Question Set 52

Question No. 1

What are the losses in steam turbines?

Answer:

1.     Residual Velocity Loss - This is equal to the absolute velocity of the steam at the blade exit.

2.     Loss due to Friction - Friction loss occurs in the nozzles, turbine blades and between the steam and rotating discs. This loss is about 10%.

3.     Leakage Loss.

4.     Loss due to Mechanical Friction - Accounts for the loss due to friction between the shaft and bearing.

5.     Radiation Loss - Though this loss is negligible, as turbine casings are insulated, it occurs due to heat leakage from turbine to ambient air which is at a much lower temperature than the turbine.

6.     Loss due to Moisture - In the lower stages of the turbine, the steam may become wet as the velocity of water particles is lower than that of steam. So a part of the kinetic energy of steam is lost to drag the water particles along with it.

Question No. 2

At what points does corrosion fatigue does show up?

Answer:

It attacks trailing edges, near the base of the foil and also the blade-root serration’s.

Question No. 3

What are the possible causes for the turbine not running at rated speed?

Answer:

The possible causes are:

1.     Too many hand valves closed,

2.     Oil relay governor set too low,

3.     Inlet steam pressure too low or exhaust pressure too high,

4.     Load higher than turbine rating,

5.     Throttle valve not opening fully,

6.     Safety trip valve not opening properly,

7.     Nozzles plugged,

8.     Steam strainer choked.

Question No. 4

What are the possible causes of excessive vibration or noise in a steam turbine?

Answer:

1.     Misalignment.

2.     Worn bearings.

3.     Worn coupling to driven machine.

4.     Unbalanced coupling to driven machine.

5.     Unbalanced wheel.

6.     Piping strain.

7.     Bent shaft.

Question No. 5

What are the stresses to which a steam turbine rotor is subjected during its service life?

Answer:

1.     Mechanical stress - The factors that contribute to mechanical stress in the shaft are the centrifugal forces and torque’s generated due to revolving motion of the shaft as well as bending arising during steady-state operation.

2.     Thermal stress - Transient operating phases i.e. start-up and shutdown the genesis of thermal stress induced to the turbine shaft.

3.     Electrically induced stress - They originate due to short circuits and faulty synchronization.

Question No. 6

What are three types of condensers?

Answer:

1.     Surface (shell-and-tube)

2.     Jet

3.     Barometric.

Question No. 7

What are topping and superposed turbines?

Answer:

Topping and superposed turbines are high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine is at the same pressure as the old boilers and is used to supply the old turbines.

Question No. 8

What design modification is adopted to reduce susceptibility of last low pressure stages to fatigue failure?

Answer:

One modification is to join the blade segments together at the shroud band.

Question No. 9

What does "upgrading" generally means in the context of steam turbines?

Answer:

Upgrading is a most widely used tern. It encompasses a variety of meanings verses life extension, modernization and up-rating of steam turbines.

Question No. 10

What does the term "ramp rat" mean?

Answer:

Ramp rate is used in bringing a turbine up to operating temperature and is the degrees Fahrenheit rise per hour that metal surfaces are exposed to when bringing a machine to rated conditions. Manufactures specify ramp rates for their machines in order to avoid thermal stresses. Thermocouples are used in measuring metal temperatures.