Know How Sharing

8/3/2019 Know How Sharing

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Avikshit Singh 1/30

Know - How Sharing

SWAS (Steam Water Analysis System)

High pres. Steam boiler and turbine are under constant attack from erosive and

corrosive element such as Silica, Sodium, Dissolved Oxygen (D.O.), Calcium,

Chlorides and Phosphates.

Without accurate and dependable on line instrumentation the monitoring becomesquite difficult and it leads to serious damage to boiler and turbine.

SWAS is combination of

1-Sample conditioning

2-Sample analysis

Sample conditioning acts as a protective wall between analyzer and parameter to be

analyzed It brings the samples physical parameter as per requirement of analyzer.

It comprises of

Sample cooling Pressure reduction with safety devices Pressure regulation

To flow meter and Analyzer

The analyzers, which are normally, are on line

1- Silica

2- Sodium

3- Hydrazine

4- D.O.

5- pH

6- Conductivity

7-Chloride, Phosphate, Chlorine, Dissolved Ozone & Nitrates etc as per operational

and conditional requirement.

Silica analyzerIt is based on optical density of silico molybdenum blue complex which is

developed by reacting with ammonium molybadate and H2SO4 in developing

yellow silicomolybdic acid complex and then reacting with Ferro ammonium

sulphate for developing intensely blue coloured silicomolybdenum blue complex .

The streams where this monitoring is done are

1-DM plant for continual monitoring of Anion and Mix bed exchanger and also

resin exhaustion

2-Boiler and Steam: Deposition of silica on the super heater tube and turbine blades

leads to loss of efficiency and permanent mechanical damage. Silica solubilityincreases with high temperature

Hence continual monitoring is crucial.

3-Condensate: For proper functioning of polishing plant and resin exhaust.

Sodium Analyzer

NaCl and NaOH in particular are known to be associated with stress corrosion

cracking of boiler and super heater tubes. Presence of sodium indicates

contamination with potentially corrosive anion (chlorides & sulphates) under high

pres. and temperatureMeasurement of sodium is recognized among other chemical parameter as an

effective tell tale to reveal the condition of high purity water/steam circuit

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The ubiquitous character of sodium in the environment makes it a useful indication

to reveal possible leak condition within the circuit and particularly in condenser

where sodium detects cooling water leak with much higher sensitivity than

conductivity measurement.

Sodium sensitive glass electrodes do measurement where in previously conditioned

Sample of pH >10 is passed. pH adjusting is done by gaseous Di -isopropylamine .

Sample conditioning is essential for limiting interference by other ion and forlowering detection limit.

Hydrazine analyzer

It is an oxygen scavenger and used as source of feed water alkalinity. It also helps

maintain a protective magnetite layer over steel surface. Under dozing hydrazine

leads to increased chances of corrosion while over dozing is an expensive waste. At

high temperature Hydrazine disintegrate into Ammonia which made process water

acidic, therefore to improve pH phosphate dozing is done in Boiler Drum. It also

helps in maintaining magnetite layer.

Dissolved oxygen analyzer

D.O. in feed water causes oxidation (rusting) of components & down streampiping especially at high temperature eventually leading to puncture or failure.

Measurement depends upon Clark cell principle. An oxygen permeable membrane

isolates the electrodes from sample water. A constant voltage supply powers two

electrodes maintaining each at constant potential. Gold working electrode

(Cathode) reduces the

DO to OH ion

O2+2H2O+4 e= 4 OH

A large counter electrode (Anode) provides the oxidation reaction

4Ag+4Br=4AgBr+4epH(Hydrogen ion concentration)

pH of water and steam must be maintained at a slightly alkaline level (between8.8

to 9.2) in order to prevent equipment corrosion.

Conductivity

To detect leakage in heat exchanger conductivity measurement is one of reliable

parameter.

Conductivity

1-To detect leakage in heat exchanger (Contamination) conductivity measurement isone of reliable parameter.

2-Concentration of simple water

3-Gaging quality of pure water

4- Measuring extent of reaction

Flow of electricity through matter is by movement of electric charges, which in

metallic conductor are electron and in electrolytic conductor are ions. In electrolytic

conductor current is usually introduced and leaves the system through metallic

electrode on surface of which chemical reaction occurs. (+ Tive ion or cation moves

towards cathode where reduction occurs and negative or anions moves towardsanode where oxidation occurs.)The conductivity of a solution depends upon the

concentration and mobility of all ions present in the solution. The ions mobility in

turn depends upon ion size and charges as well as dielectric constant of the solvent2


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and solution . To avoid polarization, A.C. potential is used in on line monitoring of

conductivity measurement. The determination of electrical conductivity (specific

electrical conductance) consists basically in measuring the a.c. resistance of a

column of solution. With a.c., concentration charges and polarization effects can be

reduced to any extent dictated by purpose of measurement.

Unit and definition

The resistance Rx of the conductivity cell of constant cross section immersed in orfilled with solution under test , is proportional to the distance L between electrode

and inversely proportional to the cross section area A of the electrolyte and

intrinsic constant of the solution.

Sc= L/A*1/Rx By dimension Sc= Cm/Cm.Cm*1/Ohm=1/Ohm.Cm= Mho/Cm

The term L/A is effective cell constant of the conductivity cell and is denoted by K.

K= Rx( measured resistance of the filled system)/ Specific resistance =Ohm/ Ohm

CM

Range of practical conductivity measurement extends from fraction of micro Mho

to 1Mho.

Gas analyzers

1-O2 in flue Gas

2-CO in flue Gas

1- H2 purity in Alternator

O2 in Flue Gas

The aim of a combustion control system is to enable fossil fuels to be burned as

efficiently as possible with minimum of pollution emission. In order to guarantee

complete combustion, it is usually necessary to feed excess air which is done at a

price. Increasing excess air level increases the stack losses.During combustion operation an important natural phenomenon occurs. Oxygen

molecules seek to migrate from higher concentration in the outside atmosphere to

the lower concentration in the furnace gases. This natural phenomenon is the

principle on which on line O2 analysis Zirconia sensor operates. Sensor is made of

Yttrium stabilized Zirconia electrolyte which has the ability to conduct oxygen ion

at temperature exceeding 650*C. Platinum electrodes on opposite surfaces of the

Zirconia electrolyte provides catalytic surfaces for the transformation of Oxygen

molecule( O2 ) to Oxygen ions(O-) which moves through the electrolyte and

recombine on opposite electrode. The movement of Oxygen ion produces a voltageacross the sensor which is the function of the relative difference in Oxygen

concentration between its outer surfaces i.e. furnace atmosphere and its inner

surface i.e. ambient air and sensor.

Emv=0.0496+(log10 p0/p1)+/- C mV p0 is reference air partial pressure

P1 is measured O2 partial pressure

C is cell constant (initially +/- 1mv)

At inner surface (Ambient air) O 2O +4e

At outer surface (flue gas) 2O+4e OCO in flue gas

If a fossil fuel is burned CO2, H2O, SO2, NOX, CO, O2 and N2 is expected in flue

gases. All these gases are present under excess air condition with exception of CO.3


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Thus it is only one which indicates that incomplete combustion is taking place.

Minimum heat loss from a boiler occurs at a point where Co is just starting to be

produced .At trace level of CO of typically 100 PPM, there is just sufficient excess

air to produce complete combustion irrespective of operational condition, type of

fuel and wear/tear of burners / control valves.

On line wet analysis type Infrared absorption analyzer are used to monitor CO in

flue gases. All hetero atomic gases absorb infrared radiation in distinct band specificto each gas.

H2 Purity in Alternator

According to Brown molecular movement there is an exchange of kinetic energy

among molecules. Te transfer of energy can be measured by the conductivity, which

is the property of substance in respect of which gases differ quantitatively.

According to kinetic theory of gases:

The thermal conductivity is directly proportional to square root of absolute

temperature and inversely proportional to square root of molecular wt. nd

independent of pressure.H2 being the lighter gas in comparison to CO2 and Air, the thermal conductivity Is

7 times higher than air and it offers best cooling medium for generator winding.Refilling in generator

H2purity meter has three scales for complete refilling sequence monitoring.a) 0 to 100 % by volume CO2 in Air

b) 100 to 0 % by volume H2 in CO2

c) 100 to 80 % by volume H2 in Air

Sequence 1- CO2 is heavier than air , therefore Generator is gradually filled from

bottom and monitoring is done by selecting scale a).Sequence 2-Displacing the CO2 by H2. H2 having lower density than CO2 is filled

in from the top until an adequate purity of 96 to 98 %is obtained. Monitoring by

Scale b) Sequence 3-H2 is maintained at a pressure.. purity is monitored by

selecting scale c).

Magnetic flow meter

Magnetic Inductive flow meter s measure the volume flow of electrically conductive

liquids and slurries. An electrical conductor in this case is the electrically

conductive medium passes through a magnetic field. the voltage U induced in this

medium is directly proportional to mean flow velocity V, Magnetic Induction B, nddistance between electrodes (nominal pipe diameter) are constant.

U=K*B*V (-ve)*D ( K=Inst .constant) V-=U/K*B*D

Volume flow rate qv can be calculated according to qv=V-*Dsqr*pi/4

qv=U*D*pi/K*B*4

Induced voltage signal is picked up either by two measuring electrodes in the

conductive Contact with the medium. A signal converter process the signal into a

pulse of 1Pulse/M3

Measuring tube is made of electrically insulating material.Flow is independent of

flow profile of the medium. i.e. pres., temperature, visco., density.Services where magnetic flow meter are installed are Reactivated carbon filter

column Anion and Cation column and pressure filters in WTP 2.

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Radar Gauge

Used for level monitoring of up and down stream of TWS

What is radar it is acronym for Radio Detection and Ranging. It typically uses an

Electromagnetic wave to determine distance and speed. Radar gauges transmit in

the frequency range between 3 to 30 G Hz. The most common type of Radar waveor signals used for level measurement are:

1- Pulse wave

2- FMCW (Frequency modulated continuous wave)

A basic principle of radar is its capacity to reflects off the surface of material base

on material Dielectric constant (D K) which is directly proportional to the amount of

Electromagnetic energy reflected from it. Any material that has D K greater than 1.8

will easily reflects radar signal. The higher the DK of the material the more signal

that is reflected and available for level measurement. In addition changes in

temperature and pressure of the material have minimum effects on the signals.

Radar gauge determines the level of a product in a tank by measuring the Ullage or

vapour space.

It is the distance from Radar gauge location to the surface of the material.

Radar electronic is the heart of radar gauge .It produces an EM wave by using an

oscillator that Converts direct currents power into a u wave frequency signal . It

also receives the return signal Basic components of a Radar gauge includes

1- Gauge housing,

2-Electronics,

3-Mounting flange

4-Wave guide,5-Antenna

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Radar gauge works well in Tubular, aerated, Solid laden, Viscous, Corrosive,

Thick paste or Slurries.

Since there is no moving parts maintenance cost is quite minimal

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Profit centred maintenance

Maintenance Management has a direct impact on Improvement, Prevention and

Correction. Maintenance Department is a +tive contributor to the bottom line of the

company. The road block to Maintenance profitability is lack of management

investment in the procedure, system and equipment Convincing management for

making this investment has become the primary challenge of Maintenance

professionals because equipment down time is a liability.The equipment merely good enough are not profitable. Machine that are kept

running in first rate condition through predictive and preventive Maintenance are

more productive than just good enough.

When presenting profitability, not only down time cost but also cost benefit of,

inventory reduction, improved product quality , reduction in repair and Maintenance

cost and improved equipment life all matters .Co-operation and knowledge sharing

are key to an efficiently run company. Maintenance leaders have to be innovative in

personnel leadership and must be able to gain commitment rather than just

consensus. Continuous training is also necessary to +ve profit centred Maintenance

dept.100 hrs of training in a year is good enough. Maintenance /Operation that is notinvesting in continuous Maintenance education and development of its people are in

danger.

Each Maintenance employee has to understand that a shared commitment for

improving the maintenance is a +ve factor. They must have +ve expectation about a

change and overcome the ve, normally associated with being resistant to change.

Some of the initiative taken at BTPS, C&I dept wrt cost centred Maintenance are

1- Installation of DCS in all the units

2- Replacement of conventional servo drive recorder with paper less recorder3-Replacement of rota meters with magnetic Flow meters in WTP2

4- Installation of radar gauges in TWS system for level monitoring

5-Installation of level monitoring inst. all the LDO and DMW tanks.

6-Installation of variable frequency drive for monitoring of speed of RCF and PCF

in Milling

7- Installation of proximity pick up for speed monitoring of speed of PCF

8-Development of different type of jigs for pre-installation calibration , speedy fault

Diagnosis and reclaiming also tool for training and innovative improvement

Air drying unit operation and maintenance

Service-To supply bone dry clean air to C&I system

Make-Chemech air drying unit

Supply- M/s Kirlosker pneumatic co in unit 5

M/S KG khosla in unit 4

Inlet- Capacity-18m3/min

Pres- 8kg/cm2

Temperature-40degre C

Moisture content-100%rhOutlet- Flow rate-16.2m3/min

Temp-52 deg.C

Pres.-7.65kg/cm27


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Dew pt at atmos.pres-(-)40 deg C

Moisture content-80ppm

Dryer unit components

1-Duel adsorption towers filled with desiccant (silica gel) and embedded 12 kW

Heater (in separate enclosure) for reactivation. While one tower is drying other will

be in adsorption mode.

2-Chang over system with 4 way 2 Audco lubricated plug valve.3-Pneumatic power cylinder operated by two solenoid valve to operate 4 way valve.

4-Control panel for Auto/Manual operation of the system.

5-A Dew pt apparatus

Cycle time for adsorption/Reactivation 6 hrs

Operation;

1- Exothermic Adsorption

2- Change over of Adsorber

3- Reactivation

After 6 hrs of adsorption i.e. drying of inst. air in 1st tower. 2nd tower which has

been under reactivation comes in adsorption mode by operation of 4 way valve.The change over takes place automatically by a timer command to operate a power

cylinder which in turn operate 4 way valve.Reactivation- Time 6 hrs

1- Depressurisation for 5 minutes by a solenoid valve which remain open for 5.30

hrs

2- Heating of silica gel for 3 hrs to raise the temperature of purge air to 180 deg

Celcius. Heaters cut off is by 6 hrs timer or temp switch contact when

temperature rises beyond 180 deg C. which ever is earlier.

3-Cooling and idling for 2hrs 20 minutes by purge air flowing through a orificeconnected across both the towers

4-Re-pressurisation takes place in last 30 minutes of 6 hrs cycle when

depressurization SV get closing command from the timer. Tower under reactivation

comes to line pressure and just after 6 hrs of cycle ends, the tower under reactivation

comes in adsorption mode.

Fault, possible causes and remedy in drier

1- Inter lock has been provided for making heater ON only when depressurization

SV is open In both auto and manual mode of operation.

2- Faulty change over takes place when limit switch of 4 way valve has notoperated due to Mechanical jamming of valve or power cylinder has not operated to

full stroke or solenoid valve of power cylinder has not operated or leakage of air is

taking place due to puncture of air piping connected with the system.

3- When control supply fails, 6-hrs synchronous motor operated timer remains in

stay put condition i.e. it will not come to initial position.

4- In auto mode heater switch must be kept in off position.

5- Faulty change over indication/alarm comes through a electronic timer

adjustable to 1 to 25 sec.

6- Heater on condition depends on permission from synchronous timer and inseries connected temperature switches.

7- Periodic dew pt measurement must be carried out to ensure optimized operation

of drying unit.8


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8- Most of the temperature switches are bimetallic type, their functioning must be

monitored otherwise they should be replaced with liquid filled system or resistance

type temperature detector.

Travelling water screen

TWS help to provide debris free in take water in suction of Raw water pumps

discharge of which is used for various usage in the power station .Travelling WaterScreen are installed in a vertical chamber forming a part of intake well.As the water

passes through the screens, refuse in the water is retained on the screen trays and as

the trays rises the refuse are washed away by flat sprays of water nozzles.

Flush water is tapped from cleaned raw water header at a minimum pressure of 0.5

kg/cm2, passes through duplex strainer and pressurized by flush water pump to not

less than 5kg/cm2. Two flush water pumps are in use .One in service and other

stand by. The pressurized water passes through the butterfly valves BFV1 or 2 into

two headers. Header no 1 connected with BFV 1 and no 2 with BFV2 .Flow nozzles

are connected to the pressurized header in head section of the screen and in turn

high velocity water jets from the nozzles clean the screen. 1st 1 to 4 and then 3to7 .The flush water along with the debris removed from the screen flows through

to refuse trench provided with 7 nos. trench jetting nozzles .first 4 Trench nozzles

are connected with header of either BFV1or 2 and rest 3 with header of BFV2.

The screens can be cleaned either manually or automatically (OMRON timers) from

control panel. Selection by A/M switch (SS-01).

Manual operation: Under manual mode entire operation has to be done sequentially

meeting the requirement of basic interlocks which are required in both mode of

operation.

1) Screen flush water pumps are interlocked with suction pressure.2) Torque and Travel limit switches for opening and closing of BFVs.

3)Auto circuit of screen drives is interlocked with corresponding Butterfly valve

position limit switch and delivery header pressure switch.

4)Under Manual mode, the operation of 1st (1 to 4)and 2nd group (5 to 7) of screen

can be controlled by group start and stop push button and also individual screen can

be controlled by local start/stop push button.

1-Type of screen Flow through

2- No of screen 7

3-Rated capacity of each screen 20,000mcu/hr 4-Speed of screen 2meter/minute

5-No of spray nozzles 18

6-No of screen wash pump 2 (Mather & Platt)

7-Capacity and Head 300Mcu/hr at 60MWC,75KW

8-Setting suction 0.2Kg/cm2 ,Diff 0.6Kg/cm2

9- Discharge 4.5Kg/cm2, Diff.!.0 Kg/cm2

10-Butterfly Valves actuator K30 F10E,BECON ROTORK

0.1KW, 24rpm (geared)

11-Time for opening closing 31.5sec12-Level Indicator Radar gauge Endress+Hauser

Range : upstream 5.65M

Down stream 6.35M9


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Differential level >200mm

Manual Operation for cleaning of screen

1-All mechanical valves are properly opened/closed

2- Selector switch on Manual mode (contactor K-102picked up)

3- Start any flush water pump(Pump selection switch)

4- Open BFV-1 and Start group-1 of screen5- Open BFV-2 and Start group-2 of screen

6- For stopping the manual operation follow the reverse path i.e.Stop the screen

and then close the respective BFV s and then stop the screen flush water pump.

Auto operation (Timer TM 101,102,103 &401) contactor K- 101 is picked up.

1-Timer TM 101 is used to set the time interval for auto start of the screen washing

system a fresh after one cycle of operation for cleaning of group 1 and 2 is finished.

2-After a set time interval flush wash pump will start and with the start of the pump

BFV1 starts opening. When delivery header pressure in header-1 is established the

1st set of screen start automatically.

3-TM-401 sets the time delay from the start of screen flush water pump upto closingof BFV-1 .i.e. desired washing time of 1st set of screen(1 to 4). With the closing of

BFV-1, 1st set of screen also stopped automatically.

4-While giving closing command to BFV-1, TM-401 simultaneously gives opening

command to BFV-2.

5-Once the BFV-2 has opened and delivery header pressure in header 2 is

established, 2nd set of screen (5 to 7) is started.

5- TM-102 set the time delay ( sum total of wash time for group1&2) for giving trip

command to running screen flush water pump and with that BFV-2 starts closing

which in turn stops the 2nd set of screen.6-Timer TM-103 (6 sec) receives command from Timer TM-102 after 2nd set of

screen are stopped for resetting of Timer TM-101 which starts counting interval

after which again washing get started.

Initial starting of the system

1- Check that whole system is in zero state i.e.flush water pumps, screen, and both

BFVs are in closed position.

2- Set the system in Auto mode (contactor K 101 picked up).and select either of

the two flush water pump.

3- Now turn the control switch (S-102) to close position from its neutral positionand bring it back to neutral position In doing so control supply is given to

timers for starting the sequence of operation in auto mode.1st cycle starts

immediately and subsequent cycle after 1st is over. Now no need of further

closing of control switch. Timers shall take care of further operation till control

supply is available in the system. However if control supply is switched off,

starting command for next cycle of operation is available only after set time

from Timer TM-01 is over after restoration of supply.

In Rest period operation4- If we require starting an immediate cycle of operation because of large amount

of debris then a fresh starting command from the control switch becomes

necessary. or timer setting to bring to minimum.10


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5- In case of control supply fails before 2nd cycle is over, then BFV-2 should be

closed after restoration of supply. Only then command for starting of cycle is

available

6- If the control switch is brought to trip position then no further screen wash cycle

shall come. However if control switch is operated to trip position during screen

wash cycle , then the system will stop only after wash cycle is over .

7-

Once closing command is given by the control switch (S-102) then the timerTM-101,102 &103 immediately start counting time.

Trouble shooting

1) If a group of screen is not starting , the reason is discharge header pressure

connecting to that group is not adequate or pressure switch is 1) not making contact

2) impulse line choking ,3) valve closed and 4)BFV not fully open5) problem with

electrical/C&I control system.

2) One particular screen is not running; it is due to electrical system problem,

ELM to be contacted.

3) Flush pump is not starting: it may be due to 1) low suction pressure, 2) pressure

switch problem 3) Timer connected with suction pressure switch is not holding 4)electrical problem

Turbine supervisory instrumentation in power station

Turbine is an active interface between steam and electric power generator, therefore

it is called prim mover. The efficient and safe operation demands careful

monitoring of all associated parameters which must be ensured as most reliable

measurement. The variables which are monitored are: -

1) Axial shift of turbine rotor

2) Differential expansions between ,a)HP turbine casing and rotor

b)IP turbine casing and rotor

c)LP turbine casing and rotor

3) 3-Eccentricity of turbine Rotor at barring gear and synchronous speed.

4) 4-RPM of turbine rotor

5) 5-Movement of position of speeder gear of turbine governing system

6)6-Over all Total expansion of HP& IP turbine

7) 7-Movement of position of control valves servo motor for steam inlet in

turbine.8)Vibration of bearing of turbine of casing and flange of HP and IP turbine.

Installation

1-For the measurement of parameter from SL. no 1 to 5, the sensors are mounted

inside the turbine casing.

2-From SL no 6 to 9 sensors are mounted at out side.

Type of sensor

3-The Eddy current loss based sensors are used for the measurement of parameter

from SL no. 1 to 4.

4-Variable resistance of 360-deg rotation is used for monitoring of parameter for SLno. 5,6 &7 .

5- Electromagnetic self generating seismic mass type velocity measuring sensors

are used for monitoring of vibration of turbine bearing casing .11


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6-Cr/Al mineral insulated thermocouple is used for monitoring the temperature of

turbine casing and flange..

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location

1- Sensors for Rotor Axial shift and Eccentricity monitoring are mounted in

the vicinity of thrust bearing of turbine .

2- Sensors for Diff. Expansions are mounted after flanged coupling of rotors

of respective turbine

3- Sensor for Position of speeder gear is mounted near seeder gear motor4- Sensor for RPM measurement is mounted against MOP (Main oil pump) shaft

were in a 30 teethed wheel is inserted.

5-Sensor for control valve servo motor travel position is mounted near the CVSM.

6-Sensors for vibration monitoring are mounted at top of bearing housing in radial

directions (vertical and horizontal).

7-All thermocouples are mounted at respective location i.e. front top, front bottom

rear, top, rear bottom, inner layer outer layer of flange, studs hole., top flange and

bottom flange.

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Now let us share about sensors working

Eddy current Pick up

When any metal moves from more flux density magnetic field to less or vice versa,

currents are induced in the metal with direction in opposite.i.e. induced current

from more to less will be opposite from less to more. These induced currents in the

metals are called Eddy current which are perpendicular to the direction of the flux

and complete their path through the core of the metal .The loss of power due to theflow of eddy current is called Eddy current loss because core offer resistance to

flow of eddy current.

Eddy current loss depends on

1- Core thickness

2-Core resistivity

3- Loss increases with square of flux density and frequency

Power loss in core =(1.64*1/P*bM2*f2*t2) watt /m2

If we multiply watt with time then energy loss =watt*time joule /volt

1.64 is form factor of the flux wave bm is max value of flux density

f is frequencyt is thickness of core

p is resistivity of core

The eddy current loss sensitivity depends upon magnitude of displacement which is

monitored .

For displacement > 4.0 mm sensitivity will be 2v/mm

In between 4v/mm

< 2.5 mm sensitivity will be 8v/mm

The measurement of axial shift of rotor, and diff. Expansions between casing androtor is done by eddy current pick-ups. The reference position of turbine rotor for 0

value of measurement is necessary in overhauled turbine, therefore rotor thrust

colour is pushed against working pad of thrust bearing up to zero mm of gap. The

pick-ups are then mounted at respective locations with pre- determined gaps

between pre- fabricated machined colour and pick ups on the rotor .The calibration

is done by recording different gaps position in mm vs. eddy current pick-up

converter out put .It must be in linear form and deviation if any should be in

permissible limits. i.e. +/- 2 .0 % of O/P.

The monitoring value for 1- Axial shift =+/-0.5 mm in exceptional cases more thanthis.

Turbine trip value is 1.7 &+1.2 mm

2-HP diff. expansion (-) 1.2 to (+) 4.o mm /+-3.0mm in 100 Mw

3-IP diff. expansion (-) 2.5 to (+) 3.0 mm

4-LP diff. expansion = (-) 2.5 to (+)4.0mm / -2.0 to +5.0 mm in 100 Mw

Some interesting observation

1-After overhauling of turbine the oil flushing is carried out at elevated temperature

of 50deg C to clean up the bearings, associated housing and piping for cleansing

from material impurities. A very alarmingly high values are seen in axial shift andHP diff. Expansion parameters. These extended values are due to the uneven

expansion of rotor (soft material) and casing in that particular location.

Measurement of speed and eccentricity is also accomplished by eddy current probe.15


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Speed measurement. Pickup is mounted against a wheel having 30 teethes . when a

protruded teeth passes the tip of the pickup, a pulsed of suitable amplitude is

generated and a counter counts the pulse generated in one minutia turbine is

rotating at barring gear speed then total pulse will be 30 *No of rotation per minute

and indicated value will be in

RPM by use of suitable divider. Like wise the system works also on synchronousspeed.

Eccentricity measurement It is the monitoring of run out of turbine at barring

gear and shaft vibration at synchronous speed. The pickup is mounted after thrustbearing on the coupling of rotor just after HPT steam seals in 2nd stage and at Mop

coupling in 1st stage turbine. A slowly varying DC voltage signal proportional to

rotor run out is generated at barring gear speed and recorded as band width of

minimum to maximum pen movement. The difference between these two values is

Eccentricity of rotor before rolling of turbine and it should match with mechanical

Dial indicator mounted locally. Some times band width is not recorded on the

width of chart of recorder because of gap which becomes < 1.25mm or more than >

1.75 mm between pickup and rotating mass. This is not unusual and in this case

minimum and max values diff. should be known through the indicators.With existing system configuration, >300 rpm turbine speed signal is necessary for

monitoring of Eccentricity at barring gear, hence speed measurement must be

healthy.

Permissible run out at barring dear should not be more than 80 um including

fundamental Run out.

Monitoring of eccentricity at synchronous speed is by the same set up but signal

coming out of converter is a super imposed AC mV of frequency as that of rotor and

amplitude is mean of varying gap between pick up and rotor. Permissible limit

depends upon thermo dynamic behaviour of rotor but should not be more than

200umm.

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Use of reading as an aid to operation.

1- Temporary deflection of eccentricity:

2- a) stationary rotor lying in a hot cylinder. This Condition applies just prior tostarting after as shut down. The temperature in vertical transverse places in the hot

cylinder of a turbine at rest is rarely uniform, and consequently the rotor lies in a

transverse gradient and bends accordingly, usually convex upward.

This state of affairs is immediately shown as high eccentricity when slow turning

begins.

Machine must be on Barring gear for long enough to allow the rotor to straighten

before run-up. Dial indicator with extension rod bearing on the shaft is used to

detect the run out in addition to on line eccentricity system.

b) Temporary thermal gradients developing in transverse sections of the rotoreven when it isrotating. These arise from non uniform heat flow in to the rotor

and most general cause is a gland sleeve which is much hotter than the shaft and

owing to diff. Expansion making contact only along a line. Heat flow into the shaft17


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is most rapid along this line and thus bending is taking place causing high

eccentricity. A gland sleeve will become much hotter than the shaft when the steam

supply is much hotter than the turbine, and the mass of steam flowing is sufficient

for rapid heating to take place. When such temporary rise in eccentricity is detected

reduction in stem flow is suggested. Some times splashes of water impinging on the

rotor causes non uniform heat flow.

Some measurable data for verification of healthiness of system working with eddycurrent Probes.1-Gap voltage depends upon sensitivity

a) Diff expansion: 2v/mm

b) Axial shift: 2v/mm or 4v/mm depending upon the pickup used

c) Eccentricity and Speed: 8v/mm

d) Eccentricity signal at synchronous speed :1.4 VAc and on barring gear4vDc for

500 um

e) In case of open detector or short circuits, o.k. light (green l.e.d.) will go off at

module facia.

With alarm of detector faulty. In this situation high/low alarm and trip circuit isinhibited.

f) Speed module must be healthy for monitoring of Eccentricity. if >300 rpm

permission is through SSM. Other wise change over is provided externally, through

a toggle switch mounted locally.

g) Power supply to eddy current pick up is always 24v DC

It gives added advantage of high range of temperature tolerance for the pickups and

system and also damage prevention in case of sort circuit.

Position measurement

1: Over all turbine traveled on Expansion.( HP& IP) Range: 0 to 50mm2; Speeder gear travelled position Range: 0 to 36 mm

3: Control valve servo motor travelled position Range: 0 to 300 mm

For all the three services remote measurement is accomplished by 360 Deg

variable resistance of 1 K ohm powered by 10 Vdc .The variable arm of the servo

pot is mechanical coupled with Pinion of rack and pinion arrangement mounted

locally for transferring Linear movement of position. Local indication is also

available of all the three services.

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Some interesting observation of over all turbine expansion: When Turbine is

under shut down for some Maintenance, the come back to original position is nevera smooth rather a staircase one. But rotor contraction is always smooth in side the

casing. This some times leads to virtual indication of diff. expansion and axial shift

against logic. One should never be alarmed for this.

The measurement of over all thermal expansion of IP turbine may give indication

which is not compatible to HP turbine. It is because of stuck up of rack rod inside

the transmitter due to exposure of high temp seal steam leaking over there leading to

indication that is not following the behavioural values. It is found that rod is not

touching the fixed plate attached with the casing while turbine reverting back .

The signal leads coming out of OTE transmitter must be routed through heatinsulated sleeve.

The pitch gap (play) between gears of pinion and servo resistance may lead to

mismatching in local and remote indication. The signal change must be indicated in

DPM, if not check for threshold of resolution of indicator in UCB. It should follow

the variation in transmitter. Because of signal conditioning, resolution may fall and

this will again results in mismatching.

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Turbine bearing vibration:

Vibration measurement offers a way of diagnosing the health of power plant

machinery to avoid the possibility of catastrophic failure, reducing the frequency of

overhaul and predicting the optimum time of overhaul. The key functions are:

1-Periodic vibration monitoring, it is routine measurement of vibration around the

power plant .The main purpose is to detect changes in level which indicate onset ofproblem and if ignored may lead to failure.

2-Continuous vibration monitoring: it is the system of permanently installed

sensors on the machinery and permanently installed instrumentation in the control

room to warn the operation personnel of change in vibration level. But because of

high cost involved, it is only suitable for critical machines like turbines and

generators. Periodic monitoring however meticulously performed, cant take the

place of installed instrumentation, because some conditions developed inside the

turbine require immediate shut down of turbine .on the other hand flexibility in

periodic monitoring offers added advantages.3-Trending: The aim of trending is to estimate from rate of change of vibration

level, how much longer a machine can safely run before being shut down. But some

times rate of change increases suddenly as a bearing approaches failure.4-Diagnosis: It is to find fault which is causing unusual vibration. Million rupees

worth of equipment will tell you nothing if you dont understand the machine you

are working with.

It is therefore important to know the machine dynamics than taking lot of readings.

When analyzing a machines vibrations signature, the first step is to look for forcing

frequencies which are caused by any mechanical problem. Serious problems occurwhen one of these forcing frequencies coincides with or is close to natural

frequencies of vibration. This condition is known as resonance and it amplifies the

effects of forcing frequencies.

Therefore 1st step is to look for forcing frequencies which may be the results of the

following

1-Unbalance; It is sinusoidal vibration at the m./c running speed.(1x).and is caused

when the centre of mass of a component does not coincide with its center of

vibration. Unbalance may be in single plane (static) or in multiple planes (dynamic).It is single frequency vibration whose amplitude is same in all radial direction and

increases with speed. It does not contain harmonics. It has single reference mark.

Unbalance depending upon severity causes bearing failure and even can shear the

shaft.

1-Misalignment between shaft of connected machines, bent shaft, and improper

seated bearings are cases of misalignment. The principal frequency of vibration is at

twice the running speed (2x) and has single, double or triple reference mark.

Magnitude of vibration is max at axial direction.

3-Uneven loading, such as belt drive of an eccentric pulley generate vibration atrunning speed. Distinguished feature is that vibration is unidirectional and varies

with load.

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4-Mechanical looseness, it generates 1x and 2x frequencies vibration but almost

always contains higher harmonics. Mechanical looseness can often be located by

taking velocity readings at several points on the machine. Measured vibration will

be highest in the vicinity and direction of looseness. Its easier to detect the

mechanical looseness by going around machines and looking for loose or broken

support. Mechanical looseness is easily mistaken for other causes of vibration.

5-Oil whirls caused by instability of rotor supported in the fluid film because oilfilm is partially carried around with the rotating shaft. This produces a self- exited

vibration that draws energy into the vibratory motion that is independent of

rotational frequency. Changes in oil viscosity change in pressure from design values

or external preloads on the shaft can produce conditions that reduce the ability of

fluid film to support the shaft.

The result is that the journal precesses around the bearing at slightly less than half of

running speed. In the extreme cases , the fluid film is no longer able to support the

shaft, and bearing wipe occurs.

6-Steam whirls; load restriction due to steam- exited rotor vibration because of

gland sleeve is a world wide problem. The vibration usually occurs on HP turbine

shaft, with a frequency very close to one of the shafts own natural resonances.

Above some critical output level, amplitude can increase suddenly so that a rapid

load reduction become necessary.

Now what is vibration analysis?

Vibration can be either a cause of trouble or the result of a trouble. Vibrations are in

the wave form originating from a source and travel to be indicated some where else

and may not directly point out the source. By the study of wave pattern, the source

can be identified and the cause rectified, this study is called vibration analysis.

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What is vibration, mechanical vibration is caused by the motion of a m/c or its part,

back and forth from its position of rest and is a periodic dynamic phenomena. By

plotting movement against time is frequency, total distance of movement from peck

peak is displacement, and maximum speed of movement is velocity of vibration.

The position of the m/c or its part at any given instant with reference to some fixed

point is the phase of vibration known by stroboscope light focussed on the m/c

rotating part If the vibration has the form of pure sinusoidal curve along a axis, theinstantaneous displacement from reference position can be represented as

x= x peak sin wt. (w is angular velocity, x peak is max displacement position from

reference point, t is time). Velocity is the time rate of change of displacement, it is

represented as

V=dx/dt=2pif cos. 2pift, the acceleration which is time rate of change of velocity is

given

as A= dv/dt= - 4pi fsq sin 2pift

peak velocity V, displacement D, and acceleration A are therefore given by

V=D/2pif; A= V/2pif; D=A/(2pif)sq.

Vibration velocity is measure of speed with which a m/c part is vibrating ascompared to the displacement which indicates how much the part is vibrating. The

energy generated by vibration is dissipated as heat and impact between components

of m/c parts, and causes internal wear failure. This energy is proportional to mean

velocity and as such velocity data are of great interest for knowing the over all m/c

condition but would not be able to pinpoint the cause of vibration.

Displacements reading are useful for detecting an increase or decrease in vibration

at specific frequency. Vibration acceleration is an indication of force acting on a

part. The max acceleration that occurs during a vibration cycle is expressed in terms

of g peak. Most of the signals from a rotating m/c are complex and may containspikes or some departure from periodic form in this case accelerometer only gives

correct picture of the vibration.

Velocity pickup is seismic velocity pickup. It consists of a coil of fine wire

supported by springs with low stiffness. A permanent magnet is firmly attached to

the case of the pickup and provides a strong magnetic field around the suspended

coil. When the case of the velocity pickup is attached to or held against a vibrating

part, the permanent magnet (being firmly attached to the case) follows the motion of

vibration. The coil of wire supported by springs with low stiffness remains

stationary in space. Under these conditions the relative motion between themagnetic field and the coiled conductor is the same as the motion of the part relative

to the fixed point in space and voltage generated is directly proportional to this

relative motion. Hence, the name Velocity pickup. The voltage output of a velocity

pickup is normally expressed in millivolts pre inch per second. This is also referred

as sensitivity of the pickup. It is 1080 millivolt peak pre inch per second for velocity

pickup being used on line at BTPS.

The sensitivity of velocity pickup is only constant over a specific frequency range.

Below 600cpm output drops exponentially.

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Accelerometer pickup

An accelerometer is a self generating device with an output proportional to

vibration Acceleration. Since accelerometer is a function of displacement and

frequency squared, they are especially sensitive to vibration occurring at high

frequencies and are much less sensitive to stray magnetic field. In accelerometer theoutput is electrical charge produced by a material when it is compressed. Such an

material is said to be piezoelectric and may be natural, synthetic or a ceramic

material. The sensitivity is expressed in Pico coulombs per g. The o/p of23


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How vibration is characterized: In practice, no m/c vibration is purely sinusoidalat a single frequency. The vibration is caused by several different frequencies

interacting with one or more natural resonant frequencies. Another complication is

that not all vibration mechanism are linear. In a linear system, the restoring force on

the m/c component is proportional to its displacement from mean position. In a non

linear system, the response is limited by some overriding influence like seal rubbing

,mechanical looseness etc. therefore any vibration signal is expressed as sum of

large no of sinusoidal vibration at different frequencies. The vibration analyzer uses

Fast Fourier Transform (FFT) analysis technique to differentiate deferent sinusoidal

frequencies and corresponding amplitudes and this is known as vibration signatureof m/c. To day with availability of digital electronic, the analysis is performed in a

fraction of second. The same instrument can tell vibration amplitude in

displacement, velocity, or acceleration and display the results on screen as a

function of frequency which is known as signature of vibration or frequency

spectrum.

Frequency is often displayed in cycle per minute rather than hertz because of the

close relationship between vibration frequencies and running speed in revolution per

minute.

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Phase angle of vibration is an important parameter because it helps to distinguish

among different causes of vibration. The vibratory motion may be in phase with the

m/c defect which caused it or it may lag by 90deg, 180deg or some other angle.

Phase angle changes as m/c speed changes. Phase angle is the angle in degrees

between the instantaneous positions of the vibrating part and a reference position.

Phase difference is angular diff. in degree between two vibrating parts. For

balancing it is necessary to know the amplitude and phase of unbalance vibration.

Due to unbalance m/c will vibrate at a frequency equal to its rotating speed andamplitude will be directly proportional to the unbalance force present. To add in

finding the angular location of rotor unbalance, use is made of a strobe light which

is triggered by vibration signal. Due to equal frequency of vibration and rotor speed,

strobe light will flash once for each revolution of the shaft and the rotor will appear

to be stationary. In m/c balancing, we observe two fundamental principles:

a) Amount of vibration is proportional to the amount of unbalance.

b) The reference mark observed with stroboscopic light will shift in a direction

opposite to the shift of heavy spot and angle that reference mark shifts is equal to

angle heavy spot shifted.

Balancing of m/c: when a unbalance rotor is taken for balancing, it is required to

know how large the heavy spot is and where on the rotor it is located. With the

vibration pickup attached to the m/c bearing housing, m/c is rotated at synchronous

speed, and unbalance data i.e. amplitude and reference angle (by the help of strobe

light) is recorded. Now suppose in a particular situation, it was 125 microns at an

angle of 90deg. Now a trial wt of say 100 gm is added at a suitable location and m/c

is again rotated at original speed. By doing so we may observe any of three to

happen.

1-Vibration amplitude may increase and reference mark remains unchanged. It

indicates that trial wt is placed directly on the heavy spot. The correct position ofadding wt for removing the unbalance will be directly opposite to the present one.

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2-If the trial wt, by chance is added at exactly the right location i.e. opposite to the

heavy spot a decrease in vibration will be seen with reference (phase) mark position

remaining unchanged. The balancing will be done by simply increasing the value of

trial wt.

3-The most usual case that can happen is that vibration magnitude and phase

changed to a new Value and position. In this case by vector analysis correct wt

position is determined. In the above example the new vibration data recorded is 75microns and phase angle shift of 120deg.

By these two data vector diagram is made (As shown in the diagram).O, represents

original

vibrations amplitude, 0 + Tnew found vibrations Amplitude and phase angle after

putting wt. Difference of vector O and O+T , gives the effect of trial wt alone and as

such marked as T on the vector. By measuring T correct balancing wt can be

determined by the formula

Correct wt = Trial wt *0mm/Tmm.In this example T=70microns Therefore

Correct wt = 100*125/70=178.5 gram. From the vector diagram angle between O

and T is 45deg, therefore trial wt is required to be moved by 45deg. From thediagram phase shift is from O to O+T i.e. clockwise hence correct wt should be

moved 45deg counter clock wise from the initial trial wt position.

Phase Analysis and Mechanical defects 11/10/11

Phase analysis not only helps unbalance correction but can be very useful in

diagnosing specific machine defects. In any rotating machine when high amplitude27

3600

900

1800

2700O

TO + T

VECTOR REPRESENTATION

OF UNBALANCE


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vibration at rotating speed in axial direction is observed, it can be due to any of the

following,

1- Imbalance of the rotor

2- Coupling misalignment

3-Bearing misalignment

4- Bend shaft

By phase analysis, the actual defect can be pin pointed. For example vibration weremeasured on motor driven fan and revealed high axial vibrations at rotating speed.

To diagnose the problem, the phase measurements are taken on two motor and fan

bearing in axial direction. If the two motor bearing have a large phase difference

between them, it indicates a bend shaft or misaligned bearing. If the two motor

bearing and two fan bearing are also in phase with each other but there is large

phase difference between motor and fan bearing then coupling is the cause of

defects. If the phase on all the bearing is same it indicates static unbalance. If large

phase difference (180deg) are seen on different position of bearing housing, it

indicates severely bend shaft badly damaged anti friction bearing.

Phase stability is indication of machine problem.Characteristics source1- Unstable unless synchronous speed Electrical

2-Stable, unless caused by uneven loading Imbalance

or cavitations. Phase follows transducer location

3-Unstable, may be highly directional Looseness

4-Stable, relation between axial phase at shaft Misalignment

ends should be approximately 180deg

5-Unstable Oil whirl

6-unstable, large phase change with change Resonancein rpm

Frequency of vibrations gives a clue to machine problem

Frequency Most likely

cause

other possible causes and remark

1*rpm Unbalance 1-Eccentric journals

2-Misalignment or bent shaft if high axial vibration

3-Reosnance

4-Reciprocatinfg forces5-Electrical problem

2*rpm Mechanical

looseness

1- Misalignment if high axial vibrations

2-Reciprocating forces

3-Resonance

2*rpm Misalignment Usually a combination of misalignment and excess

axial clearances (looseness)

Less than

1*rpm

Oil whirl (less

than1/2rpm)

1- Background vibrations

2-Sub harmonic resonance

Ac line frequency Electrical problemMany

times rpm

(Harmonically

related freq.)

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Mechanical

looseness

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Modal analysis

It is a technique for identifying the natural or resonant frequencies of a machines

structure. It is also valuable in finding out why a machine is excessively vibrating at

a particular frequency. Many cases of excessive vibration occur when a forcing

frequency coincides with a natural frequency of vibration. The only solution is to

alter or damp the natural resonant frequency of the structure to separate it from

stimulus. The 1

st

step is to find out the resonant frequency and 2

nd

is to lookout forpossible forcing frequencies.

When a structure is struck by a hammer (mallet), this is equivalent to imposing a

wide range of different forcing frequencies. If those input frequencies include a

resonant frequency of the structure, then it will vibrate in resonance for a transient

period and the frequency of its vibration can be seen on an FFT analyzer.

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