Induction 9

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CONSTRUCTION AND OPERATION

OF BOILER FEED PUMP

General :

The high-pressure boiler feed pump is a very expensive machine, which can comply

with the stated requirements, only with a careful and skilled maintenance. The safety

in

operation and efficiency of the feed pump does not only depend on the correct design and

careful manufacturing in the works, but also on the reliable operation and maintenance.

Therefore, it is important that the operating and maintenance staffs are acquainted

with these instructions properly in time. They should know perfectly the operation of the boiler

feed pump to be able to find out the cause of defect. The causes of any adverse running of

the feed pump found in the beginning may be easily removed without endangering the operation

of the power plant and also without the expensive dismantling. Any damage resulting due to

the ignorance of the operating instructions will not be the responsibility of the manufacturer

even if it occurs during the guarantee period.

1.0 Description of the feed pump :

The high pressure feed pumps of barrel type satisfy in all respects the latest developments

achieved for the design and operation of the feed pumps. It consists of the pump barrel, into

which is mounted the inside stator together with the rotor. The high-pressure cover along with

the balancing device closes the hydraulic part. The suction side of the barrel and the space in

the high-pressure cover behind the balancing device are closed along with the

balancing

device and with the stuffing box casings. The bracket of the radial bearing of the suction side

and the bracket of the radial and thrust bearings of the discharge side are fixed to the low-

pressure covers. The entire pump is mounted on a foundation frame. The hydraulic coupling

and two couplings with coupling guards are also delivered along with the pump. The water-

cooling and oil lubricating connections are provided with their accessories. All the instruments

necessary for observing the performance of the feed pump are mounted on the pump, on local

panel and on the piping.

1.01 Rotor:

The rotor of boiler feed pump consists of the shaft, impellers, distance bushes, throttle

bush, balancing disc, supporting rings, stuffing box bushes, nuts for holding the stuffing box

bushes, the disc of axial bearing with the lock nut, nuts for securing the coupling, shaft keys

and lubricating rings. For obtaining shrunk fitting of the impellers on the shaft, the impellers are

first heated and then assembled. For this purpose, stepped diameters are provided on the

shaft. On each stepped diameter two impellers are mounted.

The axial thrust of the rotor is taken up by the balancing disc, which is keyed, to the

shaft. The axial expansion of the rotor part is limited to the extent of dilation gap that exists

between the throttle bush and the supporting ring. This dilation gap should be within the limit

of 0.2 to 0.3 mm. The maximum run out of the assembled rotor, at the sealing impeller

diameters, the throttle bush and stuffing box bushings is up to 0.06 mm. The impellers are

statically balanced and the rotor as a whole is dynamically balanced within the permissible

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unbalance according to the standards of the works. Prior to dynamic balancing of the rotor,

the deflection of the rotor due to its self weight is also measured. The deflection of the rotor

due to self weight moves in the limits of 0.04 to 0.12 mm.All the measurements carried out on the rotor including the actual clearances at the

sealing rings are entered in passports delivered with every feed pump. The rotor is supported

on two part bearing shells. The brackets are connected to the low pressure covers.

The

necessary centering of rotor is done with the perfect concentricity of the stuffing box gland

bush with the bore of the stuffing box casing and for obtaining the concentricity of

the

throttle bush in the throttle space, a suitable change is made on the set of shims below the

centering keys of the low pressure cover. In order to maintain the concentricity of the centering

diameter on the keys, shims are replaced from the centering keys, which are

diagonally

opposite to each other, by removing from one place and positioning at the other.

1.01 Inside stator :

The inside stator consists of stage bodies, the diffusers and rotor. The diffusers are

centrally mounted in the stage bodies. They are secured against rotation, by locating pins.

Stage bodies are fitted with wearing rings at the place where it is likely to come into contact

with the wearing rings of impeller, and the wearing rings are secured to the stage bodies with

the help of screws. The entire inside stator is connected to the barrel. The nuts at the ends of

these bolts are tightened in order to pre stress the bolts to suit the working pressure and they

are secured with the help of locking washers.

During the operation of the pump and especially while putting the pump into operation,

shocks are developed in the pump which effect the connecting bolts and the connecting bolts

being subjected to torsion, the inside stator is secured from the suction side as well

as

discharge side. On the suction side, a key fitted and screwed on to the inlet stage a key is

provided which is fitted by a screw. This key will slide along the key way in barrel. On the

discharge side also, the outlet stage is supported on the barrel by key and keyway arrangement

on the supporting block welded to the barrel.

The centering of the inside stator is carried out by aligning the inlet stage to

the

suction side of the barrel and by aligning the high pressure cover to the end diffuser. While the

pump is in operation, the inside stator will get heated earlier than the pump barrel together

with the high pressure cover. This means that the expansion of the inside stator owing to

temperature will be more that the expansion of the barrel and of the high pressure cover. A

dilation gap of 6 mm is therefore maintained between the end diffuser an the high pressure

cover.

The sealing surfaces of the individual stages are accurately lapped using a fixture, and

they seal metal. Therefore it is important that the sealing surfaces should not be damaged

especially while dismantling the pump. If they are damaged, they must be relapped in order to

obtain the perfect mating surfaces.

1.02 Mechanical seal :

The design of the feed pump incorporates Mechanical seal. The mechanical seal eliminates

the losses of feed water in stuffing box. Working ability of the feed pump increases. With the

mechanical seal, cooling of stuffing box space should be perfect.

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Cooling is carried out by the circulation of water by means of a pumping ring through a

cooler.

Cooling of the stuffing box space is different from the seal cooler.

Even after stopping of the pump stuffing box cooling should be continued.

Coolers are designed to keep the stuffing box space temp. below 80 0C.

Pump need not be removed from base frame to replace mechanical seal.

To replace mechanical seal on discharge side, dissemble thrust bearing, and bearing

brackets along with bearing shells.

To replace mechanical seal on the suction side, dissemble the coupling along with

intermediate piece and bearing brackets along with the shells.

Follow the seal disassembly and assembly instructions strictly.

2.0 Function of the feed pump :The water with the given operating temperature should flow continuously to the pump

under a required head. It passes through the suction branch into the intake spiral and from

there it is directed into the first impeller. After leaving the impeller it passes through the

diffusing passages of the diffuser, where the kinetic energy is converted into potential energy.

Afterwards if flows over to the guide vanes to the inlet of the next impeller eye. This procedure

repeats from one stage to the other till it passes through the last impeller and the end diffuser.

Thus the feed water at the outlet of the last stage attains the required discharge head. A

small part of the feed water, i.e. about 5% which is not calculated to the guaranteed delivery

capacity, is taken off from the space behind the last impeller for the operation of the automatic

balancing device to balance the hydraulic axial thrust of the pump rotor.

The feed water passes through the balancing device and comes into the space behindthe balancing disc. Feed water is taken from this space to the feed water tank. It is evident

from the function of the balancing disc, as it disturbs the hydraulic equilibrium. Therefore the

equalizing piping must have sufficient flow capacity. For a safe operation of the balancing

device the pressure difference between the suction and balancing leak off should not exceed

more than 5 atmospheres.

3.0 Balancing device :

Balancing system of the pump takes up the entire axial thrust of the rotor, by means of

the balancing disc.

Balancing device is an important aspect of the pump as far as the design and material

selection is concerned.Balancing device consists of balancing disc, secured to the shaft and bearing

disc

fitted to the high pressure cover by mans of the tightening flange and bolts with nuts which

are locked by washers.

Axial sealing gap is formed between the bearing disc and the balancing disc.

Contact surfaces of bearing disc and balancing disc are mutually lapped against each

other.

Sealing of the individual parts is metal to metal.

Full pressure developed by the last impeller is not carried on to the balancing device

but throttled by means of the taper bush mounted on the shaft before the balancing disc.

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The feed water passes through the taper bush, through the axial sealing gap.

The axial sealing gap is approximately 0.10 mm.

Balancing disc can be dismantled without extensive dismantling of the pump in case of

damage to the balancing device.

2.0 Lubrication system :

The feed pump consists of two radial sleeve bearings and one thrust bearing. All the

bearings are force lubricated by the main oil pump or the auxiliary oil pump or the hydraulic

coupling. The thrust bearing is located at the free end of the pump.

The feed pump drive motor consists of two sleeve radial bearings.

The hydraulic coupling consists of four radial bearings and two tilting pad thrust bearings.

Before the start of the pump main motor, lubrication to the various bearings is supplied

through the auxiliary oil pump. Once the main motor attains a particular speed after start, the

main oil pump of the hydraulic coupling takes over and the auxiliary oil pump is tripped

automatically. The details of the switching 'ON' and switching 'OFF' of the oil pumps are dealt

in the hydraulic coupling operating instructions and to be followed strictly.

Two coolers are provided in the oil system. One for the working oil and the other for

lubricating oil. The pressure of oil before the radial bearing should be a minimum of 0.8 atg to

1.0 atg and for the thrust bearing 0.5 atg.

Before putting the lubrication system into operation all the oil lines and bearings should

be clean. The following instructions should be followed for flushing of the oil system.

2.01 Flusing of oil system :

1)2)

3)

4)

5)

6)

7)

8)9)

The oil pipes are cleaned thoroughly by means of wire brush.After mechanical cleaning the pipes are steam washed with steam of pressure 8 atg and

250 0C

A coating of lubricating oil is given to the pipes after steam washed with steam washing

against corrosion.

After cleaning, the pipes are assembled in position.

Fill Servo Fluid - 10" oil upto the mark indicated, through a 50 micron filter.

Remove the top halves of the bearings, tilt the bottom half bearings, such that the oil

hole in the bearing housing will allow free flow of oil through the system and protect the

ournal by covering with cloth and thin sheet of metal.

Oil filter and bearings housings are to be cleaned.

Mount all the instruments in the oil system.Heating of the oil during flushing is done by inserting steam coil or by heating elements.

The oil flushing can be started by switching on the auxiliary oil pump. After the first 4

hrs of flushing the filters should be checked and put back. The choking of filters in the initial

stages will be more and it should be cleaned from time to time and put back. The oil temperature

during flushing is maintained at about 50-60 0. After ensuring that filters are not getting choked

oil flushing is stopped.

After flushing is completed, the oil tank and coolers are drained cleaned and fresh oil is

filled.

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2.02 Thrust bearing of feed pump :

1)

2)

Quantity of oil required for each radial bearing - 7.5 lts/min

Quantity of oil required for thrust bearing - 42 lts/min. For kingsbury thrust bearing.

12 lts/min. for Mitchell thrust bearing.

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CONDENSATE EXTRACTION PUMP

GENERAL:

The WKT centrifugal pump is a hydrodynamic machine. It is characterized by the fact

that pressure is generated by a continuous flow of fluid through the pump. Mechanical work is

imparted to the fluid by a rotating impeller (109) : The impeller, diffuser and stage casing (104)

together form one stage of the pump. The suction created by the fluid discharged from the

impeller causes an equal volume of fresh fluid to flow into the suction casing (102 b) of the

pump.

The vertical barrel-type centrifugal pump consists of the pump casing, the motor

stool and the barrel. Depending on the suction head available, one column pipe is provided in

addition, if required. The pump rotor assembly and the intermediate shafts are guided in metal

bearings of bronze, liquid-lubricated, and in case of higher temperatures, in bearings of special

carbon. Under temperature conditions exceeding 100 0C (212 0F) the distribution header is fitted

with a thermal barrier.

IMPELLERS,DIFFUSERS,STAGECASING:

The impellers are designed with a neck at the entry and discharge ends. Sealing is

effected at both ends by means of renewable wear rings (122), which are press-fitted into the

stage casing (104) and into the diffuser respectively. The first stage (i.e. suction) impeller

(202a) is specially designed to prevent cavitation, and the results in a lower NPSH requirement

of the pump. The wear ring at the suction end of the first stage impeller is pressed into the

suction casing.

The backplates of the impellers are drilled to obtain a balanced pressure on both sides

of the impeller, and the residual axial thrust of the rotating assembly is consequently very

small. This residual thrust is bsorbed by an anti-friction bearing in the drive frame. The casings

are sealed by O-rings on the standard pump models, but on high temperature and pressure

pumps (cast steel casings), the sealing is effected by metal-to-metal contact of the precision

ground mating faces of the casing flanges.

SHAFTSEALING:

A pacing - type stuffing box is used as a shaft seal. During the operation of the pump

it reduces the leakage flow at the gap between the shaft and the casing. In case of a suction

pressure < than 1 atm. Abs., the ingress of air during standstill of the pump will be prevented

by admitting sealing liquid. Under higher temperature conditions, the stuffing box will be cooled

The suction and discharge piping have to be laid in such a fashion that no external

stresses are transmitted to the pump. Any sudden or abrupt change of direction or cross-

section has to be avoided. Flat gaskets should not protrude inside the pipe. Where welded

pipe lines are used, all sharp edges, welding beads and scale are to be removed

before

commissioning. The suction piping and vessels have to be thoroughly clean out and flushed

through before commissioning. The suction piping and vessels have to be thoroughly cleaned

out and flushed through before commissioning of te plant. Experience shows that welding

beads, scale and other impurities often become detached from the pipe walls only after a

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considerable period of operation, especially when the liquied pumped is hot, it is necessary,

therefore, to incorporate a strainer in the suction line to retain this foreign matter.

This strainer is to have a free flow area equal at least to 3 to 4 times the pipe cross-

section area, in order to prevent too great a pressure drop when it becomes partially clogged

with foreign matter. Cone-shaped strainers as illustrated in fig.2 have proved themselves well

in service, they should be made of corrosion-resistant material. The suction pressure is to be

carefully watched during operation. If there is a drop in pressure, the strainer is to be removed

and cleaned. After several weeks of operation, when the presence of foreign matter is no

longer anticipated, the strainer can be removed.

The vent line connects the suction compartment of the distribution header with the

apex of the suction vessel. The connection for this line, on the distribution header, is arrangedin such a fashion that any vapour or gas tending to form in the header, and which might cause

the pump to splutter, will escape through the vent line to the suction vessel.

COOLINGWATER:

According to the temperature prevailing, i.e. when he liquid being pumped has a high

temperature, the drive frame is cooled. The flow of cooling water is approx. 0.5-1 m3/h. It is

useful to have throttling means in the supply line. The discharge is to be provided to the open,

to facilitate observation of the rate of flow.

In some designs of these pumps, the bearing bracket or the packing-type stuffing-box

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in the drive frame are cooled. The system is similar to that for the frame itself.

Max. cooling water inlet emperature: approx. 20 0C (68 0F). A temperature difference of

10 0C (50 0F) between the inlet and the outlet temperature of the cooling water is allowable.

SEALINGWATER:

To prevent air from entering the system when the pump is not working, the stuffing-

box is supplied with sealing liquid. It is recommended that his be drawn from the discharge side

of the main condensate pumps.

The sealing water piping is to be fitted with a check-valve to prevent the pump from

discharging into this piping during its operation.

LEAKAGELIQUID:

There is a tapped connection provided on the motor stool to drain off any leakage. Any

leaks should be allowed to drain off freely.

FOUNDATION:

A foundation support frame consisting of steel sections is the most appropriate foundation

for the pumping set. The foundation support frame which has a machined contact face will be

leveled up to a true horizontal position, then grouted in. if this is done, the pump itself will not

require leveling up. The concrete should be allowed to set and dry out completely before the

pump is placed on its foundation. We can supply exact measurements for the foundation

support frame on request.

ERECTION OF THE PUMP :

The WKT pump is usually delivered assembled as a unit. It can be placed immediately

on to the foundation and bolted down. Then, the true position of the pupmping set is to be

verified again.

If the barrel is mounted separately from the pump, care is to be taken to ensure a

proper sealing between the barrel and the drive frame (O-ring, 542). If the set is supplied

completely assembled, the rotating assembly of the pump need only be re-aligned after previous

dismantling .

CAUTION1:

When delivered, the pumps have no oil in the bearing bracket. After the test run at our

works, the bearings are drained of oil washed, as there is no means of knowing when the pumpwill be commissioned, and the oil might oxidize if left in the bearings too long para 1.82 gives

indications about lubricants.

PACKING OF THE STUFFING - BOX :

The pup are delivered without packing in the stuffing-box. The straight lengths of the

packing rings cut off smoothly by means of a jig (see fig3) are to be of such a length that,

when fitted round the shaft sleeve, the sectional areas are in slight tough. Before insertion in

the packing area, the packing must be thoroughly soaked in oil. Together with the stuffing-box

gland, the first packing ring is inserted in the packing area and pushed to the bottom. The butt

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oints are to be taken to observe the proper sequence of packing rings and seal cage ring/

lantern ring.

The nuts of the stuffing-box bolts are slightly tightened by hand. A newlyh packed

stuffing-box is to leak appreciably at first. If this leakage does not cese of its own accord

after some time. The nuts are to be tightened slowely ad evenly during operation, until the

stuffing-box only drips slightly; this indicates that the packing is functioning correctly slightly

again. Every newly packed stuffing-box needs a certain running-in period and it is to be kept

under frequent observation during this period. After settling down, it needs only to be checked

occasionally. If the existing packing has been compressed by approx. the width of one packing

ring, the packing is to be renewed. Check also the condition of the shaft sleeve on this

occasion. It is to be replace if its surface shows signs of grooving or roughness.

STARTINGUPAND:

Always check the following points before starting up the pump for the first time and

later on. After a prolonged shutdown:

a) The bearing and stuffing-box housings must be filled with oil and the soft packing fitted

in the stuffing-box respectively.

b) Close the pump discharge valve: open the suction valve.

c) Make sure that the pump is completely primed with the liquied pumped.

d) Vent the pump (see para 1.42)

e) If applicable, turn on the cooling water and check the free flow of the cooling water.

f) Switch on the driver and check direction of rotation (anti-clockwise, when viewed from

above, looking down onto the pump).

g) Slowly, open the discharge valve.

Do not exceed the current intensity (Amps) on the motor rating plate.

SHUTTINGDOWNTHEPUMP:

The following operations should be carried out when shutting down the pump:

a) Close discharge valve.

b) Switch off driver and check that the pump runs down to a standstill smoothly and

evenly.

c) Turn off cooling water, if applicable.

d) Leave suction valve open.

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LUBRICATIONOFTHEPUMP:

The pump bearings and the column pipe bearings are lubricated by the product pumped

and require no supervision.

On no account let the pump run dry 1

The top shaft (bearing shaft) is guided within the bearing bracket. For the lubrication

of the anti-function bearings incorporated, please. If the set is fitted with oil-lubricated self-

aligning bearings or with a segmental thrust bearing. The appropriated special directions must

be followed.

The pump, the shaft and the riser piping required no attention if there are normal

working conditions. The stuffing box must leak slightly whilst the pump is running.

The drive must be maintained in accordance with the special instructions given on this subject.

LUBRICATION:

The top shaft (bearing shaft) is guided within the motor stolol. A good-quality well-

refined mineral oil should be used only to lubricate the grooved ball bearings. As a lube oil we

recommend a brand oil having the following specification:

Flash point

Pour point

Viscosity

Ash contents

Spec. gravity

: 200-220 0C (390-430 0F)

: below - 15 0C (+5 0F)

: 4-6 0E at 50 0C (30-45 c St. at 122 0F)

: below 0.05%

: 0.9 kp/dma (56 Ibs/cuft)

The oil should be changed approximately once every 2000 operating hours: in any

event however, at least once a year. The oil quantities required for one initial oil fill of the

bearing bracket (including the constant-level oiler and the fill piping) are given in table below :

Bearing size no.

Oil fill, in dm3

Fluid ounces in brackets

6311

0.3

(100)

6312

0.4

(13.5)

6315

0.5

(17)

6317

0.6

(20)

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I)

H.P. - L.P. BY PASS SYSTEM

INTRODUCTION:

a)

b)

For matching of live steam and turbine metal temperatures for a quick start up,

bypass stations have been provided, which dump the steam to the

condenser

through pressure reducing station and desuperheaters, during the period, steam

parameters at the boiler are being raised. These stations in additions to the quick

start of turbine and low noise level, also economises the consumption of

D.M.

water.

With the use of turbine by pass station, it is possible to build up the matching

steam parameters at the boiler outlet during any regime of starting, independent of

the steam flow through turbine. The steam generated by boiler, and not utilised by

the turbine during start up or shutdown, is conserved within the power cycle and

thus losses of steam into the atmosphere is cut down to the barest minimum. By

pass system enables to shorten the start-up time.

HP/LP bypass system can be broadly classified in the two groups :

1) HP bypass stations : This is utilised for the following tasks.

i) To establish flow at the outlet of superheater (SH) for raising boiler parameters

during start-up.

ii) To maintain or control steam pressure at pre-set value in main steam line during

start-up.

iii) To warm up the steam lines.

iv) To control steam temperature down stream of HP bypass at the preset value.

v) To dump steam from boiler into condenser, in case the generator circuit breaker

opens.

2) LP bypass station : The same is utilised for the following tasks.

i) Control of steam pressure after reheater.

ii) Establish flow of steam from reheat lines to condenser by its opening, proportional

to the opening of HP bypass valves.

iii) Release of steam entrapped in HPT and reheater circuit in case generator circuit

breaker opens.

The interconnections of the above stations with the turbine power cycle are as under:Description

HP bypass

Station.

LP bypass

Station.

Upstream steam Connection

Main steam lines

Ahead of MSV

Hot reheat lines

Ahead of IV

Down stream Connection

Cold reheat line.

Steam throw off device

built in condenser.

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c) Feed water is used as cooling water for H.P. bypass station and condensate from

condensate extraction pump is used as cooling water for LP bypass station.

II. H. P. BYPASS STATION

a)

b)

c)

d)

The main elements of HP bypass station are as follows,

i) 2 Nos. 127 x 160 mm. size combined pressure reducing and desuperheating

valves complete with valve mounted electro hydraulic actuator. Each valve has

been sized to pass 100 T/hr steam at 140 ata and 540 0 C on upstream side.

ii) 2 Nos. Throttling device One number after each HP bypass valve.

iii) 1 Nos. - 70 mm size spray water pressure control valve complete with valve

mounted electrohydraulic actuator.

iv) 2 Nos. - 34 mm size spray water temperature control valve complete with valvemounted electrohydraulic actuator.

v) 1 No. - Automatic pressure control loop consisting of the following elements.

1 No. Pressure master controller with proportional integrated (PI) characteristics.

2 No. Positioning loops for bypass valves.

1 No. Common set point setter.

vi) 2 Nos. - Automatic temperature control loops consisting of the elements.

2 Nos. PI controllers and positioning loops for spray water temperature control.

1 No. PI controller for spray water pressure reducing valve .

1 No. Common set point setter.

GENERALDESCRIPTION

The control components are located in a control cabinet in unit control board

(UCB). Each positioning loop may be controlled separately from the central control

desk. For supervision of the control loops, the position and Control deviation are

indicated on mosaic insert of the control desk.

OILSUPPLYUNIT

The oil supply units for the high - pressure bypass and the low - pressure bypass

are connected in parallel. Manostats control the oil pressure in the accumulators

and signal alarm "PRESSURE TOO HIGH" or "PRESSURE TOO LOW" appears in UCB if

the pressure is not in order. If the oil pressure should fall below the minimum in both

accumulators, positioning actuators will be blocked, and thereon the signal

"ACTUATOR BLOCKED" shall appear in UCB which simultaneously changes the operation

of each positioning loops from automatic to manual.

STARTUPPROCEDURE&SLIDINGPRESSUREOPERATION.

With the start-up of the boiler, the pressure set point has to be adjusted manually.

If too large a deviation between the measured pressure and the set point shall

occur, the alarms will be given. The deviation will be given for a positive as well as

a negative difference.

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The HP bypass station controls and maintains the upstream pressure at the desired

preset value when in operation. As soon as the entire steam available from the SH

outlet is swallowed by HP turbine, the HP bypass station shall get closed under

automatic controller action. The HP bypass, once closed, shall cut in under pressured

impulse only if the generator circuit breaker opens.

e)

f)

g)

h)

TEMPERATURECONTROL:

By injecting water into the HP - bypass valves, the steam is cooled to the temperature

in accordance with the preset value. To over-bridge the measuring delay of the

thermoelements, the spray temperature control valve controller is supplied with an

opening impulse at the moment when the HP - bypass valve starts to open.

To achieve favorable conditions for the bypass injection, the spray - water pressure

is reduced and adjusted to a constant value before the spray water temperature

control valves. In addition spray water pressure reducing valve remains closed

under interlock action when the HP - bypass valves are closed.

PRESSUREMEASUREMENT:

The pressure signal of HP bypass system will be taken from the main steam line.

PRESSURESETPOINTVALUE:

With sliding pressure operation, the desired pressure value has to be adjusted

manually to the actual pressure value (main steam pressure before turbine), by

adhering to a certain threshold, 5 to 8% on higher side. This follow up is effected in

such a way that the gradient of the desired pressure value is limited to the maximum

permissible pressure gradients on the boiler side. This set point can be varied from

the desk by the push button.

PRESSUREMASTER-CONTROLLER:

In accordance with the pressure control deviation the master controller works with

PI - behaviour and forms the positioning signal which is given to the slave -

positioners of the bypass valves. If the deviation between the actual pressure

signal and the set point value exceeds either a positive or a negative limit, the

monitors produce alarms. For a positive deviation alarm "AL 1" and for a negative

one alarm "AL 2" appears. These alarms do not influence the controllers, but they

will appear visually and audibly in the unit control room. With the appearing of the

those alarms, the operator has to correct the pressure set point.

The changeover from "MANUAL" to "AUTOMATIC" mode of the pressure master -

controller is linked up with the adequate changeover of the HP bypass positioning

circuits. If both of these slave positioners are in "MANUAL" mode, the master -

controller's output is switched on to the larger opening (feedback) signal from one

of the bypass systems.

This maximum selection will be performed by the MIN/MAX - selector. Furthermore,

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i)

j)

k)

it's output signal gives the basic condition for the bumpless transfer if one of the

slaves will by switched into "AUTOMATIC" mode.

HP BYPASS - VALVE POSITIONER :

With the positioners the HP - bypass valves are operated in accordance with the

output signal of the pressure master controller.

Individual control of each HP - bypass valve is possible with the push button.

The monitor transmits a continuous closing - signal to the positioner if the stroke of

valve is smaller than 2% and if the positioner is in "MANUAL" mode. In this way the

valve remains definitely closed.

SPRAYWATERTEMPERATURECONTROLVALVECONTROLLER:

The control positioners for the bypass spray temperature valves are designed in the

same way as those for the HP - bypass valves. In addition, PI - controllers are also

connected up to the control positioners. The temperature-measuring signal from

transmitter is compared at the input of the PI controller with the common set point

signal. According to the particular control deviation, the PI controller forms a rated

signal for the control positioner of the associated spray temperature valve.

Due to the hydraulic drive it is possible to attain also for the spray temperature

valves or short positioning time. This is necessary to allow the temperature control

to intervene fast enough in the event of any fast - opening of the HP - bypass

valves.

To offset the time delays of the temperature measurement and to achieve favourable

conditions when switching on the spray water cooling system (rapid adjustment to

temperature set - point), a positive reference voltage is given to the proportional

input of the injection - valve controller, by the associated bypass - valves positioning

monitor. In this way independent of the temperature-measuring signal a certain

amount of water is simultaneously injected at the opening of the bypass valve.

Manual operation of the bypass spray water temperature valve is effected by

means of the push button. The valve position and the control deviation are indicated

on the desk with indicators. With the combined PI - control positioner which influences

the pressure control valves, the spray water pressure is regulated at a preset value

before the spray water temperature control valves.

The manual control of the pressure control valve is effected with the push button.

Valve is kept closed by an interlock if the HP - bypass valves are closed.

Additionally, if the positioning loop was in "MANUAL" operation - mode, it will be

brought into "AUTOMATIC" - state, if any one of the HP - bypass valves opens.

SPRAYWATERPRESSURECONTROLLER:

The spray water pressure before the spray water temperature valves, will be

measured by the transmitter.

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Normally the valve is closed and the positioner is in "AUTOMATIC" mode. If one or

both bypass - valves open, the "CLOSE" interlock disappears and the positioner will

be brought into "AUTOMATIC" mode regardless of which mode dominated before.

However, this loop can be operated by the push button at the mosaic - desk insert.

The actual position and the control - deviation are shown by the instrument at the

mosaic - desk insert.

III) INTERLOCKS:

a)

b)

c)

d)

e)

The HP bypass system are influenced by the following interlocks.

i) Generator circuit - breaker.

ii) Condenser vacuum too low

iii) HP - valve - position a < 2% / b > 2%iv) Temperature too high at down stream of LP bypass station.

Following interlocks are produced by the bypass system and given to the other

positioning loops.

v) HP valve position more than 2%.

vi) "Close" - signal for spray water pressure control valve.

GENERATORCIRCUITBREAKER:

The HP bypass station shall come into operation at the moment the logic signal

"GENERATOR CIRCUIT BREAKER CLOSED" disappears. However this does not imply

that the HP bypass station shall be out of service if the logic signal "GENERATOR

CIRCUIT BREAKER CLOSED". appears.

CONDENSERVACUUMTOOLOW:

The HP bypass station shall close immediately in case of too low condenser vacuum

(500 mm Hg-Col). This interlock holds a first priority for the controller.

HP BYPASS VALVE POSITION LESS THAN 2% :

i) When turbine is running or not running and the control of HP bypass valve is on

manual, the memory will get closing signal through AND logic, if the valve position

is less than 2%.

ii) When turbine is running and control of HP bypass valve is on auto, the memorywill get closing signal through AND logic, if valve position is less than 2%

iii) When turbine is not running and control of HP bypass valves are on auto, there

is no closing signal to memory whatever is the position of the valve and thus

pressure control loop will actuate the valve.

TEMPERATURETOOHIGH:

If the temperature after the outlet of the HP or LP bypass station becomes "TOO

HIGH", the closing signal to HP bypass Valves is forwarded and simultaneously

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f)

g)

positioning loops change from automatic to manual mode. The interlock for this

case will be provided by the temperature supervising monitor respectively.

HP BYPASS VALVE POSITION MORE THAN 2%

If any of the HP bypass valves are opened more than 2% (which is initiated by a

part of voltage monitor) or if the position demand signal is equivalent to more than

2%, valve opening (which is initiated by a voltage monitor), a signal is available

through OR logic to indicate the valve is open and the same signal is

used to

change the control of valves BPE and BD from manual to auto if their control was on

manual.

CLOSING INTERLOCK FOR SPRAY WATER PRESSURE CONTROL VALVE

CONTROLLER:

To ensure the tight closure of spray water pressure valve so that the spray water

temperature valve are not subjected to high water pressure, during the period they

are not in operation, a tight closure signal is initiated to the valve, when HP bypass

valve opening is less than 2%

IV) L. P. BYPASS STATION: a)

b)

c)

Functionally the LP bypass control system can be divided into two parts.

1) Pressure control.

2) Temperature control.

The main elements of LP bypass stations are as follows.i) 2 Nos. 367x370 mm size pressure control valves complete with electrohydraulic

actuator. Each valve has been sized to pass 112 Ton/hr. steam at 6 ata. and

540 0C.

ii) 2 Nos. LP desuperheaters.

iii) 2 Nos. 44 mm. size spray water temperature control valves complete with

electrohydraulic actuator.

iv) 2 Nos. Automatic temperature control loop each comprising one number PI

controller and one number-positioning loop with common set point setter for

spray water temperature control valve.

v) Automatic pressure control loop with a sliding, load dependent set point formation,along with one number PI controller and two number slave positioner.

vi) Automatic valve position control loop with a master signal formed as on average

HP bypass valve position with "proportional" function.

vii) A selector unit to select any one of loop under (e) & (f) mentioned above.

GENERALDESCRIPTION:

The control components are located in a control cabinet in UCB. Each positioning

loop may be controlled separately from the central control desk. For supervision of

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d)

the control loops, the position and control deviations are indicated on mosaic insert

of the control desk.

OPERATINGMETHOD:

The system enables generally the manual selection of two operational modes with

the selector station.

Pressure control with a sliding, load - dependent set point formation with 'PI'

function.

Valve position control with a master signal formed as an average HP bypass

valve position with proportional 'P' function.

e)

During the pressure control the selector relay is energized by means of selector

station and analogue memory and the contact turns-over into position marked "R".

In this case the master pressure controller controls the valves in accordance with

the formed sliding set-point The set-point formation is load dependent and is derived

from the steam pressure after regulating stage of the turbine, measured by

transmitter. The intensity of the signal measured by this transmitter is reduced to

approx 30% of its value by the summing amplifier. A limitation to a minimum value is

realized by means of the maximum selector and motorized set-point setter.

For selecting the LP bypass valves operation, proportional to the average actual

position of HP bypass valves, the selector relay is made de-energised with the help

of push button. Thus the contact of this relay stays in the position marked 'S'. The

manual operation of LP by pass valve is possible from UCB with the help of push

button modules.

For achieving bumpless transfer of operation of LP bypass valves from proportional

to pressure mode of control, a signal UM

is always fed to PI controller, to keep thecontroller always charged. In addition to this, for achieving bumpless transfer during

pressure mode of operation, from 'manual' to 'auto' mode, the position of LP bypass

valve is fed to PI controller, through a max. selector unit. Thus the output of PI

controller is switched over to the higher valve-opening signal of any of the LP

bypass system.

TEMPERATURECONTROL&OPERATINGMETHOD:

The task of the loop is to control the steam temperature after the LP bypass valves

and before the condenser. The controlled variable is the steam temperature after

the LP bypass valves, the manipulated variable is the spray water flow.

The system receives the actual steam temperature signal from transmitter compares

it with the set-point and controls the position of the spray water valves in accordance

with the control deviation.

A feed-forward signal (actual LP bypass valve position) is introduced to the second,

P-action channel of the controller to improve the control results similar to HP

bypass system.

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To safeguard the condenser from high temperature steam, if the actual temperature

after any of the LP bypass system exceeds the permissible limit i.e. 5% above the

set value, an alarm is given through voltage monitor. Simultaneously the same

signal is fed to the slave positioners of HP bypass valves and LP bypass valves for

closing of HP/LP bypass station, and at the same time control of positioning loops of

HP/LP bypass valves change from 'auto' to 'manual' operation.

III. OPERATIONOFHP/LPBYPASSSTATIONS:

Before HP/LP bypass stations are brought into operation check the following.

a)

b)

c)

d)

e)

ESVS & IVS are closed.

Oil system of the by pass station is put into operation by switching on its motor.

Vacuum in the condenser is more than 540 mm of HgC.

Condensate flow to steam throw off devices in the condenser is established by

opening motor operated valves MC-39 & MC-57.

Interlocks for NRVS in cold reheat lines at HP turbine exhaust. When HP by pass is

opened CR1 & CR2 remain closed for 60 sec. and then will be ready to open position

closing in by power cylinder through solenoid operation.

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TG LUBRICATING OIL SYSTEM

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VARIOUS TYPES OF VALVES

1.0 Introduction :

Valves are used for different purposes. Mainly as follows:

i)

ii)

iii)

To stop or allow the flow of fluid in a pipe line

To act as a safety device.

a) To prevent excess pressure generation in a pipeline or vessel (safety valve,

Pressure Relief valve)

b) To prevent back flow of fluid into up stream side of equipment which might get

damaged due to the back flow (non-return valve, flap valve)

To regulate the quantity or pressure of the flowing fluid.

iv) Special valves for specific service conditions like Christmas tree valve for oil well,

quick operating blow down valves for boiler blow down lines.

2.0 Classification of Valves :

Valves can be broadly classified into following two types based on the relative motion of

the closing element with the direction of fluid flow or pressure.

i)

ii)

The closing element moves in the same axis in which the fluid flows.

(eg. Globe valve, Safety valve non return valve etc.)

The closing element moves in the perpendicular direction in which the fluid flows. In

other words the closing element cut across the flow of the fluid.

(eg. Gate valves, cocks, ball valves etc.)

3.0 Construction of the valves :

The valves are fabricated from different materials such as cast iron, steel, gun metal,

stainless steel etc. in order to suit the fluid they carry. The valves are classified according

to pressure class as per the relevant BSS, ASA, DIN or ISI specification. Similar type of

valves have different types of ends connection i.e. flanged, welded, socket, screwed

etc for connecting them with the pipe lines.

Valves components can be grouped mainly as.

i)

ii)

iii)

Body

Yoke, bonnet and cover.

Disk, wedge

iv) Stem

v) Gland

vi) Other parts.

i) Body :

The body of the valve serves the following functions.

a) The inlet and outlet connections are accommodated.

b) Fluid flow passage is provided in the body.

c) One part of the sealing seat is accommodated.

d) Connection to other valve parts and gasket are provided in the third flange.

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e) Wherever necessary bypass arrangement is provided on the body.

ii) Yoke Bonnet and Cover :

a) The function is to connect with the body of the third flange.

b) Houses gland and stem nut.

c) Vent plug for deaeration is located in the yoke.

d) In case of motor operated valves the motors are mounted on the yoke.

iii) Disk Wedge :

This is a moving part inside the body operated by the stem and this is the control

element to stop or regulate the flow of the medium. This contains

i) The control element seat which mates with the body seat.

ii) The body which can isolate the fluid from upstream to downstream side.

iii) Connection to the stem.

iv) Stem :

This is also a moving part and most important part as it is subjected to

high

stresses and different conditions of temperature, contact media etc. the following

functions are performed by the stem.

i) It connects with the disk or wedge.

ii) It slides against the gland packing.

iii) It has operating screw threads, which converts the torque applied at the hand

wheel to vertical thrust with the stem nut.

v) Glands:

The glands consist of the gland packings one piece or two pieces, gland cover,

gland tightening bolts and nuts. The gland tightening bolts are supported on the

yoke or bonnet. The main function of this gland is to provide sealing around the

stem and smooth sliding operation to the stem. The gland packings are generally of

knitted asbestos yarn with some solid lubricants like graphite or mica flakes.

When the gland covers are tightened they should be able to create

sufficient

pressure on the stem to prevent leakage.

vi) Other parts:

Other parts are the fasteners, used for body bonnet connection, hand

wheel,

gasket etc. fasteners are made from high carbon steel and hand wheels are made

from Cast steel.

4.0 Comparison between Globe and Gate valves:

i) In a globe valve full flow area around the periphery is obtained by a lift of d/4 where

'd' is the nominal bore of the valve. Where as in gate valve the gate has

to

completely travel across the bore of the valve and hence the valve must have a lift

of slightly more than 'd' which is about 4-times that of a globe valve. Hence the

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ii)

iii)

total height headroom required, time required to complete opening and weight of a

gate valve are comparatively more than a globe valve.

The manufacture and maintenance of a gate valve is more difficult compared to

globe valve as the two tapered seats in the wedge have to perfectly match with

that of the seats in the body.

In the globe valve the disc has to function against the full medium pressure and

hence the stem load is quite high compared to a gate valve.

iv) Due to the flow directional changes and possible turbulence in a globe valve the

flow loss is more than a gate valve in which flow is straight and with least turbulence.

5.0 Operation of Valve :

Valves can be chosen with manual hand wheel operation. In case the valves are located

away from the floor level, apart from the manual operation pneumatic, hydraulic or

electric motor operation can also be chosen.

6.0 Valve Actuators (Electrical) :

Valve actuators are of two types:

a) Rotary actuator b) Linear actuator

Rotary actuators are available in different torque capacities and different output speed.

These actuators can be fitted on gate and globe valve.

Linear actuators have a reciprocating movement and therefore have a restriction in the

operating stroke or lift of the valve. These are used in gate valve & globe valve of smaller

size.

The hand wheel is normally used for opening & closing of valve. The size of the hand

wheel is such a way selected that the torque required is sufficient to open or close the

valve.

Valves which require greater force for opening and closing and can not be provided with

larger size hand wheel, due to space limitations, are provided with actuator with a worm

drive, with chain pulley block type. In some cases electrically motor operated gear train

is mounted on the valve with reversible motor for opening and closing of valve Enclosed

figures show the various types of valves used in power station.

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