STEAM TURBINE & ITS OPERATION:
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/cm2.
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 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 up to 40% .
No comments:
Post a Comment