27 September 2022

Steam Turbine Components -Parts | Turbine Fundamentals and Basics | Basic Aspects of Steam Turbine Maintenance | Part#3 | Content-4 |

STEAM TURBINE COMPONENTS

Topics Covered:-
1.) Turbine casing (HP, IP and LP Turbine Casing)
2.) Anchor Points of Turbine
3.) Rotor (HP, IP and LP Turbine)
 4.) Blades
5.) Sealing system (Gland Seal, Steam System)
6.) Bearings & Bearing Pedestal 
7.) barring gear
8.) ESV’s, IV’s and CV’s
9.) Coupling 

1.) Turbine Casings

HP Turbine Casing
  • Outer casing: a barrel-type without axial or radial flange.
  • Barrel-type casing suitable for quick startup and loading.
  • The inner casing— cylindrical , axially split.
  • The inner casing is attached in the horizontal and vertical planes in the barrel casing so that it can freely expand radially in all directions and axially from a fixed point (HP-inlet side).
IP Turbine Casing
  • The casing of the IP turbine is split horizontally and is of double-shell construction.
  • Both are axially split and A double flow inner casing is supported in the outer casing and carries the guide blades.
  • Provides opposed double flow in the two blade sections and compensates axial thrust.
  • Steam after reheating enters the inner casing from Top & Bottom.
LP Turbine Casing
The LP turbine casing consists of a double flow unit and has a triple shell welded casing.
  • The shells are axially split and of rigid welded construction.
  • The inner shell taking the first rows of guide blades, is attached kinematically in the middle shell.
  • Independent of the outer shell, the middle shell, is supported at four points on longitudinal beams.
  • Steam admitted to the LP turbine from the IP turbine flows into the inner casing from both side

2.) Anchor Point of Turbine

Purpose: Taking care of thermal expansions and contractions of the machine during thermal cycling.
The fixed points of the turbine are as follows:
The bearing housing between the IP and LP turbines.
The rear bearing housing of the IP turbine.
The longitudinal beam of the I.P turbine.
The thrust bearing in rear bearing casing of H.P turbine.

3.) Rotors

HP Rotor:
  • The HP rotor is machined from a single Cr-Mo-V steel forging with integral discs.
  • In all the moving wheels, balancing holes are machined to reduce the pressure difference across them, which results in reduction of axial thrust.
  • First stage has integral shrouds while other rows have shroudings, rivetted to the blades are periphery.
IP Rotor
The IP rotor has seven discs integrally forged with rotor while last four discs are shrunk fit.
The shaft is made of high creep resisting Cr-Mo-V steel forging while the shrunk fit disc are machined from high strength nickel steel forgings.
Except the last two wheels, all other wheels have shrouding riveted at the tip of the blades. To adjust the frequency of the moving blades, lashing wires have been provided in some stages.
LP Rotor
The LP rotor consists of shrunk fit discs a shaft.
The shaft is a forging of Cr-Mo-V steel while the discs are of high strength nickel steel forgings.
Blades are secured to the respective discs by riveted fork root fastening.
In all the stages lashing wires are providing to adjust the frequency of blades. In the last two rows satellite strips are provided at the leading edges of the blades to protect them against wet steam erosion.

4.) BLADES

  • most costly element of turbine
  • blades fixed in stationary part are called guide blades/nozzles and those fitted in moving part are called rotating/working blades.
  • blades have three main parts  (1) Aerofoil: working part  (2) Root  (3) Shrouds
  • shroud are used to prevent steam leakage & to guide steam to next set of moving blades.
  • Three types of root arrangements are commonly used. They are (1) T-roots: for small blades; (2) Fir Tree or serrated roots – for longer blades; (3) Fork and Pin root: for longer blades shrunk on disc type rotors.
  • Integral shroud for are used for shorter blades and shrunk fitting for larger blades.
  • Lacing wires are also used to dampen the vibration and to match frequencies in the longer blades.
  • Since in the reaction type machine the pressure drop also occurs across the moving blades it is necessary to provide effective sealing at the blade tips.

 

5.) SEALING GLANDS

Steam is supplied to the sealing chamber at 1.03 to 1.05 Kg/sq.cm abs and at temperature 130 deg.C To 150 deg.C from the header.
Air steam mixture from the last sealing chamber is sucked out with the help of a special steam ejector to gland steam cooler
Provision has been made to supply live steam at the front sealing of H.P. and I.P. rotor to control the differential expansion, when rotor goes under contraction during a trip or sharp load reduction.

6.) BEARINGS:

Bearings are usually forced lubricated and have provision for admission of jacking oil
6.A) Front Bearing Pedestal :- The Front Bearing Pedestal is located at the turbine side end  of the turbine generator unit. Its function is to support the turbine casing and bear the turbine rotor. It houses the following components
a.Journal bearing
b.Hydraulic turning gear
c.Main oil pump with hydraulic speed transducer
d.Electric speed transducer
e.Over speed trip
f.Shaft vibration pick-up
g.Bearing pedestal vibration pick-up
  • The bearing pedestal is aligned to the foundation by means of hexagon head screws that are screwed in to it at several points.
  • The space beneath the bearing pedestal is filled with non shrinking grout.
  • The bearing pedestal is anchored at to the foundation by means of anchor bolts.
  • The anchor bolt holes are filled with gravel,it gives a vibration damping effect.
Bearing Pedestal(HP Rear)
The Bearing pedestal(2) is located between the HP and IP turbine.
Its function is to support the turbine casing and bear the HP IP rotor.
It houses the following components
1.Combine Journal and Thrust bearing
2.Shaft vibration pick-up
3.Bearing pedestal vibration pick-up
4.Thrust Bearing trip(electrical)
Combined Journal and Thrust Brg.
The magnitude and direction of axial thrust of the turbine depends on the load condition
The Journal bearing is elliptical sleeve bearing.
The bearing liners are provided with a machined babbit face.
Located at each end of bearing shell, babbitted thrust bad forms 2 annular surfaces.
These collars and thrust pads permit equal loading of thrust bearing.
Thrust pads are of tilting type.
Metal temperature of the journal bearing and thrust pads is monitored by the thermocouples.
IP Rear Bearing Pedestal
Arrangement:The bearing pedestal is located between the HP and IP turbines. Its function is to support the turbine casing and bear the HP and IP turbine rotors. The bearing pedestal houses the following turbine components:
  • Journal bearing
  • Shaft vibration pick-up
  • Bearing pedestal vibration pick-up
  • Hand barring arrangement
  • Differential expansion measurement device
Journal Bearing
The function of the journal brg.is to support the turbine rotor.
The journal brg. Consists of the upper & lower shells,bearing cap,Spherical block, spherical support and key.
The brg shell are provided with a babbit face.
Brg is pivot mounted on the spherical support to prevent the bending movement on the rotor.
A cap which fits in to the corresponding groove in the brg shell prevents vertical movement of the brg shell.
The brg shells are fixed laterally by key.
Each key is held in position in the brg pedestal by 2 lateral collar.
LP TURBINE
Rear Bearing Pedestal: The bearing pedestal is situated between the LP turbine and generator. Its function is to bear the LP rotor.
The bearing pedestal contains the following turbine components:
Journal bearing
Shaft vibration pick-up
Bearing pedestal vibration pick-up

7.) BARRING GEAR

The primary function of barring gear is rotate
the turbo generator rotors slowly and continuously
During startup and shutdown periods when changes
in rotor temperature occurs
> Shaft system is rotated by double row blade wheel which is
driven by oil provided by AOP
A manual barring gear is also provided with hydraulic gear
Barring speed 210/240 rpm

8.) ESV & CV

2 main stop valves and 2 control valves located symmetrically
The main steam is admitted through the main steam inlet passing first the main stop valves and then the control valves. From the control valves the steam passes to the turbine casing.
Turbine is equipped with emergency stop valve to cut of
steam supply with control valves regulating steam supply
ØEmergency stop valve are actuated by servo motor
controlled  by protection system
ØControl valves are actuated by governing system
through servo motors to regulate steam supply

9.) Couplings

Shaft is made in small parts due to forging limitation
and other technological and economic reason,so
coupling is required between any two rotors
Here using rigid coupling
Due to high torque flexible coupling can’t be used
Coupling between
HP&IP
IP&LP
LP&generator
GEN&exciter
MOP&HP
THNAK YOU

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