Turbine Auxilaries Q & A-iv

61. What is a stage in a steam turbine?
Answer: In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a "stage." A single Curtis stage may consist of two or more rows of moving blades.
62. What is a diaphragm?
Answer: Partitions between pressure stages in a turbine's casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragm is fitted into the top of the casing, the other half into the bottom.
63. What are four types of turbine seals?
Answers:
1. Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
2. Labyrinth mated with shaft serration’s or shaft seal strips.
3. 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.
4. Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.
64. In which turbine is tip leakage 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 prevent this.
65. What are two types of clearance in a turbine?
Answer:
1. Radial
- clearance at the tips of the rotor and casing.
2. Axial - the fore-and-aft clearance, at the sides of the rotor and the casing.
66. 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.
67. What is the function of a thrust bearing?
Answer: Thrust bearings keep the rotor in its correct axial position.
68. What is a balance piston?
Answer: Reaction turbines have axial thrust because pressure on the entering side is greater than pressure on the leaving side of each stage. To counteract this force, steam is admitted to a dummy (balance) piston chamber at the low-pressure end of the rotor. Some designers also use a balance piston on impulse turbines that have a high thrust. Instead of piston, seal strips are also used to duplicate a piston's counter force.
69. What is a combination thrust and radial bearing?
Answer: This unit has the ends of the babbitt bearing extended radially over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.
70. What is a tapered-land thrust bearing?
Answer: The babbitt face of a tapered-land thrust bearing has a series of fixed pads divided by radial slots. The leading edge of each sector is tapered, allowing an oil wedge to build up and carry the thrust between the collar and pad.
71. What is important to remember about radial bearings?
Answer: A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder. These bearings must be accurately aligned to maintain the close clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.
72. What is gland-sealing steam?
Answer: It is the low-pressure steam that is led to a sealing gland. The steam seals the gland, which may be either a carbon ring or labyrinth type against air at the vacuum end of the shaft.
73. What is the function of a gland drain?
Answer: The function of a gland drain is to draw of water from sealing-gland cavities created by the condensation of sealing steam.
74. What is an air ejector?
Answer: An air ejector is a steam siphon that removes non-condensable gases from the condenser.
75. How many governors are needed for safe turbine operation? Why?
Answer: Two independent governors are needed for safe turbine operation. One is an overspeed 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.
76. How is a flyball 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.
77. 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.
78. 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).
79. 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.
80. Why is it poor practice to allow turbine oil to become too cool?
Answer: If a turbine oil is allowed to become too cold, condensation of atmospheric moisture takes place in the oil and starts rust on the polished surfaces of the journal bearings. Condensed moisture my interfere with lubrication.