Induction 8

7/30/2019 Induction 8

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TURBINE STEAM AND EXTRACTION CYCLE

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CONDENSATE AND FEED WATER CYCLE

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CONDENSER COOLING WATER CYCLEAND COOLING TOWERS

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1.0WorkingPrinciple:

Whensteamisallowedtoexpandthroughanarroworifice,itabsorbsthekineticenergyatthecostofenthalpy(heatenergy).Thiskineticenergyofsteamischangedinto

mechanicalenergywhensteammovesovertheturbineblades.

Motiveforcetotheturbineisnotproducedduetostaticpressureofthesteamorfrom

anyimpactofthesteamjet.Thebladesaresodesignedthatthesteamwillglideonandoff

thebladewithoutanytendencytostrikeit.WhensteammovesovertheRotorbladesits

directioniscontinuouslychangingandcentrifugalpressureisexertedontheblade,normalto

thebladesurfaceatallthepoints.Thetotalmotiveforceactingonthebladesisthusthe

resultantofallthecentrifugalforceplusthechangeofmomentum.Thiscausestherotational

motionoftheblades.

SteamInlet

ForceF

SteamOutlet

RotorBladeProfile

2.0TYPESOFTURBINE:

Accordingtotheprincipleofactionofthesteam,turbinecanbeclassifiedas:

a)ImpulseTurbineb)ReactionTurbine

a)ImpulseTurbine:

Thesteamisexpandedinthefixednozzles.Thusthevelocityofsteamisincreased

atthecostofreductioninpressure.Thishighvelocitysteammovesovertherotorbladeand

impartsitskineticenergytotherotorblade.Nopressuredroptakesplacewhensteamglides

overtheblade.

b)ReactionTurbine:

Inthistypepressuredropsbothinfixedaswellasmovingblades.Inotherwords

steamexpandsonboth,fixedandmovingblades.Fixedbladesworkasnozzleswhereas

steamexpansiononmovingbladeproducesreaction.

Theexpansiononmovingandfixedbladeistheresultofthedesignofbladeprofile.

STEAMTURBINE

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Let

I0

-betheenthalpyofthesteamattheentrancetothefixedblade.

I1

-attheoutletoffixedbladewhichentersonthemovingbladewiththesameenthalpy.

I2

-istheenthalpyattheoutletofthemovingblade.

ThenthefactorAisknownasdegreeofReaction.

I1

-I2

A=

I0-I2

IfA0.5thentheturbineisareactionTurbine.

3.0Compounding:

Steamvelocitybecomesveryhighifsteamisallowedtoexpandinasinglestage

(singlerowofnozzleandblade).Hencetherotationalspeedoftheturbinebecomesveryhigh

andimpracticable.

Soenergyconversionofsteamisdoneinnumberofstepstoachievethepracticable

desiredspeedoftheturbine.Thisisknownascompounding.

Followingarethevarioustypesofcompounding:

a)VelocityCompounding:

Inthistypeofcompoundingentiresteampressuredroptakesplaceinonesetof

nozzle.Thekineticenergysoconvertedinnozzleisutilisedinnumberofrowmovingand

guideblades.Theroleofguidebladeisjusttochangethedirectionofsteamjetandguideit

tonextrowofmovingblades.Thistypesofturbineisalsocalledcurtisturbine.

I0

FixedBlade

I1

MovingBlade

I2

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b) Pressure Compounding :

In place of single nozzle ring, numbers of nozzle ring arranged alternatively after

moving in blade wheels. Thus instead of allowing the pressure drop in one step, It is done in

no of steps.

Steam is passed through one nozzle ring in which it is partially expanded. It thenpasses over the first moving blade wheel, where most of its velocity is absorbed. Then this

steam passes through second nozzle ring. The velocity so obtained, is again absorbed by the

second moving wheel and so on, the process is repeated till whole of the pressure is absorbed.

This type of turbine is also called Rateors turbine after its Inventor.

c) Pressure Velocity Compounding :

This is the combination of both previous methods has the advantage of allowing a

higher pressure drop in each stage and so less stages are necessary. Hence for a given

pressure drop the turbine will be shorter. But the diameter of Turbine is increased at each

stage to allow for the increasing volume of steam. This type was very popular. But it is rarely

used now as efficiency is quite low.

4.0 Multistage Reaction Turbine :

In this type, number of rows of moving blades attached to the rotor and number of

rows of fixed blade to the casing, so that each stage utilizes a portion of energy of steam.

Theoretically this may be called pressure compounded turbine as the pressure of the steam

drops over the succeeding stages. The fixed blades compared to the nozzle used in the impulse

turbine, steam is admitted over the whole circumference and passing through the first row of

fixed blades undergoes a small drop in pressure and its velocity is increased.

It then enters the first row of moving blades and as in the direction and hence momentum

giving an impulse on the blades. During the steam passes through the moving blade, it

undergoes a further small drop in pressure resulting in an increase in velocity which gives riseto a reaction in the direction opposite to that of the added velocity. In this way, the impulse

reaction turbine differs from the pure impulse turbine.

5.0 General Description of Turbine

5.1 The turbine is condensing, tandem compound, three cylinder, horizontal, disc and

diaphragm type with nozzle governing and regenerative feed water heating. The double flow

L.P. Turbine incorporates multi-exhaust in each flow.

The complete turbine assembly is mounted on pedestals and sole plates, which are

designed to ensure that the components are free to expand whilst correct alignment is

maintained under all conditions. Live steam from the Boiler enters to two Emergency Stop

Valves (ESV) of High Pressure Turbine, From ESV steam flows to the four Control valves (CV)

mounted on the casing of High Pressure Turbine (HPT) at the middle bearing side. Control

Valves in turn feed the steam to nozzle boxes located inside the HPT.

The high pressure turbine (HPT) comprises of 12 stages, the first stage being governing

stage. The steam flow in HPT being in reverse direction, the blades in HPT are designed for

anticlockwise rotation, when viewed in the direction of steam flow.

After passing through H.P. Turbine steam flows to boiler for reheating and reheated

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steam comes to the Intermediate Pressure Turbine (IPT) through two Interceptor valves (IV)

and four Control Valves (CV) mounted on the IPT itself.

The Intermediate Pressure Turbine has 11 stages. H.P. & I.P. rotors are connected by

rigid coupling and have a common bearing.

After flowing through IPT, steam enters the middle part of low pressure turbine (LPT)through two cross-over pipes. In L.P. Turbine, steam flows in the opposite paths having four

stages in each path. After leaving the L.P. Turbine the exhaust steam condenses in the surface

condensers welded directly to the exhaust part of the L.P. Turbine.

5.2 Rotors of Intermediate and low pressure turbine are connected by a semi flexible

coupling.

The direction of rotation of the rotors is clock-wise when viewed from the front bearing

and towards generator. The three rotors are supported on five bearings. The common bearing

of H.P. & I.P. rotors is a combined Journal and radial thrust bearing.

The anchor point of the turbine is located at the middle foundation frame of the front

exhaust part of low pressure cylinder. The Turbine expands towards the front bearings by nearly32 mm & towards generator by 3 mm in steady state operation at full load with rated parameters.

Turbine is equipped with a barring gear which rotates the rotor of turbine at a speed of

nearly 3.4 rpm for providing uniform heating during starting and uniform cooling during shut

down.

In order to heat the feed water in the regenerative cycle of the turbine, condensate

from the hot well of condenser is pumped by the condensate pumps, and supplied to the

deaerator through ejectors, gland steam cooler, four number L.P. heaters and gland cooler.

From deaerator the feed water is supplied to Boiler by Boiler feed pumps through three

number H.P. heaters. Extracted steam from the various points of the Turbine is utilised to heat

the condensate in these heat exchangers.

6.0 Turbine Support and Cylinder Expansion :

The complete turbine assembly is mounted on foundation frames, pedestals and sole

plates so designed that the components are free to expand or contract whilst correct alignment

is maintained during start-up, shut down and throughout the working temperature range.

6.1 Front Bearing Pedestal Support :

The front and bearing pedestal which houses a journal bearing, the main oil pump end

most of the governing system elements, rests on a sole plate secured to the foundations. This

arrangement is such that the pedestal is free to move in an axial direction with the expansion

and contraction of the turbine casing. The pedestal is held transversely in the sole plate by

axial guide key fitted along the axis of the turbine at the sliding surface between pedestal and

sole plate. Any tendency for the pedestal to lift is prevented by four inverted L shaped

clamps, two on either side of the pedestal.

6.2 Middle Bearing Pedestal support :

The pedestal rests on a sole plate secured to the foundation block. The pedestal is free

to move in axial direction, due to expansion of the machine. The transverse movement is

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restricted by key guiding axial movement. Any tendency for the pedestal to lift is prevented

by three inverted L shaped clamps, two on right side and one on the left side of the pedestal.

6.3 High Pressure Cylinder Support :

The outlet end of the high pressure cylinder is supported on the front bearing pedestaland the inlet end is supported on the middle bearing pedestal. Four lugs, two at the inlet end

and two at outlet end are cast integral with the bottom half cylinder flange at the horizontal

joint. These lugs rest on transverse keys, which are secured to the pedestal.

To maintain correct alignment and guiding for vertical expansion, vertical keys are provided

between cylinder and pedestals. Any tendency for the cylinder to lift at the supporting lugs is

prevented by L shaped clamps bolted to the pedestal pads, one at each support.

6.4 Intermediate Pressure Cylinder Support :

Four lugs, two at the inlet end and two at the outlet end are cast integral with the

bottom half of IPC flange at the horizontal joint. The inlet end of the intermediate pressure

cylinder rests on the transverse keys secured on the pads machined on the rear end of themiddle pedestal, and the exhaust end is supported on the transverse keys secured to low

pressure cylinder bottom half.

6.5 Low Pressure Cylinder Support :

The low pressure cylinder is supported on six foundation frames positioned around

bottom halves of exhaust casing. The foundation frames and the bottom halves of exhaust

part of LPC are joined by special bolts with spherical washers and clearance between the bolt

head and spherical washers allows for free expansion of the L.P. casing. The anchor points of

the turbine are located at the rear end of front exhaust part with two transverse keys.

7.0 Turbine Casing :7.1 High Pressure Casing :

The high pressure casing is made of creep resisting chromium-Molybdenum-Vanadium

(Cr-Mo-V) steel casting. The top & bottom halves of the casing are secured together at the

flange joint by heat tightened studs to ensure an effective seal against steam leakage.

Four steam chests, two on top and two or sides are welded to the nozzle boxes, which

in turn are welded to the casing at the middle bearing end. The steam chests accommodate

four control valves to regulate the flow of steam to the Turbine according to the load requirement.

Nozzle boxes and steam chests are also made of creep resisting Cr-Mo-V steel castings.

The High Pressure Turbine (HPT) comprises of 12 stages, the first stage being governing

stage. Each turbine stage consists of a diaphragm and set of moving blades mounted on a

disc.

7.2 Intermediate Pressure Casing :

The intermediate pressure casing of the turbine is made of two parts.

The front part is made of creep resisting Chromium-Molybdenum-Vanadium steel casting and

the exhaust part is of steel fabricated structure. The two parts are connected by a vertical

joint. Each part consists of two halves having a horizontal joint. The horizontal joint is secured

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with the help of studs and nut; These nuts and studs are made of creep resisting Cr-Mo-V

steel forgings. The control valves of I. P. Turbine are mounted on the casing itself.

In the Intermediate Pressure Turbine the nozzle boxes are cast integral with the casing.

There are 11 stages in the I. P. Turbine. The first stage nozzle segment of IPT is a welded

construction like other diaphragms and is mounted directly in the casing. Next two diaphragmsare also housed in casing while other 8 diaphragms are housed in three liners, which in turn

are mounted on casing.

From the Intermediate Pressure Turbine, steam is carried through two cross-over pipes

to the double flow low pressure cylinder. Each cross over pipe is provided with a compensator

for taking care of thermal expansion and to ensure that no heavy thrust or turning moments

are thrown on to the flanged connections at the intermediate pressure cylinder exhaust and

the low pressure cylinder inlet.

7.3 Low Pressure Casing :

The L. P. Casing consists of three parts i.e. one middle part and two exhaust parts. The

three parts are fabricated from weldable mild steel. The exhaust casings are bolted to themiddle casing by a vertical flange. The casings are divided in the horizontal plane through the

Turbine center line.

The lower half of the L. P. Casing has integral bearing pedestals, which houses the

following :

i) Rear bearing of intermediate pressure rotor.

ii) Coupling between IP & LP rotor.

iii) LP front & rear bearings.

iv) Generator coupling.

v) Generator bearing.

vi) Barring gear.

Steam enters the middle casing from top and then divides into two equal, axially,

opposed flows, to pass through four stages.

The last but one stages on each side are Baumanns stages. They expand a part of the

steam down to the condenser pressure and allows rest of the steam to expand through the

last stages.

7.4 Atmospheric Relief Valve :

To protect the L. P. Cylinder against excessive internal pressure, four atmospheric

relief valves are provided in the exhaust hoods.

Each assembly has 1 mm thick gasket, ring 525/755, clamped between valve seat and valve

disc. If due to some reasons the pressure at exhaust hood rises to 1.2 abs, then the valve disc

tries to lift and thereby ruptures the gasket ring, thus allowing the steam to exhaust into the

atmosphere in the turbine room.

8.0 Diaphragms and Lines

8.1 High Pressure Diaphragms & Liners

The HP diaphragms are housed in liners, which are in turn located in the grooves of the

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casing. All the liners are made of Cr-Mo-V steel castings and are in two halves connected at

the horizontal joint by bolts of suitable material. All diaphragms, designed for minimum

deflection, are divided at horizontal joint. The arrangement & support of the diaphragms

ensure correct radial alignment of the diaphragms without developing strain due to expansion.

The diaphragms are of welded construction.

8.2 Intermediate Pressure Diaphragms

The first two diaphragms are directly housed in casing. The other 8 diaphragms are

housed in three liners, which are in turn located in the grooves of the casing. All the liners are

made in two halves, split at horizontal joints and secured by studs and nuts.

Diaphragms from 14th to 22nd stages are of welded construction. 23rd stage diaphragm

is machined from high grade cast iron casting with cast-in guide blades.

8.3 Low Pressure Diaphragms:

The diaphragms are machined from high grade cast iron castings with cast in blades of

low carbon stainless steel. All diaphragms are divided on the horizontal joint fitted with keysto maintain accurate alignment. On each side, the first three diaphragms are fitted through

liners while last one is mounted directly in the casing.

The last stage diaphragms on each side of L. P. Flow are of different construction.

These diaphragms are of cast welded construction. The mild steel blades are welded to outer

ring (Steel casting) and inner plate. The diaphragms are divided at horizontal joints and are

secured with studs and nuts.

9.0 Rotors

9.1 High Pressure Rotor

The HP rotor is machined from a single Cr-Mo-V steel forging with integral discs. The

rotor is thermally stabilised to prevent abnormal deflection. The blades are attached to theirrespective wheels by T root fastening.

In all the moving wheels, balancing holes are machined to reduce the pressure difference

across them, which results in reduction of axial thrust. First stage has integral shrouds while

other rows have shroudings, riveted to the blades at periphery. The number of blades connected

by a single strip of shrouding is called a blade packet and the number of blades per packet is

decided from vibration point of view.

9.2 Intermediate Pressure Rotor

The IP rotor has seven discs integrally forged with rotor while last four discs are shrunk

fit. The shaft is made of high creep resisting Cr-Mo-V steel forging while the shrunk fit discs

are machined from high strength nickel steel forgings.

The blades on the integral discs are secured by T root fastenings while on shrunk fit

discs by fork root fastening. Except the last two wheels, all other wheels have shroudings

riveted at the tip of the blades. To adjust the frequency of the moving blades, lashing wires

have been provided in some stages.

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9.3 Low Pressure Rotor

The LP Rotor consists of shrunk fit discs on a shaft. The shaft is a forging of Cr-Mo-V

steel while the discs are of high strength nickel steel forgings.

Blades are secured to the respective discs by riveted fork root fastening. In all the stages,

lashing wires are provided to adjust the frequency of the blades.In the last two rows, stellite strips are provided at the leading edges of the blades to protect

them against wet steam erosion.

10.0 Turbine Bearing :

The three turbine rotors are supported on five bearings. The second bearing from front

pedestal side is a combined radial thrust bearing while all the others are journal bearings. The

rotors are located inside the turbine at thrust bearing. The high pressure and intermediate

pressure rotors are joined by rigid coupling and have been provided with a common bearing

while other ends are having their own bearings.

The lubricating oil to the bearings is supplied at a pressure of about 1 kg/cm2 and the

quantity of oil going to each bearing is controlled by the orifice plate fixed at its inlet end.

10.1 Thrust Bearing

The thrust bearing is of Mitchel type with bearing surface distributed over a number of

bearing pads lined with white metal. They are pivoted in a housing on the side of rotor thrust

collar. During operation, an oil film is formed between pads and thrust collar and there is no

metal to metal contact. A second ring of pads on the opposite side of thrust collar takes the

axial thrust, as may occur under abnormal conditions.

The radial thrust bearing is supported on it spherica seating at the journal bearing center line.

The inner surface of steel housing is machined spherical, matching with bearing sphere. The

bearing is in two halves bolted together. The whole radial thrust bearing is housed in middle

bearings pedestal.

10.2 Journal Bearing

The journal bearings Nos. 1, 3, 4 and 5 consist of outer shell of cast iron with an inner

shell lined with white metal. Both the shells are split at half joint and secured by bolts. The

pads on the outer shell are machined to bore diameter of bearing pedestals. For the fine

alignment, steel shims are provided under the pads.

11.0 Sealing Glands

To eliminate the possibility of steam leakage to atmosphere from the inlet and exhaust

ends of the cylinders, labyrinth glands of the radial clearance type are provided, which provide

a trouble free, frictionless sealing.

Each gland sealing consists of a number of sealing rings divided into segments, each segment

is backed by two flat springs. The sealing rings are housed in grooves machined in gland

bodies, which are in turn housed in the turbine casing, or bolted to the casing at ends.

Steam is supplied to the penultimate sealing chamber at 1.03 to 1.05 kg/cm2 abs. And at

temperature 1300 to 1500C from the header, where the pressure is maintained constant with

the help of an electronic regulator. Air steam mixture from the last sealing chamber is sucked

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out with the help of a special steam ejector to gland steam cooler.

Provision has been made to supply live steam at the front sealings of H. P. and I. P. Rotors to

control the differential expansion, when rotor goes under contraction during a trip out or

sharp load reduction.

12.0 Barring Gear :

The barring gear is mounted on the L. P. Rear bearing cover to mesh with spur gear on

L. P. Rotor rear coupling. The primary function of the barring gear is to rotate the turbine-

generator rotors slowly and continuously (Speed 3.4 rpm) during start up and shut down

periods when changes in rotor temperature occur.

When a turbine is shut down, cooling of its inner elements continues for many hours.

If the rotor is allowed to remain stand still during this cooling period, distortion of rotor begins

almost immediately. This distortion is caused by flow of hot vapors to the upper part of

casings, resulting in upper half of turbine being at a higher temperature, than lower half.

Hence to eliminate the possibility of distortion during shut down, barring gear is used to keep

the rotor revolving until the temperature change has stopped and casings have become cool.This also results in maintenance of minimum inter stage sealing clearances with higher operating

efficiency.

The same phenomenon is also observed during starting of the turbine when steam is

supplied to the sealings to create the vacuum. If the rotor is stationary, there would be non-

uniform heating of the rotor, which will result in distortion of rotors. The barring gear during

starting of turbine, would slowly rotate the turbine-generator rotor, and thereby resulting in

the uniform heating of rotor. Thus any distortion in the rotor would be avoided.

During starting period, operation of the barring gear eliminates the necessity of Breaking

away the turbine generator rotors from stand still and thereby provides for a more uniform,

smooth and controlled starting.

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231

GLANDS :

Glands are used on turbines to prevent or reduce the leakage of steam or air between

rotating and stationary components which have a pressure difference across them; this applies

particularly where the turbine shaft passes through the cylinder. If the cylinder pressure is

higher than atmospheric pressure there will be a general steam leakage outwards; it the

cylinder is below atmospheric pressure there will be a leakage of air inwards, and some sort

of sealing system must be used to prevent the air from entering the cylinder and the condenser.

You probably appreciate that glands for high and intermediate pressure cylinders have to

resist not only the ingress of air at no-load, but also the outward leakage of steam at full-load.

As most of the steam leaking from glands does no: further work in the turbine, there is a loss

of power output. For this reason every effort is made to reduce this power loss by an efficient

arrangement of seals and glands.

BALANCE PISTONS :

In high and intermediate pressure cylinders the glands may be used to neutralize part

of the axial thrust produced in the blading: in the case of 50 per cent reaction blading this is

considerable. The diameter of the gland is increased, so that the pressure of the steam acting

on the step face of the gland acts in opposition to the blading thrust; this enables smaller

thrust bearing to be used. The large diameter gland is known as a balance piston, or dummy

piston.

WATER SEALED GLANDS :

Some turbine designs incorporate a shaft gland which depends on a water seal to

prevent steam or air leakage. A typical seal arrangement (see fig.1) consists of a shaft mountedimpeller with a series of vanes or pockets machined on both faces. The impeller is contained

within an annular chamber, and , when water is admitted to the chamber, the impeller vanes

force the water to rotate at a speed approximately equal to the impeller speed. The seal is

relatively inefficient at low speeds and auxiliary labyrinth glands must be used, in conjunction

with high capacity air pumps, to raise vacuum when starting. Water isusually injected into the

seal at approximately half of the full operating speed.

The side clearances between the impeller and seal chamber must be fairly small, and

so the use of this seal is restricted to positions on a turbine where the axial differential

expansions are within the effective limits of impeller and seal chamber clearance. When this

type of seal is used on a high pressure turbine, the seal cannot absorb the full differential

pressure SO1 labyrinth glands are used to break the pressure down to a figure which thewater seal can handle.

Since a water seal absorbs and generates heat, the water contained in the annular

chamber of the water sealed gland is continuously evaporated; the water losses are made up

from a header tank.

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CARBON RING GLANDS :

The carbon ring is normally made by clamping a number of carbon segments aroundthe turbine shaft, using circumferential garter springs. The ring is fitted with a very fine

clearance around the shaft, but if a run occurs little damage is done because the ring material

is self lubricating. Carbon ring glands are not found on large modern turbines, as their maximum

operating temperature and maximum shaft speed are limited to about 1200C (2500 F) and 46

m/s (9000 ft/min). However, in spite of these limitations, they are more efficient than labyrinth

glands.

DIAPHRAGM GANDS :

We saw in part 3, Chap. 3, that a pressure drop exists across each diaphragm or an

impulse turbine. To prevent steam by passing the steam nozzles, by traveling along the shaft,

it is necessary to fit a gland at this point. Diaphragm glands, as they are called, are invariablyof the labyrinth type the length of the gland depends on the pressure difference across the

diaphragm.

LABYRINTH GLANDS :

In modern turbines the labyrinth gland has superseded the carbon ring gland, because

it can withstand high pressures and temperatures and requires little maintenance.

The labyrinth gland provides a series of very fine annular clearances, in the gap between

the cylinder wall and the shaft. The steam is throttled through this gap and its pressure

reduced step by step. In expanding through each clearance, the steam develops kinetic energy

at the expense of its pressure energy; ideally, the kinetic energy is converted by turbulence

into heat with no recovery of pressure energy. In this way, the pressure is progressively

broken down as the steam is throttled at successive restrictions. By keeping the clearance

area sufficiently small, the quantity of energy lost may be kept low, and as increases in

turbine output occur the gland leakage loss becomes proportionately less.

To reduce the clearance area, glands are made with a diameter as small as possible,

and clearances as of shaft strength and radial clearance, by the clearance within the bearing,

and by the possibility of shaft distortion.

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Turbine Gland System of 210 MW LMW Turbine :

To eliminate the possibility of steam leakage to atmosphere from the inlet and exhaust

end of Cylinders, labyrinth gland sod radial clearance type are provided, which provide trouble

free frictionless sealing.

Each gland sealing consists of a number of sealing rings, divided in to segments, each

segment is backed by two Flat Springs. The sealing rings are housed in grooves, machined in

Glands bodies, which are in turn housed in the Turbine casing or bolted to the casings at

ends.

Steam is supplied to the penultimate sealing chamber at 1.03 to 1.05 Kg/cm2 pressureand at 130 to 150 C from the header, where the pressure is maintained constant with the help

of control valve AS 55. Air/ Steam mixture from the last sealing chamber is sucked out with

the help of Special Ejector of Gland cooler No. 1.

Provision is made to supply live steam at front sealing of HP and IP Rotors to control

the Differential Expansion, when rotor goes under contraction during the Turbine Trip out or

sharp load reduction.

Glands must allow for axial expansion of the shaft and casing to take place without

causing a rub. On the other hand, if a rub does take place because of shaft vibration it is

desirable that the heat generated is minimized to prevent serious frictional heating of the

shaft and possible distortion. A typical modern gland comprises stantionary fins on spring-

loaded sectors, while the shaft is either smooth or castellated. If a rub should occur, thesectors receive the generated heat and can be replaced readily if they are damaged.

Designs of labyrinth gland at present in use are shown in Fig.2

In fig. 2 (a) the clearances are staggered to ensure that no kinetic energy is carried

over from one gap to the next. The stationary fins are axial, so that if a rub occurs, the heat

causes them to expand relative to their fixing, and they move out to increase the clearance.

Fig. 2(b) shows a resilient gland the stationary part being divided into sectors, each of

which is spring-loaded in an annular groove. If a rub occurs, the sector would give, and the

low contact pressure would ensure that only a little heat is generated.

The gland shown in Fig. 2 is of the vernier type, the fins being much finer than in the

previous designs. By making the pitch of the fins on one side 10 percent greater than the

pitch of the mating fins, only one fin in nine or ten will be opposite another fin. If a rub occurs,then only exactly opposing fins make contact, so the amount of heat generated is small.

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Turbine Bearings :

The three turbine rotors are supported on five bearings. The second bearing from front

pedestal is a combined radial thrust bearing while all other are journal bearings. The rotors

are located inside the turbine at thrust bearing. The high pressure and intermediate pressure

rotors are joined by rigid coupling and have been provided with a common bearing whileother ends are having their own bearings.

The lubricating oil to the bearings is supplied at a pressure of about 1.0 kg/cm 2 and the

quantity of oil going to each bearing is controlled by the orifice plate fixed at its inlet end.

Thrust bearing :

The thrust bearing is of Michel type with bearing surface distributed over a number of

bearing pads lined with white metal. They are pivoted in housing on the side of rotor thrust

collar. During operation, an oil film is formed between pads and thrust collar and there is no

metal to metal contact. A second ring of pads on the opposite side of thrust collar takes the

axial thrust, as may occur under abnormal conditions.

The radial thrust bearing is supported on a spherical seating at the journal bearingcentre line. The inner surface of steel housing is machined spherical, matching with bearing

sphere. The bearing is in two halves bolted together. The whole radial thrust bearing is housed

middle bearings pedestal.

Journal bearing :

The journal bearings Nos. 1, 3, 4 and 5 consist of outer shell of cast iron with an inner

shell lined with white metal. Both the shells are split at half joint and secured by bolts. The

pads on the outer shell are machined to bore diameter of bearing pedestals. For the fine

alignment steel shims are provided under the pads.

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Purpose of a condenser in a Vapor Cycle is to create sub-atmospheric pressure (i.e.

vacuum) so that steam can expand to lowest possible pressure and higher work output can be

obtained from the steam cycle. Condensation of vapor causes reduction in volume. At 0.07

Kg/cm2 absolute pressure, 1 Kg of water has a volume of 0.001 m3 whereas volume of 1 kg

water vapor at this pressure is 20.92 m3. Thus condensation of steam causes reduction in

volume by 20920 times.

Daltons Law : It states that pressure in closed vessel is equal TO THE SUM OF PARTIAL

PRESSURES of each gas in the mixture.

In condenser, mixture of air and water vapor is present.

Therefore, Pc = Pa + Ps

Where Pc is condenser absolute pressure

Pa is Air pressure

Ps is saturation steam pressure

Pressure of air will exert its own partial pressure, which will be added to the vapor

pressure, and thus absolute pressure will increase and vacuum will fall.

Condensation occurs when vapor comes in to contact with a surface that is at a

temperature lower than the saturation temperature corresponding to the vapor pressure. The

liquid thus formed due to condensation may either wet or does not wet the solid surface. If

the liquid wets the surface, the condensate flows on the surface in the form of a film. Such

condensation is called film condensation. On the other hand, if the condensate does not wet

the surface, it gets collected in the form of droplets, and the droplets falls off the surface by

gravity. This type of condensation is called drop condensation. The rate of heat transfer incase of drop condensation is very high compared to that of film condensation. Condensers in

power plants are film condensation types, as it is practically not possible to design a condenser

in which drop condensation shall take place through the period of its operating life.

Condensers used in Power Plant :

Surface condensers in which film condensation takes place are the most commonly

used condensers in thermal power plants. It consists of a air tight shell in which tightly

packed tube bundles are arranged. They are tube and shell type heat exchangers, in which,

steam condenses on the out side surface of tubes and cooling water flows through tubes. The

condenser consists of

1. Shell2. Steam Inlet with tapered steam dome

3. Cooling water inlet

4. Cooling water outlet

5. Tubes

6. Condensate Outlet

7. Air outlet

CONDENSERS

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Schematic of a two pass condenser

Condenser is composed of a steel shell with water boxes on each side, the right one

devided for two water passes. Water tubes are rolled at each end in to tube sheets. Steel

support plates are fitted at intermediate points to prevent vibrations of tubes.

Hotwell receives the condenaste and act as storage for the same. Hotwell volume is sufficient

to hold condensate formed in 1 to 2 minutes period of Turbine operation.

Layout of tubes in the Condensers (used in modern high capacity plants) is in the

shape of a Funnel. Most number of tubes and largest tube passage area is at the place where

steam enters in to the condenser. As the steam condenses and its volume decreases, there

are fewer tubes and smaller areas. Steam is made to enter the tube bundles from all sides

towards a central air cooler for deaeration. Long tubes (of the order of 30 to 50 feet length are

used. For obtaining proper and equal distribution of steam, a well-tapered steam dome is

provided above the tubes. In some designs, expansion joint is provided between Turbine

exhaust duct and condenser so that condenser can be rigidly mounted on floor. In some

designs, condenser is rigidly connected (by means of welding or bolting) to the turbine steam

exhaust duct and spring mounted on the floor.

Cooling water is passed through the condenser either in single pass or in two passes.

In two pass condenser, inlet and outlet water boxes are provided with partitions. There can be

independent inlet and outlet Cooling water connections. Such condensers are often designed

in such a way that half of the condenser can be isolated for cleaning while other half can

remain in service.

Stainless Steel 304, Admiralty, Aluminum-Brass Muntz metal, 70-30 Copper Nickel are

the widely used materials for condenser tubes. Outer tube diameters are 3/4th inch, 7/8th inch

or 1 inch.

Condenser performance is an important factor for obtaining optimum performance

levels from the plant.

Following figure explains a two-pass condenser.

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237

STARTING EJECTOR :a) The basic unit consists of a nozzle, suction chamber and a diffuser. The suction chamber

is connected with the condenser. The working steam expands in the nozzle and its pressure

energy is converted into kinetic energy thereby producing supersonic velocity jet. This

jet, discharging from the nozzle entrains the surrounding air, which finally comes from

the condenser. The air and steam mixture is then compressed in the diffuser to a pressure

slightly above atmospheric pressure and then exhausted into the atmosphere.

b) Starting ejector is recommended to be used for accelerating the initial pulling of vacuum.

During this period, starting ejector operates in parallel with the main ejectors. The

working medium for this ejector is steam of low parameters, which can be taken either

from the deaerator of auxiliary source. The pressure and temperature of this steam arenot to exceed 4.5 Kg/cm2 and 2500C respectively.

c) This starting ejector is switched off as and when the vacuum in the condenser reaches

500-600 mm of Hg column.

MAIN EJECTORS :

a) Main ejectors have been used for extracting non-condensable gases from the coldest

zone of the condenser. The working medium for these ejectors is steam of low parameters,

which can be taken either from the deaerator or auxiliary source. The pressure and

temperature of this steam are not to exceed 4.5 Kg/cm2 (g) and 2500C respectively. The

energy of steam is retrieved to the fullest possible extent as the ejectors are interposedin the feed heating cycle thereby improving the overall efficiency of the cycle.

b) These ejectors consist of three compression stages with inter coolers and after coolers.

The first stage of the suction chamber is connected to the condenser. The main assemblies

of the ejector are :

i) Suction chamber

ii) Shell

iii) Water chamber

iv) Tube system

v) Air measuring device

The suction chamber has been divided into three parts and all the compression stages

consisting of nozzle and diffuser are mounted inside this chamber.

The water box has also been divided into different zones in such a manner that cold

condensate first flows through the Ist stage inter cooler and thereafter through 2nd and

3rd stages simultaneously.

CONSTRUCTION OF STARTING EJECTOR,MAIN EJECTOR


21/35

The air-measuring device, for measurement of dry air discharge from condenser, has

been fitted at the air exit of the ejector. The design of this device is such that it enables

the measurement of dry air discharge while condenser and ejector are in operation.

c) The convergent divergent nozzle has been designed to accelerate the steam flow andthereby reduce the pressure of steam to 0.03 Kg/cm2 abs. in the 1st stage of suction

chamber. The low pressure in suction chamber sucks the non-condensable gases from

the condenser.

The steam and gas mixture enters the diffuser inlet and while passing through the

diffuser, the kinetic head is converted to pressure head. The steam and gas mixture

flows over the tube nest and steam gets condensed while non-condensable gases flow to

suction chamber of 2nd stage. The same phenomenon happens in 2nd stage and 3rd stage

during which all the steam gets condensed and gases are exhausted to atmosphere

through air measuring device.

d) The main condensate is used as the cooling medium for inter-coolers and after coolers.

e) 2 x 100% ejectors have been provided in the system, out of which one is for continuous

operation and the other one serves as a stand by unit.

f) The following fittings have been provided on the ejectors.

i) Gauge glass for indicating the condensate level (in first stage only)

ii) Pressure relief valve (water side)

iii) Angular thermometers with pockets.

iv) Pressure gauge with three-way-cock.

v) Vacuum gauge with three-way-cock.

vi) Stop valve

vii) Non-return valve

viii) Hg. Manometer for air steam mixture.

GLAND STEAM COOLER :

a) Gland steam cooler has been provided to suck and cool the air steam mixture from the

turbine gland seals. It employs a small ejector for which the working medium is steam oflow parameters, which can be taken either from deaerator or auxiliary source. The pressure

and temperature of this steam are not to exceed 4.5 Kg/cm2 (g) and 2500C respectively.

The energy of this steam is retrieved to the fullest possible extent as this gland steam

cooler is interposed in the feed heating cycle thereby improving the overall efficiency of

the cycle.

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b) The gland steam cooler comprises of the following main elements:

i) Removable water chamber.

ii) Tube system

iii) Shell

iv) Ejector.

The water chamber consists of thin walled rectangular shell having flange at the bottom

for assembly of tube system and shell. A partition has been provided in the water chamber

to make it two-path design. Tube system consists of U-shaped admiralty brass tubes

expanded into the tube plate.

Shell is a rectangular construction and is divided into two stages with the help of a

vertical partition. The ejector is connected in between the first and second stage coolers.

c) The nozzle of ejector has been designed to create a vacuum and thereby reduce the

pressure to 0.95 Kg/cm2 (abs) in the 1st stage of the cooler. The low pressure in the first

stage sucks the air steam mixture from the turbine gland seals. The steam while flowing

over the tube nest gets condensed in the 1st stage and then the remaining air steam

mixture is sucked by the ejector and is led to 2nd stage. The diffuser raises the pressure

from 0.95 Kg/cm2 (abs) to 1.05 Kg/cm2 (abs). The steam air mixture flows over the

tube nest of second stage where steam gets condensed and air is exhausted to atmosphere.

The condensate drain from the gland steam cooler is led to the condenser through a

drain expander and siphon.

d) The following fittings have been provided on the gland steam cooler.i) Gauge glass for indicating the drain level.

ii) High level alarm switches.

iii) Water box air release cock.

iv) Pressure gauge with three-way-cock.

v) Vacuum gauge with three-way-cock.

vi) Straight thermometers with pockets.

vii) Relief valve (water side)

viii) Isolating valves for level switches.

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DM AND GS SYSTEM

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This type of Heat exchanger consists of two concentric metal tubes. Hot fluid flows

through inner tube and cold fluid flows through outer tube. Heat transfer takes place along

the wall of inner tube. Fluid flow is simultaneous, but mixing of fluids do not take place.

Storage or regenerative type :

In this type of Heat exchanger, the heat transfer from hot fluid to cold fluid occurs

through a coupling medium, which generally is made of solid porous matrix. Rotary Air

Preheaters of the Boiler is an example of this type if heat exchanger. It consists of rotating

disk type matrix. Hot fluid and cold fluid flows continuously. Each element of the matrix

passes through hot stream to clod stream and back in each revolution. When the element is

in hot stream, heat energy gets stored in it. When the element passes in to cold stream, the

stored energy is transferred to cold fluid.

HEAT EXCHANGERS

Heat Exchangers are devices in which heat energy is transferred from hot fluid to cold

fluid. In Power Plant, there are many processes where heat exchangers are used, such as

Regenerative feed Cycle, Boiler, Air Heater, Oil coolers for various auxiliaries. Heat exchangers

are generally classified in three types :

1) Direct transfer or recuperative type

2) Storage or regenerative type

3) Direct contact type

Direct transfer or recuperative type : In this type of heat exchanger, cold and hot fluids

flow simultaneously, without coming in to direct contact with each other. Following figure

shows such type of heat exchanger.

Cold fluid in

Hot fluid in Hot fluid out

Cold fluid out

Sealing between

hot and cold gaspaths

Rotating diskMatrix

Hot Gas Cold Gas

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The figure above explains the direct contact heat exchanger, in which steam is the hot

fluid and condnesate is the cold fluid. Condensate heats up after mixes with steam and also

causing steam to condense. The mixture is stored in another tank. In addition to heat transfer

from steam to Condensate, mass transfer of soluble gases present in Condensate takes place

to steam.

Theory of direct transfer type heat exchangers :

These exchangers are concentric tube type (explained in the first fig) as well as tube

and shell type, which are most commonly used in power plants. The general arrangement of

these shell type of exchangers is as shown in the following figure.

Direct Contact type heat exchanger :

In this type of heat exchanger, both cold and hot fluids are in direct contact with each

other. Deaerator is one such direct contact heat exchanger in Power Plant. In Deaerator,

steam from turbine extraction is admitted to the shell and condensate is sprayed and then

made to flow over baffles and trays. In this type of heat exchangers, both the fluids should bemiscible. Cooling tower is another type of direct contact type heat exchanger, where atmospheric

air comes in direct contact with water. In addition to Trays and baffles Steam heat transfer,

mass transfer also takes place in direct contact heat exchangers.

Condensate

Steam

Storage of Hotcondensate

Trays andbaffles

TubesInlet

Shell Outlet

Baffles

TubesShell Inlet Tubes Outlet

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Theshellandtubeexchangerconsistsofnumberoftubespackedinsideacylindrical

shell.Tubeaxisisparalleltoshellaxis.Onefluidflowsthroughthetubesandanotherfluid

flowsoutsidethetubesintheshell.Thesetypesofexchangersarenormallyusedforheat

transferbetweenliquids.Heattransfersurfaceareperunitofexchangervolumeisbetween

100to500m2/m3

Fintypeheatexchangers:

Theseexchangersaresuitableforheattransferbetweengastoliquidandgastogas.

Finsareattachedtotheprimaryheattransfersurfacetoincreaseheattransfersurfacearea.

Thesearemainlyusedwhereheattransfercoefficientsarelowandsize0fheatexchanger

requiredissmall.Finnedsurfaceheatexchangersprovidelargeheattransfersurfaceareaper

unitvolumeoftheorderof700m2/m3

Tubefinheatexchangersaregenerallyusedforheattransferbetweengasandliquid

andareasshowninthefollowingfigure.Liquidflowsthroughtubesandgasflowsonthe

outersurfaceofthetubes.Internallyfinnedtubesarealsoavailable.

Classificationofheatexchangersasperflowarrangement:

Therearethreetypesoffloearrangements:

1)Parallelflow

2)Counterflow

3)Crossflow.

Inparallelflowarrangementshowninthefollowingfigure,boththefluidstreams

enteratoneend,flowthroughinthesamedirectionandexitfromtheotherend.

HEATEXCHANGER

Coldfluidout

ParallelFlowHeatExchanger

Hotfluidout HotfluidIn

Coldfluidin

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Theflowarrangementsaffecttheheattransferrate.Forthegivenflowratesandgiven

inletandoutletfluidtemperatures,heattransferrateforcounterflowheatexchangersis

highest,forparallelfloearrangementsitislowest,andtheforcrossflow,itisinbetweenthe

twoarrangements.

Numberofpasses:Thenumberofpassesthecold/hotliquidmakethroughthelengthandbreadthoftheheatexchangerscanbesingleormultiple.Theheatexchangersdiscussedtill

now,allaresinglepasstype.However,forincreasingtheeffectiveness,therecanbemultiple

passesHeatexchangers,inwhichcoldliquidispassedtwicethroughtheexchangerasshown

infollowingfigure.

CounterFlowHeatExchanger

HEATEXCHANGER

Coldfluidin

Hotfluidout HotfluidIn

Coldfluidout

Incrossfloeheatexchangers,theflowdirectionofonefluidisatrightanglestothat

ofanotherfluid.

Incounterflowheatexchanger,flowdirectionofboththefluidsisappositeasshownin

followingfigure.

CrossFlowHeatExchanger

Coldfluidin

Hotfluidout

HotfluidIn

Coldfluidout

TwoPassHeatExchangerwithtwopassesforcoldfluid

Coldfluidin

Hotfluidout

HotfluidIn

Coldfluidout

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HeatTransferCoefficientoftheHeatexchanger:

OverallHeattransfercoefficientisoftendenotedbyUandhasaunitofWatts/m2-K.

Itisgivenbytheexpression

1/U=1/h1+b/k+1/h2:whereUistheoverallheattransfercoefficientoftheheat

exchanger,h1andh2aretheheattransfercoefficientonbothsidesofthetubes,bisthetubethicknessandktheconductivityofthetubematerial.

FoulingFactor:Overaperiodoftimetheheatexchangerisinoperation,scalingandfouling

takesplaceontheheattransfersurfaces,reducingtheOverallHeattransferCoefficient.

Effectsofthedepositsareusuallyrepresentedbyfoulingfactorthatmustbeaddedtothe

otherthermalresistancesforevaluatingtheoverallHeatTransfercoefficientUasexplainedin

followingexpression.

1/U=1/h1+F1+b/k+F2+1/h2,whereF1andF2arethefoulingfactorsonboth

surfaces.ThevaluesofFoulingfactorsforvariousliquidsareasfollows:

FluidInvolvedintheheatexchangerFoulingfactor,m2K/W

DistilledWater0.0001

Boilerfeedwaterabove50C0.0002

FuelOil/CrudeOil0.001

PerformanceofaHeatexchanger:

Therateofheattransferinaheatexchangerisgivenby:

Q=UATmWatts,

WhereUisoverallheattransfercoefficientinW/m2-K,

Aistheareinm2

oftheheattransfersurfaceTmistheLogMeanTemperatureDifference(LMTD)

ExpressionforLMTDforparallelflowheatexchangerisexplainedasfollows:

ThiTHo

TCiTCo

Thi

TiTho

ToTCo

TCi

Lengthoftheheatexchanger

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(Ti-To)

LMTD=

Ln(Ti/To)

ExpressionforLMTDforcounterflowheatexchangerisexplainedasfollows:

OtherDesignfactorsfortheheatexchangers:

Mostimportantdesignfactorsfortheheatexchangeristherateatwhichofheattransfershouldtakeplace.Otherimportantfactorsare:

1)Pressuredroponeitherside

2)Sizerestrictions

3)Stressconsiderations

4)Servicingrequirements

5)Materialofconstruction

6)Cost

ThiHotFluidTHo

TCoColdFluidTCi

Thi

Ti

TCo

ThoTo

TCi

Lengthoftheheatexchanger

(Ti-To)

LMTD=

Ln(Ti/To)

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REGENERATIVESYSTEM:

Theregenerativesystemoftheturbineconsistsoffourlow-pressureheaters,one

glandcooler,onedeaeratorandthreehigh-pressureheaters.Thecondensateisdrawnby

condensatepumpsfromthehotwellofcondenserandispumpedtothedeaeratorthrough

glandcoolerandlowpressureheaterswhereitisprogressivelyheatedupbysteamextracted

fromsealsandbledpointsoftheturbine.ThedrainofcondensedsteamofL.P.heatersNo.2,3

&4flowsincascadeandisultimatelypumpedintothemaincondensatelineafterheaterNo.

2orflowstocondenser.Thefeedwaterafterbeingdeaeratedinthedeaeratorisdrawnbythe

boilerfeedpumpandpumpedtoboilerthroughhigh-pressureheaterswhereitisheatedup

bythebledsteamfromtheturbine.ThedrainofcondensedsteamofHPheatersflowsin

cascadeandundernormalloadconditionsflowstothedeaerator.

LOWPRESSUREHEATERNO.I

a)Theheaterisofhorizontalsurfacetypeconsistingoftwovalves,eachhalfhasbeen

locatedinsidetheupperpartofeachcondenser.Thetwohalveshavebeeninstalledin

parallelthesteamtobothissuppliedfromthesameextractionpoint.

b)ThehousingfortheheaterisfabricatedfromM.S.plateswithsuitablesteaminletand

drainconnections.Thetubeplateisofmildsteelandissecuredtothewaterboxand

housingbymeansofstudsandnuts.

Ushapedtubeshavebeenusedtoensureindependentexpansionoftubesandthe

shell.Theyareofsoliddrawnadmiraltybrass,19mmexternaldia,1mm&0.75mmthickandareexpandedbyrollingintothetubeplateatboththeends.Tubesystemhas

beenprovidedwithrollerstofacilitatedrawalfortubereplacement,andmaintenance.

Partitionsofmildsteelplateshavebeenprovidedforsupportingthetubesatintermediate

pointsandeffectivedistributionofheatloadinallthezonesoftheheater.

Thewaterboxisofmildsteelwithsuitablewaterinletandoutletbraches.Itisof

rectangularshapeandhasbeenprovidedwithsuitableairventanddrainconnections.

c)Thelow-pressureheaterNo.1hasbeenprovidedwiththefollowingfittings.

i)Gaugeglassforindicatingthedrainlevel.

ii)Highlevelalarmswitch.

iii)Waterboxventcock.

iv)Tubesidereliefvalve.

v)Isolatingvalvesforlevelswitch.

LOWPRESSUREHEATERNos.2,3&4:

a)Theseheaters-identicalinconstructionareofverticalsurfacetypeandaredesignedfor

FEEDWATERHEATERANDDEAERAOTRCONSTRUCTION

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thesteamtopassoverthetubesandthecondensatetoflowthroughthem.Following

aremainelementsoftheseheaters.

i)Shell

ii)Tubesystem

iii)Removablewaterbox

Shellisacylindricalconstructionwithdishedendweldedatbottomandhavingaflange

attheupperendforassemblyoftubesystemandwaterbox.Theshellisprovidedwith

suitablesteaminletanddrainconnectionsalongwithothernozzleconnectionsto

accommodatevariousfittings.M.S.bafflesareprovidedtoensureeffectivedistribution

ofsteaminthecondensingzoneoftheheater.

TubesystemconsistsofUshapedadmiraltybrasstubes,16-mmexternaldia,1mm

thickandareexpandedbyrollingintotubeplateatboththeends.Tubesystemhas

beenprovidedwithrollerstofacilitatedrawlfortubereplacement.Tubeplateisofmildsteelandissecuredtothewaterboxandshellflangebymeansofstudsandnuts.

Waterboxconsistsofthinwalledcylindricalshellhavingaflangeatthelowerendanda

dishedendweldedattop.Ithasbeenprovidedwithsuitablewaterinletandoutlet

branches.Partitionshavebeenprovidedinthewaterboxtomakeit4pathdesign.

b)Themaincondensateflowsthroughthetubesinfourpathsbeforeleavingtheheater.

TheheatingsteamenterstheshellthroughapipeandflowsovertheUshapedtube

nest.Thepartitionwallsinstalledinthetubesystemensureszig-zagflowofsteamover

tubenest.Condensateofheatingsteamreferredasdrain,tricklesdownthetubesandis

takenoutfromthelowerportionoftheshellbyautomaticlevelcontrolvalveinstalledonthedrainline.

c)FollowingfittingsareprovidedonL.P.heaters:


ii)Highlevelalarmswitch

iii)Lowlevelalarmswitch(onlyforL.P.heaterNo.2)

iv)Waterboxventcock.

v)Pressuregaugewiththreewaycock.

vi)Straightthermometerswithpockets.vii)Angularthermometerwithpockets.

viii)Isolatingvalvesforlevelswitches.

ix)Reliefvalve(shellside)

x)Reliefvalve(tubeside)

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GLANDCOOLER:

a)Glandcoolerhasbeendesignedtocondensetheleak-offsteamfromintermediate

chambersofendsealingsofH.P.&I.P.turbine.

Theconstructionofthiscoolerisidenticalwithlow-pressureheatersNo.2,3&4anditcomprisesoffollowingmainelements:

i)Shell

ii)Tubesystem

iii)Removablewaterbox

Shellisofcylindricalconstructionwithdishedendweldedatbottomandhavingaflange

attheupperendforassemblyoftubesystemandwaterbox.

TubesystemconsistsofUshapedadmiraltybrasstubes,whichareexpandedby

rollingintotubeplateatbothends.

Waterboxconsistsofthinwalledcylindricalshellhavingaflangeatthelowerendand

adishedendweldedattop.Partitionshavebeenprovidedinthewaterboxtomakeit

four-pathdesign.

b)Themaincondensateflowsthroughthetubesinfourpathsbeforeleavingthecooler.

Theleakofsteamenterstheshellthroughapipeandflowsoverthetubenest.The

partitionwallsinstalledinthetubesystemleadtozigzagflowofsteamoverthetube

nest.Condensateofleakofsteamreferredasdraintricklesdownthetubesandistaken

outfromthelowerportionoftheshellbyautomaticlevelcontrolvalve,installedonthe

drainline.

c)Followingfittingsareprovidedonglandcooler:


ii)HighLevelalarmswitch.

iii)LowLevelalarmswitch.

iv)Waterboxairreleasecock.

v)Pressuregaugewiththreewaycock.

vi)Angularthermometerswithpockets.

vii)Straightthermometerswithpockets.

viii)Reliefvalve[shellside].

ix)Reliefvalve[waterside].

x)Isolatingvalvesforlevelswitches.

DEAERATOR

a)Aconstantpressuredeaerator,peggedat7Kg/cm2absisenvisagedinturbine

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regenerativecycletoprovideproperlydeaeratedfeedwaterforboiler,limitinggases

[mainlyoxygen]to0.005CC/Litre.Itisadirectcontacttypeheatercombinedwithfeed

storagetankofadequatecapacity.Theheatingsteamisnormallysuppliedfromturbine

extractionsbutduringstartingandlowloadoperationthesteamissuppliedfromauxiliary

source.

Thedeaeratorcomprisesoftwochambers:

i)Deaeratingcolumn.

ii)Feedstoragetank.

Deaeratingcolumnisaspraycumtraytypecylindricalvesselofhorizontalconstruction

withdishedendsweldedtoit.Thetraystackisdesignedtoensuremaximumcontact

timeaswellasoptimumscrubbingofcondensatetoachieveefficientdeaeration.The

deaeratingcolumnismountedonthefeedstoragetank,whichinturnissupportedon

rollersatthetwoendsandafixedsupportatthecenter.Thefeedstoragetankis

fabricatedfromboilerqualitysteelplates.Manholesareprovidedondeaeratingcolumn

aswellasonfeedstoragetankforinspectionandmaintenance.

b)Thefeedwaterisadmittedatthetopofthedeaeratingcolumnandflowsdownwards

throughthesprayvalvesandtrays.Thetraysaredesignedtoexposetothemaximum

watersurfaceforefficientscrubbingtoeffecttheliberationoftheassociatedgases.

Steamentersfromtheunderneathofthetraysandflowsincounterdirectionof

condensate.Whileflowingupwardsthroughthetrays,scrubbing&heatingisdone.Thus

theliberatedgasesmoveupwardsalongwiththesteam.Steamgetscondensedabove

thetraysandinturnheatsthecondensate.Liberatedgasesescapetoatmospherefrom

theorificeopeningmeantforit.Thisopeningisprovidedwithanumberofdeflectorstominimizethelossofstem.

c)Deaeratorisprovidedwiththefollowingfittings:

i)Tubulartypegaugeglass.

ii)Highlevelalarmswitch.

iii)Lowlevelalarmswitch.

iv)Pressuregauge.

v)Straightthermometerswithpockets.

vi)Safetyvalve.

vii)Isolatingvalvesforstandpipes.

HIGHPRESSUREHEATERS5,6&7.

a)High-pressureheatershavebeenprovidedforheatingoffeedwaterbybledsteamfrom

theturbine.Thefeedwaterflowsthroughthetubespiralsandisheatedbysteam

aroundthetubesintheshelloftheheaters.Theseheatersarecylindricalvesselswith

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weldeddishedendsandwithintegrated,desuperheating,condensingandsubcooling

sections.Theinternaltubesystemofspiralsisweldedtotheinletandoutletheaters.In

ordertofacilitateassemblyanddisassembly,rollersatthesideoftheheaterhavebeen

provided.Bothfeedwaterandsteamentriesandexitsarefromthebottomendofthe

heaters.ThisdesignofferstheadvantagetooptimizethearrangementofpipingandthelocationoftheheatersatPowerStation.

b)HPheatersareconnectedinseriesonfeedwatersideandbysucharrangement,thefeed

water,afterfeedpumpenterstheHPH5,6&7.thesteamissuppliedtotheseheaters

fromthebleedNo.3,2,1oftheturbinethroughmotoroperatedvalves.Theseheaters

haveagroupbypassprotectiononthefeedwaterside.Intheeventoftuberapturein

anyoftheHPHandthelevelofthecondensaterisingtodangerouslevel,thegroup

protectiondevicedivertsautomaticallythefeedwaterdirectlytoboiler,thusbypassing

allthe3H.P.heaters.

c)Thecondensateofthebledsteamformedintheheateristhrowneithertothenextlowerstageheaterincascadeortothedeaeratorthroughasetofinter-lockedvalvesdepending

uponthepressureconditionsinsidetheheaters.Thereisalsoanarrangementtotake

outairsteammixturefromeachheaterincascadeandtheairsteammixtureisthrown

tothecondenserthroughtheL.P.heaters.

FollowingfittingsareprovidedontheH.P.heaters:


ii)Pressuregaugewiththreewaycock.

iii)Airventcock.

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