BASIC OPERATION OF POWER PLANT (WHRB)

POWER PLANT OPERATION

BY MUJIYONO

BOILER

Definition

As per Indian Boiler Act 1923, Boiler is defined as any closed vessel exceeding 22.75 liters in capacity which is used exclusively for generating steam under pressure and includes any mounting or accessories attached to such vessel, which is wholly or partially under pressure, when steam is shut off.

A good Boiler should have some essential qualities.

1.     Capable to meet large load fluctuations.

2. Fuel efficient i.e. to generate maximum steam with minimum fuel consumption.

3.     Ability to start-up quickly.

4.     Easy in maintenance and inspection.

5.     Occupy  less floor space.

6.     Lower friction loss in water and flue gas circuit

7.     Little attention for operation and maintenance.

Systems in a Boiler

A Boiler mainly contains following systems :

1.     Feed water system.

2.     Steam system.

3.     Air system.

4.     Flue gas system.

5.     Fuel handling system.

6.     Ash handling system.

Boiler Mountings

Fittings on a Boiler which are required for its safe and efficient operation are called mountings. These are as follows :

1.     Safety valve

2.     Water level sight glass (gauge glass)

3.     Pressure gauge

4.     Blow down valve

5.     Main steam stop valve

6.     Feed water check valve (NRV)

7.     Air Vent

8.     Start-up vent

9.     Manhole

Boiler Accessories

The devices which are used in a Boiler as an integral part and help to run the Boiler efficiently are called  Boiler Accessories.  These are :

1.     Super heater

2.     De-super heater

3.     Economizer

4.     Air Pre-heater

5.     Soot Blower

6.     Feed Pump

7.     ID and FD fans

8.     Ash Removal system

9.     Fuel supply system

10.   Dosing system

11.   Deaerator

Steam Generation In A Boiler..contd

In a Boiler fuel is burnt to get heat energy which is converted from chemical energy stored in a fuel. This heat energy is utilized to produce steam from feed water.

Fuel is fired in the furnace of the Boiler. Different fuel is used in different Boilers. Accordingly furnace is designed. Water tubes are arranged around the furnace and flue gas path. Water tube arrangement made around the furnace is called as water wall. Feed water is circulated in these tubes. Water comes to water wall from Boiler drum, and circulated back to drum after absorbing heat. Due to difference in density which is created due to difference in temperature, water circulates in these tubes naturally. Therefore, it is called Natural Circulation.

During circulation of water in tubes, steam is generated and collected at the upper part of the Drum. This is called Saturated Steam corresponding to Boiler drum pressure. This steam is further heated in Superheaters and becomes superheated steam.

Boiler Drum is filled with fresh feed water. The feed water, before entering into drum is heated at Economizer. Economizer is placed at the flue gas path. Most of the heat of the flue gas is utilized inside the Boiler. Still considerable amount of heat energy is available in it. This heat is utilized in Economizer to heat up the feed water.

For burning of fuel, required Oxygen is obtained from atmospheric air. Air is required in Boiler furnace for combustion. This is supplied by Forced Draught (FD) fan. This air is heated at air pre-heater (APH) before being sent into furnace. If cold air is used then there will be loss in energy. Air pre-heater is placed at the flue gas path after Economizer. Air pre-heater is a heat exchanger which exchanges the heat of flue gas to the cold air, which is to be used in furnace. By heating the air, burning of fuel is easier and loss of energy is minimized. If hot flue gas would not be used to heat up feed water at Economizer and air at Air Pre-heater then it would escape into atmosphere.

Finally the flue gas passes through Electrostatic Precipitator (ESP) and exhausted to atmosphere through chimney. At ESP the dust particle in the flue gas is trapped and clean gas  escapes to atmosphere.

Ash which is produced in the Boiler due to combustion of solid fuel is collected at Boiler bottom and also in Economizer, Air Pre-heater and ESP. This ash is disposed off with the help of suitable ash handling system.

Preparations for Cold Start-up

1.   All the manhole doors should be in close condition.   Tightness of the Nuts and Bolts of the man hole doors to be checked properly.

2.     All the water wall drain lines should be in close condition.

3.     All the steam drain lines should be in open condition.

4.     Start-up vent Root Manual isolation valve  should be in open condition.

5.     Drum level should be at Normal Water Level (NWL).

6.  Continuous Blow Down (CBD)  and  Intermittent Blow Down (IBD) drains should be in close condition.

7.   All the super heater vents including Drum vent and Puppy Header vent should be in open condition.

8.     Before and After Isolation valves at Feed Control Station (FCS) should be in open condition.

9.     Attemperation Control valve before and after isolation valve should be in open condition .

10.   Hydra step should be in healthy condition.

11.   Safety valves should be in healthy condition.

12.   Main Steam stop valve and by- pass valve should be in close condition.

13.   Soot blower manual isolation valve and control valve should be in close condition.

14.Boiler Drum Gauge glass steam side and water side isolation cocks should be in open condition.

15.   HP Dosing Pumps should be in Healthy condition and open suction and discharge valves of the pump.

16.   Solution in HP Dosing agitator tank should be at normal level.

17.   Boiler Feed Pumps should be in healthy condition.

18.Deaerator water level should be maintained at 60% by taking DM Transfer pump in service.

19.   Air compressors should be in healthy condition.

20.   Ash handling systems should be in healthy condition.

21.   ESP should be in healthy condition.

22.   ID fan damper should be in Zero position.

23.   All the interlocks and protection should be checked properly viz. Drum level low, Deaerator level low, Boiler Feed Pump (BFP) discharge pressure low, Flue gas temperature at Post Combustion Chamber (PCC) outlet high, silo level.

Cold Start-up process

1.     After Kiln light-up, when flue gas temperature at PCC outlet increases to more than 450 deg.C, open ID fan damper 5%. Due to natural draught created by chimney, flue gas passes through Boiler and slow heating and expansion takes place.

2.     After opening of ID fan damper, Boiler furnace temperature rises slowly. When the furnace temperature rises to 250 deg C, Open ID fan damper 10%.

3.     When Flue gas temperature at PCC outlet rises more than       600 deg C., close the ID fan damper and start ID fan.

4.     When Drum pressure reaches 5 Kg/cm2, close the Drum vent and Puppy header vent.

5.     When Boiler Drum pressure reaches 20 Kg/cm2, give blow down of the water wall to remove the deposition or sludge.

6.    By adjusting damper opening raise the Boiler pressure upto 45 kg/cm2 and 485 deg C.

7.    Start-up vent should be in open condition since the admittance of hot flue gas in boiler.

8.   Open the Main steam line drains in between Boiler Main Steam Stop Valve (MSSV) and TG MSSV.

9.  Open the MSSV by pass valve to remove all the condensate in main steam line and ensure that TG MSSV is in close condition.

10.   After removal of all the condensates in Main steam line and proper line heating, open Main Steam stop valve of Boiler.

11.   Close Super heater drains.

12.   Put Drum level controller in Auto mode.

13.   Put Attemperator controller in Auto mode.

14.   Close Start up vent as per the steam demand of TG set.

15. Charge ESP when Flue gas temperature after Economizer reaches 160deg. C

Finally the flue gas passes through Electrostatic Precipitator (ESP) and exhausted to atmosphere through chimney. At ESP the dust particle in the flue gas is trapped and clean gas  escapes to atmosphere.

Ash which is produced in the Boiler due to combustion of solid fuel is collected at Boiler bottom and also in Economizer, Air Pre-heater and ESP. This ash is disposed off with the help of suitable ash handling system.

Start-up of Waste Heat Recovery Boiler (WHRB) 
Hot Start-up

Start-up of Boiler within 2 Hrs of Tripping of Boiler is known as the Hot Start-up of Boiler.

1.     Ensure the Drum level of Boiler. It should be at Normal water   level.

2.     Start Air Compressors.

3.     Start Boiler Feed water Pump.

4.     Start ID fan with ID damper in Zero position.

5.     Open Start-up vent.

6.     Slowly open damper of ID fan. Watch Drum level.

7.     Regulate Boiler pressure by opening start-up vent.

8.     Super heater temperature has to be maintained with the help of attemperator control valve.

9.   Raise the Boiler pressure upto 45 Kg/cm2 and temperature  to 485 deg C.

10. Open the drains of Main steam line in between Main Steam Stop Valve (MSSV) of Boiler and Turbine.

11.   Open By-pass valve of MSSV.

12.   Condensate, if any, will be drained out and main steam line heating will be carried out by opening of by-pass valve.

13.   After ensuring proper Main steam line heating, open Main

14.   steam stop valve.

15.   Close all drains in main steam line.

16.  Charge ESP when flue gas temperature at Economizer outlet reaches 160 deg C

17.   Put drum level controller and attemperator controller in Auto mode.

18.   Regulate the pressure of Boiler with the help of start-up vent.

19.   Close Start-up vent as per the steam demand of TG set.

20.   Normalize ID fan damper by gradual opening and loading of Boiler.

Charging of De-areator

It removes the dissolved gases from the condensate mechanically by following two laws

1.     Henry’s Law

2.     Dalton’s Law of Partial Pressure.

·        According to Henry’s Law, Solubility of dissolved gases decreases by increasing water temperature. So by charging steam in Deaerator water temperature increases and soluble gases in condensate departs.

·        According to Dalton’s Law of Partial Pressure Pm= Ps+Pa

Where Pm= Partial pressure of Mixture

Ps=   Partial pressure of Steam

Pa=   Partial pressure of Air

·        The partial pressure of air present inside the Deaerator comes out

·        through Deaerator vent for equilibrium state.

Procedure Of Charging

1.  Ensure DM Storage Tank level is more than 60%.

2.  Start DM Transfer Pump by opening Recirculation valve.

3.  Ensure Deaerator level is 60%. If the level is less then take the make up water .

4.  Open all drain lines  of Pegging PRDS line and observe that condensate is completely drained out.

5.  Slowly open Pegging PRDS pressure Control Valve and ensure that condensate is drained out completely. Then close the drains.

6.  Gradually increase the pressure to 2.8 Kg/cm2 by increasing pegging PRDS pressure control valve.

7.  Slowly heat the Deaerator by opening the heating line isolation valve and raise the Deaerator temperature to 90 deg C.

8.  Open the before and after isolation valve of Deaerator Pressure Control valve. Then open the pressure control valve gradually. Slowly increase the Deaerator pressure upto 2 kg/cm2 .After that put the Deaerator Pressure control valve in Auto mode.

9.  Start LP Dosing pump.

10.   In LP Dosing Hydrazine is used. Hydrazine removes oxygen by chemical reaction.

11.  EQUATION- N2H4+O2=2H2O+N2

12.   By adding Hydrazine dissolved oxygen becomes water and Nitrogen gas releases.

WHRB Interlocks

1.     If Drum level becomes very low i.e. 25% then ID fan Trips and Emergency cap opens

2.     This is to protect the Boiler tubes. At low Drum level, heat flux input has to be cut off, to protect the Boiler tubes, otherwise starvation takes place.

3.     If PCC out let temperature rises to 1050 deg C then ID fan damper becomes Zero and Emergency Cap opens.

4.     This protection is incorporatedto protect the Boiler tubes from overheating.

5.     If all BFPs trip then ID fan damper becomes Zero and Emergency cap opens.

6.     When all running BFPs Trip, then Drum level falls drastically. To protect the Boiler from starvation heat flux input should be cut off.

7.     If Deaerator level becomes very low i.e.25% then All BFPs Trip.

8.     Running of BFPs at Low Deaerator Level is harmful for the Pump.

9.     If Ash Silo level is high, all ash handling systems stop.

10.  When ash Silo is at high level then conveying more ash from ash handling systems results line blockage of ash conveying line. To prevent this, it is better to stop the systems and unload ash from Ash Silo.

11.   Boiler Main steam stop valve will not open if by-pass MOV of MSSV is in close condition.

12. This protection is to avoid line hammering due to presence of condensate in main steam line and to prevent carry over of condensate towards Turbine side.

13.   Boiler Feed Water MOV will not open if by-pass MOV of Feed water MOV is in close condition.

14.  If  feed water is empty in Economizer and in the pipe line after Feed water MOV, then by opening Feed water MOV directly without opening FW by-pass, MOV will lead to overloading of BFP, resulting BFP Trip.

15.  ESP trips, if Ash Hopper level is high.

16.   ESP has high voltage. Ash has presence of combustibles.

17.   This protection is to safeguard ESP at Ash Hopper level high.

18.   ESP can not be charged without starting of Purge Air Blower.

20.    This is to Seal the ESP by the air from Purge Air Blower  before charging it.

22.  ESP can not be charged till flue gas inlet temperature reaches 160 deg C.

23.   This is to avoid deposition of moisture and oil content influe gas on ESP.

1.     Decrease in Drum level

a.     Tripping of  Feed Pump

If Boiler feed Pump trips then Feed water supply to Boiler interrupts and leads to lowering of Drum level. If this has happened then ensure that the auto stand-by Boiler feed pump has started in Auto mode. If the auto stand-by Boiler Feed pump has failed to start in Auto mode then start the Boiler feed pump manually otherwise Boiler will suffer from starvation and ultimately it will lead to Boiler trip to protect the Boiler.

b.     Tube failure in Economizer

If Boiler Economizer tube fails then water supply to Boiler Drum will be affected. This leads to decrease in drum level and Feed Control valve will open more to compensate the Drum level to Normal water level, which leads to overloading of Boiler Feed pump. 

Observe the steam flow and feed water flow. If feed water demand to drum is increasing then observe any sound from the furnace. If tube has failed inside boiler then hissing sound comes and it can be noticed from outside. Simultaneously check the smoke from the chimney. If it is of white colour then tube failure in side the furnace is confirmed.

c.      Unit getting into Island mode

When Unit comes to Island mode, it follows the load connected to the Generator.  Suppose Unit is generating more power than the Unit load and exporting to Grid.

At the time of Islanding, Generator will follow the load connected in this Unit and the Governing Control Valves would close according to load and allow the steam to pass through Turbine. The surplus amount of steam will remain in Boiler which increases the Drum pressure. This drum pressure will exert a downward thrust to the drum level and it decreases drastically.

d.     Whether CBD valve, EBD valve or IBD valve opened?

If any operating personnel has opened any of these valves without proper reason or intimation then also drum level decreases rapidly. Ensure first then close the valve or regulate it observing the drum level.

2.     INCREASE IN DRUM LEVEL

a.  Whether Cold start-up in Boiler is in progress?

During Cold start-up when water temperature reaches 90⁰ C then formation of bubble starts. This is known as swelling phenomenon. If this is the case then blow down has to be given to maintain the drum level at Normal water level.

b.    Whether Instrument air compressor tripped and air lock unit at feed control station failed?

If Instrument air compressor trips, then air lock unit of control valve at feed control station keeps the control valve at a position at which it was, before supply of instrument air. This is known as stay put condition. If air lock unit fails to keep the feed station control valve at stay put condition, then when supply of instrument air fails, it leads to 100% opening of control valve. If this happens, start the instrument air compressor as early as possible and regulate the feed station control valve.

c.      Whether Start-up vent has opened or safety valve popped up?

By opening start-up vent, when Boiler is in steaming condition, supply of steam to Turbine Drum level increases rapidly due to release of pressure in drum. If the steam demand in TG has reduced to a large extent then it results Boiler drum pressure rise quickly and at that instant drum level falls rapidly.  When start-up vent is operated to release the surplus steam or safety valve pops up,  then drum level increases rapidly. In this case at first ensure for what reason the pressure in Boiler has increased. If drum level is increasing drastically then give blow down to regulate it. Because at higher side drum level, the steam quality will be affected and carry over of water particles to super heaters and turbine will take place, which is very much harmful.

d.    Whether Start-up vent has opened or safety valve popped up? Continued….

Operation should not be carried out when Boiler is in loaded condition.  Donot close the Feed Control valve fully if drum level rises because if the control valve is closed completely, the feed water in Economizer tubes, which was passing to Drum, will became steam due to heat in flue gas and when feed water supply through Economizer will be again established through Feed control valve then hammering in Economizer tubes due to presence of steam. This may lead to Economizer tube failure. After ensuring the reason, close the start-up vent and dump the surplus steam in Condenser. Ensure that the safety valve has been reset in its position and no passing is observed.

e.      Whether drum level transmitter is malfunctioning?

If drum level transmitter is malfunctioning then observe the level in hydrastep and immediately inform shift in charge and instrument personnel about this.

f.       Whether rapid heat supply to Boiler?

If heat supply to Boiler will be increased suddenly with a huge amount then it affects the drum level and it swells. To avoid this regulate the heat input supply in a gradual loading manner. Sudden and huge amount of heat supply will overheat the grain structure of the tubes and it suffers from fatigue. In course of time tube fails.

g.     Whether stand-by Boiler Feed Pump has started?

When stand-by Boiler feed water pump has started with running Boiler feed water pump, then Drum level increases because at that opening in Boiler feed Control valve when feed water pressure increases, more feed water flows to drum due to that opening of control valve and leads to increase in drum level. This case normally happens during scheduled Equipment change over of Boiler feed water pump. At first the stand-by feed water pump is started and discharge valve of the respective feed water pump is opened.  After that the previously running Boiler feed pump is stopped. Ensure whether it is a scheduled equipment change over.

h.     Whether TG has come to Island mode?

If TG has come to Island mode then Boiler pressure increases as there is a cut off steam demand as Generator has to follow the load, connected to it in this unit. If unit was exporting the power to Grid then the surplus power will be reduced at that instant,

which the Governor of the TG set will follow. It closes the control valve and steam pressure rises in Boiler accordingly. Ensure that the unit is running under Island mode. Open the start up vent to release the pressure. Ensure that the Safety valve has popped up or not. If popped up then it has reset properly or not. Observe the drum level during this operation. Observe the Dump control valve is functioning properly or not. If it is responding properly then try to supply steam to condenser by closing start-up vent after ensuring that Boiler pressure has reduced and safety valve has reset.

h.   Whether TG tripped?

If turbine trips then steam demand in Turbine will cut off and resulting Boiler pressure rise. Ensure Dump circuit is healthy. Open the Control valve of dump and close the start-up vent after ensuring that the safety valve reset.

i.       Whether any Cooling water pump in TG has tripped?

When Cooling water pump in TG for Condenser condensate cooling trips then the vacuum in condenser drops quickly and at that instant if the auto stand-by pump fails to start then the load set point at Generator has to be reduced with immediate effect. Otherwise the TG will trip due to low vacuum. When load set point at Generator decreased suddenly then Boiler pressure increases. In this case communicate with the TG operator and open start-up vent and lower the Load set point. Try to start the Main cooling water pump manually. After restoration of cooling water pump divert the steam from start-up vent by closing it to the dump circuit and normalize the load of Generator.

3.     Decrease in Boiler Steam Pressure

a.     Whether flue gas inlet temperature has reduced?

If flue gas inlet temperature reduces then it steam generation reduces in Boiler and pressure drops. This has to be observed very carefully and the generator Load set point has to be lowered, otherwise the TG will trip when the Main steam pressure becomes low.

b.     Whether more steam demand at TG end?

If the unit is running at low load as steam generation in Boiler is low. If as a mal operation Load set point at Generator is given more than steam generation then Boiler pressure decreases. and TG is running with low load set point. Unit is importing power from Grid. If unit came to Island mode then the Generator will follow the load which is connected to it and load set point at Generator increases than the steam generation in Boiler. So Boiler pressure decreases. As we can not change the load set point of Generator by putting lower set point value, Load on the Generator has to be lowered by cutting off the load connected to it. Choose the less important load connected to Generator and cut off it as quickly as possible otherwise the unit will suffer from Black out condition due to TG trip at Main steam pressure low and Grid power is unavailable.

The same case happens when the steam generation in Boiler is low

c.      Whether superheater tube failed?

If superheater tube fails then Boiler steam pressure decreases. Observe steam flow and feed water flow. If steam flow is at lowering trend and feed water flow is at increasing trend then it indicates that tube has failed. If the tube failure has occurred in side the furnace then white smoke comes out from chimney. When steam pressure decreases then reduce the Generator set point accordingly to avoid TG trip at main steam pressure low and ensure whether tube has failed or not. If tube has failed then Boiler shut down has to be taken to replace the failed tube with a new tube.

d.      Whether ID fan damper has closed to zero position?

This case happens when flue gas temperature at Post Combustion Chamber reaches 1050⁰ C. Flue gas flow to Boiler cut off when ID damper closes. It means heat supply to Boiler has cut off. It results in less steam generation. So when ID damper closes due to high PCC temperature, immediate load reduction has to be carried out in Generator to avoid TG trip due to Main steam pressure low.

e.      Whether hand lever of Safety valve has been operated?

If any person has operated the hand lever of safety valve without proper communication with the operating personnel for sometime then Boiler steam pressure decreases and drum level increases. 

4.     INCREASE IN MAIN STEAM TEMPERATURE

a.     Whether Boiler is loaded with huge amount of heat suddenly?

Main steam temperature rises if flue gas temperature at Boiler inlet rises suddenly. As superheaters are located at convection zone,  therefore when flue gas temperature rises, it increases the superheater temperature. If attemperator control valve fails to control the main steam temperature then TG will trip due to main steam temperature going high. In order to avoid such a situation, if main steam temperature rises due to rise in flue gas temperature, then immediately attemperator control valve has to be taken to manual mode and attemperation should be increased. Also communicate with the kiln personnel about the sudden rise in flue gas temperature.

b.     Whether Soot Blowing is in progress?

During soot blowing, steam temperature rises because more steam is required for soot blowing and heat input to the Boiler has been increased by opening the ID fan damper. So during soot blowing, main steam temperature has to be observed carefully.  If attemperator control valve fails to control the rise in main steam temperature in auto mode, then it has to be controlled taking it to manual mode.

c.      Whether Attemperation control valve is in manual mode or wrong value command input by the operator?

Normally it happens when there is a high fluctuation in main steam temperature. The attemperation control valve fails to control the temperature in Auto mode. So the concerned operator has to take the attemperation control valve to manual mode to control the temperature. But if he forgets to put this control valve in Auto mode after stabilization of main steam temperature, then it will remain in manual mode and during  more heat input from Kiln, the main steam temperature would rise. Also sometimes operator puts wrong value command for attemperation control valve opening from control station in manual mode, which would result in  increase in main steam temperature.

d.     Whether forget to open before and after isolation valves of attemperation Control valve?

This situation comes during cold start-up of Boiler, if the inspection and checking was not done properly by the operation personnel. During initial period, this thing cannot be noticed but at the time of main steam temperature rise by opening attemperation control valve flow of water cannot be established as before and after isolation valves are in close condition. So care has to be taken for proper inspection and checking before start-up.

5.     DECREASE IN MAIN STEAM TEMPERATURE

a.     Whether inlet flue gas temperature has dropped?

If flue gas inlet temperature drops due to problem in Kiln side then main steam temperature decreases. So if main steam temperature is in decreasing trend then first observe the flue gas inlet temperature to Boiler.

b.     Whether Load set point is given in Generator more than the Steam generation?

If Load set point in Generator is given more than the steam generation in Boiler then main steam pressure decreases and also the main steam temperature decreases

c.      Whether valve sheet of Attemperation control valve is eroded?

This situation comes during Low Load operation of Boiler. If heat input to Boiler is low, then steam generation reduces and also the power generation. At that time, feed water passes due to eroded valve sheet of attemperation control valve and decreases main steam temperature.

d.     Whether ID damper has become Zero due to PCC outlet temperature High?

When Post Combustion Chamber temperature increases more than 1050⁰C, opening of ID damper becomes Zero. At that time heat supply to Boiler from Kiln stops suddenly. So it results in rapid decrease in main steam temperature. If this situation arrives, then attemperation control valve has to be taken to Manual mode from Auto mode and decreasing main steam temperature has to be controlled.

6.        FURNACE DRAUGHT TOWARDS POSITIVE SIDE

a.     Whether tube failure has occurred in side furnace?

In furnace, the draught is maintained at negative side to carry out the hot flue gas, ash and other suspended particles from kiln to chimney through ID fan. If Boiler tube fails inside furnace then draught goes towards positive side. As steam density is higher than air density.  Also it adds an additional load on ID fan. So ID fan takes more current in this situation.       

b.  Whether draught transmitter is showing wrong value?

This can be known if other draught transmitters in flue gas path are showing right value and one of these is showing erratic value. This problem should be brought to the notice to shift in charge and instrumentation personnel.

7.     LONG RETRACTABLE SOOT BLOWER IS NOT AT ITS ORIGINAL POSITION

a.     Whether Long Retractable soot blower’s chain has broken during Soot Blowing operation?

If chain breaks at intermediate position of lancer tube during soot blowing by LRSB, then motor will be unable to retract it to the original position i.e. home position. Check the position of lancer tube, when soot blowing operation is in progress and chain has broken. In this situation, donot cut off steam flow through lancer tube. It is because it is situated in high heat zone i.e. at convection zone. As steam acts as a coolant, it will take the heat added to the lancer tube and will protect the lancer tube from over heating and bending. The lancer tube has to be drawn out manually. After ensuring that it has been drawn to its home position, steam through the lancer tube can be cut off and chain maintenance work can be carried out.

b.     Whether home position limit switch is malfunctioning?

This may happen after completion of soot blowing by Long Retractable Soot Blower. The limit switch at home position may not give home position feed back of the LRSB due to malfunction. If this case happens then immediately the position of the lancer tube has to be checked. Limit switch at home position has to be rectified by Instrumentation department.

c.      HAMMERING OF MAIN STEAM LINE DURING CHARGING.

Usually main steam line hammering occurs if the condensate present in that line is not properly drained out and pipe line is in cold condition. If huge amount of steam is allowed to pass through that pipe line then line hammering takes place which is very much harmful for the pipe line. So to avoid this case happening always open the drain of the pipe line. Observe the condensate is drained properly from that pipe line. After completion of condensate draining, warm-up the pipe line with very less quantity of steam. Gradually increase the pipe line temperature. After confirmation that the line is properly heated, more steam flow can be allowed.

Steam Turbine

Steam turbine is a mechanical device that extracts thermal energy from steam and converts it into mechanical work.  Interiors of a turbine consists of several sets of blades. Some set of blades are fixed at casing ( Fixed Blade) and some set of blades are fixed on the rotor ( Moving Blade) .

Fixed blades convert potential energy of the steam into kinetic energy and direct the flow to moving blades. Moving blades convert this kinetic energy in to force, caused by pressure drop and result in rotation of turbine shaft. Steam is allowed to enter into the turbine through control valve. This steam after passing through different stages of blades is allowed to exhaust. The exhaust steam is condensed in a condenser and condensate then reused in boiler.

1.     Impulse Turbine

2.     Reaction Turbine

1) IMPULSE TURBINE:

In Impulse turbine instead of set fixed blades a set of nozzles are fitted in the casing. Pressure drop of steam takes place in these nozzles and velocity of steam increases. This high velocity jet of steam contains significant amount of kinetic energy. This high velocity steam is passed through a set of moving blades, where pressure of the steam remains constant and velocity decreases.

2) REACTION TURBINE:

In reaction turbine fixed blades are fixed in the casing. Shape of these blades is such that the space between the blades has cross section same as shape of nozzle. Moving blades are fixed to the rotor. Fixed blades guide the steam to moving blades . Blade shape is so designed that steam glides over the blades. Steam while gliding over moving blades produces reaction on the blade. This reaction force produce the rotates the rotor.

1.     Casing

2.     Rotor

3.     Moving Blade

4.     Fixed Blade

5.     Steam Sealing System

6.     Bearing

Ø Joural Bearing

Ø Thrust Bearing

7.     Gland

8.     Exhaust Hood

9.     Emergency Stop Valve

10.  Governing Valve And Control Valve

11.    Barring Devices.

12.    Governing Systems

v CASING

Casing of turbine plays important role for the performance of a turbine. This is the outer shell of turbine. Fixed blades and nozzles are attached to this. Casing facilitates to accommodate moving parts and provides passage for steam. Normally it is formed by casting. As the temperature of steam for operating turbine is high so, normally Cr, Mo alloy steel casting is used for casing of a turbine. Metal to metal joint sealing is done to ensure no leakage of steam.

v ROTOR

Rotor is the moving part of a turbine which extracts work from steam. This is the heaviest part of the turbine. Normally total shaft is manufactured by forging. Rotor consist of shaft moving blade and inter stage sealing labyrinth. Thrust collar is provided to take care of axial thrust of rotor during various load conditions. Rotor of the turbine is allowed to expand uniformly. Rotor of the turbine should not be allowed to remain stand still when it is hot. Due to its self weight there is a chance of sagging or deformation. Rotor

v Moving Blades

Enthalpy of steam is converted into rotational energy as it passes through turbine blade sets. In each stage of the turbine there are moving and fixed blade. As in each step pressure of steam decreases, its volume increases. The blade has to handle more volume of steam. Blade has to withstand high pressure and temperature of  steam. Good tensile and fatigue strength is required. Good vibration damping property, low ductility, resistance to corrosion and erosion is essential. Blade can be divided into three portions.

1.     Tip

2.     Profile

3.     Root

v Fixed Blades

Fixed blades facilitate expansion of steam and guide it to flow over subsequent moving blade row. Partition between pressure stages in a turbine casing are called diaphragms. It holds vane shaped nozzles or fixed it

MAIN COMPONENTS OF STEAM TURBINE

1.     JOURNAL BEARING

Journal bearing is a cylinder, which surrounds the shaft and is filled with some form of fluid lubricant. It consists of a split outer shell of hard metal and soft metal at the inner cylindrical part. In this bearing a shaft or journal rotates inside the bearing over a layer of lubricating oil, separating the shaft and bearing through a fluid film by dynamic principle. Inner surface of this bearing is coated with a soft metal called as white metal or Babbitt. This is a tin or lead based alloy.

2.     THRUST BEARING

Journal bearings are used to take radial load of the shaft. But it can’t take axial load. Shaft is permitted to float to both axial direction. But the axial float is restricted to certain limit. Excessive axial shift may damage rotating and fixed parts. For this thrust bearing is provided.

EMERGENCY STOP VALVE

Ø This valve is normally hydraulically operated. The valve opens hydraulically against a spring force. To close the valve hydraulically

Ø Fluid is drained and valve closes immediately due to force of spring. This valve is normally fully open and fully close type.

Auxiliary System Of Steam Turbine

1.     OIL SYSTEM

Ø Oil tank

Ø Oil Pump

Ø Oil Cooler

Ø Oil Filter

Ø Oil Centrifuge

Ø Oil Over Head Tank

Ø Accumulator

2.     CONDENSATE SYSTEM

3.     GLAND SEALING SYSTEM

4.     STEAM EJECTOR AND VACCUM SYSTEM

5.     CONDENSER

6.     COOLING WATER SYSTEM

Turbine Cold Startup Sequence Method

Operation of steam turbine is a complex process. Before starting the rolling of a turbine, auxiliary systems are to be properly put in service. Normally for start up of a turbine some operations are followed in sequence.

v Charging of Steam Pipe Line

From  Boiler, steam is carried to turbine main steam pipe line. In cold condition, special care is to be taken to heat up the steam line and allow gradual thermal expansion, before giving full load on the turbine.

Drain points are provided at the steam line to drain out condensate present in steam pipe line, that is formed due to condensation of steam. First of all, these drains are opened before charging steam on the pipe line. After condensate is drained out boiler main steam stop by pass valve is opened slowly .

Some steam is allowed to flow through the pipe line and it starts gaining heat from the steam and steam is condensed. At the beginning, condensate along with some steam is allowed to come out through the drain. These drains are throttled slowly and closed when no more condensate but only dry steam comes out from the drain.

Steam traps provided in the pipe line are kept in line once drains are closed. Then Main Steam Stop Valve of the boiler is opened slowly so that the line temperature is increased gradually. Ensure extraction is not restricted anywhere. Watch the temperature of bypass reaching the normal level after which stop valve of boiler can be opened fully.

To circulate cooling water in the Condenser, cooling water pumps are to be started.

Before starting pump

1.     Ensure Sump level of the cooling tower basin is normal (>80%)

2.     Keep suction valve of the pump in open condition & discharge in closed condition.

3.     Ensure inlet & outlet cooling water valves of Condenser distributer valves of cooling tower are in open condition .

4.     Ensure vents provided at Condenser water box are in open condition to remove trapped air.

5.     Start the pump & open the discharge valve .

6.     Observe whether cooling water is falling on the cooling tower or not.

7.     Ensure that distribution of cooling water in all chambers is equal, otherwise adjust the valves provided at the distribution header .

8.     Observe whether all the cooling water pumps are sharing load or not.

9.     Once Turbine is started and loaded, cooling tower fans can be started one by one as per requirement.

Starting Of M.O.P ( Main Oil Pump )

1.     Before starting of M.O.P check the healthy condition of  Main Oil Tank ( M.O.T ) low level switch from H.M.I .

2.   Before starting M.O.P,  check oil level in M.O.P oil cup as well as oil level in A.O.P  &  E.O.P oil cups.

3.     Ensure again suction & discharge valves of M.O.P, A.O.P & E.O.P are in open condition .

4.     Start M.O.P .

5.     Open J.O.P suction line coming from M.O.P  & A.O.P discharge header , then open its discharge valve .

6.     Put A.O.P, J.O.P & E.O.P in auto selection mode.

Taking Oil Cooler into Line

1.     When M.O.P starts, oil circulates to the circuit through oil cooler

2.   To ensure oil is passing through the oil cooler or not, see through the view glass after opening the air vent of oil cooler

3.    After confirming oil is passing through the vent valve to M.O.T, close the vent valve

4.     Open the oil equalizing line of standby oil cooler and wait for some time to fill it with oil, then close the equalizing valve

5.   Maintain lub oil temperature in between 42⁰C - 45⁰C by adjusting the outlet cooling water valve  of online cooler

Taking Oil Cooler into Line

1.     When M.O.P starts, oil circulates to the circuit through oil cooler

2.   To ensure oil is passing through the oil cooler or not, see through the view glass after opening the air vent of oil cooler

3.     After confirming oil is passing through the vent valve to M.O.T, close the vent valve

4.     Open the oil equalizing line of standby oil cooler and wait for some time to fill it with oil, then close the equalizing valve

5.   Maintain lub oil temperature in between 42⁰C - 45⁰C by adjusting the outlet cooling water valve  of online cooler

Checking Of Lub Oil Header Pressure and Individual Bearing Pressure

1.   Check the lub. oil header pressure from field and H.M.I . It must be more than 3Kg/cm^2.

2.     Check the individual bearing oil pressure

                                                             i.      TG Front Journal Bearing – 1.2 Kg/cm²

                                                           ii.      TG Thrust Bearing – 1.2 Kg/cm² 

                                                        iii.      TG Rear Journal Bearing – 1.2 Kg/cm²   

                                                        iv.      Gear Box – 2 Kg/cm²           

                                                           v.      Alternator Front Journal Bearing – 1 Kg/cm²       

                                                        vi.      Alternator Rear Journal Bearing – 1 Kg/cm²

3.  Check individual  bearing's  return oil line view glass whether oil is passing through it or not.

4.   Check overhead tank oil return line view glass , ensure oil flow through return oil line then close quick filling valve of overhead tank .

5.     Check healthiness of overhead tank oil level indicator .

Once the above systems  are in service, gland steam can be charged at gland. Care is to be taken while charging gland steam in a cold Turbine. As the gland area of Turbine is at normal temperature during cold condition, hot gland steam may produce thermal shock at that area. To avoid this, steam is to be charged slowly and condensate produced is to be drained through gland steam drain.

 Following steps are to be followed for gland steam charging :

1.     Charging of auxiliary PRDS (Pressure Reducing & De Superheating)

2.     Charging  of  Gland Header

3.     Charging Of Aux PRDS (Pressure Reducing And De-Superheating)

4.     Open all drain valves

5.     Open main manual isolation valve before & after PCV  (Pressure Control Valve)

6.     Open PCV by 5% from operation station

7.     Open PCV by 10% as soon as condensate comes out from line

8.     Close all drain valves

9.     Put the PCV in Auto mode with desired pressure set point

10.   Open manual isolation valve of  TCV ( Temperature Control Valve)

11.   Observe the temperature and then put TCV in auto mode with desired temperature set point

Charging of Gland Header

1.     Open all drain valves of gland steam header

2.     Open gland steam header manual isolation valve

3.     Open gland steam header PCV by 5% for line heating.

4.     Open gland steam header PCV by 10% to increase gland steam header pressure

5.     Close all drain valve in gland steam header

6.     Put  gland steam header PCV  in auto mode with desired pressure set point.

Exhaust steam of turbine is condensed at condenser with the help of cooling water. The condensate produced is evacuated from the condenser by the help of Condensate Extraction Pump (CEP). This condensate passes through gland seal condenser and ejector condenser to gain heat of the gland steam and ejector steam respectively. So the temperature of condensate increases there before feeding to deaerator for further use at boiler.

This condensate is further heated at L.P. Heater  (if provided) by using LP Steam extraction of turbine.

To put the condensate system in operation, following steps are required to be followed:

1.  Ensure condenser hot well level is adequate, otherwise fill the hot well with make up DM Water

2. Open Suction and discharge valves of the pump. Ensure differential pressure of the strainer is normal

3.     Open condensate inlet and outlet valves of gland seal condenser, ejector condenser and LP Heater

4.     Put the re-circulation control valve in auto mode

5.     Open pump gland cooling valve and start the pump

The condensate will pass through gland seal condenser & ejector condenser.  It should be re circulated to condenser again through  recirculation control valve. Once steam starts entering into turbine, discharge control valve can be put in auto mode to maintain level of the hot well.

If the condensate extraction pump is to be started and if there is vacuum inside the condenser, then vacuum balance line valve is to be opened to avoid any air trapped inside the pump.

Before Main steam enters into the turbine, there should be vacuum in the condenser. First of all, starting ejector is used to evacuate air from condenser. This is a single stage non-condensing type ejector.

      Take the following steps to build up vacuum  by starting ejector:

1.     Ensure availability of auxiliary steam at desired pressure & temperature

2.     Ensure the vacuum breaker valve of the condenser is closed.

3.   Ensure cooling water is circulating in the condenser and turbine gland is charged fully

4.     Open steam valve of the starting ejector

5.     Observe steam is vented to atmosphere

6.     Open ejector air valve

7.     Observe vacuum inside condenser is increasing slowly. 

8.   Main ejector is to be taken into line once turbine is loaded and starting ejector is to be  stopped then.

To put main ejector into line, following steps to be followed :

Main ejector is to be taken into line once turbine is loaded.  Starting ejector is to be stopped then. To put main ejector in line, following steps to be followed.

1.     Ensure Condensate Extraction Pump (CEP) is running .

2.     Ensure cooling water inlet and outlet valves of the ejector condenser are opened.

3. Vent out air from water box of  the ejector condenser by opening rotametre valve.

4.     Open ejector condensate trap before and after isolation valve

5.     Fill up the “U” tube by water locally

6.     Open flash box stand pipe isolation valve

7.     Close all drain valves of ejector

8.     Open the main isolation valve of the ejector steam line

9.  Slowly open the air line valve of the ejector and observe vacuum is increasing. 

When vacuum is stable, then the slowly ejector can be stopped by closing air valve  first  then the steam valve of ejector.

Once Auxiliary systems are in operation and full vacuum is obtained inside, condenser turbine can be started. Turbine is required to be started in two different conditions.

1.     Cold Start-Up

2.     Hot Start-Up

In cold startup turbine is started from cold condition. In this case, special care is taken for proper heating of casing and rotor for proper thermal expansion. As both rotor and casing are in cold condition it requires time for heat up. But in case of hot start up both casing and rotor are in hot condition. So it can be started within a short period.

Startup Curve

To allow proper thermal explanation of casing and rotor, the turbine manufacturer’s advise is to be followed for start up procedure.

Ø steam should not enter immediately to turbine as it may damage the turbine due to uneven expansion.

Ø Manufacturers suggest soaking time for low idle speed and high idle speed for proper thermal expansion between rotor  and casing means to hold the turbine at the particular speed for a particular time, then allow the turbine speed to higher range.

Soaking time is different for cold startup and hot startup.  Manufacturer’s advice should always be followed strictly for soaking and start up curve in cold startup and hot start up conditions.

Turbine Rolling Preparation..contd

To start rolling of turbine, some steps are followed depending upon mode of starting (Auto or Manual) and types of governing system  (Hydraulic or Electro Hydraulic)

Before rolling of turbine check, ensure the following points :

1.     Lube oil level and control oil pressure are normal

2.     Lube oil temperature is between 42 to 45⁰C

3.     Ensure gland sealing system is in operation and gland sealing pressure is normal

4.     Ensure starting ejector is in the line and condenser pressure is -0.9 kg/cm²

5.     Ensure cooling water is circulating in condenser and auxiliary cooling  water in lub. oil cooler

6.     Ensure the casing drain, TG inlet steam line drain, TG warm

7.     up vent and drain are in open condition

8.     Ensure Accumulator is in line

9.     Ensure over head oil tank is full and return oil flow is visible in the viewing glass

10.  Ensure Condensate Extraction pump (CEP) is in operation

11.   Ensure Exhaust  hood spray solenoid valve is in operating condition.

12.    Open the bypass of Turbine Steam stop valve (TSSV)

13.    Ensure complete removal of condensate from TG inlet line and ensure the temperature of TG inlet steam is rising after throttling drain valves.  Open Turbine Steam Stop Valve   (TSSV)

14.  Throttle the warm up vent as per requirement and observe steam temperature is rising. Once steam temperature reaches at desired temperature, then prepare for TG rolling.]

TG Rolling

1.     Reset the governor from wood yard SOS

2.     Reset from HMI

3.     Engage trip lever and ensure build up of trip oil pressure at governing console

4.     Open E.S.V. (Emergency Stop Valve) from H.M.I.

5.     Check physically the opening of ESV (Emergency Stop Valve)

6.     Give  run command from HMI

7.   Observe the rise in rpm gradually.  RPM goes up and after reaching 1000 rpm (Low Idle speed) automatically, it will hold for 15 minutes in hot start up and 30 minutes in cold startup (in case of auto rolling).  Otherwise hold the speed as advised by the manufacturer.

8.     Ensure oil pressure is normal. Check vibration and any abnormal sound

9.     First stop barring  gear then stop jack oil pump (J.O.P)

10.  Get the relay reset before 2000 rpm

11.  After completion of the hold time at 1000 rpm, R.P.M. goes from low idle speed to high idle speed 2500 rpm, if it is in auto mode, otherwise increase the speed manually

12.  After reaching 2500 rpm, it holds for 15 minutes in case of hot startup and 30 minutes in case of cold startup automatically. If it is not auto rolling, hold the speed as per advice of manufacturer.

13. Close the TG casing drain, inlet steam line drain, warm up vent, warm up drain

14.Check the lube oil pressure at different bearings and check bearing temperature and vibration and record it.

15. After completion of high idle speed (2500 rpm) soaking time. R.P.M. will rise up to rated speed 7500 rpm

16.Maintain lube oil pressure and temperature at different bearings as per the manufacturer’s advice

17.  Maintain TG inlet pressure and temperature as per design

18.   Give clearance to synchronize to generate power.

Turbine Protection Logic

NO	DESCRIPTION	SET POINT	TIME DILAY (sec)
1	Lube Oil header press lo lo	1.2 Kg/cm2	2
2	Lube Oil Press after filter lo lo	4 Kg/cm2	2
3	Oil Press lo - Trip device outlet	4 Kg/cm2	2
4	Turbine exhaust steam press hi hi	-0.6 Kg/cm2	2
5	Cond. Level hi hi	95%	2
6	Cond. Level lo lo	15%	2
7	Turbine overspeed	8220	2
8	Trip command from governor	-	2
9	Rotor axial displacement hi hi	+/- 0.45 mm	2
10	Turbine front brg vib hi hi	200 micron	2
11	Turbine rear brg vib hi hi	200 micron	2
12	GB pinion DE brg vib hi hi	200 micron	2
13	GB pinion NDE brg vib hi hi	200 micron	2
14	GB G shaft NDE vib hi hi	160 micron	2
15	GB G shaft DE vib hi hi	180 micron	2
16	Alternator front brg vib hi hi	160 micron	2
17	Alternator Rear brg vib hi hi	160 micron	2
18	86T relay operated		2
19	Inlet steam temp lo lo	400/440 deg C	6
20	Inlet steam temp hi hi	525 deg C	6
21	Turbine exhaust steam press hi hi	-0.6 Kg/cm2	0
22	Turbine NWS thrust brg temp hi hi	115 deg C	8
23	Turbine WS thrust brg temp hi hi	115 deg C	8
24	Turbine front brg temp hi hi	115 deg C	8
25	Turbine rear brg temp hi hi	115 deg C	4
26	MOP side thrust pad temp hi hi	115 deg C	4
27	HS Working Side temp hi hi	115 deg C	4
28	HS Non Working Side temp hi hi	115 deg C	4
29	Alt. side brg temp hi hi	115 deg C	4
30	MOP side brg temp hi hi	115 deg C	4
31	HS NWS brg temp hi hi	115 deg C	4
32	Drive end brg temp hi hi	80 deg C	4
33	NDE brg temp hi hi	80 deg C	0
34	Inlet steam press lo lo	38/39 Kg/cm2	3
35	Inlet steam press hi hi	48/50 Kg/cm2	8

NO	DESCRIPTION	SET POINT	TIME DILAY (sec)
1	Lube Oil Press lo	1 Kg/cm2
2	Control oil press lo	4.5 Kg/cm2	3
3	Trip Oil Press lo      (PSLL-305A)	2 Kg/cm2	10
4	Exhaust steam press hi	-0.4 Kg/cm2
5	Hotwell level hi hi
6	Turb speed hi hi - WWG	7865
7	Trip command from governor
8	Turb Axial shaft displacement hi hi	+/- 0.7 mm
9	Turb shaft front vib hi hi	156 microns
10	Turb shaft rear vib hi hi	156 microns
11	Turb shaft front vib hi hi	156 microns
12	GB LS shaft rear vib hi hi	340 microns
13	GB HS shaft front vib hi hi	340 microns
14	GB LS shaft rear vib hi hi	340 microns
15	Gen L/O relay 86B operated
16	Inlet steam temp lo	400 deg C	2
17	Inlet steam temp hi	520 deg C	2
18	Bearing temp hi	115 deg C
19	Thrust Brg. Temp 1	115 deg C
20	Thrust Brg. Temp 2	115 deg C
21	Thrust Brg. Temp 3	115 deg C
22	Thrust Brg. Temp 4	115 deg C
23	Tur. Front brg	115 deg C
24	Tur rear brg	115 deg C
25	GB front (HSS) brg Temp 1	115 deg C
26	GB front (HSS) brg Temp 2	115 deg C
27	GB thrust brg temp 1	115 deg C
28	GB thrust brg temp 2	115 deg C
29	GB rear (LSS) brg temp 1	115 deg C
30	GB rear (LSS) brg temp 2	115 deg C
31	Gen front brg	115 deg C
32	Gen rear brg	115 deg C
33	Inlet steam press lo	40 Kg/cm2	2
34	Inlet steam press hi	50 Kg/cm2	6
35	Exhaust temp hi	120 deg C
36	Turb speed hi hi - TSI	7860
37	Emergency trip PB1
38	Emergency trip PB2
39	Turbine trip from HMI

Turbine Auxiliary System

In Power Plant other than turbine, there are other associated systems. The systems are required for running of a turbine. Most of the  important components and systems for auxiliary systems are :

1.     Oil System

2.     Condensate System

3.     Gland sealing System

4.     Ejector and Vacuum System

5.     Cooling water System

6.     Condenser

Oil  System

Lubricating oil is supplied to the bearings and used for governing of turbine. Main function of lubricating oil is to :

1.     Lubricate the bearings.

2.     Cooling of bearings.

3.     Flush out metallic  debris.

4.     Control speed of the turbine. \

Principles of Lubrication

To maintain a film of lubricant between the surfaces in running condition any one of the following principle of lubrication  prevails.

1.     Hydro dynamic lubrication

2.     Hydrostatic lubrication

3.     Elasto-hydrodynamic lubrication

If none of the above conditions exists the condition will be of :-

Boundary lubrication

Hydrodynamic Lubrication

Also called Full Flood Lubrication/Wedge film lubrication

Wedge film formation due to geometry & speed.

a.     In hydrodynamic principle fluid viscosity is not sufficient to maintain a film between the moving surfaces & higher pressure required to support the load until the fluid film is established, the required pressure generated internally by dynamic action.

b.     The wedge film lifts the journal and allows complete separation

c.      The formation  of a thick fluid film that will separate two surfaces and support a load as the two surfaces move with respect to each other.

By feeding oil from an external source under heavy pressure into the pocket machined into the bottom of the bearings, the journal can be lifted and floated on fluid films.

When the journal reaches a speed sufficient to create hydrodynamic films the external pressure can be turned off and the bearing will continue to operate in hydrodynamic manner.                                   

Components of Lubricating Oil System

Main components of lubricating oil system are :

1.      Oil tank

2.      Oil pumps

3.      Oil filter

4.      Oil centrifuge

5.      Oil overhead tank

6.      Accumulators

Oil tank

Total oil for the system is stored in the this tank. The tank has adequate capacity to hold sufficient oil during running & stop condition. The tank base is made sloped to one side, so that the sediment in oil can be collected in the lower  area and can be drained out by opening drain valve. The tank has level measurement facility to give alarm for low oil level. Also a level glass is provided to find out tank level at any instant. Suitable tapings are provided to facilitate oil suction for oil pumps, draining of return oil from bearings and governing system, connection for oil centrifuge, fill up of fresh oil etc.

One oil mist fan is provided on the tank to vent out any oil vapor and keep the tank slightly below atmospheric pressure.  

Oil Pump

To pump oil from the oil tank to various lubrication points and controlling purpose,  oil pumps are provided. Normally three pumps are provided. These pumps are :

1.      Main oil pump ( M.O.P )

2.      Auxiliary oil pump ( A.O.P )

3.      Emergency oil pump ( M.O.P )

Oil Coolers

Normally two oil coolers of 100% capacity are provided to cool down entire oil supplied to turbine bearings,gearbox,and generator bearings for lubrication. Governing oil is not cooled at oil cooler. This oil taken out before oil cooler. One cooler is put on line and another one is kept as standby. Online changeover facility is provided to take the standby cooler in to service, without interruption of oil supply, while turbine is running.

Before changeover, it is to be ensured that the standby cooler is filled with oil and air is vented out properly. Otherwise there will be air lock and oil supply to bearings may interrupt.

Oil cooler is a shell and tube type heat exchanger. Cooling water flows inside the tube bundle and oil flows at the shell side. Cooling water for oil cooler is obtained from main cooling water system of power plant. Regulating valves are provided at the inlet and outlet of the cooling water supply line.         

To increase and decrease oil temperature, cooling water flow is decreased and increased respectively through these regulating valves. Always the cooling water outlet valve is regulated to vary flow of cooling water. At any case cooling water inlet valve is not to be throttled as sufficient cooling water will not available inside tub and tube may damage.

Drain point is provided at the cooler to drain out settled sediment at bottom of the cooler.

Oil Filters

Oil coming out from cooler is passed through oil filter to remove any contaminated particle or debris. Filter is normally basket type with removable filter cartridge. Like cooler there are two filters of 100% capacity each with suitable online changeover arrangement. The oil is filtered up to 20-25 micron level on these filters before circulating in bearings.

Differential pressure across the filter is measured which indicates the choking condition of filter cartridge. If differential pressure is high it indicates, filter is choked and needs cleaning.

Before changeover of oil filter when turbine is in operation, it is to be ensured that standby filter is completely filled and no air is trapped inside. Filter cartridge of standby filter is always to be kept clean, so that at any moment this can be taken in to line, if required.

Oil Centrifuge..contd.

Centrifuge is a machine which separates water and solid particles from oil. This is achieved by centrifugal force of a high speed rotating bowl inside the separator. Due to centrifugal force, heavier particles are displaced towards the outer periphery of the bowl and the lighter oil is displaced towards center of the bowl, where it is collected and sent back to main oil tank.

Steam Ejector And Vacuum System

Vacuum is maintained by continuously evacuating non condensing gases from the condenser with the help of  steam ejector. Pressure of non condensing gases decrease condenser efficiency. For removing non condensing gas to create vacuum in the condenser normally  steam ejector is used. This is like a pump in which venturi effect of a converging and diverging  nozzle is used to convert pressure energy of steam to velocity energy to create suction effect.

WORKING PRINCIPLE OF EJECTOR

High pressure motive steam enters to ejector chest through nozzle and then expanded. Pressure energy of steam is converted into velocity. Increased velocity causes reduced pressure  which  socks vapour.Diffuser section then compress the steam vapour mixture then exhausted to condenser.

Operating Procedure Of Ejector System

1.     Circulate condensate through ejector condenser.

2.   Open steam of ejector. So it will create vacuum in inter ejector condenser.

3.     Open steam of ejector.

4.     Open air valve of condenser.

Condenser

Condenser is an important Auxiliary equipment of any steam turbine. Exhaust steam of turbine is exhausted in to condenser, where it is condensed in vacuum. By maintaining vacuum in condenser, maximum energy can be extracted from steam and turbine efficiency increases. Condensate obtained is utilized again at boiler for steam formation.

There are different types of condenser. Some of the important types of  condensers are listed below.

1.      Jet type condenser

2.      Air condenser

3.      Surface condenser

Surface Condenser

This type of condenser is widely used at power plants. Cooling water is not mixed with condensate in this case. Condensate obtained is pure and can be used in boiler. This is a shell type and tube type heat exchanger. Shell of the condenser is closed. Tubes are arranged inside the shell in which cooling water flows. Condenser neck is connected to the exhaust hood of turbine. An expansion joint is provided in-between to facilitate thermal expansion.

Steam from turbine flows at the shell side of condenser and cooling water flows inside the tube. Main components of a surface condenser are :

- Shell                            - Hot well

- Air outlet                    - Tube

- Rapture disk                - Water box  

Overhead Tank

Oil accumulator is provided on the governing or control oil line of the turbine. This accumulator maintains oil pressure in the line during momentary fluctuation of oil pressure during oil pump change over or sudden operation of servomotor of governing valve.

In the accumulator an inert gas filled bladder is provided. Gas pressure inside the bladder is maintained slightly below the normal oil pressure.

During normal operation, oil pressure of the line compress the bladder and oil is occupied in the oil space of the accumulator. When, pressure at the line drops, the bladder is expanded, due to the inside gas pressure. So it pushes out oil of space to the line and takes care momentary oil pressure fluctuation.

Oil Accumulator

Oil accumulator is provided on the governing or control oil line of the turbine. This accumulator maintains oil pressure in the line during momentary fluctuation of oil pressure during oil pump change over or sudden operation of servomotor of governing valve.

In the accumulator an inert gas filled bladder is provided. Gas pressure inside the bladder is maintained slightly below the normal oil pressure.

During normal operation, oil pressure of the line compress the bladder and oil is occupied in the oil space of the accumulator. When, pressure at the line drops, the bladder is expanded, due to the inside gas pressure. So it pushes out oil of space to the line and takes care momentary oil pressure fluctuation.

Emergency Situation In  Steam  Turbine

Steam Turbine is a critical rotating equipment. High temperature and pressure steam is used to rotate the turbine at high speed. Mass of the rotating  part is high. There is always chance of severe misshapen leading to fatal accident and damage of high cost equipment. Incase of any system goes wrong generation of power may be interrupted for a longer period leading to heavy loss to the plant. So the power plant engineer should be trained enough to face any emergency situation, at any time and properly handled emergency situations.

1) Overspeed

Due to failure of governing system the turbine speed may become dangerously high. Rotor can rotate momentarily without damage up to 110% of rated speed. At higher speed rotor stress increases. Due to high centrifugal forces the blades which are fixed to the rotor may come out. Failure of blade root can cause severe accident and damage to turbine.   To avoid dangerous over speed turbine is provided with mechanical and electrical over speed trip arrangements. Tripping limits are set in such a way that turbine speed does not exceed 110% of rated speed. These overspeed tripping limits are to be checked regularly. Mechanical overspeed device is to be set within set limit and checked at suitable intervals. At any circumstance overspeed tripping limit is not to be bypassed. If overspeed tripping does not work, immediately stop the turbine by applying emergency trip push button. For the 18.5 MW turbine  at Tata Sponge, overspeed tripping limit is 7865 rpm.

2 ) Failure Of Lubrication Oil System :

Lubrication Oil is used to lubricate and cool down bearing metal. Sometimes the lubrication oil supply may be interrupted due to failure of pumps, leakage in oil line or choking of oil filter. This condition may damage bearings and gear box. If such an incident happens for any reason, the turbine is required to be stopped as soon as possible.  Low lube oil header pressure tripping is incorporated with turbine to trip the turbine immediately.  If lube oil header pressure becomes 1kg/cm², oil supply is to be restored as early as possible. After resuming oil supply, if possible, turbine is to be rotated manually to find out any damage (inspect bearings).

3. High Vibration

Rotor of the turbine rotates at high speed. Any deformation or unbalance of the rotor produces high vibration. Sometimes deposits on blades and damage of any rotating part may create heavy vibration. Damage of journal bearing may also produce vibration. The moving and rotating parts of the turbine are closed spaced. Due to disturbance in rotor shaft or differential expansion, there is chance of rubbing. Rubbing creates high vibration and abnormal sound, so at any case high vibration of turbine is not be overlooked. Incase of high vibration the turbine should be stopped immediately and turbine internals to be inspected to avoid further damage. High vibration protection in logic is incorporated with turbine to trip the turbine when turbine front and rear journal bearing vibration goes to 156 Micron and gear box front and rear journal bearing goes to 340 microns.

4) High Bearing Temperature   

High bearing temperature occurs due to inadequate oil flow in the bearing or metal to metal contact in between bearing and rotor. High temperature damages Babbitt material of the bearing. In case of high temperature of the bearing, a turbine is required to be stopped. Oil supply to bearing is to be checked and if required bearing is to be opened for inspection. High bearing temperature protection logic is provided to turbine. For different bearing 115⁰C is a tripping limit.

5) Failure Of Barring Device

When  turbine is stopped in hot condition, it is to be put on barring. In some situation just after stopping turbine  barring gear may be found not working. It is not recommended to keep the rotor in standstill condition. By any means rotor is to be rotated normally by hand barring arrangements provided to change the rotor position by 180◦C continuously.

6) High Condenser Hot Well Level

Due to problem in condensate extraction pumps, sometimes the condensate cannot be evacuated from hot well. So hot well level becomes high. In this situation there is possibility that water level in condenser increases and enters into turbine through exhaust hood. Condenser vacuum reduces drastically in this condition. If at any case water enters into a running turbine it creates a serious situation and damages the turbine. Load is to be reduced on turbine in this situation. If situation is not controllable, turbine is to be stopped.

9)  High Steam Parameter  

Like low steam temperature and pressure, high steam temperature and pressure is not desirable for turbine operation. High steam temperature may damage turbine as the metrology of the turbine is designed for a particular temperature.

10) Low Condenser Vacuum

Due to vacuum in condenser the steam from turbine is easily exhausted into condenser. If vacuum inside the condenser drops, it restricts exhaust of steam of turbine. This creates back pressure inside turbine. Vacuum may drop due to failure in cooling water system, failure of ejectors, or leaking condenser air line. Standby ejector or starting ejector is to be immediately taken into line. Leaking air line is to be arrested promptly or cooling water supply to be increased. If vacuum is not improved, the turbine is to be stopped immediately. Low vacuum protection logic is provided to trip the turbine when condenser vacuum drops to -0.4 kg/cm².

11) Failure Of Cooling Water Systems

Due to failure of cooling water pumps or choking in cooling water circuit, cooling water supply may be reduced or interrupted. In this case turbine exhaust steam cannot be condensed. This will increase the pressure of the condenser and drop the vacuum. Rapture disks of the condenser may rapture, heavy back pressure will be created in turbine. In this case load is to be reduced first and care is to be taken to normalize cooling water supply. If situation does not improve then turbine is to stopped.

Black Out maneuver Method for  WHRB Power Plant

Both the TG fails and Grid not available : (BLACK OUT CONDITION)

1.    In the above cases ( Total blackout condition ) ensure availability of  DG emergency power to all the emergency drives of both the CPP within 10 seconds (i.e. Boiler main steam stop valve, Auxiliary oil  pump, Barring gear, Emergency oil pump, Boiler feed pump discharge valve, CPP area lighting   &  Jack oil pump & TG steam stop valve )

2.  Ensure from field pressure gauge  that lubrication continues in both the TG by gravity method (oil flows from over head tank to all the TG bearings and returns to main oil tank by drain header )

3.  Ensure from HMI & field that Emergency oil pump is running  through DC power & oil supply continues to all the bearings.

4.    Start the Jack oil pump of TG.

5. If emergency power is not available  within 10 seconds, then immediately contact the Electrical Shift In Charge about the matter and try to resume emergency power as quickly as possible, with the help of Shift In Charge CPP & Shift In Charge Electrical.

6.    After resuming  of emergency power, close main steam stop valve of all the three Boilers  and maintain the drum pressure through start-up vent.

7.  In blackout condition, ensure that Kiln stack cap will remain 100% open till the availability of boiler  feed pump. If stack cap is closed or partially closed, then contact Kiln control rooms to open the same through Shift In Charge CPP.

8. In blackout condition, all the boilers will be in hot box-up condition.

9.    Ensure emergency stop valve of TG is in closed condition

10.  Close the TG inlet motorised valve .

11.  Close all the boilers feed pump discharge motorised valves.

12. After resuming  of emergency power, auxiliary oil pump will start in auto  mode.  Ensure the same from field & HMI, then stop the emergency oil pump from panel and put it in auto mode.

13. After resuming of 1000kva DG, power start one feed pump of CPP-1 and supply water to all three boilers and maintain the drum level upto 40% .