Schedule Q – Describes the company’s minimum requirements for the contractor quality

management system (QMS), which specifies that the contractor must inspect test and
accept all parts of the work to ensure that the work complies with all applicable
drawings, specifications and standards.
Instrumentation QA/QC – Inspector must be familiar with the international industry
codes and standards related to intrinsically safe system and electrical system for
instrumentation including:
American National Standards Institute Control (REF of SAES-J-902)
ANSI MC96.1 Temperature Measurement Thermocouple
Institute of Electrical and Electronics Engineers (REF of SAES-J-902)
IEEE 518 IEEE Guide for the Installation of Electrical Equipment to
Minimize Electrical Noise Inputs to Controllers from
External Sources
IEEE 1100 Recommended Practice for Powering & Grounding
Sensitive Electronic Equipment, The Emerald Book
International Electrotechnical Commission (REF of SAES-J-902)
IEC 60529 Degrees of Protection Provided by Enclosures
National Electrical Manufacturers Association (REF of SAES-J-902 & 903)
NEMA ICS 6 Enclosures for Industrial Controls and Systems
NEMA 250 Enclosures for Electrical Equipment (1000 Volts
Maximum)
NEMA VE 1 Metal Cable Tray Systems
NEMA VE 2 Metal Cable Tray Installation Guidelines
National Fire Protection Association (REF of SAES-J-902 & 903)
NFPA 70 National Electrical Code (NEC)
Underwriters Laboratories, Inc. (REF of SAES-J-902)
UL 94 UL Standard for Safety Test for Flammability of Plastic
Materials for Parts in Devices and Appliances

The International Society for Measurement and Control (REF of SAES-J-903)

ISA RP12.6 Wiring Practices for Hazardous (Classified) Locations
Instrumentation Part 1: Intrinsic Safety
ISA RP12.2.02 Recommendations for the Preparation, Content, and
Organization of Intrinsic Safety Control Drawings
ISA TR12.2 Intrinsically Safe System Assessment Using the Entity
Concept
As a minimum, an inspector must be able to inspect complete loop checking, wiring continuity and color
coding, start-up and troubleshooting when required. Inspector must be able to evaluate Factory
Acceptance Test (FAT) and demonstrate a thorough working knowledge of international codes and
standards related to packaged units instrumentation including Basic Process Control System (BPCS),
Integrated Process Control System (IPCS), Machinery Protection System (MPS) and Safety Instrument
System (SIS).

QC instrumentation Notes on ITP for instrument calibration:

Responsibilities: The Quality Control Instrumentation Inspector shall be responsible for conducting
surveillance and inspection duties for work and ensuring efficient recording and reporting if result as
required. He shall review all relevant documents, material certificate and non-conformance reports and
shall receive all internal request for inspection form various section of implementing work and prepare
RFI as per schedule work priorities and proceed with Quality Inspection as required. He shall also prepare
Hold Points inspections and corresponding work releases by reviewing records as proof that work
inspected conforms to specification. He shall also be responsible to ensure that an approved IFC drawing
is available prior to commencement of the work and all materials to be used are in compliance with the
approved project specification and submittal.

Notes:

"Intrinsic safety" is a design and construction method that can be applied to electrical instruments and
their interconnection wiring for safe use in a hazardous (classified) location.

Intrinsically Safe System: An assembly of interconnected intrinsically safe apparatus, associated

apparatus, and interconnection cables in which those parts of the system that may be used in hazardous
(classified) locations are intrinsically safe circuits.
Intrinsically safe systems shall only be used in Zone 0 hazardous areas or when the vendor's
standard product offering is supplied as intrinsically safe.
Intrinsically Safe Apparatus: Apparatus in which all the circuits are intrinsically safe.
Intrinsically Safe Circuit: A circuit in which any spark or thermal effect, produced either normally or
in specified fault conditions, is incapable, under the test conditions prescribed, of causing ignition of a
mixture of flammable or combustible material in air in its most easily ignited concentration.
Intrinsic Safety Ground Bus: A grounding system which has a dedicated conductor separate from the
power system and which is reliably connected to a ground electrode in accordance with Article 250 of the
NEC.
Normal Operation: Intrinsically safe apparatus or associated apparatus conforming electrically and
mechanically with its design specification.
Simple Apparatus: A device which will not generate or store more than 1.2 V, 0.1 A, 25 mW, or 20
microJ. Examples are switches, thermocouples, light-emitting diodes, and resistance temperature
detectors (RTDs).
Field termination cabinets (junction boxes) shall be NEMA Type 4X or IP 65.
Field Instrument-The intrinsically safe instrument enclosure shall be sealed to provide adequate ingress
protection (NEMA 4X or IP 65). (note NEMA 4X = Ingress Protection 56).
Enclosures for instruments and junction boxes for use in outdoor plant areas shall be NEMA Type 4 in
accordance with NEMA ICS 6 and NEMA 250 or IEC 60529, Type 65. Enclosures in severe corrosive
environments shall be NEMA 4X or IEC 60529, Type 66. Junction box materials and constructions
shall comply with SAES-P-104, Wiring Methods and Materials, Section 7 Enclosures.

All connections at the field instrument shall be made on terminal blocks. Wire nuts and spring type
terminals shall not be used. Instruments with integral terminal blocks shall be connected directly to the
field cable.
Conduit and cable entries to field junction boxes shall normally be through the bottom. Top entry
is allowable provided a drain seal is installed on the conduit within 18" of the enclosure. Side entry shall
be permitted only when space limitations dictate.
Cable Types
Cables used for instrumentation signals shall be selected per Table 2. For detailed cable construction,
refer to 34-SAMSS-913.
Wire and Cable Minimum Requirements for Instrument Circuits

A. NEC Class 1Circuits (Note 1, 2, 3,8,9, 10)

Instrument Circuit Example Field Instrument Field Junction Box
Circuit Type To Junction Box To Marshaling Cabinet
120 VAC or Switches, Solenoids, 16 AWG, 600 V, single twisted 18 AWG, 600 V, multi-twisted
125 VDC Relays, Limit switches pair, Type TC cable (Note-7) pairs cable, type TC
ARMORED:
16 AWG,600 V, single twisted
pair, TYPE MC or equivalent
B. NEC Article 725 Class-2 &3 Circuits, Conduits and Cable Tray Installations (Note 1,2,3,4,8, 10)
Instrument Field Instrument Field Junction Box
Circuit Example
Circuit Type To Junction Box To Marshaling Cabinet
24 VDC or less Solenoids, Alarms, 16 AWG, 300 V PLTC/ITC, single 18 AWG, 300 V PLTC/ITC
Discrete signals Switches, Relays, limit twisted pair or triad cable Multi-twisted Pairs or Triads,
switched, Secondary Motor ARMORED: Overall Shield
Control 16 AWG, 300 V SWA, PLTC/ITC,
twisted single pair or triad cable
Analog Signals 4-20 mA DC, RTDs, Weigh 16 AWG, 300 V PLTC/ITC, single 18 AWG, 300 V PLTC/ITC
Frequency, Pulse, or Cells twisted, shielded pair or triad Multi-twisted, individually
Transmitter Digital Speed, Vibration, Turbine cable shielded Pairs or Triads
Communication Meter, Smart digital ARMORED:
Transmitter 16 AWG,300 V SWA, PLTC/ITC
single twisted, shielded pair or
triad cable
Thermocouple Thermocouples 16 AWG, 300 V PLTC/ITC, single 18 AWG, 300 V PLTC/ITC,
Measurement twisted, shielded, thermocouple Twisted, Individually Shielded,
extension pair cable thermocouple extension multi-
ARMORED: pair cable
16 AWG, 300 V PLTC/ITC, SWA,
single twisted, shielded,
thermocouple extension pair
cable
Data Links EIA-422A Data Links, High Follow System Manufacturer's Follow System Manufacturer's
Speed Communication Recommendation (Note-6) Recommendation
networks
Color Coding--Power and signal wiring shall be color coded as follows:
 AC Su
upply Phase Black
Neutral White
Ground Green or greeen with yellow trracer
DC Su
upply Positive Red or red sleeeve over any coolor except greenn
Negative Black or blackk sleeve over anyy color except grreen
Signa
al Pair Positive Black
Negative White
Signa
al Triad Positive Black
Negative White
Third Wire Red
Therm
mocouple Positive Per ANSI MC996.1 (Yellow) / P Per IEC 584-3 (Green)
Negative Per ANSI MC996.1 (Red) / Per IEC 584-3 (Whitte)

Data Link: Any inform mation channel used for co onnecting datta processing equipment to
o any input,
output, display device, or other data
a processing equipment.
e
Drain Wiire: In a cable, the non-innsulated wire in intimate co
ontact with a shield to pro
ovide for
terminatio eld to a ground point.
on of the shie
Home-Ru un Cable: A cable, typica ally multipair/ttriad, extendiing between tthe field juncttion boxes an
nd
marshaling panels in co
ontrol or PIB buildings.
Work Pro
ocess Chart (Loop Test))
Severe Corrosive Environments: For the purposes of this standard only, severe corrosive
environments include:
a. Outdoor offshore locations, and
b. Outdoor onshore locations within one kilometer from the shoreline of the Arabian Gulf; all of
the Ras Tanura Refinery and Terminal; and within three kilometers from the shoreline of the
Red Sea.
Thermocouple Extension Wire: A matched pair of wires having specific temperature-emf properties
that make the pair suitable for use with a thermocouple to extend the location of its reference junction
(cold junction) to some remote location; alloys for such wires are specially designed and processed to
make the pair suitable for use with only one type of thermocouple.

Data links, including fiber optic cables, shall be specified and installed per system manufacturers'
recommendations.
When redundant data links are provided, the primary cable shall follow a different route from the back up
cable. Primary and backup data link cables shall preferentially enter cabinets or consoles from opposite
sides. Data link cables shall not be routed in the same conduit, duct, or tray with other instrument
cables.
DUTIES AND RESPONSIBILITIES OF A QA/QC

Contractor must comply by all means the applicable quality work as required by the Company’s Quality
standards and specifications.

WHAT IS SCHEDULE Q

Scheduled Q describes as Owner/Company’s minimum quality requirement for the Contractor’s Quality
Management System (QMS).

WHAT IS ITP

ITP (Inspection Test Plan) is a form document procedure or guidelines that require knowledge to
comply in all quality of work at site in accordance with the Owner’s Quality specification and standard.

WHAT IS QCP

QCP (Quality Control Procedure) must be in accordance with Schedule Q and that are applicable to
execute, control all site work activities as required by the Owner/Company.

WHAT IS SPECIFICATIONS

Specifications are a set of reference documents, to guide specific task of work to be implement and
achieved.

WHAT IS QA/QC

QA/QC (Quality Assurance/Quality Control) is a group/structure of the organization that requires

Approved/Qualified personnel to initiate, ensure that quality plans and procedure are meet effectively.

WHAT IS NCR

NCR (Non Conformity Report) is a form of documentation that needs identification and recording of
all Non Conformances of work as to prevent any redundant corrective actions.
 WHAT IS HOLD, WITNESS, SURVEILLANCE & REVIEW

A mandatory action that are to be recorded to perform any such activities and that are requires
Approved/Qualified personnel to meet, conduct, and planned activities in accordance with the quality
standards and specification.

Hold shall be notified of the timing of inspection or test in advance. Inspection or test shall not be
carried out without the QA/QC organization representative in attendance.

Witness shall be notified of the timing of inspection or test in advance. However, the inspection or test
shall be performed as schedule if the QA/QC organization representative is not present.

Surveillance QA/QC organization to monitor work progress without notice from Construction
Organization.

Review reviews and approval of document.

WHAT IS P&ID

Piping & Instrument Diagram shows the piping layout and detailed notes relating to piping and
instrumentation.

WHAT IS NCR, CAR, DR

NCR (Non Conformity Report) a record reporting the variation from the specified requirements.

CAR (Corrective Action Report) is a record reporting taking the actions to eliminate the effect and
causes of an existing non conformity defects or other undesirable situation in order to prevent
recurrences.

DR (Deviation Report) is a record of the Concessions granted by Client or Owner to certain deviation
from project specifications.

ILD – Instrument Loop Diagram.

MS - Method of Statement

SAES-J-903-8.2.1- Conductors of intrinsically safe circuits shall not be placed in any raceway, cable
tray, or cable with conductors of any non-intrinsically safe circuit.

SAES-J-903-8.5.3 - Light blue color coding shall be used to identify intrinsically safe wiring. The
preferred practice is to specify intrinsically safe interconnecting cables with a blue outer jacket.
Alternatively, blue sleeves slipped over the jacket at all points of termination may be used to identify I.S.
wiring.
SAES-J-902-6.1.3 Equipment shall be suitable for the supply voltages shown.

Supply Voltage
System/Device Nominal Tolerance NEC Class
Annunciator 24 VDC 21 - 28 VDC 1 or 2
Power 125 VDC 113 - 137 VDC 1 or 3
120 VAC, 60 ±2 Hz 110 - 126 VAC 1 or 3
Shutdown and 24 VDC 21 - 28 VDC 1 or 2
isolation system 125 VDC 113 - 137 VDC 1 or 3
power 120 VAC, 60 ±2 Hz 110 - 126 VAC 1 or 3
Field switch 24 VDC 21 - 28 VDC 1 or 2
contacts 125 VDC 113 - 137 VDC 1 or 3
120 VAC, 60 ±2 Hz 110 - 126 VAC 1 or 3
Analog signal 24 VDC 21 - 28 VDC 1 or 2
(loop power) (4-20 mA)
Instrumentation 24 VDC 21 - 28 VDC 1 or 2
power 120 VAC, 60 ±2 Hz 110 - 126 VAC 1 or 3

SAES-J-902-8.1.2.6 Emergency shutdown system (ESD) and fire detection system wiring shall have
dedicated cabling, junction boxes and marshaling panels.
SAES-J-902-10.2 Terminal Blocks
10.2.1 Terminal blocks shall be channel (rail) mounted, strip type, with tubular box clamp connector and
compression bar or yoke for wire termination. As a minimum, the thickness of the terminals shall be 5
mm or higher. Multi-deck and spring type terminal blocks are not acceptable.

10.2.2 Terminals shall be made of fire retardant, halogen free, high strength material such as polyamide
or equivalent in accordance with UL 94, V0. Brittle materials such as melamine shall not be used.

SAES-J-902-12.1 - Electrical systems must be connected to ground for the protection of personnel and
equipment from fault currents (safety ground) and to minimize electrical interference in signal
transmission circuits (circuit ground).Two grounding systems are required for instrumentation systems:
a) Safety Ground for personnel safety b) Instrumentation Circuit Ground
 UNDERSTANDING THE INGRESS PROTECTION SYSTEM
The IEC IP classification system designates the degree of protection provided by an enclosure
against impact and/or water or dust penetration (ingress). It has two numbers; first—protection
against solid objects, second protection against water.
EXAMPLE: IP 54

1st Figure: 2nd Figure:

protection against solid bodies protection against liquids
IP TESTS IP TESTS
0 No protection 0 No protection

Protected against Protected against

solid bodies larger vertically-falling
1
1 than 50mm drops of water
(e.g. accidental (condensation)
contact with the
hand) Protected against
drops of water
2 falling at up to 15˚
Protected against
from the vertical
solid bodies larger
2 than 12.5mm
(e.g. finger of the Protected against
hand) drops of rainwater
3 at up to 60˚ from
the vertical
Protected against
solid bodies larger
3 Protected against
than 2.5mm
(tools, wires) projections of
4
water from all
directions
Protection against
solid bodies larger
4 Protected against
than 1mm (fine
tools, small wires) 5 jets of water from
all directions

Protected against
Completely protected
5 dust (no harmful
against jets of
deposit) 6
water of similar
force to heavy seas
Completely protected
6 against dust Protected against
15 cm
1m

7 the effects of
mini

immersion

IP RATINGS DO NOT INDICATE ANY DEGREE OF CORROSION RESISTANCE.

Conversion of NEMA Enclosure Type numbers to IEC Classification Designations
(Cannot be used to convert IEC Classification Designations to NEMA Type numbers)
NEMA ENCLOSURE TYPE NUMBER IEC ENCLOSURE CLASSIFICATION DESIGNATION
1 IP10
2 IP11
3 IP54
3R IP54
3S IP54
4 AND 4X IP56
5 IP52
6 AND 6P IP67
12 AND 12K IP52
13 IP54

9
 WHAT IS A ZONE?
THE IEC HAS DEFINED 3 AREAS OF HAZARDOUS GAS
OR VAPOR RELEASE AS FOLLOWS:

ZONE 0 ZONE 1 ZONE 2

Explosive Explosive Explosive
Atmosphere Atmosphere Atmosphere
Is Is May
Continuously Often Accidentally
Present Present Be Present
Zone in which an explosive Zone in which an explosive Zone in which an explosive
mixture of gas, vapor or mist mixture of gas, vapor or mist mixture is not likely to
is continuously present. is likely to occur during occur in normal operation,
normal operation. and if it occurs will only
exist for a short time (leaks
or maintenance).

COMPARING IEC ZONES AND NEC® DIVISIONS

Z
O
N ZONE 1 ZONE 2
E
0

DIVISION 1 DIVISION 2

DETERMINING A “ZONE” REQUIRES ANSWERING 4 ESSENTIAL QUESTIONS

1
What is emission level of gas/vapor?
(a) continuous, (b) first level emission, (released during normal operation)
(c) second level emission (released during abnormal operation)

2
What type of openings currently exist?
(a) continuously open, (b) normally closed,
(c) weatherproof, (d) emergency open only

3 What is ventilation?
(a) very good, (b) good, (c) poor

4 What is level of ventilation?

(a) high, (b) average, (c) weak

2
INTRODUCTION
The Basic Process Control System (BPCS) is responsible for normal operation of the plant and in many
instances is used in the first layer of protection against unsafe conditions. Normally if the BPCS fails
to maintain control, alarms will notify operations that human intervention is needed to reestablish
control within the specified limits. If the operator is unsuccessful then other layers of protection, e.g.
pressure safety valves, inherently safe process design, or Safety Instrumented System need to be in
place to bring the process to a safe state and mitigate any hazards.
LAYERS OF PROTECTION

For this hierarchy to be effective it is critical that each layer of protection be independent or separate.
This means that multiple layers (e.g., BPCS and SIS) must not contain common components that in
the event of a single failure would disable multiple protection layers. In the case of SIS and BPCS,
the traditional design practice of separation would prevent the SIS layer from becoming disabled
when the BPCS layer experiences a problem.
Consider the following accident case history where failure of a single component, which was shared
by the BPCS and the SIS, resulted in a situation where shutdown was required and simultaneously
prevented the safety action from being taken.
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2.4 TEMPERATURE MEASUREMENT

Every aspect of our lives, both at home and at work, is influenced by Note
temperature. Temperature measuring devices have been in existence for
centuries. The age-old mercury in glass thermometer is still used today and
why not? The principle of operation is ageless as the device itself. Its
operation was based on the temperature expansion of fluids (mercury or
alcohol). As the temperature increased the fluid in a small reservoir or bulb
expanded and a small column of the fluid was forced up a tube. You will
find the same theory is used in many modern thermostats today. In this
module we will look at the theory and operation of some temperature
measuring devices commonly found in a generating station. These include
thermocouples, thermostats and resistive temperature devices.
Thermocouples (T/C) and resistive temperature devices (RTD) are generally
connected to control logic or instrumentation for continuous monitoring of
temperature. Thermostats are used for direct positive control of the
temperature of a system within preset limits.

2.4.1 Resistance Temperature Detector (RTD)

Every type of metal has a unique composition and has a different resistance
to the flow of electrical current. This is termed the resistively constant for
that metal. For most metals the change in electrical resistance is directly
proportional to its change in temperature and is linear over a range of
temperatures. This constant factor called the temperature coefficient of
electrical resistance (short formed TCR) is the basis of resistance
temperature detectors. The RTD can actually be regarded as a high
precision wire wound resistor whose resistance varies with temperature. By
measuring the resistance of the metal, its temperature can be determined.
Several different pure metals (such as platinum, nickel and copper) can be
used in the manufacture of an RTD. A typical RTD probe contains a coil of
very fine metal wire, allowing for a large resistance change without a great
space requirement. Usually, platinum RTDs are used as process
temperature monitors because of their accuracy and linearity.
To detect the small variations of resistance of the RTD, a temperature
transmitter in the form of a Wheatstone bridge is generally used. The circuit
compares the RTD value with three known and highly accurate resistors.

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R3 R1 Note
Connecting
Wires A B
Power
Supply
R2
RTD

Field Control Room

Figure 1
RTD using a Wheatstone Bridge
A Wheatstone bridge consisting of an RTD, three resistors, a voltmeter and
a voltage source is illustrated in Figure 1. In this circuit, when the current
flow in the meter is zero (the voltage at point A equals the voltage at point
B) the bridge is said to be in null balance. This would be the zero or set
point on the RTD temperature output. As the RTD temperature increases,
the voltage read by the voltmeter increases. If a voltage transducer replaces
the voltmeter, a 4-20 mA signal, which is proportional to the temperature
range being monitored, can be generated.
As in the case of a thermocouple, a problem arises when the RTD is
installed some distance away from the transmitter. Since the connecting
wires are long, resistance of the wires changes as ambient temperature
fluctuates. The variations in wire resistance would introduce an error in the
transmitter. To eliminate this problem, a three-wire RTD is used.

R3 R1

A B
Power
RW1 Supply
4-20mA
3- Wire RTD Detector
R2
RW2

RW3
Field Control Room
Figure 2
Three-Wired RTD
Figure 2 illustrates a three-wire RTD installation.

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The connecting wires (w1, w2, w3) are made the same length and therefore
the same resistance. The power supply is connected to one end of the RTD
Note
and the top of the Wheatstone bridge. It can be seen that the resistance of
the right leg of the Wheatstone bridge is R1 + R2 + RW2. The resistance of
the left leg of the bridge is R3 + RW3 + RTD. Since RW1 = RW2, the result is
that the resistances of the wires cancel and therefore the effect of the
connecting wires is eliminated.

RTD Advantages and Disadvantages

Advantages:
• The response time compared to thermocouples is very fast – in the
order of fractions of a second.
• An RTD will not experience drift problems because it is not self-
powered.
• Within its range it is more accurate and has higher sensitivity than a
thermocouple.
• In an installation where long leads are required, the RTD does not
require special extension cable.
• Unlike thermocouples, radioactive radiation (beta, gamma and
neutrons) has minimal effect on RTDs since the parameter measured
is resistance, not voltage.

Disadvantages:
• Because the metal used for a RTD must be in its purest form, they
are much more expensive than thermocouples.
• In general, an RTD is not capable of measuring as wide a
temperature range as a thermocouple.
• A power supply failure can cause erroneous readings
• Small changes in resistance are being measured, thus all connections
must be tight and free of corrosion, which will create errors.
• Among the many uses in a nuclear station, RTDs can be found in the
reactor area temperature measurement and fuel channel coolant
temperature.

Failure Modes:
• An open circuit in the RTD or in the wiring between the RTD and
the bridge will cause a high temperature reading.
• Loss of power or a short within the RTD will cause a low
temperature reading.

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2.4.2 Thermocouple (T/C)

A thermocouple consists of two pieces of dissimilar metals with their ends Note
joined together (by twisting, soldering or welding). When heat is applied to
the junction, a voltage, in the range of milli-volts (mV), is generated. A
thermocouple is therefore said to be self-powered. Shown in Figure 3 is a
completed thermocouple circuit.

Junction 1 at Junction 2 at
Temperature T1 Metal B Temperature T2

Metal A Metal A
Galvanometer Galvanometer

Figure 3
A Thermocouple Circuit
The voltage generated at each junction depends on junction temperature. If
temperature T1 is higher than T2, then the voltage generated at Junction 1
will be higher than that at Junction 2. In the above circuit, the loop current
shown on the galvanometer depends on the relative magnitude of the
voltages at the two junctions.
In order to use a thermocouple to measure process temperature, one end of
the thermocouple has to be kept in contact with the process while the other
end has to be kept at a constant temperature. The end that is in contact with
the process is called the hot or measurement junction. The one that is kept
at constant temperature is called cold or reference junction. The relationship
between total circuit voltage (emf) and the emf at the junctions is:
Circuit emf = Measurement emf - Reference emf
If circuit emf and reference emf are known, measurement emf can be
calculated and the relative temperature determined.
To convert the emf generated by a thermocouple to the standard 4-20 mA
signal, a transmitter is needed. This kind of transmitter is called a
temperature transmitter. Figure 4 shows a simplified temperature
transmitter connection.

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Temperature
Transmitter

Metal A Note
Measurement
Junction at 4-20mA
Process Reference
Temperature Junction
Metal B

Field Metal C
Control Room

Figure 4
A Simplified Thermocouple Temperature Transmitter
In Figure 4 above, the temperature measurement circuit consists of a
thermocouple connected directly to the temperature transmitter. The hot and
cold junctions can be located wherever required to measure the temperature
difference between the two junctions.
In most situations, we need monitor the temperature rise of equipment to
ensure the safe operation. Temperature rise of a device is the operating
temperature using ambient or room temperature as a reference. To
accomplish this the hot junction is located in or on the device and the cold
junction at the meter or transmitter as illustrated in figure 5.
Temperature
Transmitter
Metal C
Metal A

Measurement
Junction

Metal B

Field Control Room Reference

Junction

Figure 5
Typical Thermocouple Circuit

Thermocouple Advantages and Disadvantages

Advantages:
• Thermocouples are used on most transformers. The hot junction is
inside the transformer oil and the cold junction at the meter mounted
on the outside. With this simple and rugged installation, the meter
directly reads the temperature rise of oil above the ambient
temperature of the location.
• In general, thermocouples are used exclusively around the turbine
hall because of their rugged construction and low cost.
• A thermocouple is capable of measuring a wider temperature range
than an RTD.

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Disadvantages:
• If the thermocouple is located some distance away from the Note
measuring device, expensive extension grade thermocouple wires or
compensating cables have to be used.
• Thermocouples are not used in areas where high radiation fields are
present (for example, in the reactor vault). Radioactive radiation
(e.g., Beta radiation from neutron activation), will induce a voltage
in the thermocouple wires. Since the signal from thermocouple is
also a voltage, the induced voltage will cause an error in the
temperature transmitter output.
• Thermocouples are slower in response than RTDs
• If the control logic is remotely located and temperature transmitters
(milli-volt to milli- amp transducers) are used, a power supply
failure will of course cause faulty readings.

Failure Modes:
An open circuit in the thermocouple detector means that there is no path for
current flow, thus it will cause a low (off-scale) temperature reading.
A short circuit in the thermocouple detector will also cause a low
temperature reading because it creates a leakage current path to the ground
and a smaller measured voltage.

2.4.3 Thermal Wells

The process environment where temperature monitoring is required, is often
not only hot, but also pressurized and possibly chemically corrosive or
radioactive. To facilitate removal of the temperature sensors (RTD and TC),
for examination or replacement and to provide mechanical protection, the
sensors are usually mounted inside thermal wells (Figure 6).
Process Connecting Threads Protective Well

This portion in process

Connector Block

Figure 6
Typical Thermal Well Installation
A thermal well is basically a hollow metal tube with one end sealed. It is
usually mounted permanently in the pipe work. The sensor is inserted into it
and makes contact with the sealed end.

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