Question Set 18
Question No. 1
How is a fly-ball governor used with a hydraulic control?
Answer:
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.
Question No. 2
What is meant by critical speed?
Answer:
It is the speed at which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the vibration is caused by critical speed and not by some other trouble).
Question No. 3
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. 4
Besides lubrication, what are two functions of lubricating oil in some turbines?
Answer:
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.
Question No. 5
Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
Answer:
You should keep the cooling water circulating for about 15 miles 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.
Question No. 6
How can the deposits be removed?
Answer:
1. Water soluble deposits may be washed off with condensate or wet steam.
2. Water insoluble deposits are removed mechanically after dismantling the turbine.
3. 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.
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 many governors are needed for safe turbine operation? Why?
Answer:
Two independent governors are needed for safe turbine operation:
1. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
2. The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
Question No. 9
How will you detect that misalignment is the probable cause of excessive vibration?
Answer:
1. Coupling to the driven machine is to be disconnected.
2. The turbine is to be run alone.
3. If the turbine runs smoothly, misalignment, worn coupling or the driven equipment is the cause of the trouble.
Question No. 10
In which part of the steam turbine does corrosion fatigue occur?
Answer:
In the wet stages of the LP cylinder.