Question Set 44

Question No. 1

In which section of the steam-turbine rotors is the problem of rotor failure mostly prevalent?

Answer:

Rotor failures occur mostly on the large low-pressure rotors.

Question No. 2

In which turbines, is this pressure-velocity compounding principle employed?

Answer:

In the Curtis turbine.

Question No. 3

What are the disadvantages of velocity compounding?

Answer:

1.     Steam velocity is too high and that is responsible for appreciable friction losses.

2.     Blade efficiency decreases with the increase of the number of stages.

3.     With the increase of the number of rows, the power developed in successive rows of blade decreases. For as much as, the same space and material are required for each stage, it means, therefore, that all stages are not economically efficient.

Question No. 4

What are the main causes of turbine vibration?

Answer:

1.     Unbalanced parts

2.     Poor alignment of parts

3.     Loose parts

4.     Rubbing parts

5.     Lubrication troubles

6.     Steam troubles

7.     Foundation troubles

8.     Cracked or excessively worn parts

Question No. 5

What are the possible causes of a governor not operating?

Answer:

1.     Restriction of throttle valve reflex.

2.     Failure of governor control on start-up.

If it is found that after start-up, the speed increases continuously and the governor is not closing the throttle valve, it may be that the governor pump has been installed in the wrong direction.

Question No. 6

What are two types of clearance in a turbine?

Answer:

1.     Radial - the clearance at the tips of the rotor and casing

2.     Axial - the fore-and-aft clearance, at the sides of the rotor and the casing

Question No. 7

What is a combination thrust and radial bearing?

Answer:

This unit has the ends of the Babbitt bearing extended radially over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.

Question No. 8

What is an air ejector?

Answer:

An air ejector is a steam siphon that removes non-condensable gases from the condenser.

Question No. 9

What is the nature of circumferential cracking in shrunk-on-disc rotors in steam turbines?

Answer:

Regions of high stress concentration give birth to this type of cracking. It begins in corrosion pits and propagates towards the bore by high-cycle fatigue. It may culminate in a catastrophe, if it penetrates the bore (happily this usually does not occur).

Question No. 10

What is to be done for erosion-induced damage on high-and low-pressure stage blading?

Answer:

1.     In such cases welding repair can be a good solution and this can be carried out during a normal maintenance outage without removing the blade. Using oxyacetylene torch, Satellites is generally deposited onto the damaged site. Following this, the weld is subjected to stress-relieving and re-profiling.

2.     In case of erosion penetrating the erosion shield and extending to the base material, a filler material of consistent or identical composition of blade material is used.

3.     In some cases use is made of Inconel alloy to build up the metal base. Therefore, using welding or brazing technique, a new shield can be attached to the blade. If brazing technique is followed, the rebuilt section is stress relieved prior to the attachment of shield to it. If, on the other hand, the shield is attached by welding, then they are stress-relieved together.