Steam Turbine Basic Parts

Steam Turbine Barring device

When a turbine is left cold and at standstill, the weight of the rotor will tend to bend the rotor slightly. If left at the standstill while the turbine is still hot, the lower half of the rotor will cool off faster than the upper half and the rotor will bend upwards “hog”. In both cases, the turbine would be difficult if not impossible to start up. To overcome the problem the manufacturer supplies the larger turbines with a turning or barring gear consisting of an electric motor which through several sets of reducing gears turns the turbine shaft at low speed. The first turning gears turned the shaft at approximately 20 rev/mm, later increased to 40 and up to 60 rev/mm as proper lubrication is difficult to obtain at low speed; the same goes for the hydrogen seals of generators. Some turning gears, electric or hydraulic, turn the shaft 1 800 at set times over a period of 24 hours. Before a cold turbine is started up it should be on the barring gear for approximately three hours. When a turbine is shut down, it should be barring for the next 24 hours. If a hydrogen-cooled generator is involved the turbine should be kept on barring gear to prevent excessive loss of hydrogen, All barring gears are interlocked with a lubricating oil pressure switch and an engagement limit switch operated by the engagement handle.

Turbine Bearings

One of the steam turbine basic part is bearing. They are two types of bearings used based on the type of load act on them

• Radial Bearing

• Thrust Bearing

Radial Bearings

For small turbines mostly equipped with anti-friction type bearings. Widely used anti-friction bearings are the self-aligning spherical ball or roller bearing with flooded type lubrication is used. In the case of medium turbines used plain journal bearing. They may be ring lubricated sleeve bearings with bronze or Babbitt lining. Both flooded and force types are employed. For larger turbines, the radial bearing will be a tilting pad type. The number of pad per bearing will be selected based on the weight of the rotor. For these types of bearing forced lubrication is used.

Thrust Bearings

The main two purposes of the thrust bearing are:

• To keep the rotor in an exact position in the casing.

• To absorb axial thrust on the rotor due to steam flow.

The thrust bearing is located on the free end of the rotor or we can say at the steam inlet of the turbine. The axial thrust force is very small for impulse turbines. This is due to the presence of pressure equalizing holes in the rotor discs to balance the thrust force generated across the disc. A simple thrust bearing such as a ball bearing for small turbines and radial babbitt facing on journal bearings are commonly used in small and medium-size turbines. Tilling pad type thrust bearings are used in the large steam turbines. In the case of reaction turbine, the pressure drop across the moving blades creates a heavy axial thrust force in the direction of steam flow through the turbine. Due to greater thrust force, the heavy duty thrust bearing such as tilting pad type thrust bearings are used. The axial thrust in reaction turbines can be nearly reduced by the using off balance or dummy pistons. As we seen the purpose, the thrust bearing not only taking the thrust load and also to maintain the position of the rotor. The axial position of the rotor is very important and an axial position indicator is often applied to the thrust bearing. As a normal practice, the axial position of rotor exceeds 0.3 mm alarm and shutdown at 0.6 mm. (Readers please note these valves are thumb rule, it may change with respect to manufacturer and turbine model)

Turbine Seals

Seals are used to reduce the leakage of steam between the rotary and stationary parts of the steam turbine. Depend upon the location of seal, the seals are classified as two types, they are

• Shaft Seal

• Blade Seal

Shaft Seals

Shaft seals are used to prevent the steam leakage where the shafts extend through the casing. In the case of a small turbine (as per API 611) carbon rings are used as shaft seal up to the surface speed of the shaft is 50m/s. The carbon ring is made up of three segments butting together tightly under the pressure of a garter spring. The carbon rings are free floating in the housing and an anti-rotating pin is used to prevent the rotation of carbon ring seal.

Due to the self-lubrication properties of the carbon rings, they maintain a close clearance with the shaft. For larger steam turbines (as per API 612) labyrinth seal are used as shaft seals. In the case of condensing steam turbine to prevent the air ingression at the shaft seal by Gland condenser and ejector arrangements(as per API 612).

Blade Seals

Blade seals are used to prevent the steam leakage between the diaphragm and the shaft. The efficiency of the turbine depends largely on the blade seals. Labyrinth seals are used as blade seals in the small and large turbines. In the case of large steam turbine spring loaded labyrinth seals are used. The seals are made up of brass or stainless steel. Also, the sharp edge gives better sealing and rubs off easily without excessive heating in case of a slightly eccentric shaft. Some labyrinth seals are very simple, others are complicated.

Turbine Couplings

The purpose of couplings is to transmit power from the prime mover to the driven piece of machinery. Flexible type couplings are used in turbines. The coupling hubs are taper bore and key way to fit the tapered end of the shaft.

Governor

The governor is one of the steam turbine basic parts. Its main function is to control the operation of a steam turbine. Generally, the governor is classified as two type

• Speed-sensing governor

• Pressure sensing or load governor

Speed Sensing Governor

Speed-sensing governors are used in power generation application to maintain a constant speed with respect to the load change in governor. Droop is one of the important characteristics of this governor selection.

Pressure sensitive governor

These are applied to back pressure and extraction turbines in connection with the speed sensitive governor.

They are three types of governor used in steam turbine

• Mechanical Governor

• Hydro-mechanical Governor

• Electronic Governor

In the case of small turbine Oil relay type (Hydro-mechanical) governor NEMA class “A” is used. For the larger turbine, electronic governor NEMA class “D” is used.

Lubrication System

Oil flood lubrication is used for small turbines and pressurized lubrication is used for larger turbines. The pressurized lubrication system consists of lube oil tank, oil pump, filter, cooler, pressure regulating valve, etc., The pressurized lubrication system of turbine shall be as per API 614.