1. Rectification of defects: The defects may or may not be urgent. If immediate repairs are needed, these may be carried out on running limits or the units should be shut down. If the repairs are not urgent, it may be carried out in planned program of future work.
2. Planned preventive maintenance on running units: A system of routine maintenance is adopted to prevent the defects and breakdowns. The planning of such system needs careful consideration in order to arrive at the optimum level of maintenance. The actual work content of each maintenance routine can only be determined after experience with the plant over a period of time. After carrying out the preventive maintenance, it is recorded on a record card with full particulars or the items attended.
3. Planned preventive maintenance on shut down units: In order to minimize the maintenance cost of shut down units, it is necessary to plan the work to be done carefully in advance so that the duration of the plant shut down is reduced to a minimum. This planning process should start some weeks or months before the shutdown is due to take place.

Steam turbine overhaul:

1. Apply the load gradually.
2. Check up the oil pressure going to the bearings and control gear.
3. Observe the oil bearing temperature.
4. Observe the turbine for any noise, vibration by watching the vibration and other indicators.

C. Sequence when shutting the turbine down:

1. Application of controlled power illuminates all of the malfunction lights. This provides a check of the malfunction lights before starting the turbines.
2. Reset malfunction circuit by operating a reset switch. Malfunction lights go off and all control devices assume the condition for starting.
3. Inspect the governor mechanism, till all grease cups and oil where necessary.
4. Open the boiler stop valve to permit heating of the line and avoid condensation in the line.
5. Open header, separator, throttle and turbine casing drills.
6. Start auxiliary oil pump. This has to be stopped when the main oil pump starts delivering oil at normal pressure.
7. Adjust middle valve to secure required oil pressure for the bearing.
8. Start the circulating water pumps and dry vacuum pumps of the condenser. Operate the condensate extraction pumps as found necessary to remove water during the warming up period.
9. Turn on the turbine steam or water seal.
10. Turn on the water to the generator oil cooler and other water requiring parts.
11. Keep open all the drains ahead of the throttle valve untill all water of condensation has been removed.
12. Open the throttle or governor valve quickly to set the rotor in motion.
13. In order to check up whether the tripping mechanism operates properly or not and to prevent the turbine from accelerating too rapidly, operate the overspeeed trip valve by using the hand lever as soon as turbine starts rolling.
14. Reset the emergency overspeed valve and before the turbine comes to rest, adjust the throttle so that the turbine wheel operates between 200 and 300 rpm.
15. While the rotor is in slow motion, observe any rubbing or mechanical difficulty by using a metal rod or listening device.
16. As soon as the temperature of the oil leaving the bearing reaches about 40-48 C, start the circulating water through the oil coolers to maintain bearing oil temperature.
17. Increase the speed gradually following the manufacturer's instructions.
18. Adjust the water seal on the turbine and the atmospheric relief valve.
19. Once the machine comes under the control of governor, test the emergency governor by opening the valve in the oil line to it. See that all valves controlled by this tripping mechanism close promptly. Reset open throttle valve and restore speed to normal.
20. Close all drains.
21. Open leak off from H.P. side gland in order to flow any excess steam to the filled water heater or to one of the lower stage of the turbine.
22. Synchronize the generator and tie it in the line.
23. The speed is now under the control of governor. The turbine is now ready for load and is regulated from the turbine control panel.

1. The built-up rotor
2. The integral rotor
3. The hollow drum rotor
4. The solid drum rotor
5. The welded disc rotor

1. The Built-up Rotor:

It consists of forged steel shaft on which separate forged steel discs are shrunk and keyed. It is cheaper since the disc and shaft are relatively easy to forge and inspect for flaws and machining of these components can be carried out concurrently. Its shaft is machined with a series of stepped diameters ending with central collar. Each disc is heated and assembled on the shaft in turn, each being held in position by a form of circlip.  Relative rotation is prevented either by keys, or by hub dowels known as buttons, which locate the hubs one to another to the central collar. The number of discs depend upon the number of stages which in turn depends upon the turbine output.

2. The Integral Rotor:

The shaft and wheels of this type of rotors are formed from one solid forging. Integral rotors are expensive and difficult to forge, and there is a high incidence of rejects. Over and above, a large amount of machining time and waste material are involved.

• There is no chance of disc to become loose, particularly at high temperature end where at times the wheels may be hot and the shaft pull as found in the built-up rotor.
• This rotor is also free from the effect of creep which may cause the shrink fit of built-up rotor to disappear after a large number of running hours.
• The hoop stress is of lower magnitude as it contains a small hole meant for inspecting the forging.
• There is saving in axial length and reduction in spindle diameter over the built-up type.

3. Hollow Drum Rotor:

This type of rotor promotes even temperature distribution because it is designed with the same thickness of material as the casing.

4. Solid Drum Rotor:

This type of rotors are suitable for cylinders where there are lower temperatures but large diameters, as in I.P. cylinders without re-heat.

5. Welded Disc Rotor:

The last stage disc is the most heavily stressed part of the turbine and this is one of the main problem of L.P. rotor. The centrifugal load of the large rotating blades set up a tensile stress in the rim of disc, and this stress increases with decreasing radius, its maximum value being at the bore of the hub. If the bore is exceedingly small, the hoop stress becomes very less but of there is no hole, the hoop stresses throughout the disc are theoretically halved. Since there is no central hole in welded disc rotor, it suitable for L.P. rotors. It has two main advantages: It is less stressed and no need for large shaft forgings which are expensive and difficult to manufacture. The welding process and subsequent heat treatment should be performed with great attention. 

These 5 types of rotors are also used in gas turbines.

Also read: Balancing of Rotors