AReport on

“BASIC INSTRUMENTATION & PROCESS CONTROL”

At

Rihand Super Thermal Power Project

(NTPC Ltd.)

Submitted by:-

GHANSHYAM B-Tech. IIIrd year Electronics & Instrumentation Engineering UNITED COLLEGE OF ENGINEERING & RESEARCH, (Allahabad)

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AN OVERVIEW

NTPC

NTPC was set up on 7th November 1975, the MAHARATNA power giant today generatesmore than one fourth of the total power in the country, Ranked 5th largest powergenerating utility in the world, NTPC is the second most efficient in capacity utilizationamong the top ten thermal generating companies according to a survey conducted by Data Monitor, United kingdom. In a short span of three decades, NTPC has earned its prime status by setting up a total generating capacity of 30,644 MW. through its 15 coal and 7 gas and 4 coal & gas based power plants spread all over the country. NTPC has also entered in hydro-electric power generation, coal mining, coal washery, renovation and modernization of old plants in India as well as abroad, power distribution and consultancy for improvement of power plants .

Today, the country needs a 10 percent sustained growth in power generation to ensurethe momentum for a 7% overall growth in the economy. Recognizing this, NTPC hascommitted itself to achieving the status of a 30,144 MW company by the year2008 and40,000MW plus company by the year 2012 and power generating capacity additionprogramme of 51,000 MW (Including nuclear energy and non-conventional sources ofenergy) for the tenth plan.

Corporate Vision:

“A world class integrated power major, powering India’s growth, withincreasing global presence”

Core Values:

B-Business EthicsC-Customer FocusO-Organizational & Professional prideM-Mutual Respect and TrustI- Innovation & SpeedT-Total quality for Excellence

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RIHAND STPP:

Whole NTPC has been divided into five regions, named as Eastern region, Southern region, Northern region, Western region, and National Capital region. The foundation stone of Rihand super thermal power project was laid down on 9 February 1982. It is one of the NTPC’s best power plants, in the northern region constructed by Northern Engineering Industries (U.K.) and Bharat Heavy Electrical Ltd (INDIA). Rihand completes the power triangle with Singrauli STPS, Vidhychal STPS. It is situated in Bijpur village and in the industrial belt of the district- Sonebhadra of Uttar Pradesh, which is situated at the border of MP&UP.

This plant is situated at the south bank of Rihand Reservoir (Govind Ballabh Pant Sagar),made artificially. Its area is about 50X10 sq. Km. It is a large reservoir, having huge mass of water, from which five thermal power plants and one hydro power plant takes water for operation.

The capacity of RhSTPP, I stage of 2X500 MW and II stage of the 2X500 MW plant are inoperation. High Voltage Direct Current (HVDC) transmission system is the unique feature of this plant. It is used to transmit DC current from this plant to Delhi. It was made first in Asia. Rihand Nagar project is known for its implacable standard of quality & productivity, a hallmark of NTPC. Rihand has taken further strides to become a trendsetter in various facets of power generation, environment management, rehabilitation and resettlement, ash utilization, safety etc. Thus, Rihand is a self-contained power station with all necessary system.

NTPC Rihand is ISO 9001, ISO 9002, ISO 10000, ISO 14001 certified firm. It also meetsthe standard of SA 8000:2001.

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SALIENT FEATURE :

· Location: Bijpur village, Distt.- Sonebhadra (U.P.)

· Total capacity: 2000 MW, in 2 stages each of Proposed 2 x 500 MW each in 3rdstage

· Present capacity: 1000 MW (STAGE-1) by NEI 1000 MW (STAGE-2) by BHEL

1000MW(STAGE-3 progressing)

· Power Evacuation: +/-500Kv HVDC Bipolar line to Dadri (Delhi),400kV single circuit AC line to Shaktinagar, Allahabad and Kanpur

· Beneficiary States: UP, Haryana, Punjab, Rajasthan, JammuKashmir, Himachal Pradesh, Chandigarh and Delhi·

Major Resources:

COAL- (a) Source -Amlori mines.

(b) Maximum consumption- 43,300MT/Day

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for 3000 MW (E-Grade Coal).

(c) Mode of Transportation- MGR Rail Transportation System.

WATER - (a) Source- Rihand Reservoir.

Chimney: (i) 224.5mts (Stage 1)

(II) 275mts (Stage 2)

· Ash disposal: Ash slurry pumped to Ash dyke.

· commencement of work: 09/02/1983

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C&I

DEPARTMENT

REPORT

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RIHAND SUPER THERMAL POWER PROJECT NATIONAL THERMAL POWER CORPORATION LTD.

C&I CERTIFICATE TO WHOMSOEVER IT MAY CONCERN

This is to certify that Mr. GHANSHYAM a student of UNITED COLLEGE OF ENGINEERING & RESEARCH, ALLAHABAD (UP), has successfully completed a project on “ BASIC INSTRUMENTATION & PROCESS CONTROL LOOPS ” as a partial fulfillment towards completion of the Vocational Training at NTPC- Rihand from 14

th June to 29th July 2010.

Training Coordinator Training In-Charge

Mr. R.N. SINGH Mr. K.C. TRIPATHI

(Officer- HR-EDC) DGM (C&I)

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Project Guide

Mr. JITEN YADAV

Sr.Engineer (C&I)

Acknowlegements

I,GHANSHYAM, Vocational Trainee-2010 (C&I Dept.) of NTPC, would like to express my sincere thanks towards the following persons on completion of my project “BASIC INSTRUMENTATION & PROCESS CONTROL LOOPS” as part of the vocational trainingin this company.

First and foremost I thank my Training In-Charge Mr. K.C.TRIPATHIDGM (C&I Dept.) on a very professional as well as loving guidanceoffered during the entire course of the project. He provided thelearning material in a sequential and organized manner, based onthe interim progress of the project. He also arranged theinfrastructure required for learning and for the purpose of preparingand printing the report. Without his invaluable support andinstructions, the project definitely would not have materializedproperly.

Next, I would like to convey my gratitude towards all the members of

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C&I DEPT. NTPC-Rihand who offered advice and resources. Inparticular I would like to mention the names of Mr.JITEN YADAV (Sr.Engg.-C&I DEPT.) and Mr.SULTAN SETH (Sr.Engg.-C&I DEPT.)

Last but not the least, I thank God Almighty for his blessings.

GHANSHYAM

ELECTRONICS & INSTRUMENTATION ENGG.

UNITED COLLECE OF ENGINEERING & RESEARCH NAINI , ALLAHABAD

TABLE OF CONTENTSTABLE OF CONTENTS

Module 1Module 1:: OBJECTIVE OF ISTRUMENTATION &CONTROL………………………………10

Module 2: Module 2: INTRODUCTION TO INSTRUMENTATION EQUIPMENTS……………………………..11

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Module 3: Module 3: INTRODUCTION TO PROCESS CONTROL…………………………………..27

Module 4: Module 4: CONTROL MODES & NEGATIVE FEEDBACK CONTROLSHEMES...........30

Module 5: Module 5: MAJOR CONTROL LOOPS…………………34

i.i. FEED WATER CONTROL……………………………..34 ii. ii. FLUEL FLOW CONTROL…………………………….39

iii. iii. MILL OUTLET TEMPERATURE CONTROL……….42 iv.iv. PRIMARY AIR HEADER PRESSURECONTROL…. 46 v.v. FURNANCE DRAFT CONTROL……………………..49 vi.vi. AIR FLOW CONTROL…………………………………52

Conclusion: Conclusion: …………………………………………………..55…………………………………………………..55

MODULE 1

OBJECTIVE OF ISTRUMENTATION &

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CONTROL

▪ SAFE OPERATION OF PLANT

▪ LOWER COST OF GENERATION

▪ LONGEST EQUIPMENT LIFE

▪ MINIMUM ENVIRONMENTAL EFFECT

▪ MAXIMAM EFFICIENCY

▪ ENERGY CONSERVATION

MODULE 2

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INTRODUCTION TO INSTRUMENTATION EQUIPMENTS:

Instrumentation is the art of measuring the value of some plant parameter,pressure, flow, level or temperature to name a few and supplying a signalthat is proportional to the measured parameter. The output signals arestandard signal and can then be processed by other equipment to provideindication, alarms or automatic control. There are a number of standardsignals; however, those most common in a CANDU plant are the 4-20 mAelectronic signal and the 20-100 kPa pneumatic signal.

This section of the course is going to deal with the instrumentationequipment normal used to measure and provide signals. We will look atthe measurement of five parameters: pressure, flow, level, temperature,and neutron flux.

PRESSURE MEASUREMENT:

Pressure is probably one of the most commonly measured variables in thepower plant. It includes the measurement of steam pressure; feed waterpressure, condenser pressure, lubricating oil pressure and many more.Pressure is actually the measurement of force acting on area of surface. The object of pressure sensing is to produce a dial indication, controloperation or a standard (4 - 20 mA) electronic signal that represents thepressure in a process.

To accomplish this, most pressure sensors translate pressure into physicalmotion that is in proportion to the applied pressure. The most commonpressure sensors or primary pressure elements are described below.

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They include diaphragms, pressure bellows, bourdon tubes and pressurecapsules. With these pressure sensors, physical motion is proportional tothe applied pressure within the operating range.

Common Pressure Detectors:

Bourdon Tubes: Bourdon tubes are circular-shaped tubes with oval cross sections (refer to Figure 2). The pressure of the medium acts on the inside of the tube. The outward pressure on the oval cross section forces it to become rounded. Because of the curvature of the tube ring, the bourdon tube then bends as indicated in the direction of the arrow.

Due to their robust construction, bourdon are often used in harshenvironments and high pressures, but can also be used for very lowpressures; the response time however, is slower than the bellows ordiaphragm.

Bellows: Bellows type elements are constructed of tubular membranes that areconvoluted around the circumference (see Figure 3). The membrane isattached at one end to the source and at the other end to an indicatingdevice or instrument. The bellows element can provide a long range ofmotion (stroke) in the direction of the arrow when input pressure isapplied.

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Diaphragms: A diaphragm is a circular-shaped convoluted membrane that is attached to the pressure fixture around the circumference (refer to Figure 4). Thepressure medium is on one side and the indication medium is on the other.The deflection that is created by pressure in the vessel would be in thedirection of the arrow indicated.

Diaphragms provide fast acting and accurate pressure indication.However, the movement or stroke is not as large as the bellows

Capsules: There are two different devices that are referred to as capsule. The first is shown in figure 5. The pressure is applied to the inside of the capsule and if it is fixed only at the air inlet it can expand like a balloon. This arrangement is not much different from the diaphragm except that it expands both ways.

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Differential Pressure Transmitters: Figure 6 illustrates a typical DP transmitter. A differential pressure capsule is mounted inside a housing. One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted to outside the housing. A sealing mechanism is used where the force bar penetrates the housing and also acts as the pivot point for theforce bar. Provision is made in the housing for high- pressure fluid to beapplied on one side of the capsule and low-pressure fluid on the other.Any difference in pressure will cause the capsule to deflect and createmotion in the force bar. The top end of the force bar is then connected to aposition detector, which via an electronic system will produce a 4 - 20 masignal that is proportional to the force bar movement.Most pressure transmitters are built around the pressure capsule concept.They are usually capable of measuring differential pressure (that is, thedifference between a high pressure input and a low pressure input) andtherefore, are usually called DP transmitters or DP cells.

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Strain Gauges: The strain gauge is a device that can be affixed to the surface of an object to detect the force applied to the object. One form of the strain gauge is a metal wire of very small diameter that is attached to the surface of adevice being monitored.

For a metal, the electrical resistance will increase as the length of themetal increases or as the cross sectional diameter decreases. Strain gauges can be bonded to the surface of a pressure capsule or to a force bar positioned by the measuring element.

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FLOW MEASUREMENT: There are various methods used to measure the flow rate of steam, water, lubricants, air, etc., in a nuclear generating station. However, in this module will look at the most common, namely the DP cell type flow detector.

Flow Detectors: To measure the rate of flow by the differential pressure method, some form of restriction is placed in the pipeline to create a pressure drop .Since flow in the pipe must pass through a reduced area, the pressure before the restriction is higher than after or downstream. Such a reduction in pressure will cause an increase in the fluid velocity because the same amount of flow must take place before the restriction as after it. Velocity will vary directly with theflow and as the flow increases a greater pressure differential will occuracross the restriction. So by measuring the differential pressure across arestriction, one can measure the rate of flow.

Orifice Plate: The orifice plate is the most common form of restriction that is used in flow measurement.

With an orifice plate in the pipe work, static pressure increases slightlyupstream of the orifice (due to back pressure effect) and then decreasessharply as the flow passes through the orifice, reaching a minimum at apoint called the vena contracta where the velocity of the flow is at amaximum. Beyond this point, static pressure starts to recover as the flow

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slows down. However, with an orifice plate, static pressure downstream isalways considerably lower than the upstream pressure. In addition somepressure energy is converted to sound and heat due to friction andturbulence at the orifice plate. Figure 2 shows the pressure profile of anorifice plate installation.

Advantages and Disadvantages of Orifice Plates:

Advantages of orifice plates include:

• High differential pressure generated• Exhaustive data available• Low purchase price and installation cost• Easy replacement

Disadvantages include:

• High permanent pressure loss implies higher pumping cost.• Cannot be used on dirty fluids, slurries or wet steam as erosion will alter the differential pressure generated by the orifice plate.

Flow Nozzle: A flow nozzle is also called a half venture. Figure 7 shows a typical flow nozzle installation. The flow nozzle has a lower permanentpressure loss than an orifice plate (but higher than a venturi). The

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differential it generates is also lower than an orifice plate (but again higherthan the venturi tube). An example use of flow nozzles are the measurement of flowin the feed and bleed lines of the PHT system.

Elbow Taps: Centrifugal force generated by a fluid flowing through an elbow can be used to measure fluid flow. As fluid goes around an elbow, a high-pressure area appears on the outer face of the elbow. If a flow transmitter is used to sensethis high pressure and the lower pressure at the inner face of the elbow, flowrate can be measured. One use of elbow taps is the measurement of steam flow from the boilers,where the large volume of saturated steam at high pressure and temperaturecould cause an erosion problem for other primary devices.Another advantage is that the elbows are often already in the regular pipingconfiguration so no additional pressure loss is introduced.

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Pitot Tubes: Pitot tubes also utilize the principles captured in Bernoulli.s equation, to measure flow. Most pitot tubes actually consist of two tubes. One, the lowpressure tube measures the static pressure in the pipe. The second, the highpressure tube is inserted in the pipe in such a way that the flowing fluid is stopped in the tube. The pressure in the high-pressure tube will be the static pressure in the system plus a pressure dependant on the force requiredstopping the flow.

Annubar: An annubar is very similar to a pitot tube. The difference is that there is more than one hole into the pressure measuring chambers. The pressure inthe high-pressure chamber represents an average of the velocity across thepipe. Annubars are more accurate than pitots as they are not as position

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sensitive or as sensitive to the velocity profile of the fluid.

Square Root Extractor: The square root extractor is an electronic (or pneumatic) device that takes the square root of the signal from the flow transmitter and outputs a corresponding linear flow signal.The high and low-pressure taps of the primary device (orifice type shown) are fed by sensing lines to a differential pressure (D/P) cell. The output of the D/P cell acts on a pressure to milliamp transducer, which transmits a variable 4-20 ma signal. The D/P cell and transmitter are shown together as a flow transmitter (FT).Giving an indication of the flow rate (Q), is actually transmitting a signal proportional to the differential pressure (ΔP).

We can write this as: ΔP ∝ Q2

ORVolumetric Flow Rate = Q ∝ √ Δ P

A typical square root extractor installation is shown in Figure 13. Thissystem would produce a 4-20-ma signal that is linear with the flow rate.

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TEMPERATURE MEASUREMENT: 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.

Temperature Detectors:Resistance Temperature Detector (RTD): For most metals the change in electrical resistance is directly proportional to its change in temperature and is linear over a range of temperatures.They have positive temperature coefficient. To detect the small variations of resistance of the RTD, a temperaturetransmitter in the form of a Wheatstone bridge is generally used.

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In the case of a thermocouple, a problem arises when the RTD isinstalled some distance away from the transmitter. Since the connectingwires are long, resistance of the wires changes as ambient temperaturefluctuates. The variations in wire resistance would introduce an error in thetransmitter. To eliminate this problem, a three-wire RTD is used.

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

RTD Advantages and Disadvantages:

Advantages:

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• The response time compared to thermocouples is very fast . in theorder of fractions of a second.• An RTD will not experience drift problems because it is not selfpowered.• Within its range it is more accurate and has higher sensitivity than athermocouple.• In an installation where long leads are required, the RTD does notrequire special extension cable.• Unlike thermocouples, radioactive radiation (beta, gamma andneutrons) has minimal effect on RTDs since the parameter measuredis resistance, not voltage.Disadvantages:• Because the metal used for a RTD must be in its purest form, theyare much more expensive than thermocouples.• In general, an RTD is not capable of measuring as wide atemperature range as a thermocouple.• A power supply failure can cause erroneous readings• Small changes in resistance are being measured, thus all connectionsmust be tight and free of corrosion, which will create errors.• Among the many uses in a nuclear station, RTDs can be found in thereactor area temperature measurement and fuel channel coolanttemperature.

Thermocouple (T/C): A thermocouple consists of two pieces of dissimilar metals with their ends 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. Athermocouple is therefore said to be self-powered. Shown in Figure 3 is acompleted thermocouple circuit.

Circuit emf = Measurement emf - Reference emf

If circuit emf and reference emf are known, measurement emf can becalculated and the relative temperature determined.To convert the emf generated by a thermocouple to the standard 4-20 mA

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signal, a transmitter is needed. This kind of transmitter is called atemperature transmitter. Figure 4 shows a simplified temperaturetransmitter connection.

Advantages and Disadvantages:

Advantages:

• Thermocouples are used on most transformers. The hot junction isinside the transformer oil and the cold junction at the meter mountedon the outside. With this simple and rugged installation, the meterdirectly reads the temperature rise of oil above the ambienttemperature of the location.• In general, thermocouples are used exclusively around the turbinehall because of their rugged construction and low cost.• A thermocouple is capable of measuring a wider temperature rangethan an RTD.

Disadvantages:

• If the thermocouple is located some distance away from themeasuring device, expensive extension grade thermocouple wires orcompensating cables have to be used.• Thermocouples are not used in areas where high radiation fields arepresent (for example, in the reactor vault). Radioactive radiation(e.g., Beta radiation from neutron activation), will induce a voltagein the thermocouple wires. Since the signal from thermocouple isalso a voltage, the induced voltage will cause an error in thetemperature 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

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failure will of course cause faulty readings.

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).

The sensor is inserted into it and makes contact with the sealed end.

Minimizing the air space between the sensor and the well, however, can decrease this thermal lag.

Level Measurement: This technique obtains a level indicationindirectly by monitoring the pressure exerted by the height of the liquid inthe vessel. The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically, we have:

P = S⋅H Where

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P = Pressure (Pa) S = Weight density of the liquid (N/m3) = ρg H = Height of liquid column (m) ρ = Density (kg/m3) g = acceleration due to gravity (9.81 m/s2)The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant.When a DP transmitter is used for the purpose of measuring a level, it will be called a level transmitter.

Open Tank Measurement: The simplest application is the fluid level in an open tank. Figure 2 shows a typical open tank level measurement installation using a pressure capsule level transmitter.

If the tank is open to atmosphere, the high-pressure side of the leveltransmitter will be connected to the base of the tank while the low-pressureside will be vented to atmosphere. In this manner, the level transmitter actsas a simple pressure transmitter. We have:Phigh = Patm + S⋅H

Plow = Patm

Differential pressure ΔP = Phigh - Plow = S⋅H

The level transmitter can be calibrated to output 4 mA when the tank is at0% level and 20 mA when the tank is at 100% level.

Closed Tank Measurement: Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column

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becomes insignificant. For example, the measured hydrostatic head in a CANDU boiler may be only three meters (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter. This cover gas pressure is thus used as a back pressure or reference pressure on the LP side of the DP cell. One can also immediately see the need for the three-valve manifold to protect the DP cell against these pressures.The different arrangement of the sensing lines to the DP cell is indicated atypical closed tank application (figure 3).

Figure 3 shows a typical closed tank installation:

We have:

Phigh = Pgas + S⋅H

Plow = Pgas

ΔP = Phigh - Plow = S⋅H

The effect of the gas pressure is cancelled and only the pressure due to thehydrostatic head of the liquid is sensed. When the low-pressure impulse lineis connected directly to the gas phase above the liquid level, it is called a dryleg.

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MODULE 3

INTRODUCTION TO PROCESS CONTROL:

PROCESS CONTROL LOOP :

Process is the complex set of phenomenon that relate the manufacturing process in control loop. The various variables involve in control loops is called process variables like: LEVEL:(Transmitters, Switches), PRESSURE: (Transmitters, Switches) ,TEMPERATURE: (Thermocouples , RTD(Resistance Temperature Device)),FLOW: (Transmitters, Switches ),POSITION: (LVDT , Potentiometers),MACHINE SUPERVISION:(Vibration , Expantion ,Axial shift.) Etc.

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Overview:Control of the processes in the plant is an essential part of the plantoperation. There must be enough water in the boilers to act as a heatsink for the reactor but there must not be water flowing out the top ofthe boilers towards the turbine. The level of the boiler must be keptwithin a certain range. The heat transport pressure is another criticalparameter that must be controlled. If it is too high the system willburst, if it is too low the water will boil. Either condition impairs theability of the heat transport system to cool the fuel.

In this section we will look at the very basics of control. We willexamine the fundamental control building blocks of proportional,integral and differential and their application to some simple systems.

Feedback Control

This concept justifies the use of the word negative in three ways:

• The negative aspect of feeding the measured signal backwardsfrom the output to the input of the system. (Actual definition of

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negative feedback control).

• The control correction must be negative in that a correctionrather than a compounding of error must occur.

• The fact that an error must occur before a correction can takeplace, i.e., retrospective or negative control action.

Feedforward Control

If we wish to control our process without an error first occurring, wemust base our control on correction of the disturbances, which willeventually, cause a process error. This is termed feedforward control.Feedforward control is rarely if ever used on its own but is used inconjunction with feedback control to improve the response of control toprocess disturbances.

Summary

• Controlled Variable . output quantity of system (Level,Temperature, etc.).

• Manipulated Variable . means of maintaining controlled variableat the setpoint.

• Error signal . equals the difference between the setpoint and themeasurement. (e = SP . M).

• Setpoint . desired process level. (SP)

• Measurement . actual process level. (M)

• Closed Loop . automatic control.

• Open Loop . manual control.

• Feedback control is error correction following a disturbance.

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• Feedforward control is control of disturbances, which couldcause a process error.

MODULE 4

CONTROL MODES & NEGATIVE FEEDBACK CONTROL SHEMES

some of the possible combinations of control modes are:

Proportional only,

Proportional plus reset (integral) P + I,

Proportional plus derivative (rate) P + D.

It is also possible to use a combination of all three-control modes,Proportional plus Integral plus Derivative (P + I + D).

At a glance proportional only does not appear very attractive . we willget an offset as the result of a disturbance and invariably we wish tocontrol to a fixed setpoint.

An application of proportional only control in a CANDU system is inthe liquid zone level control system. The reason that straight proportional control can be used here is that the controlled variable is not level but neutron flux. The manipulated variable is the water level; therefore offset is not important as the level is manipulated to provide the required neutron flux.

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In general it can be said that the vast majority of control systems (probably greater than 90%) will incorporate proportional plus integral modes. (We usually want to control to a fixed setpoint.) Flow control systems will invariably have P + I control.

Derivative control will generally be limited to large sluggish systems with long inherent control time delays, (for example, that shown in Figure 18.). A good general example is the heat exchanger. The thermal interchange process is often slow and the temperature sensor is usually installed in a thermal well, which further slows the control signal response. Frequently heat exchanger temperature controllers will incorporate three-mode

control (P + I + D).

TYPICAL NEGATIVE FEEDBACK CONTROL SCHEMES

(I) Level ControlIn general we can divide level measurement into three types:(i) Open Tanks(ii) Closed TanksBubbler Systems (Open or Closed Tanks)If a differential pressure transmitter is used as a level detector, the lowpressureport will be vented to atmosphere in an open tank application. In a closed tank, where there is often a gas phase at pressure above the liquid, the low-pressure port will be taken to the top of the tank. Any gas pressure will then be equally sensed by the high and low sides and thus cancelled. Remember the closed tank installation will have either a wet or dry leg on the low-pressure sides.

Open Tank InstallationAssuming the control valve is on the inflow, the best failure mode for the valve would be to fail closed, i.e., Air to Open (A/O) valve. The pressure sensed at the base of the tank on a falling level will decrease, i.e., controller input. The valve must open more, to replenish the tank, requiring an increasing signal. The controller must be reverse acting and will usually have P + I modes. The system is shown in Figure 19 If it is necessary to mount the valve in the outflow, the best failure mode would probably be to fail open (A/C). This valve action would require an increasing signal to halt a falling tank level, again a reverse acting (P + I)controller is necessary. The same reasoning would apply to closed tank or bubbler systems, the only difference being in the sensing method employed. Remember control

modes use of derivative action on large, slow, systems.

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(ii)Flow ControlA typical flow control system requires some form of restriction to provide a pressure differential proportional to flow (e.g. orifice plate) plus a square root extractor to provide a linear signal. The controller action depends upon the choice of control valve. If an air to open valve is chosen then controller action should be reverse, as an increase in flow must be countered by a decrease in valve opening. For an air to close valve the action must of course be direct. The general format is shown in Figure 20.

The control modes will be proportional plus integral (never use derivativeon a flow control loop).

(iii)Pessure controlThe control of pressure in, say, a pressure vessel, is generally achieved in one of three ways.1. Variable Feed with Constant Bleed2. Constant Feed with Variable Bleed3. Variable Feed and BleedConsider first Variable Feed and Constant Bleed (Figure 21). The feed valve action is air to close (A/C). Increasing pressure will require an increasing valve signal to throttle the supply. The (P + I) controller is direct acting. For a variable bleed application the control valve will be transferred to the bleed application the control valve will be transferred to the bleed line and will need to be A/O if a direct acting controller is used.

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\

For variable feed and bleed we can use a split range control scheme (one controller driving two valves). This is shown in Figure 22. When at the setpoint we require feed to equal bleed. If pressure increases we require less feed action and more bleed action and vice versa. The valve actions must therefore be opposite, say feed valve A/C and bleed valve A/O. On increasing pressure the direct acting controller will supply a larger signal to the feed valve (closing it) and to the bleed valve (opening it). Pressure should thus be maintained at the setpoint with proportional plus integral control.

(iv)Temperature ControlThe general problem with temperature control is the slowness of response. For this reason the use of derivative action is fairly standard. Figure 23shows a representative heat exchanger, which cools hot bleed with coldservice water. The choice of control valve would probably be air to close, i.e., fail open, to give maximum cooling in the event of a air supply failure to the valve.

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An increase, say, in bleed temperature requires a larger valve opening, i.e., smaller valve signal. A reverse acting controller is required. Three mode, P + I + D, control is fairly usual.

MODULE 5

MAJOR CONTROL LOOPS:

1.FEED WATER CONTROL (DRUM LEVEL CONTROL):

The objective of this control system is to maintain the drum level to the normal water level of the drum at all loads. At lower loads (less than 30% MCR), the start up feed control valve will be used as final control element and at higher loads, speed control of Boiler Feed water Pumps (BFPs) will be used. Drum level is measured by three transmitters

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through temperature compensated constant head unit. The pressure compensated drum level signal may be selectedby Mid Value Auto Selection (MVAS) circuit for control.

Low load: The drum level measured signal is compared with the drum level set point. The error signal will have a proportional, integral and differential action in the single element controller. This controller output will be the position demand signal for the start up feed control valve. Auto/manual station is provided for auto/manual selection and operation. Position indicator is provided for the start up feed control valve.

High load: At higher loads the start up control valve shall be closed. The steam flow shall be measured. In order to prevent sudden response due to drum swell and shrink on load change, a time lag unit shall be included in the steam flow signal. The temperature compensated feed water flow signal is computed by adding feed water flow at economiser inlet and super heater spray water flow. The error signal produced between drum level measured signal and drum level set point shall have proportional, integral and differential action in the three element drum level controller. This will be added with steam flow signal which is the feed water flow demand signal (set point for feed water flow). This will be compared with the feed water flow in the feed water controller. Deviation if any will have a proportional and integral action in the feed water controller. This controller output will be the desired speed signal for the individual Boiler Feed water Pump(BFP) speed control system. Auto/manual station is provided for auto/manual selection and operation. Position indicator is provided for the motor driven BFP hydraulic coupling scoop position indication and speed indicator is provided for turbine driveBFP speed indication.

CONTROL LOGIC:

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DRUM LEVEL VERY HIGH: LOGIC: 2 out of 3 logic derived from hydrastep-left-1, hydrastep-right-1 and drum level transmitter > 250 mm

PURPOSE: To prevent water carry over to superheater headers andsubsequently to turbine

PROTECTION DELAY: 10 sec.

PROTECTION CHECK: Actual during cold start condition (increase feed water flow slowly until level gradually reaches trip limit)

DRUM LEVEL VERY LOW:

LOGIC: 2 out of 3 logic derived from hydrastep-left-2, hydrastep-right-2 and drum level transmitter < -375 mm

PURPOSE: To prevent BCW pumps starvation To prevent waterwall tubes starvation_

PROTECTION DELAY: 5 sec.

PROTECTION CHECK: Actual (decrease feed water flow slowly until level gradually reaches trip limit).

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Note: In Drum Level Control • This FEED WATER CONTROL is called three element control and the cause Is:

The control of make-up feed water to the boiler is a very critical aspect of boiler operation. The boiler requires that the water level in the steam drum be within a specific range to ensure proper operation. If the level is too low, circulation problems can develop and lead to localized tube failures. If the level is too high, separation of steam and water can be impaired and allow water to carry-over with the steam.A boiler will typically use one of three following feed water control schemes to maintain the specified water level.

In Single Element Control: Drum level as the single process variable.

In Two Element Control: Uses two independent process variables i.e. the drum water level and the steam flow out of the boiler. In Three element control: This control system uses three independent process variables tomaintain the water level: the drum water level, the steam flow output, and the feed water flow input.The basic idea behind a three element controller is to modulate the feed water flowbased on a comparison between the measure steam flow and feed water flow rates. In order to account for the errors resulting from the use of only the steam and feed water flow, the drum level signal is also included in the system. The drum level signal will trimthe output signal up or down in order to bring the drum level to correct value.

• Sensing level: 3+3 DP transmitters, median L&R, then average select.

• Control element: TDBFP A&B – Speed governing (electrohydraulic)

• Control action: Reverse

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• Operating Range: -50 to 50 mmwc

• Operating Limits (VL/L/H/VH): -375 / -175 / 125 / 250

• Also Affected by: BCW pumps start/stop (controlled circulation)\

BFP recirculation valves operation

Furnace fuel firing disturbances

Turbine Load / steam flow disturbances

MDBFP – Scoop (motorised)

• Controller: PID Controller (in proportional integral mode)

ADDITIONAL:

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2.FUEL FLOW CONTROL

Fuel flow demand from combustion control and air flow signal from air flow control corrected for fuel air ratio are compared and the lower is selected for the set point of the fuel flow controller. (lead-lag system). This is to ensure that under any circumstance the fuel flow should be lesser than the air flow. Fuel flow is measured by adding the feed signal of the feeders (or mills) in service and the heavy oil flow corrected for calorific value. The feeder speed/rate measured signal is hooked up to the control after a delay to suit the process lag. The actual fuel flow signal is compared with the developed set point signal above and any deviation will have proportional and integral action. The controller's output signal is the position demand signal for feeder speed regulating device. Bias unit is provided to modify the signal whenever required. An auto/manual station is provided for each feeder.

To ensure air rich furnace at all times, a maximum deviation limit system (MDL) is used. i.e. Whenever the fuel flow is more than the air flow this will automatically reduce the fuel flow and increase air flow to a safe value and both the air flow and fuel flow control is transferred to manual.

The set point of the fuel flow controller i.e. obtained on comparing the fuel flow demand from combustion control and air flow signal from air flow control corrected for fuel air ratio. This is to ensure that under any circumstance the fuel flow should be lesser than the air flow here act as compensator (lead-lag system).

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CONTROL LOGIC:

NOTES: In fuel flow control :

The feeder rate measure signal shall be delayed by an adjustable time (0-60 secs) to suit process lag.

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The gain of fuel controller shall be automatically changed according to the number of mills in service.

Fuel flow should not be allowed to on auto unless air flow control is in auto.

Here maximum deviation unit or selector is used i.e. (MDL), the main purpose of which is that whenever the fuel flow then it will automatically increase the air flow to safe value while the fuel may be either automatic or manual. If air flow or fuel flow are in automatic prior to the MDL action, they are to be rejected to manual by the MDL action.

• Sensing coal flow: 2x10 feed rate signals, average select

• Control element: Feeder feed rate (gravimetric – with eddy current clutch)

• Operating Range: 0 to 375 TPH

• Operating Limits (VL/L/H/VH): - / - / 375 / 389

• Control action: Reverse

• Controller: PID Controller (in proportional integral mode)

ADDITONAL:

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3.MILL OUTLET TEMPERATURE AND AIR FLOW CONTROL :

The objective of this control system is to adjust the mill air flow according to the feeder speed and to maintain the mill outlet temperature at the constant set value.

Mill air flow is maintained by adjusting the hot air regulating damper while the mill outlet temperature is maintained constant by adjusting the cold air regulating damper. The temperature compensated mill air flow is linearised by the square root extractor. This air flow signal is compared with variable air flow set point as a function of feeder speed. Any error between these two signals will have proportional plus integral action. Rate of change of fuel demand signal is added to provide feed forward feature. An auto/manual station with position indicator is provided.

Mill outlet temperature is measured using a thermocouple with tungsten carbide thermowell to avoid erosion. The mill outlet temperature is compared with constant set point and error will have proportional, integral and derivative action. An auto/manual station with position indicator, is provided.

Mill outlet temperature control and mill air flow control involve two dampers called hot air damper and cold air damper. Hot air damper also provides the hot air to remove the moisture from the coal so that they can easily cross the path in between mill and furnace. In the same

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manner cold air damper also maintain the mill temperature so that no coal burning occur in mill section.

CONTROL LOGIC:

NOTES: In mill air flow control and outlet temperature control :

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: MILL DAMPER CONTROLS – AIR FLOW

• Sensing air flow: 2 DP transmitters , average select & compensation

• Control element : Mill hot air damper (pneumatic)

• Operating Range: 70 to 100 TPH

• Operating Limits (VL/L/H/VH): 50 / 70 / 110 / 125

• Also Affected by: Mill cold air damper regulation

Mill hot & cold air gate operation

Hot PA header pressure regulation • Control action: Reverse

• Controller: PID controller

: MILL DAMPER CONTROLS – OUTLET TEMP

• Sensing temp: 2 thermocouples (K – Type) , average select

• Control element: Mill cold air damper (pneumatic)

• Operating Range: 80 to 90 Deg C

• Operating Limits (VL/L/H/VH): 65 / 70 / 95 / 100

• Also Affected by: Mill hot air damper regulation

Mill hot & cold air gate operation

Mill feeder coal flow regulation

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• Control action: Direct

• Controller: PID controller

ADDITONAL:

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4.PRIMARY AIR HEADER PRESSURE CONTROL

The main objective of this control is to adjust the primary air header pressure according to the feeder speed. That is, out of all the feeders, the feeder speed which is higher than that of others is considered as set value for this control.

Primary air header pressure is measured with three transmitters. One signal is selected by mid value auto selection circuit for control. The measured signal is compared with the selected feeder speed signal through a high signal selector to maintain the minimum header pressure. Deviation if any will have proportional and integral action. Separate auto/manual station and position indicator are provided for each Primary Air(PA) fan regulating device.

To have equal loading of two running PA fans, the PA fans motor current is measured,averaged and compared. The difference is used for taking corrective action. The corrective signal is used to position the PA fan regulating unit.

Refer the 'notes' in the control scheme for the interlocks.

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CONTROL LOGIC:

NOTES: In primary air header pressure control :

• Sensing hot PA header pressure: 3 pressure transmitters , median select

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• Control element: PA Fan-A&B Blade Pitch (thyristor & servo)

• Operating Range: 775 to 825 mmwc

• Operating Limits (VL/L/H/VH): 500 / 625 / 950 / 1000

• Also Affected by: Mill hot & cold air dampers regulation Mill hot & cold air gates operation

• Control action: Reverse

• Controller: PID controller

ADDITONAL:

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5.FURNACE DRAFT CONTROL :

The main objective of the control is to maintain the furnace pressure constant at the desired set value at all loads. This is achieved by changing the flow of flue gas by modulating the inlet guide vane or inlet damper and varying the speed of the ID fan by variable frequency drive system. Furnace pressure is measured by three transmitters. One signal is selected by mid value auto selection circuit for control. Excessive furnace pressure is monitored for directional block on Induced Draft(ID) and Forced Draft(FD) fans. Furnace pressure is compared with set point and error, will have proportional and integral action. Fuel demand signal is added as a feed forward feature. Master Fuel Trip(MFT) feed forward feature is provided to minimize negative furnace pressure excursion. Separate auto/manual station, and the position indicatorfor each ID fan regulating device is provided.

To have equal loading of the ID fans each ID fan motor current (sum of channel 1 and channel 2 current) is measured averaged and compared. The difference is used for taking corrective action. The corrected signal is used to position the ID fan inlet damper. The ID fan inlet damper positions between a maximum and a minimum position limit for optimised control action. If ID fan position goes outside these limits, an error signal goes to a controller, whose output is used to vary fan speed to bring back the inlet damper within the set limits.

Separate auto/Manual station and position/speed indicator are provided for each ID fanregulating device(damper/VFD).

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CONTROL LOGIC:

NOTES: In furnance draft control:

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• Sensing furnace pressure: 3 pressure, transmitters , median select

• Control element: ID Fan-A&B Inlet Guide Vanes (motorised) at startup

ID Fan-A&B Variable Frequency Drives (thyristor) on Load

• Operating Range: - 5 to -10 mmwc

• Operating Limits (VL/L/H/VH): -175 / -100 / +75 / +150

• Also Affected by: SADC dampers regulation FD fan blades regulation Mill air dampers regulation PA fan blades regulation Fuel firing regulation Flame disturbances

• Control action: Direct

• Controller: PID controller

ADDITONAL:

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6.AIR FLOW CONTROL:

The secondary air flow is measured at left and right side of the secondary air ducts to wind box by means of aerofoils. Each flow will have temperature compensation. The flow is linearised by means of square root extractors. The total PA flow measured for each mill in service , is added to obtain total air flow to the boiler. This signal is compared with the developed set point. The air flow demand from coordinated control and actual fuel flow whichever is high (lead lag system) is selected to ensure enriched combustion air. The oxygen in the flue gas at the inlet of AH is measured as primary or redundant. Transfer switch can be selected for either average value or individual value. This signal is compared with excess air set point and any error will have proportional and integral action to have better combustion efficiency. High/low limiters are used to limit the value in case the oxygen analyser is out of service. Under any circumstance the air flow should not be less than 30% MCR flow. This signal is the developed set point and the air flow signal will have proportional and integral action in the air flow controller.

This position demand signal will be selected to the corresponding FD fans in service through' auto/manual station. To have equal loading of FD fans the FD fan motors current is measured. The difference is used for taking corrective action. The corrected signal is used to position the FD fan regulating damper. Necessary interlock from FSSS, Boiler auxiliaries interlock system Maximum Deviation Limit (MDL) etc. are provided. Separate auto/manual station and position indicator for each FD fan regulating device are provided.

CONTROL LOGIC:

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Notes: In air flow control:

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• Sensing (for secondary air flow): 2+2 DP transmitters, average select & compensation

• Control element: FD fans blade pitch (thyristor & servo)

• Operating Range: 630 to 1950 TPH

• Operating Limits (VL/L/H/VH): 540 / 600 / 2000 / 2100

• Also Affected by: SADC dampers regulation

• Control action: Reverse

• Controller: PID Controller

CONCLUSION:

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A PROCESS CONTROL SYSTEM CONTROLS A LARGE SCALE PLANT SUCH AS THERMAL POWER PLANT. THIS PROCESS CONTROL SYSTEM INCLUDES A TARGET SETTING UNIT FOR SETTING AN OPERATION TARGET, A CONTROL UNIT FOR RECEIVING A SIGNAL INDICATING THE OPERATION TARGET AND FOR OUTPUTTING A CONTROLLED VARIABLE TO OPERATE THE PROCESS, AN EVALUATION UNIT FOR QUANTITATIVELY EVALUATING OPERATION CHARACTERISTICS CORRESPONDING TO THE OPERATION TARGET OF THE PROCESS OPERATED ON THE BASIS OF A SIGNAL INDICATING THE CONTROLLED VARIABLE SUPPLIED FROM THE CONTROL UNIT.

A MODIFICATION UNIT FOR EXTRACTING AN OPTIMUM OPERATION PROCESS QUALITATIVELY SQUARING OR CONFORMING WITH THE EVALUATED VALUE DERIVED BY THE EVALUATION UNIT OUT OF A MODIFICATION RULE PREDETERMINING OPERATION UNIT IN QUALITATIVE RELATION BETWEEN THE OPERATION CHARACTERISTICS AND THE OPERATION TARGET OF THE PROCESS AND FOR DETERMINING THE MODIFICATION RATE OF THE CONTROL UNIT.

A STORAGE UNIT HAVING A MODEL OF A NEURAL NETWORK FOR STORING A RELATION BETWEEN THE OPERATION TARGET AND THE MODIFICATION RATE DERIVED BY THE MODIFICATION UNIT AS A CONNECTION STATE WITHIN A CIRCUIT.

A LEARNING UNIT FOR MAKING THE MODEL OF THE NEURAL NETWORK LEARNS THE RELATION BETWEEN THE OPERATION TARGET AND THE MODIFICATION RATE.

PROCESS CONTROL LOOPS REPRESENT THE SQUENCE IN WHIT ALL SINGALS ARE PROCESED AND THE WAY THE CONTROL ACTION TAKE PLACE AS PER THE NEED OF PROCESS UNDER CONTROL. BUT TO END ON A BRIGHTER NOTE, IT CREATES FRESH CHALLENGES FOR C&I ENGINEERS IN NTPC TO EXERCISE THEIR BRAINS AND EXTRACT THE BEST PERFORMANCE OUT OF THE SYSTEM.

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