56018305 Main Components of Steam Turbine

Main components of steam turbine:

Stationary components: Almost all stationary parts are two halves. Diaphragm: Partitions between pressure stages in a turbine casing are called diaphragms. They hold the vaneshaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of a diaphragm is fitted into the top of the casing, the other half into the bottom as in Fig.

and Fig.

. The interstage

diaphragms are located in grooves in the casing accurately

Fig.

Top Diaphragm and bottom diaphragm from Michigan State University power plant [9]

Fig.

Two Diaphragm halves

Steam nozzles: Steam nozzles are installed on the peripheral of the diaphragms. There are admission Nozzles and interstage Nozzles their function is to accelerate the steam flow to high velocity by expanding it to low pressure. Located in the casing are the steam-admission nozzles which are cut into a solid block of bronze or alloy steel, depending on steam conditions. Nozzles are so proportioned as to be contributory to efficient operation and are made of corrosion- and erosion-resistant materials. This nozzle block is bolted to the steam chest, which in turn is bolted to the base of the turbine casing. The entire assembly of nozzles for one stage is called a diaphragm. The casing

assembly with the stationary blading or nozzles is referred to as the turbine cylinder. The cylinder of an impulse turbine is frequently referred to as the wheel casing. (See Fig.

Fig.

)

Nozzles, buckets, diaphragm, wheel and Shrouds [

]

Rotating parts:  Rotors for small turbines consist of a machined-steel disk shrunk and keyed onto a heavy steel shaft. The shaft is rust protected at the gland zones by a sprayed coating of stainless steel. The rotor is statically and dynamically balanced to ensure smooth operation throughout its operating range.

 Rotors for large turbines are formed from a single piece forging, including both the journals and the coupling flange. Thrust-bearing collar and oil impeller may be carried on a stub shaft bolted to the end of the rotor. Forgings of this type are carefully heat treated and must conform to specifications. Rotors are machined, and after the blades are in place, they are dynamically balanced and tested. (Fig.

)

Fig. Rotors for various types of turbines (a) rotor for condensing turbine; (b) rotor for non-condensing turbine; (c) rotor for noncondensing single-extraction turbine; (d) rotor for condensing doubleextraction turbine. (Siemens Westinghouse Power Corp.)

1.4.2.1 Wheel: A simple turbine consists of a shaft on which is mounted one or more wheels (discs). On the circumference of the wheels are located blades or buckets to receive the steam and convert it into useful work. The rims of the wheels have dovetail channels for receiving the blades. The ends of the blades are made to fit these dovetail channels. (See Fig.

Fig.

)

Rotor of a turbine in Michigan State University power plant ]

Turbine blades: On the outer portion, or circumference, of each wheel located on the shaft are blades where steam is directed and converted into work by rotation of the shaft (Fig.1.29). There are many blades in each turbine stage, and larger turbines have more stages. (Fig.1. ). 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. Figure 1.31 and Figure 1.

are a schematic

showing how the blade size varies as the steam flows through 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.

Fig.

Turbine blades of a turbine in Michigan State University power plant ]

Fig

A Turbine blade

Shroud ring: It is placed around the blades outer ends (Fig.1.26). The tips of the blades pass through holes in the shroud ring. The

ends are then welded so that they are held securely by the ring. When the blades are very long, extra lacing is sometimes used.  The function of shrouds: a. Stiffen the blades against vibration b. Confine the steam to the blade path and prevent steam axial flow.

Fig. Double-flow low-pressure turbine showing variation in blade size. (Power Magazine, a McGraw-Hill publication.)

1: Shrouds 2: Diaphragm 3: Nozzles 4: wheel 5: Blades 6: Shaft

Fig.

Turbine components

Packing: (steam Sealing) The shaft at the high-pressure end of the turbine must be packed to prevent leakage of steam from the turbine. The one at the lowpressure end of a condensing turbine must be packed to prevent the leakage of air into the condenser.  There are external steam sealing (high pressure sealing at the boiler side and low pressure sealing at the condenser side Fig.1.3 ) and Interstage steam sealing Fig.1.3 .  Types of packing:

Labyrinth packing. Water seals. Carbon packing. Flexible metallic packing.

Fig.

external steam sealing [9]

Fig.

Interstage steam sealing

Labyrinth packing Labyrinth packing is used widely in steam turbine practice. It gets its name from the fact that it is so constructed that steam in leaking must follow a winding path and change its direction many times. This device consists of a drum that turns with the shaft and is grooved on the outside. The drum turns inside a stationary cylinder

that is grooved on the inside (Fig. Fig1.3 ). There are many different types of labyrinth packing, but the general principle involved is the same for all. Steam in leaking past the packing is subjected to a throttling action. This action produces a reduction in pressure with each groove that the steam passes. The amount of leakage past the packing depends on the clearance between the stationary and the rotating elements. The amount of clearance necessary depends on the type of equipment, steam temperatures, and general service conditions. The steam that leaks past the labyrinth packing is piped to some low-pressure system or to a low stage on the turbine.

Fig. (a) Water-sealed glands and labyrinth seals as used on the high-pressure end of condensing turbines. (b) Labyrinth-type gland as used on no condensing turbines. (Siemens Westinghouse Power Corp.)

Water seals: A water-packed gland consists of a centrifugalpump runner attached to the turbine shaft. The runner rotates in a chamber in the gland casing. In some designs, water is supplied to the chamber at a pressure of 3 to 8 psi and is thrown out against the sides by the runner, forming a seal. Water seals are used in connection with labyrinth packing to prevent the steam that passes the packing from leaking into the turbine room. Such a seal is also used on the low-pressure end of condensing turbines. In this case the leakage to the condenser is water instead of air. Figure1.3 shows water-sealed glands and labyrinth seals as used on the high-pressure end of condensing turbines. They are used singly or in combination, depending on the service required. Each labyrinth consists of a multiplicity of seals to minimize steam leakage. The seal rings are spring backed and made of material that permits close running clearances with safety. The glands are usually supplied with condensate water for sealing to prevent contamination of the condensate water. Seal designs are continuously being improved to minimize steam leakage and thus improve turbine performance. The illustrated designs are typical of those found on operating turbines.

Carbon packing: Carbon packing is composed of rings of carbon held against the shaft by means of springs. Each ring fits into a separate groove in the gland casing. When adjustments are made while the turbine is cold, carbon packing should have from 0.001 to 0.002 in of clearance per inch of shaft diameter. The width of the groove in the packing casing should exceed the axial thickness of the packing ring by about 0.005 in. Carbon packing is sometimes used to pack the diaphragms of impulse turbines. Steam seals are used in connection with carbon packing. This is essential when carbon packing is used on the low-pressure end of condensing turbines, because if there is a slight packing leak, steam instead of air will leak into the condenser. In operating a turbine equipped with carbon packing, a slight leak is desirable because a small amount of steam keeps the packing lubricated. Flexible metallic packing: It is used to pack small singlestage turbines operating at low backpressure. In most cases the pressure in the casing of these turbines is only slightly above atmospheric pressure. The application is the same as when this packing is used for other purposes, except that care must be exercised in adjusting. Due to the high speed at

which the shaft operates, even a small amount of friction will cause overheating. Bearings: Bearings support and/or properly position the turbine rotor with respect to the stationary turbine parts.  Types of bearings: Journal Bearings. Thrust Bearings. .1 Journal Bearing: Their main function is to the journal or radial bearings support the weight of the rotor and position it radially. Utility turbines use journal bearing instead of ball or roller bearings. Journal bearings have a smooth surface of a soft material called Babbitt. The bearings are fed with oil as the rotor turns; it produces a pumping action that builds up pressure and a film of oil between the journal surface and the Babbitt so that in normal operation the surfaces never touch. Figure 1.36 shows the pressure distribution of the oil in the bearing.

Fig.

Formation of Oil Film in Journal Bearing

Thrust Bearing: The thrust bearing absorbs axial forces on the rotor and positions it axially with respect to the stationary turbine parts. The thrust bearing (see Figs.

and

) consists of a collar

rigidly attached to the turbine shaft rotating between two Babbittlined shoes. The clearance between the collar and the shoes is small. The piston is attached to the spindle, and steam pressure is

exerted on one side and atmospheric pressure is exerted on the other side. The difference in pressure produces a force that balances the thrust exerted on the rotating blades. If the shaft starts to move in either direction, the collar comes into contact with the shoes, and the shaft is held in proper position. Larger thrust bearings have several collars on the shaft and a corresponding number of stationary shoes. The Kingsbury thrust bearing (Fig.

) is used when a large

thrust load must be carried to maintain the proper axial position in the turbine cylinder. (The one shown in Fig. 1.37 is a combination of the Kingsbury and collar types.) The thrust collar is the same as that used in the common type of thrust bearing. The thrust shoes are made up of segments that are individually pivoted. With this arrangement, the pressure is distributed equally not only between the different segments but also on the individual segments. The openings between the segments permit the oil to enter the bearing surfaces. Almost 10 times as much pressure per square inch can be carried on the Kingsbury-type bearing as on the ordinary thrust bearing. Axial position of the bearing and turbine rotor may be adjusted by liners, located at the retainer rings, on each end of the bearing. The bearing is lubricated by circulating oil to all its moving parts. The impulse turbine does not require as large a thrust bearing as the reaction turbine because there is little or no

pressure drop through the rotating blades. However, the thrust bearing must be used to ensure proper clearance between the stationary and rotating elements. Reaction turbines that do not have some method of balancing the force caused by the drop in pressure in the rotating blades must be equipped with large thrust bearings. Turbine bearings are subjected to very severe service and require careful attention on the part of the operator. Most turbines operate at high speed (3600 rpm) and are subjected to the heat generated in the bearing itself as well as that received from the hightemperature steam. These conditions make necessary some method of cooling. In some cases the bearings are cooled by water jacketing; in others the oil is circulated through a cooler.

Fig. Main and thrust bearings: (a) main bearing; (b) section of thrust bearing and housing; (c) thrust bearing cage in place. (Siemens Westinghouse Power Corp.)

Casing: Casings are steel castings whose purpose is to

Fig.

Turbine thrust end showing balance piston and thrust bearing.

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 and also it keeps the steam in the turbine and the air out. The casing is divided into two halves upper casing and lower casing Fig.1.39. The HP/IP turbine always has shells or castings. When steam pressures and temperatures are high enough, there are two shells used to split up the pressure and temperature change. The inner shells are supported and positioned within the outer shell. The inner shells in turn support and position the other internals,

diaphragms and labyrinth seals. The shells have bolted joints at the horizontal centerline to permit assembly of the internals. In operation, the shells are covered with insulation to prevent heat loss. The low pressure turbine always has inner and outer shells or casings. Shells are most common in smaller and older units and casings on larger newer units. The outer shell or casing prevents air from entering the turbine exhaust and condenser and directs the steam from the turbine exhaust to the condenser. The exhaust hood is connected directly to the condenser, usually at the end of turbine, and so is under a partial vacuum in operation. There is a safety device (rapture disc) in the exhaust hood to prevent excessive pressure buildup if the condenser loses its vacuum.

Fig.1.39 Single-casing condensing turbine for approximately35-MW output. (Siemens Westinghouse Power Corp.).

1.8 Oil pumps: To pump the lubricating oil to the bearings. Main oil pump. AC auxiliary pump. DC auxiliary pump. Steam driven pump. Hydraulically driven pump.

Front standard: It is an extension to the turbine connected to it through a key. Also it is not insulated.  The function of the front standard : Support all control systems (Main oil pump, speed governor, over speed trip and thrust wear detector). Support all measuring equipments (Pressure indicators, Temperature indicators, Speed indicators...).

Steam chest (Nozzle box): 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. It contains the inlet control valves and the admission nozzles Fig.1. .

Fig.1

Turbine steam admission section. (Siemens Westinghouse Power Corp.) . ]

The steam chest and valve assembly shown in Fig.1.4 shows another design, and the illustration identifies the major components, including the steam inlet, the throttle valve, governor valve, and valve actuators.

Fig.1.

Turbine steam chest and valve assembly. (Siemens Westinghouse Power Corp.). [1]

The steam chest is bolted to the base and is made of iron or steel. It contains a governor valve, a strainer, and an operating hand valve that is used for manual adjustment to obtain maximum efficiency. Regardless of whether or not a hand valve is provided, the steam is made to pass through the governor-controlled admission valve contained in the steam chest. These are only typical illustrations of a small turbine design, since there are numerous designs with different features that vary between manufacturers. The multi valve steam chest (Fig.1.4 ) is cast integrally with the cylinder cover with a cored passage from each valve to a nozzle group. Single-seated valves are used, arranged in parallel within the steam chest and surrounded by steam at throttle pressure. The

governor mechanism raises and lowers the valve-lift bar in a horizontal plane, opening the valves in sequence, with an unbalanced force tending to close the valves.

Fig.1.

Simplified steam chest with multiple valves. (Siemens Westinghouse Power Corp.)

Turning gear: If a turbine is shut down and the rotor was allowed to rest in one position then due to unequal heating the spindle bends. If the turbine is a large one, vibration may occur when the turbine is started again. For these reasons most large steam turbines are provided with motor driven gear to turn the rotor slowly while the unit is out of service Fig.1.4 . and Fig. 1.44

The function of turning gear: Rotates the turbine rotor after shutdown at low speed. Rotates the turbine rotor at low speed (20-30 rpm) before starting up. Decreases the starting torque.

Fig.1.

Turning gear of a steam turbine at Michigan State University power plant ]

Fig.1.

Simplified Turning Gear