1. Introduction
1.1 History
The steam turbine has a long pedigree, stretching back to Hero¡¯s aeolipile, which was a simple reaction turbine of the ancient world, and to Branca¡¯s toy, a simple impulse turbine of vintage 1629 (Figure 1).
FIG. 12-1. Hero¡¯s and Branca¡¯s Early Turbines
Parsons designed his turbine so that the pressure drop across each stage was split into equal heat drops, or available energy, across the fixed and moving rows as illustrated in Figure 7. This became commonly known as the reaction-type turbine because of the reaction created by accelerating the flow in the moving blade. It is, in fact, more correct to designate Parsons¡¯ concept as a 50 percent reaction design because reaction is usually defined as the ratio of the heat (energy) drop across the moving row to the total heat drop over the stage.
In the Curtis?or more properly, Rateau stage turbine?the pressure drop across each stage was all taken over the fixed row (Figure 6). Thus, the impulse type stage corresponds to a zero reaction arrangement. Because of the large pressure drop across the fixed row, the nozzles are usually fitted into partitions or diaphragms having a small bore diameter where special devices are fitted to minimize flow leakage losses ; see Figure 7. With no pressure drop across the moving row, rotor thrust is minimal. The resulting geometry led to impulse turbines having so-called disc or wheel and diaphragm construction.
1.2 Basic Turbine Theory
1.2.1 Nozzles
A nozzle is a device that converts thermal energy of a fluid (gas or liquid) into kinetic (motion) energy by expanding the fluid. A typical convergent nozzle is shown in Figure 2.
FIG. 12-2. Convergent Nozzle
Assume that steam at temperature T1 and pressure P1 enters a convergent nozzle. The higher the pressure and temperature, the more thermal energy is in the steam. The steam is moving at velocity V1 before entering the nozzle. The steam leaves the nozzle at a lower pressure and temperature, T2 and P2 but at a higher velocity, V2. This is because some of the heat energy in the steam has been converted into energy of motion, called kinetic energy. Kinetic energy is a function of the square of velocity ; therefore, as the velocity increases, so does the kinetic energy.
1.2.2 Basic Turbine Types
The kinetic energy in a jet of steam is not useful as it is. The nozzle by itself cannot convert the energy in the steam to useful mechanical energy. There are two basic turbine types : impulse and reaction. Both use nozzles and rotor buckets, but in different ways.
1.2.3 Impulse Turbine
Figure 3 shows the operating principles of an impulse turbine. Steam enters an impulse turbine through a stationary nozzle that expands the steam and creates a steam jet. The steam jet strikes the rotor buckets. Note that the terms bucket and blade are synonymous, however Hanjung uses the term buckets most often.
In an ideal impulse turbine, the steam expansion occurs through the stationary nozzle ; the buckets change only steam velocity. We will see later that pure, ideal impulse turbines are seldom used in practice.
FIG. 12-3. Simplified Basic Impulse Turbine
1.2.4 Reaction Turbine
Figure 4 shown the basic operating principles of an ideal reaction turbine. The turbine rotor is forced to turn by the active force of the steam jet leaving the nozzle. In an ideal reaction turbine, the moving buckets would be the only nozzle. Therefore, all the steam expansion would occur in the moving buckets. This is impractical in large turbines because it is difficult to admit steam to moving nozzles. Thus, large turbines use fixed nozzles to admit steam to moving nozzles. Therefore, large reaction turbines use a combination of impulse and reaction principles.
FIG. 12-4. Simplified Basic Reaction
1.3 Function of Turbine Stages
Each steam turbine stage consists of a stator ring containing stationary flow guides, or nozzles, and a rotor which carries rotating blades. These components are shown in Figure 5. Nozzles and blades are both made to precise dimensions in order to accurately provide the required geometry of the flow passages in which the work of the steam is done. The nozzles and blades which form these passages are precisely designed to achieve the desired energy conversion in an efficient manner.
The blades convert the thermal energy of the steam into mechanical force. The rotor then converts this force into torque and mechanical work. This energy conversion is accompanied by a prescribed decrease in steam pressure and steam temperature, and by a corresponding increase in steam velocity, across the stage, as the steam expands.
There are two basic types of steam turbine stages : impulse stages and reaction stages. Impulse stages are used mostly at the high pressure inlet end of the turbine. In an impulse stage, the steam accelerates as it expands through the nozzle row, as shown in Figure 6. Little further expansion occurs within the rotating blade passages. Energy is then transferred to the rotor by the change in direction of the steam flow within the blade passages. The blades exert a turning force on the steam, and this force drive the rotor.
Reaction blades are customarily used toward the low-pressure end of the turbine. In a pure reaction stage, the blades and nozzles are of similar (possibly identical) profile, and the steam expands continuously as it passes through both the stationary and rotating rows. This operation is shown in Figure 7. This permits a more controlled expansion of the steam to be designed into the turbine.
FIG. 12-5. Turbine Stage Components
FIG. 12-6. Steam Flow in an Impulse Stage
FIG. 12-7. Steam Flow in a Reaction Stage
2. Turbine Component
2.1 Classification of Turbines
In Chapter 1 turbine theory and the two basic turbine types were described. Impulse and reaction turbines can be further divided into a large variety of types. To further understand turbines, it is necessary to classify turbines by their characteristics. Each of the five characteristics discussed below is applicable to both impulse and reaction turbines. These characteristics are :
¡Ü Condensing vs. noncondensing
¡Ü Extraction vs. nonextraction
¡Ü Reheat vs. nonreheat
¡Ü Single casing vs. compound
¡Ü Exhaust flows
¡Ü Peaking vs baseload
2.2 Major centerline components
This section describes the major components of the turbine which are on the turbine centerline such as the turbine casings and rotors, as well as the important supporting components such as standards and bearings. Major turbine valves and supporting systems are described later.
2.2.1 Shells and casings
The function of casings and shells in the turbine is to either keep the steam in the turbine and/or the air out. The shells and casings also support the stationary internals of the turbine and hold those parts in alignment with the rotor.
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. Appearance lagging is installed over most of the turbine. This lagging is to the turbine what the body of a car is to the frame and engine. Figure 9 shows a typical HP/IP opposed flow turbine section.
FIG. 12- 8. Tandem Compound-Two flow, Reheat, Condensing Turbine.
FIG. 12-9. Typical Opposed-Flow High Pressure Reheat Section and Intermediate Pressure Section.
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 outer shell or casing is generally referred to as the exhaust hood. The inner shell or casing supports and positions the LP turbine internals. This inner shell or casing is located inside the exhaust hood. The exhaust hood is connected directly to the condenser, usually at the bottom, and so is under a partial vacuum in operation. Figure 10 shows a typical two-flow LP section.
FIG. 12-10. Typical Two-Flow Low Pressure Section
2.2.2 Nozzles and diaphragms
The purpose of nozzles is to expand the high pressure steam to extract its energy and direct the resulting steam jets toward the rotating buckets or blades.
The nozzles are made up of many partitions that have the appearance of airfoils, similar to rotating blades. These are different in appearance from those described in chapter 1, however they function in the same manner. The partitions change the direction of steam flow to cause it to impinge on the moving blades of the rotor, as well as to increase the velocity of the flow. The partitions are held in place in a disk-like structure that, together with the partitions, is called a diaphragm. Figure 11 shows a typical diaphragm. The diaphragm fits into circumferential slots in the turbine shell inside diameter. It is split at the horizontal joint for assembly. There are labyrinth seals at the inside diameter of the diaphragm to reduce steam leakage between the rotor and the diaphragm and seal strips near the outside diameter to reduce leakage around the bucket tips.
FIG. 12-11. Typical Diaphragm
FIG. 12-12. Arrangement of Diaphragms and Buckets
(a) Cutaway
FIG. 12-13. Arrangement of Diaphragms and Buckets
(b) Cross Section
2.2.3 Turbine seals
Seals are used to control the leakage of steam. The leakage may be between turbine stages inside the turbine or where the shaft penetrates the turbine shells or casings. The interstage seals, as they are called, are mounted on the inside diameter of the diaphragms. Seals which are used to seal the penetration of the rotor through the turbine casings or one turbine section from another (as in an HP/IP opposed flow section) are mounted in packing heads or packing casings. These seals are numbered in order from the front of the turbine to the generator as N1, N2 and so on. The seals which seal the rotor at the penetration through the shells and casings are connected to the steam seal system, to be discussed later. There are a number of different seal types including carbon seals, labyrinth seals and water seals. Carbon seals are used on smaller and older units. Seals are also commonly referred to as packing.
2.2.4 Turbine rotors and buckets
The turbine buckets or blades work with the steam from the stationary nozzles to produce a torque on the rotor, or spindle as it is sometimes called. The rotor holds all the buckets and transmits the rotating mechanical energy to the generator.
FIG. 12-14. Assembly of Labyrinth Packing Ring in Hook Fit
Buckets or blades generally have the appearance of airfoil sections. The buckets at the front of the turbine are rather short but become longer through the steam path to accommodate the increasing volume of the steam as it flows through the steam path. Figure 15 shows a typical bucket. Figures 12, 13 (a) and (b) show how the bucket is installed on the rotor wheel with shroud bands at the bucket tips typing the buckets together. Shroud bands are used on almost all stages of the turbine with the exception of the last one or two stages on the LP turbine for most units. Figures 9 and 10 show typical HP/IP and LP turbine rotors.
Turbine rotors are large forgings with a bore hole at the centerline. Smaller rotors, such as those for the HP and IP sections, have wheels for the attachment of buckets machined integrally to the forging. Larger rotors, such as those used for the LP turbine, have separate wheels shrunk onto the spindle.
FIG. 12-15. Typical Turbine Bucket
2.2.5 Standards
Standards support the turbine shells and/or turbine bearings. Standards, such as the front standard, also house turbine controls and instruments.
Standards are located between turbine sections and rest on foundation plates. Some standards, particularly those for the LP turbines, are an integral part of the casing or exhaust hood and support only the bearing because exhaust hoods generally have their own support. Examples of standards are shown in Figure 8.
2.2.6 Bearings
Bearings support and/or properly position the turbine rotor with respect to the stationary turbine parts. There are generally two types of bearings, (1) journal or radial and (2) thrust. The journal or radial bearings support the weight of the rotor and position it radially. The thrust bearing absorbs axial forces on the rotor and positions it axially with respect to the stationary turbine parts. The generator also uses journal bearings which are the same design as those used for the turbine.
a. Journal bearings
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 16 shows the pressure distribution of the oil in the bearing.
FIG. 12-16. Formation of Oil Film in Journal Bearing
b. Thrust bearings
The second type of turbine bearing is the thrust bearing. Like the journal bearing, the thrust bearing builds up a thin film of oil between the bearing and a thrust runner on the rotor. Thus the rotor never touches the bearing surface in normal operation. There may be many journal bearings for each turbine ; however, there is only one thrust bearing. It is usually mounted in the front standard for smaller units or in a standard between the HP or IP section and the LP sections for larger ones.
FIG. 12-17. Thrust Bearing Details.
2.2.7 Turning gear
The turning gear turns the turbine rotor slowly, about 3-7 rpm, during shutdown, prior to starting the turbine, or when the turbine is hot. Turning the rotor slowly ensures that it is heated or cooled evenly. If the rotor is allowed to come to a rest when hot, temporary bowing and excessive vibration can result. Distortion of the turbine casing also results because the hotter steam rises to the top of the casing.
The turning gear consists of an electric motor driving a speed reducing gear train. A simplified turning gear is shown in Figure 18. The gear train drives a large ¡°clash pinion¡± or pinion gear as it is often called, that can swing toward and away from the turbine rotor. There is a ¡°bull gear¡± that is usually mounted on the outside diameter of the coupling between the turbine and generator.
FIG. 12-18. Simplified Turning Gear
2.2.8 Shaft grounding brushes
A static charge can build up on the turbine rotor due to the flow of steam over the buckets. This affect is somewhat similar to the accumulation of static charge in clouds that result in lightning. Voltages on the turbine rotor can also result from currents in the generator rotor and/or the exciter. If there were no shaft grounding brushes, there would be electrical discharges from the shaft through the bearings. When the voltage difference between the rotor and bearing became large enough to ¡°jump¡± to oil film in the bearing. The high voltage discharge (up to 150 volts) would damage the bearings. The shaft grounding brushes provide a low resistance current path from the rotor to ground. This prevents high voltages from developing and so prevents bearing damage. There are generally two types of shaft grounding brushes used.
2.3 Main steam valves
The main steam valves control the flow of steam from the boiler to the turbine under emergency and normal operating conditions. There are generally two sets of valves in series in the piping from the boiler to the turbine for each section of the turbine that gets steam directly from the boiler. On a nonreheat unit, only the HP turbine receives steam directly from the boiler, and the main stop valves and the control valves control steam flow. If the turbine is a reheat unit, then there will be a second set of valves in series at the reheat turbine inlet after the reheater, the reheat stop valves and intercept valves.
2.3.1 Main stop valves
A main stop valve is generally not a modulating valve ; it is either fully open, or fully closed. The main stop valves open fully when the turbine is reset and remain fully open until the turbine is shut down or an emergency such as overspeed trips the valves closed. The main stop valve(s) are located upstream of the control valves. There may be only one main stop valve, or as many as four, depending on the size of the unit.
Figure 19 shows a cross section of a typical main stop valve and actuator. The stem passes through the bottom of the valve, downstream of the seat through the pressure seal head. The valve has a backseat in the pressure seal head which essentially seals the valve stem when the valve in fully open. It is sealed by closely fitting bushings in the pressure seal head for situations where the valve is not fully opened or closed. There are generally connections to the steam seal system to help seal the valve. There are also drains in the valve, generally one upstream of the seat and another downstream. These are called the before and after seat drains, respectively.
2.3.2 Control valves
The control valves are located at the turbine inlet, downstream of the main stop valve(s). there are usually at least four, and as many as ten, control valves. The control valves control the flow of steam into the turbine and thus control speed before the unit is synchronized to the system, and load after it is synchronized. They may be positioned anywhere within their stroke.
FIG. 12-19. Typical Main Stop Valve Cross-Section
There are a number of different control valve designs. One of the most simple designs is called the bar lift, an example of which is shown in Figure 20. In this design, the valve stems are opened when the bar is raised. The sequence of valve opening is controlled by the length of the individual valve stems. This type of design is generally used for small units.
FIG. 12-20. Bar lift Control Valve Arrangement
The most common type of valve design is shown in Figure 21. There is a separate stem for each valve operated by a lever and cam mechanism. Often there will be a set of valves on the top of the turbine and another on the bottom.
FIG. 12- 21. Control Valve
2.4 Auxiliary steam valves
There are several kinds of auxiliary steam valves used on turbines ; specific manufacturer¡¯s information should be referenced for details. Common auxiliary steam valves discussed in this chapter are :
¡Ü Extraction nonreturn valves (positive nonreturn)
¡Ü Packing blowdown valve
¡Ü Ventilator valve
¡Ü Heating steam blocking valve
¡Ü Heating steam feed valve
¡Ü Steam pipe drain valves
2.5 Moisture separators and reheaters
One of the most significant differences between nuclear and fossil units is the steam conditions. The steam supplied to fossil units is superheated considerably at the inlet and usually reheated in the boiler and so the turbine operates with dry steam through all but the last two or three stages.
The steam supplied to nearly all nuclear units is dry but at or near saturation conditions. Reheating of the steam is possible, however the maximum reheat temperature is limited to the saturation temperature of the steam at the turbine inlet. This temperature is usually about 545 deg F as compared to the 1000 deg F reheat temperature for fossil units. The result is that nuclear units operate with wet steam throughout all or nearly all of the steam path. The steam at the turbine inlet is generally saturated but dry. At the HP turbine exhaust the steam may have as much as 15% moisture. Operation with wet steam causes inefficiency and erosion.
Moisture separators are installed between the HP turbine exhaust and the low pressure turbines. Moisture separators are large vessels with special panels which force the wet steam to follow a zig zag path. These panels are made up of ¡°chevron plates¡± as shown in Figure 22. The water droplets cannot follow the same path since they are so much more dense than the steam. The droplets fall out and are drained from the vessel. These moisture separators can remove all but a small fraction of (less than 1%) the moisture in the steam.
FIG. 12-22. Chevron Plate Moisture Separator Elements.
FIG. 12-23. Moisture Separator Reheater.
3. Turbine Supporting Systems
The turbine, line most large pieces of equipment, requires support from a number of subsystems for operation. This chapter describes these systems and their operation. The systems discussed are as follows :
¡Ü Lube oil system
¡Ü EHC hydraulic system
¡Ü Steam and water seal systems
¡Ü Exhaust hood cooling system
4. Generator System
4.1 Generator construction
There are two main components in the generator, the rotor and the stator. The generator must also have a source of DC current to magnetize the rotor, usually called the exciter. The generator may have several components and subsystems, depending on its particular features, including :
¡Ü Compartment coolers
¡Ü Gas control system
¡Ü Seal oil system
¡Ü Stator winding cooling system
Figure 24 shows a typical large, modern generator with compartment coolers and a shaft driven exciter.
4.1.1 Generator stator and windings
The generator stator, also called the armature, supports the iron core and windings, the rotor, and the compartment coolers. The stator consists of a steel plate casing called the ¡°wrapper¡± that covers a frame that in turn holds the iron core. An iron core is used in order to produce a stronger magnetic field for the generation of voltage. There are tubes within the wrapper to help distribute cooling gas. Older units use air at atmospheric pressure for cooling. Newer generators use hydrogen under pressure (from 15 to 75 psig) for cooling. Hydrogen is more effective than air in carrying away heat, and the higher its pressure, the more effective it is in removing heat. Figure 25 shows a typical stator. The core is made up of thousands of laminated steel sheet metal punchings, each of which is insulated from the others. Note that the core is referred to as ¡°iron¡± even though it is made up of these steel punchings. The insulation is necessary to avoid creating large currents in the core which would cause it to heat up to an unacceptably high temperature. The punchings are ¡°stacked¡± with spaces between groups of punchings to allow for cooling ventilation. Figure 26 shows a simplified stator core construction.
FIG. 12-24. Modern generator with shaft driven exciter.
FIG. 12-25. Typical Generator Stator
FIG. 12- 26. Simplified Stator Core Construction
4.1.2 Generator Rotor
The rotor acts as a large electromagnet. When it turns inside the stator, it induces a voltage and current in the stator windings. The rotor takes the form of a long cylinder with slots machined along its length. Copper windings fit into these slots and are held into the slots along their length by wedges that slide into the top of the slots. The slots are insulated from the windings, and each turn of the winding is insulated from the next turn. The windings are held at the ends of the rotor by retaining rings. The wedges and windings often have holes or slots in them to allow cooling gas to flow.
Most units operate at 3600 RPM. These units have what is known as a two pole winding. That is there is one winding in the field and it acts as one, large electromagnet with two poles as shown in Figure 27. Some units operate at half the normal speed, 1800 RPM. In order for these units to produce power at the same frequency as the 3600 RPM units, it is necessary to use a four pole winding. There are two sets of windings in the field, producing, in effect, two large electromagnets, for a total of four poles. The result is that the windings in the stator ¡°see¡± change of magnetic flux at the same frequency as they would if there were a two pole unit operating at 3600 RPM.
FIG. 12-27. Two Pole Generator Rotor
FIG. 12-28. Simplified Generator Rotor Construction.
4.2 Exciters
4.2.1 Types of Excitation Systems
The exciter provides the DC electric power necessary to magnetize the generator rotor. There are many types of exciters. Sometimes the exciter is separate from the generator, taking the form of a DC generator driven by an AC motor or a small steam turbine. Such a unit is said to be separately excited. In a power plant with more than one turbine-generator, there might be as many separate exciters as turbine-generators, plus a spare exciter. Such arrangements are usually found in older plants and are not so common for modern units. Most modern turbine-generators produce their own excitation and are thus called self-excited.
4.3 Generator Support Systems
4.3.1 Generator Gas Cooling Systems
The gas cooling system consists of the following :
¡Ü Gas coolers
¡Ü Rotor fans
¡Ü Gas ducts
¡Ü Stator and rotor gas passages
Systems supplying gas and measuring gas purity are not considered here but are discussed later. Figure 29 shows a typical generator gas cooling system.
FIG. 12-29. Generator Gas Cooling System Arrangement
SCCK.TK (http://www.kntc.re.kr/)
1.1 History
The steam turbine has a long pedigree, stretching back to Hero¡¯s aeolipile, which was a simple reaction turbine of the ancient world, and to Branca¡¯s toy, a simple impulse turbine of vintage 1629 (Figure 1).
FIG. 12-1. Hero¡¯s and Branca¡¯s Early Turbines
Parsons designed his turbine so that the pressure drop across each stage was split into equal heat drops, or available energy, across the fixed and moving rows as illustrated in Figure 7. This became commonly known as the reaction-type turbine because of the reaction created by accelerating the flow in the moving blade. It is, in fact, more correct to designate Parsons¡¯ concept as a 50 percent reaction design because reaction is usually defined as the ratio of the heat (energy) drop across the moving row to the total heat drop over the stage.
In the Curtis?or more properly, Rateau stage turbine?the pressure drop across each stage was all taken over the fixed row (Figure 6). Thus, the impulse type stage corresponds to a zero reaction arrangement. Because of the large pressure drop across the fixed row, the nozzles are usually fitted into partitions or diaphragms having a small bore diameter where special devices are fitted to minimize flow leakage losses ; see Figure 7. With no pressure drop across the moving row, rotor thrust is minimal. The resulting geometry led to impulse turbines having so-called disc or wheel and diaphragm construction.
1.2 Basic Turbine Theory
1.2.1 Nozzles
A nozzle is a device that converts thermal energy of a fluid (gas or liquid) into kinetic (motion) energy by expanding the fluid. A typical convergent nozzle is shown in Figure 2.
FIG. 12-2. Convergent Nozzle
Assume that steam at temperature T1 and pressure P1 enters a convergent nozzle. The higher the pressure and temperature, the more thermal energy is in the steam. The steam is moving at velocity V1 before entering the nozzle. The steam leaves the nozzle at a lower pressure and temperature, T2 and P2 but at a higher velocity, V2. This is because some of the heat energy in the steam has been converted into energy of motion, called kinetic energy. Kinetic energy is a function of the square of velocity ; therefore, as the velocity increases, so does the kinetic energy.
1.2.2 Basic Turbine Types
The kinetic energy in a jet of steam is not useful as it is. The nozzle by itself cannot convert the energy in the steam to useful mechanical energy. There are two basic turbine types : impulse and reaction. Both use nozzles and rotor buckets, but in different ways.
1.2.3 Impulse Turbine
Figure 3 shows the operating principles of an impulse turbine. Steam enters an impulse turbine through a stationary nozzle that expands the steam and creates a steam jet. The steam jet strikes the rotor buckets. Note that the terms bucket and blade are synonymous, however Hanjung uses the term buckets most often.
In an ideal impulse turbine, the steam expansion occurs through the stationary nozzle ; the buckets change only steam velocity. We will see later that pure, ideal impulse turbines are seldom used in practice.
FIG. 12-3. Simplified Basic Impulse Turbine
1.2.4 Reaction Turbine
Figure 4 shown the basic operating principles of an ideal reaction turbine. The turbine rotor is forced to turn by the active force of the steam jet leaving the nozzle. In an ideal reaction turbine, the moving buckets would be the only nozzle. Therefore, all the steam expansion would occur in the moving buckets. This is impractical in large turbines because it is difficult to admit steam to moving nozzles. Thus, large turbines use fixed nozzles to admit steam to moving nozzles. Therefore, large reaction turbines use a combination of impulse and reaction principles.
FIG. 12-4. Simplified Basic Reaction
1.3 Function of Turbine Stages
Each steam turbine stage consists of a stator ring containing stationary flow guides, or nozzles, and a rotor which carries rotating blades. These components are shown in Figure 5. Nozzles and blades are both made to precise dimensions in order to accurately provide the required geometry of the flow passages in which the work of the steam is done. The nozzles and blades which form these passages are precisely designed to achieve the desired energy conversion in an efficient manner.
The blades convert the thermal energy of the steam into mechanical force. The rotor then converts this force into torque and mechanical work. This energy conversion is accompanied by a prescribed decrease in steam pressure and steam temperature, and by a corresponding increase in steam velocity, across the stage, as the steam expands.
There are two basic types of steam turbine stages : impulse stages and reaction stages. Impulse stages are used mostly at the high pressure inlet end of the turbine. In an impulse stage, the steam accelerates as it expands through the nozzle row, as shown in Figure 6. Little further expansion occurs within the rotating blade passages. Energy is then transferred to the rotor by the change in direction of the steam flow within the blade passages. The blades exert a turning force on the steam, and this force drive the rotor.
Reaction blades are customarily used toward the low-pressure end of the turbine. In a pure reaction stage, the blades and nozzles are of similar (possibly identical) profile, and the steam expands continuously as it passes through both the stationary and rotating rows. This operation is shown in Figure 7. This permits a more controlled expansion of the steam to be designed into the turbine.
FIG. 12-5. Turbine Stage Components
FIG. 12-6. Steam Flow in an Impulse Stage
FIG. 12-7. Steam Flow in a Reaction Stage
2. Turbine Component
2.1 Classification of Turbines
In Chapter 1 turbine theory and the two basic turbine types were described. Impulse and reaction turbines can be further divided into a large variety of types. To further understand turbines, it is necessary to classify turbines by their characteristics. Each of the five characteristics discussed below is applicable to both impulse and reaction turbines. These characteristics are :
¡Ü Condensing vs. noncondensing
¡Ü Extraction vs. nonextraction
¡Ü Reheat vs. nonreheat
¡Ü Single casing vs. compound
¡Ü Exhaust flows
¡Ü Peaking vs baseload
2.2 Major centerline components
This section describes the major components of the turbine which are on the turbine centerline such as the turbine casings and rotors, as well as the important supporting components such as standards and bearings. Major turbine valves and supporting systems are described later.
2.2.1 Shells and casings
The function of casings and shells in the turbine is to either keep the steam in the turbine and/or the air out. The shells and casings also support the stationary internals of the turbine and hold those parts in alignment with the rotor.
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. Appearance lagging is installed over most of the turbine. This lagging is to the turbine what the body of a car is to the frame and engine. Figure 9 shows a typical HP/IP opposed flow turbine section.
FIG. 12- 8. Tandem Compound-Two flow, Reheat, Condensing Turbine.
FIG. 12-9. Typical Opposed-Flow High Pressure Reheat Section and Intermediate Pressure Section.
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 outer shell or casing is generally referred to as the exhaust hood. The inner shell or casing supports and positions the LP turbine internals. This inner shell or casing is located inside the exhaust hood. The exhaust hood is connected directly to the condenser, usually at the bottom, and so is under a partial vacuum in operation. Figure 10 shows a typical two-flow LP section.
FIG. 12-10. Typical Two-Flow Low Pressure Section
2.2.2 Nozzles and diaphragms
The purpose of nozzles is to expand the high pressure steam to extract its energy and direct the resulting steam jets toward the rotating buckets or blades.
The nozzles are made up of many partitions that have the appearance of airfoils, similar to rotating blades. These are different in appearance from those described in chapter 1, however they function in the same manner. The partitions change the direction of steam flow to cause it to impinge on the moving blades of the rotor, as well as to increase the velocity of the flow. The partitions are held in place in a disk-like structure that, together with the partitions, is called a diaphragm. Figure 11 shows a typical diaphragm. The diaphragm fits into circumferential slots in the turbine shell inside diameter. It is split at the horizontal joint for assembly. There are labyrinth seals at the inside diameter of the diaphragm to reduce steam leakage between the rotor and the diaphragm and seal strips near the outside diameter to reduce leakage around the bucket tips.
FIG. 12-11. Typical Diaphragm
FIG. 12-12. Arrangement of Diaphragms and Buckets
(a) Cutaway
FIG. 12-13. Arrangement of Diaphragms and Buckets
(b) Cross Section
2.2.3 Turbine seals
Seals are used to control the leakage of steam. The leakage may be between turbine stages inside the turbine or where the shaft penetrates the turbine shells or casings. The interstage seals, as they are called, are mounted on the inside diameter of the diaphragms. Seals which are used to seal the penetration of the rotor through the turbine casings or one turbine section from another (as in an HP/IP opposed flow section) are mounted in packing heads or packing casings. These seals are numbered in order from the front of the turbine to the generator as N1, N2 and so on. The seals which seal the rotor at the penetration through the shells and casings are connected to the steam seal system, to be discussed later. There are a number of different seal types including carbon seals, labyrinth seals and water seals. Carbon seals are used on smaller and older units. Seals are also commonly referred to as packing.
2.2.4 Turbine rotors and buckets
The turbine buckets or blades work with the steam from the stationary nozzles to produce a torque on the rotor, or spindle as it is sometimes called. The rotor holds all the buckets and transmits the rotating mechanical energy to the generator.
FIG. 12-14. Assembly of Labyrinth Packing Ring in Hook Fit
Buckets or blades generally have the appearance of airfoil sections. The buckets at the front of the turbine are rather short but become longer through the steam path to accommodate the increasing volume of the steam as it flows through the steam path. Figure 15 shows a typical bucket. Figures 12, 13 (a) and (b) show how the bucket is installed on the rotor wheel with shroud bands at the bucket tips typing the buckets together. Shroud bands are used on almost all stages of the turbine with the exception of the last one or two stages on the LP turbine for most units. Figures 9 and 10 show typical HP/IP and LP turbine rotors.
Turbine rotors are large forgings with a bore hole at the centerline. Smaller rotors, such as those for the HP and IP sections, have wheels for the attachment of buckets machined integrally to the forging. Larger rotors, such as those used for the LP turbine, have separate wheels shrunk onto the spindle.
FIG. 12-15. Typical Turbine Bucket
2.2.5 Standards
Standards support the turbine shells and/or turbine bearings. Standards, such as the front standard, also house turbine controls and instruments.
Standards are located between turbine sections and rest on foundation plates. Some standards, particularly those for the LP turbines, are an integral part of the casing or exhaust hood and support only the bearing because exhaust hoods generally have their own support. Examples of standards are shown in Figure 8.
2.2.6 Bearings
Bearings support and/or properly position the turbine rotor with respect to the stationary turbine parts. There are generally two types of bearings, (1) journal or radial and (2) thrust. The journal or radial bearings support the weight of the rotor and position it radially. The thrust bearing absorbs axial forces on the rotor and positions it axially with respect to the stationary turbine parts. The generator also uses journal bearings which are the same design as those used for the turbine.
a. Journal bearings
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 16 shows the pressure distribution of the oil in the bearing.
FIG. 12-16. Formation of Oil Film in Journal Bearing
The second type of turbine bearing is the thrust bearing. Like the journal bearing, the thrust bearing builds up a thin film of oil between the bearing and a thrust runner on the rotor. Thus the rotor never touches the bearing surface in normal operation. There may be many journal bearings for each turbine ; however, there is only one thrust bearing. It is usually mounted in the front standard for smaller units or in a standard between the HP or IP section and the LP sections for larger ones.
FIG. 12-17. Thrust Bearing Details.
2.2.7 Turning gear
The turning gear turns the turbine rotor slowly, about 3-7 rpm, during shutdown, prior to starting the turbine, or when the turbine is hot. Turning the rotor slowly ensures that it is heated or cooled evenly. If the rotor is allowed to come to a rest when hot, temporary bowing and excessive vibration can result. Distortion of the turbine casing also results because the hotter steam rises to the top of the casing.
The turning gear consists of an electric motor driving a speed reducing gear train. A simplified turning gear is shown in Figure 18. The gear train drives a large ¡°clash pinion¡± or pinion gear as it is often called, that can swing toward and away from the turbine rotor. There is a ¡°bull gear¡± that is usually mounted on the outside diameter of the coupling between the turbine and generator.
FIG. 12-18. Simplified Turning Gear
2.2.8 Shaft grounding brushes
A static charge can build up on the turbine rotor due to the flow of steam over the buckets. This affect is somewhat similar to the accumulation of static charge in clouds that result in lightning. Voltages on the turbine rotor can also result from currents in the generator rotor and/or the exciter. If there were no shaft grounding brushes, there would be electrical discharges from the shaft through the bearings. When the voltage difference between the rotor and bearing became large enough to ¡°jump¡± to oil film in the bearing. The high voltage discharge (up to 150 volts) would damage the bearings. The shaft grounding brushes provide a low resistance current path from the rotor to ground. This prevents high voltages from developing and so prevents bearing damage. There are generally two types of shaft grounding brushes used.
2.3 Main steam valves
The main steam valves control the flow of steam from the boiler to the turbine under emergency and normal operating conditions. There are generally two sets of valves in series in the piping from the boiler to the turbine for each section of the turbine that gets steam directly from the boiler. On a nonreheat unit, only the HP turbine receives steam directly from the boiler, and the main stop valves and the control valves control steam flow. If the turbine is a reheat unit, then there will be a second set of valves in series at the reheat turbine inlet after the reheater, the reheat stop valves and intercept valves.
2.3.1 Main stop valves
A main stop valve is generally not a modulating valve ; it is either fully open, or fully closed. The main stop valves open fully when the turbine is reset and remain fully open until the turbine is shut down or an emergency such as overspeed trips the valves closed. The main stop valve(s) are located upstream of the control valves. There may be only one main stop valve, or as many as four, depending on the size of the unit.
Figure 19 shows a cross section of a typical main stop valve and actuator. The stem passes through the bottom of the valve, downstream of the seat through the pressure seal head. The valve has a backseat in the pressure seal head which essentially seals the valve stem when the valve in fully open. It is sealed by closely fitting bushings in the pressure seal head for situations where the valve is not fully opened or closed. There are generally connections to the steam seal system to help seal the valve. There are also drains in the valve, generally one upstream of the seat and another downstream. These are called the before and after seat drains, respectively.
2.3.2 Control valves
The control valves are located at the turbine inlet, downstream of the main stop valve(s). there are usually at least four, and as many as ten, control valves. The control valves control the flow of steam into the turbine and thus control speed before the unit is synchronized to the system, and load after it is synchronized. They may be positioned anywhere within their stroke.
FIG. 12-19. Typical Main Stop Valve Cross-Section
There are a number of different control valve designs. One of the most simple designs is called the bar lift, an example of which is shown in Figure 20. In this design, the valve stems are opened when the bar is raised. The sequence of valve opening is controlled by the length of the individual valve stems. This type of design is generally used for small units.
FIG. 12-20. Bar lift Control Valve Arrangement
The most common type of valve design is shown in Figure 21. There is a separate stem for each valve operated by a lever and cam mechanism. Often there will be a set of valves on the top of the turbine and another on the bottom.
FIG. 12- 21. Control Valve
2.4 Auxiliary steam valves
There are several kinds of auxiliary steam valves used on turbines ; specific manufacturer¡¯s information should be referenced for details. Common auxiliary steam valves discussed in this chapter are :
¡Ü Extraction nonreturn valves (positive nonreturn)
¡Ü Packing blowdown valve
¡Ü Ventilator valve
¡Ü Heating steam blocking valve
¡Ü Heating steam feed valve
¡Ü Steam pipe drain valves
2.5 Moisture separators and reheaters
One of the most significant differences between nuclear and fossil units is the steam conditions. The steam supplied to fossil units is superheated considerably at the inlet and usually reheated in the boiler and so the turbine operates with dry steam through all but the last two or three stages.
The steam supplied to nearly all nuclear units is dry but at or near saturation conditions. Reheating of the steam is possible, however the maximum reheat temperature is limited to the saturation temperature of the steam at the turbine inlet. This temperature is usually about 545 deg F as compared to the 1000 deg F reheat temperature for fossil units. The result is that nuclear units operate with wet steam throughout all or nearly all of the steam path. The steam at the turbine inlet is generally saturated but dry. At the HP turbine exhaust the steam may have as much as 15% moisture. Operation with wet steam causes inefficiency and erosion.
Moisture separators are installed between the HP turbine exhaust and the low pressure turbines. Moisture separators are large vessels with special panels which force the wet steam to follow a zig zag path. These panels are made up of ¡°chevron plates¡± as shown in Figure 22. The water droplets cannot follow the same path since they are so much more dense than the steam. The droplets fall out and are drained from the vessel. These moisture separators can remove all but a small fraction of (less than 1%) the moisture in the steam.
FIG. 12-22. Chevron Plate Moisture Separator Elements.
FIG. 12-23. Moisture Separator Reheater.
The turbine, line most large pieces of equipment, requires support from a number of subsystems for operation. This chapter describes these systems and their operation. The systems discussed are as follows :
¡Ü Lube oil system
¡Ü EHC hydraulic system
¡Ü Steam and water seal systems
¡Ü Exhaust hood cooling system
4. Generator System
4.1 Generator construction
There are two main components in the generator, the rotor and the stator. The generator must also have a source of DC current to magnetize the rotor, usually called the exciter. The generator may have several components and subsystems, depending on its particular features, including :
¡Ü Compartment coolers
¡Ü Gas control system
¡Ü Seal oil system
¡Ü Stator winding cooling system
Figure 24 shows a typical large, modern generator with compartment coolers and a shaft driven exciter.
4.1.1 Generator stator and windings
The generator stator, also called the armature, supports the iron core and windings, the rotor, and the compartment coolers. The stator consists of a steel plate casing called the ¡°wrapper¡± that covers a frame that in turn holds the iron core. An iron core is used in order to produce a stronger magnetic field for the generation of voltage. There are tubes within the wrapper to help distribute cooling gas. Older units use air at atmospheric pressure for cooling. Newer generators use hydrogen under pressure (from 15 to 75 psig) for cooling. Hydrogen is more effective than air in carrying away heat, and the higher its pressure, the more effective it is in removing heat. Figure 25 shows a typical stator. The core is made up of thousands of laminated steel sheet metal punchings, each of which is insulated from the others. Note that the core is referred to as ¡°iron¡± even though it is made up of these steel punchings. The insulation is necessary to avoid creating large currents in the core which would cause it to heat up to an unacceptably high temperature. The punchings are ¡°stacked¡± with spaces between groups of punchings to allow for cooling ventilation. Figure 26 shows a simplified stator core construction.
FIG. 12-24. Modern generator with shaft driven exciter.
FIG. 12-25. Typical Generator Stator
FIG. 12- 26. Simplified Stator Core Construction
4.1.2 Generator Rotor
The rotor acts as a large electromagnet. When it turns inside the stator, it induces a voltage and current in the stator windings. The rotor takes the form of a long cylinder with slots machined along its length. Copper windings fit into these slots and are held into the slots along their length by wedges that slide into the top of the slots. The slots are insulated from the windings, and each turn of the winding is insulated from the next turn. The windings are held at the ends of the rotor by retaining rings. The wedges and windings often have holes or slots in them to allow cooling gas to flow.
Most units operate at 3600 RPM. These units have what is known as a two pole winding. That is there is one winding in the field and it acts as one, large electromagnet with two poles as shown in Figure 27. Some units operate at half the normal speed, 1800 RPM. In order for these units to produce power at the same frequency as the 3600 RPM units, it is necessary to use a four pole winding. There are two sets of windings in the field, producing, in effect, two large electromagnets, for a total of four poles. The result is that the windings in the stator ¡°see¡± change of magnetic flux at the same frequency as they would if there were a two pole unit operating at 3600 RPM.
FIG. 12-27. Two Pole Generator Rotor
FIG. 12-28. Simplified Generator Rotor Construction.
4.2.1 Types of Excitation Systems
The exciter provides the DC electric power necessary to magnetize the generator rotor. There are many types of exciters. Sometimes the exciter is separate from the generator, taking the form of a DC generator driven by an AC motor or a small steam turbine. Such a unit is said to be separately excited. In a power plant with more than one turbine-generator, there might be as many separate exciters as turbine-generators, plus a spare exciter. Such arrangements are usually found in older plants and are not so common for modern units. Most modern turbine-generators produce their own excitation and are thus called self-excited.
4.3 Generator Support Systems
4.3.1 Generator Gas Cooling Systems
The gas cooling system consists of the following :
¡Ü Gas coolers
¡Ü Rotor fans
¡Ü Gas ducts
¡Ü Stator and rotor gas passages
Systems supplying gas and measuring gas purity are not considered here but are discussed later. Figure 29 shows a typical generator gas cooling system.
FIG. 12-29. Generator Gas Cooling System Arrangement
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