instructors pressure sensors and detectors 08 nov 08

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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KFI03Lr02_Pressure Sensors and Detectors 08Nov08

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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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ACAD BASICCURRICULUM

INSTRUCTOR GUIDE



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:



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:




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ACAD BASICCURRICULUM

INSTRUCTOR GUIDE



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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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.

instructors pressure sensors and detectors 08 nov 08

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