Chola Turbo Steam Turbines

Back pressure turbines can either be single stage or multi-stage which are often used in industrial plants. The turbine serves as a reducing station between boiler and process steam header. These turbines can either be used for drive application (sugar mill drive, sugar fibrizor / shredder drive, pump drives and so on.) or power generation application in which case the turbine drives the generator.

These turbines are straight-back pressure type. These type of turbines find application where back pressure steam is fully utilized to meet process demands. The power generation is incidental to the process steam demand.

The back pressure turbine may also have bleed points (uncontrolled extractions) to satisfy steam demands at intermediate pressures. This provision is applicable when the bleed (medium pressure) steam volume demand is low and pressure variations can be tolerated. These turbines are of bleed cum back pressure type.

The back pressure turbine with one controlled extraction point is possible. This extraction steam is also used to meet process steam demand at intermediate pressure when volume demand is high and pressure variations cannot be tolerated. These turbines are of extraction cum back pressure type.

The condensing turbines take high pressure steam, expand it in turbine nozzles and blades, and exhaust it to a condenser at lower than atmospheric pressure. It is principally used when power must be generated with minimum steam consumption.

The condensing turbine may also have bleed points (uncontrolled extractions) to satisfy steam demands at medium intermediate pressures. This provision is applicable when the bleed (medium pressure) steam volume demand is low and pressure variations can be tolerated. These turbines are of bleed cum condensing type.

These type of turbines find application mainly for power generation where bleed steam is required for feed water heating (HP heater, LP heater and Deaerator requirement) and / or where bleed steam is utilized to meet small process demands.

The extraction condensing turbines are used when a constant pressure steam flow has to be extracted for process purposes. The constructional characteristics of these turbines are very similar to condensing turbines. From the second admission valve chest it is possible to inject low pressure steam, available in the plant to supply the power required by the plant. These turbines are typically used for co-generation where the turbine meets both the power and steam demand of the process plant. In these type of turbines the power generated can be maintained more or less at a steady level despite variation in process steam demands.


In some cases it may be necessary to have both bleed and controlled extraction steams from the condensing turbine. These turbines are of bleed cum extraction condensing type.

The HP casing is made of cast alloy steel and is characterized by its simple cylindrical shape. Pre-stressed bolts hold the two sections together at the horizontal flange. The design of the flange is such that it ensures complete tightness of the joint without sealing materials.

The pattern for casting the HP casing is assembled according to the size of turbine, taking into account the extraction pipes and extraction control valve flanges.
The HP casing contains the standardized components, such as nozzle plates, shaft seal box and diaphragms all flexibly suspended to allow for thermal expansion.

The rotors of turbines are made from single piece forgings. The shape of the rotor depends on the type of turbine and may possess up to nineteen impulse wheels in a double extraction turbine.
Thrust Bearing is provided to absorb the axial thrust caused by the steam.
The rotor, complete with blading, is dynamically balanced and also undergoes a test at 10% above normal speed. Design of the shafts is aided by modern computer programs.

At the inlet end the rotor is fitted with a combined axial and radial bearing & at the exhaust it is supported by a radial bearing. the Bearing shells are fitted in housings supported by the lower part of the turbine casing.
The forces caused by the weight and reaction, are transferred in this design from the turbine casing to the foundations. In the event of the casing temperature changing, the bearings and the rotor can accommodate the movement due to thermal expansions without any change in their relative positions. By arranging the axial bearing at the inlet end, the working clearances for the HP shaft seal and that part of the blading which, with the smallest dimensions reacts most sensitively to changes in geometry, are almost constant.