What is a steam turbine?

A steam turbine is a heat engine, which enables the pressure and temperature energy of steam to be transformed into useful work. This transformation is achieved by expanding the steam through nozzles giving a high kinetic energy, which is converted into useful work by impinging on moving blades (changing the direction of the steam) mounted on the rotor. In order to convert large amounts of energy into useful work with maximum efficiency, it may be necessary to expand the pressure energy of the steam in a series of steps (pressure compounding). Each such step contains a set of nozzles and moving blades and is referred to as a stage. Turbine contains two or more stages are known as a multistage turbine. The steam enters at one end of the turbine, which is known as the Steam End and flows axially relative to the rotor to the exhaust end. It then enters in to a water cooled condenser, where it is condensed and returned to the boiler as feed water.

For maximum efficiency, a high speed is required for the turbine rotor, whereas a relatively low speed is usually required for the machine to be driven. In such case, it is necessary to use a Speed Reduction Gear Box between the turbine and the driven machine. A frame or baseplate with pedestals equipped with bearings is necessary to support the rotor. A casing or cylinder surrounds the rotor to contain the steam. This cylinder is also supported from the baseplate and is concentric with the rotor. It is divided along its length by plates containing the nozzles. These plates are referred to as Diaphragms. The diaphragms are held at their periphery in the casing to prevent them rotating. They are sealed at their inner diameters (where the rotor passes through) by a series of fins set close to the rotor.

These fins are termed Labyrinth packings and form the interstage seals, which controls the leakage of steam between stages. Similar labyrinth packings are necessary, where the rotor shaft passes through the ends of the cylinder in order to prevent steam leakage to atmosphere. To accommodate differential expansion (the relative growth of components subjected to differing temperatures) and also to maintain correct meshing of the gear teeth under all operating conditions, a flexible coupling is provided between the turbine rotor and the gear box pinion (input) shaft. Certain accessories are necessary for the control and continuous operation of the turbine. These include:-
a. Speed Governor – to control the amount of steam being fed to the turbine to suit the load and thus to maintain constant turbine speed and constant electrical frequency when the driven machine is an alternator.
b. Lubrication System.
c. Control System
d. Safety and Protection Devices.
e. Gland Condenser – for steam leak-offs from control valve spindle and labyrinth glands. This improves turbine room habitability and helps conserve feed water, which would otherwise be lost by direct steam leakage.
f. Steam Condenser – to condense the exhaust steam from the turbine. Certain auxiliaries are required on the condenser. … Air Ejectors to extract air entrained in the steam entering the condenser and thus maintain the vacuum. … Extraction Pump – for extract the condensate from the condenser.

Depending on steam conditions, power and speed, the materials used in turbine construction may vary. Always consult the turbine name plate on the turbine before connecting it to a steam inlet or exhaust supply, to ensure that the turbine is rated for the prospective conditions.