Steam Turbines 

• A steam turbine converts the energy of high-pressure, high temperature steam produced by a steam generator into shaft work.
• The energy conversion is brought about in the following ways

• The high-pressure, high-temperature steam first expands in the nozzles emanates as a high velocity fluid stream.
• The high velocity steam coming out of the nozzles impinges on the blades mounted on a wheel. The fluid stream suffers a loss of momentum while flowing past the blades that is absorbed by the rotating wheel entailing production of torque.
• The moving blades move as a result of the impulse of steam (caused by the change of momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words they also act as the nozzles.
• A steam turbine is basically an assembly of nozzles fixed to a stationary casing and rotating blades mounted on the wheels attached on a shaft in a row-wise manner.
• Steam turbines are employed as the prime movers together with the electric generators in thermal and nuclear power plants to produce electricity.
• Turbines can be condensing or non-condensing types depending on whether the back pressure is below or equal to the atmosphere pressure.

Types of Steam Turbines 

• Impulse Turbines 
• Reaction Turbines 
• Impulse-Reaction Turbines

Impulse Turbines 

• In this turbine expansion of the steam takes place in one set of nozzles
• As the steam flows through the nozzle its pressure falls from steam chest pressure to condenser pressure
• Due to this relatively higher ratio of expansion, the steam coming out at a very high velocity through the fixed nozzles impinges on the blades fixed on the periphery of a rotor.
• The blades change the direction of the steam flow without changing its pressure
• The resulting motive force (due to change in momentum) gives the rotation to the turbine shaft.
• It is evident that the velocity of the steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzles.
• The loss of energy due to this higher exit velocity is commonly “carry over loss” or “leaving loss”.

Reaction Turbines 

• In this type of turbine, there is a gradual pressure drop and takes place continuously over the fixed and moving blades
• The function of fixed blade is that they alter the direction of the steam as well as allow it to expand to a larger velocity
• As the steam passes over the moving blades its kinetic energy is absorbed by them
• The volume of steam increases at lower pressure therefore, the diameter of the turbine must increase after each group of blade rings
• It may be noted that in this turbine since the pressure drop per stage is small, therefore, the number of stages required is much higher than an impulse turbine of the same capacity.

Impulse-Reaction Turbines

• The steam expands both in fixed and moving blades continuously as the steam passes over them.
• Therefore the pressure drop occurs gradually and continuously over both moving and fixed blades.
• Example: Parson’s turbine

Impulse-Reaction Turbines vs Reaction Turbine

Impulse-Reaction Turbines

• In impulse turbine all hydraulic energy is converted into kinetic energy by a nozzle and it is the jet so produced which strikes the runner blades.

Reaction Turbine

• In reaction turbine only some amount of the available energy is converted into kinetic energy before the fluid enters the runner.

Impulse-Reaction Turbines

• The velocity of jet which changes, the pressure throughout remaining atmosphere.

Reaction Turbine

• Both pressure and velocity changes as fluid passes through a runner. Pressure at inlet is much higher than at outlet.

Impulse-Reaction Turbines

• Water-tight casing is not necessary. Casing has no hydraulic function to perform. It only serves to prevent splashing and guide water to the tail race.

Reaction Turbine

• The runner must be enclosed within a watertight casing.

Impulse-Reaction Turbines

• Water is admitted only in the form of jets. There may be one or more jets striking equal number of buckets simultaneously.

Reaction Turbine

• Water is admitted over the entire circumference of the runner.

Impulse-Reaction Turbines

• The turbine doesn’t run full and air has a free access to the bucket.

Reaction Turbine

• Water completely fills at the passages between the blades and while flowing between inlet and outlet sections does work on the blades.

Impulse-Reaction Turbines

• The turbine is always installed above the tail race and there is no draft tube used.

Reaction Turbine

• Reaction turbine are generally connected to the tail race through a draft tube which is a gradually expanding passage. It may be installed below or above the tail race.

Parts of Steam Turbines

• Rotor
• Casing/shell
• Bearings
• Shaft seals
• Steam control system
• Oil system

Rotor

• This is the main moving elements of a turbine
• In impulse turbine it is a shaft on which the wheels carrying the blades are mounted.
• The rotor of a reaction turbine is a drum
• It will be stepped or tapered in order to increase its diameter towards the lowpressure end
• The rotor is supported through bearings mounted on both sides of the turbine
• It is coupled tot the rotor of the alternator which is again supported through bearings mounted on both sides of the alternator
• The converted rotational energy of the turbine is transmitted to the alternator rotor which in turn, converts the mechanical rotational energy into electricity
• A simple single stage turbo-alternator will have at least four bearings supports

Casing/Shell

• Casing is the principal stationary element, often called the shell
• It surrounds the rotor and holds, internally any nozzles, blades and diaphragm that may be necessary to control the path and the physical state of the expanding steam
• The bearings, auxiliaries and steam lines are attached to the casing or are an integral part of it
• It is also shaped to become the main frame and support of the assembled turbine
• The casing is normally thermally insulated from outside to prevent radiation losses
• The casing/shell temperatures are measured at different locations for monitoring purposes

Bearings

• The main bearings of a single stage turbine are two in number (excluding alternator bearings which are additional in two number), placed outboard of the shaft seal
• Most journals run in plain bearings
• Some small turbines are ring-oiled from reservoirs, other follow large turbine practice with pressurized oiling system
• Thrust is carried by separate thrust bearings, plain or ball
• Where the large end thrusts are produced as in the case of reaction turbines, they are mainly neutralized by steam loaded balance plates of the rotor
• The amount of heat energy dissipated because of the friction in the bearings  depends mainly on the turbine load conditions
• This may result in increase of bearings temperature which can be controlled through lubrication of the bearings
• The bearings temperature measurement is also a way of detecting deterioration in bearings before complete mechanical failure
• Hence a lubrication system with pressure / flow, temperature and /or oil tank level controls become part of the bearings system

Shaft Seals

• Where the shaft emerges from the casing it needs sealing to prevent steam outflow at the high pressure end and air inflow at the vacuum end
• On small non-condensing turbines, this is accomplished by mechanical sealing rings; however these are not practical if the shaft diameter is large
• Labyrinth glands with steam or water sealing at the condenser end are employed on all large turbines
• Multistage internal turbines must also be internally sealed between the shaft and diaphragms

Steam Control System

• Flow of the steam through a turbine is usually regulated so as to produce constant rotative speed in the presence of variable power demand
• This is always the case where the turbine is used for electric generation
• Control is exercised by varying the quantity and pressure of the steam flowing through the turbine
• If quantity control could be had alone, it would be employed, but the turbine has fixed size nozzles and pressure control is the most practical method of varying quantity
• In large turbines, power is varied with minimum throttling by subdividing the first stage nozzles into groups which come into action in sequence as load is increased
• However, beyond the first stage the entire nozzle group is always in action, and pressure and quantity are variable when power is changes

Oil System

• Oil is required for lubricating the bearings
• Most turbines use the oil pressure system for both bearing lubrication and  governor servomechanism operation
• An integral oil pump, driven from the main shaft, provides the pressure oil relays and governor valve operating cylinders
• The same oil, when reduced somewhat in pressure, serves for circulating to the bearings
• An oil reservoir, oil filter and oil cooler are included in this system
• Sometimes a separately driven emergency oil pump is provided even though the fault were immediately detected and the emergency valve tripped
• This is because of the enormous store of energy in the massive rotor running at 1500 or 3000 rpm.