What is steam turbine?

 

Intoduction-Steam turbine is mechanical device that convert kinetic energy from pressurised steam into useful mechanical work. It is generate rotary motion directly.

            Sir Charles Algemon Parson , a British engineer, patented modern steam turbine in 1884. The crude from of a steam turbine was made by Hero Alexandra during 1^st century AD, called Hero ball or Aeolipile .It was the first from of reaction steam turbine. This device was consisting of an airtight chamber(sphere) with bent or curved pipes, the sphere starts rotating.

Now days, large steam turbine are used to generate 100MW electricity. Interior of a turbine consists of several sets of blades. The blades fixed on the casing are called fixed blades and some blades are fixed on rotor shaft is called rotating or moving blades. Clearance between these two blades sets is very minimum. steam is allowed to expand in these blades.

Fixed blades convert the potential energy of the steam into kinetic energy and directs the flow to the moving blades . Moving blades convert this kinetic energy into force and rotate the turbine shaft. Steam turbine are mostly axial flow type it means steam flow over the blades in the direction parallel to axis of rotor.                                 

Steam turbine blade- turbine blades classified in two type.

·      Moving blades

·      Fixed blades( daiphragm)

Moving blades-we are discussed in earlier section those blades are fixed on rotor shaft it is called moving blades and This blades are convert kinetic energy into force. In each stage of the turbine, there are moving and fixed blades. At each stage the pressure of steam are drop .

       A blades has to withstand higher pressure and temperature at the inlet stage. The pressure and temperature of steam decrease gradually. Toward the last stage of blade, the steam becomes wet and its volume become more. So, the blade size at the exit stage is large and it has to withstand erosion.

v Properties of a good blades- These are some properties are available in a turbine blades.

·      Good tensile and fatigue.

·      Toughness and ductility at working temperature.

·      Resistance to corrosion and erosion.

·      Good machinability.

·      Low ductility.

·      Good vibration damping property.

 Fixed blades(Diaphragm)- The partitions between pressure stage in a turbine’s casing are called diaphragm. They hold the vane-shaped fixed blades which facilitate the expansion of steam and guide it to flow over the subsequent moving blade row. One-half of diaphragm is fitted to the top casing and second half diaphragm is fitted into the button casing.

         Diaphragm is comprises of following three main section

·      Outer ring

·      Blade ring

·      Inner ring or web