Question No. 1

What are the two basic types of steam turbines?

Answer:

1. Impulse type.

2. Reaction type.

Question No. 2

How can steam turbines be classified?

Answer:

The classification of steam turbine are as follows:

By the action of steam:

1.     Impulse.

2.     Reaction.

3.     Impulse and reaction combined.

The number of step reductions involved:

1.     Single stage.

2.     Multi-stage.

3.     Whether there is one or more revolving vanes separated by stationary reversing vanes.

The direction of steam flow:

1.     Axial.

2.     Radial.

3.     Mixed.

4.     Tangential.

5.     Helical.

6.     Re-entry.

The inlet steam pressure:

1.     High pressure.

2.     Medium pressure.

3.     Low pressure.

The final pressure:

1.     Condensing.

2.     Non-condensing.

The source of steam:

1.     Extraction.

2.     Accumulator.

Question No. 3

Why should a steam or moisture separator be installed in the steam line next to a steam turbine?

Answer:

All multistage turbines, low-pressure turbines, and turbines operating at high pressure with saturated steam should have a moisture separator in order to prevent rapid blade wear from water erosion.

Question No. 4

Besides lubrication, which are two functions of lubricating oil in some turbines?

Answer:

In large units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

Question No. 5

Do the radial axial-bore cracks occur in the LP rotor/shaft alone?

Answer:

These are also known to occur in the HP as well as HP rotors.

Question No. 6

Do you think that turbine blade failure is the only cause of unreliability of steam turbines? Does upgrading of turbine means replacement of blades and/or improvement of blade design?

Answer:

1.     Like the blades, the steam-turbine rotors are highly stressed components. They are subject to cracking by a variety of failure mechanisms. Rotor failures do occur. And when they occur the result is catastrophic with the complete destruction of the unit and the total loss of generating capacity.

2.     Therefore, special attention should be given to rotor upgrading and repairing techniques.

BLADE FAILURES

●      Unknown 26%

●      Stress-Corrosion Cracking 22%

●      High-Cycle Fatigue 20%

●      Corrosion-Fatigue Cracking 7%

●      Temperature Creep Rupture 6%

●      Low-Cycle Fatigue 5%

●      Corrosion 4%

●      Other causes 10%

Besides, many damage mechanisms operate in combination of, poor steam/water chemistry, certain blade design factors that vary from one turbine manufacture to other, system operating parameters,

Question No. 7

How do the problems of vibration and fatigue arise with steam turbine blades?

Answer:

1.     These arise due to flow irregularities introduced because of manufacturing defects, e.g. lack of control over tolerances.

2.     System operating parameter, e.g. low flow may excite various modes of vibration in the blades.

Question No. 8

How does the dirty safety trip valve trip the safety trip at normal speed? What is the remedy to it?

Answer:

Dirt may find its way to the safety trip valve and get deposited around the spring end cap end. This will block the clearance between the safety trip valve and the spring end cap. As a result the steam pressure in the spring cap gets lowered allowing the valve to close.

The remedy is,

The spring end cap as well as safety trip valve should be cleaned.

Question No. 9

What maybe the possible causes for the safety trip tripping during load variation?

Answer:

1.     Light load and high inlet steam pressure.

2.     Safety trip set very close to the operating speed of turbine.

Question No. 10

What is the safe maximum tripping speed of a turbine operating at 2500 rpm?

Answer:

The rule is to trip at 10 percent over speed. Therefore, 2500 x 1.10 = 2750 rpm