How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation:
One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
How will you detect that misalignment is the probable cause of excessive vibration?
Coupling to the driven machine is to be disconnected.
The turbine is to be run alone.
If the turbine runs smoothly, either misalignment, worn coupling or the driven equipment is the cause of the trouble.
In which part of the steam turbine does corrosion fatigue occur?
In the wet stages of the LP cylinder.
In which part of the steam turbine does stress corrosion cracking (SCC) occur?
In the wet stages of the low-pressure turbine.
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.
What are the types of thrust bearings?
Babbitt-faced collar bearings
Tilting pivotal pads
Tapered land bearings
Rolling-contact (roller or ball) bearings
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.
What are the basic causes of the problem of rotor failure?
Fatigue failure due to high stress.
Aggressive operating environment
What are the differences between impulse and reaction turbines?
The impulse turbine is characterized by the fact that it requires nozzles and that the pressure drop of steam takes place in the nozzles.
The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row of blades mounted on a drum. The drum blades are separated by rows of fixed blades mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of correcting the direction of steam onto the moving blades.
In the case of reaction turbines, the pressure drop of steam takes place over the blades. This pressure drop produces a reaction and hence cause the motion of the rotor.
How can problems of "excessive vibration or noise" due to piping strain be avoided on steam turbines?
The inlet as well as exhaust steam lines should be firmly supported to avoid strains from being imposed on the turbine.
Adequate allowance should be made for expansion of steam pipes due to heat.
How the deposits in turbine be removed?
Water soluble deposits may be washed off with condensate or wet steam.
Water insoluble deposits are removed mechanically after dismantling the turbine.
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.
How 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.
How the misalignment of Flanges be rectified?
The bolts holding the flanges together are to be tightened. The coupling is to be checked for squareness between the bore and the face. At the same time axial clearance is to be checked.
How the problem of excessive speed variation due to throttle assembly friction be overcome?
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.
How the problems of vibration and fatigue arise in steam turbine blades?
These arise due to flow irregularities introduced because of manufacturing defects, e.g. lack of control over tolerances.
System operating parameter, e.g. low flow may excite various modes of vibration in the blades.
How does solid-particle erosion occur?
Solid-particle erosion, i.e. SPE occurs in the high-pressure blades. And it takes place when hard particles of iron exfoliated by steam from superheater tubes, reheater tubes, steam headers and steam leads strike on the surface of turbine blades.
How does the internal efficiency monitoring lead to the detection of turbine deposits?
Process heat drop.
Adiabatic heat drop.
The process heat drop and adiabatic heat drop are obtained from a Mollier-Chart for the corresponding values of steam parameters – pressure and temperature – at initial and final conditions.
How is a flyball governor used with a hydraulic control?
As the turbine speeds up, the weights are moved outward by centrifugal force, causing linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a spring-loaded piston. The movement of the piston controls the steam valves.