detector occurs when the equalization valve is either open or leaking. When this occurs, it is similar to losing the reference leg force. The difference in pressure decreases, resulting in the indicated level being higher than the true level. Example Refer to the drawing of a D/P level detection system below for a pressurizer at normal operating temperature and pressure. Calibration of the level detector took place under normal conditions. The high-pressure side of the detector connects to the reference leg and upon opening the equalizing valve, the indicated pressurizer level will be greater than the actual level because the forces exerted by the reference leg and the variable leg approach each other. This results in a minimum D/P and a maximum indicated level. Figure: Steam Generator Level Detector Now consider a transient condition where the reference leg temperature decreases. This will result in higher density of the reference leg fluid. The force exerted on the reference leg side of the D/P detector is a result of the height of the fluid and the density. If the density increases, the resultant force will increase, resulting in a higher differential pressure and lower indicated level than the true fluid level. Rev 1 57
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. A. condensing pot; higher B. pressurizer; higher C. condensing; lower D. pressurizer; lower Duration 15 minutes Logistics Use PowerPoint slides 145 149 and the IG to review TLO 3. Use directed and nondirected questions to students, check for understanding of ELO content, and review any material where student understanding of ELOs is inadequate. TLO 3 Summary The three major reasons for utilizing remote level indication are as follows: It may be necessary to take level measurements at locations far from the main facility. The level to be controlled may be a long distance from the point of control. The measured level may be in an unsafe/hazardous area. Gauge glass: A transparent tube is attached to the bottom and top (the top connection is not needed in a tank open to atmosphere) of the tank that is monitored. The liquid height in the tube will be equal to the height of the liquid in the tank. Magnetic bond level detector: 58 Rev 1
The detector consists of a magnetic float that rises and falls with changes in level. The float travels outside of a nonmagnetic tube, which houses an inner magnet connected to a level indicator. When the float rises and falls, the outer magnet will attract the inner magnet, causing the inner magnet to follow the level within the vessel and actuate the level indicator. Conductivity probe: The probe consists of one or more level detectors, an operating relay, and a controller. When the liquid makes contact with any of the electrodes, an electric current will flow between the electrode and ground. The current energizes a relay, which causes the relay contacts to open or close depending on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control valve, or a combination of the three. D/P detector: A D/P detector uses a pressure detector connected to the bottom of the monitored tank. The difference between the higher pressure in the tank and a lower reference pressure (usually atmospheric) yields the tank pressure. This pressure comparison takes place in the D/P detector. Density compensation: If a vapor with a significant density exists above the liquid, it is necessary to add the vapor hydrostatic pressure to the liquid hydrostatic pressure to obtain accurate transmitter output. The three options for density compensation are as follows: Electronic circuitry Pressure detector manual input Instrument calibration The environmental effects on level detection are as follows: Density of the fluid Ambient temperature changes Humidity The basic block diagram of a D/P level instrument: A D/P transmitter consists of a diaphragm with the high-pressure (HP) and low-pressure (LP) inputs on opposite sides. As the differential pressure changes, the diaphragm will move. The transducer changes this mechanical motion into an electrical signal. An amplifier amplifies the electrical signal generated by the transducer and sends it to the level indicator. Rev 1 59
A level indicator displays the level indication at a remote location. Failure mode of an indirect level detector depends on details of the HP and LP D/P cell connections. Now that you have completed this lesson, you should be able to do the following: 1. Describe the three functions for using remote level indicators. 2. Describe the operation of the following types of level instrumentation: a. Gauge glass b. Magnetic bond c. Conductivity probe d. D/P 3. Describe density compensation in level detection systems, why systems need it, and how it is accomplished. 4. State the purpose of basic differential pressure detector-type level instrument blocks in a basic block diagram: a. D/P transmitter b. Amplifier c. Indication 5. Describe the environmental conditions that can affect the accuracy and reliability of level detection instrumentation. 6. State the various failure modes of level detection instrumentation. 7. Analyze detector installation and applications to determine the effects of transients on level indication. Duration 4 hours 30 minutes Logistics Use PowerPoint slides 150 152 and the IG to introduce TLO 4. TLO 4 Flow Detectors Overview 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) can be 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. Objectives Upon completion of this lesson, you will be able to do the following: 1. Describe the theory of operation of a basic head flow meter. 2. Describe the basic construction of the following types of head flow detectors: a. Orifice plates 60 Rev 1
b. Venturi tube c. Dall flow tube d. Flow nozzle e. Elbow meter f. Pitot tube 3. State the typical failure modes for head flow meters including the effects of vapor on a flow instrument. 4. Describe the necessity for density and/or temperature compensation to include: a. Why it may be required b. Compensation methods 5. Describe the following types of mechanical flow detectors, including the basic construction and theory of operation. a. Rotameter b. Nutating disk 6. Describe density compensation of a steam flow instrument to include the reason density compensation is required and the parameters used. 7. State the purpose of typical flow detection device blocks used on a simple block diagram: a. D/P transmitter b. Extractor c. Indicator 8. Describe the environmental conditions that can affect the accuracy and reliability of flow sensing instrumentation. ELO 4.1 Flow Meter Theory of Operations Introduction Head flow meters operate on the principle that placing a restriction in a line will cause a pressure drop from the upstream side of the restriction to the downstream side. Head flow meters operate by quantifying the pressure drop, and converting the drop to a flow rate. Industrial applications of head flow meters incorporate a pneumatic or electrical transmitting system for remote readout of flow rate. Generally, the indicating instrument extracts the square root of the differential pressure and displays the flow rate on a linear indicator. Duration 25 minutes Logistics Use PowerPoint slides 153 160 and the IG to present ELO 4.1. Flow Meter Theory of Operation There are two elements in a head flow meter; the primary element is the restriction in the line, and the secondary element is the differential pressuremeasuring device. The figure below shows the basic operating characteristics of a head flow meter. Rev 1 61
Figure: Head Flow Instrument Flow path restriction results in a differential pressure across the restriction. A mercury manometer or a differential pressure detector measures this pressure differential. From this measurement, flow rate is determined from known physical laws. The restriction will cause a downstream increase in fluid velocity and decrease in pressure. The volumetric flow rate remains unchanged the same amount of fluid passes through per unit time both upstream and downstream of the restriction. The change in fluid pressure is proportional to the square of volumetric flow rate. 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. Where: volumetric flow rate K flow constant for the meter D/P differential pressure caused by restriction The head flow meter actually measures volumetric flow rate rather than mass flow rate. Mass flow rate is easily calculated or computed from volumetric flow rate by knowing or sensing temperature and/or pressure. Temperature and pressure affect the density of the fluid and, therefore, the mass of fluid flowing past a certain point. If the volumetric flow rate signal compensates for changes in temperature and/or pressure, a true mass flow rate signal results. Thermodynamics describes that temperature and density are inversely proportional, while pressure and density are directly 62 Rev 1
proportional. To show the relationship between temperature and pressure, one of two forms of the mass flow rate equation is used: ( ) Where: = mass flow rate A = area D/P = differential pressure P = pressure T = temperature K = flow coefficient The flow coefficient is constant for the system based mainly on the construction characteristics of the pipe and type of fluid flowing through the pipe. The flow coefficient in each equation contains the appropriate units to balance the equation and provide the proper units for the resulting mass flow rate. Calculating volumetric flow rate uses the area of the pipe and differential pressure. As stated above, compensating for system temperature or pressure converts this volumetric flow rate to mass flow rate. Example A cooling water system is operating at steady-state conditions indicating 900 gpm with 60 psid across the flow transmitter venturi. If cooling water flow rate is increased to 1,800 gpm, flow transmitter venturi delta-p will be approximately psid? We know that the flow meter described operates on the principle of. The volumetric flow was increased by a factor of 2. Therefore, taking that factor and squaring it means that the D/P must increase by a factor of 4. To calculate the actual change: ( ) ( ) ( ) ( ) ( ) Rev 1 63
Knowledge Check Flow detectors (such as an orifice, flow nozzle, and venturi tube) measure flow rate using the principle that flow rate is... A. inversely proportional to the D/P squared. B. inversely proportional to the square root of the D/P. C. directly proportional to the square root of the D/P. D. directly proportional to the(d/p squared. Duration 2 hours 10 minutes Logistics Use PowerPoint slides 161 174 and the IG to present ELO 4.2. ELO 4.2 Flow Meter Construction Introduction There are several designs of flow meters that work on the theory that flow is proportional to the square root of the D/P. This section discusses some of those designs, including: Orifice plates Venturi tubes Dall flow tube Flow nozzle Elbow flow meter Pitot tube Manometer Orifice Plates The orifice plate is the simplest of the flow path restrictions used in flow detection, as well as the most economical. Orifice plates are flat plates 1/16 to 1/4 inch thick. They are normally located between a pair of flanges and in a straight run of smooth pipe to avoid disturbance of flow patterns from fittings and valves. The figure below shows key dimensions of an orifice plate. 64 Rev 1
Figure: Orifice Plate When the fluid reaches the orifice plate fluid is forced to converge through the small hole; the point of maximum convergence actually occurs slightly downstream of the physical orifice, at the vena contracta point. The velocity increases and pressure decreases. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. The difference in fluid pressure between the normal pipe section and at the vena contracta provides the necessary data to determine the volumetric and mass flow rates. Three kinds of orifice plates are used: concentric, eccentric, and segmental; the figure below shows their flow sections. Segmental and eccentric orifice plates are functionally identical to the concentric orifice. Concentric Orifice Plate Figure: Orifice Plate Types The concentric orifice plate is the most common of the three types. As shown above, the orifice is equidistant (concentric) to the inside diameter of the pipe. Flow through a sharp-edged orifice plate results in a velocity change. As the fluid passes through the orifice, the fluid converges, and the velocity of the fluid increases to a maximum value. At this point, the pressure is at its minimum value. As the fluid diverges to fill the entire pipe area, the velocity decreases back to the original value, however, the pressure Rev 1 65
increases only to about 60 percent to 80 percent of the original input value. This pressure loss is irrecoverable; therefore, the output pressure will always be less than the input pressure. The pressures on both sides of the orifice are measured; the measured differential pressure is proportional to the flow rate. Segmental Orifice Plate The circular section of the segmental orifice is concentric with the pipe. The segmental portion of the orifice eliminates damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe. Depending on the type of fluid, the segmental section is located on either the top or bottom of the horizontal pipe to increase the accuracy of the measurement. Eccentric Orifice Plates Eccentric orifice plates shift the edge of the orifice to the inside of the pipe wall. This design also prevents upstream damming in the same way as the segmental orifice plate. Disadvantages of Orifice Plates Orifice plates have two distinct disadvantages; they cause a high permanent pressure drop of 20 percent to 40 percent (outlet pressure will be 60 percent to 80 percent of inlet pressure), and they are subject to erosion, which will eventually cause inaccuracies in the measured differential pressure. They yield inaccurate readings for fluids that may have gases or vapors in solution. The gases and vapors tend to collect at the top of the upstream face. This could cause changes in the density thereby causing erroneous readings. When gas or steam passes the orifice flow detector, the fluid density, and corresponding pressure fluctuates. These fluctuations cause transients on the D/P cell and make the reading very difficult and inaccurate. Venturi Tube The venturi tube is the most accurate flow-sensing element when properly calibrated. The figure below shows a typical venturi tube, with a converging conical inlet, a cylindrical throat, and a diverging recovery cone. It has no projections into the fluid, no sharp corners, and no sudden changes in contour. 66 Rev 1
Figure: Venturi Tube The inlet section decreases the area of the fluid stream, causing the velocity to increase and the pressure to decrease. In the center of the cylindrical throat, the pressure will be at its lowest value, and neither the pressure nor the velocity is changing; low-pressure measurements occur here. The recovery cone allows for some pressure recovery such that total pressure loss is only 10 percent to 25 percent. This is the lowest pressure drop of any of the head flow meters. The high-pressure measurements occur upstream of the entrance cone. The major disadvantages of this type of flow detection are the high initial costs for installation and difficulty in installation and inspection. Dall Flow Tube The Dall flow tube has a higher ratio of pressure developed to pressure lost than the venturi flow tube. The figure below shows a cut-a-way of a Dall flow tube. It is more compact and more commonly used in large flow applications. Figure: Dall Flow Tube The tube consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube. This section is the inlet shoulder; a converging inlet cone and a diverging exit cone follow the inlet shoulder. A slot or gap between the two cones separates them. The lowpressure measurements occur at the slotted throat (area between the two Rev 1 67
cones). The high-pressure measurements occur at the upstream edge of the inlet shoulder. The Dall flow tube is available in medium to very large sizes. In the large sizes, the cost is normally less than that of a venturi flow tube. This type of flow tube has a pressure loss of about 5 percent. The equation below shows the relationship between flow rate and pressure drop. Where: = volumetric flow rate K = constant derived from the mechanical parameters of the primary elements ΔP = differential pressure Flow Nozzle The flow nozzle is similar to the venturi and normally used for high velocity flow. The figure below shows a cross-section of a flow nozzle. It has a smooth contoured flow restriction, but does have a relatively high permanent pressure loss similar to the orifice. It does not allow for collection of particulate as an orifice does. Figure: Flow Nozzle Flow nozzles are common measuring elements for air and gas flow in industrial applications. Because of their relatively smooth contoured flow restriction, flow nozzles are appropriate for measuring flow of fluids containing particulates. The Steam Flow Detection section includes more detail on the flow nozzle. 68 Rev 1
Elbow Meter The elbow meter is another head flow meter using a developed differential pressure to determine flow, as shown in the figure below. When fluid encounters a piping bend, the fluid traveling on the inner radius does not have to travel as far as the fluid next to the outer radius, which creates a slight differential pressure within the bend. The bend's difference in surface area will create a low-pressure area on the inner pipe wall and a higherpressure area on the outer pipe wall. This change in pressure created by the elbow is a small change. This pressure difference is proportional to the volumetric flow rate squared. Figure: Elbow Meter The small pressure difference created by the elbow meter allows high accuracy even at high flow rates. The differential pressure instrument used is more costly than some other head flow meters. The elbow meter is a simple design and can measure flow in either direction, which is a big advantage. Pitot Tube The Pitot tube is another primary flow element used to produce a differential pressure for flow detection. In its simplest form, it consists of a tube with an opening at the end. The small hole in the end is located such that it faces the flowing fluid. The velocity of the fluid at the opening of the tube decreases to zero. This provides for the high-pressure input to a differential pressure detector. A pressure tap provides the low-pressure input, as shown in the figure below. Figure: Pitot Tube Rev 1 69
The Pitot tube actually measures fluid velocity instead of fluid flow rate. However, the equation below shows the volumetric flow rate calculation. Where: = volumetric flow rate A = area of flow cross-section v = velocity of flowing fluid K = flow coefficient (normally about 0.8) Calibration is required for Pitot tubes for each specific application, as there is no standardization. Pitot tubes are versatile instruments; they can measure fluid velocity even when the fluid is outside a confined pipe or duct, such as the exterior of an airplane. 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? A. Point D B. Point B C. Point C D. Point A Duration 5 minutes Logistics Use PowerPoint slides 175 179 and the IG to present ELO 4.3. ELO 4.3 Failure Modes Introduction The head flow meters are reliable for long-term continuous operation. The leakage of differential pressure cell connections is one of the most common problems with head flow meters. 70 Rev 1
Failure Modes Condition Indication Discussion 1. Leak on highpressure connection 2. Leak on lowpressure connection 3. Orifice plate erosion 4. Loss of density compensation input 5. Steam pressure input fail low 6. Steam pressure input fail high Indicated flow less than actual Indicated flow more than actual Indicated flow less than actual Indicated flow less than actual Indicated flow less than actual Indicated flow more than actual Leak on the highpressure tap would result in a lower D/P, which corresponds to lower indicated flow. Leak on the low-pressure tap would result in a higher D/P, which corresponds to higher indicated flow. Orifice size will increase due to erosion. This results in a lower D/P for the same flows. 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. Apparent density has decreased, less mass is sensed passing the flow detector. 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. Rev 1 71
Knowledge Check Knowledge Check Knowledge Check The most probable cause for fluctuating indication from a liquid flow rate differential pressure detector is... A. unequal temperature gradients in the liquid. B. gas or steam being trapped in the liquid. C. vortexing of the liquid passing through the flow device. D. the valve on the high-pressure sensing line being partially closed. Which one of the following will cause indicated volumetric flow rate to be lower than actual volumetric flow rate using a differential pressure flow detector connected to a calibrated orifice? A. The orifice erodes over time. B. Debris becomes lodged in the orifice. C. System pressure decreases. D. A leak develops in the low-pressure sensing line. Refer to the drawing of a pipe elbow used for flow measurement in a cooling water system below. 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. 72 Rev 1
A. increase; smaller B. decrease; larger C. increase; larger D. decrease; smaller ELO 4.4 Density/Temperature Compensation Introduction The flow measurement of gases and fluid/vapor mixtures requires compensation for density. The density of these types of compressible fluids changes as their surrounding environmental conditions change. Duration 10 minutes Logistics Use PowerPoint slides 180 182 and the IG to present ELO 4.4. Density/Temperature Compensation When steam passes through a head flow meter, the pressure change is less than when a liquid passes through. The steam pressure will react like liquid with the velocity increasing and pressure decreasing. However, when steam pressure decreases, so does its density as its molecules expand outward. Since there is less mass per unit of volume (density), the velocity also increases because of both the change in density and the change in area. The change in density takes place between the high- and low-pressure taps in the head flow meter. As the system forces a change in pressure, there is a change in density. To compensate for density properly, it is necessary to know the density values in both the high- and low-pressure areas. Density compensation converts volumetric flow rate to mass flow rate. Knowledge Check Knowledge Check Density input is normally used in steam flow instruments to convert into. A. differential pressure; volumetric flow rate B. volumetric flow rate; mass flow rate C. mass flow rate; volumetric flow rate D. mass flow rate; differential pressure If steam pressure input to a density-compensated steam flow instrument fails low, the indicated flow rate will... Rev 1 73
A. decrease, because the density input has decreased. B. decrease, because the density input has increased. C. increase, because the density input has increased D. increase, because the density input has decreased. Duration 5 minutes Logistics Use PowerPoint slides 183 193 and the IG to present ELO 4.5. ELO 4.5 Mechanical Flow Detectors Introduction Area Flow Meters The head causing the flow through an area meter is relatively constant such that the rate of flow is directly proportional to the metering area. The rise and fall of a floating element produces the variation in area. Mounting of this type of flow meter must be such that the floating element moves vertically and friction is minimal. Displacement Flow Meter In a displacement flow meter, all of the fluid passes through the meter in almost completely isolated quantities. A register counts the number of these quantities and indicates them in terms of volume or weight units. Rotameter The rotameter is an area flow meter so named because a rotating float is the indicating element. The rotameter consists of a metal float and a conical glass tube, constructed such that the diameter increases with height. The figure below shows an example rotameter. Figure: Rotameter When there is no fluid passing through the rotameter, the metal float rests at the bottom of the tube. As fluid enters the tube, the higher density of the 74 Rev 1
float will cause the float to remain on the bottom. The space between the float and the tube allows for flow past the float. As flow increases in the tube, the pressure drop increases. When the pressure drop is sufficient, the float will rise to indicate the amount of flow. The higher the flow rate the greater the pressure drop. The higher the pressure drop the farther up the tube the float will rise. The float should stay at a constant position at a constant flow rate. With a smooth float, fluctuations appear even when flow is constant. By using a float with slanted slots cut in the head, the float rotates in the flow, but maintains a constant position with respect to flow rate. Typically, a rotameter measures only low flow rates. To read a rotameter, line up the widest portion of the float with the calibrated scale, and read the value aligned with the widest portion of the float. Nutating Disk The most common type of displacement flow meter is the nutating disk, or wobble plate meter. The figure below shows a typical nutating disk. Typically, water services, such as raw water supply and evaporator feeds use nutating disk flow meters. Figure: Nutating Disk The movable element is a circular disk attached to a central ball. A shaft protrudes from the ball, and a cam or roller holds the shaft in an inclined position. A chamber surrounds the disk that has spherical sidewalls and conical top and bottom surfaces. Fluid enters an opening in the spherical sidewall on one side of the partition and leaves through the other side. As the fluid flows through the chamber, the disk wobbles, or executes a nutating motion. The shaft describes an inverted cone shape as the disk nutates. It takes a known volume of fluid to make the disc complete one revolution; the total flow through a nutating disc is the product of the number of disc rotations and the known volume of fluid for one rotation. To measure this flow, the motion of the shaft generates a cone with the point, or apex, down, as shown in the cut-a-way below. The top of the shaft operates a revolution counter, through a crank and a set of gears, calibrated to indicate total system flow. It is possible to add a variety of accessories that can perform functions in addition to measuring the flow, such as automatic count resetting devices, to the fundamental mechanism. Rev 1 75
Hot-Wire Anemometer Figure: Nutating Disk Cutaway The hot-wire anemometer, principally used for gas flow measurements, consists of an electrically heated, fine platinum wire that is immersed into the fluid flow. As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases. The fluid flow stream cools the wire electrode, causing its electrical resistance to change. Anemometers may be either constant-current or constant-resistance. In a constant-current anemometer, the fluid velocity is determined from a measurement of the change in wire resistance. In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, constant resistance. Electromagnetic Flowmeter The electromagnetic flowmeter operates on a principle similar to an electrical generator. A pipe placed between the poles of a magnet such that the flow of the fluid in the pipe is normal (perpendicular) to the magnetic field, replaces the rotor of the generator. As the fluid flows in the pipe and through this magnetic field, the fluid gains an electromotive force that will be mutually normal to both the magnetic field and the motion of the fluid. A galvanometer or an equivalent, with electrodes attached to the pipe measures this electromotive force. For a given magnetic field, the induced voltage will be proportional to the average velocity of the fluid. In order for this type of flow detection device to be most effective, the fluid should have some degree of electrical conductivity. Ultrasonic Flow Equipment Ultrasonic flow devices use the Doppler frequency shift of ultrasonic signals reflected from discontinuities in the fluid stream to obtain flow measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream. The sensor clamps on the outside of the pipe, and an ultrasonic beam from a 76 Rev 1
piezoelectric crystal passes through the pipe wall into the fluid at an angle to the flow stream, shown in the figure below. A second piezoelectric crystal located in the same sensor detects signals reflected off flow disturbances. An electrical circuit compares transmitted and reflected signals, and the corresponding frequency shift is proportional to the flow velocity. Knowledge Check Figure: Ultrasonic Flow Detector What type of flow meter is depicted in the cross-section below? A. Analog B. Ultrasonic C. Nutating disk D. Rotameter ELO 4.6 Steam Flow Density Compensation Introduction Measurements of steam flow normally use a steam flow nozzle, as shown in cross-section in the figure below. The flow nozzle is most applicable for the measurement of steam flow and other high-velocity fluid flow measurements where erosion may occur. It is capable of measuring Duration 20 minutes Logistics Use PowerPoint slides 194 201 and the IG to present ELO 4.6. Rev 1 77