Temperature Basic

8/3/2019 Temperature Basic

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AL JUBAIL PETROCHEMICAL COMPANY

L.L.D.P.E PLANT

Basic Training Material

TEMPERATURE

MEASUREMENT

Instrument Section


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KEMYA, L. L. D. P. E. PLANT INSTRUMENT MAINTENANCE DEPARTMENT

POST QUALIFICATION PROGRAM

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TEMPERATURE

Preface

This session deals with the instrumentation system involved in measurement andtransmission of temperature. The criteria used in describing the topic are generalawareness of process and the relation to instrument system. This session

prepared for a level of technical skilled personnel who are familiar with theprincipal idea of instrument technology in measurement and control ofcontinuous process plant. This session is divided into five sessions each detailed

separately. Also attached some objective questions in the last of this session

Instructional objectives.

The Objectives of this session stress understanding general techniques used intemperature measurement. After going through the session you should be ableto:

1. Define thermal energy.2. The relation of temperature scales to thermal energy.3. Temperature scale calibration.4. Transform a temperature reading between the Celsius and Fahrenheit

temperature scale.5. Define different methods used in temperature measurement.

INTRODUCTION

Process control is a term used to describe any condition, natural or artificial, by

which a physical quantity is regulated. Industrial temperature regulation hasalways been of paramount importance and becomes even more so with theadvance of technology. In this chapter we will be concerned first with developing

an understanding of the principles of thermal energy. Then developing a workingknowledge of the various thermal sensors employed for temperaturemeasurement.

Temperature of a substance or medium is a phenomenon expressing its degreeof hotness or coldness. That is the average energy per molecule of a material. Incase, the name used to describe the thermal energy per molecule of a material isrelated by the statement that the material; has a certain degree of temperature;

the different sets of units are referred to as temperature scales.


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Calibration

To define the temperature scales, a set of calibration points are used; the

average thermal energy per molecule is well defined through equilibriumconditions existing between solid, liquid, or gaseous states of various pure

materials.Rate of phase change of water, oxygen, gold are took as the standard calibrationpoints

Industrially, we will discuss Celsius scale and Fahrenheit scale in temperaturemeasurement. Other than this there are Rankine scale and Kelvin scale.

Relation temperature scales.The relative temperature scales differ from absolute scales only in a shift of the

zero axis. Thus, when these scales indicate a zero of temperature, of the sampleis not zero. To transform from Celsius to Fahrenheit, we simply note the scalediffer by the size of the degree and a scale shift to 32 separates the two; thus

To covert Celsius value to Fahrenheit value

Examples

A. Convert 144.5C to temperature in F

(T)F = 9/5 x C + 32

=9/5 x 144.5 C +32= 292.1F

B. Convert 317F to temperature in C

(T)C = 5/9 (x C - 32)=5/9 (317C-32)

=158.33C

(T)F = 9/ 5 x C + 32

(T)C = 5/ 9 (x C - 32)


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Industrial temperature sensors

Here some listed temperature sensors common in the process industries;

Resistance temperature detector (RTD)Thermo- couples.Thermal expansion type temperature sensors

Definit ions

Resistance temperature detectors (RTD) are electrical type sensors, whichvaries its resistance directly proportional to the change in temperature.

Thermo- couples are devices, which measure temperature on the basis ofElectro-motive force (emf) that generated in a closed circuit, dissimilar junction.

Thermal expansion type temperature sensors. The mechanical expansionof certain metals, like, liquids are used in measurement of temperatures. Bimetalstrips and liquid filled type are examples for these types.


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L.L.D.P.E PLANT

Advance Training Material

TEMPERATURE

MEASUREMENT

Instrument Section


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TEMPERATURE MEASUREMENT

Page 1 of 7

TEMPERATURE

Preface

In this session temperature measurement technology is detailed to an limitedextend up to the calibration and trouble shooting of measuring instruments. Thissession is prepared for a level of technician who are familiar with the principal

idea of instrument technology and control. This session is divided into fivedivisions each detailed separately.

Instructional objectives

The objective of this session is to detail the general techniques in temperature

measurement. After you read this chapter, you should be able to:1. Describe the principal function of Thermo-couples.2. Describe the principal function of RTDs.3. Describe the principal function of Expansion type temperature sensors.4. Calibrate and trouble-shoot temperature transmitters, pneumatic and

electronic.

5. Calibrate & Repair temperature switches.INTRODUCTION

Temperature is one of the fundamental parameters, denoting a physicalcondition of matter, similar to mass, length, and time. However, temperaturedenotes basically an intensive property of the matter. It is a measure of mean

kinetic energy of the molecules of the substance and represents the potential ofheat flow.We discussed earlier, the methods used in measurement of temperature include:

1. Generation ofemfat the junction of two dissimilar metals2.Variation in electric property (Resistance)3. Change in physical dimensionsThere are other methods in measurement of temperature, which are notdiscussed here.

Resistance Temperature Detectors (RTD)

The two basic devices used are the resistance-temperature detectors (RTD),based on the variation of metal resistance with respect to temperature, and the

thermistors, based on the variation of semi-conductor resistance withtemperature. Here we discuss RTD only.


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Construction of RTDs: An RTD is simply a length of wire whose resistance is to be

monitored as a function of temperature. The construction is typically such thatthe wire is wound on a form to achieve small size and improve thermal

conductivity. The RTD is protected from environment by a sheath or protectingtube. The standard RTD commonly used is Pt100, which shows 100 ohms at 0CThe manufacturers of RTD supply the proportional chart of temperature versus

resistance, which shows almost linear curve.

THERMO-COUPLE TEMPERATURE SENSORS

A current flow in a closed circuit made up of two dissimilar metals if the junctions

of two metals are kept at different temperature. This effect is known as 'Seebeckeffect'. Thermocouple works in this principle.Construction of thermocouple. A thermocouple is simply welded or twisted

junction between two metals, which are in construction. The thermocouple are

protected by a protective sheath covering or even sealed in glass to protect theunit from a hostile environment.Range. Thermocouple has a greatest range of all the type considered. The table

below will illustrates it.

METAL ALLOYS USEDTYPE

+ ve -veRANGE

TYPE K Chromel Constantan 175C to 1200C

TYPE J Iron Constantan 150C to 745C


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FILLED TYPE TEMPERATURE SENSOR

Filled type temperature sensor is an example for sensor changing physicaldimension with temperature. Gas/Liquid fill temperature sensors are in the formof an immersible bulb, an elastic-measuring element, tube, spiral spring or

diaphragm, coupled to the bulb through a capillary tube, and an indicating orrecording or switch assembly attachment.

PNEUM ATIC TEMPERATURE TRANSM ITTER

Principle of operation

Any change ion the sensor temperature of the gas filled thermal element causes achange in the gas pressure and, therefore, a change in the force being applied to

the bottom of the force bar. The force bar pivot about a cross-flexure, and anymotion of the force bar causes a change in the clearance between the nozzle and

the top of the force bar. This produces a change in the output pressure from therelay to the feedback bellows, until the force exerted by the bellows balances the

force exerted by the thermal system.Two additional forces act on the force bar. The elevation spring is used to elevateor suppress the zero transmitter, and the compensating bellows is used to

compensating bellows is used to compensate for ambient temperature andatmospheric pressure variation.


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The output pressure, which establishes the force balance, is the transmitted

pneumatic signal. This pressure is proportional to the measured temperature. Thepneumatic signal is transmitted.

Calibration of pneumatic temperature transmitter (FOXBORO 12A Series)

1. Prepare two calibrating baths maintained at temperatures near upper andlower limits of range.

2. Calculate output signals corresponding to temperatures of calibrating baths.For example, if the output range of the transmitter is 3 to 15 PSI and thetemperature range is 100C to 300C, then output at 200C should be 9 PSI

3. Remove sensor from the process and immerse it in the test bath. At leastentire sensitive length must be submerged.

4. Regulate air supply to 20 PSI.5. Immerse sensor in lower temperature bath. When the output pressure is

stabilized, note reading. If necessary, change reference adjustment to obtaincorrect output pressure.

6. Immerse sensor in higher temperature bath. When output pressure isstabilized, note reading. If correct proceed step 9. If not correct, continue withstep 7

7. Turn off air supply and loosen feedback bellows lock-screw. If the output islow, rotate feedback bellows, to the optimum output. Check the out put afterturn on the air supply.

8. Repeat steps 5 through 7 until outputs are correct readings are obtainedwithout readjusting at both temperatures.


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ELECTRONIC TEMP ERATURE TRANSM ITTER.

Here we will discuss both RTD & T/C transmitters

Calibration procedure RTD transmitters

Calibration procedure

1. If RTD is connected, remove all RTD lead connections.2.Attach the calibration equipment as shown in the figure.3. Turn power on.4. Set decade box to the resistance corresponding to the desired base

temperature. Adjust zero potentiometer until the output is 4 mA.

5. Set decade box to the resistance corresponding to the desired full-scale value.Adjust the span potentiometer until output is 20 mA.6. Repeat until both 4 and 20 mA readings are obtained without readjusting


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Calibration procedure T/ c transmitters

Calibration procedure

The following steps describe the procedure for calibrating a thermocoupletransmitter.1. If a thermocouple is already connected, remove all the thermocouple lead

connections.2. Install equipment as shown in the figure and allow the thermocouple junction

to stabilize at the ice point.

3. Determine the thermocouple millivolt levels at the desired base and full scaletemperatures. (See Monograph, Temperature vs. millivolts)

4. Turn power on.5. Set millivolt source until monitoring voltmeter reads the emf corresponding to

the desired base temperature. Adjust zero pot until output is 4 mA.6. Set the millivolt source to the desired full-scale temperature. Adjust span pot

until output is 20 mA.7. Repeat steps 5 & 6 until both the 4 and 20 mA readings are obtained without

readjusting the pots.

8. Disconnect the thermocouple reference junction and read out. Reconnectthermocouple and power leads. Replace terminal cover.


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TEMPERATURE SWI TCHES

Here discussed Temperature switches are of filled type sensors. The switchescan be divide to two-part sensor element, and snap switch assembly.

Operation: The temperature switches operate on the vapor pressure principle.

In actual operation the vapor pressure produced is applied to a pressure-sensingelement. The element in turn actuates a snap action switch. This type ofoperation is called SAMA Class II system. Various filling materials are used,

including Propane, Butane, Freon, Propyl alcohol, and Xylene.

Calibration:

Aim: To check the contact changes occur on the desired set value.

1.Prepare calibrating bath maintained at temperatures near to desired set-point.2.Remove sensor from the process and immerse it in the test bath. At least

entire sensitive length must be submerged.3.Keep tester on the contacts. Normally Open contact for Low Switches and

Normally Close contacts for High Switches4.Immerse sensor in temperature bath. For high switch increase the temperatureto set point, and for low switch decrease the temperature to the set point.

Adjust tension of adjusting spring by rotating adjusting nut.5.In the case of Low switch increase the temperature gradually beyond the set

point to verify reset of the switch, and in the case of High switch decrease

gradually below the set point. If required adjust the tension of adjusting springby rotating adjusting nut.

6.Repeat steps 4 through 5 until proper function is obtained without readjusting.

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Temperature Basic