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8/6/2019 Instructors Pressure Sensors and Detectors 08 Nov 08
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This sample review copy provided for evaluation of the training material
from General Physics Corporation, at http://nucleartraining.gpworldwide.com/gfe.aspx
ACADBASIC CURRICULUM
INSTRUMENTATION &CONTROLCHAPTER3
PRESSURE SENSORS AND DETECTORS
INSTRUCTORGUIDE
REV 2
TM
2003 General Physics Corporation, Elkridge, Maryland
All rights reserved. No part of this book may be reproduced in any form or by
any means, without permission in writing from General Physics Corporation.
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2003 GENERAL PHYSICS CORPORATION 2 of 28 I&C - CHAPTER 3 -
REV 2 PRESSURE SENSORS AND DETECTORS
www.gpworldwide.com [email protected]
THIS DOCUMENT IS A SAMPLE OF THE INSTRUCTOR GUIDEprovided for evaluation of the training material
from General Physics Corporation, at http://nucleartraining.gpworldwide.com/gfe.aspx
for more information contactJohn Galamback - JGalamback @gpworldwide.com 410.340.3463
Sam Shoppell - [email protected] 803.295.5209Jeff Klein - [email protected] 803.295.5210
ACAD BASICCURRICULUM INSTRUCTOR GUIDE
VOLUME: INSTRUMENTATION & CONTROL TIME hrs
INSTRUCTOR GUIDE: PRESSURE SENSORS AND
DETECTORS
REV 2
RESPONSIBILITY SIGNATURE TITLE DATE
Origination _____________________ _____________________ _____________
Review/Concurrence _____________________ _____________________ _____________Review/Concurrence _____________________ _____________________ _____________
Approval/Objectives _____________________ _____________________ _____________
Approval/Final _____________________ _____________________ _____________
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2003 GENERAL PHYSICS CORPORATION 3 of 28 I&C - CHAPTER 3 -
REV 2 PRESSURE SENSORS AND DETECTORS
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ACAD BASICCURRICULUM
INSTRUCTOR GUIDE
VOLUME: INSTRUMENTATION & CONTROL TIME hrs
INSTRUCTOR GUIDE: PRESSURE SENSORS AND
DETECTORS
REV 2
OBJECTIVESUpon completion of this chapter, the student will be able to perform the following objectives at a minimum proficiency
level of 80%, unless otherwise stated, on an oral or written exam.
1. EXPLAIN how a bellows-type pressure detector produces an output signal including:
a. Method of detection
b. Method of signal generation
2. EXPLAIN how a bourdon tube-type pressure detector produces an output signal including:
a. Method of detection
b. Method of signal generation
3. STATE the three functions of pressure measuring instrumentation.
4. DESCRIBE the three alternate methods of determining pressure when the normal pressuresensing devices are inoperable.
5. STATE the three environmental concerns that can affect the accuracy and reliability ofpressure detection instrumentation.
6. LIST the parameters that can be measured using a differential pressure (D/P) cell.
7. For a D/P sensing instrument, EXPLAIN the purpose and use of an equalizing valve.
8. DESCRIBE instrument response to equalizing valve operation.
9. EXPLAIN how a strain gauge pressure transducer produces an output signal including:
a. Method of detection
b. Method of signal generation
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ACAD BASICCURRICULUM
INSTRUCTOR GUIDE
VOLUME: INSTRUMENTATION & CONTROL TIME hrs
INSTRUCTOR GUIDE: PRESSURE SENSORS AND
DETECTORS
REV 2
K/A - CROSS REFERENCE MATRIX
The table below shows a matrix of INPO documents and the fundamental knowledge statements listed
as areas for study in this topic.
OBJ
90-003
STA
90-016
NLO
92-008
MNT
93-008
RADPRO
97-012
CHEM
98-004
ESP
00-003
LOPressure Sensors 1,2,3 XXX XXX
Vacuum Sensors 1,2,3 XXX
Differential Pressure Sensors 6,7,8 XXX
Describe the principles of operation, typical applications, and failure modes of the following detectors andinstrument strings:
Bourdon Tube Pressure/Differential PressureDetector 1,2 XXX
Pressure/DifferentialPressure Switches 6,7 XXX
Describe the effect of environmental conditions on the following instrumentation:
Reactor Coolant SystemPressure 4,5 XXX
NOTE TO INSTRUCTOR This text refers to taking a differential pressure gauge out of service
and returning it to service. Determine if the audience you are training will be involved with
operating differential pressure gauge isolation and equalizing valves. Different facilities have
different rules about who is allowed to operate the valves. Ensure your presentation addresses the
facility rules for operation and control of differential pressure gauge isolation and equalization
valves.
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KEY POINTS, AIDS,
QUESTIONS/ANSWERSINSTRUCTOR GUIDE
I. LESSON INTRODUCTION
A. Greet students
1. Introduce self and lesson title.
2. Explain overall structure and format of lesson.
3. Encourage questions.
4. Encourage student contribution ofinformation.
5. Explain sequence of presentation.
OBJECTIVES TP 3-1 ,2 B. Read the objectives.
1. Relate the objectives to the job.
2. Objectives define the lesson requiredknowledge.
3. Objectives will be reviewed during the lesson.
4. Objectives are directly related to examquestions.
C. Create interest.
II. PRESSURE SENSORS AND DETECTORS
REVIEW A. Two types of pressures are monitored in industrialfacilities and power plant systems.
Ptotal = Pstatic + Pdynamic 1. The two types are dynamic and static.
Pdynamic = v2/2gc a. Dynamic pressure is that of a fluid in
motion and static pressure is that of a fluid
at rest.
This chapter looks at static pressure
Chapter 5 Flow looks at dynamic pressure
2. Pressure sensors and detectors can be used forother applications besides pressure
applications.
a. Pressure sensors and detectors can bescaled and calibrated to indicate level andflow signals.
3. Pressure measurement is a necessary functionin the safe and efficient operation of industrial
facilities and power plants
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QUESTIONS/ANSWERSINSTRUCTOR GUIDE
B. Pressure Detector Functions
1. Although the pressures that are monitored varyslightly depending on the details of facilitydesign, all pressure detectors are used to
provide up to three basic functions:
Objective 3 1) Indication
2) Alarm
3) Control
2. Since the fluid systems may operate at bothsaturation and sub-cooled conditions, accurate
pressure indication must be available tomaintain proper cooling.
3. Some pressure detectors have audible andvisual alarms associated with them when
specified preset limits are exceeded.
4. Some pressure detector applications are used asinputs to protective features and controlfunctions.
5. If it is necessary to provide isolation betweena system and a pressure sensor (i.e., to
transmit a signal) then a tube, bellows, ordiaphragm may be used.
6. To provide a pressure signal, pressure-electrical switches and transducers are used.
a. The pressure signal trips bistables tocontrol processes and displays, and toannunicators in a control room.
7. In a plant, pressures are measured:
a. On the suction and discharge sides of
pumps
b. Inside containers, such as a pressure vessel
c. Across orifices that measure fluid flow
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8. The measurements are made to:
a. Determine volumetric quantities in tanks
b. Control turbine inlet steam
c. Set safety-relief valves
d. A host of other reasons
C. Manometers
1. Manometers are one of the oldest, simplest,and most accurate sensors used to measure
pressure.
a. A manometer is a direct way to measurepressure.
b. Frequently manometers are used as adifferential pressure detector.
c. The manometer differential pressuredetector in conjunction with a flowrestriction (orifice) can be used to measure
flow.
2. The simplest type of manometer is the U-tubemanometer.
a. It consists of a tube (usually made of glassor another transparent material) bent into
the shape of a U with both ends of the
tube open.
Figure 3-1 / TP 3-3APPLIED
PRESSURE
ATMOSPHERIC
PRESSURE
GRADUATED
SCALE
ATMOSPHERIC
PRESSURE
b. The tube is then filled with a liquid,usually water or mercury.
1) A basic U-tube manometer is shown inFigure 3-1.
c. With both ends of the tube subjected to thesame pressure, the fluid levels on each
side will be equal.
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Figure 3-2 / TP 3-4
PRESSURE
DIFFERENTIAL
LOW
PRESSURE
TAP
ORIFICE PLATE
HIGH
PRESSURE
TAP
d. If the pressure applied to one of the tube
ends is increased, the water column in theopposite tube end will increase.
e. The difference in height between the twolevels is equal to the differential pressure.
Figure 3-3 / TP 3-5
ATMOSPHERICRESSURE INPUT=
ATMOSPHERICPRESSURE INPUT =
2PSIG ATMOSPHERIC
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
3. A commonly used manometer is the well-typemanometer (Figure 3-3).
a. Well-type manometers operate on thesame basic principle as the U-tube
manometers.
b. The major difference is that the tube endconnected to the desired area of pressuremeasurement has a fluid reservoir with a
larger area than the tube.
Figure 3-4 / TP 3-6
PRESSUREINPUT
0
1
2
3
4. Another type of manometer is the inclinedtube manometer or draft gauge (Error!
Reference source not found.).
a. It is a variation of the well-typemanometer.
b. The water column is positioned almosthorizontally, not vertically.
c. In effect, this lengthens the graduations ofthe scale.
d. This allows smaller changes in pressure tobe measured.
e. The inclined tube manometer is veryaccurate but is cumbersome due to its
large size.
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Figure 3-5 / TP 3-7P1 P2
FLUID (S)
LIQUID (T)
A B
FLUID (S)
R
z
D. U-Tube Manometer
1. A simple, yet accurate, device for measuringdifferential pressure is a U-tube manometer,represented by Figure 3-5.
a. It consists of a clear U-shaped tube filledwith a liquid (T) that is heavier than the
fluid (S) in the system.
2. If the pressures at point 1 (P1) and point 2 (P2)are equal, the level in the manometer will be
equal.
a. As the flow rate increases, the head lossincreases.
b. The pressure at point 2 will be lower thanthe pressure at point 1.
3. If the pressure on one leg (P1) is greater thanthe pressure on the other leg (P2), the level of
liquid (T) rises in the lower pressure leg andlowers in the higher pressure leg.
a. The larger the differential pressure, thelarger the height difference between thelevels in the two legs.
b. This difference in the levels is called thereading (R) of the manometer.
4. The relationship between the reading and thedifferential pressure can be developed bymaking a pressure balance at positions at the
same level in both legs, shown in Error!Reference source not found., as Points A and
B.a. Since this fluid is not moving, the pressure
at Points A and B are equal.
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b. The pressure at point A also equals
pressure P1 plus the pressure due to fluid Sabove point A.
c. The pressure at point B equals pressure P2plus the pressure due to liquid T and fluid S
above point B.
Equation 3-1 / TP 3-8
)Rz(PP S1A ++=
d. These relationships can be expressedmathematically as shown in Error!Reference source not found..
RzPP TS2B ++=
BA PP =
RzPRzP TS2SS1 ++=++
)(RPP ST21 =
)(RP/D ST =
Where:
PA = pressure at point A (lbf/ft2)
P1 = system pressure at location P1
(lbf/ft2)
S = density of fluid S (lbm /ft3)
z = distance between liquid T andlocation of P2 (ft)
R = manometer reading (ft)
PB = pressure at point B (lbf/ft2)
P2 = system pressure at location P2
(lbf/ft2)
T = density of liquid T (lbm /ft3)
D/P or (P1-P2) = differentialpressure (lbf/ft
2)
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5. Disadvantages if the difference in pressure is
too large or the high pressure applied tooquickly the fluid can be pushed out of the
manometer tube and into the system.
a. The manometer must be refilled to be usedagain.
6. Advantages Inexpensive, simpleconstruction, good for measuring differential
pressure, which allows the device to be used
to measure pressure, flow, or level.
E. Bellows-Type Detectors
1. The need for a pressure-sensing element thatwas extremely sensitive to low pressures and
provided power for activating recording and
indicating mechanisms resulted in thedevelopment of the metallic bellows pressure-
sensing element.
a. The metallic bellows is most accurate whenmeasuring pressures from 0.5 to 75 psig.
Objective 1a, 1b 1) However, when used in conjunctionwith a heavy range spring, some
bellows can be used to measure
pressures of over 1,000 psig.
Figure 3-6 / TP 3-9 2. Figure 3-6 shows a basic metallic bellowspressure-sensing element.
3. The bellows is a one-piece, collapsible,seamless metallic unit that has deep foldsformed from very thin-walled tubing.
a. The diameter of the bellows ranges from 0.5to 12 in. and may have as many as 24 folds.
4. System pressure is applied to the internalvolume of the bellows.
5. As the inlet pressure to the instrument varies,the bellows will expand or contract.
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a. The moving end of the bellows is connected
to a mechanical linkage assembly.
b. As the bellows and linkage assemblymoves, either an electrical signal is
generated or a direct pressure indication is
provided.
6. The flexibility of a metallic bellows is similar incharacter to that of a helical coiled compression
spring.
See Materials Science for more details 7. Up to the elastic limit1 of the bellows, therelation between increments of load and
deflection is linear.
a. However, this relationship exists only whenthe bellows is under compression.
8. It is necessary to construct the bellows such thatall of the travel occurs on the compression side
of the point of equilibrium.
a. Therefore the bellows must always beopposed by a spring, and the deflectioncharacteristics will be the resulting force of
the spring and bellows.
F. Bourdon Tube-Type Detectors
Objective 2a, b 1. The bourdon tube pressure instrument is one ofthe oldest pressure sensing instruments in usetoday.
Figure 3-7 / TP 3-10 a. The bourdon tube (refer to Figure 3-7)consists of a thin-walled tube that is
flattened to produce a cross-sectional areaelliptical in shape, having two long flat
sides and two short round sides.
1) The tube is bent lengthwise into an arcof a circle of 270 to 300 degrees.
2) The tube is permanently fastened at oneend.
1 More detail on elastic limit provided in Material Science.
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a. Recall from basic electrical theory that the
current flow through an inductor is found bythe applied voltage divided by the inductive
reactance.
b. The inductive reactance is directly
proportional to the frequency and theinductance.
c. Inductance is a characteristic of the coil
itself AND the core.
d. If more of the magnetic core moves into the
coil, the inductance increases.
e. Less in the core and inductance decreases.
f. Therefore, the current through the coil will
increase as the inductance decreases (core
moves out of coil) and decrease as the
inductance increases (core moves into coil).
Figure 3-16 / TP 3-21
AC
AC
NON-CONDUCTIVE
TUBE
COILS
CENTER
TAP
MOVEABLE
IRON CORE
PRESSURE DETECTOR
FORCE
4. For increased sensitivity, the coil can be
separated into two coils by utilizing a center
tap, represented by the line between the AC
sources shown in Figure 3-16.
a. As the core moves within the coils, theinductance of one coil will increase, while
the other will decrease.
Figure 3-17 / TP 3-22
MOVEABLE
IRON CORE
NON-CONDUCTIVE
TUBE
PRESSURE DETECTOR
FORCE
AC INPUT
PRIMARY COIL
OUTPUT
5. Another type of inductance transducer,illustrated in Figure 3-17, utilizes two coils
wound on a single tube and is commonly
referred to as a Differential Transformer.
6. Recall from Electrical Science that to generatean electrical current the only things required
are a magnetic field, a conductor to carry the
current, and relative motion between themagnetic field and the conductor.
a. In this arrangement the primary coil
supplied with an AC current provides the
magnetic field.
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b. As the alternating current flows through
the primary coil the magnetic field createdexpands and collapses.
c. The magnetic field is expands and
collapses across the secondary coils,
which creates relative motion.
d. The wire in the secondary coils providescurrent carrying conductor, thus the
requirements to generate electricity have
been met.
e. The primary coil is wound around the
center of the tube.
f. The secondary coil is divided with one half
wound around each end of the tube.
1) Each end is wound in the opposite
direction, which causes the voltagesinduced to oppose one another.
g. A core, positioned by a pressure element, is
movable within the tube.
h. When the core is in the lower position, the
lower half of the secondary coil providesthe output.
i. When the core is in the upper position, the
upper half of the secondary coil providesthe output.
7. The magnitude and direction of the output
depends on the amount the core is displaced
from its center position.
a. When the core is in the mid-position, thereis no secondary output.
D. Variable Capacitance Transducers
1. A variable capacitance detector is a
differential pressure cell that uses a
capacitance type pressure sensor.
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2. It converts differential pressure to flow, level,
or pressure.Figure 3-18 / TP 3-23 3. Capacitive-type detectors, illustrated in
Figure 3-18, consist of two flexible conductive
plates and a dielectric.
a. In this case, the dielectric is the fluid.
4. As pressure increases, the flexible conductive
plates will move farther apart, changing thecapacitance of the transducer.
a. This change in capacitance is measurable
and is proportional to the change inpressure.
IV.SUMMARY
Direct trainees to refer to learning
objectives in the student text.
Point out where answers may be found in
student text.
A. Review all objectives ensuring studentunderstanding
B. Evaluation
Ask Summary Questions following to
ensure trainees mastery of objectives.
1. Informal evaluations during class
2. Formal evaluations with
a. Quizzes
b. Chapter Tests
c. Final Examination
C. Closing Remarks
Point out how the lessons learned in thisclass are applicable to future lessons and
plant applications.