What is a Turbine ?

-A Turbine is a device which converts the heat energy of steam into the kinetic energy & then to rotational energy.

-The Motive Power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate.

-The basic cycle for the steam turbine power plant is the Rankine cycle. The modern Power plant uses the rankine cycle modified to include superheating, regenerative feed water heating & reheating.

RANKINE CYCLE

PROCESS A-B: PROCESS B-C: PROCESS C-D: PROCESS D-E: PROCESS E-A: RANKINE CYCLE D' C B WORK DONE A HEAT REJECTED ISENTROPIC/ADIABATIC COMPRESSION PROCESS FEED WATER TO BOILER IS PRESSURIED TO BOILER CONSTANT PRESSURE PROCESS FEED WATER HEATED UPTO SATURATION TEMP T1 CALLED SENSIBLE HEATING POINT C IS INTERMEDIATE POINT OF STEAM GENERATION CONSTANT PRESSURE & TEMPERATURE PROCESS FEED WATER IS VAPOURISED CALLED LANTENT HEAT OF VAPOURISATION POINT D IS STEAM IS DRY & SATURATED.

ENTROPY, S ISENTROPIC/ADIABATIC EXPANSION PROCESS EXPANSION OF STEAM TO VACCUM CONSTANT PRESSURE & TEMPERATURE PROCESS REJECTION OF HEAT TO CONDENSOR TO CONDENSE THE STEAM.

AT POINT D, STEAM IS DRY & SATURATED.

D E E'

B A RANKINE CYCLE (Reheat Cycle) E F C D WORK DONE G HEAT REJECTED ENTROPY, S

Impulse & Reaction Turbine

1.

Based on Blading Design

a) Impulse turbine

There is no pressure drop across moving blades. Steam energy is transferred to the rotor entirely by the steam jets striking the moving blades. Since there is no pressure drop, negligible thrust is

produced.

b. Reaction turbine

Steam expands in both the stationary & moving blades. Moving blades also act as nozzles. High axial thrust is produced.

c) Combination of Impulse & Reaction turbine

IMPULSE TURBINE

a

IMPULSE REACTION TURBINE

DIFFERENCES

Impulse & Reaction Turbine

On the Principle of working 1.

2.

Impulse Turbine Reaction Turbine Impulse Reaction Pressure drops in nozzles and not in moving blade -Constant blade channel area -Profile type blades -Restricted round or incomplete admission of steam -Diaphragm contains nozzles -Occupies less space for same power -Higher efficiency in initial stage -Suitable for small power requirements -Blade manufacturing is not difficult -Velocity of steam is high -Pressure drops in fixed blade as well as in moving blades -Varying blade channel area -Aerofoil type blades -All round or complete admission -Fixed blades similar to moving blades attached to casing serve as nozzles and guide the steam -Occupies more space for same power - higher efficiency in final stages.

-Suitable for medium or high power requirements.

-Blade manufacturing process is difficult.

-Velocity of steam is less.

Turbines Classification

Based on Inlet & Outlet Steam Condition

• Back pressure turbines –

The Exhaust steam from the turbine flows out of the steam piping at medium or low pressure. Basically, the exhaust steam can be used effectively in any other machines or equipment in the plant.

• Condensing turbines –

Full steam quantities entering into the turbine are exhausted, and converted to condensate in a condenser. The exhaust steam pressure is lower than the atmospheric pressure.

• Extraction turbines –

Medium or low pressure steam required by the process plant is extracted from the intermediate stage of a condensing or back pressure turbine.

CONSTRUCTIONAL FEATURES

•

CASING :

MADE OF CAST STEEL EXCEPT CONDENSING STAGE WHICH IS MADE OF CAST IRON. IT IS MOUNTED ON THE FRONT-END BEARING PEDESTAL. EXPANSION OF CASING IS TOWARDS FRONT END.

•

ROTOR :

MACHINED FROM A FORGED BLANK OF ALLOY STEEL. ROTOR IS A SINGLE FORGING INCORPORATING THE THRUST BEARING COLLARS. IT IS SUPPORTED ON TWO PRESSURE LUBRICATED JOURNAL BEARINGS. EXPANSION OF ROTOR IS TOWARDS REAR END.

•

CONTROL STAGE :

CONSISTS OF MOVING BLADES & NOZZLES. NOZZLES ARE MACHINED FROM SOLID BLANKS.

THE MOVING BLADES ARE MACHINED FROM SOLID BAR STOCK & HAVE INVERTED TEE ROOTS. TEE ROOTS ARE INSERTED INTO GROVES IN THE TURBINE ROTOR & CAULKED WITH BRASS STRIP.

1. External Losses

ESV & strainer losses

LOSSES

Governing losses (throttling losses) Leaving Energy Losses (Latent heat of exhaust steam in condenser) Radiation Loss to the surroundings

2. Internal Losses a) Blade losses

i) Primary Losses: Friction loss due to profile surface finish ii) Secondary Losses: - Impingement loss

Losses in Turbine (contd.) b) Inter stage Tip Leakages:

Steam throttles in the inter stage seals without doing work

c) Residual velocity Losses:

Kinetic energy of the leaving steam of one stage will be carried over to the next stage. As the axial clearances increase between the stages (or stage groups) part of the kinetic energy will be lost.

CROSS-SECTIONAL DRAWING OF BLADED ROTOR H.P.BLADES

INTER STAGE SEALING

Blade Profiles

• Impulse A A SECTION A-A (SIMPLIFIED DIAGRAM) • Reaction A • Twisted Blade B SECTION A-B ( SIMPLIFIED DIAGRAM ) A A18 A13 A10 A7 A4 A1 Y A18 A SECTION A16-A16 U A A4 A1 A13 A SECTION A13-A13 A A10 A7 A SECTION A7-A7 A SECTION A3-A3 A

DEFINITION OF GOVERNING SYSTEM

“A SYSTEM WHOSE PURPOSE IS TO CONTROL A PRIME MOVER” The function of governor is to control the speed of the turbine.It does this by controlling the flow of steam to the nozzles.When the governor reacts to speed, it controls the steam flow and steam flow by defination is power. Therefore, through speed governing shaft output speed is governed with variable power output.

Types of Governor

1. Mechanical Governor 2. Hydraulic Governor 3. Combination of mechanical & Hydraulic 4. Pneumatic Governor 5. Electronic Governor

Protection Requirements

• Over speed • Low Lube oil pressure • Low vacuum • Axial shift • High Vibration • Bearing temperature • High/low extraction pressure • Exhaust temperature • Generator / Compressor protections • Manual / Remote trip

EMERGENCY STOP VALVE

STOP VALVE

DRAIN TRIP OIL START-UP OIL CYLINDER WITH TEST PISTON TESTER-OIL

GOVERNING VALVE ASSEMBLY WITH ACTUATOR

HP GOVERNING VALVE ASSEMBLY

YOKE ASSEMBLY

SERVOMOTOR

PILOT VALVE

MECHANICAL TRIPPING DEVICE (VERTICAL MOUNTING) MECHANICAL OVER SPEED GOVERNOR