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MAJOR COMPONENTS OF A TURBINE



The usual turbine consists of four fundamental parts: the rotor which carries the blades or buckets; the stator consisting of cylinder and casing, which are often combined and within which the rotor turns; the nozzles or flow passages for the steam, which are generally fixed to the inside of the cylinder; and the frame or base for supporting both the stator and the rotor, the latter being carried in bearings.In addition, bearings, shaft glands, lubrication equipment, devices for the control of turbine speed, flexible couplings, and in some cases, reduction gears are required.

The turbine rotor carries the various wheels around which are mounted the blades. The steam decreases in pressure as it passes along the shaft and increases in volume requiring progressively larger blades on the wheels. The astern turbine is mounted on one end of the rotor and is much shorter than the ahead turbine. The turbine rotor is supported by bearings at either end; one bearing incorporates a thrust collar to resist any axial loading.

The turbine consists of a shaft, which has one or more disks to which are attached moving blades, and a casing in which the stationary blades and nozzles are mounted. The shaft is supported within the casing by means of bearings that carry the vertical and circumference loads and by axial thrust bearings that resist the axial movement caused by the flow of steam through the turbine. Seals are provided in the casing to prevent the steam from bypassing the stages of the turbine.

Blades. On the outer portion, or circumference, of each disk located on the shaft are blades where steam is directed and converted into work by rotation of the shaft. There are many blades in each turbine stage, and larger turbines have more stages. Blades generally are made from low carbon stainless steel; however, for high-temperature applications and where high moisture is expected, alloy steels are used to provide the strength and erosion resistance needed. Special coatings on the blades are often used where high erosion is anticipated.

As the steam flows through the turbine, it expands and its volume increases. This increased volume is handled by having longer blades and thus a larger casing for each stage of the turbine.

The turbine efficiency, as well as its reliable performance, depends on the design and construction of the blades. Blades not only must handle the steam velocity and temperature but also must be able to handle the centrifugal force caused by the high speed of the turbine. Any vibration in a turbine is significant because there is little clearance between the moving blades and the stationary portions on the casing. A vibration of the moving blades could cause contact with the stationary components, which would result in severe damage to the turbine. Vibration has to be monitored continuously and corrected immediately when required.

Rotor shaft and bearings. The rotor shaft is supported at each end by bearings. These are normally ball bearings on small turbines; however, on the larger turbines, a pressure-lubricated journal bearing is used. Because of the axial thrust along the shaft that results from the difference in steam pressure across the stages of the turbine, thrust bearings are used to maintain the clearances between the moving blades and the stationary portions in each stage of the turbine.

Casings and seals. Casings are steel castings whose purpose is to support the rotor bearings and to have internal surfaces that will efficiently assist in the flow of steam through the turbine. The casing also supports the stationary blades and nozzles for all stages.

At the turbine inlet, steam enters through a stop valve and steam chest. In high-temperature turbines these components are separate from the main turbine structure. In smaller units, the steam chest is usually mounted directly on the casing. At the outlet of the turbine, an exhaust hood guides the steam from the last stage to the condenser inlet.

In addition to being designed to support the weight of the stationary nozzles and blades, the casing also must resist the mechanical stresses that are caused by the reaction forces on these nozzles and blades as well as the thermal stresses that are caused by the steam temperature differentials that occur during operation in the various stages of the turbine.

Since the shaft penetrates through the casing, seals are necessary to minimize the leakage of steam. In small, low temperature turbines, carbon packing ring seals are used. These seals are located directly on the shaft and are held in place by a spring assembly.

In larger turbines, labyrinth seals are used to control steam leakage.

In many turbine designs, a combination of the two types of seals is used at the ends of the shaft.

EXERCISES

 


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