Question Set 46
Question No. 1
In which turbine tip leakage is a problem?
Answer:
Tip leakage is a problem in reaction turbines. Here, each vane forms a nozzle; steam must flow through the moving nozzle to the fixed nozzle. Steam escaping across the tips of the blades represents a loss of work. Therefore, tip seals are used to prevent this.
Question No. 2
What are four types of thrust hearings?
Answer:
1. Babbitt-faced collar bearings.
2. Tilting pivotal pads.
3. Tapered land bearings.
4. Rolling-contact (roller or ball) bearings.
Question No. 3
What are some conditions that may prevent a turbine from developing full power?
Answer:
1. The machine is overloaded.
2. The initial steam pressure and temperature are not up to design conditions.
3. The exhaust pressure is too high.
4. The governor is set too low.
5. The steam strainer is clogged.
6. Turbine nozzles are clogged with deposits.
7. Internal wear on nozzles and blades.
Question No. 4
Why is it necessary to open casing drains and drains on the steam line going to the turbine when a turbine is to be started?
Answer:
To avoid slugging nozzles and blades inside the turbine with condensate on start-up; this can break these components from impact. The blades were designed to handle steam, not water.
Question No. 5
What steam rate is as applied to turbo-generators?
Answer:
The steam rate is the pounds of steam that must be supplied per kilowatt-hour of generator output at the steam turbine inlet.
Question No. 6
What is the operating principle of a reaction turbine?
Answer:
A reaction turbine utilizes a jet of steam that flows from a nozzle on the rotor. Actually, the steam is directed into the moving blades by fixed blades designed to expand the steam. The result is a small increase in velocity over that of the moving blades. These blades form a wall of moving nozzles that further expand the steam. The steam flow is partially reversed by the moving blades, producing a reaction on the blades. Since the pressure drop is small across each row of nozzles (blades), the speed is comparatively low. Therefore, more rows of moving blades are needed than in an impulse turbine.
Question No. 7
What is a multi-port governor valve? Why is it used?
Answer:
In large turbines, a valve controls steam flow to groups of nozzles. The number of open valves controls the number of nozzles in use according to the load. A bar-lift or cam arrangement operated by the governor opens and closes these valves in sequence. Such a device is a multi-port valve. Using nozzles at full steam pressure is more efficient than throttling the steam.
Question No. 8
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. 9
By monitoring the exhaust steam temperature, how can the blade deposition be predicted?
Answer:
1. Immediately after the 1st commissioning, the different values of exhaust temperature for different steam flow rates are precisely determined and plotted against steam flow. This will produce the first actual graph. This is for a clean turbine.
2. Similar graphs are to be drawn at later periods for comparing with the initial graph.
3. A rise in exhaust steam temperature under the same conditions refers to deposit formation.
4. An increase of exhaust steam temperature by more than 10% in the range of 70 to l00% steam flow indicates inadmissible blade depositions. Shutdown is to be taken and blades are to be washed off deposits.
Question No. 10
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 15mills 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.