BASICS

• What is Instrumentation?• Basic Terminologies• Process & its Control• Field Instruments & its principles• Valves & its working

What is Instrumentation?• Instrumentation is about measurement and

control.

• Instrumentation engineering is the engineering specialization focused on the design and configuration of process systems.

• Instruments are devices which are used in measuring attributes of process systems.

Basic Terminologies• Process:

Series of continuous or regularly recurring steps or actions intended to achieve a predetermined result, as in heat treating metal, or manufacturing acid.

• Transducer(sensor):Element which converts one form of Energy to Other form.

• Primary Transducer:Transducer which converts the Process parameter to a form

readable by Secondary Transducer.Eg: Orifice plate

• Secondary Transducer:Transducer or transmitter which responds to a measured

variable and converts it to a standardized transmission signal which is a function only of the measurement. Eg: DP Transmitter

• Signal:

The signal is the event or phenomenon that conveys data from one point to another.

• Loop:

A Loop is a combination of one or more interconnected instruments arranged to measure a process variable. It shall comprises the whole chain from Primary element to Correcting Element.

• Controller:

A device that operates automatically by use of some established algorithm to regulate process variable(PV) according to the set point(SV). The controller input receives information about the status of the process variable and then provides an appropriate output signal(MV-manipulated variable) to the final control element(eg-valves etc.,).

•Interlock:It refers to the set of plant conditions(eg. Level of a tank, temp of furnace,

position of furnace or a valve, flow of a fluid, etc) which are to be satisfied before operating(starting, stopping, opening ,closing ,etc)of any instrument or equipment.

ANALYSER:

•Monitor pollutant gas emissions from industrial processes.

•Gas analyzer is a gas comparator providing high linearity of signal transformation function.

WEIGHFEEDER:

•Controller multiplies the signal from the load cell(belt load,kg/m) with that from the speed transducer(belt speed,m/s) to get the feed rate.

•The controller then either changes the belt speed or belt load to get the set feed rate.

CONVEYOR:

Conveyors are used as components in automated distribution and warehousing. A belt conveyor consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler.

BELTWEIGHER:

• Material flowing over the belt may be weighed in transit using a beltweigher.A belweigher or belt weigher is a piece of industrial control equipment used to gauge the mass or flow rate of material travelling over a conveyor belt.

Process & its Control

• Process Parameters:– Pressure– Level– Temperature– Flow

Pressure Measurement

• PRESSURE A force applied to or distributed over a surface. The

pressure (P) of a force (F) distributed over an area(A) is defined as :

P = F / A

Standard Unit of Pressure is Pascal

Other units of pressure are psi kg/cm2

bar atmosphere

torr1 Pa = 1 N/m2

Pressure Measurement

• Primary Pressure Measuring Devices:– Diaphragm– Bellows– Manometer

• Pressure measurements can be divided into three different categories:

– absolute pressure– gauge pressure and – differential pressure

GAUGE PRESSURE

• Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

In measurements a gauge is used to record the pressure difference between the system and the atmospheric pressure. This is called gauge pressure and can be stated by the following equation:

Pg=Pa+PowherePg= gauge pressurePo = atmospheric pressure

• If the pressure of a system is below atmospheric, it is called vacuum pressure.

•When pressure is measured by a gauge, the quantity obtained usually excludes the ambient atmospheric pressure and is therefore called overpressure,

Poverpressure = Pgauge

ABSOLUTE PRESSURE

I f atmospheric pressure is included, then the result ing pressure is called absolute pressure

Pabsolute = Patmospheric + Pgauge

The absolute pressure is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum.

P=Pg+Po

P=absolute pressure, Pg=gauge pressure,Po=atmospheric pressure.

DIFFERENTIAL PRESSURE

Differential pressure is the difference in pressure between two points.

ATMOSPHERIC PRESSURE

•The atmospheric pressure is the pressure in the surrounding air at or "close" to the surface of the earth.

•The atmospheric pressure varies with temperature and altitude above sea level.

•Atmospheric pressure is the pressure exerted at the surface of a body by a column of air in an atmosphere.

1 atmosphere on Earth = 760 mil l imeters of mercury (760 Torr) and 101,325 Pascals.

STANDARD ATMOSPHERIC PRESSURE :

The Standard Atmospheric Pressure (atm) is used as a reference for gas densit ies and volumes.

The Standard Atmospheric Pressure is defined at sea-level at 273 oK (0oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293 oK (20oC) is also used.

Types of Pressure Measuring Devices

Manometer Bourdan Gauge

Contd..

Strain Gage Types

Capacitive

Peizo Electric

LVDT

Level Measurement

• Some of the most commonly used liquid-level measurement methods are:

• RF capacitance• Conductance (conductivity)• Hydrostatic head/tank gauging• Radar• Ultrasonic

Level Measurement

Level Measurement using Pressure Transmitter

P = ρgh

Open Tank Closed Tank

RF CapacitanceConductive Type

Hydrostatic Head

RADAR TypeUltrasonic Type

Flow Measurement

• Principle:– Flow is measured by measuring velocity through a known

area.with this indirect method,the flow measured is the volume flow rate Q .

Q = A x V

Where A is the cross sectional area of the pipe V is the fluid velocity

Unit of low is m3/hr or litres/hr

Flow Measurements

• Types:– Head Type Flowmeters– Mechanical Flowmeters– Electronic Flowmeters– Mass Flowmeters

Different Type of Head Type Flowmeters

• Orifice Plate• Venturi• Flow Nozzle • Pitot Tube• Elbow

Orifice• Service: Clean Liquids, Gases Steam,(no slurries or corrosive)• Scale: Square Root• Accuracy: 1% Full Scale• Permanent Pressure Loss: High• Cost: Low Basic Equation :V=k*(h/D)0.5

Venturi• Service: Clean Liquids, Gases Steam Slurries and Dirty Fluids• Scale: Square Root• Accuracy: 1% Full Scale• Rangability: 3:1• Permanent Pressure Loss: Low• Cost: High

Variable Area Meters

Mechanical Flowmeters

Electronic Flowmeters

Vortex Flowmeters

Ultrasonic Flowmeters

Temperature Measurement

• Temperature:

– Webster’s defines temperature as “the degree of hotness or coldness measured on a definite scale.

Various units of temperature are related as

C = 5/9 (F – 32)

F = 9/5 (C ) + 32

K = 273 + C

R = 460 + F

Temperature Measurement

• Types of Temp Measurement:– RTD– Thermocouple– Thermistor– Thermopile– Pyrometer

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Steam

ColdWater

HotWater

Load Disturbance

TT

TIC

I/P

Temperature terminologyTemperature Control Loop

• Temperature Loop Issues:– Fluid response slowly to change in input heat– Requires advanced control strategies

• Feedforward Control

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• Example: Thermistors• RTD (discussed later)• Thermistors• Semi-conductors made from specific mixtures of pure oxides of

nickel, manganese, copper, cobalt, and other metals sintered at very high temperature.

• Used with Wheatstone Bridge which amplifies small change in resistance - in a simple circuit with a battery and a micro-ammeter.

• Stability -• Linearity -• Slope of Output -

Temperature Measurement Technology

Change in RESISTANCE with response to change in TEMPERATURE

ModeratePoor (Logarithmic)Negative

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Temperature SensorsRTDs

• What is an RTD ?– RResistance TTemperature DDetector

Platinum resistance changes with temperature

Rosemount’s Series 78, 88

Rosemount’s Series 68, 58Series 65Two common types of RTD elements:

Wire-wound sensing elementThin-film sensing element

» Operation depends on inherent characteristic of metal (Platinum usually): electrical resistance to current flow changes when a metal undergoes a change in temperature.» If we can measure the resistance in the metal, we know the temperature!

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Temperature SensorsRTDs

• How does a RTD works?– Resistance changes are Repeatable– The resistance changes of the platinum wiring can be

approximated by an ideal curve -- the IEC 751

0

50

100

150

200

250

300

350

-400 -200 0 200 400 600 800Re

sist

ance

(Ohm

s)

Temperature (oC)

oC Ohms0 100.0010 103.9020 107.7930 111.67

International Resistance vs. Temperature Chart:

IEC 751

IEC751

IEC 751 Constants are :- A = 0.0039083, B = - 5.775 x 10 -7, If t>=0°C, C=0, If t<0, C = - 4.183 x 10 -12

Example: RT = R0 [1 + At + Bt2 + C(t-100)t3]= 103.90

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ProcessProcessTemperatureTemperature

Hot junction

– Two dissimilar metals joined at a “Hot” junction

Cold junction+

-MV

– The wires are connected to an instrument (voltmeter) that measures the potential created by the temperature difference between the two ends.

DT

“40 millivolts!,” Tommy Seebeck yelled in a heated

debate.

The junction of two dissimilar metals creates a small voltage output proportional to

temperature!

What is a Thermocouple ?

Temperature SensorsThermocouples

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• How does a Thermocouple work ?– The measured voltage is proportional to the temperature temperature

difference difference between the hot and cold junction! (T2 - T1) =∆T. +

-MVHeat

Hot junction Cold junction

oC Millivolts0 0.00010 0.59120 1.19230 1.801

Thermoelectric Voltagevs. Temperature Chart:

TYPE E THERMOCOUPLE

∆T

-20

0

20

40

60

80

-500 0 500 1000

Vol

tage

(m

V)

Temperature (oC)

IEC584

Measurement Measurement JunctionJunction

TT22

Reference Reference JunctionJunction

TT11

Temperature SensorsThermocouples

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� Type J– Iron / Constantan

• White, Red• 0 to 760 °C• Least Expensive

Types of Thermocouple

� Type K– Chromel / Alumel

» Yellow, Red» 0 to 1150 °C» Most Linear

� Type T– Copper /

Constantan» Blue, Red» -180 to 371 °C» Highly resistant to

corrosion from moisture

+ -

+ -

+ -

Temperature SensorsThermocouples

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• Better Accuracy & Repeatability– RTD signal less susceptible to noise– Better linearity– RTD can be “matched” to transmitter

(Interchangeability error eliminated)– CJC error inherent with T/C’s; RTD’s lead wire

resistance errors can be eliminated

Why choose RTD over Thermocouple ?

Better Stability– T/C drift is erratic and unpredictable; RTD’s drift

predictably– T/C’s cannot be re-calibrated

Greater Flexibility– Special extension wires not needed– Don’t need to be careful with cold junctions

Temperature SensorsComparison

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• Applications for Higher Temperatures• Above 1100°F

• Lower Element Cost • Cost is the same when considering temperature point

performance requirements• Faster response time

• Insignificant compared to response time for T-Well and process

• Perceived as more rugged• Rosemount construction techniques produce extremely

rugged RTD

Why choose thermocouple over RTD ?

Temperature SensorsComparison

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RANGE OFFER -200 to 500º C RTD

500 to 1100º C Thermocouple Type K

>1100º C Special Thermocouple R, S or B

Temperature SensorsTemperature SensorsComparisonComparison

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• What is a thermowell (T-well) ?– A unit that protects a sensor from process

flow, pressure, vibrations, and corrosion– Allows for sensor removal without process

shutdown– Slows response time (by 5 times)

Why are there different material types ?– To handle different corrosive environments– To handle different temperature and pressure limits

Sensor accessoriesThermowells

Control Valves

• The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

• The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator.

• Control valves are valves used within industrial plants and elsewhere to control operating conditions such as temperature, pressure ,flow, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "set point" to a "process variable" whose value is provided by sensors that monitor changes in such conditions.

• The opening or closing of control valves is done by means of electrical, hydraulic or pneumatic systems.

CONTROL VALVES:

They are basically pneumatically operated valves which require around 4 to 5 kg/cm2 of air pressure to operate the valve.

I / P Converter POSITIONER CONTROL VALVE

SUPPLY AIR

Pneumatic signal

CURRENT

SUPPLY AIR

Control Valve Types

Valve Body Types

• Diff. types of Valve Body:• Butterfly Valve• Globe Valve• Ball Valve• Plug type Valve• Needle Valve

Positioner & its accessories• Pneumatically operated valves depend on a positioner to take

an input signal from a process controller and convert it to valve travel.

• A pneumatic signal (usually 3-15 psig) is supplied to the positioner. The positioner translates this to a required valve position and supplies the valve actuator with the required air pressure to move the valve to the correct position.

• Analog I/P Positioner—This positioner performs the same function as the one above, but uses electrical current (usually 4-20 mA) instead of air as the input signal.

Automation (ancient Greek: = self dictated), roboticization or industrial automation or numerical control is the use of control systems such as computers to control industrial machinery and processes, replacing human operators.

The most commonly used automation systems are :

• DCS - Distributed Control System

• PLC - Programmable Logic Controller

• SCADA – Supervisory Control And Data Acquisition System

DCS• Distributed control system (DCS) refers to a control system usually of a

manufacturing system, process or any kind of dynamic systemdynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers are connected by a network for communication and monitoring.

• DCS is a very broad term used in a variety of industries, to monitor and control distributed equipment.

• A DCS typically uses computers (usually custom designed processors) as controllers and uses both proprietary interconnections and protocols for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (a.k.a. field) and output modules transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexers/demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles.

ARCHITECTURE OF DCS

OperatorWorkstation 1

OperatorWorkstation 2

OperatorWorkstation 3

Controller 1 Controller 2 Controller 3 Controller 4

Sensor 1 Actuator 1 Actuator 2 Sensor 3 Actuator 3 Sensor 4 Actuator 4

Database

Input Module

OutputModule

Sensor 2

Input Module

Input Module

Input Module

OutputModule

OutputModule

OutputModule

HIS HIS ENGSTATION

FCS

NIU NIU

BCV

MFCD

RL BUS

V NET

ETHERNET

FIELD INSTRUMENTS

RIO BUS

V NET

FCS

MOPL

JB 1JB 2

FIELD INSTRUMENTS

MAR MAR

DCS : BASIC CONFIGURATION

MAR

V NET

JB 3

BASIC TERMINOLOGIES OF DCSHIS: Human Interface StationThe HIS is mainly used for operation and monitoring-it displays process variables,control parameters and alarms necessary for users to quickly grasp the operating status of the plant.

NIU: Node Interface UnitThese are remote I/O units which all the Instruments are connected.these units in turn are connected to FCS through RIO bus.

FCS: Field Control StationIt is the main control unit which controls the plant.there can be more than one FCS which then communicate with each other and also communicate with the HIS from where the Operator is operating.

Vnet:The Vnet real time control system BUS links station such as FCS,HIS,BCV andCGW.

ETHERNET:Ethernet is used to link HIS,ENG and supervisory systems.it is also used for transferring data files to supervisory computers and for HIS data equalization.

RL Bus: This a control system BUS(communication link) which connects Field control units,operators stations.

CGW: Communication GatewayThis unit links the Vnet control system BUS to an ETHERNET BUS

BCV: Bus ConverterThe communication bus of one version of DCS may not communicate with the newer versions so BUS CONVERTER is used to convert the BUS to suitable mode.In our plant our existing RL BUS is converted to newer system bus Vnet by the Bus Converter kept in Engineering room near central control room.

Programmable Logic Controller (PLC)

•Programmable Logic Controller (PLC) is a microprocessor based system that uses programmable memory to store instructions and implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and processes.

•Unlike Personal Computer, PLC does not contain peripherals, such as display or keyboard, that allow user to directly interact with PLC. In order to facilitate interaction, separate computer is provided, normally taking form of a standard PC. Through this external computer, operator can re-program PLC, provide set-points and view trends of process variables that are controlled and manipulated by PLC.

PLC Actuator Process

Sensor

ExternalComputer

Power Supply

Communication Module

Microprocessor + Memory

Analogue Input (AI) Module

Discrete Output (DO) Module

Discrete Input (DI) Module

Analogue Output (AO) Module

Analogue Sensor

Discrete Sensor

Analogue Actuator

Discrete Actuator

Operator Workstation

Programmable Logic Controller Architecture

PLC

Communication Module

Microprocessor

Input Module

External Computer

Programmable Logic Controller Architecture

PLC

Output Module Actuator Process

Sensor

PLC consists of the following components:

• Microprocessor – This is the brain of PLC. It reads input signals, executes control program and communicates results (decisions) of control program as action signals to the outputs.

• Memory – It stores control program that is to be executed at a prescribed rate.

• Power Supply – This component is used to convert the mains AC voltage to the low DC voltage (e.g. from 240V AC to 5V DC). This unit powers the processor and the circuits in the input and output modules.

• Input Module – This component receives information from external devices (sensors). It contains circuitry that provides electrical isolation and signal conditioning functionalities. Input module can be analogue input (AI) or discrete input (DI) module. AI module receives continuously changing signal whose amplitude is proportional to the current value of the measured process variable. DI module receives discrete/digital (ON/OFF) information from discrete sensors, for example push button (ON if button is pressed, OFF if button is not pressed). Note that DI is much more frequently used than AI.

• Output Module – This module communicates control actions to external devices (actuators). It contains circuitry required to interface PLC with actuators (e.g. digital-to-analogue converter and power amplifier). Like input module, output module can be analogue output (AO) or discrete output (DO) module depending on the type of actuator used.

• Communication Module – This component allows PLC to communicate with external devices using sophisticated multiple-bit digital communication protocols (e.g. Ethernet).

Programmable Logic Controller (PLC)

PLC Programming

• Ladder Diagram - most common• Structure Text Programming (ST)• Functional Block Programming (FB)• Instruction List (IL)• Sequential Function Chart (SFC)

• SCADA system performs the following tasks

• Collection of data from field devices, which can be sensors, actuators and controllers.

• Transfer of field devices’ information via communication link to the central site (master station)

• Execution of any necessary analysis and supervisory control calculations, all of which are taking place at the master stations.

• Display process information on a number of operator screens.

• Convey any required supervisory control actions back to the field devices.

Supervisory Control and Data Acquisition (SCADA)