pam and tmd

8/10/2019 pam and tmd

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PAM

PLANT AUXILIARY

MAINTENANCE

WATER CIRCULATION SYSTEM

Theory of circulation

Water must flow through the heat absorptionsurface of the boiler in order that it be evaporated into

steam. Indrum type units (natural and controlled circulation) the water iscirculated from the drum

through the generating circuits and then back to the drum where the steam is separated and directed to


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thesuper heater. The water leaves the drum through the down comersat a temperature slightly below

the saturation temperature. Theflow through the furnace wall is at saturation temperature.

Heatabsorbed in water wall is latent heat of vaporization creating amixture of steam and water.

Types of boiler circulating system:

Natural circulation system

Controlled circulation system

Combines circulation system

Natural circulation system

Water delivered to steam generator fromfeed heater is at a temperature well below the saturation

valuecorresponding to that pressure. Entering first the economizer it isheated to about 30-

40Cbelow saturation temperature. Fromeconomizer the water enters the drum and thus joins the

circulationsystem. Water entering the drum flows through the down comer and enters ring heater at

the bottom. In the water walls a part of thewater is converted to steam and the mixture flows back to

thedrum. In the drum, the steam is separated, and sent to superheater for super heating and then sent to the high pressure turbine.Remaining water mixes with the inc

oming water from theeconomizer and the cycle is repeated.The circulation in this case takes place onthe

thermo-siphon principle. The dowm comers contain relativelycold water whereas the riser tubes contain

a steam water mixture.Circulation takes place at such a rate that the driving force and thefrictional

resistance in water walls are balanced


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As the pressure increases, the difference in density between water and steam reduces. Thus the

hydrostatic head available will not beable to overcome the frictional resistance for a flow

correspondingto the minimum requirement of cooling of water wall tubes.Therefore natural circulation

is limited to the boiler with drumoperating pressure around 175 kg/cm.

Controlled circulation system

Beyond 80 kg/cm of pressure, circulationis to be assisted with mechanical pumps to overcome the

frictionallosses. To regulate the flow through various tubes, orifice platesare used. This system is

applicable in the high sub-critical regions(200 kg/cm)

Combined circulation system

Beyond the critical pressure, phasetransformation is absent, and hence once through system isadopted.

However, it has been found that even at super critical pressure, it is advantageous to recirculate the

water through thefurnace tubes and simplifies the start up procedure. A typicaloperating pressure for

such a system is 260 kg/cm

CHPH

(CONTROL STRUCTURE PUMP HOUSE)

The control system has following pumps:-

Chlorine pump-2(for chlorination of water)

HP pump-6(for boiling of water)

LP pump-3(for EP pump house)

Fire pump-(incase of fire breakdown)

TWS pump-3(for screening of water)

CRW pump-3(supply water for water treatment)

This house is known as control house because amount of water to be supplied for treatment is

controlled from this house with thehelp of these pumps. Generally 2 CRW pumps out of 3pumpsremains

open.similarly,1 FS ,2 LP,4 HP,1 TWS pumps remainsopen. If more water is needed then others pumps

are opened.

WATER TREATMENT PLANT

As the types of boiler are not alike their working pressure and operating conditions vary and so do the

typesand methods of water treatment. Water treatment plants used inthermal power plants are

designed to process the raw water towater with vary low in dissolved solids known as


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"dematerializedwater". No doubt, this plant has to be engineered very carefullykeeping in view the type

of raw water to the thermal plant, itstreatment costs and overall economics

Actually, the type of demineralization processchosen for a power station depends on three

main factors:

The quality of the rawwater.

The degree of de-ionization i.e. treated water quality

Selectivity of resins.

Water treatment process which is generally made up of two sections:

Pretreatment section

Demineralizationsection

Pretreatment section

Pretreatment plant removes the suspendedsolids such as clay, silt, organic and inorganic matter, plants

andother microscopic organism. The turbidity may be taken as of twotypes of suspended solids in water.

Firstly, the separable solids andsecondly the non separable solids (colloids). The coarsecomponents,

such as sand, silt etc, can be removed from the water by simple sedimentation. Finer particles however,

will not settle inany reasonable time and must be flocculated to produce the

large particles which are settle able. Long term ability to remainsuspended in water is basically a

function of both size and

specificgravity. The settling rate of the colloidal and finely divided(approximately 001 to 1 micron)

suspended matter is so slow thatremoving them from water by plain sedimentation is tank

shavingordinary dimensions is impossible. Settling velocity of finelydivided and collide particles under

gravity also are so small thatordinary sedimentation is not possible. It is necessary, therefore, touse

procedures which agglomerate the small particles into

larger aggregates, which have practical settling velocities. The term"Coagulation" and "flocculation"

have been used indiscriminately to describe process of turbidity removal. "Coagulation" means to bring

together the suspended particles. The process describes theeffect produced by the addition of

a chemical Al (SP) g to acolloidal dispersion resulting in particle destabilization by areduction of force

tending to keep particles apart. Rapid mixing

isimportant at this stage to obtain. Uniform dispersion of thechemical and to increase opportunity for pa

rticles to particlecontact. This operation is done by flash mixer in thec1ariflocculator. Second stage of

formation of settle able particlesfrom destabilized colloidal sized particles is termed a"flocculation".

Here coagulated particles grow in size by attachingto each other. In contrast to coagulation where the

primary force iselectrostatic or intrinsic, "flocculation" occurs by chemical bridging. Flocculation is

obtained by gentle and prolonged

mixingwhich converts the submicroscopic coagulated particle intodiscrete, visible & suspended particles.


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At this stage particles arelarge enough to settle rapidly under the influence of gravityanomaly be

removed.

If pretreatment of the water is not done efficiently thenconsequences are as follows:

Si02 may escape with water which will increase the anionloading.

Organic matter may escape which may cause organic foulinginthe anion exchanger beds. In the 'pre-

treatment plantchlorine addition provision is normally made to combatorganic contamination.

Cation loading may unnecessary increase due to addition of Ca(OH)2 in excess of calculated amount for

raising the pH of the water for maximum floe formation and also

AKOrDgmay precipitate out. If less than calculated amount of Ca(OH)2 is added, proper pH flocculation

will not beobtained and silica escape to demineralization section willoccur, thereby increasing load on

anion bed.

Demineralization

This filter water is now used for demineralizing purpose and is fed to cation exchanger bed, butenroute

being first dechlorinated, which is either done by passingthrough activated carbon filter or injecting

along the flow of water,an equivalent amount of sodium sulphite through some stroke pumps. The

residual chlorine which is- maintained in clarification plant to remove organic matter from raw water is

now detrimentalto action resin and must be eliminated before its entry to this bed. Normally, the typical

scheme of demineralization up to the .mark against an average surface water,is three bed system with a

provision of removing gaseous carbondioxide from water before feeding to Anion Exchanger. Now, let

us see, what happens actually in each bed when water is passedfrom one to another.Resins, which are

built on synthetic matrixof a styrene divinely benzene copolymer, are manufactured in sucha way that

these have the ability to, exchange one ion for another,hold it temporarily in chemical combination and

give it to a strongelectrolytic solution. Suitable treatment is also given to them insuch a way that a

particular resin absorbs only a particular group

of ions. Resins, when absorbing and releasing cationic portion of dissolved salts, is called cation, exchang

er resin and whenremoving anionic portion is called anion exchanger resin. presettrend is of employing

'strongly acidic cation exchanger resin andstrongly basic anion exchanger resin in a DM Plant of

modernthermal power station. We may see that the chemically activegroup in a cationic resin is SOx-H

(normally represented by RH)and in an anionic resin the active group is either tertiary amine

or quaternary ammonium group (normally the resin is represented byROH). The reaction of exchange

may be further represented as below The water from the ex-cation containscarbonic acid also

sufficiently, which is very weak acid difficult to be removed by strongly basic anion resin and causing

hindrance toremove silicate ions from the bed. It is therefore a usual practice toremove carbonic acid

before it is led to anion exchanger bed. Theex-cation water is trickled in fine streams from top of a tall

tower packed with, rasching rings, and compressed air is passed from the bottom. Carbonic acid breaks

into C03 and water

mechanically(Henry's Law) with the carbon dioxide escaping into theatmosphere. The water is accumula


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ted in suitable storage tank below the tower, called degassed water dump from where the sameis led to

anion exchanger bed, using acid resistant pump.The ex-anion water is fed to the mixed bedexchanger

containing both cationic resin and anionic resin. This bed not only takes care of sodium slip from cation

but also silicaslip from anion exchanger very effectively. The final output fromthe mixed bed is Exira-

ordinarily pure water having less than0.2/Mho conductivity 7.0 and silica content less than 0.02 pm.

Anydeviation from the above quality means that the resins in

mixed bed are exhausted and need regeneration, regeneration of themixed bed first calls for suitable,

back washing and settling, so thatthe two types of resins are seperated from each other. Lighter

anionresin rises to the top and the heavier cation resin settles to the bottom. Both the resins are then

regenerated separately with alkaliand acid, rinsed to the desired value and air mixed, to mix the

resinagain thoroughly. It is then put to final rinsing till the desiredquality is obtained.It may be

mentioned here that there are two types of strongly basic anion exchanger. Type II resins are slightly

less basic than type I, but have higher regeneration efficiency than typeI. Again as type II resins are

unable to remove silica effectively,type I resins also have to be used for the purpose. As such, thegeneral

condition so far prevailing in India, is to employ type IIresin in anion exchangers bed and type I resin in

mixed bed (for the anionic portion).It is also a general convention to regenerate theabove two resins

under through fare system i.e. the caustic sodaentering into mixed bed for regeneration, of type I anion

resin, isutilized to regenerate type II resin in anion exchanger bed. Thecontent of utilizing the above

resin and mode of regeneration isnow days being switched over from the economy to a higher costso as

to have more stringent quality control of the final D.M.Water.

Internal Treatment

This final D.M effluent is then either led tohot well of the condenser directly as make up to boilers, or

beingstored in D.M. Water storage tanks first and then pumped for makeup purpose to boiler feed.As

the D.M. Water has a good affinity toabsorb carbon dioxide and oxygen, and both are extremelyharmfulto metal surfaces for their destruction like corrosion, these have

to be removed before it is fed to boiler. This is being done indesecrator. Still the residual oxygen which is

remaining in thewater is neutralized by a suitable doze of hydrazine, at the pointafter desecrator. To

have further minimum corrosion, the pH of feed water is to be maintained at around 9.0 for which

purposeammonia in suitable doze is added to this make up water at a pointalong with hydrazine as

stated above


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TMD

TURBINE MAINTENANCE

DEPARTMENT

Operating Principles

A steam turbine's two main parts are the cylinder andthe rotor.As the steam passes through

the fixed blades or nozzles itexpands and its velocity increases. The high-velocity jet of

steamstrikes the first set of moving blades. The kinetic energy of thesteam changes into

mechanical energy, causing the shaft to rotate.The steam then enters the next set of fixed

blades and strikes thenext row of moving blades.As the steam flows through the turbine, its


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pressure andtemperature decreases, while its volume increases. The decrease in pressure and

temperature occurs as the steam transmits .energy tothe shaft and performs work. After

passing through the last

turbinestage, the steam exhausts into the condenser or process steamsystem.

The kinetic energy of the steam changes intomechanical erringly through the impact (impulse)

or reaction of thesteam against the blades.

Main Turbine

The 210MW turbine is a tandem compounded typemachine comprising of H.P. & I.P. cylinders.

The H.P. turbinecomprises of 12 stages the I.P. turbine has 11 stages & the L.P. hasfour stages

of double flow. The H.P. & I.P. turbine rotor are rigidlycompounded & the I.P. & the I.P. rotor by

lens type semi flexiblecoupling. All the three rotors are aligned on five bearings of whichthe

bearing no.2 is combined with thrust bearing.The main superheated steam branches off into

twostreams from the boiler and passes through the emergency

stopvalve and control valve before entering, the governing wheelchamber of the H.P. turbine.

After expanding in the 12 stages inthe H.P. turbine the steam returned in the boiler for

reheating.

The reheated steam from the boiler enter I.P. turbine viainterceptor valves and control valves

and after expanding entersthe L.P. turbine stage via 2 numbers of cross over pipes.In the L.P.

stage the steam expands in axially oppositedirection to counteract the trust and enters the

condenser

placeddirectly below the L.P. turbine. The cooling water flowingthroughout the condenser tube

s condenses the steam and thecondensate collected in the hot well of the condenser.The

condensate collected is pumped by means of 3*50% duty condensate pumps through L.P.

heaters to

deaerator from where the boiler feed pump delivers the water to boiler through H.P. heaters th

us forming a closed cycle.

TURBINE CYCLE

Fresh steam from boiler is supplied to the turbinethrough the emergency stop valve. From the

stop valves steam issupplied to control valves situated on H.P. cylinders on the front bearing

end. After expansion through 12 stages at the H.P. cylinder steam flows back to boiler for

reheating and reheated steam from

he boiler cover to the intermediate pressure turbine trough twointerceptor valves and four

control valves mounted on the I.P.turbine.After flowing trough I.P. turbine steam enters the

middle part of the L.P. turbine through cross over pipes. In L.P. turbine theexhaust steam

condenses in the surface condensers welded directlyto the exhaust part of L.P. turbine. The


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selection of extraction points and cold reheat pressurehas been done with a view to achieve

the highest efficiency. Theseare two extractions from H.P. turbine, four from I.P. turbine

andone from L.P. turbine. Steam at 1.10 to 1.03 g/sq cm Abs issupplied for the gland sealing.

Steam for this purpose is obtainedfrom deaerator through a collection where pressure of steam

isregulated.From the condenser condensate is pumped with the helpof 3*50% capacitycondensate pumps to deaerator through the low pressure regenerative equipments.

Feed water is pumped from deaerator to the boiler through the H.P. heaters by means of

3*50% capacity feed pumpsconnected before the H.P. heaters.

DESCRIPTION OF MAIN TURBINE

Main Components of Turbine:

Emergency Stop Valve

Steam from the boiler is supplied to the turbine throughtwo emergency stop valves. The

emergency stop valve operated byhydraulic servomotor shuts off steam supply to the turbine

whenthe turbo set is tripped. The emergency stop valves connected

tothe four control valves through four flexible loop pipes of Chromium-Molybdenum-Vanadium

steel.

H.P. Cylinder

It is made of creep resisting Cr-Mo-V steel castingmade of two halves joined at the horizontal

planeThe horizontal joint is secured with the help of studsand nuts made of high creep resisting Cr-

Mo-V steel forgings.

Toensure H.P. tightness the studs are tightened by heat to a predetermined temperature with

the help of electric heater.

H.P. Rotor

The H.P. rotor has discs integrally forged with the shaftsand is mechanical forming single Cr-

Mo-V steel forging. A special process to prevent abnormal rotor deflection thermally

stabilizesthe rotor forging.

L.P. Rotor

It consists of shrunk fit discs on a shaft. The shaft is aforging of Cr-Mo-V steel while the discs are

of high strength Nisteel forging.The H.P. rotor is connected by rigid couplings whole theI.P.

rotor and L.P. rotor are connected by semi-flexible lens typecoupling. The rotors are

dynamically balanced to a very precisedegree.


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Boiler Feed Pump

: Three per unit of 50% capaCity eachlocated in the '0' meter level in the TG bay.

High Pressure Heaters

: Normally three in number and aresituated in the TG bay.

Drip Pumps

: Generally two in number of 100% capacity eachsituated beneath the LP heaters.

Turbine Lubricating Oil System

: This consists of Main OilPump (MOP) Starting Oil Pump (SOP), AC standby oil pumpsand

emergency DC' oil pump and Jacking Oil Pump (JOP) (oneeach per unit).

Boiler Feed Pumps

This pump is horizontal and of barrel design driven byan Electric motor through a hydraulic

coupling. All the bearings of pump and motor are forced lubricated by a suitable oil

lubricatingsystem with adequate protection to trip the pump if the lubricationoil pressure falls

below a preset value.The high-pressure boiler feed pump is very

expensivemachine which calls for a very careful operation and skilledmaintenance. The safety

in operation and efficiency of the feed pump depends largely on the reliable operation and

maintenance.Operating staff must be able to find out the causes of defect at thevery beginning

which can be easily removed without endangeringthe operator of the power plant and alsowithout the expensivedismantling of the high pressure feed pump.The feed pump consists of

pump barrel, into which ismounted the inside stator together with rotor. The hydraulic part is

enclosed by the high pressure cover along with the balancingdevice. The suction side of the

barrel and the space in the high pressure cover behind the balancing device are enclosed by the

low pressure covers along with the stuffing box casings. The

bracketsof the radial bearing of the suction side and radial and thrust bearing of the discharge

side are fixed to the low pressure

covers.The entire pumps are mounted on a foundation frame. Thehydraulic coupling and two

claws coupling with coupling guardsare also delivered along with the pump. Water cooling andoillubricating are provided with their accessories.

Turbine Driven Boiler Feed Pump

The single cylinder turbine is of the axial flow type.The live steam flows through the emergency

stop valve and thenthrough the main Control Valves 5 nos. (Nozzle governing). Thesevalves

regulate the steam supply through the turbine in accordancewith load requirements. The


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control valves are actuated by a lift bar which is raised or lowered via a lever system by the

relaycylinder mounted on the turbine casing axial thrust to the maximum extent. Since the axial

thrust varieswith the load, the residual thrust is taken up by the thrust bearing.The leak off

from the balancing piston is connected back to theturbine after 9th stage.The turbine is

provided with hydraulic andelectro-hydraulic governing system. A primary oil pump is used asaspeed sensor for hydraulic governing and shall Probes are used asa speed sensor for electro

hydraulic governing.Whenever steam is drawn from the cold reheat line

or auxiliary supply, steam flow is controlled by auxiliary controlvalve. During this period the

main control valves (4 nos.) willremain fully opened and the bypass valve across it will

remainclosed. (Bypass remains closed for a short period when change,over from IP steam to

CRH takes place).The steam exhaust for the BFP- Turbine is connected tothe main condenser

and the turbine glands are sealed by glandsteam axial thrust to the maximum extent. Since the

axial thrust varieswith the load, the residual thrust is taken up by the thrust bearing.The leak off

from the balancing piston is connected back to theturbine after 9th stage.The turbine is

provided with hydraulic andelectro-hydraulic governing system. A primary oil pump is used asa

speed sensor for hydraulic governing and shall Probes are used asa speed sensor for electro

hydraulic governing.Whenever steam is drawn from the cold reheat line

or auxiliary supply, steam flow is controlled by auxiliary controlvalve. During this period the

main control valves (4 nos.) willremain fully opened and the bypass valve across it will

remainclosed. (Bypass remains closed for a short period when change,over from IP steam to

CRH takes place).The steam exhaust for the BFP- Turbine is connected tothe main condenser

and the turbine glands are sealed by glandsteam.

High Pressure Heaters

These are regenerative feed water heaters operating

athigh pressure and located by the side of turbine. These aregenerally vertical type and turbine

bleed steam pipes are connectedto them.HP heaters are connected in series on feed

watersideand by such arrangement, the feed water, after feed pump entersthe HP heaters. The

steam is supplied to these heaters form the bleed point of the turbine through motor operated

valves. Theseheaters have a group bypass protection on the feed waterside.In the event f tube

rupture in any of the HPH and the level of thecondensate rising to dangerous level, the group

protection devicediverts automatically the feed water directly to boiler, thus bypassing all the

3 H.P. heaters

Following fittings are generally provided on the HP heaters

Gauge glass for indicating the drainlevel.

Pressure gauge with three way cock.


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Air Vent cock.

Safety valve shell side.

Seal pot.

Isolating valves.

High level alarm switch.

Speed Governor

It is directly coupled to the turbine rotor

throughcoupling and has been designed to maintain automatically thespeed of the turbo set. It

is located with the front pedestals

Turbine Oil Lubricating System

This consists of main oil pump, starting oil pump emergencyD.C. oil pump and each per unit.

TYPES OF VALVES USED AND MAINTAINED IN TMD

Gate Valve

Regulating Valve

Non-Return Valve

Safety Valve

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