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CONDENSER, ST COOLERS & LP HEATERS

STEAM TURBINE & AUXILIARIES

PART II A

BY

M.K.LAHOTI N.P.MATHURDGM(HXE) DGM(HXE)

HEAT EXCHANGER ENGINEERINGBHEL HARDWAR


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SURFACE CONDENSER

1.0 INTRODUCTION :

Condenser is used to condense the exhaust steam from L. P. Cylinder of

Steam Turbine and to produce the deepest possible vacuum in order to

increase the heat drop and the turbine output. It also facilitates reuse of

condensate thus obtained.

CONDENSER (KWU DESIGN) :

Optimum utilisation of steam space by providing rectangular cross section

of tube-nest is an added feature of this condenser. For ensuring equitable

loading of condenser tubes in the bottom rows without incurring

appreciable steam side pressure drop, tubes have been segregated insmall bunches leaving wide lanes between them. Tube bundle has been

kept 1/2o inclined towards front water box for its self-draining during C.W.

pump tripping and shut down.

The condenser water chambers have provision for the isolation of half of

the condenser for leak detection of tubes without tripping the plant.

CONDENSER (LMW DESIGN) :

This is a twin shell condenser, each connected with exhaust part of low

pressure casing. A bypass branch pipe interconnects the two shells.

Special feature of this condenser is the tube layout which has been

evolved after a long and intensive research work in laboratories with a

view to ensure efficient heat transfer from steam to cooling water flowing

through the tubes.

Bowing of tubes has been used for its self-draining during C.W. pump

tripping and shut down.

2.0 CONSTRUCTION :

CONDENSER (KWU DESIGN) :

It is a rectangular surface condenser having suitably stiffened dome,

consisting of stiffening pipes, welded on either side with the opposite side

wall of dome shell. Except tubes, the remaining construction is of

fabricated type. The tubes have been expanded into main tube plates and

are supported by the tube support plates at intermittent points to prevent

their sagging and to curb the flow induced vibrations. Non-condensable


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gases are continuously extracted in order to maintain vacuum in the

condenser.

Water box removal/handling arrangement has been provided to facilitate

the removal of covers for enabling re-tubing and cleaning of tubes etc.

Water boxes are further provided with suitable manholes for carrying outmaintenance works without removing them.

Water box inside surface and circulating water side of water chamber

including carbon steel main tube plates have been protected against

corrosion by application of protective coating over the surface in contact

with cooling water.

LP Heater No. 1 has been installed inside the condenser neck (mainly in

500 MW sets) to save floor spare and piping cost. Two numbers Steam

throw devices have been provided in each condenser for dumping steam

during start-up and sudden load throw off. Steam dumping system isdesigned for continuous operation with 60% by-pass load.

CONDENSER (LMW DESIGN) :

Each condenser shell has been divided into upper and lower parts. Front

water box, shell and rear water box constitute the lower part. Two tube

plates and six support plates are located inside the lower body of the

condenser.

Front water boxes have been divided into two parts to make the

condenser two pass. End cover of water boxes is kept removable forfacilitating repairs and replacement of tubes. Manholes have been

provided for routine maintenance and visual inspection. Condenser tubes

are secured to the end tube plates by expanding and flaring of tube ends

which provides good sealing against entry of cooling water into steam

space. The tubes have been so arranged that there is equal distribution of

steam on the tube nest with minimum pressure drop.

With a view to allow relative expansion between tubes and the body of

lower part, lens type compensator has been provided in the body itself at

the rear water box end. The arrangement prevents deformation of the

body and damage to joint between tube and end tube plate.

Upper part of condenser has been designed to allow smooth flow of steam

over tube nest. It consists of steel flat walls strengthened from inside by

gratings of longitudinal and transverse rods and from outside by channels.

These rigid bars help the condenser to retain its shape against


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atmospheric pressure. Bellow has been provided in upper part to take

care of relative expansion between turbine and condenser.

Two sections of Low pressure heater no. 1 have also been located inside

the upper part of condenser. Condenser is supported on springs in order

to allow expansion along the height. The weight of condenser is taken bythe springs whereas weight of CW and condensate collected in hotwell as

well as the thrust of springs during operation is transferred to turbine

foundation.

Steam throw device has been incorporated in each condenser for

dumping of steam into condenser during start up and sudden load throw

off. Steam dumping system is suitable for continuous operation with 30%

by-pass load.

3.0 OPERATION REGIMES :

Condenser back pressure at rated conditions with clean tubes is normally

as follows : 76 mm Hga with CW inlet temperature of 33o C and 89 mm

Hga with CW inlet temperature of 36o C. Back pressure changes with

change in water inlet temperature. CW temperature rise across condenser

is normally kept as 8-10o C and TTD as 4-5o C. When we add up CW inlet

temperature, temp. rise across condenser and TTD we arrive at saturation

temp. of steam in condenser. Condenser back pressure can be further

worked out for the saturation temperature of steam by referring to steam

table. The available back pressure of condenser can be compared with

the calculated back pressure to know the health of condenser. Further,

CW side pressure drop across condenser varies depending upon tube

length, water velocity in tubes, cleanliness of tube etc. Higher pressure

drop in condenser indicates fouling of tubes.

4.0 CRITICAL COMPONENTS :

(i) Tubes(ii) Tube plates(iii) Rubber cord(iv) Bellows, Springs(v) Steam throw device

5.0 FAILURE MECHANISM :

(i) Tubes : Erosion/corrosion/fouling of tubes caused by poormaintenance / non-operation of COLTCS / poor quality of coolingwater resulting in poor condenser vacuum / frequent shut downsdue to tube failures.


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(ii) Tube plates : Corrosion/pitting of tube plates and other surfacescoming in contact with raw water. Damage to surface of tube plateduring re-expansion of tubes.

(iii) Damage to protective coating / lining in water boxes.(iv) Cracks in Bellows/Springs which are constantly expanding/

contracting.(v) Cracks/pitting of weld joints.(vi) Deformation of rubber cord.(vii) Erosion of Steam throw device holes.(viii) Mechanical damage to condenser internals and other stressed

components.

6.0 CONDITION ASSESSMENT :

(i) Condenser tubes :

Visual examination to assess tube condition such asdiscoloration, bending/deformation and fouling.

Hydraulic test / Water fill test to identify tube failure and re-expansion work.

Eddy Current Testing (ECT) of approx. 10 % tubes ofcondenser, especially if history indicates frequent tubereplacements.

(ii) Water Boxes, Water chamber and Tube Plates :

Visual inspection of tube plate for any mechanical damage or

corrosion marks. Visual inspection of water box, water chamber walls for

corrosion, pitting and condition of protective coating/painting.

Visual inspection of all weld seams on internal and externalsurfaces.

DPT / 10% UT of welds.

Visual examination of bolts/fasteners for any damage, samplehardness/microstructure test for their strength.

(iii) Shell Internals, Stiffening pipes / Rods :

Visual inspection of tube bundle for bending or any other type ofdamage to baffles, tube hole and tube interface area for anymechanical damage etc.

Visual inspection of weld joints, tie rods / stiffening pipe / rodjoints etc.

Visual inspection of air ducts / air pipes & their seal welds.

DPT / 10% UT of welds depending upon their requirement.


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(iv) Bellows, Springs, STD holes :

Visual inspection of bellows, welding joint of involutes, springelements, spring cages, STD holes, dimensional check ofsprings.

DPT / 10% UT of bellow joints.

(v) Condenser vacuum :

Water fill test of condenser steam space.

Helium Leak Detection of vacuum system.

MULTI-FREQUENCY EDDY CURRENT TESTING (ECT)

Eddy Current Testing is based on inducing electric currents in a conducting

material using a high frequency alternating current through a coil. Thechanging magnetic field induces circular currents known as eddy currentsin the job. Magnetic field produced by eddy currents opposes the primarymagnetic field which gives rise to change in impedance. Surface defectssuch as cracks, pits, change in dimensions alter the eddy current pattern.These variations are electro-magnetically decoded to produce the flawpattern in the material. ECT has been widely used world over to assesstube condition and integrity, especially the remaining wall thickness. TheECT System essentially consists of multi-frequency eddy current generator,a transducer, a pneumatically driven probe driver mechanism for travel intothe tubes, signal recorder, software, calibrated probes etc.

The failure signals received during eddy current testing in-situ is marred bysignals picked up from the interface surfaces of tube and support plate. Byemploying multi frequency testing technique, such additional signals areneutralized producing accurate and true results of defects.

HELIUM LEAK DETECTION TECHNIQUE

Various methodsare being / have been adopted by power station maintenanceengineers for arresting air leakage. Water fill tests, smoke test, steam pressuretest, ultrasonic test, freon tests are some of the common ones. However, each isassociated with one or the other deficiencies and therefore are not independentlyeffective. For example, the water fill test requires a shut down of the unit andincase of an error, cannot be repeated; it requires large amount of DM water andcan be less effective if stations other water systems are also leaking. Also, dueto environmental regulations, some of these are banned.


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Helium Leak Test is practically found to be the most sensitive andconvenient method. As the detection can be conducted during operation ofthe TG Set, it is repeatable and therefore finds favour with O&M staff. Useof helium is

very sensitive. The test kit includes a base unit which is complete withhelium sensor, probe, helium pump, LCD indicators, console, power unitetc; portable leak indicator integral with spray arrangement and portable

(remote) receiver. Helium is advantageous because it is non toxic, noninflammable, relatively inexpensive and quickly diffuses through smallleaks. In addition, helium is non reactive with other chemicals and its verylow presence in atmosphere, makes leak detection

The process of leak detection is carried out during operation of TG set. Nospecific preparation is required before the test. The helium detector isplaced near the vacuum pump / ejector with detector probe connected totheir (air evacuating devices) exhaust. The portable indicator is connectedto a portable helium cylinder while the remote sensor is fastened to the beltof the personnel carrying the indicator. Alternately, welding torch with extra

long hoses is connected to helium cylinders and carried along for sprayingthe gas at the suspected joints / parts. As helium is lighter than air,spraying commences from the operating floor such as the L P T joints,glands moving down to lower floors till the ground / condenser level is verysensitive. The test kit includes a base unit which is complete with heliumsensor, probe, helium pump, LCD indicators, console, power unit etc;portable leak indicator integral with spray arrangement and portable(remote) receiver. Helium is advantageous because it is non toxic, noninflammable, relatively inexpensive and quickly diffuses through smallleaks. In addition, helium is non reactive with other chemicals and its verylow presence in atmosphere, makes leak detection nonvenient.

The process of leak detection is carried out during operation of TG set. Nospecific preparation is required before the test. The helium detector isplaced near the vacuum pump / ejector with detector probe connected totheir (air evacuating devices) exhaust. The portable indicator is connectedto a portable helium cylinder while the remote sensor is fastened to the beltof the personnel carrying the indicator. Alternately, welding torch with extralong hoses is connected to helium cylinders and carried along for spraying

AIR, VAPOUR &HELIUM

EXHAUST TO

HELIUM LEAK DETECTION


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the gas at the suspected joints / parts. As helium is lighter than air,spraying commences from the operating floor such as the L P T joints,glands moving down to lower floors till the ground / condenser level isreached. Sprayed helium enters the vacuum system along with the air andtravels into condenser, to the vacuum pump / ejector and finally exhausted

into atmosphere. Here, the probe of the detector allows part of the mixtureto get sucked towards the base unit where the helium is sensed and asignal is received both in the base unit and the remote unit. This helps theoperator to cover a large area with ease. The time taken by helium to travelfrom being sprayed till receive of signal, is 20 30 seconds.

Advantages : The following are benefitsof adopting helium leak test :

Detects the smallest of air leakage thus improving condenser

vacuum.

Possible to carry-out test during unit operation; shutdown not

required. Minimum manpower required; two persons are sufficient.

Attending leakage and simultaneous retest possible to gauge

effectiveness.

Conducting test is economical.

7.0 R&M AND LIFE EXTENSION :

Based on 6.0 as above a comprehensive component material defectinformation is generated. This is compared with and supplemented byoperational and historical data. Wherever applicable, mechanical and

thermal calculations are also performed to assess remaining life.The above analysis shall form the basis for redesign, replacement andrepair in order to meet improved performance and life expectancy. TheRLA study will also bring out the need to have better maintenance andinspection schedules.

*****


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CONDENSER(LMWD

ESIGN)


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CO

NDENSER(LMWD

ESIGN)


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S.T. COOLERS

TURBINE OIL COOLER (210/250/500 MW)&

CONTROL FLUID COOLERS (500 MW)

1.0 INTRODUCTION :

TURBINE OIL COOLER:The function of the oil cooler in the bearing oil circuit is to cool the oil usedfor lubricating and cooling the turbine/generator bearings.

CONTROL FLUID COOLER:The function of the control fluid cooler is to cool the control fluid used for

the control system of turbine.

2.0 CONSTRUCTION :

TURBINE OIL COOLER:Essentially, the oil cooler consists of tube-nest, inner and outer casings,upper and lower water boxes. The oil to be cooled enters the cooler outercasing through the inlet nozzle at the top, flows to the inner casing, andthrough the disc and doughnut baffle openings. The baffles are held inposition by spacers mounted on steel rods. This arrangement forces theoil to follow a cross flow pattern while flowing to the outlet nozzle at the

bottom. The inner casing, holding the doughnut baffles, is in two halves,bolted together and is attached to the lower tube plate by screws. Tubesare roller expanded in the tube plates.

The lower water box, with partition between cooling water inlet and outletbranches, is bolted with the shell flange and lower tube plates. The tubenest is free to expand upwards due to differential thermal expansion.Dummy rods have been provided in the tube nest in line with the partitionwall to check oil bypassing within the tube nest.

CONTROL FLUID COOLER:

The cooler consists of tube-nest, shell & upper and lower water boxes.The fluid to be cooled enters the cooler shell through the inlet nozzle atthe top and flows through the baffles. The baffles are held in position byspacers mounted on steel rods. This arrangement forces the fluid tofollow a cross flow pattern while flowing to the outlet nozzle at the bottom.Tubes are roller expanded in the tube plates.


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The lower water box with partition between cooling water inlet and outletbranches, is bolted with the shell flange and lower tube plates. The tubenest is free to expand upwards due to differential thermal expansion.


Each TG set is provided with 2X100% coolers. While one cooler is put inoperation the other one remains as stand-by.Oil cooler is designed with temperature of oil at inlet to cooler as 65 oC and45oC at outlet. These temperatures are 55oC and 45oC respectively forLMW set.

Control fluid cooler is designed with temperature of fluid at inlet to cooleras 55oC and 49oC at outlet.

4.0 CRITICAL COMPONENTS :

(i) Rubber O-rings(ii) Cooling tubes(iii) Tube plates

5.0 FAILURE MECHANISM :

(ix) O-rings : These are provided between parting planes for sealingpurpose and get damaged due to wear & tear. Whenever the cooleris disassembled for attending to leakage or servicing, these are tobe replaced.

(x) Tubes : Erosion/corrosion of tubes is caused by poor maintenance /

poor quality of cooling water resulting in tube failure.(xi) Tube plates : Corrosion/pitting of tube plates coming in contact withraw water. Damage to surface of tube plate during re-expansion oftubes. Poor maintenance of water box internal surface / protectivecoating.

6.0 CONDITION ASSESSMENT :

(i) Visual examination to assess tube condition such as discoloration,bending/deformation and fouling.

(ii) Hydraulic test to identify tube failure and re-expansion work.(iii) Visual inspection of tube plate for any mechanical damage or

corrosion marks.(iv) Visual inspection of water box for corrosion, pitting and condition of

protective coating/painting.(v) Visual inspection of all weld seams on internal and external

surfaces.(vi) DPT / 10% UT of welds depending upon their requirement.


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(vii) Visual examination of bolts/fasteners for any damage, samplehardness/microstructure test for their strength.

(viii) Visual inspection of tube bundle for bending or any other type ofdamage to baffles, tube hole and tube interface area for mechanicaldamage etc.


Based on 6.0 as above a comprehensive component material defectinformation is generated. This is compared with and supplemented byoperational and historical data. Wherever applicable, mechanical andthermal calculations are also performed to assess remaining life.The above analysis shall form the basis for redesign, replacement andrepair in order to meet improved performance and life expectancy. TheRLA study will also bring out the need to have better maintenance andinspection schedules.


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L.P.HEATERS (210/250/500 MW)

1.0 INTRODUCTION :

L P Heaters are employed in regenerative feed heating cycle of Turbine.

Their function is to heat up the condensate flowing through the tubes byusing the heat of extracted steam on shell side so as to improve theoverall cycle efficiency.

2.0 CONSTRUCTION :

Two types of LP Heater designs generally are provided i.e. Vertical LPHeaters and Horizontal LP Heaters based on layout requirement of powerstation. Where as vertical LP Heaters have been provided only in 210 MWthermal sets, horizontal LP Heaters are being provided in 210 / 250 / 500MW sets due to their better design features.

2.1 VERTICAL LP HEATER:

This type of heater is vertically mounted, with bolted joint between shell,tube system and water box, which enables the withdrawal of tube bundlefor maintenance work by dismantling the bolted joint.

Shell is made of carbon steel with cylindrical construction thataccommodates steam inlet, shell drains, drains inlet from higher heater,shell vent and shell relief valve connections. Dished end welded at lowerend and upper end is provided with flange which is bolted with tube plate

and water box. Steam inlet connection is kept near upper end of shell.Supports are also provided for vertical mounting of heater.

Tube nest consists of Admiralty Brass U-tubes for LP Heater No.1&2 and90/10 Cu-Ni U-tubes for LP Heater No.3, Carbon steel tube plate, baffles,impingement baffles for steam inlet and drain inlet. Tubes are roller-expanded in tube plate holes. Suitable arrangement is provided forcontinuous venting of non-condensable which ensures efficient use ofsurface area provided. Impingement plate is provided at the steam inletconnection to avoid direct impingement of steam on the tubes.

Water box is welded carbon steel construction provided with dished end atupper end. Pass partitions are welded in water box to make it a four-passdesign. Condensate inlet and outlet connections are provided on waterbox. Sentinel type water expansion valve is provided to protect the heateragainst excessive pressure. Suitable lifting lugs are provided for handlingof water box and heater assembly.


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2.2 HORIZONTAL LP HEATER:

This type of heater is horizontally mounted, with shell welded to the tubeplate through a skirt. Instead of tube bundle withdrawal, shell withdrawal isprovided for major maintenance work for which a shell cutting-line is

provided.

Carbon steel cylindrical shell is completely welded type thataccommodates steam inlet, shell drains, drains inlet from higher heater,shell vent and shell relief valve connections. Dished end is welded at oneend and other end is welded with tube plate through shell skirt. Steam inletconnection is kept at thermal balance of tube Nest. Lugs are provided fortransportation and lifting during its withdrawal.

Tube nest consists of welded stainless steel U-tubes to ASTM-A688TP304, Carbon steel tube plate, baffles, impingement baffles for steam

inlet and drain inlet. Tubes are roller-expanded in steel tube plate holes.For best utilization of tube surface, central air duct is provided forcontinuous venting of non-condensable. With central air ducting, steamfrom all round is directed to center, thus air blanketing is avoided. A shortskirt, which form part of shell during assembly, is welded to tube plate.Stainless steel ring is welded over the tube nest at the location of shellcutting to avoid approach of flame at the tubes while shell cutting.

Water box of carbon steel plate is welded to tube plate on the other side.Pass partition is welded to tube plate on one side and bolted to innercover on other side, so that tube plate is accessible for plugging and

inspection. Suitable supports are provided for mounting of heater.


Four LP Heaters are provided in 210 MW Russian variant, i.e. LP HeaterNo. 1,2,3 and 4 where as three LP Heaters are provided in 210 / 250 / 500MW KWU variant, i.e. Heater No. 1,2 & 3. Heater No. 1 may be providedinside or outside the condenser dome based on layout of the powerstation however LP Heater No. 1 for 500 MW set is invariably provided incondenser dome.

Shell side of LP Heaters is designed for 7.0 Kg/cm2 (g) & 3.5 Kg/cm2 (g)pressure for Russian & KWU sets respectively. The tube side designpressure corresponds to shut off head of the condensate extraction pump.

A typical value of temperature rise of condensate between lowest LPHeater and highest LP Heater is given as below:

500 MW set: 480 C to 123.50 C (from LPH-1 inlet to LPH-3 outlet)


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210 MW/250 MW (KWU): 48.50 C to 1210 C (from LPH-1 inlet to LPH-3outlet)210 MW (Russian): 51.50 C to 1570 C (from LPH-1 inlet to LPH-4 outlet)

4.0 CRITICAL COMPONENTS:

(i) Tube bundle (Vertical LP Heaters with non-ferrous tubes speciallywith admiralty brass tubes. Tube bundle in horizontal heaters withwelded SS tubes though critical in application but no failure hasbeen reported so far from any site indicates better reliability andrather maintenance free operation of horizontal design.)

5.0 FAILURE MECHANISM:

(i) Erosion / corrosion of non-ferrous tubes (mainly admiralty brass

tubes) is caused under following circumstances :

a) During by passing of one the heater in the chain, additionalflows are coming to heater under operation leading to highersteam velocities resulting in tube failure.

b) Dezincification of admiralty brass tube is caused due to copperpick leading to tube failures.

c) In case parameters of extraction steam (especially in LPH-4 ofRussian variant) are not properly controlled then it will exposethe tubes to high temperature leading to tube failure.

(ii) Tube failure may also be caused due to excessive vibration underreasons given at (i) a) and (i) c) above.(iii) Wear and tear and mechanical damages are also reasons of failure

of tubes and other components e.g. baffles etc.

6.0 CONDITION ASSESMENT :

(i) Visual inspection of tube bundle for failure/damage of tubes anddamage to other parts e.g. baffles etc.

(ii) Hydraulic testing to identify tube failure and plugging of failedtubes.

(iii) Visual inspection of tube plate for any mechanical damage.(iv) Visual inspection of water box and shell internal surfaces for any

damage.(v) Visual inspection of all weld-seams.(vi) DPT of weld seams to assess soundness of the welds.


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Based on 6.0 as above a comprehensive component material defectinformation is generated. This is compared with and supplemented byoperational and historical data. Wherever applicable, mechanical and

thermal calculations are also performed to assess remaining life.

The above analysis shall form the basis for redesign, replacement andrepair in order to meet improved performance and life expectancy. TheRLA study will also bring out the need to have better maintenance andinspection schedules.


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