SOMETHING ABOUT TURBINE

EVOLUTION OF STEAM TURBINE:

•          Concept of steam turbine was developed in 120BC.

•          Boiler-Turbine concept was developed in 1629.

•          Practical steam turbine was invented in 1889.

WORKING PRINCIPLE:

When steam is allowed to expand through a narrow orifice, its heat energy (enthalpy) is converted to the kinetic energy. This kinetic energy is converted to rotational energy through the impact or reaction of the steam on the blades.

As the steam moves over the blades its direction changes continuously & centrifugal pressure exerted on the blades. This motive force is combination of centrifugal force & change of moment and its direction is always normal to the blade surface.

TURBINE TYPES:

1.       Impulse Turbine

2.       Reaction Turbine

COMPOUNDING:

·         Velocity of steam is directly proportional to the square root of the heat drop in the nozzle or fixed blade.

·         So complete expansion in one stage steam velocity touches the supersonic speed.

·         To avoid this concept of compounding is adopted.

• As the pressure drops the specific volume of the steam increases. To accommodate this huge volume of steam the blade height gradually increases from one stage to the next stage.
• As there is a pressure drop in each stage, so this force the steam to pass through the clearances of fixed & moving blades. Thus the inter stage sealing is needed to improve the turbine efficiency.
• The pressure drop across the stages creates an axial thrust on the rotor towards low pressure side. This thrust is counter balanced by providing a balance piston & thrust bearing.

DESIGN CONSIDERATION OF TURBINE:

·         To over come these facts generally a combination of both the types are considered in designing a turbine.

·         Generally in high pressure side impulse stages were kept as leakage loss is less & subsequently the percentage of reaction increases towards low pressure side. In low pressure turbine complete reaction stages were adopted. 

CLASSIFICATION OF TURBINE:

1. According to the direction of flow:

            a) Axial flow turbine

            b) Radial flow turbine

            c) Single flow

            d) Double flow

2. According to the no. of cylinders:

            a) Single cylinder

            b) Double cylinder

            c) Three cylinder

            d) Four cylinder

3. According to the method of governing:

            a) Throttle governing

            b) Nozzle governing

            c) Bypass governing

4. According to the principle of action:

            a) Impulse

            b) Reaction

5. According to the heat balance arrangements:

            a) Condensing with regeneration type

            b) Condensing type

            c) Back pressure type

6. According to the steam condition:

            a) Low pressure turbine (1 to 4ksc)

            b) High pressure turbine (4 to 40ksc)

            c) Intermediate pressure turbine ( >40ksc)

7. According to the shaft arrangement:

            a) Tandem compound

            b) Cross compound

8. According to the extraction type:

            a) Automatic extraction type

            b) Non-automatic type

TURBINE LOSSES:

·         External losses & Internal losses

·         External losses are bearing frictional losses & are same for all turbines.

·         Internal losses:

                a) Frictional loss due to fluid flow

                b) Leakage loss due inter stage passing

                c) Leaving loss due to speed of steam at exit from blade

·         Friction loss is minimised by providing smooth curved & aerofoil shaped blades.

·         Leakages losses are minimised by providing inter stage sealing arrangements in the blade tips.

·         Leaving loss is proportional to the square of the velocity & minimised by reducing the velocity. The reduction in velocity is achieved by increasing the   blade height, so the annular space increases towards the last stages