Mechanical Interview Questions 11 contains questions about
101. Besides lubrication, what are two functions of
lubricating oil in some turbines?
102. Do you stop cooling-water flow through a steam
condenser as soon as the turbine is slopped?
103. How can the deposits be removed?
104. How can the fatigue damage on high- pressure blades
be corrected?
105. How will you detect that misalignment is the
probable cause of excessive vibration?
106. In which part of the steam turbine does corrosion
fatigue occur?
107 In which part of the steam turbine does stress
corrosion cracking (SCC) occur?
108. In which zone of steam turbines has temperature-creep
rupture been observed?
109. What are the types of thrust bearings?
110. What are the types of turbine seals?
101. Besides lubrication, what are two functions of lubricating oil in some turbines?
In larger 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.
102. Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
You should keep the cooling water circulating for about 15 mill or more so that the condenser has a chance to cool down gradually and evenly.
Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.
103. How can the deposits be removed?
o Water soluble deposits may be washed off with condensate or wet steam.
o Water insoluble deposits are removed mechanically after dismantling the turbine.
o Experience shows that water soluble deposits are embedded in layers of water-insoluble deposits. And when the washing process is carried out, water soluble parts of the deposit dissolve away leaving a loose, friable skeleton of water- insoluble deposits which then break loose and wash away.
104. How can the fatigue damage on high- pressure blades be corrected?
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.
105. How will you detect that misalignment is the probable cause of excessive vibration?
o Coupling to the driven machine is to be disconnected.
o The turbine is to be run alone.
o If the turbine runs smoothly, either misalignment, worn coupling or the driven equipment is the cause of the trouble.
106. In which part of the steam turbine does corrosion fatigue occur?
In the wet stages of the LP cylinder.
107 In which part of the steam turbine does stress corrosion cracking (SCC) occur?
In the wet stages of the low-pressure turbine.
108. In which zone of steam turbines has temperature-creep rupture been observed?
Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is, it has been found to occur in the control stages of the high-pressure and intermediate-pressure turbines where steam temperature sometimes exceed 540°C.
In the reheat stage, it has been observed that creep has caused complete lifting of the blade shroud bands.
109. What are the types of thrust bearings?
o Babbitt-faced collar bearings
o Tilting pivotal pads
Tapered land bearings
Rolling-contact (roller or ball) bearings
110. What are the types of turbine seals?
Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
Labyrinths mated with shaft serrations or shaft seal strips.
Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.