28 September 2022

TURBINE AT A GLANCE

TURBINE AT A GLANCE

1.) WORKING OF STEAM TURBINE

A steam turbine works on the principle of conversion of High pressure & temperature steam into high Kinetic energy, thereby giving torque to a moving rotor.

For above energy conversion there is requirement of converging /Converging-Diverging Sections

Such above requirement is built up in the space between two consecutive blades of fixed and moving blades rows.

2.) TYPE OF TURBINE

IMPULSE TURBINE =   In a stage of Impulse turbine the pressure/Enthalpy drop takes place only in Fixed blades and not in the moving blades.

REACTION TURBINE  =  In a stage of Reaction Turbine the Pressure/enthalpy drop takes place in both the fixed and moving blades.

 

3.) DEGREE OF REACTION=

               ( Heat drop in Moving stage)

( Heat drop in moving blade + Heat drop in fixed blade)

In impulse stage ,degree of reaction is  O

Single stage impulse turbine is called as De-laval Turbine.

Series of impulse stages is called as Rateau Turbine

Double Stage Velocity Compounded impulse turbine is called as Curtis Stages

50% Reaction turbine is called as Parson Turbine

Practically the degree of reaction of a stage can   be 0  60%  over the  different stages of a turbine

 

Velocity Compounded Turbines   Here the High temperature, Pressure Steam is expanded in a single row of fixed blades into very high velocity which is then fed to 2 or 3 rows of moving blades with one each guide/turning row placed  in between the two moving stages.

Pressure compounding Turbines Here the pressure is dropped in stages and employs low velocity of Steam in each stage. Each stage consists of Fixed blade( nozzles)  and moving blades.

 

4.) LOSSES IN STEAM TURBINE

Friction losses

Leakage losses

Windage loss( More in Rotors having Discs)

Exit Velocity loss

Incidence and Exit loss

Secondary loss

Loss due to wetness

Loss at the Bearings (appx 0.3% of total output)

Off design losses

MAIN LOSSES IN TURBINE

FRICTION LOSS  It is more in Impulse turbines  than Reaction Turbines,because impulse turbines uses high velocity of steam and further the flow in the moving blades of the Reaction turbines is accelerating which leads to better and smooth flow(Turbulent flow gets converted to Laminar flow)

LEAKAGE,S LOSS It is more in Reaction turbines  than Impulse turbines because there is Pressure difference across the moving stage of reaction turbines which leads to the Leakages. In Impulse turbine such condition is not there.

Leakage loss predominates over friction losses in the High Pressure end of the Turbine

Friction Losses predominates over the Leakage’s Loss in the Low Pressure end of the Turbine.

It is observed that the Efficiency of The IP Turbine is the maximum followed by The HP and LP Turbine .

LMZ Turbines are more impulse in nature

KWU Turbines are more reactive in nature

Sparing Rateau and Curtis stages, all other stages of  turbine is a mixture of Impulse and Reaction with varying degree of reaction.

Pressure/Enthalpy drop is more in Impulse stage than in reaction.

Comparatively Reaction Blade is more efficient than the Impulse blade.

Impulse turbine requires fewer no. of stages than reaction turbine for same condition of steam and power requirement.

MODIFICATIONS IN THE BLADESIN TERMS OF EFFICIENCY

  T2       It is comparatively flat blade with Thinner inlet

  T4       It is More Curved and thicker inlet

  Tx       More Curved, thicker inlet but  thinner   outlet

  3DS    Three Dimensional with reduction in  secondary losses

 

LUBRICATION

    Turbine bearings are hydro dynamically lubricated. For this to happen following things are important

  1. Viscosity of oil which is directly related to the oil Temperature.

  2. Rotation/speed of the Rotor.

  3. Desired Clearances/Converging Wedge in the Bearings. (convergence should be in the direction of rotation)

    In fact the Pressure of the Lube oil is mainly just to ensure that oil reaches the Bearing.  However it is also very important and requires to be maintained as per design.

LUBRICATION

As rotor rotates at low speed ,initially there is no film lubrication  but as its speed increases there is conversion of boundary layer lubrication into Film lubrication.

From zero speed to appx. 540 RPM there is no continuos film between rotor and bearings and there is chance of rubbing between rotor and Bearing. Therefore JOP is used to prevent the contact between rotor and bearings.

At Above 540 RPM the JOP can be Switched off, as film lubrication comes into picture.

It is to be noted that when the surfaces are parallel the volume flow rate  at inlet  is less than the outlet flow rate and film can,t sustain.

Therefore for a stable film, area needs to be convergent to ensure equal volume flow throughout the length.

The minimum clearance depends upon following

– Viscosity of oil

– Speed of Rotor

– Load on the rotor

TYPE OF BEARINGS

Cylindrical Bearing( Single wedge ring) This has single oil inlet

Elliptical Bearing( Double wedge bearing) This has double oil inlet

Segment Bearing( Multi  wedge Bearing)

BEARINGS

Cylindrical bearings are normally used  for system where no transients are envisaged particularly in turbines without controlling stage, whereby one side radial impulse due to steam forces is not there.

Multi wedge bearings are  used by installing bearings in segments . Each Segment will have its own wedge.

Multi wedge  bearings can take more load ,can dampen the sudden disturbance on shaft and there is no formation of Oil Whirl and low frequency vibration components.

BEARINGS

Acceptance of  Dye Penetrant testing ,DPT & Ultrasonic testing, UT results should be strictly as per BHEL (Hyderabad) AA 0850126 standard which is available on COS website.

Always carry out DPT test before UT Test

If any old bearing fails then replace with New/ Rebbabited bearings (both the halves needs to be replaced)

If New / Rebbabited bearing is used then ensure blue matching of pads with respect to their female seats.

Blue matching of pads of old good or New/ rebaabbited bearing with pedestal should be more than  > 90 % with all the sides touching all around .

Bearing should not be touched with hand  S.O.C and T.O.C  of bearing should be made on machine only. In no case babbit to be removed  manually more than  0.1 mm ( across dia)

Bedding should be 30-35 degrees all around on both sides.

   -Less bedding will lead to more oil leakages from the side of the bearing

  – Extra bedding will lead to more friction loss.

No of shims in each pad should not be more than 4 nos. If it is more , substitute with suitable equivalent sheet of stainless steel only.

The difference in the values of SOC and TOC  when measured from two sides (front and  rear) of bearing should not be more than 0.05 mm.

Thumb Rule : In any bearing the side oil clearance on one side will be appx. equal to  D/500 mm, where D is the rotor Dia in mm.

OVERSHOT OIL GROOVE IS VERY-2                                                           

   IMPORTANT.  ALWAYS CHECK FOR IT

While measuring S.O.C ensure that the feeler should go into the gap neither less and nor more than appx. 25 mm.

Never  Exchange the bearing shell of thrust bearing with the yoke of other bearing and viceversa. Always use their  Original set in pair.

BEARINGS AND TOROUS/PEDESTAL CONTACT

Three types of contacts are there

  1. Cylindrical-cylindrical

  2. Spherical- spherical

  3. Spherical -Cylindrical

Cylindrical-cylindrical contact can be obtained by the manual scrapping, but Spherical- spherical and Cylindrical- Spherical should be brought with the machining only. In Urgency spherical-spherical can be rectified at site provided correction is very small.

INTERFERENCE OF THE BEARINGS WITH THE PEDESTAL

Interference is kept to ensure proper loading on the Bearing Top  to ensure its proper clamping which helps in the damping of the bearing Vibration

It is in the  limit 0f  0.15 to 0.35 mm

This check is carried out after the completion of the TOC checking of the bearing.

ALIGNMENT

  • Alignment readings are taken at four positions at 12,3,6,9,12  O clock position  by rotating the couplings together

Readings for Radial   Alignment is taken by Dial Gauges at four locations.

Readings for axial alignment ie the gap between the couplings is taken by the means of the slip Gauges at four location of couplings ie L,R,T,B  each at four position of clock. Axial reading is averaged for four position of the Coupling locations

ALIGNMENT

  1. Always Ensure Bedding of the rotor with the bearing.

  2. During alignment process for IP-LP and LP-GEN coupling , condenser is to be kept on springs and water filled upto the tubenest level. During the final correction/ checking place LPT top half in position and tighten intermittently

  3. Always take readings for two sets.

  4. Best compromise between alignment and slope is to be achieved with priority for alignment.

  5. Always refer to the  Dismantling  readings of Alignment , Seal Bore and Slope  during the process of alignment.

ALIGNMENT

  Alignment Deemed to be over

If  the readings obtained are as per design values

Readings obtained in two consecutive sets are appx. Identical

 Check whether readings of  L+R = T+B . If difference is there then readings are not proper.

Ensure the final radial position of couplings is same as of set datum  initially. If it is more than 0.01 mm then retake the reading.

SEAL BORE

Seal Bore readings are the Rotor Centering readings with respect to the Bearing Pedestals.

It is measured by means of inside micrometers from the oil guard Seat surface  to the surface of the rotor Journal. At each location it is measured at the three points, Left, Right, bottom

Seal bore readings are the reference readings for any correction and normally should not change.

But over a period of time due to change in foundation characteristics it can lead to change in seal bore readings.

CATENARY

When rotors are resting in free condition on their bearings, there will be individual sags in them due to its dead weight and  dimensions.

Due to above the mating coupling faces  of the rotors shall be at an angle. In this condition it is very difficult to couple the rotors.

To align the coupling faces the rotors are given vertical adjustment through bearing or pedestal adjustment so that they become parallel and coupling becomes easier. One rotor normally LP rotor is kept as MASTER.

From above adjustment the deflected lines of rotors  also approaches the Center line which reduces the Centrifugal forces on the rotor.

CATENARY /SLOPE ADJUSTMENT

Correction / Adjustment in the Slope is carried out by making changes in the Pads / Keys of  the bottom  half of the bearing  .

In New design it is carried out by Spring Deck Correction whereby the whole pedestal lift is affected by means of adding/ removing plates/Sheets in the spring deck foundation.

CATENARY /SLOPE

 Slope is measured by Microlevel. Before taking slope readings , MICROLEVEL should be calibrated. It should be calibrated on a Known surface like  Lathe machine Bed and then sealed  with lacquer to avoid any disturbance.

 After above slopes should be taken in the two direction(to minimize the any error of the level). Second one in 180 degree opposite to first position but at the same location . For this ensure the vertical level is in the center

 Final slope reading is the average of the two readings

 Slopes are measured by a micro level one division of which corresponds to slope of  0.1 mm per mtr length

 Slope to be taken in uncoupled position of rotors

CENTERING

Centering is the Radial clearances between the Diaphragms/Casing blades with the Rotor

In LMZ,SKODA,GE Turbines it is measured by means of False Rotor shaft, where by reading are measured at three position L,R,B at each stage of Diaphragms and Glands. Readings are measured by Inside micrometer.

Correction is affected by means of Keys in the Diaphragms/casing

CENTERING

  1. Before taking centering readings ensure radial play of diaphragm in Liner and gland boxes in no case should be more than 0.08 mm

  2. During centering reading push the components towards Left side

  3. No of shims packers below each  key  should not be more than 4. If it is more than it then replace it with suitable sheet of equivalent size of  SS

                       CENTERING

In KWU turbines  it is measured by actually lifting ,downing and moving left and right the inner casing w.r.t  rotor which is  rotated in the bearings.It is also called as ROLL CHECK .At the moment where  touching/Resistance is   observed  gives  value of  the clearance between casing and rotor. Inner casing movement is affected by  means of  Hydraulic Jacks and Screws. It is a faster means of Centering check

Correction is affected by means of changing Dimensions of keys of the Casing.

IMPORTANCE OF SEALS

Sealing fins reduces the leakage’s of steam through inter stage and turbine ends there by leading to high efficiency and better Heat Rate

They also result in balancing of the Thrust.

Sealing fins deemed to be reject/damaged

  1. When the desired clearances have increased

  2. When their Knife edge is lost or found bend

  3. When they are broken at the intermittent places

SEALINGS RINGS

During Overhauling ,Even if the Old  Seals

   are used, never use the old  Helical/ Flat Springs

Butt Clearances between sealing segments should be strictly as per Design values.

SEAL FIN  CUTTING

      It is checked by Tape Test

Gland Sealing segment Radial clearance are checked by a putting Suitable Medical Adhesive tape sealed on rotor and colored by blue color and then rotating the rotor by 360o. If the fins are not touching with tape then the clearance achieved is equal to the Thickness of the  tape.

Wherever rubbing in the tape is observed the corresponding row of segments are taken out , further grinded and rechecked sane as above to get the designed clearances.

SWING CHECK

Swing check is a measurement which shows the mating accuracy of the  coupling faces of the two rotors coupled together.

It is the measurement of the radial throw at the free end due to coupling face geometric form of the two rotors coupled together.

It is carried out by removing the Real bearing and putting a false bearing/Lifting tackle on which the rotor is supported. This false bearing has larger side oil clearances. Dial is put at the horizontal level and then rotors are rotated . Readings are taken at many positions, The max throw is the Swing of the rotor.

SWING CHECK

Readings are to be recorded after  elongation of all coupling bolts of  HP-IP and IP-LP coupling.

Allowable Swing value depends upon the diameter of coupling and length of rotor.

The final dial gauge reading at start position should be same as the Initial set  Dial gauge reading at start position

If the  swing check is on very high side then same is to be corrected by following method

     — By interchanging the coupling position

Improving facial run out

Honing and reaming & fixing with new coupling bolts.

ELEVATION TRANSFER

It is the measurement taken between the two mating parts which are themselves assembled in two halves.

This is taken to ensure proper fitting of the components  so that there is no fouling among themselves.

This measurement is taken by Height Gauge at the parting plane on both the sides.

BUMP CHECK

It is Maximum total axial  gap  of the steam path of the turbine Cylinders . Normally carried out for HP& IP Cylinders

It is measured by moving the entire rotors from the zero position on either side.( Zero position is Rotor collar on the working thrust pads in LMZ turbines and Rotor collar in the 50% float Position of the thrust Bearing in KWU Machines)

The Reading is taken by Dial Gauges

Total sum of the Left and right movement is the BUMP Value

BEARING FLOAT

It is the total axial movement of the Rotors between the working and the Non working Pads of the Thrust Bearing.

It is measured by moving the rotors from the extreme end of the working pads to the extreme end of the  Non working pads or vice versa Reading is measured by Dial Gauges .

Its value varies from 0.3 mm to 0.55 mm depending upon the turbine design

Value less than the design value leads to Lubrication problem

BEARING FLOAT

Value more than the design value can lead to accelerated wearing of the Babbit pads

HORN DROP TEST

By horn drop test the loading of the casing on each corner is determined. It is taken on HP and IP Outer Casing.

In a horn drop test a drop is measured on an individual corner of the casing with the help of dial indicator by removing the support of the individual corner and then it is compared with the opposite corner.

Procedure . During  the  horn drop  test  the individual  packer  of the  casing is  removed and  the  load  of  that  corner  is  supported  over the  jacking  screws  of  the  casing . Now gradually the jacking screw is

HORN DROP TEST relieved with the help of Hydraulic jacks on that corner and drop is recorded. This is repeated for each corner of the casing.

In case of variation in the left and right side reading the drop is adjusted by adjustment of the shims from left to right or vice versa . No addition/subtraction of shims is allowed from the outside.Correction is carried out till the equal loading is observed.

Above  Complete test  at four corners is again repeated after joining all the Pipe lines with the Casing

The variation in horn drop reading of left and right side of the casing may be permitted up to difference of 50%.

If the variation is much , then no correction is made by adjustment of casing packers. The variation at this stage is caused due to piping pull /push only and hence if any correction is required then the same is to be carried out in the piping joints and its Hangers support only. Sometimes it is necessary to cut the piping joint for the necessary correction.

ANCHOR POINT

Anything when heated will expand. Same is true for  Turbine rotors and Casings.

Problem with rotors is more complex as it is also subjected to the axial thrust also.

To allow their controlled motion during operation and to prevent any eventuality between rotor and casing they are required to be anchored

Rotors are anchored at Bearing no 2(between HP &IP) by means of thrust bearing. In some Turbines they are also anchored at free end

Thrust bearing( Anchor point) is always located near High temperature end to minimize the differential expansion.

ANCHOR POINTS

Casings which are connected together by means of pedestals and keys  have LPT front as the anchor point.

It is to be noted that rotor rests on the bearings and bearings further rests on the pedestals

The pedestals can be moving / sliding or can be fixed. When it is sliding it carries bearing along with it which further carries rotor along with it.

    When pedestals are  fixed the necessary casing expansion is taken by the bellows at the ends.

Total Expansion of Turbine can be calculated which is appx. Equal to LαΔT where

    L = Length of Turbine

    α = coefficient of thermal expansion

    ΔT= Difference of temperature.

DIFFERENTIAL EXPANSION

It is the difference between the rotor expansion and the Casing Expansion

It  is +ve if rotor expands more than casing

It is -ve if casing expands more than rotor

During initial startup diff Exp. is +ve as rotor mass is less and expands at more faster stage than casing. After full load this gap reduces.

Diff. Expansion pick up is mounted at the farthest point  from the anchor point to record the max difference.

AXIAL SHIFT

Axial shift on the Rotors comes due to two components

Direct Pressure Thrust: It is the axial thrust due the diff of the pressure X  Area component across the moving stage blading and Discs

Velocity Component: It is due to diff. of the velocity X Mass component across the moving stage blading.

Total summation of all the above components leads to the Axial Thrust.

AXIAL SHIFT

Axial Thrust is +ve if it is towards generator

Axial Thrust is -ve if it is towards HPT front

There are many ways of balancing axial thrust

  1. Double flow self balanced cylinder

  2. Mutually balanced individual cylinders

  3. By putting balancing drum in the HP Module

  4. Having holes in the Discs of the blading

  5. Reverse flow in individual cylinder after partial Expansion.

  6. Optimizing Steam Extraction Locations

AXIAL SHIFT

  1. Sealing fins are also important component of balancing axial thrust

  The Residual thrust  is taken care by the Thrust pads.

 Tripping Limits of Axial shift

The Max. + ve  axial shift tripping value is just less than the minimum axial gap in the HP Casing which is in its first stage.

 The Max. – ve  axial shift tripping value is just   less than the minimum axial gap in the IP Casing which is in its first stage.

GOVERNING SYSTEM

To regulate the speed of the Turbine at various loads in a isolating set

To regulate the load as per the frequency demand when working in a grid network.

To prevent and protect the turbine from over speeding.

To allow  on line testing of various safety protections .

To enable the tripping of TG set in the event of actuation of protections.

Safe and healthy loading of the TG Set

TYPES OF GOVERNING-I

Nozzle Governing

Throttle governing

Bypass Governing

   Governing is effected by varying the amount of steam which is further done by changing the positions of control valves.

   Any Throttling of Steam  will lead to losses . In governing system along with the change of quantity of steam the quality of the steam also changes. Therefore Throttling should be minimum.

BYPASS GOVERNING

It is employed in small capacity turbines running on high pressure conditions and with small blading dimensions.

Here the loading up to appx  80% ( Economic loading ) is met by normal control valves feeding the First stage. For higher loading , to supply more steam which is not possible due to small blading dimensions ( can lead to operational problems) the extra quantity of the steam is fed to the intermediate section of turbine bypassing the initial high pressure stages.

TYPES OF GOVERNING-II

Constant Pressure Mode: Here pressure upstream of control valves is kept constant and change is made by changing the position of control valves.

Variable Pressure mode:Here control valves are in full open position and pressure upstream of  control valves varies proportionately with the load requirement.

Response of  Constant pressure Mode is much faster than Variable pressure mode, but Constant pressure leads to more losses.

TYPE OF GOVERNING -III

Mechanical: Transducer is mechanical centrifugal speed governor which actuates controls valves through mechanical linkages.

Hydro mechanical: Here transducer is a centrifugal speed governor .It is connected to a Hydraulic system where the signal is amplified so that Control valves servomotor can be actuated.

Hydraulic: Here speed transducer is a centrifugal pump whose discharge pressure is proportional to square of speed. This signal is sent to hydraulic converter which generates

    a signal which is proportional to valve opening/Closure is required. Before applying it to control valves servomotor this signal is suitably modified.

Electro Hydraulic:Here transducer can be electrical or Electronic. The generated signal after processed electronically and electrically is fed to a Electro- hydraulic converter which converts electrical signal into Hydraulic signal.Hydraulic signal before applying to control valve servomotor is suitably amplified.

REGULATION

Regulation % of Turbine is defined as

  (No load Speed – Full Load speed)  X 100  %

                 ( Nominal Speed)

It varies from 2.5% to 8%

Normally it is 4 to 5 %

Turbine having less regulation will be more sensitive in the grid and hence will share more load  and vice versa

Normally base load plant has high regulation and Peak load plant has small regulation.

VIBRATION

Its manifestation in the form of amplitude of Oscillation, indicate some problem with the equipment e.g. Fever in a body

The impact of high vibration  usually are loss of integrity of components and associated generation loss because of forced outage either as a preventive measure or consequence of any failure due to  high vibration/vice versa.

  CAUSES, TYPE OF PROBLEMS

 AND CORRECTIVE ACTIONS

INFORMATION REQUIRED TO DIAGNOSE THE PROBLEM

Frequency  components causing change  in amplitude of vibration (low frequency,1x, higher  frequency etc.)

Vector position (Phase and Amplitude) of 1x across different bearings in H,V and A directions, at different speeds.

Bode plots,Cascade,Waterfall,Orbit plots,time waveform

Process parameters  correlation with load,speed,vacuum, etc.

LATEST DEVELOPMENTS-ONLINE DIAGNOSTIC SYSTEMS

FEATURES

Bar Graph

Time waveform,Orbit

 Trend, Multi variable trend,  XY plots

Frequency spectrum,

Acceptance region

Bode,Water fall, Cascade

Shaft average Centreline plot

MCM/EXPERT SYSTEM WORKING

Leave a Reply

Your email address will not be published.