Apr 27

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  • How many governors are needed for safe turbine operation? Why?

Two independent governors are needed for safe turbine operation:

  • One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
  • The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
  • How will you detect that misalignment is the probable cause of excessive vibration?
    • Coupling to the driven machine is to be disconnected.
    • The turbine is to be run alone.
    • If the turbine runs smoothly, either misalignment, worn coupling or the driven equipment is the cause of the trouble.
  • In which part of the steam turbine does corrosion fatigue occur?
  • In the wet stages of the LP cylinder.

    • In which part of the steam turbine does stress corrosion cracking (SCC) occur?

    In the wet stages of the low-pressure turbine.

    • In which zone of steam turbines has temperature-creep rupture been observed?

    Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is, it has been found to occur in the control stages of the high-pressure and intermediate-pressure turbines where steam temperature sometimes exceed 540°C. In the reheat stage, it has been observed that creep has caused complete lifting of the blade shroud bands.

    • What are the types of thrust bearings?
    • Babbitt-faced collar bearings

    • Tilting pivotal pads

    • Tapered land bearings

    • Rolling-contact (roller or ball) bearings

  • What are the types of turbine seals?
    • Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.

    • Labyrinths mated with shaft serrations or shaft seal strips.

    • Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.

    • Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.

  • What are the basic causes of the problem of rotor failure?
    • Normal wear.

    • Fatigue failure due to high stress.

    • Design deficiency.

    • Aggressive operating environment

  • What are the differences between impulse and reaction turbines?
    • The impulse turbine is characterized by the fact that it requires nozzles and that the pressure drop of steam takes place in the nozzles.

    • The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row of blades mounted on a drum. The drum blades are separated by rows of fixed blades mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of correcting the direction of steam onto the moving blades.

    • In the case of reaction turbines, the pressure drop of steam takes place over the blades. This pressure drop produces a reaction and hence cause the motion of the rotor.

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