In this article, we will discuss steam turbine basic parts. The steam turbines are widely used in power generation, refineries and petrochemical industries.
- 1 Turbine Casings
- 2 Turbine Rotors
- 3 Turbine Blades
- 4 Stationary Blades (Diaphragms) and Nozzles
- 5 Shrouds
- 6 Turbine Barring device
- 7 Turbine Seals
- 8 Turbine Couplings
- 9 Governor
- 10 Lubrication System
The casing shape and construction details depend on whether it is a High Pressure(HP) or Low Pressure (LP) casings. For low and moderate inlet steam pressure up to 120 bar, a single shell casing is used. With a rise in inlet pressure the casing thickness as to be increasing. Handling such heavy casing is very difficult also the turbine as to slowly brought up to the operation temperature. Otherwise undue internal stress or distortions to the thick casing may arise. To over this for high pressure and temperature application double casing is used. In the double casing inner casing is for High pressure and the outer casing is for hold the low pressure.
Most of the turbine have casings with horizontal split type. Due to horizontal split it easy for assembling and dismantling for maintenance of turbine. Also, maintain proper axial and radial clearance between the rotor and stationary parts.
Usually, the turbine casings are heavy in order to withstand the high pressures and temperatures. It is general practice the thickness of walls and flanges decrease from the inlet to exhaust end due to the decrease in steam pressure from inlet to exhaust.
Turbine Casing MOC
Large casings for low-pressure turbines are of welded plate construction, while smaller L. P. casings are of cast iron, which may be used for temperatures up to 230°C.
Casings for intermediate pressures are generally of cast carbon steel able to withstand up to 425°C. The high-temperature high-pressure casings for temperatures exceeding 550°C are of cast alloy steel such as 3 Cr 1Mo (3% Chromium + 1% Molybdenum.) The turbine casings are subjected to maximum temperatures and under constant pressure. Hence the material of casing shall subject high “Creep”. (Click here to read more about material properties)
The casing joints are made of steam tight by matching the flange faces very exactly and very smoothly, without the use of gaskets. Dowel pins are used to secure exact alignment of the casing flange joints.
The casing contains grooves for fixing the diaphragms (for impulse turbines) or for the stationary blades (reaction turbines). (Click here to read more about impulse and reaction turbines
The steam turbine rotors must be designed with the most care as it is mostly the highly stressed component in the turbine. The design of a turbine rotor depends on the operating principle of the turbine.
The impulse turbine, in which the pressure drops across the stationary blades. The stationary blades are mounted in the diaphragm and the moving blades fixed or forged on the rotor. Steam leakage is in between the stationary blades and the rotor. The leakage rate is controlled by labyrinth seals. This construction requires a disc rotor.
The reaction turbine has pressure drops across the moving as well as across the stationary blades. The disc rotor would create a large axial thrust across each disc. Hence disc rotors are not used in the reaction turbine. For this application, a drum rotor is used to eliminate the axial thrust caused by the discs, but not the axial thrust caused by the differential pressure across the moving blades. Due to this, the configuration of reaction turbine is more complicated.
Disc Type Rotors
This type of rotor is largely used in steam turbines. The disc type rotors are made by forging process. Normally the forged rotor weight is around 50% higher than the final machined rotors. Refer above figure for disc type rotor.
Drum Type Rotors
Initially, the reaction turbines rotors are made by solid forged drum-type rotor. The rotors are heavy and rigid construction. Due to this, the inertia of the rotor is very high when compare with the disc-type rotor of the same capacity. To overcome this nowadays the hollow drum-type rotors are used instead of solid rigid rotors. Usually, this type of rotor is made of two pieces construction. In some special cases, the rotor is made up of multi-piece construction.
The drums are machined both outside and inside to get perfect rotor balance.
The efficiency of the turbine depends on more than anything else on the design of the turbine blades. The impulse blades must be designed to convert the kinetic energy of the steam into mechanical energy. The same goes for the reaction blades, which furthermore must convert pressure energy to kinetic energy.
The blades are strong enough to withstand the following factors
- High temperatures and stresses due to the pulsating steam load
- Stress due to centrifugal force
- Erosion and corrosion resistance.
Depend upon the pressure region the blades are also classified as follow. Refer above figure for rotor pressure region
- High Pressure (HP) blades
- Intermediate Pressure (IP) blades
- Low Pressure (LP) blades
The turbine blades are made up of chromium-nickel steel or 17 Cr’13 Ni – steel.
Stationary Blades (Diaphragms) and Nozzles
Nozzles are used to guide the steam to hit the moving blades and to convert the pressure energy into the kinetic energy. In the case of small impulse turbine, the nozzles are located in the lower half of the casing. But in the case of the larger turbine, the nozzles are located on the upper half of the casing.
Stationary Blades (Diaphragms)
All stages following the control stage have the nozzles located in diaphragms. The diaphragms are in halves and fitted into grooves in the casing. Anti-rotating pin or locking pieces in the upper part of the casing prevent the diaphragm to rotate.
All modern diaphragms are of an all-welded construction. The stationary blades in reaction turbines are fitted into grooves in the casing halves; keys as shown lock the blades in place. In some cases, the blades have keys or serration on one side of the root and a caulking strip on the other side of the root is used to tighten the blades solidly in the grooves.
After turbine blades are machined through the milling process. Then the blades are inserted in the rotor groove. Depend upon the application the blade root section varies
Blade roots are subject to take four types of stress
- Tensile stress due to the centrifugal forces
- Bending stress due to fluid forces act on the blade in tangential direction
- Stress due to vibration forces.
- Thermal stress also due to the uneven heating of the blade root and the rim.
This type of blades is used in the last stage of a large multistage steam turbine. These are the largest blade in turbine and contribute around 10% of the turbine total output. Due to larger in size, these types of blades are subjected to high centrifugal and bending forces. To overcome these forces twisted construction is used.
Shrouds are used to reinforce the turbine blades free ends to reduce vibration and leakage. This is done by reverting a flat end over the blades refer figure. In some cases especially at the early stages, the shroud may be integral with the blade. When the blades are very long as in the case of the last stage of LP turbine. The rotor blades are further reinforced by using lacing wires (caulking wire) which circumferentially connects all the blades at a desired radius and shrouding is eliminated.
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.
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
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.
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)
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 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. Refer below figure.
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 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.
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.
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.
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.