Turbine Rotor and shaft coupling types
In this article we discussed about the turbine rotor and shaft coupling types used the steam turbine .rotor is the moving part of the steam turbine, which extract the work from the steam. This is the heaviest part of the turbine. Rotor consist of shaft , moving blades and inner stages sealing labyrinths . Thrust collar is provided to take care of axial thrust of rotor during various load conditions. Shaft is a solids forging of suitable material to withstands inlet steam pressure and temperature we discussed about it further in this article .The rotating parts of turbine are referred as rotor, including spindle, rotary table (or wheel drum), rotor blade and shaft coupling, etc.
Rotor is one of the most important parts of steam turbine, the functions of which is to convert power forms and deliver torque. The working conditions of rotor is rather complex: it is in high-temperature working conditions and rotates at high-speed, and not only bear great centrifugal force resulted from the weight of blades, impeller and main shaft, but also bear thermal stress caused by unevenly distributed temperature, and also it have to bear enormous dynamic stress generated by huge torque and shaft torsional vibration; therefore, rotor is required of high strength.
Rotor is flexible rotor; impeller and main shaft use sleeve installation structure, including thrust collar and shaft coupling. Different-2 turbine have different types of rotors specifications according to their operating conditions like LP , HP and medium types of rotors .
Different types of turbine rotors
High pressure rotors: the HP rotors are machined from a single Cr- Mo -V steel forging with integral discs. The rotor forging is thermally stabilized to prevent abnormal defection. “T” root fastening are attached to their respective wheels by the wheels. In all the moving wheels, balancing holes are machined to reduce the pressure difference across them which results in reduction of axial thrust. First stage has integral shrouds while other rows have shrouding riveted to the blades are periphery. The number of blades connected by a single strip of shrouding is called a blade packet and the number of blades per packet is decided from ration point of view.
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Intermediate pressure rotor:
The IP rotor has seven discs integrally forged with rotor while last four discs are shrunk fit. The shaft is made of high creep resisting Cr – Mo-V steel forging while the shrunk fit disc are machined from high strength nickel steel forging. The blades on the integral disc are secured by “T“ root fastening while on shrunk fit disc by fork foot fastening. To adjust the frequency of the moving blades, lashing wires have been provided in some stages.
Low pressure rotor
The LP rotor consists of shrunk fit discs a shaft. The low pressure rotor shaft is made by forging of Cr-Mo-V steel while the rotor discs are made by high strength nickel steel forgings blades are secured to the respective discs by riveted fork root fastening.
In all the stages lashings wires are providing to adjust the frequency of blades. In the last two rows satellite strips are provided at the leading edge of the blades due to protect from high volume and also from the corrosion
Types of turbine shaft coupling
Shaft coupling is an important part to connect turbine and generator rotor, which drives generator rotor to rotate at high speed via delivering torque and converts mechanical energy into power.
Shaft coupling is generally divided into 3 types: rigid, semi-flexible and flexible. The connection of turbine rotor generally uses rigid shaft coupling.
Flexible shaft coupling has strong flexibility and permits rotors to be connected to have relatively large off-center and is not sensitive to vibration. However, for the structure of which is complex, torque delivered is small.
The structure of rigid shaft coupling is simple, which has features such as high connection rigidity, short axial dimension, reliable, no need of lubrication, no noise, besides delivering relatively large torque; it can also deliver axial pushing force and radial force. The disadvantage of which is that it doesn’t allow the two connected rotors to have relative displacement in axial and radial direction, thus it has strict requirements on the coaxiality of the two axises: small divation in manufacturing and installation will cause shaft coupling to bear unnecessary subsidiary stress and cause the unit large vibration. And it is very sensitive to vibration transmission and increasing difficulty to on site vibration reason searching.
Structure of turbine shaft coupling installed in WHG (Waste Heat Recovery Generation ): using rigid shaft coupling and connected with turbine and generator rotors respectively, using bolts for connection.
Turning gearing bull gear is installed between turbine rotor and generator shaft coupling; shaft coupling delivers torque, axial pushing force, horizontal shear load and bending torque.Accurate centering and correct installation method between the two shaft couplings are especially important.
After accurate centering, all components of shaft coupling and accessory screw holes shall be cleaned. Before rotor is placed on bearing shaft, all shaft coupling ends shall be checked. If any burr or scratching is found, all should be trimmed, however rasper shall not be used. Check all screw holes and scratched surface and remove found burr.When this work is finished, operator shall clean shaft coupling and match all screw holes. When rotor is hoisted and in position, operator shall make sure that it aims accurately at all marks and put shaft coupling bolts in correct screw holes.
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