SELECTION OF TURBINE MATERIALSFACTORS GOVERNING SELECTION OF MATERIALS• Creep resistance • Strength at high temperature• Erosion resistance• Working at high temperature

MATERIAL RESPONSIBLE FOR STRENGTH• Nickel increases strength & erosion resistance.• Chromium increases resistance to corrosion & erosion• Molybdenum increases strength at high temperatures• Vanadium increases strength and fatigue resistance.

• Manganese provides hardness.

MATERIALS USED FOR STEAM TURBINES• H.P. casing 3% Molybdenum cast steel or

0.5% Molybdenum, 0.3% Vanadium cast steel.• L.P. casing Cast steel or mild steel• Rotors Forged chrome – molybdenum steel or 0.5% molybdenum steel• Blading 0.12% carbon, 12% chrome, 1% nickel stainless

ironor 12% chrome stainless steel or12% chrome, 36% nickel steel

• Nozzles Stainless iron or 28% copper monel metal• Glands Cupro nickel or leaded nickel bronze (65% copper)• Diaphragms Mild steel or 0.5% molybdenum steel• Gears / pinions Forged steel & rims case hardened with

0.15~0.4% carbon, 0.2~0.3% silicon, 0.6~1.5% manganese and 0.15~0.3% molybdenum

METHODS OF BLADES FIXING• Turbine blades are made of corrosion

resistance steel with integral root and integral tenon for the shroud.

• Blades may have various types of root fastenings which include serrated root, straddle T , inverted T, fir tree or straddle fir tree, multi root etc. Blades may be mounted in segments or individually.

• In case of segmental blading, 8~12 blades are assembled together with distance pieces and shroud before fitting them on the rotor. Number of blades are determined as per vibration consideration.

AXIAL ENTRY BLADING• In case of integral blading, each

blade is an entity, it has it’s own root and are fitted singly.

• The blades are secured in the grooves, machined in the rotor to complement the shape of the root.

• Due to high centrifugal forces, root design is very complex.

• The tips of the adjacent blades are usually attached by a shroud in groups of 6~8 to provide stiffness to blade against vibration.

• Circumferentially mounted blades are inserted through gateway into machined groves on the wheel. Closing blade is fitted into gate is riveted into position by an axial pin through root and rotor.

TURBINE ROTORS• Turbine rotors may be solid,

hollow, built up or gashed. Gashed rotors are mainly used in modern turbines although some IP turbines utilize built up construction.

• For built up rotor, the shaft is stepped down from middle on either side and wheels are shrunk on with keys.

• Gashed rotors are solid, allow expansion of rotor and casing and maintain closer axial clearance.

DOUBLE CASING ARRANGEMENT

DOUBLE CASING ARRANGEMENT (CONTINUED)• Use of high temperature and pressure steam causes problems in design,

this arrangement is a solution to these problems.

• The outer casing has to be thick to resist high pressures and acts as main support girder for turbine rotor. The inner casing is made to expand freely and has ability to absorb thermal stresses due to large expansion caused by the high temperature steam.

• The inner casing is fully supported by the outer casing which acts as a support cradle.

• The outer casing is subjected to exhaust steam pressure and temperature and maintain concentricity and alignment irrespective of expansions.

• Exhaust steam between the casings acts as a steam jacket and reduces heat losses from the surfaces which improve thermal efficiency.

DIAPHRAGMS

DIAPHRAGMS (CONTINUED)• Diaphragms provide separations between the discs and hold

either nozzles or fixed blades.

• Allowance for expansion must be provided for diaphragms such that they also maintain the shaft gland clearance to a minimum.

• Lower half of diaphragm is supported in lower casing by two lugs

resting in casing and recessed in diaphragm. Upper half is supported by two plates slotting into upper joint and recessed into diaphragm. Centering being done by radial keys.

• Present day diaphragms are of all welded construction. Nozzles are positioned by close fitting bands which are then welded together to form a continuous structure.

GLAND SEALING ARRANGEMENT

GLAND SEALING ARRANGEMENT (CONTINUED)• In order to prevent steam leakage, most steam turbines use labyrinth packing

where the shaft is coming out from casing.

• The inward pointing circumferential ridges on the packing rings are not making a contact with shaft but are separated by a small clearance.

• Thus the packing does not completely stop leakage but limits it to tolerable amount.

• As steam is forced out of the casing under pressure, each ridge produces an

obstacle to the flow of steam, thereby dropping it’s pressure available for subsequent ridge.

• In large turbines , leak off steam along-with air is led to recovery condenser.

• A single ring of labyrinth packing is used as inter-stage packing in the diaphragm of impulse turbines.

GEARING IN MARINE USE• Gearing is used on marine turbines for combining the power of

HP, IP and LP turbines and to transmit power to propeller at low revolutions.

• Turbines operate at high RPM at which propeller efficiency is low. In order to improve propeller efficiency, RPM of output shaft is maintained between 90~150 by reduction gearing.

• Helical spur gears are used which offer better distribution of load, quite and smooth operation. Single helical gear introduce axial thrust but double helical gearing overcomes this difficulty by canceling the axial thrust on each other.

• Various arrangements of gearing are used in marine field.

EPICYCLIC GEARING• In parallel shaft gears, the axis are

fixed whereas in the epicyclic at least one axis moves relative to another fixed axis.

• An epicyclic gear consists of a sun wheel on the central axis, a internally toothed ring called the annulus, planet or star wheel carrier and planets or star wheels which revolve on spindles attached to the carrier.

• This arrangement can give speed reductions of 3:1 to 12:1 depending upon relative gear wheel dimensions.

• Main difference between parallel shaft gears and epicyclic is of the axis whether fixed or movable.

DOUBLE REDUCTION GEARING• Double reduction gearing is commonly used

where turbine speeds go upto 7000 RPM.

• Articulated gearing allows easier removal of individual trains and gives greater flexibility in carrying out alignment.

• Interleaved design is an alternative arrangement where the primary wheels straddle the secondary wheels and this design is more compact. A typical double reduction gear could give speed ratio of 66:1.

• Where large slow turning propeller is used, triple reduction gearing would be employed for turbine speed of 6000 RPM or more.