Question Set 50

Question No. 1

In which turbine is velocity compounding utilized?

Answer:

In the Curtis turbine.

Question No. 2

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

Answer:

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.

Question No. 3

Is there any adverse effect off full-arc admission operation?

Answer:

At low loads, this results in a heat-rate penalty, due to throttling over the admission valves.

Question No. 4

Is there any other type of racking occurring in HP/IP rotors and causing rotor failures?

Answer:

1.     Blade-groove-wall cracking.

2.     Rotor-surface cracking.

Question No. 5

Of all the factors that contribute to the unreliability of steam turbines, which one is the most prominent?

Answer:

It is the problem of turbine blade failures that chiefly contribute to the unreliability of steam turbines.

Question No. 6

Rim cracking continues to be a problem of shrunk-on-disc type rotors in utility steam turbines. Where does it occur?

Answer:

Cracking has been located at the outer corners of tile grooves where the blade root attaches to the rotor.

Question No. 7

So can you recommend this technique as a permanent measure?

Answer:

No, this can be recommended in extreme cases or at best temporarily.

Question No. 8

What should be the more sound approach in the case of steam turbine blade failure?

Answer:

The more reasonable and better approach is to replace the damaged blades with new ones that are stiffened by:

1.     Secreting the interface surface of individual blades so they interlock, or

2.     Welding the blades together.

3.     In some cases, a single monolithic block is machined out to manufacture the blades in a group.

4.     In some other cases, blades themselves are directly welded into the rotor.

Question No. 9

Steam blowing from a turbine gland is wasteful. Why else should it be avoided?

Answer:

It should be avoided because the steam usually blows into the bearing, destroying the lube oil in the main bearing. Steam blowing from a turbine gland also creates condensate, causing undue moisture in plant equipment.

Question No. 10

What are the consequences of turbine depositions?

Answer:

The consequences of turbine depositions have three effects.

A. Economic Effect:

1.     Reduction in turbine output

2.     Decrease in efficiency requiring higher steam consumption.

B. Effect of Overloading and Decreasing Reliability in Operation:

1.     Pressure characteristic in the turbine gets disturbed with the effect that thrust and overloading of thrust bearing increase.

2.     Blades are subjected to higher bending stresses.

3.     Natural vibrations of the blading are affected.

4.     Vibration due to uneven deposition on turbine blading.

5.     Valve jamming due to deposits on valve stems.

C. Corrosion Effect:

1.     Fatigue corrosion

2.     Pitting corrosion.

3.     Stress corrosion.