44
STEAM POWER PLANT TURBINE
A steam power plant continuously converts the energy stored in fossil fuels ( Coal, Oil or natural gas ) or fissile fuel ( uranium, thorium ) into shaft work and ultimately into electricity. The working fluid is water, which sometimes in the form of liquid and sometimes in the state of vapour phase during its cycle of operation. Energy released by burning of fuel is transferred to water to generate steam at high pressure & temperature, and then it expands in turbine to a low pressure to produce shaft work. The steam leaving the turbine is condensed in a condenser. Thus it follows the cycle - LIMITATIONS OF HIGHER EFFICIENCY:
To achieve higher efficiency turbine inlet steam temperature should as high as possible and exhaust steam temperature of LP turbine should be as low as possible. Considering the metallurgical limitations turbine inlet steam pressure & temperature should be around 130-150 kg/cm2 & 540 0C. LP turbine exhaust pressure & temperature depends upon quality of cooling water available at site. However maximum wetness of LP turbine exhaust steam should be restricted to 12%.
In India capacity of Thermal Power Plants are ranging from 50 MW to 500 MW. Most of the Turbines are either Russian design LMW sets ( Leningrad Machine Works) or German design KWU sets ( Kraft Werk Union).
210 MW LMW TURBINE :
210 MW KWU TURBINE :
Steam Turbines are of two types : 1. Impulse Turbine
2. Reaction Turbine.
1. Impulse Turbine :
In impulse turbine fixed nozzles are used for expanding steam to convert steam pressure into kinetic energy of steam. The high velocity steam impulse acts on the moving blades to rotate the turbine shaft.
Steam after gliding over the moving blades changes its direction of flow. This change of direction of flow causes change of momentum, which create rotational motion of turbine shaft.
Steam pressure is dropped across the fixed blades to develop high velocity of steam. No pressure is dropped while gliding over the rotating blades
2. Reaction Turbine :
When steam expands over the fixed blades, heat energy is converted into kinetic energy. This high velocity steam acts on moving blades. This results in change in momentum and reaction effects.
Some heat energy of steam is converted into kinetic energy when it moves over the moving blades due to its typical shape. It is a continuous process in reaction turbines to convert pressure / heat energy into kinetic energy.
The turbines may also be classified as: 1. Axial Flow Turbine.
2. Radial Flow Turbine. 1. Axial Flow Turbine.
Steam flows along the axis of the shaft while passing through the turbine. Most of the turbines are axial flow type. Steam inlet & outlet pressure drop develops axial thrust on the turbine shaft along the direction of steam flow.
2 Radial Flow turbine :
Steam flows along the radius of the blade in Radial Flow Turbines. It is also called
L’lungstorm turbine. Normally it rotates two separate shafts and two generators are driven. It can be started very quickly and its capacity.
Its maximum limited to 50 MW.
Turbines may also be classified based on flow paths: 1. Single Flow Turbine.
2. Double Flow Turbine. 3. Reverse Flow Turbine.
1. Single Flow Turbine:
In this system steam expands in one direction and exhausted. 210 MW LMW HPT & IPT and 210 MW KWU turbines are single flow turbines.
In single flow turbine the difference of steam inlet & outlet pressure develops a huge amount of axial thrust on the shaft. To reduce this axial thrust a dummy piston is placed at the inlet to produce opposite force. Moreover a thrust bearing is provided in between HPT & IPT.
2. Double Flow Turbine :
In this system steam inlet is through middle portion of the turbine and after that steam expands in both direction. After work done on turbine shaft it is exhausted.
Normally specific volumes of steam at different points are:
Sl.No. Location of Steam Specific Volume ( m3 / kg)
1. H.P. Turbine inlet 0.0245
2. I.P. Turbine inlet 0.135
3. I.P. Turbine exhaust 0.385
4. L.P. Turbine exhaust 22.5
From above it is seen that steam expansion in L.P. Turbine is 58 times. 210 MW LPT and 210 MW KWU IPT & LPT are of double flow turbines.
In some big size turbine steam expand in first few stages in reverse direction to reduce axial thrust on the turbine shaft.
Main components of steam turbines :
1. Turbine shaft. 2. Turbine casing.
3. Moving blades fixed on rotor. 4. Fixed blades fixed on casing. 5. Turbine bearing to support rotor. 6. Dummy Piston in HP Turbine. 7. Gland sealing.
8. Rupture Diaphragm in LP turbine.
Turbine Shaft & Moving Blades:
The rotor shaft transmits the torque generated by the change of momentum of steam. Turbine shaft or rotor holds turbine blades. The moving blades are used to convert kinetic energy of steam into driving force of the shaft. Shroudings are provided at the last stage of turbine blades to avoid vibration due to long length of blades. In impulse turbine moving blades change the momentum of steam to generate torque. No pressure / temperature drop occurs while passing through moving blades of Impulse Turbine. Impulse blades are compact, heavy and robust.
In reaction turbine driving force is generated by reaction force of steam as it accelerates through the moving blades. In this turbine pressure drops both in the nozzles / fixed blades as well as in the moving blades since shape of the moving blade channels are also nozzle shaped. Due to the expansion of steam while flowing through the blades, there is an increase of kinetic energy, which gives rise to reaction in the opposite direction (Newton’s third law of motion). Blades rotate due to both impulse effect of the jets (due to change in momentum) and the reaction force of the exiting jets impressed on the blades in the opposite direction. Such turbines are called impulse-reaction turbines, or to distinguish from impulse turbine, simply reaction turbines.
Outer Casing:
It is the stationary part of the turbine. It holds the fixed blades/ diaphragm of the turbine. Casing may be single shell or double shell. Metal thickness of single shell casing is high to withstand the temperature difference of steam inlet temperature and atmospheric temperature 210 MW LMW turbine is of single shell type. To reduce metal thickness high pressure turbines are made of double casings. Each casing is designed to withstand relatively low temperature difference. This reduces metal thickness. 210 MW KWU turbines are made of double shell type.
Fixed Blades:
The fixed blades convert heat energy of steam into kinetic energy. The first stage of HP impulse turbine fixed blades is nozzles shaped to perform as nozzles. Steam pressure drop takes place in these nozzles.
In reaction turbine small amount of heat drop takes place in each stage, hence size of each stage is smaller than of impulse turbine and no. of stages is high. One pair of fixed blade and rotating blade forms a stage of turbine.
Front end pedestal:
The following accessories are located in front end pedestal: 1. Main Oil Pump.
2. Over-speed trip device. 3. Hydraulic speed transmitter. 4. Shaft position measuring device. 5. Journal bearing of turbine ( Brg. No. 1 ) 6. Electric speed transmitter.
7. Speeder Gear.
Dummy Piston:
High pressure & temperature steam when expands in a single flow HP turbine along the axis of the shaft, it develops a huge amount of axial thrust acting on the shaft in the direction of steam flow. To reduce / minimize this axial thrust a dummy piston is provided at the inlet of turbine .A force opposite to the axial thrust is generated in dummy piston. The value of axial thrust is more in a reaction turbine than in an impulse turbine.
Thrust Bearing:
Thrust bearing is provided to absorb unbalance axial thrust generated in turbine.
It is located in between HP & IP turbine at central pedestal. The bearing is called as ‘Michel type’ tilting pad bearing. The wedge shaped cavity between thrust pad & shaft collar is filled with oil.
Diaphragms :
Diaphragms are located inside inner casing of turbine to house stationary blades. The fixed blades in an impulse turbine have the form of nozzles mounted in diaphragms. Diaphragms are fixed in both upper & lower halves of the cylinder casing by means of keys so that during expansion of the cylinder fouling with the shaft seal is avoided. LP turbine diaphragms are cast type.
The cast type diaphragms are made of iron castings with steel nozzle plates embedded in them. To avoid leakage of steam across the diaphragms seals are provided in between shaft and diaphragms.
TURNING GEAR / BARRING GEAR :
• Turning gear motor drives the TG shaft at a slow speed before steam rolling to avoid the need of large quantity of steam flow to get the rotor moving from rest. This avoids unwanted thermal stress on the rotor shaft.
• After shutdown of turbine, various parts of it remain at high temperature and uneven cooling of the same takes place, which in turn may cause shaft bending and misalignment. To avoid this TG shaft is rotated at slow speed through barring gear for uniform cooling.
In LMW turbine Turning Gear is mounted on LP – Generator coupling. It rotates TG shaft at 3-4 rpm. by a turning gear motor. In KWU turbine the shaft is rotated by a hydraulic turbine at around 120 rpm.
Before putting the TG on barring gear the following must be ensured : • Generator seal oil is established. ( to avoid rubbing of seal block). • Turbine lub oil system is established.
• Jacking oil pump is running properly.
There is a provision of hand barring to rotate turbine shaft by engaging hand barring manually in case of failure of barring gear driving mechanism.
Metallurgy of Turbine :
Turbine operates at high temperature & pressure. Hence Creep & fatigue failure of the shaft may occur. Considering these factors the metallurgy of turbine shaft is selected.
When any component is under thermal stress at high temperature for a considerable period creep stress occurs. The material may fail due to this creep stress. It varies directly with the duration of time. Creep failure occurs at the blade attachment with the rotor shaft.
Thermal fatigue develops after repeated cyclic reversal of thermal stresses. This happens due to repeated heating & cooling of turbine. If steam temperature is fluctuated repeatedly at the inlet of HP & IP turbine fatigue failure may occur.
Considering the above the metallurgy of the turbine should be : • Good creep resistance at high temperature.
• High fracture toughness. • Good mechanical toughness. • Good proof strength.
LP rotors requirement is low temperature high tensile strength and high toughness. For this it is made of 3.5Ni Cr M0 V material.
Expansion of Turbine:
The turbine casing and rotor expands and contract due to heating and cooling. Due to difference of metal mass of rotor and casing the rate of expansion and contraction are different. The casing may expand both in radial and axial directions. Radial expansion is controlled by radial fixing points. The maximum expansion in axial direction at full load is called the absolute expansion. The casing may expand over the sliding support. The bearing pedestal is allowed to slide axially on keyways fitted between bedplates / soleplates and pedestal. Some cases pedestal bearings are fixed with the foundation and the casings are allowed to expand to one end by means of supporting feet.
Axial Expansion of Rotor:
For KWU 210 MW turbine the anchor point is at central pedestal i.e. at bearing no. 2. HP turbine rotor expands towards bearing no. 1, whereas IP & LP turbine rotors are expanded towards bearing no. 5.
In LMW turbine HP-IP coupling is rigid one and IP-LP coupling is semi-flexible. The axial expansion of rotor is difference between HP & IP rotor expansion since IP for LMW turbine is single flow type. In 210 MW KWU turbines HP-IP & IP-LP both couplings are rigid. Permissible axial shift is 1.7 mm towards front end pedestal and 1.2 mm towards LP turbine.
Axial Expansion of Casing:
HP cylinder casing is anchored at bearing no. 2, so its casing expansion takes place towards bearing no. 1. IP cylinder casing is anchored at bearing no. 3, so it expands towards bearing no. 2. LP cylinder is anchored at middle of casing, so it expands towards bearing no. 4 & 5.
Expansion bellows are provided I LP turbine to accommodate LP casing expansion.
Differential Expansion of Rotor:
Both rotor & casing of turbine expands due to increase of temperature. Due to difference of geometry and mass of metals the expansions are different. The difference of expansion of rotor & casing is called differential expansion. Its value may be (+) ve or (-) ve. Positive expansion indicates Rotor is expanded more than casing and opposite in case of (-) ve expansion. The
TURBINE BEARINGS :
LMW 210 MW TG Bearing No. 1, 3, 4 & 5 are journal bearings, whereas bearing no. 2 is a combined journal cum thrust bearing.
KWU 210 MW TG bearing No.1, 3, & 4 are journal bearings, whereas bearing no. 2 is a combined journal cum thrust bearing.
Journal Bearings :
Turbine Rotor is supported on journal bearings. The bearing is splitted into two halves. White metal is used as bearing metal. The bearings are spherical in shape to take over small deflection of shaft.
Combine journal cum Tilting Pad Thrust Bearing :
Michel type bearings are universally used to withstand heavy axial thrust of turbine shaft. It is accomplished by using oil film. Function of thrust bearing is to bear thrust load and to keep the shaft in position to maintain stator & rotor blade clearance. This bearing has a provision with a step on the back to tilt. Due to this tilting ability an oil film is formed in between shaft collar and white metal lining of thrust pad.
Pads are usually fitted on both sides of the collar. This thrust bearing is combined with a journal bearing for supporting the shaft.
45
THERMAL POWER STATION TURBINE PLANT LAYOUT DESIGN & ITS
LOCATION
Introduction – Power station layout is concerned with the logical and economic use of space and the relationship of one piece of plant with another. The design & layout of Steam Turbine station building, which contain Main Turbine, Generator and Turbine auxiliary equipment and its system like condensing system and equipments, regenerative system & equipments, cooling system & equipments, lube oil system & equipments, feed system & equipments, compressed air system & equipments, etc are very much important for the erection cost control & easiness of operation and maintenance, good and safe working condition. An efficient turbine plant and system layout minimizes losses and running cost. Ideally, plant items should be located as close as practical considering adequate access for operation and maintenance. The best layout design of turbine plant & its system involves the correct balance between lowest cost and best arrangement from both constructional and operation point of view.
The basic layout of a standard 210 MW capacity Turbine Station has already been standarised considering above stated aspects. However, the Turbine plant arrangements within the building envelope vary due to the design and manufacture of main plant items particularly Turbine, Generator, Condenser and its auxiliary equipments.
The basic pattern consists of an integral building structure with Boiler and Turbine houses arranged in parallel but separated by annexe containing mechanical, electrical auxiliary plant items and its systems including instrumentation.
1. Main Turbine Plant Orientation – With fossil fired plant, the important determination and consideration for T.G. Plant layout design are –
a) The overall dimensions of Turbine Generator with the condensing, feed heating and general Turbine auxiliary plant with walkway and passages.
b) Provision for adequate access, unloading bay, lay down area for machine parts. c) Turbine hall dimension & space for lifting generator and turbine with casing. d) E.O.T. crane for lifting and installation of equipments.
e) Regenerative heaters position and location. f) Type and design of B.F. Pump, C.E. Pump, etc.
g) Piping layout from Boiler to Turbine & Turbine to Boiler.
Beside this, one of the most important decisions, which influences overall station layout, is the choice of the relationship between T.G. and Boiler, since both the Boiler center line spacing and dimensions of the Turbine hall can be significantly influenced by this decision. The final out come depends on
traditional expenses of plant layout, choice of plant supplier, relationship between civil, electrical, mechanical cost, scope of supply of particular contractor, site problem & concerned engineers liking/disliking.
2. Turbine Generator Island Concept – It is conventional practice for T.G. to be supported by foundation block, which are elevated above ground level of power station. The height (13 mtr. for Bk.T.P.P, 9 mtr. for K.T.P.P.) at which T.G. is located is termed as operating floor elevation. The operating floor layout of T.G. alone is termed as island layout. This layout is the most popular layout preferred by most power station manufacturers and designers.
The preference is based on need to provide –
a) Clear and unhindered access to main plant items. b) Direct crane access to all parts of the Turbine hall.
c) Good maintenance access, efficient lighting and ventilation to all area. d) Opportunity for equipment location in intermediate floor levels. e) Better facility for Turbine maintenance and ample lay down space. f) Pleasing and uncluttered appearance of the Turbine hall.
g) Control room is in same elevation of T.G. floor better monitoring facility. h) Most economic and practical layout.
i) Best access and plant maintenance.
3. Turbine – Generator Auxiliary System – The following system are included – a) Feed heating system.
b) Condenser and condensate system. c) Compressed air system.
d) Cooling water system. e) Circulating water system.
f) Main lube oil, jacking oil & control oil system. g) Turbine oil purification system.
h) Generator seal system, Gas system. i) Gland sealing cooling & Vacuum system.
4.1 Feed Heating System – It forms an integral plant of the generating process by raising the temperature and pressure of the condensate returning from the Turbine to Boiler end. In this system, a number of pumps (Boiler Feed Pumps, C.E.Pumps, Drip Pumps, etc.) and heaters (H.P. & L.P. Heaters) arranged in saris and which are linked by pipe work system. The location of each of the components in the system follow logical and defined sequence and it is important in terms of overall system economics and hydraulic performance. Each elements of this system should be correctly located in relation to the other and to the Turbine in particular.
4.2 Condenser & Condensate System – The condenser is an integral part of the Turbine as it serves as heat sink & creates vacuum. Recently more conventional under-slung arrangement is preferred. In this arrangement condenser, it self located directly beneath the L.P. Turbine. The main advantages of this location are:
a) Easy condenser box erection facility. b) Access for tube insertion & withdrawal.
c) Convenient C.W. pipe, condensate pipe connection & optimized C.W. pipe length. d) Good water distribution and uniform flow through condenser.
e) Convenient efficient air extractions system related to condenser.
4.3 Circulating Water System – It serves to transform L.P.Turbine exhaust steam to water. The system-piping network located below ground and connected to condenser and Cooling Tower through C.W.Pumps.
4.4 Auxiliary Cooling Water System & D.M.C.W. System – Cooling water system maintains temperature of Turbine Oil, Seal Oil, Generator Gas, etc. of T.G. system and working oil of B.F.Pumps.
4.5 Compressed Air System – Compressor are vital equipment for the control of Turbine, Boiler and auxiliary equipment operation, testing, cleaning. In Bk.T.P.P. compressors are located in ground floor between ‘AO’-‘A’ Row. Air system piping distributed from compressor drier discharge.
4.6. Turbine Lube Oil, Jacking Oil, Control Oil System, Oil Purification System & M.O. Tank – The location of the above system is very much important from standpoint of fire protection, cleanliness and safety.
4.7 Generator Seal Oil System & Gas System – Generator seal oil system is very much important for generator shaft sealing to prevent Hydrogen gas leakage from generator or ingression of air into generator. Gas system is important for maintenance of Hydrogen pressure in generator, H2 gas purging
out by CO2 gas system & gas filling.
4.8 Gland Sealing, Cooling and Vacuum System – Turbine gland sealing, cooling & vacuum system are separate system provided with Turbine auxiliary system to maintain vacuum in condenser, to seal the Turbine gland and to cool gland
Steam through condensate water. The above systems are connected with Turbine separately and located below Turbine floor for erection cost control and operation maintenance facility. System equipments like blowers, coolers, ejectors are located below Turbine floor.
Some sub-systems like HP & LP Bypass system, oil vapour extraction system, on-load tube cleaning system are also include with the main system of steam system (MS, CRH, HRH) piping, Turbine lube oil system and circulating water system in modern power station like Bk.T.P.P. for assistance service to the system. The important sub-system are located within the main system separately and positioned in the Turbine house. Another most important location is piping and valves layout. More than 200 nos. power operated valves are installed and located in the powerhouse for remote / local operation of a standard 210 MW thermal power station.
46
THEORY OF STEAM TURBINE, SPECIAL FEATURES AND
CONSTRUCTION DETAILS OF TURBINE
1. Introduction: The Steam Turbine is a most popular prime mover of generator, which requires steam as
a working fluid, a source of high-grade energy and a sink for low-grade energy. When the steam flows through the Turbine, part of the energy content continuously extracted and converted into useful mechanical work. The main objective of the Turbine designer are to ensure that this process of conversion of heat energy of steam to mechanical work is carried out with – maximum efficiency, maximum reliability and minimum cost. Second objectives are that the plant should require minimum supervision and minimum starting time.
Steam Turbine offers many advantages –
a) From thermodynamic point of view, the Steam Turbine occupies a favourable position as it can translate into mechanical work from the expansion of steam in the Turbine. Its thermal economy also good especially in Turbine of large output and operating at high pressure.
b) From mechanical point of view, the Turbine is ideal because propelling force applied directly to the rotating element of machine.
2. Theory of Steam Turbine :
2.1 Modern Steam Cycle – Thermal Power Station operates by using steam in closed power cycle, where water undergoes various thermodynamic processes in a cycle process. One-half of the cycle consists of the boiler cycle (heat source) and the other half is turbine cycle, consists of turbine, condenser, feed pump and feed water heaters. Feed water supplied to the boiler drum where water is boiled and converted into saturated steam and that steam further superheated to the super heaters and then fed to the turbine where the steam expands and give-up heat energy, a high proportion of which is transferred into work energy on the turbine shaft due to change of momentum in the moving blade and increment of velocity in the nozzles and/or moving blades. The exhaust steam from turbine, which is termed as carry over loss, passes to the condenser where it condense by transferring its latent heat of vapourisation to the cooling water. This condensed water enters into hotwell and the same is pumped by condensate extraction pump through L.P. Heaters, coolers to the Deaerator and Feed Storage Tank. Boiler Feed Pumps, in turn, takes feed water from Feed Storage Tank and pumps the feed water to the boiler drum routing through regenerative heaters. In this way water cycle and steam cycle is completed. This cycle follows modified Rankin Cycle. In this cycle steam after HP Turbine outlet sends for reheating to the boiler again. The reheated steam further expanded in the IP & LP cylinder for increasing the thermal cycle efficiency.
2.2 Development of various types of Turbine and their theory of operation –
a) Simple Impulse Steam Turbine – Here steam expansion takes place in the set of nozzle and the pressure drops to about condenser pressure after expansion in nozzle. The expansion of steam results very high velocity and due to this high velocity steam jet blade/rotor velocity reaches maximum. In this Turbine carry over/exhaust loss also high and efficiency is poor. This type of Turbine is not economical and can be employed for small power generation.
b) Pressure Compounded Impulse Turbine – This type of Turbine construction is arrangement of number of simple impulse Turbine in series on the same shaft, allowing the exhaust steam from one Turbine to enter the next Turbine nozzle. Each simple impulse machine is termed as a stage of Turbine. Each stage comprising of a set of nozzles and blades. Here total pressure drop in the Turbine is the summation of pressure drop in each stage nozzle & for that reason the Turbine is termed as pressure compounded. Here nozzles are fitted into partition, termed as diaphragm. It separates one
wheel chamber to another. Due to pressure compounding, steam velocity, blade velocity, rotor speed and leaving loss is lower that that of simple impulse Turbine and its efficiency is higher.
c) Velocity Compounded Impulse Turbine – This Turbine comprises a set of nozzle with two or more rows of moving blades & number of guide blades. Guide blades are fixed blade, fitted with stator, placed after first stage and in between moving blades for guiding steam & to direct in the next stage. The guide blades are set in reverse manner. In this Turbine, steam expands in the first stage nozzle & nozzle outlet pressure reaches about condenser pressure. There after steam passes through rows of guide blades and moving blades. In guide blade, there is a slight velocity drop due to friction. In this Turbine leaving loss is small; being about 1–2% of initial available energy of the steam with the ordinary nozzle arrangement, the most efficient speed of blade of this Turbine is 0.15 of steam speed. d) Pressure Velocity Compounded Turbine – This type of Turbine constructed on basis compounding both pressure and velocity in a single rotor. The wheel carried two rows of blades between nozzles. The efficiency of the Turbine is not so high.
e) Pure Reaction Turbine – This type of Turbine is not a practical type though “Parson” tried to develop.
f) Axial Flow Impulse Reaction Turbine – This Turbine is a joint application of the impulse & reaction principles of operation. Here, in every stage & steam expands in nozzles & enters in moving blade where it suffers a change in direction i.e. momentum and undergo small drop in pressure which gives rise to a reaction in the opposite direction of increased velocity. Thus, gross propelling force is vector sum of impulse & reaction force. This type of Turbine is very efficient & successful Turbine.
2.3 Factors of Turbine performance and sizing –
Turbine performance and sizing is affected by followings - i) Initial Steam Pressure.
ii) Initial Steam Temperature.
iii) Whether reheat is used or not, and if used reheat pressure and temperature. iv) Condenser Pressure.
v) Regenerative Feed Water Heating. i) Initial Steam Pressure:
With increase in the initial steam pressure at constant temperature & constant condenser pressure, wetness of steam in the last stages of turbine increases, thereby reducing internal efficiency of these stages. Usually 1% moisture in steam in a particular stage results in 0.9 to 1.2% reduction, erosion becomes so severe that life of turbine in endangered.
With increase in initial steam pressure, blade heights of initial stages get reduced. If blade heights of initial stage blades are less than 25 mm, this stage becomes very inefficient due to three dimensional flow and vortex formation etc. Sometimes this problem is overcome by partial admission in first or first few stages.
With increase in pressure, shell thickness of casings and size of flange and flange bolts increase. Bigger and thicker flanges and flange bolts implies non-symmetric casing resulting in higher incremental stress, thereby restricting rate of speeding loading of Turbine. This problem to certain extent can be solved by using multi (double) shell design for casings or by flange and flange bolt heating arrangement.
In light of above considerations lower initial steam pressures and used for smaller Turbines (resulting in simple design and quicker start ups) and higher initial steam pressure are used for larger Turbines (resulting in higher efficiency).
The following are typical recommended value of initial stream pressure for various rating Turbines:- 50 MW - 50 to 90 ata (non-reaheat)
50 – 100 MW - 90 to 130 ata. 100 – 200 MW - 130 ata. 200 – 300 MW - 130 to 170 ata. 300 – MW & above - 130 to 240 ata. ii) Initial Steam Temperature:
As initial temperature increase, the thermal cycle efficiency increases and hence from thermodynamic (theoretical considerations) there is no upper limit for initial steam temperature.
Material considerations do restrict the initial steam temperature. Up to 4000C plain carbon steels can be used and up to 4800C low alloy steels can be used. Above 4800C and up to 6000C heat resistant ferritic steels can be used. It gives limiting value of initial steam temperature to be 5650C (Leaving margins for temperature swings).
During operation of power plants, it was found that plant outages due to boiler failure with initial steam temperature 5650C were enormous as compared with initial steam temperature 5350C. Now-a-days, practical limit for initial steam temperature is 5350C to 5400C.
Above 5400C temperature, austenitic steels could be used, which have higher coefficient of thermal expansion & lower thermal conductivity but poor machineability and weldability as compared to ferritic steels. For these reasons, use of austenitic steels is not preferred.
iii) Reheat:
Reheating the steam after it has partially expanded, improves the thermal cycle efficiency by 4 to 5% as a more efficient cycle is added to original cycle.
With the reheat, available heat drop (for conversion to work) increases by almost 12% per unit mass of working fluid, resulting in almost corresponding reduction in mass flow of working fluid for generating same power output. This results in smaller auxiliary equipment (like condenser, heaters, condensate and boiler feed pumps), resulting in savings in investment.
Reheating reduces moisture in last stage of Turbine, thereby improving the internal efficiency of Turbine. Reheat is universally used for unit ratings higher than 100 MW. Sometimes, double reheat is also employed for supercritical pressure units.
Steam after partial expansion is usually reheated to initial steam temperature at pressure 0.15 to 0.3 times initial pressure. Absolute increase in thermal cycle efficiency and thermal plant efficiency by reheating is approximately 1.5 to 2% respectively.
iv) Condenser Pressure:
A condenser provides heat sink, low vacuum and preserves working fluid (Condensate water). Lower condenser pressure implies lower mean temperature at which heat is rejected to sink, thereby increasing the thermal cycle efficiency. Lower condenser pressure also means larger volumetric flow of steam at Turbine exhaust, resulting in larger L.P. Turbine and larger condenser. The increase in capital cost of L.P. Turbine and condenser due to lower condenser pressure is usually offset by increase in efficiency.
v) Regenerative Feed Water Heating:
In modern power thermal power station, usually feed water is heated to 0.65 to 0.75 times saturation temperature in 5 to 9 heaters by regenerative heating for achieving optimum efficiency. As a consequence of steam extraction for feed water heating, increased steam flow through Turbine is required to generate the same power. Usually thermal cycle employing regenerative feed water heating will have 30% higher flow at stop valves and 30% lower flow at Turbine exhaust as compared to thermal cycle without regenerative feed water heating. This makes regenerative feed water heating even move attractive due to following reasons:
a) Increased steam flow in initial stages results in increased blades heights resulting in improving internal efficiency of Turbine.
b) Reduced flow at Turbine exhaust demands lesser exhaust area, resulting in smaller blades in last stages blades, which limiting factor in Turbine design.
c) The decrease in steam flow at Turbine exhaust also reduces flow of working fluid through auxiliary equipment (like condenser, condensate pumps, ejectors and low-pressure heaters), thereby reducing their sizes and saving in capital investment.
3. Construction of Steam Turbine:
Main components of a Steam Turbine are i) Rotors, ii) Cylinders or casings, iii) Emergency Stop Valves and Control Valves, iv) Liners and Diaphragms, v) Blades, vi) Bearings, vii) Sealings, Viii) Barring Gear, ix) Governing and Protection System, x) Turbine Supervisory System.
i) Rotor:
If the Turbine is impulse type the rotor is disc type, i.e. blades are carried in the discs, which may be integral forged with shaft or shrunk on the shaft. If the Turbine is reaction type, the rotor is drum type, i.e. blades are directly carried on the rotor.
In the integral forged rotors discs and shaft are machined from one single forging. The main advantage of integral rotor is that there is no disc loosening problem and as such are commonly used in high temperature zones (H.P. & I.P.) integral rotors are expensive and difficult to forge and there is relatively high incidence of rejections. Also more material is to be removed by machining. Now a days, it is becoming more and more popular for fossil fuel Turbines.
Build-up type rotors can be of two types (a) Shrunk-on disc rotor - It is used when rotor is too heavy to be forged in single piece. Shaft and disc are forged separately and are assembled after machining is shrinking of discs on shaft. (b) Welded (Hollow drum) rotors – In this case discs are separately forged, rough machined and welded together at periphery to make the complete rotor. Radial holes are drilled in the rotor so that steam can go inside the voids of rotor and help in uniform heating of the rotor. Welding and subsequent heat treatment has to be performed with extreme care.
Rotors are coupled by means of coupling. Earlier semi flexible coupling were used because these allowed a limited amount of misalignment and required original rigidity. In this case, each rotor required its own set of bearing. Now-a-days trend is to use rigid coupling, because in this case only one bearing is required between two rotors (because whole rotor system behaves as a single rotor).
ii) Casings:
Turbine casings are essentially pressure vessels, their weight being supported at each end. These are therefore, designed to withstand and hoop stresses in transverse plane and to be very stiff in longitudinal direction to maintain accurate clearance between stationary and rotating components.
Usually casings are of two designs (a) Single shell casings, (b) Multi (double) shell casings.
Single shell casings take pressure drop from steam pressure to atmospheric pressure in single shell and hence required thick wall and heavy flanges at parting planes. This causes very large incremental thermal stresses during transients, resulting in slower start-ups and shutdowns. This problem to certain extent is solved for flange and stud heating.
In Multi (double) shell casings, there is intermediate pressure (approximately 25% pressure of main steam) between the shells and hence two shells resulting in thinner walls and lighter flanges at parting planes share pressure drop. This type of casing has lower incremental thermal stresses during transients resulting in quicker start-ups and shutdown. Multi (double) shell casings are now commonly used for H.P. and L.P. Turbines.
iii) Emergency Stop Valves and Control Valves:
Turbines are equipped with emergency stop valves to cut off steam supply and with control valves to regulate steam supply. In case of reheat Turbines emergency stop valves are also provided in the hot reheat line.
Emergency stop valves are actuated by servomotor controlled by the protection system. ESV remains either fully open or fully close. Control valves are actuated by the governing system through servomotors to regulate steam supply as required by the load.
iv) Liners and Diaphragms:
In reaction Turbines, guide blades are directly carried in the casings and hence liners and diaphragms are not generally used.
In impulse Turbines, most of the pressure drop of a stage takes in guide blades resulting in higher deflection guide blades. Additional bending strength to guide blades is provided by diaphragms. Welded diaphragms are used in higher temperature zones while cast diaphragms are used in low temperature zones.
v) Blades:
Blades are single most costly element of Turbine. Blades fitted in the stationary part are called guide blades or nozzles and fitted in the rotor are called moving or working blades:
Blades are of three types:
a) Cylindrical (or constant profile) blade.
b) Tapered cylindrical (tapered but similar profile) blades. c) Twisted (twisted and varying profile) blades.
This type of blade is used for very long blades. Blades have three main parts
(a) Aerofoil: it is working part of blade and is one of the types described above. (b) Root: It is portion of the blade which is held with disc, drum or casing.
Three type of root arrangement are commonly used; (b1) T-Roots: from small blades; (b2) Fir-tree or Serrated Roots: for longer blades; (b3) Fork and Pin Root: for longer blades but it can be used with shrunk on disc type rotors only.
(c) Shroud: It can be either riveted to main blade or it can be integrally machined with the blade. Now-a-days trend is towards integral shroud for shorter blades and free standing for larger blades. Some times lacing wires are also used to dampen the vibration and to many frequencies in the longer blades (e.g. LMW machine – 210 MW)
vi) Bearings:
Journal bearings are manufactured in two halves and usually consist bearing body faced with antifriction tin-based babbiting to decrease coefficient of friction. Bearing body match with adjustable seating assembly in the pedestal. Bearings are usually forced lubricated and have provision for admission of jacking oil.
The thrust bearing is usually Mitchell type and is usually combined with a journal bearing, housed in spherically machined steel shell.
vii) Sealings:
Sealings in Turbine casings are provided to check steam leakage from H.P. and I.P. Turbines and air leakage into L.P. Turbine.
Sealing of Turbines are usually Multi-labyrinth type, which provide maximum amount of throttling in a given axial length.
viii) Barring Gear:
Barring gear rotates the Turbine as high speed when Turbine is being started or shutdown, thus allowing uniform heating or cooling of the rotors to avoid any distortion of rotors.
.
ix) Governing and Protection System:
Governing system is provided on utility steam Turbines to maintain rated speed (within steady state regulation) at all the loads and to provide predetermined load sharing among the Turbines operating in the grid. Governing system has speed control in parallel to speed governing to maintain constant speed at all the loads when set is running is isolation and change load share of Turbine when running in parallel.
x) Turbine Supervisory System:
Instrumentation is provided for indication/recording of important parameters like vibrations, eccentricity, differential expansion, overall expansion, valve position and stresses in major components. Hooked up with indicators are suitable alarms and tripping mechanisms for cautioning the operating men and tripping the Turbine, if these values reach alarmingly inadmissible values. 4. Special features of Bk.T.P.P. Turbine (Similar to KWU design):
1. Fuji Electric Company, Japan make, Tandem compound condensing type, three-cylinder reaction turbine with reheat cycle and regenerative feed water heating system. No. of stages in H.P. = 23, in I.P. = 17 and in L.P.T = 8 x 2.
2. Individual rotors of H.P., I.P. and L.P. cylinder are rigidly coupled. 3. All rotors are machined from single forging.
4. H.P. cylinder – Barrel type design and permit flexible operation, rapid start-up and high load change. The casing has no flanges and hence thermal stress is minimized. The inner casing is vertically split. 5. H.P. Turbine employed with throttle governing without a control stage.
6. H.P. Turbine main steam pipe connection after valve designed by easy detachable breech nut and ‘U’-seal ring elements. Cold reheat pipe designed as single pipe construction tapped from H.P. Turbine exhaust for materials saving and erection advantage. The pipe divided into two sections at boiler area. 7. I.P. Turbine is single flow double shell construction fitted with two nos. combine reheat stop valves and
intercept valves to control steam flow from reheater to turbine (I.P.) and to prevent acceleration during trip-out of turbine by the remaining steam of reheater pipe and flow back to H.P. Turbine.
8. I.P. inlet steam pipe fixed at outer casing vertically is provision of expansion in all direction fixing special type ‘L’-ring.
9. Double flow, three casing L.P. Turbine is connected with I.P. Turbine through cross around pipe and Turbine exhaust is multi exhaust type and extraction (L.P.) tapped at difference stages for moisture control and sizing control. The L.P. exhaust directly connected to the twin condensers.
10. L.P. Turbine gland packing with connected through bellows for keeping turbine rotor centre unaffected. 11. H.P. & I.P. rotors, exposed to high temperature, are designed as integrally forged reaction blades with
shroud and formed as rigid shaft. Therefore, blade vibration chance can be avoided.
12. The entire turbine blading is provided with reaction blading for achieving highest efficiency. The stationary blades are inverted ‘T’ or ‘L’ and shrouds are machined from solid. Last three stages of L.P. Turbine blades are designed without shroud ring and of taper twisted type.
13. Critical speed of H.P. and I.P. rotors are designed to be above normal rated speed. Critical speed of this Turbine is 3700 and 1739 rpm.
14. The bearing pedestals of the L.P. Turbine are mounted on the foundation. L.P. cylinder carries shaft seal housing are joined with outer casing by diaphragm. Here the flexibility factor of each bearing is greater than double point bearing and the bearing type is dynamically stable.
15. The journal bearing is double wedge type. At the bottom of bearing centre oil hole with nozzles provided for jacking of shaft.
16. The turning gear, positioned between I.P. & L.P. rotor and at bearing no.-3, is provided with oil hydraulic type turbine for high-speed turning (80 – 120 rpm) by oil pressure.
17. Turbine is provided with two nos. oil coolers of each capacity 100%.
18. The Turbine oil pumps for lubrication and governing is directly mounted on main oil tank and oil pipe connections are welded type routed through oil canal.
19. Turbine equipped with Electro-hydraulic governor with provision of a standard hydraulic governor as a back up.
20. Governor impeller serves as speed detector and generates primary oil pressure corresponding to speed, which in turn regulates speed governor bellows.
21. High pressure jacking oil system for Turbine rotor jacking during coasting down of speed or rising of speed or during turning operation is provided to eliminate chance of bearing Babbitt material rubbing.
22. The axial shift of each Turbine is about to ‘zero’ irrespective of loading. The differential expansion/contraction problem is avoided in this design for multi casing construction of Turbine cylinder.
23. Large load dumping of Turbine, islanding and house operation, quick start-up and shutdown facilities are provided with incorporation of 60% capacity H.P. & L.P. bypass system.
24. Important parameters:
a) Main Steam Inlet -149 Kg/Cm2, 5370 C. b) RSV Inlet - 33.9 Kg/Cm2, 5370C. c) Exhaust Pressure - 0.103 Kg/Cm2. d) No. of Extraction - 6 nos.
5. Special features of Turbine (LMW design):
1. 210 MW capacity, BHEL make, Tandem compound condensing, three cylinder, horizontal, disc and diaphragm type with nozzle governing.
2. High pressure Turbine comprises 12 stages, 1st stage is governing stage.
3. After H.P. Turbine steam flows for reheating and fed to I.P. Turbine of 11 stages. 4. I.P. Turbine coupled with L.P. Turbine by semi-flexible coupling.
5. L.P. Turbine is double flow with a multi exhaust is each flow. No. of stages is 4x2 and in pen-ultimate stage is Baumanian exhaust provided for large moisture separation. L.P. Turbine handles 88% dry saturated steam.
6. Three Turbine rotors are supported on five bearings.
7. Turning gear with motor is mounted on L.P. rear bearing cover to mesh with spur gear & to rotate rotor at 3.4 rpm during start-up and shutdown.
8. High response hydro-mechanical governing is provided for speed/load control. 9. 30% H.P. & L.P. bypass system provided for start-up.
10. Initial steam unloading gear is provided for unloading Turbine when main steam press drops more than 10% of rated steam pressure.
11. Flange & stud heating system is provided for casing heating during cold start-up and rotor heating is provided for hot start-up.
12. Main steam parameters at inlet at rated load – 130 kg/Cm2, 5350C. Rated steam flow – 670 T/hr, CW flow – 27000 T/hr, Turbine exhaust pressure – 0.08 Kg/Cm2.
*** *** ***
47
TURBINE START UP FROM COLD, WARM, HOT, VERY HOT CONDITION
ALONG WITH ITS AUXILIAIRES.
TURBINE START – UP
(a) At least one CW pump and ACW pump are running
(b) Turbine lub. Oil system is healthy i.e. Auxiliary oil pump is running and AC EOP & D.C. EOP are in auto. AC JOP is running and D.C. JOP is in auto.
(c) Generator seal oil system is healthy with normal H2 pressure (2Kg/cm²) and normal H2 purity (>
99.5%).
(d) Turbine is running at barring speed (80-120 rpm) with turning oil supply valve full open. (e) Turbine interlock system is healthy.
(f) MOT oil level normal. MOT vapour extractor running and lub. Oil temp. at oil cooler outlet is more than 35°C, Lub. Oil temp. control in auto & set point 40°C.
(g) DMSW pump is running and DMCW make up tank level is normal (h) DMCW (T) pumps running (at least one).
(i) Service Air and instrument air compressors running. (j) C.T fans are running
(k) At least one condensate pump (CEP) and boiler feed Pump (BFP) running on recirculation and also condensate path and feed water path are through. Check the quality of Hot well and deaerator water, if necessary take fresh DM water.
(l) Hot well level and deaerator level are normal and respective controllers are on auto.
2). MAIN TURBINE PRE-WARNING:
(a) Check that M.S line and H.P. bypass drain valves to unit flash tank is open. All the drains connected to HP. Flash tank should be closed and alternate drains to unit flash tank will be opened till condenser vacuum min. 320 mm Hg is achieved. HPBP warm up line valves are closed till condenser vacuum pulling.
(b) Charge M.S. lines at boiler outlet pressure 10 kg / cm² and temperature 200°C (c) Warm up M.S. lines for 30 minutes.
(d) Raise M.S. pressure to 35Kg/cm² and temp. 280°C
(e) Put the TAS system in service and maintain its normal pressure & temp. In auto (>9kg/cm² & 270°C)
3). VACUUM UP OPERATION: (a) Gland steam exhauster is running.
(b) Put starting ejector in service after achieving gland steam header temp. more than 21°C and steam pr at starting ejector >7Kg/cm².
(c) At condenser vacuum. 135mm of Hg, put gland steam controller in service and set G.S. header pressure at 0.068 kg in auto
(d) At condenser vacuum 200 mm of Hg, close vacuum breaker and put on auto.
(e) Close all atmospheric drains (connected to UFT) and open their drains to HP flash tank. Watch continuously LPT exhaust temp.
(f) Check drip line loops and ejector loops are sealed with normal water level and established. (g) When vacuum reaches 680mm of Hg withdraw starting ejector.
4). HP-LP BYPASS SYSTEM OPERATION:
(a) After achieving vacuum (>670mm of Hg) set the HP bypass downstream temp. at 320°C. (b) Ensure water supply before spray control v/vs of HPLP BP
(c) After warning HP-LP bypass lines through warm up lines gradually open HP-LP bypass valves up to 5% and allow steam to flow through these lines.
(d) Set the HP BP and LPBP down stream pressure at 25 kg/cm² respectively and put them on auto. If required increase boiler firing rate.
5). CHECK THE FOLLOWING CONDITION OF DRAIN VALVES: (a) All the drain valves of M.S. lines are open to UFT.
101,102,103,104,109,110,115,116).
(c) Open drains of MSV warming (L) –MAL - 11 and MSV warming (R) –MAL-12 up to 25%. (d) Open HP connection pipe L & R drain valve (MAL-13, MAL-14) full.
(e) Full open HPT casing drain v/v (MAL-22).
(f) Full open drain of ICV warming (L&R) MAL -26, MAL -31 (g) Full open drain of IP connection pipe (L&R) MAL-27 & MAL 32 (h) Full open drain of cross side pipe and balancing pipe MAL-40 (i) Full open gland steam pipe drain MAL-18
(j) Full open HRP steam strainer (L&R) drains
(k) Full open before & after seat drains of all extraction steam lines to flash tanks. (l) Observe M.S. pressure and temp. reached at least 35kg/cm² and 300°C. 6). CHECK THE METAL MATCHING CONDITION :
(a) Check the boiler outlet temp. and R.H. outlet temp. superheat is more than 50°C (b) Check the steam purity
PH - 9.0 to 9.3 ; conductivity < 1.0 µ mho/cm. Silica < 0.05 ppm ; total iron < 0.05 ppm
It required sufficient steam dumping may be done to achieve above steam purity.
(c) Raise the M.S. parameters for metal matching condition of MSV, HPC and IPS according to the following graphs.
(SEE FIGURE – 1 TO 5 ATTACHED AT THE END). 7). TURBINE STEAM ROLLING:
Depending upon the HP turbine outer casing metal temp. at 50% metal thickness, the type of turbine start up can be chosen:
Cold start up : < 250°C Warm start up : 250 -410°C Hot start up : 410 -460°C Very hot start up : > 460°C (A) Cold start up (HP casing temp. <250°C )
Manual rolling : Select EHG Governor
1) All the trip devices and generator relays are in reset condition Speeder gear (65M) : 100%
EHG speed setter (65F) : 0% EHG power setter (65P) : 0%
2) Slowly bring the starting device (77M) to 0% and reset the turbine (TLR). Slowly increase 77M from 0 to 42%.
3) At 77M 0% auxiliary starts up oil will reset the trip devices and start up oil will reset stop (MSV, RSV) valves.
4) At 42% starting device position start up oil and aux. start up oil pr. will be reduced to zero. As trip oil pr. develops at the bottom portion of stop valves, pistons, stop valves (MSV, RSV) start opening. Thus heating of MSV & RSV starts as MAL drain valves are previously opened. 5) Full open MSV (L& R) warming drains valves (MAL-11, MAL-12).
6) Checking the warming of MSV and RSV casing
Check the MSV (L) casing metal diff. temp. ∆Ø< limit value (30°C) Check the HP turbine casing neutral diff. temp. ∆Ø < limit value (30°C) Check the IP rotor metal diff. temp. ∆Ø < limit value (30°C)
Check the diff. temp. of turbine casing between top & bottom HPT < 55°C
IPT < 65°C
7) Close the MSV (L&R) warming drain valve from 100% to 25%. 8) Check the shaft eccentricity < 75 µM.
10) Now increase the starting device (77M) position to 100%
11) Confirm the condition for speed up from curve and metal matching condition for MSV, HPT, and IPS.
12) Select TSC influence: ON and turbine speed raise mode selected slow (200 rpm/ min.). Selection of speed raise mode is given below:-
Speed change rate Start up mode
Zero to heat soak (1080RPM) Heat soak to rated (3000RPM)
Cold (<250°C) 200 rpm/ min. 500 rpm/min.
Warm (250-410°C) 500 rpm/min. 500 rpm/min.
Hot (410-460°C) 500 rpm/min. 500 rpm/min.
V. Hot (460°C above) 500 rpm/min. 500 rpm/min.
13) Set the EHG speed setter (65F) : 1080 RPM (soaking speed). This increases EHG output and thus develops secondary oil pressure to open CVs proportionately.
Don’t hold the turbine between 400 to 1050 rpm . In case of any trouble, decrease the speed to turning speed.
14) Confirm that at turbine speed more than 400 rpm, turning oil supply valves closes automatically. At speed more than 500 rpm AC JOP stops on auto.
15) Hold the turbine at soaking speed 1080 ± 30 rpm for 30 min. for heat soaking. 16) Check turbovisory readings are within limit
HPT diff. Exp. + 4.5 to -3.0 LPT DE + 7.5 to - 2.0 LPT DE +18.5 to - 2.0 17) Check the following :
(a) Turbine Bearing metal temp. <90°C (b) Lub. Oil temp. maintaining at 40°C (c ) HPT exhaust steam temp. < 480°C (d) LPT exhaust steam temp. < 90°C (e) Shaft vibration < 125 µM
(f) Bring pedestal vibration < 62 µM (g) Steam purity < limiting value (h) HPT top bottom diff. temp. < 55°C (i) IBT top bottom diff. temp. <65°C
(j) Check HPT casing, HPT rotor metal diff. temp. <30°C (k) IPT rotor metal diff. temp. < 30°C
(l) MSV (L) casing metal diff. temp. <30°C
18) Confirm the condition for turbine speed up as per the curve for turbine speed up. 19) Set the EHG speed setter (65F) to 3000 RPM (rated).
Speed rise rate will be 500 rpm/min.
Don’t hold the turbine between 1110-2850 rpm. Turbine critical speeds are at 2439 rpm and 3700 rpm.
In case of any abnormality such as vibration, brg. Temp. etc. crosses limit, decrease the speed to heat soak speed (1080 rpm).
20) At rated speed (3000rpm) hold the turbine for 5 min.
Close the MSV (L&R) warming drain valves and ICV(L&R) warming drain valves. At 2950 rpm AOP stops automatically.
Control oil pr. > 8.5 kg/cm² .
21) Charge DM water to generator H2 gas cooler and exciter air cooler.
22) Check all the turbine parameters similar to the checking done after soaking (1080 RPM) speed. 23) Now synchronize the turbo-generator with grid as per the synchronizing procedure.
Keep the load at 10.5 MW (5%) for 55 min. observe M.S. pr. and temp. as 70 kg/cm² and 370°C . 24) Close HP connection pipe (L&R), MS line & HPC drain valves.
25) Refer to the start up diagram, confirm the condition for load increase and check the following limiting values.
(a) Steam purity
(b) Differential expansions (c) Shaft vibrations
(d) Brg. Metal temp.
(e) Lub. Oil Temp. at maintaining at 45°C
(f) MSV (L) casing, HPT casing, HPT rotor and IPT rotor metal differential temp...
26) Just after synchronization, EHG will change over from speed controller to power controller. To increase load, target load set point is given to power setter (65P) and load rate (65PD) set point is given manually.
27) After keeping 55 min. at 10.5 MW load, further increase of load up to 27% (56MW) at a rate of 0.37% / min. (0.8MW/min.) can be done. After that load can be increased up to 100% at 0.91% / min. (2MW/ min.). Hence, at least 195 min. is required to increase load to 100% from synchronization.
(SEE FIGURE FOR COLD START UP MODE ) For cold start up to synchronization form BLU – 205 min. From synchronization to full load - 195 min. Total = 400 min.
28) At 42 MW load gradually take LPHS in to service and close drain valves for extraction lines. Whenever load is increased, MS pressure decreases if HPLP Pp is in auto; HPLP system closes to
maintain the pressure. Otherwise, if on man mode HPBP valves are to be closed manually. But HPBP valves should not be closed below 2% before reaching 30% (63MW) load) to avoid MFR tripping through R.H. protection logic. At 42 MW load check all MAL drains auto closing. 29) At 50MW load 6.6 KV auxiliary supplies are changed over from station to UAT.
30) Increase load to 60MW and change deaerator pegging from TAS to CRH. Change over TAS from HC to LC.
31) Increase load up to 84MW. LPBP closes automatically.
At 84MW, put ALFC on auto (ALFC –auto load limiter follow up controller). So that load limiter can follow the current load on auto.
32) 80-90 MW load but HPHS in service.
Change deaerator pegging from CRH to 4th extraction (extraction from IPT exhaust). Gradually increase the load to 125MW (2MW/min).
TURBINE WARM START UP
HP Turbine outer casing metal temp. at 50% is between 250 -410°C (32 hours after unit shut down).
1) Condenser vacuum is achieved more than 670 mm of Hg.
2) All the drain valves of MS lines, CRH, HRH, HPBP, LPBP including MAL drains are opened similar to cold start up.
3) Put HP-LP bypass in operation and achieve M.S. parameters for metal matching of MSV, HPC, and IPS. Confirm the metal matching condition from metal matching curve for speed up. Observe the MS temp. and pr. as 70Kg/cm² and 410°C.
4) Select TSE influence ‘ON’ speed fast and start turbine to rated speed at 500 rpm / min.
5) After achieving 3000RPM check the turbovisory parameters etc. as per cold start up condition and synchronize the turbo generator with grid.
6) Hold the turbine at 5% (10.5MW) load for 10 minutes. (SEE FIGURE FOR WARM START UP MODE)
After that load can be increased up to 24% (50MW) at rate of 1.9 % / min (4MW/min) and then increased up to 100% load at 0.95% / min. (2MW/min.). Hence at least 100 min. is required to achieve full load.
7) LPHS, HPHS, UAT, TAS LC, deaerator pegging etc are taken in to service similar to cold start up.
TURBINE HOT STARTS UP.
HPT outer casing metal temp. at 50% depth is between 410 -460°C ( 8 hrs. after unit shut down). 1) Increase boiler firing to achieve M.S. temp. for metal matching of MSV, HPC, IPS as per metal
matching curve. Check all the O.K. mark on start up page in CRT.M.S. temp. & pr. required as 435°C & 70 kg/cm².
2) All MAL drains are in open condition as per MAL drain chart.
3) Select EHG, hot mode, TSE – ON, speed rise fast and give command for rated speed (3000 rpm). Speed rise 500 rpm / min.
4) Synchronize and increase load up to 20% load (42MW) at 2.5%/ min (5MW/Min.), then up to 30% (63MW) at 0.87%/Min. (1.8MW/min) and then full load at 1.4% / min (3MW/Min.). Hence, at least 68 min. is required to achieve full load.
5) Put LPHs, HPHs, UAT, TASLC, deaerator pegging etc. similar to cold start up procedure
TURBINE VERY HOT START UP
HPC outer casing metal temp. At 50% more than 460°C
1) Steps up to synchronization similar to hot start up mode. M.S. Pr. & temp. 149 kg/cm² & temp. 537°C.
2) Machine load can be increased just after synchronization up to 100% load at a rate of 2.0% / min (4.2MW/min). Hence, at least 48 min. is required to achieve full load.
3) LPHS, HPHs, TASLC , UAT, deaerator pegging etc are charged similar to hot start up procedure . TURBINE NORMAL SHUT DOWN
1) Decrease load at 2% /min (4.2 MW/Min.).
2) At 100-90 MW deaerator pegging change over from EX-4 to CRH. 3) At 83-60MW LPBP control valves open on auto.
4) At 60MW load deaerator pegging change over from CRH to TAS. Before this TAS HC to be put in to service.
5) Change over auxiliaries bus (6.6 KV) from UAT to station. 6) At 40MW take HPHs out of service.
7) Check turbine master drain valve (MAL valves) in auto. 8) At 20MW take LPHs out of service
9) At 10.5MW(5%) turbine is tripped by operating TLR. Ensure S.V. and C.Vs are closed . Ensure voltage has come down to zero. Ensure turbines speed is decreasing, Note down the turbine coasting down time.
10) Check whether MAL drains have operated according to the drain chart. 11) Check AOP starts at 2850 RPM.
12) Check JOP auto starts at 400 RPM
13) Turning gear oil valve opens at 300 RPM and turning is established at around 150 to 200 RPM. 14) Close HPBP manually
15) Close LPBP after reducing HRH pr. to zero. 16) Isolate H2 coolers and excite air coolers.
17) In case of vacuum breaking (a) Withdraw main ejector (b) Vacuum breaker is opened.
(c) Gland sealing steam CV closed at 140 mm of Hg. 18) TAS out of service
19) Boiler outlet valve close
20) Gland steam condenser exhaust fan stop. 21) Open M.S. lines drain valves.
22) During coasting down check the turbovisory parameters such as vibration, D.E. Brg. Metal temp., lub. Oil temp etc.
The turbine stress controller generates a hold function i.e. it does not allow turbine to increase a decrease load or speed if metal matching conditions are not satisfied. This prevents the turbine from excessive stress during load change or speed change. The turbine stress controller calculates the temp. margin from the midwall temp. and surface temp, of main stop valves (MSV), HPC, HP shaft and IP shaft and if these margin falls below 15°C if generates a trigger to the EHG and consequently EHG blocks any increase or decrease of load during such conditions. Here it must be noted that this ‘HOLD’ function is generated only if turbine stress controller is made ‘ON’.
It is obvious that during load increase, dt will be positive and during load decrease dt will be negative. If we plot a curve between tm & dt, then the instantaneous point can be defined on it. The point must be within allowable limit defined in the curve.
Say for tm being same as that for the current point, the max. allowable dt for positive side is dtu and for negative side is dtl.
Then upper temp. margin a dtu –dti and lower temp. margin =dti … dtl which should not fall below 15°C . TURBINE INTERLOCKS AND PROTECTIONS :
1) AOP auto starts at 2280 RPM and pr.< 5.1 ksc. 2) AOP auto stops at 2850 RPM and pr. > 8.0 kg/cm² 3) AC EOP auto starts at 1.1 kg /cm²
4) AC EOP auto starts at 1.1 kg /cm² with time delay 5) AC JOP auto starts at 400 RPM
6) AC JOP auto stops at 500 RPM Similar for DC JOP 7) Turning oil stop valve auto open at 300 RPM
8) Turning oil stop v/v auto close at 400 RPM 9) MOT oil capacity -20m³, type of oil ISOVG32
10) Turbine over speed 3330 RPM (Mech.) 3360 RPM (Elect.) (Trip) 11) Cond. Vac very low - 542 mm of Hg(T) - 608 mm (A)
12) Control oil pr. low < 5.0 k Sc. (A), < 2.0 Ksc (T) 13) Lub. Oil pr. low < 1.0 Ksc (A), trip 0.3KSc(T) 14) Shaft vibration high 125 µM (A) , trip 250 (T)
15) LPT exhaust temp. high 90°C (A), 117°C (T/S),115°C(G/S) trip. 16) M.S. temp. high - 545°C (A), 565°C (T),
17) M.S. temp. low - 440°C (A), 430.8°C (T)
18) Turbine I/L pr. low - 63.5 KSC (A), 60 KSC (T) , when Turbine load > 40% 19) HPT diff. Exp. + 4.75 - 3.02 (A)
+ 5.69 - 3.96 (T) 20) IPT DE + 7.6 - 2.0 (A)
+ 8.4 - 2.9 (T) 21) LPT DE + 18.5 - 3.0 (A) + 19.2 - 3.8 (T) 22) Axial Thrust + 0.51 - 0.53 (A) + 0.98 - 1.03 (T) 23) Tur electricity high> 76 µM (A)
48
ROUTINE CHECK UP OF TURBINE AND EMERGENCY OPERATION.
The following routine check-up should be done (for turbine area) from time to time at CRT and at least once local checking must be done in a shift.
TURBINE LUB OIL AREA:
At CRT checking’s Normal value Alarm value Trip value
I. MOP disch pr 9.0 Kg/cm²(i.e ksc
ii. Turbine lub oil pr 2.8 ksc 1.1 ksc(low alarm) 0.8ksc(V.Lo-Tur Trip) iii. Control oil pr 8.5 ksc 2.0 ksc (low alarm) 2.0ksc(V.lo-tur trip
Check Secondary oil pr at HPCVs and IPCVs.
Check AC AOP, AC EOP, DC EOP, AC JOP, DC JOP, Turning oil supply V/V, MOT Vapour extractor (stand-by), turbine drain v/v Master are in AUTO mode.
Lub oil temp before & after cooler 62ºC/45ºC & 55ºC (Hi at cooler O/L) iv) Lub oil temp control v/v position.
v) Brg drain oil temp 65ºC 75ºC (High)
vi) Brg metal temp : brg-1:100ºC (high) 120ºC(v.hi- Tur trip manually) brg-2: 90ºC ,, --- do---
brg-3,4 : 105ºC ,, ----do---
At LOCAL checkings-
1. Look for any leakage of oil, air, steam, water etc and hear for any abnormal sound and also check approach, cleanliness of equipments.
2. Check lock out switches of AC AOP, AC EOP, AC JOP, Turning oil supply v/v, stand-by equipments, drain v/vs are in released condition. All available equipments must be in lined-up
3. Condition.
4. check turbine lub oil local pr gauge ,DP across lub oil filter (normal value below0.5ksc& high at 0.9ksc)
5. check MOT level(Normal: 0.0cm,high/low value :+7.5cm/-7.5cm),MOT oil temp,MOP disch pr,tur lub oil temp &cooling water temp before /after tur lub oil cooler,cooling wtr control v/v position,stand-by cooler status(whether back charged or not etc )
6. Check all brg drain oil temp gauges.
7. DP across control oil filter (Normal: 0.2ksc,high:1.0ksc).
8. both JOPs are lined –up from MOT & tur lub oil disch hdr,DP across jacking oil filter (normal: 0.2ksc, high:0.5ksc)
TURBO-VISORY PARAMETERS:
At CRT checkings-
Sl.No. Description Normal value High value V. High & (Trip) value 1. Axial Shift -0.30mm ⍨0.5mm(high) ⍨1.0mm(tur-trip)
2. Differential expansion of HPT -1.1mm +4.5mm/-3.0 mm +5.5mm/-4.0mm 3. Differential expansion of IPT +3.7 mm +7.5 mm/ -2.0mm +8.5 mm / - 3.0 mm 4. Differential expansion of LPT +11.0 mm +18.5 mm / -2.0 mm +19.5 mm / -3.0mm 5. Diff temp bet top&
bottom of HPT
