SIMPLE, SINGLE STAGE STEAM TURBINE
SELECTION OF TURBINE MATERIALSFACTORS GOVERNING SELECTION OF MATERIALSCreep resistance Strength at high temperatureErosion resistanceWorking at high temperature
MATERIAL RESPONSIBLE FOR STRENGTHNickel increases strength & erosion resistance.Chromium increases resistance to corrosion & erosionMolybdenum increases strength at high temperaturesVanadium increases strength and fatigue resistance.Manganese provides hardness.
MATERIALS USED FOR STEAM TURBINESH.P. casing 3% Molybdenum cast steel or 0.5% Molybdenum, 0.3% Vanadium cast steel.L.P. casingCast steel or mild steelRotorsForged chrome molybdenum steel or 0.5% molybdenum steelBlading0.12% carbon, 12% chrome, 1% nickel stainless ironor 12% chrome stainless steel or12% chrome, 36% nickel steelNozzlesStainless iron or 28% copper monel metalGlandsCupro nickel or leaded nickel bronze (65% copper)DiaphragmsMild steel or 0.5% molybdenum steelGears / 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 FIXINGTurbine 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 BLADINGIn case of integral blading, each blade is an entity, it has its 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.
4TURBINE ROTORSTurbine 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
6DOUBLE 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
10GLAND 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 its 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 USEGearing 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 GEARINGIn 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 GEARINGDouble 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.