CONCEPT OF INSTRUMENTATIONIN THERMAL POWER STATION

Prepared by :- Unknown

*Instruments should be independent for their working

*The total instrumentation should be interdependent to each other in assessing the process conditions.

*Instrumentation should be sufficient to provide adequate information to the operators for

*Cold start of the unit

*Warm/hot start of the unit

*Shut down, both planned and emergency shut down

Thermal Power Stations employ a great number of equipment performing number of complex processes, the ultimate aim being the conversion of chemical energy into Electricity. In order to have stable generating conditions, always a balance is maintained that Heat input = Electricity output + losses

But this balance is frequently disturbed due to *grid troubles external to the process and machines, * the troubles in the process itself * the troubles in the equipments. When the balance is disturbed, all the process variables deviate from their normal values.

Which calls for the following:*Instruments: To measure and indicate the amount of deviations.*Automatic Control: To correct the deviation and bring back to normalcy.*Annunciation : To warn about the excessive deviations if any.*Protection: To isolate the equipments or process from dangerous operating conditions caused due to such excessive deviations.

The proportionate cost of instrumentation during seventies was about 2.3 to 2.5% of the total cost of boiler, turbine and their Auxiliaries for the unit sizes up to 60/100 M.W. This has become about 7% for 210 M.W. and is reaching about 10-12% may even higher in the near future for the same capacity units.

increase in instrumentation cost is due to

*Increase in unit capacity, operating the unit at higher parameter for economic reasons.

*New inventions, improvements, modernization of instruments and equipments.*Expected change in the duty cycles of the boiler and turbine facilitating two-shift operation, quick run up etc.*Improved awareness among the personnel about the utility of the instruments.

INDICATORS*Local: indicators are self-contained and self-operative and are mounted on the site. *Remote: used for telemetering purposes and mounted in the centralised control room or control penal.

**The indicators both local and remote are sometimes provided with signaling contacts wherever required.

The remote indicators may be electrical, electronics, and pneumatic or hydraulic based for their operation and accordingly they are named. The indicators can be classified as analogue or digital on the basis of final reading.

Indicators are available for single & multi point measuring systems.

RECORDERS*Recorders are necessary wherever the operating history is required for analysing the trends and for any future case studies or efficiency purposes.*single point or multipoint Multipoint :multipoint continuous recorders or multipoint dot recorders.(dot recorders select the point one after the other in sequence)

PRESENTATION OF INFORMATIONS

information measured and received from the various parts of the plant/process are presented in three categories.*Vital information's*becomes vital whenever some sections of the plant start malfunctioning.*information required occasionally to efficiency engineers, which is given by recorders mounted on back panels or local panels.

Vital informations

which is required by operators at all times for the safe operation of the plant. The information is presented through single point indicators/recorders, placed on the front panels. Main steam pressure, temperature, condenser level, vacuum, drum level, furnace pressure etc. are some such parameters.

The second group of information is generally not vital of the plant. But becomes vital whenever some sections of the plant start malfunctioning. Such needs are met through multipoint indicators/recorders placed in the front panels. Eg: Temperature and draft across the flue gas path, bearing temperatures of the motors fans etc.

The last group of information is not required by the operators but for the occasional need of the efficiency engineers. These informations are given by recorders mounted on back panels or local panels. Eg: D.M. make up quantity, fuel oil flow quantities etc.

CODING OF INSTRUMENTSIn order to distinguish the parameters required from the other instantly, a shape coding of instrument face is being adopted. A general approach could be as below:

Level instruments - Horizontal edgewiseTemperature inst - Horizontal edgewisePressure inst - Circular

ARRANGEMENT OF INSTRUMENTS

*Master Panel arrangement: In this arrangement, instruments measuring important parameters are provided in one panel. *All instruments in this panel will be of circular shape with normal rated value marked at 12oclock position. * This will help the operators to quickly notice any deviation from the rated values.

SELECTION OF INSTRUMENTS*Instrument engineers are required to work in close association with the system design as well as equipment design engineers in selecting instruments and sensing systems.

*The instrument and system design engineers decide the location for the measurement of various parameters such as level, pressure, flow, differential pressure, temperature etc. based on the system design and layout conditions.

factors influences to select the appropriate instruments:*Range of measurement*Required accuracy of measurement*The form of final data display required*Process media*Cost*Calibration & repair facilities available *Layout restrictions*Maintenance requirements/availability*availability of skill persons.

TEMPERATURE MEASURIINSTRUMENTS

* Accurate measurement of temperature is required to assess the material fatigue, heat balance, heat transfer etc.

* The measurement ranges from ambient temperature viz. air at inlet of FD fan to 1300oC to 1400oC inside the furnace zone.* Temperature measurement is to be made for water /steam, oil (fuel oil and lubricating oil), air, flue gases, hydrogen gas, bearing babbit metal, turbine casing, generator winding and cores, S.H. tube metal etc.

*Filled system thermometry such as mercury in glass, mercury in steel, vapour filled, or bimetallic thermometers used for local indication of temperature.

*The selection of thermometer depends upon the range of the temperature to be measured.

*These instruments are available with electrical contacts for setting up annunciation and protection system wherever required.

*Resistance thermometers made up of platinum, copper resistance type, secondary instruments used in conjunction are cross coil indicators or electronic bridges.

*Resistance thermometers are generally used up to 250oC.*Above 250oC, thermocouples are used as primary sensors. *The secondary instruments for thermocouple sensors are pyrometric millivolt meters or electronic potentiometers.

*Null balance electronic bridges are used for the very accurate measurement of millivolts from thermocouples ,either as indicators, or indicator cum recorders with alarm /protection contacts and with remote transmission facilities.

PRESSURE MEASURING INSTRUMENTS

*The pressure measurement in Thermal Power Station ranges from less than atmospheric at condenser to hydraulic test pressure of boiler.*Pressure of steam/water, lubricating oil, fuel oil, air flue gases, hydrogen etc are measured.*For local indication of pressure and differential pressure, bourdon tube and diaphragm type gauges or liquid manometers & for remote measurement transmitter, either/electric/electronic or pneumatic coupled with a secondary instrument are used.

Transmitters

*mechanical movement of sensing elements such as bourdon, bellows, diaphragm etc. are employed.*pressure causes an displacement/movement in the sensor which causes pneumatic or electrical out put . *which is measured by the secondary instruments,such as indicators or recorders. Some may incorporate signaling contacts.

LEVEL MEASUREMENT*is generally carried out as differential pressure measurement. *level measurement in open tanks such as D.M.water storage and fuel oil and lube oil tanks and in closed tanks such as deaerator, condenser hot well, boiler drum, L.P. & H.P. heaters etc.are made.*Gauge glasses and floats are used for local indication and the transmitters with the secondary instruments for remote level measurements.

*boiler drum poses many problems because of varying pressures and temperatures, which continuously change the density of media calls for corrections to be made in order to get correct levels.

* Hydrastep improves the accuracy and reliability of the drum level measurement.

* The nucleonic level gauges or the capacitance and resistance type sensors serve for continuous level measurement of the raw/pulverised coalbunkers and dust collectors hoppers.

FLOW MEASUREMENT

*Flow measurements of solids, liquids and gases are required in Thermal Power Stations for carrying out safe and optimum operation. *liquid flow measurements can be made within reasonable accuracy. *steam flow measurement requires density correction under varying pressures. *The air and flue gas flow measurements suffer accuracy and reliability due to variation in pressure, temperature, duct leakage, dust accumulation etc.

*flow measurements are based on inferential principles carried out by placing suitable throttling devices in the flow path of the fluids ie pipes/ducts.. *The throttling devices are suitably selected depending upon the media, flow quantity etc. among orifice, venturi, flow nozzle, dall tube etc. *The differential pressure developed across such sensing devices is proportional to the square of the flow quantity.

ANALYTICAL INSTRUMENTS Feed water quality assessed by conductivity, pH, dissolved oxygen, and sodium parameters, steam quality by conductivity, silica and pH analysers. The combustion quality is assessed by the percentage of oxygen, carbon monoxide or carbon dioxide in the flue gases. The purity of hydrogen inside the generator housing is measured by utilising the thermal conducting capacity of the hydrogen gas.

*The water and steam purity is measured as the electrolytic conductivity by electronic bridge method & conductivity cell dipped into the medium.*The percentage volume of oxygen in combustion gases is measured by utilising the paramagnetic properties of oxygen,& the carbon monoxide percentage by the Absorption of Electromagnetic radiation principle*Recent developments are on line in situ instruments for these two parameters where the problem of sampling is dispensed with.

TURBOVISORY INSTRUMENTS The turbovisory instruments have become very important for modern day turbines where the materials have been stressed nearer to the yield points and the internal clearances have become minimum.

Shaft eccentricity, vibration (both shaft and bearing pedestal), differential expansion of shaft and cylinders, over all thermalexpansion of the cylinder, speed, & axial shift etc. are some of the turbovisory measurements. all These measurements are interrelated and interdependent.

Temperature measurement locations*Steam temperature at boiler outlet, super heater stages, steam legs before ESVS, IVs, and -H.P. cylinder outlet, hot reheat and exhaust hood temperatures. *Temperature of condensate/feed water along the flow path from condenser* Metal temperature of turbine casing ,super-heaters and reheaters . *indicators/indicator-cum-recorders with alarm and protection facilities in control room&with multi-point selection as per requirement.

*Flue gas temperature in various zones of boiler-indicator and indicator cum recorder in control room.*Air temperature at inlet and outlet of air preheater.*Turbine bearing oil drain temperature-indicator cum recorder in U.C.B.*Generator winding and core temperature-indicator cum recorders in control room.*Temperature of auxiliary equipment bearings such as mill ID, FD and PA fans etc. indicator cum recorder in U.C.B.

PRESSURE MEASURING LOCATIONS

Condensate pressure after condensate pumps and before the ejectors, - indicator in U.C.B. Deaerator pressure - indicator cum recorder in U.C.B. with electrical contacts for interlocking facilities.Feed water pressure after feed pumps individual indicators for each pump.Feed water pressure before and after feed regulating stations-indicators in U.C.B.Drum pressure-indicator cum recorders in U.C.B. with alarm signaling

*Super-heater steam pressure at boiler outlet 2 Nos. indicators one for each side in U.C.B. and at local with alarm protection facilities. Measurement is done at the outlet of superheater and before stop valves. *Steam pressure-1 No. indicator cum recorder in one of the lines before turbine stop valve in U.C.B. *Steam pressure at emergency stop valves and IVs. *Steam pressure after control valves indicators in local panel for each valve. *Steam pressure at Curtis wheel-indicator cum recorder in U.C.B. with alarm contacts.

*Steam pressure of H.P. turbine exhaust indicator in U.C.B. for cold reheat steam. *Vacuum in condenser indicator cum recorder in U.C.B. with alarm facilities and separate vacuum relay for protection. *Hot reheat pressure indicator in U.C.B with signaling contacts. *Steam pressure at the exhaust of I.P. cylinder-indicators in local panel. *Heavy oil pressure-indicators in U.C.B. with signaling contacts. Measurement is made before and after pressure regulating valves.

*Light warm up oil pressure Measurement is made before and after the flow control valves.indicators in U.C.B with signaling contacts. *Igniter oil pressure-indicator in U.C.B. *Governing oil pressure-indicator in U.C.B. with signaling contacts. *Lubricating oil pressure Measurement is made after oil coolers -indicator in U.C.B.. *Air pressure-indicators in U.C.B. for secondary air, primary air measured before and after air heaters. *Wind box pressure indicators in U.C.B.

*Furnace draft-indicators and recorders in U.C.B. (Measurement is made averaging left and right side drafts).

*Flue gas draft before and after economiser-indicators in U.C.B.

*Draft after air-heaters indicators in U.C.B.

*ID fan suction-indicators in U.C..B

LEVEL MEASUREMENT*Drum level-indicators and indicators cum recorders (total 3 Nos. from different tapping) in U.C.B. with alarm and protection facilities.

*Local gauge glass

*Remote indirect measurement

*Drip level in H.P. and LP heaters-indicators in U.C.B. with alarm and protection facilities.

*Condensate level in condenser-indicator in U.C.B with alarm facilities.

*Deaerator level-indicator in U.C.B. with signaling contacts for alarm and protection.

*The various storage tank levels such as D.M. water, fuel oil, lubricating oil etc. are measured by local direct gauge glasses.

FLOW*Condensate flow to deaerator-indicator/recorder in U.C.B. with integrator unit for totalizing in two locations (i) between air ejectors and L.P. heater (ii) between the final L.P heater and deaerator.*Feed water flow indicator/recorder in U.C.B. with integrator unit. Measurement is made between final H.P. heater and feed regulating valves.*Super heated steam flow 2 Nos. indicators cum recorders one for each pipe with integrator unit in U.C.B.*Re-heater steam flow-2 Nos. indicators cum recorders one for each side of the boiler. Measurement is made at the inlet to reheater.

*Air flow-2 Nos. indicator cum recorders one for each FD fan in U.C.B. Measurement is made at the discharge of the FD fans.

*The fuel oil flow to the unit is given by two indicators cum recorders in U.C.B. one measuring the oil in the incoming line and the other in the return line.

Normally the coal flow is measured for the whole station by the belt conveyor weighers.

AUTOMATIC COTNROL Boiler control loops :*Steam pressure always called as Boiler Master Control.*Combustion control*Furnace draft control*Boiler feed regulation or drum level control*Super heater/reheater steam temperature control*Auxiliary steam pressure control*Mill group control*Feed pump speed control.

TURBINE AUTOMATICS*Condenser hot well level regulation*Drip level control in L.P. and H.P. heater

*AUTOMATIC TURBINE RUN UP

BURNER MANAGEMENT *For higher capacity boilers, fuel-firing rate is also higher. Explosion can occur within 1 to 2 secs of fuel accumulation. *AS human reflexes are slower. A complete automatic burner management system called furnace safeguard supervisory system (FSSS in short) has been introduced to manage the present day boilers. *This system takes care that every increment of fuel input corresponds to the available ignition energy inside the furnace.

The following are functions of automatic burner management system.

*Furnace purge supervision*Igniter control*Warm up oil control*Pulverizer control*Secondary air damper control*Boiler trip protection*And also condition of the plant whether the cold or hot start

A process can be defined as a series of manufacturing stages. Which could be either mechanical, electrical, physical or chemical or combination of all these, that the feed material would have to undergo to be transformed in to desired products

Flow, pressure, level, temperature, and similar quantities are called parameter of the process variables.

The separation between the minimums maximum values of measurement is called the range and the difference between lower and upper range values is the measurement span

A control loop in general comprises a measuring device, a controller having the desired value (set point) that can be set by the process operator, and controlled devices. However when the set point of one controller is set by another controller or a computer it is termed as remote set point, some times also called the cascade set point, such a combination is called a cascade loop

A controller must include; a measuring unit, a set point and a comparator to determine the difference between set point and measurement to generate the error. A control unit that operates on the error and produces an out put and an out put unit to drive the correcting device. A controller always responding with an action that will tend to bring the measurement and desired value to coincidence

The controller has two operating modes, auto and manual. In manual the controlled out put is by passed and the control device is driven directly by the operator

A mimic or process graphic is pictorial representation of the process. It can be static showing picture only or dynamic when includes live process parameter data.

A distributed control system (DCs) is a micro processor based control system with centralized hard wired inputs and outputs that are soft ware connected and configured to provide control and computation. To communicate with the system and the process a DCs is always provided with visual display (VDU) unit + a computer key board and printer

InstrumentationInstrument : a thing used in performing an actionInstrumentation: provision of use of mechanical instruments to perform different tasks in industry.Such as :measurement,control, protection,etc.

measurement: is a comparison between known standard to un- known magnitude.

EVOLUTION OF MEASUREMENT TECHNOLOGY & AUTOMATION:

The ability of logical reasoning and quest to understand the nature led to the development of tools & techniques by the humans ,application of them, analyzing the results ,paving the way for constant development in all round the measurement techniques.

Economic constraints coupled with product improvement ,is calling for increased sophistication in measurement &control techniques.

A Process of Manufacturing Any Product Is Comprises of Series of Manufacturing Stages Which Could Be Either Mechanical,physical, Chemical, Electrical or Combination of All or Any of These.Flow,pressure,level,temperature,ph Etc Are Called Parameters of the Process Variables.

Energy can nether be creatednor destroyedmerely converted.

Energy available in fuel = 100%

During conversion:90% is given to steam.(10% is lost to stack, unburned fuel etc.)from 90%:50% is lost to condenser.Conversion efficiency of a TPS=35.8%.This or above conversion efficiency will be possible only when assistance of proper instrumentation is available.

Efficiency's of :turbine :85% 15% lossalternator :98.5% 1.5%lossgen.transformer:95% 5% loss

Instrumentation in whole is used to improve /increase:* safety of plant and personnel.* availability of plant* efficiency of plant and equipment.* efficient use of process variables.* to alert the operator/engineer facilitating them to take timely corrective actions.

the economic reasons forced the developments in the technology enabling the realization of larger plant sizes. But the larger plant increased the complexity of control and this in turn increased the need for better and faster measurements,to increase the availability & reliability of the plant.

Indian thermal power plants

Unit capacity

Pr kg/cm2

Temp 0c

30 Mw

59.8

482

50-62.5

90.0

510-535

80-100

90.0

535 mostly non-reheat

110-140

130

535-535 rh typ

210

150

535-535 ,,

500

178

,,

The evolution of Industrial MeasurementTechnology seen from +1-2% accuracytransmitter to +0.2% accuracy.

earlier transmitters are based on dominant transducer technology.

There are two types of advances taking place in the Instrumentation area.

* the traditional sensors like thermocouples, RTDs etc are being made more and more intelligent.

* new sensors with better accuracy and noise immunity are being invented.

Development

1930s The discrete devices used for analog control were governors and mechanical controllers.

1940s direct connected pneumatic controllers

50s transmitter type of pneumatic controllers

50s. discrete devices used for digital control were relays and stepping switches

Development PneumaticElectricalElectro-pneumaticElectronicsp basedComputer based (DDCs.)

Transmission lag

replacement of relays and pneumatic controllers with their solid state equivalents, resulted in the development PLCs&PIDcontrollers.Realization of computer capabilitiesfirst led to Data Acquisition, then to Supervisory control and finally to Direct Digital-Control.

With continued developments in technology, these two streams have now merged into the present day computer based control systems(distributed digital control).

SignalsPneumatic : 3 - 15 psi or 0.2 - 1.0 Kg/cm2 electrical : 4 - 20 ma (normally) 1 - 5 voptical signals with fiber optic systems/when a direct line of sight exists.

Advantages of pneumatic system

Simple ,hence less skilled person can be utilised for servicing,Less expensive on initial cost.Reliable and more rugged components are available.Higher motive force actuator is readily available.Flame proof, suitable for hazardous surroundings. Operation is smooth,over load proof due to compressibility.Requires large quantity of clean compressed air(moisture&dirt free).Permits wider ambient temperature operations.

Advantages of electronic system

High speed of signal transmission,hence less time lag.Higher amplification is possibleeasily adaptable to complex & integrated controlgreater accuracyfeed back from more number of variablescheaper if the system is adapted for large plants.

-First supplied range of instrumentationTechnical collaboration of USSRinstrumentation was mostly voltage based,& controllers were electronic pulse type.Transmitters employed LVDT,1-0-1v AC signal limitations of above instrumentationthey are bulkyDPTs range above 6.3 kg/cm2 were not availablerange adjustment was not possible at sitecompensation for pressure & temp. was not possible

Controllers suffered from

Feed forward signals/actions not possiblefuel-air ratio and lead-lag features could not be provided in combustion control.Biasing facility not availableauto run - back feature was not available.

transducersVoltage base - /four wire transducers. (o/p:1-5,0-5,0-10 v)Current base - two wire transmitters (o/p:4-20,0-20,0-50 ma)pneumatic - o/p: 0.2-1kg/cm2 / 0.2-1.0 bar/ 3 - 15 psi/

Live zero

dead zero

Measurement cycle

sensing

transmitting

Indicating /&

Recording /&controlling.

sensingSensors: Pr : diaphragm, level: manometers, capsule, float mechanisms bourdon tube, head pressure,temp: thermometers resistance etc. thermocouples,flow : primary elements, mechanical meters.

Transmitting stageSignal conditioning suitable to next stage,i.e.: amplifying, converting, attenuating, etc.

Final stageIndicating Indicating & annunciation,/ protection.Indicating & recordingrecording & annunciation,/protection.Indicating/& controlling.Controlling.

Variables in electrical technologyResis tance, capaci tance, induc tance, reluc tance, conduc tance, reac tance, &impedence in signal condition circuits.

Developments in Indicators & RecordersGravity control replaced with controlling torque techniques in indicators.Miniaturization to present more information to operator in a reachable distance,for controlling a system.(modular & panel arrangement).Edge wise indicators (to accommodate more no.of instruments in a given place).Digital indicators for easy reading.Multi-pen recorders,(continuos & dot metric recorders)multiple functions in rec. & indicators .

Primary measuring element selection&characteristics.The intended use of the sensor.Range: normal range over which the controlled variable might vary?.are there extremes to this.?Response time: time required for a sensor to respond,to a change in its input.Accuracy: how close the sensor comes to indicating the actual value of the measured value.Sensitivity: how small change in the controlled variable the sensor can measure.

Precision: how consistent the sensor is in measuring the same value under the same operating conditions over a period of time.Dead band: how much of a change to the process is required before the sensor responds to the change?Costs: what are the costs involved. (i.e. Purchase cost,installation cost, operating cost,and calibrating etc.)installation problems: special installations ( i.e.corrosive fluids,explosive mixtures,size and shape constraints,remote transmission requirements etc.)

Transducer requirementsShould sense the desired input signal.Should be insensitive to other signals present simultaneously in the measurand.Should be amenable to modifications with appropriate processing and display devices.It should not alter the event to be measured.Should be able to with stand hostile environment while maintaining the rated accuracy.Should be easily available and reasonably priced.

Accuracy 1. Percentage of true value: = measured value-true value X 100 true value

percentage of full scale deflection : = measured value-true value X 100 maximum scale value

* F.S.D. is less accurate than % of true value.

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