Question No. 1

What is combined-cycle cogeneration?

Answer:

A combined cycle using a gas turbine or diesel, usually driving a generator in which the exhaust gases are directed to a waste heat-recovery boiler or heat-recovery steam generator (HRSG). The steam from the HRSG is then directed to a steam turbo-generator for additional electric power production. The use of the exhaust heat from a gas turbine improves the overall thermal efficiency. In cogeneration, electric power is produced, but part of the steam from the HRSG or from extraction from the steam turbine is used for process heat, hence the term cogeneration-the simultaneous production of electric power and process heat steam.

Question No. 2

What is important to remember about radial bearings?

Answer:

A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder.

These bearings must be accurately aligned to maintain the close clearances between the shaft and the shaft seals, and between the rotor and the casing. lf excessive bearing wear lowers the rotor, great harm can be done to the turbine.

Question No. 3

What is the cause of axial-bore cracks?

Answer:

Inadequate toughness of rotor steel and transient thermal stresses.

Question No. 4

What is the cause of turbine deposits?

Answer:

The turbine deposits are steam-born foreign matters settled on turbine blades. Substances dissolved in the BFW transfer partly from the water to steam, during the process of evaporation. They get dissolved in the steam and are carried into the steam turbine.

Question No. 5

What is the definition of a steam turbine?

Answer:

A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or produced by the fixed blades.

1. Neilson definition: The turbine is a machine in which a rotary motion is obtained by the gradual change of the momentum of the fluid.

2. Graham's definition: The turbine is a prime mover in which a rotary motion is obtained by the centrifugal force brought into action by changing the direction of a jet of a fluid (steam) escaping from the nozzle at high velocity.

Question No. 6

What is the harm if the rotor is over speed?

Answer:

Over speed rotor grows radially causing heavy rub in the casing and the seal system. As a result, considerable amount of shroud-band and tenons-rivet head damage occurs.

Question No. 7

What is the nature of rotor surface cracks in steam turbines?

Answer:

They are shallow in depth and have been located in heat grooves and other small radii at labyrinth-seal areas along the rotor.

Question No. 8

What is the remedy for a bent steam turbine shaft causing excessive vibration?

Answer:

1. The run-out of the shaft near the centre as well as the shaft extension should be checked.

2. If the run-out is excessive, the shaft is to be replaced.

Question No. 9

What is the remedy of the damage to blade profiles?

Answer:

Upgrading the turbine and depending on the extent of damage, upgrading may involve:

1. Weld repair of affected zones of the blade,

2. Replacement of damaged blades by new ones and of new design,

3. Replacement of base material,

4. Application of protective coatings to guard against corrosion and erosion damage.

Question No. 10

What is the solution to the problem of SCC/corrosion fatigue of steam turbine blades?

Answer:

It involves changing the blade material as well as minimizing the presence of corrodents in steam to a permissible level.