Question Set 43
Question No. 1
What is the possible cause of slow start up of a steam turbine?
Answer:
This may be due to high starting torque required by the driven equipment.
Question No. 2
What is the remedy for rotor-surface cracking?
Answer:
Current rotor/shaft should be machined off (skin-peeling).
Question No. 3
What is to be done in case of cracks originating at the lacing-wire holes?
Answer:
These are to be weld-repaired. However the following factors must be considered:
1. The length of the crack that appears on the pressure and/or suction face.
2. Whether the cracks propagate towards inlet end, discharge end and or both.
Question No. 4
How does the damage to turbine-blades tell upon the efficiency of the unit?
Answer:
The damage to blade profiles changes the geometry of steam flow path and thereby reducing the efficiency of the unit.
Question No. 5
How can the problem of excessive speed variation due to throttle assembly friction be overcome?
Answer:
The throttle should be dismantled. Moving parts should be checked for free and smooth movement. Using very fine-grained emery paper, the throttle valve seats and valve steam should be polished.
Question No. 6
How can damaged tenons be repaired?
Answer:
By adopting modern welding techniques, tenons can be rebuilt This in some cases results in extended blade life.
Question No. 7
How can the fatigue damage on high-pressure blades be corrected?
Answer:
Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc admission. This can be corrected by switching over to full arc admission technique.
Question No. 8
How does pressure monitoring ensure detection of turbine deposits?
Answer:
1. Pressure of steam expanding in the turbine is measured at characteristic points, i.e., at the wheel chamber, points of pass-out, inlet/outlet of HP, IP and LP stages of the turbine.
2. The turbine manufacturer provides the pressure characteristics in the form of graphs.
3. At 1st commissioning, the user supplements these theoretical curves with those derived from actual measurements. These are actual pressure characteristics for a clean turbine. Now these pressure characteristics are compared with those obtained during operation in the later period.
4. Under identical conditions, an increase in pressure shows the formation of deposits.
5. For a steam throughput in the range 70-100%, an increase in wheel chamber pressure of more than 10% indicates severe blade depositions.
Question No. 9
How is oil pressure maintained when starting or stopping a medium-sized turbine?
Answer:
An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.
Question No. 10
In which cases does moisture-impingement and washing erosion occur?
Answer:
1. These are encountered in the wet sections of the steam turbine.
2. For nuclear power plants, these wet sections can involve parts of high pressure cylinder.