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Steam Turbine Generator Fundamentals .

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STEAM TURBINE-GENERATOR FUNDAMENTALS THERMODYNAMIC PRINCIPLES Chapter 1

STEAM TURBINE THEORY Chapter 2

STEAM TURBINE UNIT DESCRIPTION Chapter 3

STEAM TURBINE MAJOR COMPONENTS Chapter 4

STEAM TURBINE VALVES Chapter 5

STEAM TURBINE AUXILIARY SYSTEMS Chapter 6

GENERATOR THEORY Chapter 7

GENERATOR CONSTRUCTION Chapter 8

GENERATOR AUXILIARY SYSTEM Chapter 9

© 1999 - TG201J5.0_June09, Printed: 3/1/11

THERMODYNAMIC PRINCIPLES Chapter 1

TERMINAL OBJECTIVE:

The goal of this chapter is to provide an understanding of the Power Plant Thermodynamics Principles.

ENABLING OBJECTIVES:

At the completion of this section the participant should be able to: 1. 2. 3. 4. 5.

List the energy conversions which take place in the power plant cycle. Explain the First Law of Thermodynamics. Explain the Second Law of Thermodynamics. Describe the difference between an Open System and a Closed System. Explain the phases of water.

© 1999 - TG201J5.0_June09, Printed: 12/1412010

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201

TABLE OF CONTENTS 1.0

INTRODUCTION ................................................................................................................................... 3 1.1 1.1.1 1.2 1.3 1.3 .1 1.4

2.0

The First Law of Thermodynamics ........................................................................................... 11 The Second Law of Thermodynamics ....................................................................................... 14 T-S Diagrams ............................................................................................................................ 15

WATER AND STEAM ......................................................................................................................... 16 3.1 3.2

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3 3 7 8 8 9

LAWS OF THERMODYNAMICS ...................................................................................................... 10 2.1 2.2 2.3

3.0

Energy ......................................................................................................................................... Types of Energy ...................................................................................................................... Units of Energy and Work ........................................................................................................... Work and Power .......................................................................................................................... Work ....................................................................................................................................... Energy Conversion in a Power Plant.. .........................................................................................

Properties of Water ................................................................................................................... 17 Steam Tables and the Mollier Diagram ..................................................................................... 18

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THERMODYNAMIC PRINCIPLES 1.0

INTRODUCTION

Thennodynamics is the science that describes and defines the conversion of one fonn of energy into another. Examples include the conversion of chemical energy into thennal energy, which occurs during the combustion process, and the transformation of thermal energy into mechanical energy, which takes place in the turbine. Each step in the conversion of energy is termed a "process" and several processes constitute a thermodynamic system or cycle. The thennodynamic cycle that is used in conventional power plants is used to produce work to turn a generator that make the final conversion of energy into electrical energy. The water and steam used in the conventional power plant is the working fluid of the thennodynamic cycle. The working fluid conveys energy between different components and is used in each process. The steam undergoes several changes in the conversion of energy. This chapter relates energy, work, and heat with the working fluid of a power plant. The concepts presented in this module will provide a better understanding of power plant operation and efficiency.

1.1

ENERGY

Energy is a fundamental aspect of all fonns of matter and all systems. One of the most important aspects of energy is expressed as a physical law; the Law of Conservation of Energy. This law states that energy can be changed from one type to another, but it cannot be created or destroyed. Energy can be thought of as the ability or capacity to do work. When work is done, energy is frequently changed from one type to another in accordance with the Law of Conservation of Energy.

1.1.1

Types of Energy

A power plant may be thought of as an "energy conversion factory" that converts one type of energy to another type. There are many different types of energy. Four types of energy used in the power plant cycle are chemical energy, mechanical energy, heat energy, and electrical energy.

Chemical Energy Chemical energy is the energy locked in the molecular bonds of a chemical compound (fuel in the case of a power plant). The chemical energy is released by a chemical reaction, such as that which occurs when oxygen and heat are supplied to bum the fuel. The chemical structure of the fuel is changed and the combustion products that result are at a lower energy level. The difference in the chemical energy level of the fuel and the combustion products is converted to heat energy.

Mechanical Energy Mechanical energy is made up of two different components, potential energy and kinetic energy. Potential energy is the energy an object has as a result of its distance from the center of the earth, or its elevation. The higher the elevation of an object the more potential energy it has. Kinetic energy is the energy that a substance has as a result of its velocity. The higher the velocity of a substance the more kinetic energy it has. In fact, kinetic energy in a substance is proportional to the square of its velocity. Thus, if one were to double the velocity of an object like a bal1, its kinetic energy would increase by a factor of four. An object, such as a bal1, may have both potential and kinetic energy. This is true, for instance for a bal1 that has been thrown into the air and is 20 feet above the ground and has a velocity of 40 feet per second. The sum of the potential and kinetic energy of the ball is its mechanical energy.

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 There are changes in both potential and kinetic energy in the power plant cycle. The role that potential energy plays in the overall energy conversion, however, is relatively unimportant when compared to the other types of energy used in the power plant cycle. Accordingly, kinetic energy is usually the only type of energy considered in mechanical energy.

Heat Energy, Temperature and Enthalpy Heat energy is the energy in a substance that is caused by temperature and pressure. Heat energy is actually made up of two different types of energy; internal energy and pressure-volume (P-v) energy. Internal energy in a substance depends upon its temperature. The motion of molecules of a substance is internal energy. The molecules of a substance are constantly rotating, vibrating, and moving from place to place at high velocity. The amount of motion is determined by the temperature of the substance. The higher its temperature, the greater the molecular motion and thus the greater its internal energy. Temperature can be expressed in many different scales. In the English system, the Fahrenheit scale is defmed with the freezing point for water at 32°F and the boiling point (at sea level) at 212°F. Another scale important in thermodynamics that is significant with regard to internal energy is called the Rankine scale. The "zero point" for the Rankine temperature scale is "absolute zero. " Absolute zero is the temperature at which, in theory, all molecular motion stops (-459 .67°F). The internal energy of any substance at absolute zero would be zero since internal energy is determined by molecular motion. The Rankine temperature scale must be used in some areas of thermodynamics that are described later in this module. Temperature can be converted from degrees Fahrenheit to degrees Rankine by adding459.67 to the temperature in Fahrenheit. Thus, for example, 1000°F is 1459.67°R. In the English system internal energy is expressed in a unit called the British Thermal Unit (BTU). The BTU is defmed as the amount of heat required to change the temperature of one pound of water one degree Fahrenheit. Increasing the temperature of a pound of water by 1°F, therefore, increases its internal energy by 1 BTU.

Different substances have different amounts of internal energy at the same temperature. For example, to increase the temperature of 1 pound of steel at 60°F by 1°F, it takes 0.118 BTU; 1 pound of petroleum, 0.5 BTU. Therefore, 1 pound of water has more internal energy than 1 pound of steel or petroleum at the same temperature. The state of a substance, solid, liquid, or gas, also has considerable influence on its heat energy. For instance, water at the freezing point has much more heat energy than ice at the freezing point. As heat energy is added to ice and the ice changes state to water, the molecular structure of the ice becomes more random. One way of considering this change is to say that the ice molecules must fmd room to move. Similarly, steam at the boiling temperature has more heat energy than water at the same temperature. The molecules of steam are in motion and freer to move than those of a liquid. Because the gas molecules are at a higher energy level and are free to separate and move, gases like air or steam are compressible. This means that their volume can be greatly reduced if put under pressure. Compression of a gas increases its internal energy. This P-v energy can be put to work by expanding the gas. Since gases are compressible and can retain P-v energy, they also have a greater enthalpy (total energy) than a solid or liquid. P-v energy in a substance depends upon its pressure and specific volume. The higher the pressure of a fluid, such as steam for instance, the greater its energy. A substance at a given pressure and temperature occupies a fixed volume that can be determined by the parameter-specific volume. Specific volume is the volume occupied by one pound mass of a substance. Specific volume in the English system is expressed in terms of cubic feet per pound mass. Specific volume is also the inverse of density. The product of the pressure and specific volume ofa substance is a measure of the P-v energy. Pressure in the English system is measured in pounds per square inch. There are two variations in pressure units. The most common of these is expressed in pounds per square inch gauge (psig). Atmospheric pressure is defined as zero psig. Most pressure measurements in everyday situations, including power plants, are made in psig (the pounds gauge scale).

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THERMODYNAMIC PRINCIPLES The other variation in pressure measurement is pounds per square inch absolute (psia). The difference between psig and psia is the zero point of the scale. For psia zero is a perfect vacuum. Thus, atmospheric pressure (which is zero psig) is 14.69 psia. To convelt from psia to psig, atmospheric pressure (14.69 psia) is added to the reading in psia. The absolute pressure scale is much less common than the pounds gauge scale, however it is important because it is used for most thermodynamic calculations. The amount of heat energy in a substance is usually measured as its enthalpy. The enthalpy ofa substance is the sum of its internal energy and its P-venergy. This is expressed by the equation:

Equation 1-01 where

h u Pv 778

enthalpy (BTU/lb) internal energy (BTU/lb) the pressure-volume energy (pressure x specific volume) = conversion factor (778 ft-lb/BTU)

= = =

The specific volume expression used in calculating the P-v energy above is defined as the volume per unit mass of a substance. A foot-pound (ft-Ib) is the unit of work. However, because both heat and work are forms of energy, a conversion factor 778 ft-lb/BTU, can be used to convert the units. Figure 1-01 illustrates the concept of heat energy. Energy from the burning candle is transferred to the air . in the sealed container. The candle converts energy from the combustion ofparaffm and air. The air in the container absorbs this energy in two forms: (I) the internal energy of the air in the container increases as its temperature increases, and (2) the pressure-volume energy increases because its pressure increases. This example illustrates that while heat energy is thought of as two different types of energy, these two types of energy are closely related. The reason that the pressure of the air in the sealed container in the example increases is due to the increase in temperature.

-

THERMOMETER

AIR

Figure 1-01 Heat Energy

CONTAINER

./

HEAT TRANSFER

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Heat energy is difficult to use to do work. Heat energy is usually converted to mechanical energy because mechanical energy can be used more easily. Figure 1-02 shows heat energy being converted to mechanical energy using steam in a piston and cylinder arrangement. The steam is under pressure. Pressure is produced by the steam's molecules colliding with the cylinder walls and piston. The steam does work by exerting a force on the piston, which causes the piston to move. As the piston moves out, the volume of the steam increases as the pressure and heat energy decrease. The temperature of the steam also decreases, also causing a decrease in heat energy. This process is called expansion. The difference in the heat energy of the steam before and after the expansion is the energy that was converted to mechanical energy. . .

Expanding Steam

Figure 1-02 Conversion of Heat Energy to Mechanical Energy Expansion of steam for energy coiwersion is used in power plant steam turbines. Steam enters the turbine at high pressure (typically around 2400 psig) and is expanded to a very low pressure, nearly a vacuum. The temperature of the steam also falls considerably in expansion through the turbine; typically from lOOO°F to about 80°F to lOO°F. In the steam turbine expansion process, the heat energy in the steam is converted to mechanical energy to do the work of turning the generator rotor. Electrical Energy Electrical energy is a result of electrons flowing through a conductor. The amount of electrical energy flowing through a conductor is determined by the amount of electron flow, or current (measured in amps) and the "electrical pressure," or voltage, against which the electrons must flow. There are two types of electricity used in power plants, direct current (DC) and alternating current (AC). In DC electricity, the electrons always flow in the same direction. In AC electricity, the direction of the flow of electron changes continuously, reversing itself 60 times per second for 60 HZ power. There is a relationship between the current and voltage in a conductor for DC electricity called Ohm 's Law. Ohm's law may be written as:

E=I

X

R

Equation 1-02 Where: E = voltage in volts I = Current in amps R = Resistance in Ohms


THERMODYNAMIC PRINCIPLES The greater the current for a given voltage, the greater the electrical energy flowing through an electrical conductor. Similarly, the greater the voltage for a given current, the greater the electrical energy. Units of electrical energy are watts. Electrical power (for direct current circuits) can be detennined from the following circuits using the following equation p= E x I

Equation 1-03 Where: P = Power in watts E = Voltage in volts I = Current in amps The two equations above apply only to DC electricity. Similar relationships exist for AC electricity and these are explained later in this course. Electrical energy is usually expressed in terms of watt-hours. Watt-hours are the product of power and the time for which it is generated. This is true for both AC and DC electricity. Electrical power can be produced using mechanical force through the use of magnetism. When a magnetic field is moved near a conductor, voltage is induced in the conductor. This voltage results in current to the load. In most power plant generators the rotor is a large electromagnet. It is rotated inside the stationary armature which has many conductors. As the torque that is exerted on the generator rotor increases, current increases and thus power electrical generation is increased. The details of generators and how they work are covered in detail later in this course.

1.2

UNITS OF ENERGY AND WORK

Units are used to describe the size and magnitude of various properties of matter. In the discussion of temperature earlier in this Section, for example, it was explained that the unit degree Fahrenheit can be used to express the temperature of a substance. Work, energy and properties of substances are expressed in many different units. Many quantities and properties can be expressed using more than one type of unit. As an example, temperature can be expressed in degrees Fahrenheit or degrees Rankine. The choice of units often depends on the discipline being considered. When working with electrical equipment it is convenient to use electrical units such as volts, amps and watts. When working with mechanical components, it is convenient to work in mechanical units such as pounds, feet, footpounds, and BTUs. Since the same parameter may be expressed in different units, it is often necessary to "conveli" the units through the use of conversion factors. An example of a conversion factor is that used to convert temperature from degrees Fahrenheit to degrees Rankine . The conversion factor 459.67 is added to degrees Fahrenheit to obtain degrees Rankine. In many cases conversion factors must be used by multiplying or dividing rather than adding or subtracting. Conversion factors are published in many different places. Prefixes are also commonly used with units . Common examples of prefixes are "kilo," which means one thousand, and "mega" which means million. A conversion factor is implied when these prefixes are used. For example one kilowatt is equal to 1,000 watts. The conversion factor in this instance is 1000 watts per kilowatt. It is also common to use abbreviations with units. Examples of common abbreviations are "OF" for degrees Fahrenheit, "KW" for kilowatts and "BTU" for British Thermal Units. Conversion tables usually provide these abbreviations as well as the conversion factors .

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Conversion factors are used in the following example in which the efficiency of a power plant is determined. A power plant bums coal that has a heating value of 13,000 BTU/lb at a rate of220,000 pounds per hour and produces 250,000 KW of electricity. It produces 2.86 billion BTU per hour through the conversion of chemical to heat energy. The power plant also produces 250,000 KW -hours (KWH) of electrical energy per hour. The efficiency of the plant is defined as the ratio of the energy supplied to the plant to the useful energy produced. It is necessary to express the energy supplied and the useful energy produced in the same units in order to make this calculation. Since the energy is expressed in BTUs and the energy produced is expressed in different units, the conversion factor 3413 BTUIKWH must be used as shown ill the following equation. .

2jO,OOOKWH - :( - J 4 1 ~ IHV I KWH

E.lJldem:v- - - - - - - - - - - - - - x - Iom{' = 29.8% 2.860,000,000 - BTU

Eauation 1-04

1.3

WORK AND POWER

A full understanding of energy conversion in power plants requires that various concepts related to energy be understood as well. Among these concepts are work, energy and entropy.

1.3.1

Work

Energy can be defined asthe capacity to do work. Another way to defme work is in terms of mechanical energy. Work in terms of mechanical energy is the action of a force moving an object over a distance. In fact, work is often considered as energy in motion since moving an object increases its kinetic energy. Work can also be thought of as a way to convert one type of energy to another. The turbine, for example does work on the generator by exerting a force (torque) on the generator as it moves (rotates). The generator then converts the mechanical energy from this work to electrical energy. Figure 1-03 illustrates a small steam turbine that being used to lift a weight. The steam turbine converts the heat energy of the steam into mechanical energy to lift the weight. The weight has more potential energy after it has been lifted to a higher elevation through the work of the turbine. The turbine has converted heat energy to potential energy by working. STEAM IN

l TURBINE

l

STEAM OUT

WEIGHT

Fi::ure 1-03 Wei::ht Lffted by Small Steam Turbine


THERMODYNAMIC PRINCIPLES Another example of work involved in conversion of energy occurs In a pump. A prime mover, such as a motor or a turbine, transforms energy (electrical or heat) to mechanical energy to rotate the pump. The pump uses this mechanical energy to do work on the fluid, increasing the energy of the fluid. The result of the increase in the fluid's energy is generally seen as an increase in the pressure of the fluid. There may be other changes in energy as well, such as an increase in the velocity of the fluid or an increase in its temperature. The increase in velocity results in an increase the kinetic energy of the fluid, whereas, the increase in temperature results in an increase in the internal energy of the fluid. Typically, this temperature increase is very small.

1.3.2

Power

It is useful to know how much energy is necessary to make a process occur. The amount of energy alone is not enough to describe many processes, however. The rate at which the energy is delivered to or generated from a process is also important. Power is the rate at which work is done. For example, in Figure 1-03, if the weight is lifted at a speed that is twice the original speed, then twice as much power is being used. Regardless of the rate, however, the same amount of work is performed and the same amount of energy is used if the weight is lifted the same distance.

1.4

ENERGY CONVERSION IN A POWER PLANT

A power plant receives fuel and burns it to convert the chemical energy of the fuel into heat energy. In a gas turbine, this energy is converted directly to mechanical energy as the hot gases expand to drive the turbine. Some of the mechanical energy of the turbine is transferred through the shaft to the compressor to increase the pressure and temperature of the air used in the gas turbine. The rest of the mechanical energy is transmitted through the shaft to the generator where it is converted to electrical energy. In a combined cycle plant,hot gases from the gas turbine are exhausted to a heat recovery steam generator (HRSG) where additional energy conversion takes place. The heat energy of the gases is transfelTed to the water in the HRSG, steam is formed and then superheated. The heat transfer takes place in the tubes inside the HRSG. The internal energy of the steam is increased through the absorption of heat. The pressure increases because the volume of the gaseous steam is limited. The heat energy in the steam from the HRSG is converted to mechanical energy in the steam turbine. The turbine uses the mechanical energy from the steam to turn the generator, which then converts the mechanical energy to electrical energy. The steam expands and cools in the energy conversion in the steam turbine. A small fraction of the steam condenses in the steam turbine and appears as small water droplets. The mixture of steam and water exhausts from the steam turbine to the condenser where the remaining steam is condensed into water, usually refelTed to as condensate. The heat required to change state between steam and water, called the heat of vaporization, is rejected to the circulating water through heat transfer in the condenser. The condensate is then pumped back to the HRSG through heat exchangers designed to capture more heat through heat transfer. The process is then repeated.

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 2.0

LAWS OF THERMODYNAMICS

The conversion of heat to work is based on two fundamental principles generally referred to as the First and Second Laws of Thermodynamics. The First Law is simply a restatement of the Law of Conservation of Energy that describes the relationship between heat and work. The Second Law describes the availability of heat energy to do work. Regardless of the type of work or the type of energy under consideration, the terms heat, work, and energy have practical significance when viewed in terms of systems, processes, cycles and their surroundings. In the case of expansion work in a steam turbine, the system is a fluid (water/steam) capable of expansion or contraction as a result of pressure, temperature or chemical changes. The way in which these changes take place is referred to as the process. A cycle is a sequence of processes that produces net heat flow or work when placed between an energy source (fuel) and an energy sink (condenser). When dealing with energy and the means of converting energy from one form to another, it is convenient to draw a boundary around the system. Everything within the system boundaries is part of the system, and everything outside of the boundaries is called the surroundings. Energy can be transferred across the system boundaries between a system and its surroundings. There are two types of systems: closed systems and open systems. A closed system, as shown in Figure 104, has no transfer of mass to or from its surroundings. For example, the feedwater/steam piping in a power plant is the boundary of a closed system. It is used to collect water (mass) and isolate it from the surroundings. Energy is transferred into the system in the HRSG and out of the system in the turbine and condenser. The mass of the working fluid in the system (steam/water) stores the energy.

oundary

Energy'n

Figure 1-04 Closed Svstem

Energy Out

An open system, as shown in Figure 1-05, transfers both mass and energy to or from its surroundings. An example of an open system is a cogeneration power plant where some steam produces electrical power in a closed loop process, but some steam is extracted from the turbine and used in some other process (say building heating) and is not returened. Mass In

Energy In

•

-.~ ~L

¥

BOUndary

Energy Out

~~ -~ I ~~ L~'-:==="~~~ •

Fif.!ure 1-05 Open Svstem


Mass Out

THERMODYNAMIC PRINCIPLES The conventional power plant steam/water cycle (often called the Rankine cycle) is a closed system used to convert the heat of combustion into mechanical work. The mass of the system is water, and the system boundary consists of the boiler tubes, turbine casing, condenser tubes, pump casings, and the interconnecting piping.

2.1

THE FIRST LAW OF THERMODYNAMICS

From the principle of conservation of energy, whenever there is any net transfer of energy inward across .the boundary ofa system, the stored energy of the system increases by an amount equal to the net energy transferred. Conversely, if there is a net transfer of energy out of the system during any process, the stored energy of the system decreases by an amount equal to the net energy removed. This principle relates to the First Law of Thermodynamics which states that the sum of all energy entering a system must equal the sum of all energy exiting, recall that energy can neither be created nor destroyed. In the case of a closed system, the first law of thermodynamics can be applied by using an energy balance, as shown in Figure 1-06. From this energy balance the following equation can be written:

I where

Q

w

EJ E2

Q - W == E2 - EI

I

Equation 1-05 net heat transferred to the system net work done by the system = stored energy of the system at the stmi of a process = stored energy of the system at the end of the process

= =

Figure 1-06 Energy Balance (Closed System)

Q -~

This equation states that the difference between the net heat energy added to a closed system and the net work done by the system is seen as a change in the amount of energy stored in the system. This general "energy" equation is one form of the First Law of Thermodynamics. Application of this equation to a system is called writing the energy balance for the system. An energy balance is written by evaluating the three terms of the general energy equation. These include the heat Q and work Wadded to or removed from the system, as well as change in total energy possessed by the system. The energy in the system includes potential energy, kinetic energy, internal energy and P-v energy. The changes in potential and kinetic energy in 'most closed systems are very small compared to other changes and so to sirnplify the equation, they are assumed to be zero. Thus, the change in the total energy equals the sum of the changes in internal energy and pressure-volume (P-v) energy, which equals the change in enthalpy. This can be represented by the following equation:

I Q - W = Hr HI = DH Equation 1-06 Where: Q W

HJ H2

net heat transferred to the system net work done by the system enthalpy of the system at the start of a process = enthalpy ofthe system at the end of the process

= = =

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 It is important in using this equation that the energy units used are the same. It is also important to adopt "sign conventions" for heat and work are used in applying these relationships. Usually the following sign convention is used. Ifheat is added to the system, a positive value (plus sign) is used for Q; ifheat is removed from the system, a negative value (minus sign) is used. If work is done by the system, a positive value is used for W; if work is done on the system, a negative value is used.

The general energy equation also applies to open systems, as shown in Figure 1-07. The type of open system most frequently encountered in practical systems is called a steady flow system.

Energy ---1=~~-------1 In -

Q --~:

.

-

Energy Out

Figure 1-07 General Energy Equation in Open Systems In this case, the mass flow into the system equals the mass flow out. Thus, no mass is collected by the system. In addition, the potential and kinetic energy changes of the working fluid can be eliminated since they are essentially the same at the inlet and outlet conditions. Thus, the change in the total energy equals the sum of the changes in internal energy and P-v energy entering and leaving the system. This equals the change in enthalpy of the working fluid . The general energy equation can be rewritten as follows:

Q+Hi = W+Ho

Equation 1-07 where

Q W Hi

Ho

= = = =

net heat transferred to the system net work done by the system enthalpy of the working fluid entering the system enthalpy of the working fluid leaving the system

This equation can be applied to the entire power plant without examining the details of the process within the plant. The equation can also be applied to individual components and processes in the power plant such as the HRSG, steam and gas turbines, boiler feed pumps and so on. For example, a turbine is designed to extract energy from the working fluid to do work in the form of turning a shaft. This shaft work is converted to electrical energy by the generator. Figure 1-08 shows a simplified diagram of a turbine. A simple turbine is a steady flow system in which, ideally, no heat is transferred to or from the system (Q = 0). The general equation for a simple turbine is written as follows:

H; = W + Ho

Equation 1-08

Because the turbine in this example is a steady flow system, the energy equation must be written for some selected time interval. This is accomplished by writing the equation in terms of rates of energy transfer, in BTU per unit time, as follows : (Note: the· above the letter is an engineering designation for a rate. In other words, m=mass, where M

is mass flow.

Equation 1-09


THERMODYNAMIC PRINCIPLES This form of the general energy equation is particularly impOitant because the rate of work done by the system w is the power output of the system. The other terms in this form of the equation, namely the mass flow M rate and the enthalpies, hi and ho, are measured quantities. The lower case "h" is used rather than the upper case "H" as in the previous equation because the specific enthalpy, BTU per pound, is used rather than the "gross" enthalpy.

Hln---.

Figure 1-08 Simplified Turbine Diagram

Wt

Another example of a heat transfer system is a boiler (or HRSG) of a power plant which is used to take high pressure, low temperature water and generate high pressure, high temperature steam. Figure 1-09 shows a simple boiler as open boundaries. Applying the general energy equation to the boiler, the following equation can be written:

Q+H;=W+Ho

Q is the amount of heat transferred through the boiler tubes and absorbed by the water and steam. Since the boiler does not do work, the work term W in the equation is zero. Thus, the equation can be simplified and written for a steady flow system:

•

where

Q is the rate of heat transferred to the working fluid .

•

Q = m(ho -hJ

HIGH PRESSURE _HIGH TEMPERATURE STEAM mho

Qb (FUEL) EXIT GAS

Figure 1-09 Simple Boiler

COMBUSTION

L.:=:::::=-_______.....:::::::::si="

HIGH PRESSURE LOW TEMPERATURE WATER mhj

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2.2

THE SECOND LAW OF THERMODYNAMICS

The First Law of Thermodynamics describes the relationship between he(1.t, energy and work. While the First Law is useful in describing, for example, how much heat is required to produce a given amount of work, it is not sufficient by itself to describe all aspects of conversion of energy. For example, consider a rotating flywheel which is brought to rest by friction in its bearing. The temperature of the bearing rises . The increase in the internal energy is equal to the original energy of the rotating flywheel. Can the flywheel start rotating as the bearing cools down until the bearing temperature is restored to its original value and the flywheel once again has its original kinetic energy? There is nothing in the First Law that helps to answer this question, none the less, the answer is no . It 'is evident that there must be some natural principle, in addition to the First Law of Thermodynamics, which determines the direction of a process. This is where the Second Law of Thermodynamics applies. The First Law is a statement of the equivalence of various forms of energy and says that energy must be conserved in a process; however, it gives no indication of whether or not difficulties will be encountered in making the conversion from one energy form to another. The Second Law of Thermodynamics is not restricted to interchanges of heat and work, but rather is a broad philosophy on the behavior of energy and energy transformations. The Second Law concentrates on the feasibility of energy conversion processes. Consider a power plant cycle as shown in Figure 1-10 which consists of the boiler, turbine, condenser, and feedwater systems. Heat is added to this cycle in the boiler. Energy leaves the system in the form of work done by the turbine. However, not all ofthe energy is removed from the steam in the turbine, and the steam that enters the condenser must be condensed. In order to condense the steam in the condenser, the latent heat of vaporization of the steam must be rejected from the system. If this heat were not rejected, the condenser pressure and temperature would begin to increase lowering the work output of the turbine. This rejected heat is more than half of the total heat added to the cycle in the steam generator.

Q

added

SECONDARY FLANT CYCLE SYSTEM

w Figure 1-10 Power Plant Cycle

... Q rejected This cycle appears to be very inefficient. It would seem that the cycle could be made more efficient by using the heat rejected in the condenser rather than "throwing it away." Unfortunately, the heat rejected from the condenser is at a relatively low temperature; typically around 100°F. Most thermodynamic processes used in power generation, such as generating steam in the HRSG, require much higher temperatures. In fact, apart from using warm circulating water from the condenser for heating greenhouses and melting snow from sidewalks, there are very few ways in which it is practical to use the heat rejected in the condenser. In thermodynamics, the heat rejected from the condenser is said to have low availability.


THERMODYNAMIC PRINCIPLES The example of the power plant cycle in Figure 1-10 illustrates a fundamental consequence of the Second Law of Thermodynamics. It is impossible to convert all ofthe energy supplied to a thermodynamic system to useful work. Some of the energy is lost or rejected. The more energy that can be converted to useful work, the more efficient the system. In thermodynamics, the opposite view is often taken; a thermodynamic cycle can be made more efficient by minimizing the heat rejected. The example of the power plant cycle in Figure 1-10 also demonstrates that, when dealing with energy conversion; it is not enough to know the amount of heat transferred to describe a thermodynamic process. The temperature at which heat is transferred is also impOitant because the availability if heat energy in a substance depends upon its temperature. The lower the temperature of a substance, the less the availability of its heat energy to do work. This concept is so important that another property is defined to describe both the amount of heat transferred and the temperature at which it is transferred. This property, entropy, represented by S is defined as the ratio of heat transfelTed to the absolute temperature at which it is transferred. This can be written in the following equation:

where

~S

Q T

== change in entropy of a system during some process (BTU/oR) amount of heat added to the syStem during the process (BTU) = absolute temperature (OR)

Entropy is a property as is pressure, temperature, volume or enthalpy. Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy as part of the information tabulated.

2.3

T-S DIAGRAMS

The definition of the change in entropy can be visualized by considering a process in which heat is added to a substance. If this process is carried out at a constant temperature, the change in entropy(DS) equals the heat added (Q) divided by the absolute temperature (Tabs) . The usefulness of entropy can be illustrated by describing thermodynamic processes on a diagram called a T-S diagram and using the defmitionof entropy. The following equation can be written by rearranging the equation that defines entropy:

Q =Tx~S where

~S

Q

T

= change in entropy of a system during some process (BTU/o R) = amount of heat added to the system during the process (BTU) = absolute temperature (OR)

The amount of heat required for a thermodynamic process can be thought of as the area under a curve plotted on a T-S diagram. That area can be determined through a mathematical process called integration.

1-15-1.1 HPC Technical Services

T.

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Figure 1-11 shows a T-S diagram for two different thermodynamic processes in which the temperature T and the entropy S of a substance both change as heat is added.

PATH · B

ENTHROPY Figure 1-11 T-S Diagram The heat required for the process represented by path A can be found by determiriing the area under the 'curve A between the limits of SI and S2. The second thermodynamic process, represented by path B has the same endpoints but is different from path A; because all of the points on curve B are lower than those on curve A. Accordingly, the area under curve B is less than that under curve A and so the heat required for the process represented by curve B is less than that for the process represented by curve A. Thus, as this example demonstrates, it is not enough to know the endpoints of a process in order to determine the .amount of heat required for that process; the path must also be known. The work done by or on a thermodynamic system and the heat added to or removed from the system can be easily visualized on the T-S diagram. T-S diagrams are, therefore, frequently used to analyze energy transfer cycles. In the following Chapter, two cycles that are used in power plants, the Brayton and Rankine Cycles, are depicted on a T-S diagram to determine efficiency. The Brayton and Rankine Cycles thermodynamically represent the gas turbine and water/steam cycle of a combined cycle power plant.

3.0

WATER AND STEAM

Water is the primary substance used to transfer energy in a power plant. The steam is used to drive the steam turbine-generator which produces electrical power. Water is a key resource because of its wide availability, nontoxic nature, and favorable properties. The properties discussed in this section are: • • • • • • •


States or phases Heat capacity (specific heat) Heat of fusion Heat of vaporization Saturation temperature Saturation pressure Superheat

THERMODYNAMIC PRINCIPLES

3.1

PROPERTIES OF WATER

Water can exist in any of the three states: solid, liquid, and gas. These three states are also called phases. The state or phase of water depends on its temperature and pressure. At atmospheric pressure, water below 32°F is solid (ice), water above 32°F and below 212°F is liquid, and water above 212°F is gaseous (steam). Heat must be transferred to or from water to change both its temperature and state. Figure 1-12 shows the amount of heat at atmospheric pressure needed to change OaF ice to 212°F steam and beyond. When heat is transferred to ice, its temperature increases until the ice reaches the freezing point of 32°F. The amo.!lot of heat required to change the temperature of ice is determined by a parameter called specific heat. The specific heat of ice is 0.505 BTU/lboF and so one pound of ice must absorb 0.505 BTU of heat to raise its temperature by 1°F. In Figure 1-12, OaF ice is heated to 32°F by adding approximately 16 BTU of energy.

500·_------------------------..,

HEAT OF VAPORIZATION

'SATURATION POINT/

16 -144- 1-

18o-

J- - - - - - 9 7 0 - - - - - -,- - 1 3 8 -

BTU

Figure 1-12 Water Phase Diagram When more heat is added beyond this point, however, the temperature of the ice does not change. Additional heat energy instead melts the ice. The process of melting ice to water is called a phase transformation or change of state. The heat required for the change of state from ice to water is called the heat of fusion or latent heat. The heat of fusion is the difference in internal energy of ice and water. The amount of heat needed to change 1 pound of ice at 32°F to water at 32°F is 144 BTU. Once all of the ice changes state to water, as more heat is added, the temperature of the water increases. The increase in temperature occurs at a rate of about I OF rise for each BTU added, since the specific heat of water is about 1 BTU/lb- OF. In fact, the specific heat of water changes slightly as its temperature changes. The specific heat is exactly 1 BTU/lb-oF when the temperature of the water is at 60°F. To increase the temperature of 1 pound of water from 32° to 212°F, 180 BTU of heat are required. This heat addition is called sensible heat, since the heat addition can be "sensed" as a temperature change. At 212°F, another phase transformation begins. If more heat is added, the water starts to boil. Boiling is the change of state from water to steam. The temperature at which water boils, for a given pressure, is called the saturation temperature. Water at the saturation temperature is called saturated liquid, and steam at the saturation temperature is called saturated steam. At saturation temperature, water as a liquid and a gas exist together. The heat required for the change of state from water to steam is called the heat of vaporization. The heat of vaporization is the difference in internal energy of water and steam. The amount of heat needed to change I pound of water at 212°F to steam at 212°F is 970.3 BTU.


T

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The saturation temperature (boiling point) of water depends on its pressure. At atmospheric pressure, the saturation temperature is 212°P. The saturation temperature of water decreases as its pressure decreases and increases as its pressure increases. Por example, if the pressure is lowered to 1 psi a (compared to atmospheric pressure of 14.69 psia) the saturation temperature of water is 10 l. 7°P. If the pressure is increased to 100 psia, the saturation temperature of water is 327.8°P. There is a unique relationship between pressure and temperature of water at saturation conditions. That is, for any given saturation pressure, there is one and only one saturation temperature. Thus; at saturation if the pressure is known, the temperature is also known and vice versa. Once all of the water changes state to steam, further addition of heat to the steam increases its temperature above the saturation temperature. Steam that is above saturation temperatUre is called superheated steam. The specific heat of steam is 0.490 BTU/lb-oP at saturation at atmospheric pressure and so 0.490 BTU is needed for each degree of superheat for 1 pound of steam. The specific heat of steam changes as its pressure and temperarure change. The difference in the temperature of superheated steam and the saturation temperature for its pressure is called the superheat or degrees of superheat of the steam. P or example, steam at atmospheric pressure that has been heated to 222°P has 10 degrees of superheat. As water boils and changes to steam, a mixture of steam and water at the same temperature exists. A new parameter, steam quality (often referred to simply as quality), is necessary to describe the mixture of steam and water. Steam quality is defmed as the mass percentage of steam present in the steam-water mixture at saturated conditions. If, for example, 90% of the water in a mixture of steam and water were steam, the quality of this mixture would be 90%. Quality is only useful in saturation. This is because water that is below the saturation pressure (and thus has no steam) has zero quality and superheated steam has a quality of 100%.

3.2

STEAM TABLES AND THE MOLLIER DIAGRAM

The properties of water have been studied more than those of any other substance. The properties of water that are most useful in thermodynamics of power plants are specific volume, enthalpy and entropy. Tables have been developed listing the changes of each property with changes in pressure and temperature. The two tables most used in power plant work are the saturated steam tables and superheated steam tables. The saturated steam tables provide the values of properties of steam and water at saturation conditions while the superheated steam tables provide the values of properties of steam above saturation temperature. Some steam tables also provide the values of properties of water below saturation temperature (called subcooled water). All of these tables of properties are, together, referred to as steam tables. These tables are commonly published as a book. The saturated steam tables give the values of properties of saturated water and saturated steam for temperatures from 32°P to 705.47°P and for the corresponding pressures from 0.08865 to 3208.2 psia. Water below 32°P and 0.08865 psia is ice rather than saturated steam or water. Water at 705.47°P and 3208.2 psia is at the critical point. At the critical point there is no difference in the density or other properties of water and steam and thus saturation no longer has meaning.

1-18

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THERMODYNAMIC PRINCIPLES Normally, two sets of saturated steam tables are provided, temperature tables and pressure tables. Temperature tables list values of properties according to saturation temperature in even increments of temperature. Pressure tables list values of properties according to saturation pressure in even increments of pressure. Both the temperature and pressure tables have the same information, however the information is organized differently for convenience. The temperature tables are easiest to use when the temperature is known, and the pressure tables are easiest to use when the pressure is known. Table 1-01 shows a portion of a saturated steam temperature table. Table 1-02 shows a portion of a saturated steam pressure table. Temp. Press. of I psia

Enthalpy, Btunbm Volume, ft3nbm Entropy, Btunbm x R Water I Evap. ISteam Water I Evap. ISteam Water I Evap. ISteam 8g

1133.38 1115.36 1097.55 1079.96 1062.59

v, 0.02207 0.02201 0.02194 0.02188 0.02182

8'g

560.0 558.0 556.0 554.0 552.0

0.36607 0.37230 0.37966 0.38715 0.39479

0.38714 0.39431 0.40160 0.40903 0.41660

562.4 559.8 557.2 554.6 552.0

625.3 628.8 632.0 635.3 638.5

1187.7 1188.4 1189.2 1189.9 1190.6

0.7625 0.7600 0.7575 0.7550 0.7525

0.6132 0.6177 0.6222 0.6267 0.6311

1.3757 1.3777 1.3797 1.3817 1.3837

560.0 558.0 556.0 554.0 552.0

550.0 548.0 546.0 544.0 542.0

1045.43 1028.49 1011.75 995.22 978.90

0.02176 0.02169 0.02163 0.02157 0.02151

0.40256 0.41048 0.41855 0.42677 0.43514

0.42432 0.43217 0.44018 0.44834 0.45665

549.5 546.9 544.4 541.8 539.3

641.8 645.0 648.1 651.3 654.4

1191.2 1191.9 1192.5 1193.1 1193.7

0.7501 0.7476 0.7451 0.7427 0.7402

0.6356 0.6400 0.6445 0.6489 0.6533

1.3856 1.3876 1.3896 1.3915 1.3935

550.0 548.0 546.0 544.0 542.0

540.0 538.0 536.0 534.0 532.0

962.79 946.88 931.17 915.66 900.34

0.02146 0.02140 0.02134 0.02129 0.02123

0.44367 0.45237 0.46123 0.47026 0.47947

0.46513 0.47377 0.48275 0.49155 0.50070

536.8 534.2 531.7 529.2 526.8

657.5 660.6 663.6 666.6 669.6

1194.3 1194.8 1195.4 1195.9 1196.4

0.7378 0.7353 0.7329 0.7304 0.7280

0.6577 0.6621 0.6665 0.6708 0.6752

1.3954 1.3974 1.3993 1.4012 1.4032

540.0 538.0 536.0 534.0 532.0

V'g

Vg

hr

h'g

hg

8,

Temp. of

Table 1-01 Saturated Steam Temperature Table

M-

1-19HPC Technical Services

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Press.

Temp.

psia

F

1200.0 1190.0 1180.0 1170.0 1160.0 1150.0 1140.0 1190.0 1120.0 1110.0

r

Volume, ft3nbm

Enthalpy, Btunbm

Entropy, Btunbm x R Energy,Btullbm

Iwater IEVap. ISteam Iwater IEvap. ream IWater IEvap. ISteam Iwater Isteam V,

V'g

567.19 0.02232 0.34013 566.13 0.02228 0.34371 565.06 . 0.02254 0.34734 563.99 0.02221 0.35103 562.91 0.02217 0.35478

Vg

h,

h'g

h9

S,

Sfg

S9

U,

Ug

0.36245 0.36599 0.36958 0.37324 0.37695

571.9 570.5 569.0 567.6 566.2

613.0 614.8 616.6 618.5 620.3

1184.0 1185.3 1185.7 1186.1 1186.6

0.n14 O.n01 0.7688 0.7674 0.7661

0.5969 0.5993 0.6017 0.6042 0.6066

1.3683 1.3694 1.3705 1.3714 1.3727

566.9 565.5 564.2 562.8 561 .4

1104.3 1104.7 1105.0 1105.3 1105.6

561.82 560.73 559.63 558.52 557.40

0.02214 0.02210 0.02206 0.02203 0.02199

0.35869 0.36247 0.36641 0.37041 0.37449

0.38073 0.38457 0.38847 0.39244 0.39648

564.8 563.3 561 .9 560.5 559.0

622.2 624.1 625.9 627.6 629.6

1187.0 1187.4 1187.8 1188.2 1188.7

0.7647 0.7634 0.7620 0.7606 0.7592

0.6091 0.6115 0.6140 0.6165 0.6190

1.3738 1.3749 1.3760 L3nl 1.3783

560.1 558.7 557.3 555.9 554.5

1106.0 1106.3 1106.6 1106.9 1107.2

1100.0 556.28 1090.0 555.14 1080.0 554.00 1070.0 552.86 1060.0 . 551.70

0.02195 0.02192 0.02188 0.02184 0.02181

0.37863 0.38285 0.38714 0.39150 0.39694

0.40056 0.40476 0.40902 0.41335 0.41n5

557.5 586.1 554.6 553.1 551 .6

631 .5 . 1189.1 633.4 1189.5 635.3 1189.9 837.1 1190.3 639.0 1190.7

0.7678 0.7564 0.7550 0.7536 0.75"..2

0.8216 0.6241 0.6266 0.6292 0.6318

1.3794 1.3805 1.3817 1.3828 1.3840

553.1 551 .7 550.2 548.8 547.4

1107.6 1107.8 1108.1 1108.4 1108.7

1050.0 1040.0 1030.0 1020.0 1010.0

550.53 549.36 548.18 546.99 545.79

0.021n 0.02174 0.02170 0.02166 0.02163

0.40047 0.40507 0.40967 0.41454 0.41941

0.42224 0.42681 0.43146 0.43820 0.44103

550.1 548.6 547.1 545.6 544.1

640.9 642.8 644.7 646.6 646.5

1191 .0 1191 .4 1191 .8 1192.2 1192.6

0.7507 0.7493 0.7478 0.7463 0.7449

0.6344 0.6370 0.6396 0.6423 0.6449

1.3851 1.3663 1.3874 1.3886 1.3898

545.9 544.5 543.0 541 .5 540.0

1109.0 1109.3 1109.6 1109.9 1110.1

1000.0

980.0 970.0 960.0

544.56 543.36 542.14 · 540.90 539.65

0.02159 0.02155 0.02152 0.02148 0.02145

0.42436 0.42942 0.43457 0.43982 0.44518

0.44596 0.45097 0.45609 0.46130 0.46882

542.6 541.0 539.5 537.9 536.3

650.4 652.3 654.2 658.1 658.0

1192.9 1193.3 1193.7 1194.0 11£4.4

0.7434 0.7419 0.7406 0.7389 0.7373

0.8476 0.8503 0.6530 0.6557 0.6584

1.3910 1.3922 1.3934 1.3946 1.3958

538.6 537.1 535.6 534.0 532.5

1110.4 1110.7 1111 .0 1111 .2 1111.5

950.0 940.0 930.0 920.0 910.0

538.39 537.13 535.85 534.66 533.26

0.02141 0.02137 0.02134 0.02130 0.02127

0.45054 0.45621 0.46190 O.48nO 0.47363

0.47205 0.4n59 0.48324 0.48901 0.49490

534.7 533.2 531 .6 530.0 528.3

860.0 881 .9 683.8 685.8 667.7

1194.7 1105.1 1195.4 1196.7 1198.1

0.7358 0.7242 0.7327 0.7311 0.7295

0.8812 0.8840 0.6688 0.8696 0.6724

1.3970 1.3982 1.3995 1.4007 1.4019

531.0 52S.4 527.9 526.3 524.8

1111 .7 1112.0 1112.2 1112.5 1112.7

900.0 890.0 880.0 870.0 860.0

531 .95 530.63 529.30 527.96 526.60

0.02123 0.02119 0.02116 0.02112 0.02109

0.47968 0.48586 0.49218 0.49883 0.50522

0.50091 0.50706 0.51333 0.51975 0.52631

526.7 526.1 523.4 521.8 520.1

669.7 671.6· 673.6 675.6

1196.4 119B.7 1197.0 1197.3 6n.6 1197.7

0.7279 0.7263 0.7247 0.7230 0.7214

0.6753 0.6762 0.6811 0.8840 0.6869

1.4032 1.4045 1.4057 1.4070 1.4083

523.2 521 .6 520.0 518.4 516.7

1113.0 1113.2 1113.4 1113.7 1113.9

850.0 840.0 830.0 820.0 810.0

525.24 523.86 522.46 521.06 519.64

0.02105 0.02101 0.02098 0.02094 0.02091

0.51197 0.51886 0.52592 0.53314 0.54052

0.53302 0.53988 0.54889 0.55408 0.56143

518.4679.5 516.7681.5 615.0683.5 513.3685.6 611 .6687.6

1198.0 1198.2 1198.5 1196.8 1199.1

0.7197 0.7160 0.7183 0.7146 0.7129

0.8889 0.6920 0.6959 0.8990 0.7020

1.4094 1.4109 1.4122 1.4138 1.4149

515.1 513.4 611.8 610.1 508.4

1114.1 1114.3 1114.5 1114.8 1115.0

800.0 790.0 780.0 no.o 760.0

518.21 516.76 515.30 513.34 512.34

0.02087 0.02083 0.02080 0.02076 0.02072

0.54809 0.55894 0.563n 0.57191 0.58025

0.58896 0.57667 0.58457 0.59267 0.60097

509.6889.6 508.1 691 .5 506.3693.6 504.6695.7 502.7697.7

1199.4 1199.7 1199.9 1200.2 1200.4

0.7111 0.7094 0.7076 0.7058 0.7040

0.7051 0.7082 0.7114 0.7146 0.7178

1.4183 1.4176 1.4190 1.4204 1.4218

506.7 505.0 503.3 501 .5 499.8

1115.2 1115.4 1115.5 1115.7 1116.9

750.0 740.0 730.0 720.0 710.0

510.84 509.32 507.78 506.23 504.67

0.02069 0.02065 0.02061 0.02058 0.02054

0.56860 0.59757 0.60657 0.61581 0.62530

0.60949 0.61822 0.62719 0.63639 0.64585

500.9699.8 499.1701 .9 497.2703.9 495.4706.0 493.5706.1

1200.7 1200.9 1201 .2 1201.4 1201 .6

0.7022 0.7003 0.6985 0.6966 0.6947

0.7210 0.7243 0.7176 0.7309 0.7343

1.4232 1.4246 1.4260 1.4275 1.4290

498.0 496.2 494.4 492.6 490.8

1116.1 1116.3 1116.4 1116.6 1116.8

m.o

Table 1-02 Saturated Steam Pressure Table Both variations ofthe saturated steam tables are tabulations of pressure P, temperature T, specific volume v, specific enthalpy h, and specific entropy s. The term "specific" means that the value of the property is given as "per pound." Subscripts are used to distinguish between water in different phases. The subscript " f ' (for fluid) is used for water. The subscript "g" (for gas) is used for steam. The subscript "fg" is used to denote the difference between the same property for water and steam. Thus hfis the enthalpy of water at a given pressure/temperature, hg is the enthalpy of steam at the same temperature, and hfg is the difference between hf and hg. It should be evident that hfg is the heat of vaporization.


THERMODYNAMIC PRINCIPLES Table 1-03 is a summary of the notation used in steam tables. Both saturated tables list the values of properties of water as a saturated liquid and a saturated vapor for the specified temperature/pressure condition. They also list the change in each property between the liquid and vapor states. For example, referring to the saturated steam temperature table (Table 1-01), the saturation pressure for steam at 540°F is 962.79 psia. On the same line, the specific volume, enthalpy and entropy for water and saturated steam at this temperature can be found. T

-

Temperature

P

Pressure (psi)

v

-

Specific volume of saturated liquid (cu ftIlbm

vf

-

Specific volume of saturated liquid (cu ftIlbm

Vg

-

Vfg

- Specific volume change of vaporization (cu ftllbm

hr ~ h

htg S

Specific volume of saturated vapor (cu ftIlb m

Specific enthalpy (BTU/Ibm) Specific enthalpy of saturated liquid (BTU/Ibm) Specific enthalpy of saturated vapor (BTUllbm) - Specific enthalpy of change ofvaporization (BTUllbm)

-

Specific entropy (BTU/Ibm-OF)

Sf

-

Specific entropy of saturated liquid (BTUllbm-OR)

Sg

-

Specific entropy of saturated vapor(BTU/Ibm-OR)

SCg

-

Specific entropy of change of vaporization (BTU/Ibm-OR)

Sh

-

Number of degrees of superheat (OF)

Table 1-03 Steam Table Notation Tables I-Oland 1-02 show values for three properties of steam, enthalpy, entropy and specific volume. The values for enthalpy are given in units of BTU/Ibm' The values for entropy are given in units of BTU/oR-Ibm' The specific volume, v, of steam is the inverse of its density p at a given temperature and pressure: Specific volume v =

1 -

p Density is the amount of weight a substance has per unit of volume, usually expressed in Ib/f3. Specific volume is the volume of a unit mass of a substance or ft3/1b. Understanding that the density and specific volume of water change with temperature and pressure is impOliant because some steam plant equipment takes advantage of this characteristic of water. For example, the steam drum, water tube, and downcomer arrangement shown in Figure 1-13 uses density changes in water for natural circulation. Since this portion of the boiler (or HRSG) boils or evaporates water to form steam, it is often referred to as the evaporator in HRSG's. In a conventional fired boiler, this portion of the boiler is called the water walls because the boiler tubes make up the walls of the furnace.

1-21~

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om

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 In Figure 1-13, saturated water at 548°F from the steam drum ( 1 ) flows through the downcomer (2). Saturation pressure for this temperature is 1028.49 psia. The saturated steam temperature table (only a portion was shown in Table 1) gives the specific volume for saturated water at 548°F as v = 0.02169 x cu ft/lb m • ~:=-_~ STEAM 548°

1028 psia STEAM DRUM (1)

l

t

DOWNCOMER, OUTSIDE OF BOILER WALL (2)

HEAT FROM COMBUSTION (Qb)

HEADER

Figure 1-13 Boiler Water Circuit The water from the downcomer is distributed to the water wall tubes by the header (3), then flows up the water wall tubes (4) located in the walls ofthe boiler. The water in the tubes absorbs heat from combustion in the boiler. However, since the water is already at saturation temperature, the heat added causes some of the water to boil, making saturated steam. From the saturated steam temperature table, the specific volume for saturated steam at 548°F is 0.43217 x cu ft/lb m • The ratio of the specific volume of water to steam at this temperature is about 19.9. In other words, the water is about 19.9 times more dense that the steam. As a result of this difference in density, the steam bubbles rise in the tubes. Thus, there is a mixture of steam bubbles and water in the evaporator tubes. There is only water in the downcomer, however, since the mixture of water and steam in the evaporator tubes is less dense that the water in the downcomer, there is greater pressure at the bottom of the downcomer than the bottom of the evaporator tubes. The pressure difference causes water to circulate from the drum to the downcomer, upward through the evaporator tubes and back to the drum. This phenomenon is called natural circulation. Mixtures of saturated water and steam like that in the waterwall or evaporator tubes occur often in a power plant. Another example is the steam leaving a turbine and entering a condenser. This steam is actually a mixture of water that has condensed in the turbine steam path. steam quality x is the property used to express that amount of steam present in a steam-water mixture. As an example, if the steam at the turbine exhaust has a quality of 87%, each pound of the steam-water mixture leaving the turbine contains 0.87 pounds of steam and 0.13 pounds of water.


THERMODYNAMIC PRINCIPLES Superheated steam tables give values of properties of superheated steam for a given pressure and temperature. Table 1-04 is a pOition of a superheated steam table. Abs. Pras. Sat Water Sat Water Tcmperature - DcgRCS Fahrenheit IbISq in. (Sat TCql) 550 650 600 sb

700

750

800

850

900

1000

61.61 0.5485 1255.J 1.4557

111.61 0.5993 1294.4 1.4921

161.61 0.6449 1329.3 1.5228

211.61 0.6871 1361.5 1.5500

261.61 0:7272 1392.0

311.61 0.7656 1421.5

1.S748

1.S977

361.61 0.7656 1450.3 1.6193

461.61 0:8753 1507.0 1.6595

55.42 0.5137 1249.3 1.4457

105.42 0.5636 1290.1 1.4833

155.42 0.6080 1325.9 1.5149

205.42 0.6489 1358.7

255.42 0.6875 1389.6

305.42 0.7245 1419.4

1.S426

1.S677

1.S908

455.42 0.7603 1448.5 1.6530

465.42 0.8295 1505.4 1.6530

299.47 0.6872 1417.. 3 1.5842

299.47 0.6872 1417.3 1.6062

449.47 0.7881 1403.9 1.6469

343.72 0.6865 . 1444.7 1.6000

433.72 0.7S0S 1502.4 1.6410

y . 950 (538.39)b s

0.02141 534.74 0.7358

0.4721 1194.7 1.3970

11.61 0.4883 1207.6 1.4098

Sh 1000 y (514.58) b s

0.02159 542.66 0.7434

0.4460 1192.9 1.3910

5.42 0.4535 1199.3 1.3973

0.02177 550.15 0.7507

0.4222 1191.0 1.3851

49.47 0.4821 1243.4 1.4358

99.47 0.5312 1285.7 1.4748

149.27 0.5745 1322.4 \..S072

199.47 0.6142 1355.8 1.5354

249.47 0.6515 1387.2

0.4006 1189.1 1.3794

43.72 0.4531 1237.3 1.4259

93.72 0.S017 1281.2 1.4664

143.72 0.S440 1318.8 1.4996

193.72 0.5826 1352.9 1.5284

243.72 0.6188 1384.7

293.72 0.6S33

1.S542

1.5779

Sb 1050 y (650.53) b

s Sb 1100

y

0.02195

(SS8.28) b

SS7.SS

s

0.7S78

1.S608

141S.2

Table 1-04 Superheat Steam Table There are two differences between the saturated and superheated steam tables . First, there is only one superheated steam table, not two as with the saturated tables. Second, both the pressure and temperature of the steam are required to determine the values of properties of superheated steam. With saturated steam, either the pressure or the temperature was sufficient to find the values of properties of water or steam. The superheated steam tables are organized as a grid with pressure along the vertical axis and temperature on the horizontal axis. As with the saturated tables, specific volume v, enthalpy h, and entropy s are tabulated in the superheated steam tables. Also given is the saturation temperature for each incremerit of pressure and number of degrees of superheat, shown as Sh, for each temperature and pressure. Not all tables give values for Sh. To calculate this value, the saturation temperature T is subtracted from the temperature of the superheated steam.

I Sh = T - T where

Sh T Tsat

sat

number of degrees of superheat eF) = temperature of superheated steam (OF) = temperature of saturated steam at the same pressure as the superheated steam.

=


~

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Another method of making the values of properties of steam available is the Mollier Diagram (also called the Mollier chart). Figure 1-14 is a Mollier Diagram. The Mollier Diagram is a graphical presentation of the properties of saturated and superheated steam. It is a graph of specific enthalpy h versus specific entropy s. On this h-s diagram, there is a line that curves downward like a hill or a dome. Above this saturation dome, as it is often called, the steam is superheated. Below the saturation dome, there is a mixture of saturated steam and water. In the superheated area, there are lines of constant temperature (called isotherms), lines of constant pressure (called isobars), and lines of constant superheat. In the saturated area of the Mollier diagram, there are lines of constant pressure and constant quality (moisture) percent. . ENTROPY (Btunb -OF)

1.0

1.2

1.4

1.6

1.B

2.0

2.2

1600

1500

1400

1300

1>

I

~1200

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ifi

1100

toOO

1000

900

BOO

1.0

1.2

1.4

1.6

1.B

2.0

2.2

ENTROPY (Btu/lb -OF)

Figure 1-14 Mollier Diagram The values of properties of steam can be determined directly from the Mollier Diagram. In many cases, the diagram can be easier to use than the steam tables because the values can be read directly from it, rather than interpolated or calculated. The accuracy of steam properties from a Mollier diagram is not always as good as that from the steam tables, especially if small versions ofthe Mollier chart are used.

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STEAM TURBINE THEORY Chapter 2 TERMINAL OBJECTIVE:

To familiarize the student with the theory of Steam Turbine Operation. ENABLING OBJECTIVES:

At the completion of this section, the student should be able to: 1. 2. 3.

Describe the Impulse Principles. Describe the Curtis Stage of a Turbine. Describe the Reaction Principles.

© 1999 - TG201J5.0_June09, Printed: 12/1412010



INTRODUCTION ................................................................................................................................... 3 1.1 1.1.1 1.2 1.2.1 1.2.2

_v . 2-2 ~

Turbine Principles ....................................................................................................................... 3 Nozzles and Their Principles .................................................................................................. 4 Basic Turbine Types and Their Principles ................................................................................... 4 Impulse Turbine ...................................................................................................................... 5 Reaction Turbines ................................................................................................................... 7


STEAM TURBINE THEORY 1.0

INTRODUCTION

Steam turbines are used to convert the heat energy in the steam into mechanical energy. Ifthe steam turbine drives a generator, then this mechanical energy will be further converted. and then into electrical energy. The steam turbine is, by itself, a very simple machine with few moving parts. This is desirable because it allows the steam turbine-generator to have very good reliability. It is not unusual for a steam turbine to run for more than a year without shutdown. Current practice in some areas calls for steam turbine to have major maintenance outages about once every five years. On some equipment, the interval between major overhauls has been extended to more than ten years. While very reliable, the large steam turbine-generator is a complex machine with many components and supporting systems. This chapter covers the following: • • • • • •

Turbine main steam valves Turning gear Turbine lube oil system Turbine EHC fluid system Turbine gland steam system Turbine controls

Operation of the steam turbine requires consideration of many aspects including thermal stress, requirements for generator synchronization, and values ofcritical parameters such as the lube oil header temperature and gland steam header pressure. The turbine manufacturer provides detailed starting and loading instructions to provide the operator with guidance on all of these aspects of operation. This chapter describes the principles used in the steam turbine, the centerline components and supporting systems of the turbine.

1.1

TURBINE PRINCIPLES

The power plant is often described as an energy conversion factory in which the chemical energy in the fuel is transformed in a series of steps into electrical energy, with the turbine-generator as one part of the power plant. The function of the steam turbine is to convert the energy in the high pressure and temperature steam from the boiler or HRSG into mechanical energy. It is common to refer to the energy conversion that occurs in the turbine as happening in a single step. The conversion of energy in the turbine actually occurs in two steps.This Section describes both of these processes. •

First, the heat energy in the steam is converted into kinetic energy of a steam jet by nozzles.

•

Second, the steam jets are used with buckets or blades mounted on a rotor to produce a mechanical force and torque.

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1.1.1

Nozzles and Their Principles

A steam turbine nozzle is a device that converts heat energy of steam into kinetic energy (energy of motion) by expanding the steam. A simplified, convergent nozzle of the type most often used in steam turbines is shown in Figure 2-01

Figure 2-01 Simplified, Convergent Nozzle

P1, T1

V1

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 Vlbefore entering the nozzle. The steam leaves the nozzle at a lower pressure and temperature, T z and P2 but at a higher velocity, V z. 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. The ratio of the pressure upstream and downstream of the nozzle is critical in the efficient operation of the nozzle. It is designed to operate with a constant pressure ratio for best efficiency in energy conversion. If turbine conditions change the pressure ratio, inefficiency results. Also, if changes to the nozzle such as erosion occur, the design is upset and inefficiency results. Common problems with nozzles which occur in operation are erosion from debris in the steam and deposits from contamination of the steam

1.2

BASIC TURBINE TYPES AND THEIR PRINCIPLES

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 (also called blades), but in different ways.


STEAM TURBINE THEORY

1.2.1

Impulse Turbine

Figure 2-02 illustrates 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 the term buckets is used most often for impulse turbines. Turning Ro~or (Mechanical Energy)

Flowing Steam _ _ _-I

Figure 2-02 Impulse Turbine Operating Principles H.. t

H..

In an ideal impulse turbine, the steam expansion occurs through the stationary nozzle; the buckets change only steam velocity. Ideal impulse turbines do not exist in practice, however turbines that are nearly ideal impulse turbines are often used.

: s~~e

Figure 2-03 shows axial and radial views of an ideal impulse turbine stage. Each set of nozzles and rotor buckets is called a stage. The graph in Figure 2-03 shows that all the pressure drop in the stage occurs at the nozzles, and the velocity and volume of the steam increase in the nozzles. The expanded steam strikes the buckets, forcing them to rotate and reducing the velocity of the jet of steam. The force of the steam on the buckets produces the mechanical energy needed to turn the generator. This mechanical energy comes from the jet of steam which has its velocity reduced considerably.

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Steam Chest"-... Equal/zing Hoie

Figure 2-03 Ideal Impulse Stage In large modern power plants, there is considerable thermal energy in each pound of steam delivered to the turbine. It is impractical and inefficient to build a single nozzle and rotor large enough to convert all the steams thermal energy into useful work. Therefore, large modern turbines are usually multi-staged, with each stage converting part of the steams thermal energy to mechanical energy. In a basic multi-staged steam turbine, steam enters through the first-stage nozzle, which converts part of the thermal energy in the steam into kinetic energy.

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The steam jet from the first-stage nozzle strikes the first-stage rotor buckets. After leaving the first-stage rotor buckets, the steam passes through the second-stage nozzle. Some of the remaining thermal energy is then converted to kinetic energy. The second-stage rotor buckets are forced to rotate by the steam jet leaving the secondstage nozzles. Impulse turbines can be multi-staged in two ways. The first is the Curtis (or velocity compounded) stage shown in Figure 2-04. A velocity compounded stage has one set of nozzles with two or more rows of moving buckets. There are stationary buckets between each row of moving buckets. Each set of nozzles and buckets makes up one stage. In passing from the nozzle exit through one set of buckets, the velocity of the steam decreases because of the work it does on the buckets. The steam then passes through a row of stationary buckets that change the direction of the steam without changing its pressure or speed. The new steam direction is approximately parallel to the original steam direction leaving the nozzles. The steam then strikes a second row of buckets that are attached to the same wheel as the first row. This process may be repeated through as many as four rows of moving buckets in one stage. Most Curtis stages, however, are limited to two rows of moving buckets.

Absolute Pressure LB. I SQ. IN Absolute Velocity

Nozzle

Steam Chest

Figure 2-04 also shows that in an ideal Curtis stage, the Equalizing Hole e.ntire pressure drop occurs through the nozzle, and the pressure remains constant across the buckets. This is a characteristic of impulse turbines. The velocity, on the other hand, drops in steps as it passes through the moving buckets. Figure 2-04 Ideal Curtis Stage

In a sense, Curtis staging is not multi-staging. This is because, as pointed out above, no matter how many rows of moving buckets a Curtis stage has, it is still only one stage. It is possible, however, to have a second Curtis stage behind the fust. Absolute Pressure

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The second way that impulse turbine stages may be arranged is the Rateau (or pressure compounded) stage. A Rateau turbine consists of a series of nozzles and buckets. Each set of nozzles and buckets makes up a stage. Figure 2-05 shows a four stage, pressure compounded, impulse turbine. The steam pressure in a series of Rateau stages drops in steps through each set of nozzles.

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Figure 2-05 Arrangement of Typical Rateau Stages



Abso lute

Many old, multistage, impulse turbines consist of both Rateau (pressure compounded) and Curtis (velocity compounded) stages. Usually, the fIrst stage (and sometimes the second stage) is a velocity compounded stage with two rows of moving buckets on its wheel. The remaining stages are then pressure compounded stages as shown in Figure 2-06. Newer turbines seldom use Curtis . staging, however, otherwise the multi-staging is the same. It is not unusual to have as many as 20 stages in an impulse turbine.

"""',,-: --,-----'-.

pressure=---,---:.~_ _ _---:.-c-:;-.:--;.._---,-,---,== =

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Figure 2-06 Combination of Curtis and Rateau Staging

1.2.2

Reaction Turbines

Figure 2-07 illustrates 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 nozzles. 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, practical, large reaction turbines use a combination of impulse and reaction principles.

HEAT

HEAT

HEAT

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Figure 2-07 Reaction Turbine Operating Principles

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The typical impulse-reaction turbine has stationary nozzles and moving nozzles. The moving nozzles are created by varying the cross section of the openings between adjacent buckets (usually called blades in such turbines) as shown in Figure 2-08 . Reaction turbines can be classified by the percentage ofthe energy conversion that occurs in the moving nozzles. Typically, turbines that are called reaction turbines have about 50% reaction and 50% impulse. Turbines which use a combination of impulse and reaction principles are often referred to simply as reaction turbines to distinguish them from the impulse turbines.

Seal

Flow

Rotating Blade

Figure 2-08 Reaction Turbine Blading

: STEAM INLET

-++-+--I~

MOVING NOZZLES

FI XED NOZZLES

FIXED NOZZLES

EXHAUST

- ' """-

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Figure 2-09 shows a series of reaction turbine stages. Each stage consists of a set of fixed nozzles and a set of moving nozzles. Thepressure drop occurs over both the fixed and moving nozzles. Reaction turbines are multistaged by alternating sets of fixed and moving nozzles and are basically pressure compounded turbines with reaction. Each pair of fixed and moving nozzles makes up one stage. Many times, reaction turbines have one Curtis impulse stage as the first stage of the turbine. Figure 2-10 shows a typical arrangement.

MOVING

"""-~ NOZZLES

-'"",,-J"",,-J"",,-J"""----".Y '-""""-J"""-J"""-J"",,-J""'< -,"""-J"""-J ,J"""- J ,

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CHECK YOUR UNDERSTANDING Questions:

1.

The two components that make up a turbine stage are:

2.

Describe the function of the components in a steam turbine stage.

3.

The three things that happen to steam as it flows through a turbine stage are:


T.

STEAM TURBINE UNIT DESCRIPTION Chapter 3

TERMINAL OBJECTIVE: To familiarize the student with steam turbine unit description. ENABLING OBJECTIVES: At the completion of this section, the student should be able to: 1. 2. 3.

Describe the various designs of steam turbines. Describe steam flow through various steam turbines. Describe different ways of prewarming starting and loading steam turbines.

© 1999 - TG201J5.0_June09, Printed: 12/14/10

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 TABLE OF CONTENTS

1.0

CLASSIFICATION OF STEAM TURBINES ........................................................................................ 3 1.1 1.2 1.3 1.4 1.5 1.6

2.0

COMPARISON OF TURBINE TYPES AND MANUFACTURERS .................................................... 7 2.1 2.2 2.3

Aerodynamic Efficiency .............................................................................................................. 7 Number of Stages ........................................................................................................................ 7 Stage Design ................................................................................................................................ 7

3.0

UNIT DESCRIPTIONS .......................................................................................................................... 8

4.0

TURBINE OPERATION ...................................................................................................................... 15 4.1 4.2 4.3 4.4 4.5 4.6

~ 3-2 ~

Condensing versus Non-condensing ........................................................................................... 3 Extraction versus Non-extraction ................................................................................................ 4 Single pressure versus mUltiple pressure .......................... ;.......................................................... 4 Reheat versus Non-reheat ............................................................................................................ 5 Single Casing versus Compound ................................................................................................. 5 Exhaust Flows ............................................................................................................................ 6

Prewarming ............................................................................................................................... Starting and Loading ................................................................................................................. Full Arc Admission ................................................................................................................... Partial Arc Operation ................................................................................................................. Turbine Supervisory Instruments (TSI) ..................................................................................... Overspeed Protection .................................................................................................................


15 15 16 16 17 17

STEAM TURBINE UNIT DESCRIPTION

1.0

CLASSIFICATION OF STEAM TURBINES

In the previous section, turbine theory, the two basic turbine types were described. Impulse and reaction turbines can be further divided into a large variety of types using important characteristics. Each of the six characteristics discussed below is applicable to both impulse and reaction turbines. These characteristics are: • • • • • •

1.1

. Condensing vs. non-condensing Extraction vs. non-extraction Single pressure vs. multiple pressure Reheat vs. non-reheat Single casing vs. compound Exhaust flows

CONDENSING VERSUS NON.CONDENSING

One characteristic for classifying steam turbines is whether they are condensing or non-condensing. In a condensing turbine, the steam is exhausted into a condenser. By condensing the steam, the turbine exhaust pressure and temperatures can be very low. Low exhaust pressure allows the turbine to make maximum use of the thermal energy in the steam and makes the power plant more efficient. Nearly all large steam turbines are of this type. In non-condensing turbines, the exhaust steam is not condensed. The steam may simply be allowed to blow into the atmosphere or (more often) it may be used for some useful purpose such as heating buildings. If a noncondensing turbine exhausts to a pressure greater than atmospheric pressure, it is called a backpressure ·unit. This type of turbine is most often seen in process plants such as steel mills, refmeries and paper mills. Sometimes the non-condensing turbine is referred to as a "topper". It reduces the pressure from a high pressure boiler output to a lower usable value. In the process, electricity may be produced as a by-product. Figure 3-01 illustrates a comparison of these two classifications ..

CONTROL VALVE

LEGEND

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,,-

,, \

,

BEARING COUPUNG SHAFT PACKING STEAM FLOW

STEAI~

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EXHAUST STEAM TO PROCESS

Figure 3-01 Condensing versus Non-Condensing

.

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1.2

EXTRACTION VERSUS NON-EXTRACTION

A second way turbines Can be classified is by extraction or non-extraction. Extraction turbines are sometimes called ;'bleeder"tui-bines. An extraction turbine is a multi-stage turbine where some of the steam is exhausted, or bled, from between turbine stages at extraction points. This extraction steam may be used for regenerative feedwater heating or other purposes. In most power plant applications the extraction steam is uncontrolled. In industrial applications the extracted steam may be controlled (this difference will be highlighted later). See Figure 3-02 for the differences.

EXTRACTION TO FEEDWATER HEATER

EXHAUST

EXHAUST

Figure 3-02 Extraction versus Non-Extraction

1.3

SINGLE PRESSURE VERSUS MULTIPLE PRESSURE

Most turbines have a significant variation of steam pressure in the steam path. This pressure variation has a direct impact upon construction technique. The result is separately defmed sections as illustrated in Figure 3-03.

-

High - Pressure Section

High-Pr~ssure_

, ------I

Section

-- - -- - - - - -~

LowPressureSection

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Figure 3-03 Single Pressure versus Multiple Pressure

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1.4

REHEAT VERSUS NON-REHEAT

, A third way that turbines can be classified is reheat or non-reheat. A reheat turbine is a multistage turbine in which the steam is directed from some intermediate stage of the turbine back to the boiler. In the boiler; the steam is reheated and then piped back to the turbine. Some large turbines return the steam to the boiler to be reheated a second time. This is called a double reheat turbine. There are two advantages to reheating steam. First it makes the power plant more efficientthermodynamically. Second, it delays the start of steam condensation in the turbine. Nearly all modem power plant large steam turbines use reheat. See Figure 3-04 for a figure highlighting the differences. " ', ', ',

STEAM FROliIlAIN BOItB!

----+--,

IP EXHAUST OR CROSSOVER STEAM

,, STEAM FLOW TO IIAIN CONDENSER

,, ,

LEGEND ~

-if.L

~

BOILER REHEATER

,,

BEARING COUPLING SHAFT PACIIING STEAM FLOW

,.

EXHAUST STEAM

Figure 3-04 Reheat versus NOll-Reheat

1.5

SINGLE CASING VERSUS COMPOUND

Another way to classify turbines is as single casing or compound turbines. A single casing turbine has all the stages of the turbine in one casing as shown schematically in Figure 3-05(a). As turbines become larger, it is not practical to have all the stages in one casing. Therefore, they are divided into two or more casings. These machines are known as compound turbines. There are two different types of compound turbines, tandem-compound and crosscompound. A tandem-compound turbine is shown in Figure 3-05(b). The turbine sections are in line with one another and the sections are on the same shaft. The tandem compound turbine shown has two different sections. Large , modem units may have as many as five separate sections.

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 A cross-compound turbine is shown in Figure 3-05 (c). In this case, the different turbine sections are on different shafts. For power plants, this means that two separate generators are used. This can be an advantage for very large turbine generators since it may be easier to build and ship two half-size generators than one very large generator. Some large cross-compound units have two or more turbine sections on each shaft, and thus they are a combination of cross-compound and tandem"cOInpound . . . Nearly all large steam turbines are multiple casing units. The tandem-compound arrangement is most common. Cross-compound turbines are often designed for large units and in cases where the advantage in efficiency of a cross-compound unit over a tandemcompound cim be justified.

(a) SINGLE CASING

STEAM EXHAUST

STEAM IN

(b) TANDEM - COMPOUND

STEAM IN

STEAM EXHAUST

- -+-+1+1-+;.<.,.]

Figure 3-05 Comparison of Turbine Arrangements

(e) CROSS - COMPOUND

1.6

EXHAUST FLOWS

Condensing turbines can be further classified by the exhaust flow. A single-flow condensing turbine passes . all of its exhaust steam to the condenser through one exhaust opening. However, the low pressure sections of a large compound turbine become so large that they must be split up into more than one section because of design limitations. Turbines with as many as six flows are not uncommon. See Figure 3-06 for an illustration of a fourflow unit. Notice four (4) parallel paths to the condenser. 1_ _ _ LOW-PRESSURE SECTION _ _ _ _ LOW-PRESSURE SECTION (TURBINE END) (GENERATOR END)

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STEAM TURBINE UNIT DESCRIPTION 2.0

COMPARISON OF TURBINE TYPES AND MANUFACTURERS

There are as many different variations in steam turbines as there are manufacturers, and many manufacturers build different types of turbines as well. This Section describes some of those differences with reference to the two large steam turbines manufacturers that have historically dominated the market in the United States, Westinghouse and General Electric. There are some key differences in the design of impulse and reaction staging that are important in understanding other design features of the turbine. General Electric (GE) turbines are mostly impulse turbines while Westinghouse turbines are principally reaction turbines. The reason for this is historical. In the early history of the turbine-generator business in the United States, GE bought rights to the patent for the impulse turbine from C.G. Curtis and Westinghouse bought rights to the reaction turbine from Sir Charles Parsons. Those patents have, of course, long since expired. Both Westinghouse and GE now build turbines that incorporate both reaction and impulse features. It is still true, however, that GE turbines are principally of the impulse design and most Westinghouse turbines are principally the reaction design. A comparison of the two designs is useful in understanding how steam turbines work.

2.1

AERODYNAMIC EFFICIENCY

The fIrst difference between the two designs is in aerodynamic efficiency. The aerodynamic efficiency of the impulse stage is less than that of the reaction stage but the reaction stage efficiency falls off sharply when it is not in operation at its design point. Aerodynamic effIciency becomes more important as a design consideration as the stage becomes longer (as is the case in the low pressure turbine). A consequence ofthese differences in effIciency is seen in the fIrst stage of many Westinghouse turbines. The fIrst stage of the turbine is one in which the conditions under which the turbine conditions change considerable over the range of operation because the valves used to control the flow of steam to the turbine open just ahead of the fIrst stage nozzles. The fIrst stage turbine buckets (or blades) are the shortest in the turbine. Accordingly it is common for Westinghouse turbines, that are principally reaction otherwise, to have a Curtis stage (also sometimes referred to as the control stage) as the fIrst stage in the turbine. In GE turbines, on the other hand, the last stage buckets are as long as those in Westinghouse turbines. Since, for long buckets, a pure impulse design would have a performance penalty due to aerodynamic losses, most GE turbines have a signifIcant degree of reaction in the long buckets of the LP turbine.

2.2

NUMBER OF STAGES

Another difference between the impulse and reaction designs is the amount of energy that can be absorbed in a single stage. The impulse design can absorb more energy than the reaction stage. The consequence of this fact is that more stages are required in a reaction turbine as compared to the impulse design. For units that have the same initial steam conditions more stages are required in a reaction turbine than a comparable impulse turbine. For example, in a plant with two sister units, one Westinghouse and the other GE, both rated at about 300 MW and both operating with the same steam conditions, the GE unit has 22 stages while the Westinghouse unit has 37 stages.

2.3

STAGE DESIGN

As described earlier, all of the pressure drop in the impulse turbine is (ideally) across the stationary nozzles. In the reaction design, the pressure drop is split between the stationary and moving nozzles. The consequence of this fact is the nozzles in the GE design are rather massive while the buckets are relatively less massive. In the Westinghouse design the stationary and rotating blades are of roughly equal strength. This is illustrated in Figure 307. Figure 3-07 (a) shows the reaction stage while Figure 3-07(b) shows the impulse stage. The stationary nozzles of the impulse design are in fact rather massive. In fact, when one looks down at the horizontal joint of the impulse turbine, the buckets appear to be running in a compartment formed by adjacent nozzles. The impulse turbine is sometimes said to be "compartment design" as a result.

3_7-1.{ HPC Technical Services

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 There are two other differences in design illustrated in Figure 3-07. First, note that the labyrinth seals for the impulse design are concentrated at the inside diameter of the nozzle with relatively little at the tips of the buckets. This is consistent with the fact that there is little tendency for steam to leak past the tip of the bucket since there is little pressure drop there. On the other hand there is a large pressure drop across .the nozzle and so seals with many teeth are required to seal the inside diameter of the nozzle. The second difference in the two designs illustrated by Figure 3-07 is the balance hole in the wheel of the impulse turbine. Labyrinth packing does not prevent all leakage; it can only control and reduce leakage. Thus, there is some leakage of steam into the wheel space. Also, there is some steam which leaks by the root of the bucket. If there were no balance hole, the steam would build up pressure on the upstream side of the wheel. This would produce axial force on the wheel (discussed below). The steam would also tend to flow from the wheel space through the bucket at a right angle compared to the rest of the steam flow, thus disturbing the steam flow and reducing efficiency. The balance hole prevents this flow from occurring.

There is no balance hole in the reaction stage, of course, since there is a pressure difference across the moving blades by design. A balance hole in the reaction stage would bypass steam from its normal flow path. BUCKET

Figure 3-07 Comparison of Reaction and Impulse Stage Designs (a) Reaction Stage Design

3.0

LABYRINTH PACKING (b) Impulse Stage Design

UNIT DESCRIPTIONS

As stated earlier in this text, steam turbines are custom designed by the OEM to the meet the oWners' application. It is not unusual to [md a wide range of turbine designs within one utility or even one plant. Each plant is constructed to fit a particular need. Recognize, also, that a power plant built in the 1960's may have been upgraded during the last decade to meet the increased demand. A utility may decide to upgrade an existing power plant or build a brand new one. The utility makes this decision based on projected power requirements into the future as well as many permitting and environmental , issues .. The projected megawatt requirement will dictate what size generator(s) will be installed. The selection of the steam turbine to drive the generator will be based on: • • • • •

Boiler Capacity (lbs. per hour) Steam Conditions (pressure-temperature) Environmental Conditions Turbine Efficiency Size Limitations

It should be noted that size limitations can come about based upon unit length or area (square feet of foundation) as an example.

What follows in this section are illustrations and descriptions of steam turbine arrangements. The data given in the description on steam conditions and generator output are only examples . .~ ~

.



_HIGH PRESSURE_ _ _ _

SECTION

SECTION

TO CONDENSER

,.,

LEGEND

X 181

Figure 3-08 illustrates a tandem compound double flow turbine. This is a nonreheat turbine. This design has some unique characteristics.

LOW PRESSURE _ __

JOURNAL BEARING

"1

2.

Figure 3-08 Tam/em Compound Double Flow Turbine

1

SCALE IN FEET

~~~;~:~a~oBuE~NR~~O -i I- COUPL1N"

• •

• • • •

The high pressure and low pressure rotors are coupled together and contained in a single shell arrangement. The turbine rotors are supported on three journal bearings. The bearing arrangement is unique. The # 1 and #2 bearings support the HP rotor. The #3 bearing supports the attached generator. The LP turbine section coupled to the HP turbine and generator does not require bearing Sl,lpport. Steam inlet valves are mounted top and bottom of the turbine shell. Steam flows through the HP section, over and into the LP section and exhausts to the condenser. Typical rated operating steam conditions is 850 psi at 900°F. Typically these turbines would drive generators producing 55 MW.

Figure 3-09 illustrates another non-reheat tandem compound double flow unit. Note the differences between this design and the previous one.

Figure 3-09 Tandem Compound Double Flow Turbine

•

•

• • • • •

,. High pressure rotor coupled to a 15 2.J 1 LEGENO low pressure rotor contained in a SCALE IN FEET X JOURNAL BEAR.ING single shell arrangement. Three bearing locations to support ~ ;~~~~:~s~O~:-N~~G the turbine shafts. Bearing #1 and -II- COUPLING #2 locations are similar to the Figure 3-08. Bearing #3 supports the aft end of the LP rotor, inboard of the generator coupling. Steam inlet valves mounted.top and bottom. Steam flows through the HP section, over and into the LP section and exhausts to the condenser. Typical rated operating steam conditions are 850 psi at 700°F. Typically this turbine would drive a generator producing 60 MW. The fabricated internal crossover is a very common and fundamental feature of older units ofthis type . .

3-9


~. -

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Figure 3-10 illustrates another nonreheat tandem compound double flow unit similar to the previous two designs but with the following changes:

Figure 3-10 Tandem Compound Double Flow Turbine

,

,

To

To

Condenser

10

Condenser

15 I

legend

X

Joumal Bearing

rgJ

Combined Journal and Thrust Bearing

~

• • •

20 I

Scale In Feet

I-Coupllng

The first few stages of the HP section are enclosed in a separate inner shell. Higher rated operating steam conditions: 1450 psi at 1050°F. Driving higher output generators: 70 MW to 100 MW.

Figure 3-11 illustrates a tandem compound double flow condensing reheat turbine.

l e~~;;;tl- se~~'on

Intercept Valve

~

_____ _ _ . . .

:

~ -~

I"

,

IP

't·.. · · ·-~···-1

:

-

LP

;.

Sec~on

.

Rotor

!

r

~~m

• ~------~---,

. :

L:::,~---

To

To

Condenser

Condenser Legend

x ~ -0-

Joumal Bearln; Combined Joumal and Thrust Bearing Coupling Steam Path

3-10


Figure 3-11 Tandem Compound Double Flow Condensing Reheat Turbine

STEAM TURBINE UNIT DESCRIPTION Characteristics of this design are: • • • • • •

• • •

Three separate turbine sections: the high pressure, intermediate or reheat section, and the low pressure section. Three bearings to supportthe rotors. Separate steam control and intercept valves. Inner and outer turbine shell arrangements for the HP turbine. HP and IP turbine rotors share a common shaft. Steam flow enters the HP turbine through the control valves. Steam exhausted from the HP section enters the reheater (located in the boiler) then flows through the intercept valves to the inlet of the . IP or reheat turbine section. Exhausted from this section, it flows over and into the double flow LP turbine section and exhausts to the condenser. Typical operating steam conditions are 1800 psi at 1000°F. Typically driving generators rated at 60 to 125 MW. Fabricated external crossover.

Figure 3-12 illustrates a tandem compound, triple flow, condensing, reheat design. The scale shows how much larger this design is as compared to the previous ones. The design characteristics of this unit are:

-

High Pressure Section -

Reheat Low Pressure Sectlon:....,·---- Section -

- - -+-

, I

'-./\/VV'- - - - - - - -Reheater

Legend

x

Jouml!l Bearing

~

Combined Joumal and Thrust Bearing

-11-

Coupling

Bearings

o I

S

10

15

20

I

I

I

I

Scale In Feet

Steam Path

Figure 3-12 Tandem Compound, Triple Flow, Condensing, Reheat Turbine

• • • • •

There are three separator rotors coupled together. There are two bearings per rotor making this a six bearing machine. The high pressure section has an inner and outer shell arrangement. The control valves are shell mounted. The intermediate pressure (reheat) section is enclosed in a shell separate from the HP section. The rotor is divided into two sections, an intermediate pressure and a low pressure section.

.

3-11


J.k ~

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 • •

• •

The low pressure section contains a double flow low pressure rotor. Steam flow enters the HP section through the control valves and exhausts to the reheater located in the boiler. Reheated steam enters the IP section through the intercept valves. Some steam from the IP turbine flows directly into the LP turbine aft of the IP. The remaining steam flows through the crossover into the double flow turbine section. Steam from all LP sections exhausts into the condenser. Typical operating steam conditions are 2400 psi at 1000°F. Typically driving generators rated at 150 to 200 MW.

Figure 3-13 illustrates a large complex design known as a tandem compound, four flow, condensing, reheat turbine. The scale shows how large this machine is. The characteristics of this design are:

.'\~~;'ro,r","" .'~m ·TJfl1~~~m High Pressure -+-

, ~ Reheat ~

I

Section

_

I

Low Pressure

I I

I

I

\ :\ ve

Low Pressure SectlonB

~I

Section A

rttt .: : ::.

•• • •• _ _ _ _ • • J _ o ·

·_ .".--------1

1

'

t '............. ...... .~

t

-.•.. . . __. __.• . '.,- -_._ -_. _-- _. ! •. _-- -_._- _._-_._-

.........

l1. ·..·. . .

:· ·.... ·R~~~....

, ··j

To Condenser

o !

5!

10 !

15 I

20 I

Scale in Feet ! .ntercept Valve

LEGEND

X

o

-il-

JOURNAL BEAR.ING

TliRUST DEARlNG COUPUHG

STfAH PATH

Figure 3-13 Tandem-Compound, Four Flow, Condensing, Reheat Turbine • • • • • • • • •

3-12

Four separate rotors coupled together. There are two bearings per rotor making this an eight bearing machine. The high pressure section has an inner and outer shell arrangement. The control valves are mounted separately off the shell. The intermediate pressure (reheat) section has an inner and outer shell arrangement. The rotor is of the double flow design. There are two separate low pressure sections. Each LP section contains a double flow LP rotor. Steam flow enters the HP through the control valves and exhausts to the reheater located in the boiler. Reheated steam enters the IP section through the intercept valves. Steam exhausted from the IP section flows through the crossover into the two LP sections. Steam from the LP sections exhausts through two separate openings to the condenser. Typical operating steam conditions are 2400 psi at 1000°F. Typical application would be driving generators rated at 500 to 800 MW.


STEAM TURBINE UNIT DESCRIPTION Figure 3-14 illustrates a tandem compound, six flow, nuclear turbine design. The scale provided gives a reference on how large this machine is. Nuclear turbines operate at 1800 rpm driving four pole generators. Because of the way water is heated to produce steam in a nuclear plant, the steam is of a lower value than in a fossil fuel plant. The characteristics of this design are:

r

p:~~re Section

Steam

..

-~-

l

-(" .... f"'- .

,',',!(

:~~~"'1- Main Stop . 0. . . ': ci"':""b-': a ': Valve

6

-..:.-- -' '-- -:.___~;I~:.~I ,

.. ... _______ _______ ... _____ ... ~---------+----- --

i

,...., ..... ,: '

I

I

,,

I

+

.

"

Intercept Valve

" j'mo", \-- '-~-":

. !

(- -- -'-~-": l, + I

-- ..

+

t t

T LP~A + I

, I

I

. ____ ~-----"~--- . . --- -.. '

',, ___

l

__ _

~ ________

X

JOURNAl BEARING

OJ

THRUST BEARING

-I ... -

..

T LP~C +

I.. _____

+ __ _::.. .. __ ~ ___ ~,

A

+ I

I I

:

:

i--.... --,

..

-

...

I

:

:

110--" ..

[

--.... -,

....... - .. "

;

•

:

.,

To Condenser

-

.... .. .. ............... . . ...... .... ,.. .. .. - . " " " ........ .. .... .. .......... . . ,

LEGEND

-

T LP~ B + A ___

_~.;_

I I

~----- -. . -------~-----~~~:- -~ - . Moisture r Oo ....-.. j Separator(s)

_ ..

~

I

T

~

o I

COUPUNG

5 I

10

15

!

I

20 I

Scale in Feet

STEAM PATH

Figure 3-14 Tandem Compound, Six Flow, Nuclear Turbine • • •

One high pressure section, three separate low pressure sections. No intermediate (reheat) section. Four separate rotors coupled together, two bearings per rotor making this an eight-bearing machine. The high pressure section has a single turbine shell arrangement. The high pressure rotor is a double flow ~~Q

• •

•

• •

'

.

Each low pressure section contains a double flow LP rotor. Each LP section is equipped with an opening into the condenser. Control and intercept valves are not shell mounted. The control valves are mounted on separate steam chests adjacent to the turbine. The reheat intercept valves are located in the piping from the reheater near the turbine. Steam flow enters the HP section through the control valves. Exhaust steam from the HP section enters a moisture separator(s). From the moisture separator(s) it flows through the intercept valves for each LP section. Steam exhausted from the LP sections flows into the condenser. Typical operating steam conditions are 800 psi at525°F. Typical application would be driving a generator rated at 800 to 1000 MW.


T.

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 Figure 3-15 illustrates a cross-compound unit. This arrangement is designed to drive two generators. The lIP-IP element operates at 3600 rpm and drives a two-pole generator. The LP element operates at 1800 rpm and drives a four-pole generator. The characteristics of this design are:

I H~h"""~

Section-

Intercept , __ _ _ .... __ _ _ .... _~ Vallie .

:

y- -,

Control Valve

I

HP/IP element operates at 3600 rpm and drives a two-pole generator

:

i Inner Shell HP Section

I I I I I I

1 I I I

,

I

I

~-

t

__ .... -"NN#-- _' Reheaters

----- .... ----

LP element operates at 1800 rpm and drives a four-pole generator L.EGEND

..

,-" 1 :

T

-

I I I I

t

+ I

I

+ + To Condenser

-t ~

COUPliNG

1- - - - - - - Low Pressure _ _ _ _ _ _...1 Element

07_M...150_esnOl

Figure 3-15 Tandem Compound, Six Flow, Nuclear Turbine

• • • •

•

• •

The lIP-IP rotor is a single rotor. The rotor is supported on two bearings with the forward journal bearing combined with the thrust bearing. The lIP & IP sections are contained in separate inner shells. These two sections together create an opposed flow turbine. The lIP section is equipped with shell mounted control valves, top and bottom. The IP section is equipped with separate mounted intercept and reheat stop valves. The LP section or element is an opposed flow design. Four bearings support this arrangement with the indicated middle journal bearing combined with a thrust bearing. The LP section is equipped with a single opening to the condenser. Steam flow enters the HP section through the control valves and flows axially through the turbine. Exhaust lIP steam flows through a reheater and enters the IP section through intercept and reheat stop valves. IP exhaust steam exits through a single crossover which splits and flows the steam to opposite ends of the LP section. One side of the LP rotor is a mirror image of the other. Steam flows from the ends of the LP section to the center and exhaust to the condenser. Typical operating steam conditions are 2000 psi at 1000°F. Typical application would be drivrng generators rated at 200 MW total output.



4.0

TURBINE OPERATION

The initial start-up and loading of a turbine involves many steps that must be performed in sequence. A major concern during starting and loading is the level ofthermal stress imparted on the rotor. Thermal stress occurs when an object is heated or cooled. The greater the rate of heating (or cooling), the greater the thermal stress. Excessive thermal stress can cause cracks, either on the surface or the bore of the rotor. 4.1

PREWARMING

Before a steam turbine is started (rolled on steam to a high speed) it is very important that the turbine rotor temperatures be some minimum value (often 300°F). The most important concern is the rotor bore temperature. It must be at some minimum value before the turbine is brought to rated speed. 300°F is typically the transition point where the turbine rotor metal changes from brittle to ductile. A ductile rotor is more capable of withstanding temperature change and corresponding thermal stress. Getting the rotor bore temperature up to this value may require rotor prewarming on turning gear. Another approachis to have a very slow roll up to turbine speed. Different manufactures approach this need differently. Prewarming the rotor is accomplished by admitting main boiler steam into the HP section. Main stop (GE) or throttle (W) bypass valves are throttled to control the flow of warming steam. The main control valves are fully opened to allow the steam flow into the HP section. On a reheat machine the intercept(or) and the reheat stop valves would be fully closed to prevent steam flow through the IP section. During prewarming, the rotor is spinning slowly (3 to 4 rpm) on turning gear. Prewarming continues until the rotor bore temperature has achieved a preset minimum value. Prewarming can be accomplished by: I. 2. 3. 4.

4.2

adjusting throttle steam temperature according to turbine metal temperature; roll time; speed and load holds (soaks) and load changing rates.

STARTING AND LOADING

With the prewarming of the turbine complete, starting of the turbine can begin. Initially the steam flow is controlled by the main stop (GE) or throttle (W) valves. As steam flows through the entire turbine, it will roll off turning gear and begin to accelerate. Throughout this process the turbine will be operated under specific "Starting and Loading Procedures". The rotor speed will be held at specific values to allow temperature differentials to normalize. Rotor axial growth and vibration will be monitored. Shell expansion will be monitored. At each speed hold, the heat transfer process should be complete before allowing the turbine to accelerate to the next speed hold. Temperature increases are controlled against a time line as dictated by the operating instructions. By following the "Starting and Loading" procedures, the thermal stresses endured by the rotor are controlled. With the rotor spinning at 3600 rpm and the heat soak completed, the generator can now be synchronized. Steam flow through the turbine is still being controlled by the main stop or throttle bypass valves. The main control valves are still wide open allowing full arc (or single-valve) admission. The stop valve bypass valves are only allowing a small percentage of the main boiler steam to drive the turbine at full speed - no load. Just after generator synchronization the steam flow to the turbine should be slightly increased. This will insure a positive load on the turbine. Steam load increase is accomplished by the bypass valve disk opening a little further. No other valve settings change.


~


4.3

F ULL ARC ADMISSION

Throughout the prewarming and starting and loading phases of operation, main steam has been admitted into the turbine through all segments of the nozzle. Known as full arc admission, the benefit to the nozzle and rotor is that they are evenly heated, thus reducing thermal and vibrating $tresses during this phase of operation. See Figure 3 -16 for an illustration of this full arc admission mode of operation.

Full Arc Admission Throttled Bypass or

Wide Open

Pilot Valves

AdmisSioll

F IIII~c Valve Legend

I-STOP

'-----' CONTROL

VALVES

VALVES

Figure 3-16 Full Arc A dmission 4.4

PARTIAL ARC OPERATION

As the generator approaches one third of its rated output, steam turbine valve settings begin to change. At . this point the bypass valve has nearly reached the full open position and cannot increase steam flow through the turbine. At this transition point, the control valves begin to close sequentially. As they close they throttle the steam flow . The main stop or throttle valve will then be allowed to open. Main steam to the turbine is now controlled by the control valves;

Partial Arc Admission VALVES WlOE OPEN

t:><:J

Valve Open

~

valve Oosed

VALVES

~

PARTIALLY OPEN

V""'IIII

Valve Partlaly OpenJCIosed

V,,1ve legend

I-STOP

VALVES

'----~ I

CONTROL

VALVES

Figure 3-17 Partial Arc Admission


STEAM TURBINE UNIT DESCRIPTION The control valves are designed to open or close sequentially. A typical arrangement would include four valves each controlling steam flow to one of four sections of the nozzle. During normal turbine operation steam flow to all four quadrants of the nozzle is not required. Therefore the valve opening arrangement opens only the valves required to allow steam flow to meet turbine demand. Flowing steam through part of the nozzle only is called partial arc admission.

4.5

TURBINE SUPERVISORY INSTRUMENTS (TSI)

Thermocouples are assembled into the HP turbine shell or cylinder cover. These thermocouples are used to detect and transmit the inner surface shell or cover temperature to the control room. This is just one of many locations where thermocouples are installed on the turbine. Thermocouples, expansion and vibration detectors are part of the turbine supervisory instrumentation (TSI). TSI readouts are found in the control room where the turbine operator uses the information throughout turbine operation. Adjustments are made to the turbine control system by the operator as required based on information read through the TSI system. Figure 3-18 illustrates approximately where these instruments will be found.

THRUST BEARING WEAR DETECTOR

SHELL EXPANSION DETECTOR

TO CONDENSER ROTOR EXPANSION DETECTOR

Figure 3-18 Typical TSI Instrument Locations

4.6

OVERSPEED PROTECTION

Turbine overspeed protection is provided by the overspeed trip system. The functions of the overspeed and emergency trip system is to limit and control shaft rpm during conditions of potential overspeed, and to close steam valves automatically when necessary during emergencies. The turbine-generator will overspeed whenever the electrical load on the generator disappears suddenly. The tendency to overspeed will be greater when there is more load on the generator.

V

3-17· HPC Technical Services

T.

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The control system is built to take care of this situation. When it works properly it will limit the overspeed to a safe value and when conditions have stabilized, leave the unit running with no load and in a condition where it could be synchronized and re-loaded quickly and easily if desired. This kind of an event is called a "normal overspeed" . Ifthe control system does not work properly when the load is lost, the speed will approach a dangerously high level. Then the overspeed trip will work, closing all the steam valves through the trip system. The system will need to be reset before normal operation can begin again. This kind of an event is called and "emergency . overspeed" . The overspeed trip system is often testable. These tests should be conducted routinely during unit operation (most units allow this test to be conducted without a unit trip). This test actually exercises components to help make it more like the device(s) will function properly.

HPC Technical Services 500 Tallevast Road· Suite 101 Sarasota, FL 34243 USA (941) 747-7733 • FAX (941) 746-5374 www.hpcnet.com

_v . 3-18

T


STEAM TURBINE MAJOR COMPONENTS Chapter 4

TERMINAL OBJECTIVE:

To familiarize the student with identifications and functions of steam turbine major components.

ENABLING OBJECTIVES:

At the completion of this section, the student should be able to: 1. 2. 3. 4.

Given the identity ofa steam turbine component, describe the function of that component. Describe the steam flow through the turbine. Identify the four areas where steam is sealed and describe how each area functions. Define the terms "opposed flow" and "double flow" in relation to steam turbine design.

© 1999 - TG201J5.0_June09, Printed: 6/23/2009

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201

TABLE OF CONTENTS

1.0

TURBINE SECTIONS ............................................................................................................................ 3 1.1 1.1.1 1.1.2

2.0

TURBINE ROTORS .............................................. ,................................................................... :............. 7

3.0

BLADE RINGS AND DIAPHRAGMS ................................................................................................ 11

·4.0

STEAM EXTRACTIONS ..................................................................................................................... 13

5.0

STEAM SEALS .................................................................................................................................... 14

6.0

BEARINGS ........................................................................................................................................... 16

7.0

TURBINE THERMAL EXPANSION AND CONTRACTION ........................................................... 23

.M - 4-2

T

Turbine Section Components ...................................................................................................... 3 High Pressure - Intermediate Pressure Sections ..................................................................... 3 Low Pressure Section ............................................................................................................. 5


STEAM TURBINE MAJOR COMPONENTS .

1.0

.

TURBINE SECTIONS

Large steam turbines are divided into pressure sections to take full advantage of the entire range of operating steam conditions. Each section contains the turbine stages designed to operate under steam conditions unique to that section. A turbine with three separate pressure sections is shown in Figure 4-01 . The first section of the turbine is the high pressure (HP) section. This section contains the stages that will see the highest pressure and temperature steam from the main boiler. The second section is known as the intermediate pressure (IP) or reheat section. The steam flowing through this section is of a lower pressure but at a similar temperature to the steam in the high pressure section. This is accomplished by directing the steam exhausted from the HP section through a reheater in the main boiler. Notice also that this turbine has an HP & IP section on the same rotor. Steam flow is in opposite directions through each section giving the name of this design "opposed flow" . The last section of the turbine is the low pressure (LP) section. The steam flowing 'through this section is lower in pressure and temperature than it is in the IP section; Not all turbine designs have a reheat section (e.g., smaller units and nuclear units). Turbines without an IP section are classified as non-reheat turbines. Turbines are also classified by section arrangement. Figure 4-01 illustrates a "tandem compound" arrangement. All of the turbine rotors are coupled together to drive one generator. The steam path through the turbine shown in Figure 4-01 would be as follows : Intermediate Pressure

Steam from Main Boiler

----~-__,

IP Exhaust or

Crossover Steam

low

Steam from the main boiler enters and expands through the high pressure HP section. Exhausted steam from the HP section (called "cold reheat" steam) is returned to the boiler reheater section for additional heating.

High Pressure

Steam from the boiler reheater (called "hot reheat" steam) enters and expands through the IP turbine section.

HP Exhaust (Cold Reheat)

Steam Flow to Main Cond enser

Boil er Reheater

Steam exhausted from the IP turbine flows to the LP section via a crossover pipe, cross around pipe or casing. The low pressure section exhausts the steam down into the condenser.

Figure 4-01 Three Pressure Section Steam Turbine

1.1

TURBINE SECTION COMPONENTS

1.1.1

High Pressure - Intermediate Pressure Sections

Steam from the main boiler enters the high pressure section through a set of control valves. Typically, the control valves are mounted on a steam chest. The steam chest may be an integral pm1 of the HP turbine shell or a . separate forging. Specific steam valve designs, their function and how they mounted on or off the turbine will be discussed later in this chapter.

M-

4_3HPC Technical SCI'vices

~

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The function of these valves is to control the flow of main boiler steam into the HP turbine. General Electric and Westinghouse call these valves control valves. The operiing and closing of these valves is a function of the turbine control system. The steam chest directs the flow of steam into the HP section nozzle box, which directs the flow of high pressure steam into the first turbine stage. The nozzle box is a circular assembly made up of an inner and outer ring with the nozzle vanes supported between them. The nozzle bog is an extremely rigid and durable assembly designed to withstand the high steam pressures and temperatures. The nozzle box ring assembly is qivided into four to six segments. Steam flow to each segment is controlled by one or more control valves and steam chest arrangement. A typical HP tUrbine section will contain six or seven stages. As steam flows through the HP turbine, it expands and loses pressure. The steam path through the HP section accommodates this expansion . The stationary and rotating blades in the first stage of the HP section are smaller than the blading in the last stage. The high pressure turbine section is enClosed in a shell. The shell is made in two halves, an upper and lower half and are. held tightly together with bolts assembled through the horizontal bolting flanges . .some HP shell designs include an inner and outer shell. GE calls these parts shells; Westinghouse calls these parts cylinder covers. Refer to Figure 4-02. to

N2SEALS

CROSSOVER

BEARING

from

CONTROL VALVES

Figure 4-02 HP-IP Section Components Figure 4-02 illustrates a typical opposed flow HP-IP turbine. Refer to this figure and note: •

Shell mounted control valves. In this figure the steam chest is an integral part of the outer turbine shell. The valve components which control steam flow into the turbine are located in the steam chest.

•

The HP and IP sections are designed with inner shells.

•

The nozzle box and partitions

•

The shell is fitted with seals located over the rotating blades or buckets. These seals prevent steam · from escaping the steam path over the rotating parts. Wheel space seals are located between the turbine wheels to prevent steam leaks around the rotor body.


STEAM TURBINE MAJOR COMPONENTS •

There are steam seals located around the rotor shaft at each end where it exits the shell. These seals prevent steam from escaping along the rotor shaft to the turbine room atmosphere.

•

The rotor journals and bearings are outside the turbine shell. The forward or # 1 bearing is a combined journal and thrust bearing arrangement.

•

The forward bearing assembly is located and supported in the front bearing standard. The aft bearing assembly is located and supported in the mid standard. Standard is a GE term. Westinghouse refers .to these supports as pedestals.

•

The lower half of the outer shell is bolted to and suspended by the upper shell.

•

The upper outer shell is supported at the forward end by the front standard and at the aft end by the mid standard. The supports are designed to allow the shell assembly to grow along the turbine axial centerline during thermal expansion.

•

The outer shell is designed with openings for steam admission into the turhine, HP exhaust steam out to the reheater, admission of reheated steam into the IP section, IF exhaust steam to the crossover, and extraction steam ports.

So far the components have been described as a combined HP~IP section contained in a single outer shell arrangement. In some turbine designs the IP or reheat section will be contained in an entirely separate shell arrangement. The IP section components will be similar to what has been described thus far but are designed specifically for the steam conditions they must operate under. The rotor will be supported by two bearings located at each end of the HP-IP turbine shell. The steam path components become progressively larger from inlet to exhaust to accommodate the expanding steam. The turbine shell may be a single or an inner and outer arrangement. The shell will be equipped with seals which prevent steam from escaping along the rotor shaft. Seals are also installed over the turbine buckets or blades and between the wheel spaces to maintain the steam within the steam path. The lower half shell is bolted to and suspended by the upper half shell. The upper half shell is supported by two mid standards, one at each end. The shell supports axe designed to maintain the turbine location and weight and also allow it to grow along the axial centerline during thermal expansion. In a tandem compound turbine design, the IP turbine section is coupled to the HP section and, the LP section. Steam flow from the reheater to the IP section passes through a set of valves prior to entering the turbine. GE and Westinghouse call these valves reheat and intercept stop valves. The function of these valves is to cut off the steam flow to the lP section should there be a turbine trip. Actuation of these valves is through the turbine control system.

1.1.2

Low Pressure Section Figure 4-03 is a cross sectional view ofa typical double flow low pressure section. Refer to Figure 4-03

and note: •

The turbine rotor is supported by two bearings.

•

Steam enters at the center of the turbine and flows in equal proportions to both sides. One side of the turbine is a mirror image of the other.

• . The steam path components grow progressively larger to accommodate the expanding steam. •

Where the first stage HP turbine buckets (or blades) may be 2 or 3 inches in height, the last stage LP buckets (blades) could be as large as 52 inches from root to tip.

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 •

Steam exhausted from the LP section flows directly into the condenser.

•

The entire LP seCtion is contained in an outer casing called an exhaust hood by GE, cylinder cover by Westinghouse.

•

The steam path stationary components are located and supported by the inner casing (GE) or inner LP cover (Westinghouse).

•

The hood, casing or covers are made in upper and lower halves which bolt together at the horizontal joint.

•

Flow to Condenser

Fi~ure

The LP section is supported by pads which are an integral part of the lower exhaust hood (OE) or lower cylinder cover (Westinghouse). The pads run the full length of both sides of the LP section. They sit on sole plates which are grouted into the plant foundation.

Flow to Condenser

4-03LP Section Components

Atmosph ere Rel ief

•

Shown in Figure 4-04, the upper exhaust hood · or cylinder cover is equipped with rupture or atmospheric relief diaphragms. This device consists of a flexible material stretched under a knife edge around its circumference. Should the situation arise where condenser vacuum is lost and steam pressure builds in the LP section, the flexible material is pushed up against the knife edge and ruptures. The steam is released into the turbine room.


or Rupture Diaphragm

Figure 4-04 Rupture Diaphragm

STEAM TURBINE MAJOR COMPONENTS

Atmosphere Relief or Ruptu re Diaphragm

Flow to Condenser

·Steam '

Bearing .

Seal

Figure 4-05 Seals & Tuming Gear

2.0

•

Steam seals are installed in the inner casing or cylinder cover where the rotor shaft exits. These seals prevent air from entering the turbine along the rotor shaft.

•

Seals are located over the top of the rotating blades or buckets to prevent steam from escaping the steam path.

•

In a tandem compound turbine the LP rotor is coupled to the IP rotor and to the generator. The LP rotor-to-generator coupling is the location for the turning gear drive mechanism. A large gear sandwiched between the coupling face engages with a gear driven by the turning gear motor as can be seen in Figure 4-05.

TURBINE ROTORS

The rotor is one of the major components inside a steam turbine. It is through the rotor that thermal energy is conve11ed to mychanical energy. Compared to the other major components within a turbine, the rotor is quite simple. GE refers to this component as a rotor; Westinghouse calls it a spindle. Figure 4-06 illustrates an HP section turbine rotor assembly. This rotor was machined from a single forging. The journal bearing surfaces, steam seal areas and wheel sections were all configured during the machining process.

Bearing Journal

'"~

/

Wheel Sections

Bearing -l __ _ 1 Journal

Figure 4-06 Steam Turbine Rotor

This rotor has two journal surfaces, one at each end. During the machining process, the journal surfaces are machined and ground to an extremely smooth surface finish . This is necessary to eliminate as milch friction in the bearings as possible.

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The area adjacent to the journal surface is of a slightly larger diameter. The bearing oil seals, which are part of the bearing housing, ride on this surface. Their function is to keep the lube oil in thejoumal bearing area. Moving in from the journal surfaces is another larger diameter area on the rotor shaft. This area has steps machined into it. The name for these steps is lands. These lands spin within the stationary steam seals assembled in the turbine casing or cylinder cover. The steam seal and lands work together to effectively eliminate steam leaks along the rotor shaft. Moving alongthe rotor body we come to the wheel sections. The wheels of the rotor shown in Figure 4-06 are an integral part of the rotor body. Some rotor designs will have the wheels machined as separate discs, shrunk on and keyed to the rotor shaft. Other designs may have a combination of integral and shrunk on wheels.

Attached to the circumference of the wheels are the turbine blades or buckets as shown in Figure 4-07. Turbine manufacturers use a. variety of methods for attaching buckets or blades to the rotor or spindle.

Figure 4-07 Turbine Rotor

Figure 4-08 illustrates the circumferential assembly method. A dovetail profile is machined around the turbine wheel circumference and a "notch" is machined into the wheel fit to allow assembly of the buckets to the wheel. A corresponding dovetail profile is machined into the base of the bucket. This bucket assembly method is found on General Electric HP, IP and the shorter stages of the LP rotors.

Figure 4-08 GE Circumferential Entry Bucket Assembly


STEAM TURBINE MAJOR COMPONENTS Figure 4-09 illustrates the . finger dovetail. The bucket dovetail is really a row of fingers which fit into corresponding grooves machined circumferentially into the turbine wheel. Pins assembled through the dovetail area lock the buckets to the turbine wheel. This fmger dovetail is the method used for attachment of the last stages of buckets and GE LP rotors.

Figure.4-09 GEFillger Dovetail Bucket Assembly

Wheel Dovetail Finger

Figure 4-10 illustrates finger dovetail turbine buckets loaded on a turbine wheel

Figure 4-10 GE Dovetail Buckets Assembled onto Wheel

Figure 4-11 illustrates a dovetail commonly referred to as the "Christmas Tree." This dovetail is machined axially on the turbine wheel permitting side entry of the turbine blades to the wheel. This blade attachment method is found on Westinghouse HP and IP spindles and the last stage of the LP spindle.

Figure 4-/1 W Axial Elltly Blade Assembly

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STEAM TURBINE-GENERATOR FUNDAMENTALS- TG201 Figure 4-12 illustrates the blade assembly method found on Westinghouse "drum" type rotors. The dovetail groove is machined circumferentially into the rotor body. A corresponding dovetail patterIi on the turbine blade (called the blade root) locks the blade to the rotor.

Shroud Strips

L--~I

I

0

Figure 4-12 W "T-Root" Blade Assembly

Fastening

Some rows of buckets or blades will have shrouds connecting their tips as shown in Figure 4-13 . Turbine efficiency is maintained by keeping the moving buckets as rigid as possible. High steam velocity and pressure causes a great amount ofturbulence around the buckets. This turbulence causes the moving buckets to vibrate. Bucket vibration is decreased by tying each row together at their tip 'with shrouds. The shrouding is a metal band that surrounds the bucket outer circumference.

Figure 4-13 Shrouding Over Bucket Tips

The shrouding is usually fitted over "tenons" (small raised knobs on the top of the bucket) as shown in Figure 414, and then welded or riveted in place. Thus, both ends of the buckets are secured. The bottom (root or dovetail) is secured by the rotor and the tip by the shrouding.

Figure 4-14 Shrouding & Tennons


STEAM TURBINE MAJOR COMPONENTS Not all bucket groups are shrouded. The last stage in the LP sections is too large for shrouding to be effective. As shown in Figure 415, tie wires (used on GE LP buckets) are fitted between each bucket to reduce vibration. The wires are laced through holes machined into the airfoil section of the buckets and welded in place. Another method of making the large airfoil section of an LP blade more rigid is a lashing lug. The lashing lug is T.I.G. welded on the airfoil section. With the blades assembled in the rotor, the lashing lugs contact each other and work to keep the large airfoil sections rigid. Lashing lugs are found on Westinghouse LP last stage blades.

Figure 4-15 Tie Wires

3.0

BLADE RINGS AND DIAPHRAGMS Shown in Figure 4-16, the stationary steam path components in a GE turbine are called diaphragms. These steam path components are an assembly of an inner ring and an outer ring with blades or partitions supported between theni .

Outer Ring --~--,~

Horizontal Joint

Figure 4-16 Diaphragms

---!==:

The stationary assemblies are split at the horizontal centerline. Each assembly is located and supported by the turbine shell or cylinder cover. The outer ring of the assembly is machined to fit a corresponding groove in the turbine shell or cylinder cover. Nozzles or partitions convert the thennal energy contained in the steam into kinetic energy in the form of steam at high velocity.

The diaphragms are constructed of a strong inner and outer ring, with the nozzle partitions in between. They can either be of cast or welded construction. A "T". slot is machined on the inner ring 10 to accommodate a . spring-loaded seal ring or·"packing." · . .


Each diaphragm is mounted in a groove, and supported by its associated lower shell. The position can be individually adjusted to compensate for misalignment between the rotor and the diaphragm. The diaphragms are split in half at the horizontal joint and are either bolted or keyed together. This is illustrated in Figure 4-17 .

. Figure 4-17 Diaphragms In Shell

Westinghouse turbines have their nozzles or blades fitted into blade rings. The blade ring assembly consists of stationary, nozzle shaped blades fitted into grooves machined into the blade ring as shown in Figure 418. The stationary blades are locked in place in the grooves with "L" shaped caulking strips that are rolled into place. The tips of the stationary blades will have either integral shrouds or, in some cases, shroud strips similar to those used on the rotating blades. Stationary Blades

Figure 4-18 Westinghouse Blade Rings

GENERATOR END

GOVERNOR END

As can be seen in Figure 419, there may be several rows of nozzles in each blade ring. In the LP turbine, the last couple of rows of stationary blading may be fixed in the inner casing instead of a blade · ring.

Figure 4-19 Rotor & Blade Rings


STEAM TURBINE MAJOR COMPONENTS GENERATOR END

Sealing of the nozzles is accomplished by either "T" shaped seal rings inserted in the ends of the stationary blading or seal strips caulked into shallow grooves in the rotor. These strips ride in close proximity to the shrouds on the . stationary blades. These seal are illustrated in Figure4-20.

GOVERNOR END

Retaining Screw at Horizontal Joint

(l- ll )

Rotating Bl ad e

Figure 4-20 Se{il Rings

4.0

STEAM EXTRACTIONS

In earlier discussions it is implied that all the boiler steam that enters the turbine at the HP inlet will eventually leave the turbine at the LP turbine exhaust. This is not actually the case. Steam is extracted at various locations along the turbine steam path. This extracted steam is put to work elsewhere within the plant and does not do any more work within the turbine. Reheater

Superheater

The extraction steam is used mainly to heat the condensate/feedwater going to the boiler in closed type feedwater heaters. Although most of the extraction steam is used for closed feed water heating, extraction steam is also used in the plant deaerators. The steam is extracted from all three of the turbine sections (HP, reheat, and LP) described previously.

Figure 4-21 Steam Extraction Points

The extraction outlets are referred to as "uncontrolled" extraction points. By uncontrolled we mean that there is no pressure control. The extraction line pressure will be whatever the pressure is at the turbine stage where the steam was extracted. As the turbine load changes, the extraction pressure changes. These extractions take away from the mechanical energy being transmitted by the turbine. More · impOltantly, however, ·these extractions increase the overall efficiency of the power plant. Thus, while turbirie power is lost, moreBTUs are gained in the overall plant heat rate. This result is more efficient use of fuel.

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 5.0

STEAM SEALS As stated earlier, the function of the stealll seals is to prevent steam from escaping the steam path. This is necessary for maintaining turbine efficiency. Figures 4-22and 4-23 illustrate steam path and wheel space seals. .

The seal strip mounted in the outer diaphragm ring blocks steam from escaping over . the top of the buckets. A seal tooth is located on the forward side of the inner diaphragm ring. It rides over a seal surface, which is an integral part of the bucket casting. This seal area prevents steam from escaping below the steam path.

Figure 4-22 Sieam Path Seals

Spring

Figure 4-23 Wheel Space Seals

Seal Teeth

The wheel space seals shown in Figure 4-23 provide an additional barrier against steam escaping the steam path. The seal teeth on the packing segments work with the lands machined on the rotor to effectively stop any steam flow through this area.

Shaft seals are located where the rotor exits the turbine shell or cylinder cover. The seals are part of the turbine steam sealing system. In the HP or IF section of the turbine, the steam sealing system prevents the high pressure steam from escaping into the atmosphere. In the low pressure section, the sealing system prevents air from entering the turbine.

4-14 "PC Technical Services

STEAM TURBINE MAJOR COMPONENTS A typical arrangement for HP or IP shaft seals is shown in Figures 4-24 and 4-25. The arrangement for an LP section is shown in Figure 4-26. Shaft Seal Castellations

Figure 4-24 HP Steam Paths E

Steam from Seals at Higher Loads

X H 1\ U

~

S T E R

E A M

Steam Seal Header

The systems works like this; the lands machined on the HP or IP rotor shaft ride adjacent to the steam packirig seal teeth. As the steam travels the difficult path between teeth and lands, it loses pressure. The more teeth and lands, the greater the pressure drop. With the pressure reduced to 1 psig, the steam encounters an opening in the packing where a constant flow of sealing steam is introduced at 3 psig. With the sealing steam present at a greater pressure, it has blocked the main steam from leaking. Just beyond the opening where the sealing steam is introduced is another opening. This second opening, very close to the end of the packing, has a negative pressure or vacuum on it. The sealing steam and any air that may have entered the seal area are drawn off here and vented to atmosphere.

T

Fif!ure 4-25 N-J Packinf! This description of the HP-IP stearn seal operation is typical of start-up and low load turbine operation. At full load operation, the main steam that travels through the seals is drawn off at the first opening it encounters and is piped through the steam seal system to the LP shaft seals.

0

5 T E

A M I N

@

G L A N

0 E X

H A U

In the LP section, slanted seal teeth and a smooth rotor surface, as shown in Figure 4-26, work with the steam seal system to prevent air from entering the turbine, which is under vacuum.

5 T E

R

Figure 4-26 LP Steam Seals

The steam seal description here is very basic. The steam seal system in fact is rather complex. It is equipped with valves and controls for regulating steam seal header pressure throughout the turbine operating range. This system will be discussed in greater detail later in this text.

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BEARINGS

Each turbine rotor is supported at its journals by a set of bearings. The journal is a section of the rotor that is machined to very fme tolerance. This finely machined area rides within the bearings. Journal bearings keep the rotor from moving excessively up or down (vertical movement) and from side to side (transverse movement). The journal bearings function to support and position the rotor in the radial direction. The bearings are positioned in the standards or pedestal or in a housing made integral to the turbine casing. The rotor journal rides in the elliptical bearing on either a thin oil film or an oil wedge, depending on the speed of the rotor. A typical journal bearing is illustrated in Figure 4-27.

Figure 4-27 Typical Journal Beating Installation

Bear!ng Ring

In Figure 4-28 we see the rotor at slow speed, maybe on turning gear. The rotor rides on a thin film of oil. The film thickness is typically about two-thousandths of an inch.

Figure 4-28 Slow Speed Oil Film

07J..1I676l701

,, w

As the speed of the rotor increases, the rotor will carry more oil into the converging area between the bearing surface and the journal. This converging area causes a highpressure oil wedge to fonn as shown in Figure 4-29, and physically lifts the rotor so that it is no longer riding on a thin (.002) oil film .

The thickness ofthe oil wedge will be determined by the amount of oil in the bearing, the viscosity of the oil; and the weight the rotor exerts on the wedge. Under nonnal . operating conditions, the thickness of the oil wedge will be no more than .005 thousandths of an inch.

Slight Negative Pressure

! t

.005 -----::---~~'t'"'t"1i' Thick

Figure 4-29 High Speed/Pressure Oil Wedge


STEAM TURBINE MAJOR COMPONENTS Ring

There are two primary types of journal bearings as illustrated in Figure 4-30. One is the tilt pad bearing and the other is the plain or elliptical type.

Babbitt lined Steel Pads

Figure 4-30 Two (2) Bearing Types

The tilt pad gets its name from the pads pivoting to create multiple oil wedges, as seen in Figure 4-31 . This design surrounds and better controls radial movement of the turbine shaft and are usually found at the #1 and/or #2 bearing position(s). Tilt pad bearings consist of from 4 to 6 Babbitt lined steel pads, depending on the manufacturer and application.

Bearing Ring Dowel Pin j Bearing Support Ring Upper Half Bearing Ring Upper Half Babbitt Lined Steel Pads

Figure 4-31 Multiple Pad (4), Tilt Pad Bearing

Bearing Support Ring Lower Half

The RED shaded areas in Figure 4-32 show the oil wedges formed at each of the six (6) pad in this example. Note that while all of the pads have oil wedges, the lowermost pad still carries the heaviest load as can be seen by the longer "force" arrows.

Figure 4-32 Multiple Oil Wedges (6)

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Tilt pad bearings are typicallY only used on rotors that are expected to experience a . significant elevation change, like those found on HP and IP turbine rotors. By having a tilt-pad in this type application, the pads maintain parallelism with the rotor during transient operation. A five pad, tilt pad bearing is illustrated in Figure 4-.33.

Figure 4-33 Tilt Pad Bearing

Elliptical bearings (Figure4-34) split at the horizontal joint. The term elliptical applies to the bearing clearances in the assembled state. Bearing liner to rotor journal clearance is greater in the horizontal position (HI & ill) then in the vertical (VI). Elliptical bearings are used at the remaining turbine-generator bearing positions.

Figure 4-34 Elliptical Journal Bearing

Adjusting Pad

Elliptical bearings are used in numerous turbine-generator applications. As a matter of fact, if a plain or elliptical bearing will do the job, it is the preferred bearing as it is the simplest by design.

Figure 4-34 Elliptical Bearing Half


STEAM TURBINE MAJOR COMPONENTS All bearings must be continuously lubricated and cooled. This is done by the bearing oil that constantly . flows through the small space between the bearing and the rotor journal surface. This oil is constantly recycled by the lube oil system. The oil cannot be allowed to leak out along the rotor surface. Therefore, oil deflectors are assembled to prevent the escape of oil on the rotor from the bearing housings. Pumps are used to maintain the flow of oil at a minimum pressure, filters to keep the oil clean, and coolers to maintain the proper temperatme. Steam turbines require some way of controlling the axial forces that are applied to them by the flow of steam through the steam path. This is accomplished through the use of a thrust bearing. Figure 4-35 illustrates two different thrust bearing configurations as well as the axial thrust forces generated by the direction of steam flow through the turbine. · . . The thrust bearing absorbs axial forces and positions the rotor · axially in relation to the stationary components of the turbine. The thrust bearings consist of a rotating collar or collars on the rotor and stationary pads positioned in thethrust bearing housing . in either the front Uournal bearing #1) or mid Uournal bearing #2) standard.

inIi ill'- -_.Ii -

Turbine Rotor

~

Thrust

Bearing

.......IIiIII_ Axial Forces _ _ _ __

Cage

R u n n e

Figure 4-35 Thrust Bearing Configurations

Radial Journal Bearing Shaft

R

u n n e

-,-,--."..--.,--,;;=---,--=----. - - - - J

Bearing

~Housing

R u

n n e

Radial Journal Bearing Shaft

I ·

R u

The thrust bearing in Figure 4-36 consists of two thrust runners. This is a typical arrangement when the bearing is actually a combined journal-thrust bearing as shown in the figure.

n n

e

Figure 4-36 Double Runnel' Thrust

Copper Thru st Plates

I Bearing ~H o u sing

Figure 4-37 shows a typical single runner type thrust bearing. This type of thrust is usually found on units where the thrust is housed in the front standard or pedestal.

Figure 4-37 Single Runnel' Thrust

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 -

.016

Typical axial thrust clearances might be about 16 thousandths (.016) of an inch.

.016

n

Copper Thrust Plates

Also, regardless of the thrust collar configuration, the thrust bearing will have an "active" and "inactive" thrust face with Babbitted surfaces.

n

Shaft

n

Figure 4-38 Thrust Clearances

Thrust Spacer Plates

As with the journal bearings, there is more than one thrust bearing design to meet the needs of various turbine types. These are "tapered land" and "tilt pad" or "Kingsbury" ® type bearings which are illustrated in Figure 4-39.

A -A

------

Tapered Land

Rotation

Th ru~t

Ring

Tilt Pad Rotating View

Stationary View

Figure 4-39 Thrust Bearing Types Ba bb i tt ~

Figure 4-40 is and illustration of the "tapered land" thrust bearing. The stationary bearing surface is divided into an equal number of pads separated by oil feed grooves. Each pad is tapered in a circumferential and radial direction. The motion of the runner will wipe oil into the contracting wedge shaped area and build up load carrying oil pressures.

Tapered Land Thrust

Oil Groove

-

Land

R otation

Figure 4-40 Tapered Land Thrust Bearing


A -A


T

S H A T R B

AU

B I

T 5 I T 0

N A R Y

T T

P L A T

F A C E

E '--- -

Illustrated in Figure 4-41, the motion of the thrust runner and the shape of the thrust land, forces the oil into a "wedge" shape that separates the runner from the lands during normal operating conditions. This type of thrust bearing is quite common, ifnot exclusive, on GE "fIxed speed" steam turbines. By fIxed speed we mean turbines that drive generators.

T

B

1

R

0 T

0 I

A

1

T

l

I N G

!

.

H R U

5 T R U ·N N E R

Figure 4-41 Tapered Land Oil Wedge Formation

Thrust Collar

Ring

Figure 4-42 is an illustration of the tilt pad or "Kingsbury" thrust bearing. The individual pads are mounted on a series of blocks designed to permit the pads to pivot. This pivoting action allows the force transmitted by the runner to be applied equally to all of the pads. The pivoting action also allows for some misalignment. Between the thrust runner and the thrust bearing assembly.

Rotating View

Stationary View

hrust

Colla~

Runne

Babbitt ~

Figure 4-42 Tilt Pad Thrust

Tilt Pad Thrust

As stated previously, thrust bearings are arranged to carry the axial load of the rotor in both directions. One arrangement is to locate the thrust bearings on each side of the runner. Another arrangement is to use two thrust runners several inches apmt with the bearings assembled between and facing each runner surface. Both types are illustrated in Figure 4-43. The thrust pads are carried in a thrust casing, which can be moved axially. The casing is shimmed initially to position the rotor within the shell. The goal is to obtain correct clearances between moving and stationary parts during normal operation.

Single Runner T H R U

5 T R U

Figure 4-43 Tilt Pad Bearing Types

N N E

R

Double Runner

STEAM TURBINE~GENERATOR FUNDAMENTALS - TG201 The thrust bearing is flooded with bearing oil under pressure at all times with the turbine in service. In addition; Figure 4-44 illustrates a thrust bearing wear detector (trip device) that is installed to trip the turbine automatically if there is excessive rotor movement in either axial direction.

IjTLI

Test

Handwheel ===il==~==::::S

Connections for Test Gages for Setting of Pressure Switches

Figure 4-44 Thrust Wear Detector

I Follower Piston

~«

:

.

;.. :. Strainer Rotating Collar on . ' .' Turbine Shaft _ _ _ ~

Figure 4-45 is a tapered land thrust plate. The plate itself is constructed from copper with a Babbitt material (silver colored material) brazed to it as the bearing surface.

Figure 4-45 Tapered Land Thrust Plate (Bearing)

Figure 4-46 is one (1) pad of the many pads used in a tilt pad bearing. The pad itself is constructed from steel with the Babbitt material brazed to it as the bearing surface.

Figure 4-46 Tilt Pad Bearing, Pad


STEAM TURBINE MAJOR COMPONENTS 7.0

TURBINE THERMAL EXPANSION AND CONTRACTION

We have all seen examples ofmatetials, especially metals that expand and contract when they are heated or cooled. This type of expansion explains why the running of hot water on ajar lid can help you get the jar open. This is illustrated in Figure 4-47.

•

Figure 4-47 Expalldillg Jar Lid

A steam turbine is heated to temperatures up to the 1000°F range. The temperature is 1OOO°F at the HP turbine inlet (as well as the reheat turbine inlet). Way down on the other end (at the LP turbine exhaust), the temperature is a lot lower, aroundJ 00 -120°F, as shown in Figure 4-48.

Crossover Pi ing

LP Turbine exhaust temperature is 100 to 120 0 F

Figure 4-48 Temperature Differences The turbine sees these extreme temperature differences during normal operation, and also going through rapid temperature transitions during start ups, load swings, and shut downs. This can cause problems because of the thermal expansion and contraction of the various turbine components. The larger turbine assemblies may expand as much as two inches. If not controlled, turbine damage can occur from excessive stresses and contact of the moving and stationary parts.

Crossover Piping

Crossove r Piping

Most of the axial growth occurs as shown by the green arrow on the lower turbine diagram in Figure 4-49. /

Figure 4-49 Thermal Expansion

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Part of the problem is created by the different rate of thermal expansion between the stationary and moving components. The stationary components (shells) are for the most part dense (thick), and take a while to obtain a uniform metal temperature. On the other hand, the moving rotor assembly (Figure 50) does not have as much mass and heats and cools more rapidly.

Figure 4-50 Different Expansion Rates

Thus the two (stationary and moving) components will expand and contract at different rates. When the rotor is growing at a more rapid rate than the shell we refer to this condition as being rotor long. When the rotor is contracting at a more rapid rate than the shell we refer to it as going rotor short. A rotor long condition should then be expected when the unit is being loaded. A rotor short situation might be expected when the unit is being cooled. See Figure 4-51. Crossove r Pipin g

Figure 4-51Rotor Long andlor Short

Expansion and contraction movement problems are partially avoided by good turbine design. As for turbine stationary components design, side to side (transverse) and up and down (vertical) movements are maintained within acceptable limits by the use of bolts, anchors, keys, and dowels at critical locations like the ones shown in Figure 452. The turbine LP casings are keyed to prevent back and forth (axial) movements. Thus the adjacent HP-IP shell with its components is free to slide away from the generator towards the front standard.

HP/IP Shell Rests at Centerline on Front and Mid Standards

Crossover Pi ing

Front Standard Sole Plate

Axial Keys Hold LP Casings in Place

Figure 4-52 Axial Keys



.

Notice the term "slide." The HP-IP shell moves axially from the LP anchor. The shell expands as it is heated and contracts when cooled. The standards, which are attached to the shell for support, slide freely on metal plates upon the turbine foundation as seen in Figure 4-53. This arrangement allows the turbine to grow or shrink axially without restriction. Heavy grease is used between a sliding standard and the metal plate that it slides . on.

Figure 4-53 S(and(lI'(/ Slide

That takes care of the stationary components, but what about the rotating assembly? The journal bearings maintain side to side and up and down rotor position in bearing housings. The thrust bearing is used to set the correct axial position of the rotor in the shell using thrust bearing shims. Rotor axial expansion is away from the thrust bearing. However, the turbine rotor expands and contracts at a different rate than the shellsicasings that surround it, especially during a unit startup or shutdown. What must be done is to manage the two rates of axial expansion so that the rotor, while spinning at high speeds, will not contact stationary parts. This problem is avoided partly by design and partly by proper operation procedures. The designers, in consideration of these problems, allow the thrust bearing to locate the rotor at an optimum point, and thereby allow ample clearances between the rotating and stationary elements during normal conditions. If it were possible to control expansion and contraction so that all parts of the turbine expanded and contracted at the same rate, the turbine internal clearances would remain the same during all phases of operation. However, since this type of control is not always possible, there are established rates at which the turbines can be heated and cooled. These rates ensure that the internal clearances within the turbines are properly maintained. These rates are both temperature limits (rate of rise, etc.) and roll times.

STEAM TURBINE VALVES Chapter 5

TERMINAL OBJECTIVE: The goal ofthis section is to familiarize the participant with steam valve maintenance procedures and cautions.

ENABLING OBJECTIVES: At the successful completion ofthis section the participant should be able to: 1. . 2. 3.

Identify the various steam valves on a steam turbine . Describe the operation of the various steam valves on a steam turbine. Describe the Auxiliary Valves on a steam turbine.

© 1999 - TG201J5.0_June09, Printed: 12/14/10

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 TABLE OF CONTENTS

1.0

INTRODUCTION ................................................................................................................................... 3

2.0

GE TYPE VALVES ................................................................................................................................ 4

3.0

WESTINGHOUSE VALVES ............................................................................................................... 19

4.0

AUXILIARY VALVES ................... ;....................................................................................... ;............. 25

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STEAM TURBINE VAL VES 1.0

INTRODUCTION

Controlling the flow of steam through the tw-bine is a function of the steam valves. All steam turbines will be equipped with a valve arrangement for controlling steam .flow into the HP section. Turbines with a reheat section will be equipped with an additional set of valves for controlling the reheat steam flow. As with other components studied thus far, GE and Westinghouse use different names for valves serving similar functions . Figw-e 5-01 is a simplified schematic diagram of the valve arrangement for a multisection reheat tw-bine.

Intermediate · Pressure Steam from M.ain Boiler - - - -....--~

IP Exhaust or Crossover Stea m

Low

Control

Main boiler steam enters the steam chest through the main stop (GE) or throttle (W) valves. These valves serve two functions, control the flow of steam into the chest and completely shut off the flow of steam during a turbine trip.

High Pressure

HP Exhaust Intercept Valve Steam Flow to Main Condenser

Depending on tw-bine design, the steam chest may be an integral part of the HP turbine shell or separate castings mounted adjacent to the HP turbine.

Boiler Reheater

Figure 5-01 Valve LOClltiOils

Mounted on the steam chest are the main control valves. The function of these valves is to control the flow of steam from the chest into the HP nozzle assembly. These valves are grouped and opened sequentially to admit steam to certain sections of the nozzle. Reheat steam enters the IP tw-bine section through combined reheat or separate reheat stop and intercept valves on a GE unit, separate reheat stop and interceptor valves on a Westinghouse unit. Valve operation is through the turbine control system. The mechanical hydraulic or electro-hydraulic (MHC or EHC) control systems use hydraulic cylinders to operate the valves. These control systems, for both GE and Westinghouse, are described later in this text. The following pages illustrate and describe steam valves found on steam turbines of General Electric and Westinghouse manufacture. NOTE: Through the production history of Westinghouse turbines, the control and intercept valves have been called governor and interceptor respectively.

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GE TYPE VALVES

The purpose of the Main Stop Valve is to shut off steam flow, to the turbine, in an emergency. To facilitate this positive closure, the Main Stop Valve is an unbalanced valve. By being an unbalanced valve, the steam flow , being shut off by the main stop valve, will assist in the closure of the main stop valve.

Steam Flow

,--------'~ 1/

Inlet

Figure 5-03 illustrates a GE main stop valve assembly. The primary function of this valve is to quickly shut off main steam flow to the turbine under emergency conditions. Refer to the figure and note:

Steam Dam (A~ti-Swirl

\

Figure 5-02 Unbalanced Valve

Cover Nut:==~~""",,,,~~ Cover Spacer

Steam Strainer

..

Swing Check Valve

An exceIJent example of an unbalanced valve, although not representative of most modern stop valves, is a swing-check valve as illustrated in Figure 5-02.

Cove r

)

~/

\

Partition)

Press ure Seal Head Stem

Val ve Seat

Coupling Assembly

Val ve stel J Stop Valve Bypass Disk ,.' Actuator Assembly

.

Steam Path

0 ,~

.

Section A-A Main Va lve Di sk

Figure 5-03 Main Stop & Bypass Valve

w ith Bypass

Val ve Open

A bypass valve within the main valve disk opens and controls steam flow during turbine start-up and loading. The main disk remains seated at this time. As more steam is required, the main disk is raised through the internal contact between the main disk and bypass valve. Refer to internal detail in Figure 5-04.

Figure 5-04 Bypass Valve


Stop Valve Bypass Disk

Valve Ste

~1'-'1ti:77iI Steam Path

Section A-A Main Valve Disk with Bypass Valve Open

STEAM TURBINE VAL VES The steam strainer (Figure 5-05) is in place to remove palticles of boiler slag (114 inch and larger) from the steam .

Figure 5-05 Steam Strail1er

Steam 'D am (Anti-Swirl Partition)

Pressure Seal Head

Valve action is through the actuator assembly coupled to the valve stem as shown in FigW'e 5-06. Oil pressW'e from the tW'bine control system works a hydraulic piston in the actuator against the valve closing spring.

Coupling Assembly

Actuator Assembly

Figure 5-06 Actuator Assembly

07_ K_ I0'CeST20 2

Cap Bypass Disk

Ca p bolt

Ma in Disk

As shown in FigW'e 5-07, a backseat located in the pressure seal head prevents steam from escaping down the valve stem when the valve is in the full open position.

Stem Sea t Areas (Back Seat )

Bushings

Stem Against Backseat

"'"

Pressure Sea l Head

Figure 5-07 Stem & Seal Head Backseat

Closed Position

Open Position

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This is a separately mounted valve. It is welded into the main boiler steam pipe upstream and adjacent to the main control valves. Figure 5-08 shows a set of GE Main Stop Valves in place on a unit and concentrates on the right hand (RH) valve. The large steam lead in the upper right hand position is the main steam inlet to the valve.

Figure 5-08 Main Stop Valve Set In-Place

Cover Nut-_ Cover

_

spacer:::==~~~~~,,~ Cover

Steam Strainer

A steam dam or anti-swirl partition prevents the steam from circulating within the valve body as shown in Figure 5-09.

_

Steam Dam (Anti-Swirl Partition)

Inlet Pressure Seal Head Stem

Valve Seat

Coupling Assembly

Figure 5-09 Allti-Swirl Dam Actuato< Assembly


STEAM TURBINE VAL VES .

.

The purpose of the control valves is to regulate steam flow to the steam turbine. Their position in the steam path is shown in Figure 5-10. IP Exhaust or Crossover Steam

Steam from Main Boiler Stop Valve Control Valve

Steam flow to Main Condenser

Boiler Reheater

Figure 5-10 Steam Path Location ojControl Valve(s)

A GE, separate steam chest mounted control valve is shown in Figure 5-11. The plan view of a typical arrangement of separate mounted control valves illustrates how they are grouped and assembled to the steam chest. The valves and their steam chest are located just below the turbine deck alongside of the HP turbine. Steam enters at both ends of the steam chest after leaving the Main Stop valves.

Tension Valve CloSi ng

sp" " g -~-1ffi"4tIi

Push Rod link (Com pression)

Steam

Actuator Valve Disc

AS"m bl l' ~'"

Valve Seat

Assemblv

"m_-I<>'7,,-

-~~fJ

Seat Pin -

Steam Inlet

Steam Inlet

Control Valves Plan View

Figure 5-11 Separate Moul1ted COlltrol Valves

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The function of these valves is to control steam to meet turbine load demands. Valve operation is as follows: (Refer to Figure 5-12.)

The operating mechanism utilizes a spring assembly which acts directly on top of the valve crosshead assembly to keep the valve Closed. Valve Closing :o. prmg --

-

-

-f!f.""'TH

Tension Rod

Crosshead Guide

Push Rod link

Pressur.e Head ~ Steam Chest Actuator Assembly Valve Disc

Asse mblv-~V//;'iI

Figure 5-12 Control Valve Assembly

The valve is opened by the hydraulic cylinder lifting the end of the lower lever. The other end of the lower lever is linked to the upper lever. The upper lever is supported on the spring housing. The opposite end of the upper lever is held down by tension rods attached to the hydraulic cylinder. The crosshead connects the valve stem to the lower lever. High pressure oil through the control system acts on the actuator assembly. The actuator assembly and hydraulic cylinder maintain valve opening allowing steam flow to meet turbine load demands. The valve disk is raised and lowered within the steam flow chamber of the valve chest. The valve seat is pinned in place in the lower half of the valve chest. Seals in the pressure head assembly prevent steam from escapmg up the valve stem.

Some control valves are designed with an internal pilot valve as shown in Figure 5-13 . Opening of the pilot valve within the control valve main disk equalizes the steam pressure on both sides of the main disk. With the pressure balanced, less force is required to lift the main disk off the seat.

Steam Chest

Figure 5-13 Piloted (Balanced Pressure) Control Valve Valve Open


STEAM TURBINE VAL VES A GE combined reheat (CRY) valve is shown in Figures 5-\4 and 5-\5 . As the name implies, there is two valves in one, the intercept valve and the reheat stop valve. The primary function of this valve is to provide emergency trip protection to the turbine by shutting off the steam supply from the reheater. These valves are located in the reheat steam piping leading to the lower half shell of the IPturbine; one per each side and very close to the turbine shell.

Crosshead

Steam In Intercept Valve Actuator . Assembly

Figure 5-14 CRV, Intercept Valve Portion

Cross head

Steam I n Intercept Valve Actuator Assembly Reheat Stop Valve Disk Pressure Seal Head

Figure 5-15 CRV, Reheat Stop Valve Portion Reheat Stop Valve Hydraulic Actuator Assembly

Ba lance

The reheat stop valve disk is lifted from the seat through the action of the hydraulic actuator coupled to the valve stem. The reheat stop valve is an unbalanced valve. Asa resu lt this reheat stop valve has to be opened before the intercept valve. The intercept valve keeps the steam .pressure off the top of the reheat stop valve disk. Springs in the actuator close the valve upon loss of hydraulic oil pressure due to a turbine trip.

Reheat Valve Closed

Reheat Valve Open

Figure 5-16 Reheat Stop Valve Positions

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 During normal operation of the turbine-generator under load, the intercept valve is wide open. The same is true of the reheat stop portion of the valve. . The reheat stop valve disk (Figure 5-17) is lifted from the seat through the action of its hydraulic actuator coupled to the valve stem. Springs in the actuator close the valve upon loss of hydraulic oil pressure due to a turbine trip. As shown in Figure 5-17, seals in the lower pressure seal head prevent steam from escaping along the reheat stop valve stem.

Steam In

Stem Backseated

Figure 5:'17 Open Valves

CRV Closed Position

Upper Pressure Head

Steam In Intercept Valve Actuator Assembly

Valves Open

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e5rnl1

The intercept valve disk is lifted from the seat through the action of its hydraulic actuator connected by push rods with the upper lever as shown in FIgure 5-18. The upper lever lifts the disk through the' crosshead and valve stem assembly. The lower lever working with the upper lever eliminates high hydraulic cylinder forces to be transmitted to the valve casing or upper pressure head bolting.

Figure 5-18 Intercept Valve Actuator Reheat Stop Valve Hydraulic Actuator Assembly

01~l..119

STEAM TURBINE VAL VES . .

The steam strainer shown in Figure 5-19 eliminates any particles (114 inch or larger) of reheater tube or steam pipe slag from entering the turbine.

Crosshead

Depending on the vintage of the unit, the Intercept valve stem mayor may not have a stem leakoff line. If it has a stem leakoff line it directs any leakoff steam to a closed feedwater heater or the condenser .. Ifit does not have a stem leakoffline it will be equipped with seals that prevent steam from leaking past the stem and into the turbine room .

Steam I n Intercept Valve

Pressure Seal Head Stem Leakoff

---~ -~'I:l1

Figure 5-19 Steam Strai11er & Upper Pressure Head . Reheat Stop Valve Hydraul ic Actuator Assemb ly

In either case once the valve is in the fully open position, a backseat on the valve stem and a cOITesponding seating surface on the upper pressure head, prevent steam from leaking out into the turbine room.

Valve

Position

The intercept valves throttle close in an overspeed. The setting, however, is such that the intercept starts closing after the control valves have started to close (or are closed) as shown in Figure 520. RHSV

95

100

105

107

Turbine Speed (% Rated)

110

These intercept valves are pmt of what is known as the I st line of defense. Should they fail to shut off steam flow sufficiently to control the turbine in an overs peed, then the reheat stop valve will close as pmt of turbine trip scheme. In a recovery from an overspeed (where a trip did not occur), the intercept opens, prior to the main control valve, to blow down the reheater before the control valves admit steam from the superheat section of the boiler.

Figure 5-20 Valve Operati0l1{l1 Graph

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The intercept valve is equipped with a balance chamber as shown in Figure 5-21(see enlarged detail view) that has sealing rings and balance holes in the disk such that the pressure above the disk is always equal to the pressure below the disk (downstream from the valve seat). This allows the intercept valve to be positioned accurately anywhere in its travel, regardless of the upstream pressure. The intercept valve is a balanced valve. The reheat stop valve, being unbalanced, must be opened before the intercept valve is opened.

Balance Chamber

Reheat Stop Valve Disk

Reheat S~eam

Reheat Stop Valve

.

Hydraulic Actuator Assembly

Figure 5-21 Intercept Valve Balance Chamber

Intercept

}

Valve Disk Detail View Reheat Steam Path Through Balance Chamber

Some of the newer GE units are equipped with . combined main stop and control valves that are a smaller version of the CRY valves described here and an example of these valves is shown in Figure 5-22.

Figure 5-22 New Design Main Stop & Control Valve Combination


STEAM TURBINE VAL VES A GE separately mounted reheat stop valve is shown in Figure 5-23. The function of this valve is to shut off the steam flow to the IP turbine on a turbine trip. This valve is located in the reheat steam pipe adjacent to and upstream of the intercept valve. This valve is fully open during normal turbine operation. Refer to the figure and note:

Cover Bushing Valve Holder Pin Flange Link Clevis Nut --...fzl:::::=~~ Pin , Pressure Head

Valve Casing ,Disk

The view in the figure is looking downstream through the valve with the disk fully closed.

Stem

Coupling Bolt Steam (Water) Shield Coupling ,

Figure 5-23 Sepal'([te Mounted Reheat Stop Valve Hydraulic Actuator

Cover

Under normal operating conditions the disk is maintained open by the actuator coupled to the valve stem pushing up and rotating the disk around the valve holder pin. Loss of hydraulic oil pressure to the actuator piston will allow the spring in the actuator to lower the valve stem and rotate the disk into the closed position. This is shown in Figure 5-24

Valve Holder Pin Flange Link Clevis Nut ---fzt;:::::<~~ Pin Pressu re Head

Closed

Stem

Figure 5-24 Closed and Opened Positions ojthe Valve Disk Hydraulic Actuator

Opened

5-13~


~


Cover Bushing Valve Holder Pin Flange

A backseat located in the pressure seal head and a corresponding one on the valve stem, prevent steam from escaping down the valve stem when the valve is in the fully open position. While the valve is moving from the fully closed to the fully opened position, a stem leakoff line directs any leakoff steam to a closed feedwater heater or the condenser.

Link Clevis Nut ---fzt=~R Pin

BaCkseat~~"'~~~~5< Pressure Head

. Leakoff

Hydraulic Actuator

As with the reheat stop valve associated with the CRY, this separate reheat stop valve cannot open against full reheat steam pressure.

Figure 5-25 Backseat & Stem Leakojj

Balance Chamber

Figure 5-26 illustrates a GE separate mounted intercept valve assembly. The function of this valve is to control the flow of reheat steam to the IP turbine section in a pre-emergency overspeed condition. Unlike the open or shut reheat stop valve, this valve has some throttling capability and is governor controlled. The valve will also trip closed on a turbine trip . This valve is located in the reheat steam piping between the reheat stop valve and the turbine shell. Operation ofthis valve is as follows : (Refer to the figure .) The valve is designed with a bypass valve identified as the middle sleeve. During normal turbine operation the main valve is wide open. Should an overspeed situation develop, the valve begins to close. With the main disk seated, an unequal pressure is created on each side of the valve disk.

Piping

.....1'4--i---f:~F-+-- Middle Sleeve

Main Valve Disk

Hydraulic Actuator

Figure 5-26 Separate Mounted Intercept Valve


STEAM TURBINE VAL VES Valve action is through the actuator assembly coupled to the valve stem. Oil pressure from the turbine control system works a hydraulic piston in the actuator. Loss of oil pressure allows the actuator valve closing spring to overcome the hydraulic piston and close the valve.

Balance Chamber Piping

+.fH H---i'S-lftt-t-- - M 'OOlle

Steam Inlet

-

Sleeve

As the actuator pushes up the valve stem, the middle sleeve pilot valve unseats first. This allows immediate pressure equalization on both sides of the main valve disk. Continued upward valve stem movement raises the main valve disk to the fully opened position

Steam Out

Coupling

Hydraulic Actuator

Figure 5-27 Sepal'{lte Intercept Valve

Balance Chamber Piping

The balance chamber piping shown in Figure 5-27, allows for smooth operation of the . valve over the complete range of its travel once the main disk is off its seat. ~.~~II----tAt_-- Middle Sleeve

Steam Inlet

Steam Out Main Valve 0

S t e r n - - - - - -. . .

Coupling

Figure 5-28 Balallce Chamber Pipillg Hydraulic Actuator


. A steam strainer eliminates particles of reheater tube or steam pipe slag V-t" or larger from entering the turbine A backseat located in the pressure seal head and a corresponding one on the valve stem, prevent steam from escaping down the valve stem when the valve is in the fully open position. These items are shown in Figure 528.

-

Steam Out

Coupling

Figure 5-29 Strainer & Backseat

Figure 5-29 illustrates a GE shell mounted control valve and a valve arrangement. As shown, the valves ·are mounted directly over the steam chest which is an integral part of the HP turbine shell. One set of valves is located on the upper shell, another set on the lower shell.

lift Pin Lifting Lever Pivot Pin Lever Support

.~I;fti===:- ;~~~s=.~~~ Pin

rO'SSheadl-->~~Ii~

C,

The function of these valves is to control the flow of the main boiler steam into the HP turbine. Refer to the figure as the valve is described here. The valve disk is assembled to the end of the valve stem located within the steam chest. The opposite end of the valve stem is connected to a crosshead arrangement, which is raised by the upward motion of the lifting lever.

Val ve Closing Spring

Spring

Valve Stand Pressure Head Valve Stem

- ----rr.;om. HP Turbine Shell and Steam Chest

Control Valve

Valve Seat

Disl,-~~

----;~~'N

Plan View Multiple Control Valve Arrangement

Figure 5-30 Shell Mounted Control Valves


Ever Bolts Support

STEAM TURBINE VAL VES

Spring Retainer

Lifting Lever Pivot Pin Lever Support

Pressure on the lifting lever to keep the valve closed is provided by the valve closing springs shown in Figure 30. Rotation of the cam raises the lifting lever against spring pressure, opening the valve. Thus the amount of valve opening is directly related to cam rotation.

Crosshea Valve Stand Pressure Head Valve Stem ----#;~~~ 1I'~~l5f':l"" HPTurbine " Shell and Steam Chest Control Valve Disk

Va Ive Seat

---I7~~, 1

----
Figure 5-3/ Valve Closing Springs, Li/ting Lever & Cam

Upper Control Valve(s)

As shown in Figure 5-31 , a hydraulic cylinder located in the turbine front standard through a lever called the "D" rod rotates the camshaft and cams thus controlling valve position. Another lever called the "E" rod operates the valves located on the lower shell for those units so equipped. Located at the end of both rods is a rack gear that engages and turns a pinion gear attached to the outboard end of the valve camshafts.

Figure 5-32 Control Valve Operating Mechanism

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The plan view in Figure 5-32 shows a threevalve arrangement on the top of the turbine shell. Many units also have an equal amount of valves on the bottom half of the turbine shell as indicated in Figure 5-25. The number of valves can vary from . unit to unit with the configuration ofthe valves dependant on both the size and/or vintage of the unit. . Each valve opens over a steam path in the turbine shell that directs the flow of steam to a specific area of the nozzle. The valves control the flow of steam to each of their designated areas of the nozzle.

Figure 5-33 Control Valve Plan View

Plan View Multiple Control Valve Arrangement

As shown in Figure 5-33, the valves pressure head is equipped with high and low pressure stem leakoff lines that direct steam leaking past the valve stem to different locations in the steam system depending on pressure. The high pressure stem leakoff may go to the steam seal system while the low pressure stem leakoff is directed to either the condenser or a Low Pressure feedwater heater. 2nd Leakoff

High Pressure 1st Leakoff ---+!=

Valve


Figure 5-34 Stem Leakoff


WESTINGHOUSE VALVES

For the initial admission of steam, a typical Westinghouse turbine generator unit is provided with two throttle valve-steam chest assemblies. Each assembly is located along each side of the HP turbine cylindei·. A typical steam chest has one throttle valve and three to four governor valves. Four governor valves are shown in Figure 534. The throttle and governor valves provide redundant protection at the high pressure steam inlet of the turbine. Governor Valves (Control) - - -

Throttle

Throttle -ValveActuator

1-- -

Figure 5-35 Combined Throttle Valve & Governor Valve Steam Chest Arl'{lllgemellt

Figure 5-35 illustrates a Westinghouse throttle valve. The valve is horizontally mounted at one end of the steam chest. The purpose of the throttle valve is to provide emergency shut down of the steam turbine in an overspeed or any threatening condition. The throttle valve is hydraulically opened and spring closed.

Spring Housing Springs (4)

J

/.~

Steam Chest

.

Valve Stem Bushing Valve Stem Valve Bushing Valve (Main)

~ "" Jil~~fjl~~~~P~ilotvalve Steam Outlet

'f'

Lever~

operating' Stop

Figure 5-36 Throttle Valve

V~~I::~~~:~

B~r o

--;':-::-3

Throttle Valve Seat

_~_.=_

===C::

.

Operating Levers

(In Pairs)

Valve Stem Guide

5-19Ji:


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~~

valve

.

Spring Housing

Springs (4) .

Link

Bonnet

Valve Guide

Steam Chest Valve Stem Bushing

.

Valve Stem Valve Bushing Valve (Main)

II

Pilot Valve Seal Ring

Steam Outiet

Lever~

operating'

Stop B L r

o

Operating

(In Pairs)

Figure 5-37 Seal Rings & Stem Leakoff

-:.:-=-_~ -

~=Le:e~s - -

Valve

V

S tern Guide

As shown in Figure 5-36, the throttle valve main disk is designed with a sealing ring so that, in the wideopen position, the seal ring reduces steam leakage through the stem leakoff lines to a minimum when it enters the provided chamber in the valve bonnet.

Stem Leak Off Lines

The throttle valve is an unbalanced steam valve, meaning the steam flow it is shutting off will assist in the valve closure as shown in Figure 5-37. As shown in Figure 5-38, the throttle valve has an integral pilot valve that can open against full boiler pressure. By doing so, the pressure is equalized across the throttle valve disk and the throttle valve can then be opened.

Steam In

Figure 5-38 Unbalanced Main Valve Disk

Pilot Valve Spring

On some machines, this pilot valve is also used for turbine start up.

Fif:ure 5-39 Pilot Valve



As can be seen in Figure 5-39, a permanent steam strainer prevents foreign objects 14" or larger from entering the turbine. During initial operation or after boiler re-tubing, an additional "temporary" fine mesh strainer is added to the OD of the permanent strainer.

Link

Spring Housing Springs (4)

~

""

Steam

Chest

Valve Stem Bushing Valve Stem Valve Bushing Valve (Main) Pilot Valve

Steam Strainer

Steam Outlet

Figure 5-40 Steam Strainer

Figure 5-40 illustrates one type of a Westinghouse governor valve assembly. In this type of arrangement the valves are mounted on the steam chest in parallel sequence and are lifted or positioned by a single lift bar and a single servomotor (actuator) or hydraulic cylinder.

Throttle

-Valve-

Throttle Valve-

Actuator

Steam Flow to Nozzles

Figure 5-41 Single Actuator Valve Arrangement


Figure 5-41 illustrates another type of governor valve that is lifted or positioned by an individual servomotor or hydraulic cylinder. This valve is of the single-seated plug type and is controlled by the servomotor that responds to oil pressure signals from the control system.

Spring Housing Assembly

Crosshead Pin

Bracket Bonnet Leak Off Lines -~*~::lJ

Valve

{ctuatO~/servo Motor

I

Figure 5-42 IndividualActuator Valve Arrangement

I

LJJ

Single .:.seated plug


Cross head Bushing

Crosshead Pin

Bracket

Bonnet Leak Off Lines

-.~~~

Valve Stem Bushing __-J / . /

Valve

_[

Steam Chest

ctuator1servo Motor

The valve operating lever is connected through the crosshead and pin to the valve stem. The lever is fulcrummed so that upward movement of the servomotor raises the lever opening the valve while downward movement of the servomotor lowers the lever closing the valve. This can be seen in Figure 542.

/.. - .

Stem ---j~C:~rII~ I Valve Nut Disk --¥7'77>:J

Figure 43 Governor Valve Operation

Spring Housing

Assembly

Heavy springs, shown in Figure 5-43, are provided to assist in the closing of the valve. Bracket

Figure 44 Governor Valve Closing Springs

Ji: .•



Another (smaller) spring (Figure 5-44) ensures the operating lever moves down as the hydraulic cylinder moves down, and independent of the valve closing springs.


__-m,ovy Springs

__ 0

Crosshead Bushing

--~

Bonnet

Leak 'OFf Lines -¢~~~ Steam Chest Valve Stem Bushing

Figure 45 Govel'1lor Valve Control Level' Spring

Valve Nut Disk -..,..,..,...,..,""

The function of the different types of Governor Valves is to precisely regulate the speed and load of the turbine by controlling the steam flow to the turbine. Figures 5-45 & 5-46 illustrate a Westinghouse interceptor valve. These valves are located in the steam piping between the reheat stop valve and the reheat turbine. The function of the interceptor valve is to limit the flow of steam to the reheat turbine in the event of a turbine overspeed, where the turbine has not tripped. E:::':l_ - - - - A c t u a t o r 1_ _- - Switch

_ - - - - Actuator

, - ' _ - - Switch

Bonnet

=" I~~~f~~

Stem

-

In'E''''''''''''-

I~~~~~~- Leakoff

Leakoff Stem

Pressure

Pressure

Seal Rings

Seal Rings

Steam Str;'lne!r-++----+l1 Steam

Spring

r -w --lf1I*1---- Stem

Steam Str;'ine!r-++----+ll Steam inle,t++-

~~--If1I*1----Stem

Valve

Valve

Figure 5-46 Interceptor Valve (Closed)

Figure 5-47 Interceptor Valve (Opened)

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Spring Adjust Spring

E:: !:I _-----

Actuator , _ - - - - Switch

:,

Refer to Figures 5-47 & 5-48 while the valve operation is described here: During normal turbine-generator operations under load,the valve is wide open.

~~~~~~- Leakoff Pressure Seal Rings Steam Strainer-t-t-~----1'f1 Steam Inle,t++--

'I-I~---

Stem

The valve disk is lifted from the seat through the action of the hydraulic actuator coupled to the valve stem. . . The interceptor valve disk is a balanced design . Pressure equalization holes in the valve disk ensure the pressure on top of the valve will be equal to the pressure downstream of the operating valve. Springs in the housing above the valve bonnet close the valve upon loss of hydraulic oil pressure due to a turbine trip

Valve Ive

07_I'C1SLeSTl02

Figure 5-48 Interceptor Valve Operation I

=----

e;:::;J ....

Actuator _ - - Switch

Steam leakage up the valve stem is limited by a closing fitting stem bushing. This valve is of the single seat plug-type design. Each valve is operated by a separator actuator. The valve is surrounded by a permanent steam strainer to keep foreign material from entering the turbine. Pressure Seal Rings

Steam Strainer·--1H-- -+I Steam

--=§f.H+*- - - S t e m

Tn l,·t -l--I--

Figure 5-49 Interceptor Valve Operation IJ Valve

Steam



AUXILIARY VALVES

The following valves discussed here function to protect the turbine from overspeed after a trip or to reestablish turbine operation after a trip. Not all GE turbines have all of the valves listed here. Though this description is specific to GE turbines, similar valves are found on the Westinghouse units.

The EqualizerValve, as shown in Figure 5~49 , is mounted on one combined reheat stop valve on units equipped with the trip anticipator. Upon generator load . rejection, the reheat stop valves will close, thus stopping the flow of reheat steam into the reheat turbine section. Reopening the reheat stop valve against full reheater pressure would take some effOlt. The equalizer valve will automatically open and . direct to the condenser, the reheat steam that has been trapped between the intercept valve and reheat stop valve disk. When reheat steam pressure has been reduced to approximately 15% of full load, the reheat stop valves are capable of reopening, at which point the equalizer valve will close.

Intercept Valve

Reheat Stop Valve

Figure 5-50 Equalizer Valve

• ,

Blowdown Valve

The Slowdown Valve is shown in Figure 5-50. The function of this valve is to release the steam that will be bottled up in the HP turbine section and reheater should a full load trip or load rejection occur. In this situation, the steam will be trapped between the closed main control valves and intercept valves. The vacuum existing in the IP and LP turbine sections will draw the bottled up steam through the steam seals between the HP-IP sections. If the steam seal packing is worn, this would allow the trapped HP steam to enter the reheat section and drive the turbine to overspeed. The blowdown valve opens the steam sealleakoff annulus directly to the condenser so as to divert most of the steam leakage.

Figure 5-51 BloJlldoJIII1 Valve

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STEAM TURBINE-GENERATOR FUNDAMENTALS ~ TG201 Steam from Main Boller - - - -__- - - - - - . Intermediate

Pressure

The Ventilator Valve shown in Figure 5-51 is provided to protect the HP turbine from overheating. Serious heating of the HP turbine would result from windage losses should it be allowed to spin in high pressure bottled up steam. This valve connects the main steam lead direCtly to the condenser and automatically opens when the main stop and control valves close in a turbine trip situation. When opened the ventilator valve will draw steam from the reheater backwards, through the HP turbine and to the condenser. This reverse flow of .. steam keeps the HP turbine cooler and helps bleed down the reheat pressure.

Ventilator Valve

IP Exhaust or Cross over Steam

X lf - - - - --I

HP Exhaust (Cold Reheat)

Intercept . Valve Steam Flow to Main Condenser

Boil er Reheater .

......-

....- - - - - - - -......-------I~ To Condenser

Figure 5-52 Ventilator Valve



STEAM TURBINE AUXILIARY SYSTEMS Chapter 6

TERMINAL OBJECTIVE: To familiarize the reader with the steam turbine auxiliary systems and their operation. ENABLING OBJECTIVES: At the completion of this section the student should be able to: 1. 2.

Identify the steam turbine auxiliary systems. Describe the function of the steam turbine auxiliary systems.

© 1999 - TG201J5.0_June09, Printed: 12/14/2010



INTRODUCTION ................................................................................................................................... 3

2.0

LUBE OIL SYSTEMS ............................................................................................................................ 3 2.1 2.2 2.3 2.4 2.5

3.0

TURBINE SHAFT SEAL SYSTEMS .................................................................................................. 11 3.1 3.2 3.3 3.4 3.5 3.5.1 3.5.2

4.0

Basic Principles ............. :...................................................................................... ,....................... 3 GE MHC Systems ....................................................................................................................... 5 GE Schenectady EHC Lube Oil System ...................................................................................... 7 Westinghouse MHC Lube Oil Systems ....................................................................................... 8 Westinghouse EH Lube Oil System .......................................................................................... 10

Introduction .......................................................... ;.................................................................... Basic Principles ......................................................................................................................... Steam Packing ........................................................................................................................... Water Seals ................................................................................................................................ System Descriptions .................................................................................................................. Steam Seal Systems .............................................................................................................. Water Seal System ................................................................................................................

EH FLUID SYSTEM ............................................................................................................................. 21 4.1 4.2 4.3 4.4

Introduction ............................................................................................................................... 21 Hydraulic Fluid .......................................................................................................................... 21 Westinghouse System ................................................................................................................ 22 GE System ................................................................................................................................. 24

_1.f - 6-2

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11 11 12 15 16 16 19


STEAM TURBINE AUXILIARY SYSTEMS

1.0

INTRODUCTION

In this section we will study two "auxiliary systems" that are needed for every steam turbine generator, and a third that is needed for some units. These are the lubrication or lube oil and the turbine shan't seal systems, and the electro-hydraulic or "EH" fluid system. We will study each of these in tum. In each ofthese systems there are some basic principles that apply to all units, and some principles and construction details that are not the same in GE and Westinghouse units, and even in different units built by the same manufacturer. As we study each system, we will start with the basic principles that apply to all units and then explain the main differences between manufacturers and units.

2.0

LUBE OIL SYSTEMS

2.1

BASIC PRINCWLES

The purpose ofthis system is to supply clean oil at the proper pressure and temperature for: •

lubricating and cooling the thrust andjoumal bearings.

•

lubricating and cooling the turning gears and the exciter reduction and governor drive gears, if any.

•

the generator shaft seal system.

•

cooling couplings on some units.

•

the turbine control and/or emergency trip system on some units.

Cooling and lubricating the thrust andjoumal bearings takes much more oil than all the other uses combined. The main parts of the system are a tank or reservoir, pumps, ejectors (Westinghouse), coolers, pressure regulating devices, connecting piping, and test devices, instruments, etc. All modem turbine-generator lube oil systems have these main features: •

The main oil tank is at or near the lowest level of the power plant, below the high pressure turbine section. This is usually at least ten feet and often more than twenty feet below the turbine-generator centerline elevation. The tank is at this location because:Oil will drain back to the tank easily. The tank will be well away from and below hot steam pipes, valves, etc. so leaks are less likely to be a fire hazard. The space close under the turbine is usually very crowded with steam pipes and valves, which must be placed as close to the turbine as possible. The largest oil pipes connect to the front and mid standards of the turbine, so the tank should be as close to that area as possible considering the other limits.


_1.1 -

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•

The main pump that supplies all the oil during normal operation is a centrifugal pump driven directly by the main turbine shaft. This is because: The main shaft is the most efficient and reliable drive there is. The discharge pressure of a centrifugal pump running at a set speed changes very little even if the flow changes quite a lot. This is good in a system where the flow can change due to leaks, control system actions, etc. A centrifugal pump is simple, easy to maintain and cheap to build.

•

On the other hand, a centrifugal pump built to run at a set speed puts out very little pressure if its speed is much below normal. Also, they are not very good at lifting liquid to their suction; they really need a positive pressure at the suction to work well.

•

The above means that on all turbine oil systems there must be: An auxiliary pump to supply oil at speeds below the. operating range of the main shaft pump - usually about 90% of rated speed.

A way to pump oil up from the tank to the main pump so its suction will be at a positive pressure. •

There would be very serious damage to the turbine generator if the bearing lube oil supply should fail when the shaft was turning. In fact, the unit could be destroyed if it should fail at full speed. Therefore, there must be one or more back up supplies that would start automatically to guard against this danger.

•

The main source of heat in the lubricating oil is heat from the shaft and friction. The primary function ofthe lube oil passing through the bearing is for heat removal and not for lubrication.

•

The oil pressure must be kept in a set range so the amount flowing to all parts of the system will be correct.

•

Older turbine control systems ("MBC" or mechanical hydraulic controls) use lube oil at a pressure much higher than the pressure needed for lubrication, etc. Modem "ERC" (electrohydraulic) controls use a separate high-pressure fluid system. This means there are important differences in the lube oil system ofMBC units compared to ERe.

With these principles in mind, let us study some actual lube oil systems.

_1J ~ T



2.2

GE MHC SYSTEMS This description is typical of GE MHC systems and uses regular lube oil for its working fluid.

High pressure oil is needed from the lube oil system in order to start the turbine-generator, because the control system uses it to open the steam valves. This oil comes from the auxiliary oil pump which also supplies the oil needed by all other parts of the system whenever it is in operation. The auxiliary oil pump is normally driven by an AC motor of75 to 200 horsepower depending on unit size. Some large units use two AC motor driven pumps in parallel and some small, old units use a small steam turbine to drive the auxiliary pump. The turbine/pump assembly is often called the "turbo pump". Figure 6-01 shows part of the system during a start up. As shown, the pump itself is inside the tank below the oil level. The driving motor or turbine is on top of the tank.

fROM MAIN 011. PUMP

10 MAIN OIt.PUMP SUCTlON

DISCHARGE 10

B....RINGS AUK.

OIL COOLER

OIL PUMP

?!i=====r-===4=:::::!~====F-

- - TOP_""_TAM!..

AH....D"" ~

.

- _ _ - __

B....RING REUEFVALVE

;/

Figure 6-01 GE Auxiliary Oil and Booster Pumps The auxiliary pump draws oil from the tank and pumps it at about 200 psi into a "T" connection. The upper branch of the T goes to the control system hydraulic header. Beyond that, the branch is closed off by a closed check valve in the main shaft pump discharge. The lower branch of the T divides. One branch goes through the "bypass baffler" valve, past the bearing relief valve and to the transfer valve. The transfer valve sends oil to one of the two oil coolers (the other being in reserve) and on to the bearings. The other branch goes through the "booster baffler" valve into the "submerged oil driven booster" pump.


_M -

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The "booster" is a centrifugal pump driven by an oil-powered turbine. The high pressure oil drives the turbine and in so doing, loses much of its pressure. The "used" oil goes into the bearing header, joining the other oil that came through the bypass baffler. The bypass baffler and the booster baffler are adjusted to drive the oil turbine at the correct speed while also supplying more than enough oil for the bearings. The bearing relief valve discharges oil as needed to keep the correct bearing header pressure. There is no way to adjust the oil pressure in the control system. It is set by the pump construction. The centrifugal pump that is driven by the oil turbine, draws oil out of the tank and pumps it up to the suction of the main shaft pump in the turbine front standard. When the shaft pump speed gets high enough (about 90% of normal), it will begin to pump. Its full output pressure is about 50 psi higher than the auxiliary pumps', so it will force open the check valve in its discharge and force the check valve in the auxiliary oil pump discharge closed. At this point the main pump has "taken over" and the system is in the normal on-line running condition. The auxiliary pump is then shut down but put in standby, ready to start automatically if the high pressure oil supply should fall below normal. Figure 6-02 shows more details of the system.

L-..._ _ _ _ :........ ........

Figure 6-02 GE Oil System It shows everything that was on Figure 6-01 plus the main shaft pump itself, two more motor-driven pumps, and the electrical controls for all three motor driven pumps. One of the "new" pumps is the "AC T.G." or AC turning gear pump. It draws oil directly from the tank and as shown, pumps it directly into the bearing header upstream of the transfer valve. This pump is used when the unit is on turning gear and high pressure oil is not needed. The bearings take less oil on turning gear than they do while running; this, plus the much lower discharge pressure, means that the AC T.G. needs only about one-tenth the power of the auxiliary pump. When the AC T.G. is not needed, it is put in standby as an automatic back up to the auxiliary pump.

_1.! - 6-6

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STEAM TURBINE AUXILIARY SYSTEMS The other "new" pump in Figure 6-02 is the DC pump. It is like the AC T.G. pump except driven by a DC motor. It backs up all the other pumps in case the AC electric power supply fails . . Not shown on Figure 6-02 is a "vapor extractor." It is a small motor-drivel;l fan qr blower mounted on top . of the oil tank. It constantly draws air out of the tank from above the oil surface and discharges it out a vent pipe to the outdoors. This draws off water vapor and oil fumes. The slight vacuum drawn by the vapor extractor goes up all the bearing drain pipes and draws in air through all the oil deflectors, where the turbine-generator shafts pass out of . their housings and into the turbine room. This keeps oil vapor from coming out Qfthese clearance spaces. Refer to Figure 6-03. BEARING METAL THERMOCOUPLE

I

OIL RETURN

OIL GUARD PIPING

The bearing header itself is simply a long feed pipe that runs the length of the turbine-generator. It starts out at the oil tank quite large in size and narrows as it precedes the length of the unit: A branch leads off to the header to each bearing or other device that uses oil. There is an orifice in each of the individual bearing feed lines that control the rate of flow to the individual bearing. The orifice is sized based on the oil requirement of that bearing. On the drain side of the bearing there is an oil drain line that feeds back into the bearing drain system. The bearing drain system is an even larger pipe thatfully encloses the oil feed line inside it. This is protection against fire should the feed line break or leak. Also, located on the bearing drain side are a thermometer and a telltale. The thermometer shows the oil drain temperature. The telltale draws off part of the drain oil as visible evidence that there is flow to the bearing. All of the drained oil returns hack to the Tube oil tank. At the bearing itself, there is often a thermocouple, which monitors bearing metal temperature.

Figure 6-03 Bearing Header

The lube oil system supplies oil for other purposes, depending on the unit. These may include the governor drive and exciter reduction gears, the turning gear, coupling cooling sprays, generator shaft seal system, etc.

2.3

GE SCHENECtADY EHC LUBE OIL SYSTEM

The lube oil system for GE units with an EHC control system is the same as it is for MHC units except for one substitution of pumps, as follows: •

There is no auxiliary oil pump, since high pressure oil is not needed to start the unit.

•

There is a "new" pump, the "motor suction pump." It is driven by an AC motor and is physically much like the AC turning gear pump but about 50% larger.

•

The motor suction pump draws oil directly from the tank and pumps it into the pipe that connects the booster pump and the main shaft pump suction.

The motor suction pump is turned on when the unit is being started up. When the turbine-generator speed is


_1.1 -

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 low, the motor suction pump pressure closes the check valve on the booster pump discharge and supplies oil to the main shaft pump suction. As the main shaft pump comes up to speed and begins to pump, the booster pump will be driven by its oil turbine and begin to build up discharge pressure. At about 90% speed, the booster pump discharge pressure overcomes that of the motor suction pump. The booster pump discharge check valve opens, the motor suction pump discharge check valve is forced closed, and the unit is in normal operation. The motor suction pump is then shut down and put in automatic standby. . Figure 6-04 shows the system. It is the same as Figure 6-02, except that: •

The auxiliary oil pump is now titled "motor suction pump."

•

MHC system piping that does not exist in EHC units is shown like this

+++++. •

EHC system piping that is NOT in MHC systems is shown as a heavy RED line.

10

BEARINGS

OIL COOLER

•• ••

~ ~

•

...-•• . ..•.. .. ~ ~

..•

.-•

•• ••• '!"•• •••

t t ... . . . ..

~t :'-~lSYSTEII

Figure 6-04 GE Oil Systems

Figure 6-05 shows a simplified Westinghouse system. Note, the "eductor" and the three motor driven oil pumps are in the main oil tank and draw oil from it. •

There is no oil driven booster pump as in the GE system. Instead, one or more "eductors" are used. These work on the same principle as a steam jet air ejector. That is, ajet of high pressure oil passes

_1.-i - 6-8

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STEAM TURBINE AUXILIARY SYSTEMS through a carefully shaped passage (called a 'Venturi section") which creates a vacuum. The vacuum draws in oil from the tank which mixes with the high pressure jet oil. The combined flow then passes on down a pipe at a pressure much below the original jet pressure but still high enough to do the job. The eductor has no moving parts. The eductor uses a small quantity ofHP oil to move a large quantity of LP oil required for the shaft pump suction and LO header. •

As the figure shows, the oil from the eductor supplies both lubricating oil and main shaft pump suction.

•

The AC auxiliary oil pump has exactly the same purposes as in the GE system; to supply lube oil and high pressure oil during start up. In both systems, the high pressure oil is needed for the control system. Also, the high pressure oil drives the booster pump in the GE system and the eductor in the Westinghouse system.

•

The Westinghouse auxiliary oil pump is really two pumps on one shaft driven by the same motor, as the figure shows.

•

Bearing header pressure is adjusted with the relief valve in the Westinghouse system. You will recall, coordinated adjustments are needed in the GE system. Neither system has any adjustment for the high oil pressure.

OIL COOLERS

Figure 6-05 Westinghouse Oil System


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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 WESTINGHOUSE EH LUBE OIL SYSTEM

2.5

Westinghouse units with the EH turbine control system, like the GE EHC units, have a separate high pressure fluid system for working the steam valves. They do not need high pressure oil from the lube oil system for start up so there is no auxiliary oil pump. However, during start up, oil at about 100 psi is needed for the emergency trip system and as back up for the generator shaft seal system. This oil comes from the "seal oil back up pump." The flow needed is not very large; the seal oil back up pump is a positive displacement pump much smaller than the . other motor driven pumps. However, it is in the same place, on the oil tank top. The rest ofthe system is very much like the Westinghouse system for MH units already explained. Figure 6-06 is a schematic diagram.

CONTROL DEVICES

1--. SEALOIL BEARING OIL HEADER SEAL OIL BACK UP PUMP

OIL RETURN SCREEN

HP OIL

t

L.. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .....

Figure 6-06 Westinghouse Oil System - EH Units

_M - 6-10

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3.0

TURBINE SHAFT SEAL SYSTEMS

3.1

INTRODUCTION

As we will explain, there are two main types of turbine shaft seal systems, but both have the same purpose and most of the basic principles are the same. One type uses only steam for sealing. It is used on all Westinghouse turbines and the newer GE turbines. It is called the "steam gland sealing" system on the W units and the "steam seal" system on GE units. Older GE units use a system called the "water seal" system. It is actually a combination of a steam seal system and a "water seal", which works on an entirely different principle. Older Westinghouse units also used combination steam and water seal systems. The purpose of the system is to seal the clearance space between the turbine shaft and casings where the shaft passes through the outer walls of the casing. Also, the clearance space between the stem and the body of some of the main steam valves must be sealed. The system must keep steam from leaking out when the pressure inside the casing is above atmospheric, and it must keep air from leaking in when the internal pressure is below atmospheric. These clearance spaces must be sealed because: Steam escaping out into the room is unpleasant ifnot dangerous. Also, it might get into nearby bearing housings thus getting water into the lube oil system. Escaping steam is a waste of energy and demineralized water. •

Escaping steam can cut or erode the shaft or valve stem and other parts.

•

Air entering the turbine will increase the exhaust pressure (lower the vacuum) thus hurting efficiency.

•

Air entering the turbine could cause harmful cooling of hot turbine ports.

The principles of operation and the main features of the system will be explained in the following pages.

3.2

BASIC PRINCIPLES

The following principles are basic to all turbine shaft sealing systems: •

There is no practical way to make a "steam" seal that will completely stop the flow of steam or air through the clearance space between the turbine shaft and casing outer walls. The best that can be done is to make a seal that will greatly restrict or reduce the flow, and then to capture and control this flow in a way that will suit our purposes.

•

"Water" seals can completely stop the flow of air or steam, but they are not practical in all locations.

•

The inside of the turbine low pressure sections is always under vacuum when the unit is running; air is always trying to leak ill.

•

The pressure inside the HP and IP turbine sections and the main steam valves changes with operating conditions. When the unit is being prepared for start up and running below rated speed,


_1.1_

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 the pressure inside is below atmospheric and air tries to leak in. As power output (load) increases, the steam pressure inside each seal rises, fIrst becoming equal to atmospheric and then going above it so steam will try to leak out. Also, the load where each seal "goes through zero" pressure and the highest internal pressure that is reached, may be different for each point. However, the internal pressures always rise as load increases. •

3.3

There is always a waste of energy whenever steam leaks or flows from a higher to a lower pressure without doing useful work. Therefore, whenever this happens in the steam sealing system, we should try to keep the waste as low as possible by reducing the amount of flow and/or the pressure drop. · . .

STEAM PACKING

Steam "packings" are used to reduce or restrict the flow of steam through the clearance spaces of the turbine. This is true in those places where the shaft passes through the outer casing walls and also, many other places inside the turbine where the shaft passes through a dividing wall that has a pressure difference from one side to the other (such as diaphragms). However, only those packings in the outer casing walls are part of the seal system. Figure 6~07 shows a typical packing, simplifIed. Note how the steam must zigzag around the teeth to work its way downstream.

-----t~ STEAM FLOW

Figure 6-07 Steam Packing

Across each tooth there is a pressure drop. In order to explain why this is true we will look at a different type of a restriction to flow, an "orifIce." This is simply a hole through which a fluid (liquid or gas) will flow if there is a pressure difference across the hole.

Figure 6-08 shows a pipe with an orifIce in it. If fluid flows in the pipe, the orifIce will cause the pressure to decrease as the fluid flows through the orifIce. In this fIgure the pressure at P2 is less than the pressure at Pl. The pressure drop from PI to P2 depends on three things: (1) the rate of fluid flow, (2) the size of the orifIce and (3) the shape of the orifIce. For the time being we will assume all three to be fIxed.

2 P2

P1

V0

------------------' Figure 6-0.8 An Orifice

/ \(.) r-----~

n :-I

.

PRESSURE TO VELOCITY CONVERSION

-y - 6-12 ~


EDDIES AND TURBULENCE FLOW

( . ) VELOCITY DISSIPATED

I

STEAM TURBINE AUXILIARY SYSTEMS Just suppose, for example, each orifice causes a pressure drop of 50%. In other words, if P 1 was measured to be 800-psig then P2 will be 400-psig. The pressure at P2 is 50% of the pressure measured at PI. Now consider a series of orifices, one right after the other as shown in Figure 6-09. There are eight orifices in a row. Also, we will continue to assume a 50% pressure drop across each orifice. Our pressure begins after the previously assumed 800-psig. Let us see what the fmal pressure will be (note the rounding to simplifY the numbers).

Orifice Number 1 2 3 3 5 6 7 8

Pressure Before 400 psig 200 psig 100 psig 50 psig 25 psig 12 psig 6 psig 3 psig

Pressure After 200 psig 100 psig 50 psig 25 psig 12 psig 6 psig 3 psig 1 psig

Table 6-01

PRESSURE

PRESSURE TO VELOCITY

Figure 6-09 Multiple Orifices

The pressure of the fluid was decreased from 400 psig to 1 prig by passing through eight orifices in series. That is a great pressure drop! The fmal pressure is only 0.25% of the original pressure. However, there was still some pressure at the end of the last orifice. We could add many more orifices and there would still be a positive pressure at the end. In other words, there will still be flow through the pipe, though it will be much less because of the orifices. We have just explained the principle of steam packing. It can greatly reduce the flow through the space where a shaft passes through a stationary part, but it cannot stop it completely. Of course, there is a practical limit to the number of "orifices" (or packing teeth) we can put into real machines. We noted earlier, the flow through an orifice (or a set of packings) depends on three things (1) the pressure drop across the packing (2) "the size ofthe orifice" (the clearance) and (3) "the shape of the orifice."

6-13 HPe Technical Services

_\J -

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Figure 6-10 shows how the flow through an orifice depends on its shape. The figure shows water flow but the principle is the same for steam or any liquid or gas.

~~ -~

Figure 6-10 Nozzle and Orifice

~or--~

(A'

(8'

NOZZLE

SHARP EDGED. ORIFICE

In (A) the orifice has a well-rounded opening so the fluid flows smoothly and the water jet has the same diameter as the orifice or hole. In (B) the orifice has sharp edges. This causes the flow to "neck down" as shown so the water jet is smaller than the hole and flow will be less than in (A). In fact, an orifice like (B) will pass less than 2/3 as much flow (theoretically, 61 % as much) as one like (A).

Orifices can have edges rounded by any amount so the flow can vary from 61 % to 100% of the amount that would go through a well rounded, full flow orifice like (A). You can see why it is important for turbine efficiency to keep all the packing clearances as small as possible and to keep the teeth sharp. During maintenance outages all the teeth should be sharpened, even jfthe clearances are not reduced and may even be increased very slightly.

Figure 6-11 shows a typical shaft packing assembly and the nearby parts. The shaft usually has grooves in it as shown in the detail, with matching high and low teeth on the packing rings. This forms a more zigzag path for the steam flow, increasing the restriction. This construction is called "labyrinth" ("lab-ee-rinth") which means "an intricate structure of passages through which it is difficult to fmd one's way." The packing rings are usually made of a rather soft bronze alloy that will wear away without damaging the shaft in case there is rubbing. They are made in segments; two, three or more in both the upper and lower half of the packing casing.

OIL DEFLECTOR

BEARING ROTOR

The springs shown in the figure are flat leaf-type springs that press the segments radially inward, but they will "give" in case the rotor should rub the packing.

.

.

_1.1. - 6-14

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...JPA~~GL,

Figure 6-11 Shaft Packing

~ ROTOR ·

STEAM TURBINE AUXILIARY SYSTEMS 3.4

WATER SEALS

Many GE turbines built before the early 1960s had water seals as part of their shaft sealing system, in addition to the steam seal packings we have studied so far. Water seals, like the one shown in Figure 6-12, work on an entirely different principle than steam seals. This type is called a "non-circulating" water seal and is used in the turbine exhaust hood, usually the #3 and #4 seals. EXHAUST HOOO

The "impeller" is simply a ring on the shaft, made of bronze: It turns inside the casing much like a centrifugal pump impeller turns inside a pump casing and it works something like a pump with the discharge shut off. /

CONOENSA110111

,---~~--,

Condensate is piped into the casing, filling the space around the impeller. When the shaft is stopped or turning slowly, the water leaks through the clearances between the teeth and the shaft, in both directions. The water that leaks in simply joins the rest of the condensate inside the exhaust hood. The water that leaks out goes into a drain and is lost from the system.

~

I ~----~~~ ' M ~~~~------~

P TURBINE ~----~~,

L L

E

SHAFT r.~~~------~

R OIWN

As the unit speeds up the impeller begins to work better, throwing the water outward against the impeller casing and building up a pressure - just like a centrifugal pump. Finally, the pressure is great enough to "seal." That is, water will no longer leak outward to drain. Also, the flow into the exhaust hood is greatly reduced because the "pump" pressure is high enough to overcome the pressure difference between the outside air and the condenser vacuum.

Figure 6-12 Water Seal

Water seals work very well when they are "sealed." Also, they are very simple, inexpensive, and do not take up much space (keeping the turbine shaft short). Temperature differences are not a problem in the low-pressure parts ofthe turbine. However, water seals do not seal well until the shaft is up to about half speed, making it harder to pull vacuum on start up. Also, the outward flow of water at low speeds may erode the shaft; this is expensive to repair. The non-circulating water seal just explained will not work where the surrounding temperatures are too high, because the water will boil and spoil the sealing action. A somewhat different style, called the "circulating" water seal is used instead. ' , , The circulating seal has a discharge pipe coming out of the casing, with a hand-operated valve in it. The inlet and discharge valves are regulated to keep water flowing through the seal fast enough so it will not boil. The flow must also be regulated to keep the pressure inside the casing high enough for the pumping action to work, and also keep the flow low enough to keep the assembly from running too cool. There is a sight glass and a thermometer in the discharge pipe and a casing pressure gauge, which the operator uses to regulate the inlet and discharge valves. Also, the drain on circulating seals has a sight glass in it, unlike the non-circulating seals.


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SYSTEM DESCRIPTIONS

Now that we have studied the purpose of the turbine shaft sealing systems, the basic principles, and the three main types of seals (steam; non circulating water and circulating water), we will study how they are used in the two types of systems named in the Introduction- "steam seal" and "water seal" systems.

3.5.1

SteaJIl Seal Systems

The steam seal system description that follows is typical of both GE and Westinghouse units. The main differences are in the terminology and in some mechanical parts. In this section we will use GE terminology and a typical GE system for a model. We will point out important terminology and hardware differences as we come to them. Recall how a set of steam packings breaks down the pressure and reduces the amount of the steam flowing through the clearance space between the shaft and the turbine casings. Refer to Figure 6-13. Suppose there were enough packing teeth on the left so that at a certain pressure inside the casing, the pressure was reduced to 4 psig at the point shown. If we then pipe steam from the steam seal supply into the space "X" at 4 psig, there will be no steam flowing out of the turbine. This would take care of one main task of the system - stopping steam from leaking out of the turbine.

Figure 6-13 Sealing Steam Supply

1

FROM STEAM SEAL SUPPLY

Of course, the problem is not really solved. We still have steam leaking from the turbine. Only, now it is coming from the steam seal supply instead of the turbine. This does not yet eliminate any of those problems that we were discussing earlier.

SEALING

t

STEAM

l

EXHAUST

Figure 6-14 Steam Supply & Exhaust

_1J. - 6-16

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Refer to Figure 6"14. We have added an exhaust outlet between the steam seal header and the atmosphere. The pressure in this outlet will be slightly less than atmospheric. In other words, we are going to draw a slight vacuum on this particular annulus. Steam from the steam seal header now escapes via the exhaust. It can no longer enter the atmosphere. Air is also being pulled in to the exhaust. Air flow inward guarantees against any possible steam flow outward. A mixture of steam from the left and air from the right is being drawn out of the exhaust.

STEAM TURBINE AUXILIARY SYSTEMS Enough on the orifices. Look at what we really have in a typical turbine. Figure 6-15 shows the shaft packings that were mentioned in earlier paragraphs. On this drawing you should also locate the steam seal header and the exhaust. The steam seal header pressure is controlled at 4 psig. Those packing teeth between the inside of the turbine and the inlet of the sealing steam are designed such that the header pressure of 4 psig can, indeed, seal the turbine against leakage. The steam seal header pressure, however, should not be allowed to exceed 4 psig by much. The pressure must not over-pressure the exhaust header. Since the steam pressure inside the turbine varies with turbine load we must be able to adjust the system accordingly. At higher loads this high pressure steam is great enough to cause the pressure at the steam seal header to increase above the 4 psig. So we need a device that will actually remove steam from the header. The above statement about the excess steam holds true only on the high pressure and I.P. turbine sections. On the low pressure turbine we will have to add steam at the header at all times. These other devices that we are about to begin studying will not only remove steam from that high pressure element but they will also redirect some of it to the low pressure elements. In effect, they will make the turbine self -sealing at the higher loads.

CONDENSATE

;::::jl

STEAM FLOW @LOWLOAD

STEAM FLOW @HIGHLOAD EXHAUST HEADER SEAL STEAM HEADER

INLET REGULATOR

Figure 6-15 Gland Seals

F~

Figure 6-16 illustrates some of these devices we just referred to. These are the devices that control the steam seal header pressure. They not only control the pressure but they also direct the steam as the pressure varies. Steam enters the header from two different sources.

, OLANO

.. _. .... ,,,.. , .. ... ".. ........l ".... 'f ....

EXHAUSTERS ill

~.,

~

~

¥:"~"

....... ,.", ........ ,

EXTRACTION

TOLP. FWHEAOER

STEAM SEAL HEADER

LIVE STEAM VALVE

UNLOADER VALVE STEAM SEAL REGULATOR

Figure 6-16 SSH Low Load Operation

EXHAUST REGULATOR

One source is used during low load operation and is external to the turbine. The other source is used at high load operation and is coming directly from the high pressure turbine. Figure 6-16 illustrates the system at low load operation only. During low load operation steam is directed into the steam seal header from the main boiler, through the steam seal regulator live steam valve. On more modem units this valve is simply called the steam seal feed valve. In order for the boiler to be used it must have enough pressure to supply the required steam, about 114 of rated. During low load operation the live steam valve regulates steam flow into the header.


_1.1 -

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Steam flow is from the source through S3 and Sl to the live steam valve. The live steam valve regulates pressure (flow to the header). S2 on the diagram is the manual bypass valve. It is used if 1. 2.

the live steam valve should fail closed, or if additional flow were required to maintain steam seal header pressure (i.e., due to low boiler pressure).

Some turbines can get steam from an auxiliary boiler and/or from the HP turbine exhaust (cold reheat) as well as from the main steam supply. At high loads there is an excess amount of steam in the high pressure and IP sections. This steam finds its way into the steam seal header. There is so much steam flowing into the header that the steam seal header pressure will begin to raise. In Figure 6-17 the live steam valve closes to try to control pressure but it cannot. It winds up being full closed and the header pressure is still rising. The unloader valve will begin to open as a result of the rising pressure. This valve will dump the excess steam from the header, thereby reducing the pressure back to 4 psig. On newer units, this valve is known as the steam packing unloading valve.

EXTRACTION

TOLP. FWHEAOER

STEAM SEAL HEADER

Figure 6-17 SSH at High Load UNLOADER VALVE STEAM SEAL REGULATOR

STEAM SUPPLY

This unloader valve directs the steam to a, low pressure feedwater heater and to the condenser. By doing this the steam will continue to do useful work as far as the plant heat rate is concerned. Valve B is a manual bypass valve used to increase flow from the header to the LP extraction, if necessary. Before going further, let us point out two main differences between the typical GE system we have been explaining and a typical Westinghouse system. 1.

The normal Westinghouse steam seal header pressure is about 18 psig rather than 4 psig.

2.

Most Westinghouse units have a "desuperheater" in the gland steam feed to the LP turbine glands.

This is simply a temperature-controlled valve that sprays cool condensate into the gland sealing steam if its temperature goes above a set limit. This is needed because the steam temperature at the glands can go higher than in the GE system because of (a) the higher header pressure and (b) the GE sealing steam is cooled by running the uninsulated steam piping inside the cool exhaust hood for some distance before it connects to the seal. The exhaust header vacuum we were discussing earlier is created by a steam packing or gland exhauster. The gland exhauster is a blower fan and condenser arrangement as shown in Figure 6-17. This fan, as mentioned previously, pulls a mixture of air and steam from the steam packings.

_1.lt - 6-18

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STEAM TURBINE A UXILIARY SYSTEMS The steam is condensed across the condenser tubes. Air is vented. The amount of vacuum can be adjusted by the repositioning of the butterfly valves found before and around the fans. These are all of the primary components that make up the steam seal system. For the most part, the operation is automatic once placed into service.

3.5.2

Water Seal System

As already noted, the GE units use a combination of steam seals, circulating water seals and non circulating water seals to make up their turbine shaft seal system. Non-circulating seals are used in the turbine exhaust hood, where the pressure inside the seal varies between atmospheric and full condenser vacuum. These are the #4 and #5 packings. Whenever the steam pressure inside the seal goes above atmospheric during operation, the seal assembly consists of a steam seal on the inside and a circulating water seal on the outside. The steam seal breaks down the pressure of the steam trying to leak out (to about 1 or 2 psig) and this leakage steam is then piped away. The circulating water seal keeps the 1 or 2 psig steam from coming out of the seal. The pressure inside the combined steam and circulating water seal assembly goes below atmospheric during start up and low load operation, so air tries to leak in. In this condition, steam is piped into the space between the steam and the water seal, and the water seal works (more or less) nor These combination seals are used at each end of the HP turbine casing. That is, one seals the place where the shaft comes out of the front of the casing and goes into the front standard. The other combined seal is between the crossover and the #2 turbine bearing, inside the exhaust hood structure. These two combined assemblies are numbered #1 and #2. Figure 6-18 shows these locations. Note, the #2 bearing is vented to the outside and sealed off from the turbine steam path even though it is inside the exhaust hood structure. This means the pressure across the #2 seal assembly is always atmospheric on the bearing side, and varies during operation from full condenser vacuum up to full crossover steam pressure. IPROTOR

,- -\- ' - -:~_.r=t" I :-: - - - -.- --, I

I

I

I

I

-5P>

STEAM_ : MAIN STOP I VALVE I

-'

.

r

Y

I

r.- ' I

I

I

r------c:-~-I

,

INTERNAL FABRICATED CROSSOVER

REHEATER

I

I

V TO CONDENSER

I

V TO CONDENSER

~----->2-~0"...---->-- ..: INTERCEPT VALVE

Figure 6-18 GE Steam Turbine Design Code D2


_1.1 T.

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The pressure across the #3 seal (non-circulating) is always atmospheric on the side towards #2 bearing, and varies from atmospheric to full condenser vacuum on the other side. Thus, the #3 seal sees the same conditions as the #4 in the turning gear end of the exhaust hood. Figure 6-19 is a schematic ofthe"steam seal" part of the combined system. .VENT

HP LEAKOFF 10

DEAERATOR

Figure 6-19 Steam Seal System

PRESS GAUGE

I?==
As we have explained, the steam packing teeth break down the pressure and greatly reduce the flow of the steam leaking through. The # 1 steam packing has an "HP leakoff" which takes the steam that has passed through the highest-pressure sets of teeth, to an extraction. This improves efficiency by putting this steam to use rather than making it "leak through" all the way. The steam that passes through the lower-pressure teeth of the #1 steam packing, and all the steam that passes through the #2 steam packing, is piped outside the turbine. The two leakoffs are joined in a manifold. The figure shows two other connections to the manifold. One comes from the main steam supply just ahead of the main stop valve. A manual valve in this line is opened to pressurize the manifold during start up and low load operation. The other manifold connection goes to two unloading valves in parallel. These unloading valves are simply spring loaded valves something like a relief valve, that open as much as they have to in order to maintain a set upstream pressure. One unloading valve is set to hold 1 to 2 psig upstream pressure. It discharges into a low pressure heater. Some useful energy is recovered from this steam. The other unloading valve is set to hold 2 to 3 psig upstream pressure. It discharges directly into the exhaust space at condenser pressure. Since it is set higher, it only opens after the first unloading valve is wide open and still not able to keep the pressure below 2 psig. Not shown in the figure are connections to the manifold from the valve steam leakoffs of the main stop and control valves .

.\it . 6-20 ~


STEAM TURBINE AUXILIARY SYSTEMS 4.0

EH FLUID SYSTEM

4.1

INTRODUCTION

The purpose of the EH Fluid System is to supply high pressure fluid directly to the turbine main steam valves. This high pressure hydraulic fluid is used to position the steam valves. In addition to the positioning of these valves, the hydraulic fluid relays trip signals to the valves from the devices in the turbine trip and overspeed protection system. As already noted, the modem GE and W turbines have an "EHe" (electro-hydraulic control) system. These systems do not use lube oil as the working fluid as the older units do. Rather, they use a special hydraulic fluid that comes from a separate auxiliary system. The system has a reservoir or tank, pumps, filters, pressure regulating devices, oil coolers and heaters, servo and dump valves, and piping connecting all parts of the system. Most of the main parts are on or in the tank which is usually one or two levels below the HP turbine section. The normal fluid working pressure is about 1800 psig. All the systems are basically the same. The main differences are in some of the hardware. All the systems must have the features listed below in order to maximize the system reliability: •

Temperature and cleanliness is continuously controlled.

•

The fluid chemical properties are maintained by active filters.

•

Two independent pumping systems allow the turbine to operate while maintenance work is being performed on the system.

•

Various alarms and pressure switches are built into the system which will auto-start the standby pumping system, trip the turbine should a major loss of pressure occur, or simply provide you the necessary feedback on the system's safe operation.

•

The system allows for both steady-state and transient flow requirements.

The pages that follow will cover the EH fluid itself, a typical Westinghouse system, and a very brieflook at the GE system compared to the Westinghouse system.

4.2

HYDRAULIC FLUID

The fluid is synthetic; you will not find it in a natural state like you would a normal oil. Its chemical name is phosphate ester. This synthetic fluid has good lubricating properties and good stability. The normal operating pressure of the system varies between 1600 and 1900 psig. This fluid has to be routed to all of the turbine steam valves in order to operate those valves. This brings some very high-pressure fluid very close to some very hot turbine components. A ruptured pipe near the turbine could be very hazardous. This synthetic fluid has the advantage of having a very high flash point. The flash point is so high that one could say that the fluid is fire resistant. There are a few facts that you should understand before working on these systems: 1.

This fluid is harmful if swallowed. Smoking or eating should not be allowed in work areas where this fluid is in use.


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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 2.

Contact with the eye is dangerous. Eye protection should be worn as a means of preventing eye injuries. Any contact should be flushed with water.

3.

Skin.irritation may occur as a result of contact with this fluid. Any contact area should be washed with water.

4.

Fumes, produced when fluid is in contact with hot surfaces, are irritating and mildly toxic. Combustion products from a fITe can cause lung damage after prolonged exposure.

5.

Finally, this fluid seems to take a great dislike to rubber. The fluid will attack and soften ordinary rubber material. Workers should be aware that this fluid may destroy their rubber soled shoes.

The bottom line is that the hydraulic fluid, like any fluid that you may work with, deserves its own special consideration. Be aware of the fluid that you are dealing with and respect it.

4.3

WESTINGHOUSE SYSTEM

Refer to Figure 6-20 as you read the following thumbnail sketch. We will be discussing the normal operation of the EH Fluid System

DRAIN

RETURN

SUC""'"

sTRAINER

Res

R

1 ---,-------8•• .---------i>
a _________________ _

D

ACCUMULATOR U.lGa1_OS_H

Figure 6-20 Westinghouse EH System The EH reservoir contains all the hydraulic fluid. The fluid pumps take their suction from this reservoir. The fluid is fITst strained and then pumped to its high operating pressure. The hydraulic fluid then passes through a 10 micron filter, to an unloading valve, a check valve and on to the main header. The hydraulic header at this point branches off in two directions, one to the turbine valves hydraulic cylinders and the other to a polishing filter. This polishing filter provides for the proper cleanliness of the fluid. We must keep the hydraulic fluid at the proper temperature. This is done by two heat exchangers that are located on the drain return. The temperature in the tank is held between 110 and 130 degrees. A manually operated control valve regulates the cooling water flow through the coolers in order to meet this temperature requirement. The fluid is pumped from the reservoir through one of two AC motor driven constant displacement pumps. Each pump is capable of meeting 100% of the system requirements.

-y - 6-22 ~


STEAM TURBINE AUXILIARY SYSTEMS One pump is in service at a time. The pump that is not in service is in standby. It should be ready to operate if the running pump fails to maintain pressure. The discharge pressure of each pump is monitored by a pressure gage. There is also a pressure switch that monitors the header pressure and will start the standby pump if the pressure falls below 1350 psig. Once started, the pump will stay in service until you turn it off. These pumps should be alternated on a once per week schedule. Variations in system demand are handled by the unloading valve which directs pump discharge back to the ERC reservoir when system pressure rises to about 1950 psi and directs pump discharge to the HP ERC header when pressure falls to about 1600 psi. Another component of the EH Fluid System is the accumulators. The pressure side accumulators are connected to the RP fluid header. They provide an immediate available reserve of fluid if there should be a sudden flow requirement. Drain side accumulators maintain a positive drain pressure to push the drain return fluid through the coolers and back to the reservoir. The high pressure accumulators are the piston type, having a free piston in a closed cylinder. The cylinder is connected to the high pressure fluid pipe on one side of the piston; the other side is full of nitrogen gas under a pressure near the fluid pressure. As fluid pressure rises, it forces the piston to the nitrogen side, compressing the gas. As fluid pressure falls, the nitrogen pushes the piston back, forcing fluid back out into the pipe. In this way, the accumulator dampens pressure surges and also acts as a reserve supply of fluid. The low pressure accumulators have the same purpose as the high pressure ones, but they work in the fluid drain piping at a pressure of about 30 psi. Instead of a piston, there is a "bladder" - a flexible diaphragm in the piston. The fmal output of the parts we have explained so far, is a supply of high pressure hydraulic fluid to the valve actuators and trip or dump valve. Each of the steam valves has on it, an actuator that controls its position basically a hydraulic cylinder and pilot valve assembly. The dump valves are "on-off" type valves and are in the turbine control system. When "on" (closed) they allow fluid pressure to build up in the system. When off, they "dump" fluid from the system. No pressure can build up and the steam valves cannot open .

.................. """""c_ _ _ ,, ___ •• -8-----.8~8 1I: l • -

8 8

AU. PORtS CLOSfD

C
r--

==g~§i_'AS

SlAAttR

STMlER

."'...

::;::===::

Figure 6-21 GE EHe System


_1J -

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4.4

GE SYSTEM The GE system is very much like the Westinghouse system just explained. The main differences are: •

The GE system operates at 1600 psi.

•

The pumps are a variable displacement type that have an internal pressure compensator. There is no unloading valve.

•

There are no low pressure accumulators.

The system has a small reservoir (tank), AC motor driven pumps, filters, pressure regulating devices, coolers and piping. Normal fluid working pressure is in the order of 1600-psig (110 mpa). Some units regulate this pressure constantly while others make use ofunloader valves which allows this pressure to cycle. Temperature and cleanliness is continuously controlled and monitored. Fluid chemical properties are maintained by active filters. See Figure 6-21 as we proceed (or examine your own system drawing). Typically, there are two independent pumping systems providing redundancy. Each pump is driven by an AC motor with auto start pressure switches starting the standby pump with a loss of pressure. On the suction side of the pump there is a strainer to protect the pump. On the discharge side of the pump there is another high pressure (usually non-collapsible) strainer that protects the system from pump failure. Often times there is differential pressure switch across this discharge strainer to alarm should it plug. On some type units the pressure is regulated by a pressure compensator internal to the variable displacement pump. On other units an unloading valve in the pump discharge regulates downstream pressure. In either case the pump typically cannot meet short term transient demands generated by large valve movements. An accumulator is required for thIS purpose (the number and size of accumulators is a function of the hydraulic system demand). These accumulators may be of the piston or bladder type and are pre-charged with nitrogen to a pressure approximately 60% hydraulic pressure. Return flow from the turbine valves is through a cooler to the reservoir. There is a relief valve in the line to the cooler to relieve excess pressure should there be resistance to the flow to drain.

CHECK YOUR UNDERSTANDING Questions 1.

The turbine-generator main lube oil pump is driven by_ _ _ __ _ _ _ __ _ A. an electric motor B. the generator shaft C. the turbine shaft D. an air motor

2.

In some power station arrangements, the turbine lube oil system will also serve the: A. generator bearings B. gland seal system C. boiler feed pump turbine D. generator hydrogen seals E. MIlC system

_1.1 - 6-24

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IIPC Technical Services


3.

The function of the shaft steam seals on the HP or IP turbine is to_______ the turbine. A. prevent air from entering B. maintain vacuum in C. prevent steam from escaping

4.

The combination of rotor lands and seal packing teeth work together effectively in _ _ _ _ _ _ _ of the escaping steam. A. lowering the temperature B. reducing the pressure C. completely stopping the flow D. reversing the direction

5.

The flow of escaping turbine steam is completely stopped in the seals by the introduction of A. sealing steam B. sealing air C. a vacuum seal

6.

The primary function of the ERC system is to_ _ _ _ _ _ _ __ A. trip the turbine B. synchronize the generator C. back up the MRC system D. position the steam control valves

7.

The normal operating fluid pressure in the EHC system is maintained at_ _ _ _ _ _ psig. A. 1000 B. 1600 C.700 D.500

8.

Unlike the MHC system, the ERC system uses _ _ _ _ _ _as the hydraulic system fluid. A. a synthetic fluid B. water C. turbine lube oil D. condensate


_1.1 T

GENERATOR THEORY Chapter 7

TERMINAL OBJECTIVE: Upon completion ofthis chapter, the student will be able to discuss the principles and fundamentals associated with the generation of AC power.

ENABLING OBJECTIVES:· At the completion of this section, the reader should be able to: 1. 2. 3. 4. 5. 6. 7. 8.

State the purpose of a generator. Describe inputs/outputs of an operating generator. List three (3) variables that determine the voltage in a conductor as it passes through a magnetic field. Of those three (3) variables listed in objective #3 above, state which is controllable by an operator. Describe how an alternating current is derived as a 2-pole magnetic field rotates. Describe how a 3-phase generator is typically wound. State the difference between induced voltage and generator terminal voltage. Given the # of poles ofa generator, state the speed this generator operates at to generate a 6O-Hertz output. 9. Describe how armature reaction is derived. 10. Describe how the magnetic fields interact in the air gap, given a change in load. 11. Define the term power factor and describe its components.

© 1999 - TG201J5.0_June09, Printed: 12/14/10


TABLE OF CONTENTS

1.0

FUNDAMENTALS OF GENERATOR DESIGN ..................................... :... "........................................ 3 1.1 1.2 1.3 1.4 1.5 1.6 1. 7 1.8 1.9

2.0

SYNCHRONISM (OR, WHAT IS HAPPENING IN THE AIR GAP) ................................................ 13 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 2.3.4

3.0

_1.1 - 7-2

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Purpose of the Generator ... ..... ... ..... ...... ..... ......... .. ......... ..... .......... .... ........ ........ .. ...... .. ................. 3 Air Gap .......................... :... ......... .... ...... ....,' ....................................... ...... .... ........ .... :...................... 4 . Magnetic Field .......... ............................ .... ....... ........ .. ....................... ........................................... 5 Rotor Electromagnet ....... ... .... ...... ..... ;...... ........... .. ........ .... ... ..... ... ............................................ ;... 6 'Speed - Frequency Relationship ................................................................................................. 7 Stator Winding ...................................................................................................................... ...... 8 Three-Phase Generator ...... ........................... ..... ......... ................................................ ........ ... .. .... .... 9 . Induced Generator Voltage .. .......... .. .......... ........ ........ .. ........................................... . .................. 11 Armature Reaction ......... ....................................................... ..................................................... 12

Air Gap Magnetic Fields ..................................................................................... :........ .. ...... .. ... Active Power (Watts) ......... ... ....... ............. .. ... ........... .. ................ .. ........ .... .. .............................. Reactive Power (VARs) ...................................................................... :............ .. ....................... . Inductive Loads .... ........................................................ ....... ... ........ ........ ......... ...................... Capaciti ve Loads ... ............. ........ ......... ............. .. ..... ........... .. .... ..... ....:.... ............. ...... ...... ..... Mechanical Analogy ....... ............ ....................................... ..... ................. ... ........... ... ..... ....... Active / Reactive Power Flow .................................................. .... :.......................................

13 15 16 16 17 18 19

POWER FACTOR ......... .... ..... ........................................................ .............. ............ ...................... ... ... 21

.


GENERATOR THEORY

1.0

FUNDAMENTALS OF GENERATOR DESIGN

1.1

PURPOSE OF THE GENERATOR

The purpose of your generator set is to take mechanical energy input from the prime mover and produce electrical energy for consumers' use. The generator transforms mechanical energy to electrical energy. Inputs to the generator, from the prime mover, are: • Speed • Torque Mechanical energy is the product of speed times torque as illustrated in Figure 7-01. Outputs from the generator are: • •

Volts Amperes

Electrical energy is the product of volts times amperes.

Figure 7-01 Generator Power Conversion

Look at the nameplate of your generator. You will see, as in Figure 7-02, the output of the generator is rated in volt-amperes (kVA or MVA on larger units). In this particular situation the unit is rated at 204,500kV A. 204,500-kV A is the design output ofthe generator.

Figure 7-02 Generator Nameplate


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1.2

AIR GAP

The author has often asked: "Where does your power plant connect to the grid?" Almost 100% of the time, the answer is in the switchyard orat the generator breaker. We'd like to take exception to that answer, even though it is most certainly understandable. Consider this answer to the question asked: "Your power plant connects to the grid in the air-gap of the generator." Think about the argumept that follows. The generator stator, or armature, is hardwired and/or magnetically connected to the load. When one thinks of the grid, one thinks about your generator "connected" to and being part of that grid. The generator rotor, on the other hand, is coupled to the prime mover that is driven by the fuel, water, or steam thatis an integral part of the power plant. This argument is illustrated in Figure 7-03 . In order to accomplish this energy transforrilation we are going to need a medium, something in between that "ties" the speed/torque of the turbine (or prime mover) to the Voltage/current of the generator. That something is magnetic energy. This "tie" is magnetism. We need a magnetic field so we can transform speed/torque from the prime mover to voltage/current on the generator. This magnetism is located between the generator rotor driven by the turbine (or . prime mover) and the generator stator. So the argument is that the power plant connects to the grid at the generator air-gap.

Generator

Reheater

r-+-t--..........,.....,

...

-;c======::::::l~ ~' - Magnetism: Rotor Transforms SpHdlTorquo Into

VoltagolCur",nl

FOSSil

Stator

Fuol · Air

Cooling Water

Furnace

L--.!::::;----;;:1

QQQOOOOO -r -

Cond,nuta Pump

..•• ':.·.':.D;:;:.~~ ••

Feedwlter Pump

.....Q . .:......... :: • Coal : ::: Station

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::::

Hydro Station

I' -I I --

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Southern 11e

_ _ 500-kV •••••• 230-kV • • ••••• 138-kV

Figure 7-03 Generator Air Gap


I

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_1.1 - 7-4

: ....

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Southeast Substation

GENERATOR THEORY Figure 7-04 provides us a more pictorial view of this transformation of energy. This is a picture of a large turbine driven generator. Notice the stationary windings (stator), the rotor windings (in the partially removed rotor), and the air gap. It is in this air gap where the magnetic fields interact and energy is transformed. We will be discussing all these components in detaila$ we proceed through the materials.

VOLTAGE CURRENT (Output)

Figure 7-04 Generator Major Components 1.3

MAGNETIC FIELD

We are all well acquainted with what we can do with the horseshoe magnet. The atoms that make up all matter can be considered tiny magnets. In some materials, such as iron, these tiny magnets can easily be made to align themselves all in the same direction. When this is done, the magnetic strengths are added to one another, and the bar is considered to be "magnetized." The region within the influence of the magnet is called a magnetic field. It will be noted that magnetism appears to leave at one end and enter at the other end. These ends are called poles, north and south poles. See Figure 70S .

Now, if we were to move a conductor through the magnetic field we would induce a voltage in that conductor. The magnitude of the induced voltage within that conductor will be a function of these variables:

• • •

•

Direction of Motion

Fixed Conductor

Figure 7-05 Generation of Electricity

Strength of the magnetic field, Speed of the magnetic field over the conductor (or conductor through the magnetic field), and the Length ofthe fixed conductor.


_1.1 T.


ROTOR ELECTROMAGNET

SLOTS (WHERE THE

COPPER WINDINGS GO)

POLAR AXIS

Earlier it was mentioned that we needed magnetism to "tie" the turbine speed/torque input to the generator voltage/current output. To do this, we need to create a magnetic field. (Only very small generators make. use of a permanent magnet). We will use an electromagnet. First, we need (l rotor that can support copper windings (see the top drawing of Figure 7-06). Notice how these slots are machined into the rotor surface longitudinally. The copper windings are laid into these slots and connected in series so that we have one continuous winding that wraps around the generator rotor. This continuous winding is represented in the lower drawing, except in this drawing we see only one winding (coil) per slot. This was to simplify the drawing for educational purposes. Current is passed through these copper windings (in the slots). The result of this current is that the rotor becomes an electromagnet. The strength of this electromagnet is a function of two things:

Figure 7-06 Rotor Electromagnet

1.

Number of turns within the windings (in other words, how many times the copper is wrapped around the forging), and

2.

The amount of current within these windings.

The resultant magnetic field is measured as ampere-turns. The strength of the magnetic field is described as the amount of current, times the number of turns of copper windings wrapped around the generator rotor.

CORE

Figure 7-07 Path of Magnetic Field

-y - 7-6 T


This rotor magnetic field flows from the North-pole to the South-pole as seen in Figure 707. In the path of the magnetic field we place a conductor (the conductor is the darker rectangle above the North-Pole). That conductor is contained inside the core. The core provides an efficient ~eturn path for the magnetic, field flow from the North-Pole to the South-Pole. In a 6O-Hertz system, it is required that this electromagnetic field be rotating at some speed such that the North and South Poles pass the same point every 1/60th of a second. This single point on our drawing is represented by the dark dot (conductor) above the N-Pole. To meet this requirement, rotor speed is dependent upon the number of poles within the rotor. For a two- . pole rotor, this speed will be 3600 RPM; whereas, for a four-pole rotor, the speed will be 1800 RPM.

GENERATOR THEORY 1.5

SPEED - FREQUENCY RELATIONSHIP

The essential feature that makes a generator different from other electrical machines is the synchronous link that exists between the stator and rotor magnetic fieldS. Because of this synchronous link, there is a fixed . relationship between rotor speed and the frequency of induced electromotive force (voltage) in the stator. They can be related by this equation (as we seen before):

N

= (120*

IX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Equation 1.01

where, N = speed of the rotor in rpm; f = frequency of induced emf in the stator in Hz; and P = number of poles 4-, S-, and 24-pole machines are illustrated in Figure I-OS.

4·POLE 1800.rpm 1500'rpm

a·POLE 60·Hertz 50·Hertz

gOO·rpm 750·rpm

60·Hertz 50·Hertz

24·POLE

Figure 7-08 Multiple Pole Units


_1.1 T.


STATOR WINDING

Now, for the sake of our education, we will assume a single phase of out generator stator armature winding has 24 conductors brazed together to form a coil, as seen in Figure 7-09. Every loop in the shown coil represents one pass of an armature bar. We will also assume that as the rotating electromagnetic field passes through one of these conductors we induce l-kV. Since these conductors are all in series with one another, we have induced 24-kV in our generator windings. This is our generator voltage, Eg •

Figure 7-09 Sum of Induced Voltages

Compare this to a set of DC batteries (Figure 7-10). If you take two 12VDC batteries and connect the (-) terminal of one to the (+) terminal of the other, you will have 24VDC across the remaining two terminals. Figure 7-10 also illustrates why you cannot measure the induced voltage. If you were to put an instrument at the end ofconductor-lO, you would see (maybe) lO-kV. We have 24-kV, however, at the terminals.

1 kV per conductor

Figure 7-10 Voltages in Series

_1.1 - 7-8 T.


GENERATOR THEORY

1.7

THREE-PHASE GENERATOR

In the actual generator the voltage generated is built up to a practical amount by connecting a number of conductors in series. The conductors are so joined together to make up what is called a phase winding. In the modem generator, there are usually three such groups of conductors spaced symmetrically in the slots of the stationary core, from which the generator gets its name, "three-phase" generator. It has been found most economical to generate and distribute large blocks of electric power with three phase circuits. Figure 7-11 illustrates a simple 3-phase generator winding.

Figure 7-11 Three-Phase Winding As the rotor revolves through 360°, a voltage is induced in each of the three-stator windings. Because the windings are 120° apart, the three waveforms also are 120° apart. (See Figure 7-12). The poWer output of each winding consists of two pulses of power each cycle as the current and voltage reach their two maximums and minimums. The combined three-phase power output consists of six pulses and is much smoother and more efficient than a single-phase generator. Note that the voltage maximums in the three waveforms are 120° apart, and all three voltages are equal.

PHASE A

PHASE B

PHASE C

I

I

:-120°-:- 120o~ 120o~

Figure 7-12 Three-Phase Generator Voltage Wave Forms

.

7-9


~. ~

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Figure 7-13 shows the three stator (armature) windings and the six leads from the armature. The windings are labeled A -AI, B-Bb and C- C I and are 120° apart. AI, Bb and C I are connected together to form the "neutral". The generator is "Wye" connected (the windings are shaped like the letter "Y"). This results in higher voltage (less current) for a given kVA rating . .

NEUTRAL

B

Figure 7-13 Three-Phase Generator wlLoads The voltages in a three-phase generator are 120° apart. The three-phase waveforms show that Phase Band C voltages exactly balance Phase A voltage (Figure 7-14a and Figure 7- 14b). The net effect is a large voltage between A, Band C coil ends, but no voltage between AI, B I and C I coil ends. The sum of all the voltages (or currents) is zero in a balanced three-phase circuit.

+

A

o~ lt ~

...

...

I

"

I

... ..J;. ;

(a)

,,"

(b)

Figure 7-14 Balanced Three-Phase Generator Voltages A three-phase system requires much less wire than a single-phase system. Three phase systems permit large amounts of power to be transmitted at lower cost than single-phase systems. When single-phase power is required, it is available from a three-phase system. Large three-phase motors and inductive devices, such as transformers, are more efficient and less costly than single-phase equipment with the same power ratings. All large power systems are designed as three-phase systems with customer loads balanced across the phase systems as closely as possible. No current exists in the neutral unless something happens to disturb the balance. It is customary to connect the neutral to a "ground" such as the earth.

_1.1 - 7-10 T.


GENERATOR THEORY

1.8

INDUCED GENERATOR VOLTAGE

In Figure 7-15 we combine our rotating 2-pole electromagnetic field with our 3-phase generator windings. It is still simplified but far more representative of what these two devices actually look like. In the figure shown, we see the north pole directly underneath phase-A, meaning we are inducing the maximum voltage in that phase. The rotor is turning clockwise, therefore the voltage will be on the rise in phase-B, and be decreasing in phase-C.

Voltage is a function of the speed that the rotor magnetic field cuts through the stationary armature windings (we discussed this earlier). 120'

Fi/.lure 7-15 Rotatill/.l Maj(lletic Field thru 3-Plwse WilUlb'j(S We will refer to the voltage we induced within the generator windings as the Generator Voltage and will label the generator voltage as Eg • We will refer to the voltage measured at the generator terminals as the "Terminal Voltage" and will label this terminal voltage as E t • See Figure 7-16. Examine this figure and it would appear that the induced and terminal voltages are equal, but we will demonstrate that the induced and terminal voltages are NOT the same as we continue through the materials. It is also worth noting that we cannot measure the generator voltage E g . Any physical measurements we take are what we see at the terminals, E t • Control room instrumentation does not indicate Eg , they indicate the terminal voltage only, E t !

Eg

........ Not Measurable

Et Terminal Voltage

........

Monitored in Control Room

GENERATOR

Figure 7-16 Induced versus Terminal Voltage .

7-11 ~


~


1.9

ARMATURE REACTION

The next step in our study is to connect the 3-phase generator winding to a load. With a load being applied there will be current. Often it is said that "cUrrent comes from the generator" . This is true, but is very important to recognize that this load current, in fact, goes through the generator. The generator stator windings are part of the circuit. If there is load current going through the g~nerator windings there will be a voltage drop (will be .looked in more detail in later paragraphs). Current in any conductor will produce a magnetomotive force (MMF) about that conductor. The current in each phase coil will, therefore, produce a time varying magnetomotive force (MMF) within that coil. The magnetomotive force is a property that gives rise to magnetic fields. The standard unit of mea"surement is the ampere-turn (AT) .. This stator magnetic field (magnetomotive force) is referred to as the "Armature Reaction" as the magnetic field is in reaction to the armature current in the windings. The strength of this Armature Reaction is a function of the number of turns within the coil and the amount of current through the coil.

Armature Re action = NxI .......................................... ..... ............................... Equation 1.02 where, N = number of turns, and I = current The load current will be 3-phase as is the generator voltage. 3-phase, alternating current is shown in Figure 7 -17, the current in each of the three phases, peaks 120 0 apart from each of the other phases.

+1

;--- .... ,

+.5

Ib

? ,

...........

.......\ ' JJ....

.. , ... ,, ...

.

Ie

,

O ~~--~~~---r--~--~------~--~--~----~

6~..

270'

...'., ,,

'.

-.5

,, -1

.-..._--,'

,

,

t ••

'.

....

..............

Figure 7- 17 Three-Phase Generator Sine Waves

_1.1 - 7-12

~


GENERATOR THEORY Armature In Figure 7-18 we see the 2-Pole rotor with the rotor magnetic field extending from the North-Pole. Superimposed on this image is the magnetic force (Armature Reaction) that exists in the air gap as aresult of the load current. Look closely in the air-gap and there is an Armature Reaction operating at some angle. This Armature Reaction is broken into its components (making a right triangle). One component is tangent to the rotor surface and at a right angle to the rotor magnetic field. The other component is vertical (downward) in the direction opposite that ofthe rotor magnetic field coming out of the North-Pole. The rotor is driven by the prime-mover, this rotating torque is the output of the power plant. The Armature Reaction is a reaction to current being pulled off of the generator stator due to the load applied. When these two forces are equal (they are always opposite) the generator speed will be constant. Your understanding of the interaction of these magnetic fields and prime mover torque is critical to understanding how a generator functions.

CORE

Figure 7-181nteraction of Mflgnetic Fieltls III tfle Air Gap

2.0

SYNCHRONISM (OR, WHAT IS HAPPENING IN THE AIR GAP)

2.1

AIR GAP MAGNETIC FIELDS

We now have two rotating fields in our air gap. The first is generated by us, operators of our power plant. We provide current through the rotor windings to generate a magnetic field. Then we drive that rotor by what we refer to as a prime mover. Figure 7-19 illustrates this rotor magnetic field being driven by a small steam turbine. MASS FLOW INTO TURBINE

Figure 7-19 Turbine Driven Magnetic Heltl We have seen that this magnetic field induces a voltage on the stationary windings. When we close the circuit breaker we have current to the load. This current results in a rotating magnetic field, in the air gap (this has been the subject of discussion for the last few pages). The strength and orientation of this rotating stator magnetic field (Armature Reaction) is a result of the type of load applied (we will study this a little later). Let's freeze the rotation of our two magnetic fields. Now we can examine a little better exactly what is happening.


_M -

T.


The horizontal component of the Armature Reaction (that opposes rotation of the rotor) distorts the magnetic field .emanating from the rotor windings. See Figure 7-20. This distorted rotor magnetic field is forced to run "out-of' its preferred .magnetic center. The distorted magnetic field will, therefore, exert a force on the conductor (the rotor forging) to try to back the rotor up (this force is much like the force exerted on a conductor by a horseshoe magnet that is in the path ofthe magnetic field). This tangential force trying to backup the rotor is opposed by the torque delivered by the prime mover. If these two forces are equal, Armature Reaction and torque, then the speed .ofthe rotor is constant.

TANGENTIAL ORCEOUETO

AR ATURE REACTION (ARJ TORQUE FROM ·

PRIME MOVER

Recognize that, in this figure, we have shown the Armature Reaction (AR) as if it is from a single stator bar. The AR is actually the some of the . forces Armature Reactions from each of the three phases. We show the AR at the air gap as this is where it is forcing the rotor magnetic field out of its desired magnetic center. This "Armature Reaction" is rotating at the same speed as the rotor. So, in Figure 7-20, we are looking at a snapshot in time, a stopped motion. A mechanical analogy of these air gap forces is illustrated in Figure 7-21. In this figure the Armature Reaction analogy is a spring that is exerting a synchronizing torque, trying to get the rotor to backup, to return to a load angle of zero. Torque applied on the rotor via the prime mover, opposes this synchronizing force. So, again, when the forces in the air gap are equal, the speed of the rotating rotor is constant.

_1.1 - 7-14 T.

.


-

)

TORQUE FROM PRIME MOVER

SUM OF TANGENnAl FORCES FROM STATOR

THUS SPEED IS CONSTANT

Figure 7-20 Air Gap Forces on Rotor

SYNCHRONIZING TORQUE

TURBINE TORQUE

LOAD ANGLE

Figure 7-21 Mechanical Analogy of Synchronous Forces in Air Gap

GENERATOR THEORY

2.2

ACTIVE POWER (WATTS)

Active power is that generator output that does work. This is energy that is transformed into light, heat, or motion. Active power is the result of current through the resistive component of the load applied (copper in the motor copper windings). We have seen that the current to the load results in an Armature Reaction (AR) within the generator windings. This armature current is in-phase with the generator terminal voltage, as seen in Figure 7-22.

I

V

RESISTIVE CIRCUIT .

Figure 7-22 Resistive Load Characteristics

This in-phase current results in the Armature Reaction being at a right angle to the magnetic field emanating from the rotor North-Pole. Torque from the prime mover is driving the rotor. These forces are all illustrated in Figure 7-23. Looking at the figure one should recognize that if the prime mover torque and the generator armature reaction were to be equal, then the speed of the prime mover and generator will be constant. If the load were to be increased (someone, somewhere, turned on a light), then the armature reaction would increase. The generator speed would slow. It is up to the governor of the prime mover to adjust mass flow through the prime mover to halt the speed decay.

TORQUE

~1IIl---AR

Figure 7-23 Armature Reaction Jill Active Load


_M -

~


2.3

REACTIVE POWER (V ARS)

There are two types of reactive power that need to be studied. One is due to the application of inductive loads (motors and transformers) and the other is due to the application of capaCitive loads (transmission lines).

2.3.1

Inductive Loads

when a pure inductive load is connected to the armature windings; the load causes the phase current to lag behind the voltage by 90°. The reader should recognize that there is no pure inductive load. All coils found in motor windings, for example, are made of copper. Copper has resistance. This copper results in a resistive load, discussed in the previous paragraphs on active loads. Here, we are examining, only the inductance aspects of this load, so for these purposes it can be considered to be pure. Because of the effect of the inductive load, the phase voltages is 90° ahead, or, stated differently, the current lags the voltage by 90°. This means that the current is 90° out of phase from where it was when a resistive load was applied! See Figure 7-24 for the orientation ofthe load current and terminal voltage. If the current phase changes by 90°, then the angle of the armature reaction, AR, must also shift by 90°, AR is the result of current. We can represent this effect in a model by merely rotating the AR 90° back from the position shown for an active load. Figure 7-25 shows the relative positions ofthe rotor MMF and armature reaction for the case of the generator loaded with pure inductance. Notice that the Armature Reaction (AR) directly opposes the rotor MMF. As a result the "Net Air Gap" magnetic field strength will decrease, the induced generator voltage will decrease, meaning the terminal voltage will decrease as well. It will be the role of the voltage regulator to detect and compensate for this voltage decrease.

V

~L-~

_ __.

_1_ ---'1 X.

INDUCTIVE CIRCUIT

Figure 7-24 Phase CurreIU wi Pure Inductive Load ""-'-"

TORQUE

AR

Figure 7- 25 A R wi Pure Inductive Load

This current is 90° out of phase with the terminal voltage because of the inductive element of the load windings. A magnetic field is created within these windings and it takes current out-of-phase with the voltage to create this magnetic field.

_1.1 - 7-16

~

.


GENERATOR THEORY The term "VAR" (seen in a meter shown in Figure 7-26) is used to describe the "reactive power" flow to create this magnetic field in the winding. A V AR is a unit of myasure of "reactive power", a . VAR is the current, that is 90° out of phase with the terminal voltage, times the voltage. This current is 90° out of phase with the voltage as it must "create and sustain" this magnetic field within the inductive device (motor or transformer) so that device can do work . .

The function of REACTIVE POWER is to establish mid sustain a magnetic field in (lit inductive device, or an electric fieldin a capacitive device, such that the stated device can perform useful work! .

By convention, inductive V ARs are assigned a · positive (+) sign. These are VARs that must be supplied by the generators to meet the system demand. Positive V ARs are referred to as flowing "Out" of the generator to the system. These inductive, or lagging, VARs are used to excite the magnetic fields of transformers, motors and transmission lines. Sometimes we referred to this as over-excitation. Without these Lagging/ Inductive/Positive VARs, these magnetic devices would not function as designed; they would not work. Inductive elements in a power system are usually referred to as "sinks" of VARs, in other words, VARs flow to them. See Figure 7-27.

(+) ·VAR

Figure 7-26 VARs

•

VAR's "Out" Lagging Power Factor

Figure 7-27 Inductive VARs 2.3.2

Capacitive Loads

When a pure capacitive load is connected to the stator windings, the phase current is made to lead the phase voltage. With a pure capacitive load, the current leads the voltage by 90°. This is illustrated in Figure 7-28 .

V)\ -

I

1

--1T Xc

'-(''_______

CAPACITIVE CIRCUIT

Figure 7-28 Capacitive Circuits


_1.1 -

T.

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Because of the effect of the capacitive load, the phase voltages is 90° behind, or, stated differently, the current leads the voltage by 90°. This means that the current is 90° out of phase from where it was when a resistive load was applied, only in the other direction from the inductive loading. As we have seen, the angle of the armature reaction, AR, must also shift by 90° as AR is the result of current. Figure 7-29 shows the relative positions ofthe rotor MMF and armature MMF for the case of the generator loaded with pure capacitance. .

Torque

Notice that the Armature Reaction (AR) "adds-to" the rotor MMF. As a result the "Net Air Gap" magnetic field strength will increase, the induced generator voltage will increase, meaning the terminal voltage will increase as well. It will be the role of the voltage regulator to detect and compensate for this voltage increase.

eG20'_004 .

Figure 1-29 Capacitive Loading

By convention, capacitive V ARs are assigned a negative (-) sign. Negative V ARs are referred to as flowing "In" toward the generator. Capacitive loads, such as capacitive banks and lightly loaded transmission lines, are referred to as sources of V ARs. This may also be referred to as under-excitation. See Figure 7-30 for an illustration.

(-) VAR ......-11. _-

VAR's "IN" Leading Power Factor

Transmission Lines

Figure 7-30 Capacitive VARs

2.3.3

Mechanical Analogy

The mechanical analogy to a VAR would be a "wheel barrel". See Figure 7-31. The purpose of a wheel barrel is to do work more efficiently. In comparison, the purpose of a motor is to do work more efficiently. To do work with a wheel barrel one first puts a load into a wheel barrel, then lifts up on the handles to acquire a comfortable leverage. Lifting up on the wheel barrel puts "potential energy" into the wheel barrel, but in the casual defmition of wanting to do work, no work was done, the load was not moved toward its destination. As the operator of the wheel barrel, however, one can feel the energy in one's arms and shoulders. Again, there was energy exerted, but no work was done by our defmition of wanting to move a load across the room. Now that a comfortable leverage has been achieved, the operator can move the load to its destination. Workhas been accomplished.

_1.1 - 7-18

~

.


Figure 7-31 Wheel Barrel

GENERATOR THEORY

We would argue that the lifting ofthe wheel barrel to a comfortable leverage is analogous to a VAR flowing to a motor winding. Reactive current to the motor winding results in the creation of a magnetic field within the motor .air gap. This magnetic fielq is necessary for the motor to function, the magnetic field requires current to be created, that current to create a magnetic field is 90° out-of-phase with the voltage supply. No work has been done. Now that a magnetic field exists within the motor, active current (current that is in-phase with the voltage -watts) can exist to cause the rotor to rotate. Once the rotor turns, work is being performed.

2.3.4

Active / Reactive Power Flow

When both inductance and capacitance exist in a circuit, and they usually do, some or all the VARs required by the inductive elements will be supplied by the capacitive elements. In other words, the capacitive V ARs compensate for the inductive VAR loading. If there were exactly the same amount of inductive and capacitive loads, then the capacitive load would meet the inductive load requirement, and the inductive load would meet the capacitive load requirement. The generator reactive power flow would be zero. Zero reactive power flow is illustrated in Figure 7-32.

, t

VAR

(+)

MW~

VAR=Q Generator

Figure 7-32 Zero Reactive Power We will now consider a situation where there is, in fact, a combination of active and reactive loads. A simple example would be a motor. We require reactive power to the motor to create and sustain the magnetic field in the motor air gap. Then we require active power to the motor to do the work. This is illustrated in Figure 7-33.

Active power now to motor results In torque ouiput of motor rotor.

Figure 7-33 Motor Load


_M -

T.

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Figure 7-34 represents the MFR and AR in this active/reactive loading demonstration. Notice that, the AR is no longer on the horizontal axis. It is configured to be slightly higher on the right hand side of the drawing. This configuration of the armature reaction (or AR) means there is a horizontal and a vertical component. The horizontal component is pushing from right to left. The vertical component is pushing from top down. The horizontal component of armature reaction (AR) opposes the motion of the rotor - meaning prime mover torque is required to maintain stability. Again, when the horizontal component ofthe armature reaction is exactly equal to the torque applied, the rotor frequency will be constant · . The vertical component or amiature reaction (AR) opposes the rotor field strength (MFR). This will result in a lesser voltage being induced in the generator A lesser voltage will be realized at the generator terminals as a result

/

OPPOSESMFR (decreasing net air gap magnetic field strength)

\ OPPOSES MOTION of ROTOR (Torque)

Figure 7-34 Active/Reactive Induction Loading Ifwe were to examine a case where there is a capacitive load (see Figure 7-35), the difference would be that the vertical component of armature reaction would add to the rotor magnetic field strength, causing a rise in generator terminal voltage.

OPPOSES MOTION

of ROTOR ~"'U'I

Figure 7-35 Active/Reactive Capacitive Loading

_M - 7-20

~


ENHANCES MFR (increasing net air gap magnetic field strength)

GENERATOR THEORY

3.0

POWER FACTOR

... Figure 7-36 illustrates all the components of the power triangle for your inspection. Take note of the following items as you proceed. •.

• • •

The hypotenuse is the "Apparent Power" and its measurement is in Kilo-VoltAmperes. The horizontal is the "Real Power" and its units are Kilo Watts. Finally, there is the vertical, labeled "Reactive Power" with units in Kilo VARs. Now lets proceed at understanding this power triangle.

Kilo Watts

REAL POWER Figure 7-36 Power Trifmg/e

We have seen that inside the generator we have a rotating magnet that is created by current passing through the field windings. This rotating magnet "cuts" through the stationary armature bars and "induces" a voltage in these armature bars (Figure 7-37). Now, when a load is applied across the armature bars, there will be current passing through that load. .

Figure 7-37 Gellerator Load We have stated it before, and will state it again: "Voltage" and "Current" is what is the output of the generator. "Voltage" is induced by the rotating magnet, and "Current" is passes through the applied load. We have seen that the load could be resistive, like the copper in the wire. The load could be inductive; motor and transformer windings are an example. The load could be capacitive; transmission lines capacitance to Earth is an example. The current through that load may be "out-of~phase" with the voltage, how much out-of-phase and in which direction, depends upon the load applied.


_1.1. -

T.

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 N •

Let's just say that the load applied is some large motor through some transformers. This means the current will "lag" the voltage as illustrated in Figure 7-38.

o

I

T

CURRENT

The vector relationship of the voltage applied across the motor windings and the current through the motor windings is illustrated in this figure on the screen. Notice the current and voltage are not in-phase.

I

I

~I I P I

H I A . S

E

I

VOLTAGE

V

10&3014

Figure 7-38 Voltage and Current Load Vectors We want to study this current in more detail. Let's take our "Current" vector and move it over so we can analyze it a bit. See Figure 7-39. Notice we did not change direction or magnitude, no laws of vectors were violated. The current is now standing alone for our analysis.

Figure 7-39 Current Vector •.~,__ In Figure 7-40 we see the "Current" vector reduced to its components. Notice the "horizontal vector" plus the "vertical vector" equal the "original current vector". Since this is true we can relate this . statement algebraically. The "current vector - I" is equal to the sum of the "horizontal current vector - IH" plus the "vertical current vector - Iv". Study this triangle to satisfy yourself that this is true. Now that we've broke this down into an algebraic statement let's apply another law of algebra. Now this other algebraic law is that you can multiply one side of the equation by anything, "providing" you mUltiply the other side of the equation by the same thing. For example we could multiply both sides by two, or

Iv

Figure 7-40 Current Vector Components

eighteen, if we desire. In our case, we're going to mUltiply both sides of the equation by "V". "V" for voltage, assumirig the voltage output of our 'generator is fixed (a valid assumption for our purposes). The fmal expression is shown on Figure 7-41. The expression reads: "Current" times the voltage is equal to "Horizontal Component of Current times the voltage" plus the "Vertical Component of Current times the voltage". Notice, that the "horizontal component of the current, times the voltage" is parallel to the original voltage vector outputting our generator. ' Also, the "vertical component of the current vector, times the voltage" is perpendicular to the voltage vector outputting our generator.

_1.1. - 7-22

~


Figure 7-41 Current times Voltage Vectors

GENERATOR THEORY

This is pretty meaningful. If you' 11 recall from your study of alternating current circuits, in a purely inductive circuit, the current lags the voltage by 90 degrees. In a purely resistive circuit, the current is exactly in phase with the voltage. Why is this important? Think about it. There is no device in the control room that directly controls generator kVA output. Instead, we control the vectorial components that make up the generator output. We have one controller that "controls the horizontal component of the current, times the voltage", this is the prime-mover "Governor". We have another controller that controls the "vertical component of the current, times the voltage", this is the Automatic Voltage Regulator (A VR). So lets give the horizontal and vertical components a name. See Figure 7-42. The horizontal component we will call "real power" and its units is kiloWatts. One kiloWatt (kW) is of course 1,000 watts. A Watt, is the "voltage" TIMES the "current that is in-phase with the voltage". This output is controlled by the governor. This "real power" component represents that portion of the generat<.>r output that is doing "real work". The output is being transformed into heat, light, or motion. Output is adjusted in response to frequency deviation.

APPARENT POWER .

REACTIVE POWER KVAR

IV V

Automatic Voltage Regu lator (AVR)

REAL POWER KW

(

Governor)

~

IV Figure 7-42 Power Triangle

The vertical component we will call "reactive power" and its units is kiloV ARs (kV ARs). A V AR is an acronym for Volts-Ampere-Reactive, and is the "Voltage" TIMES the "Current that is 90 degrees out-of-phase with the voltage". This output is controlled by the voltage regulator. This "reactive power" component represents that portion of the generator output that is required to magnetize inductive circuits or to generate an electric field in capacitive circuits. By controlling the components of the generator output, we, in-effect, control the generator output. Before we conclude, lets do a little housekeeping regarding our explanation of the power triangle. We use the term "power factor" quite regularly. Let's define it in our schematic on the screen. "Power Factor." What does this term mean? Well, in the electric power industry, what we are often most concerned about, is the generation of "real power", kiloWatts. Yet the generator output is "apparent power", or Kilo-Volt-Amperes. It might be useful to know what is the relationship between this "real power" and "apparent power". It might be useful, and it is! We call this relationship the "Power Factor".

POWER FACTOR

PF

-

KW

The "Power Factor" describes the ratio between Kilo Watts and KilO-Volt-Amperes. See Figure 7-43.

KVA

Kilo Watts Figure 7-43 Power Factor


_1.1. -

T.


A more mathematical term used for "Power Factor" is that it is the cosine of the angle between the "adjacent side" and the "hypotenuse" ofthe "Power Triangle" . For those with trigonometric backgrounds you will recall that the cosine of an angle is the "adjacent side" divided by the "hypotenuse". It is . the "Power Factor" . See Figure 7-44. .

Kilo Watts KW

cos (angle) =KVA = Power Factor

As we have seen in our studies, the "Power Factor" may be lagging or it may be leading, usually it lags. A lagging power factor means that the current lags the voltage in order tomeet the requirement of inductive loading. A leading "Power Factor" means the current leads the voltage in order to meet the demand of a capacitive circuit. A unity "Power Factor" means the current is in-phase with the voltage, reactive power output is zero.

Figure 7-44 PF Derivation


_1.l - 7-24

~


GENERATOR CONSTRUCTION Chapter 8

TERMINAL OBJECTIVE: To better acquaint the participant wIth how a generator is constructed so that we develop a better understanding of how components are placed at risk during normal/abnormal operations.

ENABLING OBJECTIVES: At the completion of this section, the student should be able to: l. 2. 3. 4. 5. 6.

Given a cross sectional drawing of the generator, be able to identity the major components. Describe the primary function of each major component of the stator assembly. Describe the primary function of each major rotor assembly component. Provide a basic description of the material that is used for the construction of each major component. Describe how heat is removed from the generator components. Describe how the type of excitation used impacts generator construction.

© 1999 - TG201J5.0_June09, Printed: 12/14/2010

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 TABLE OF CONTENTS . 1.0

INTRODUCTION ................................................................................................................................... 3

2.0

STATOR ASSEMBLY ........................................................................................................................... 3 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3.0

Stator Frame ................................................................................................................................ 6 Stator Core .................................................................................................................................. 10 Stator Windings ......................................................................................................................... 14 End Shield! Bearing Bracket ..................................................................................................... 23 Hydrogen Seals .......................................................................................................................... 24 Bearings ..................................................................................................................................... 26 Grounding Brush ....................................................................................................................... 28

ROTOR ASSEMBLY ........................................................................................................................... 28 3.1 3.2 3.3

Rotor Body ................................................................................................................................ 29 Field Windings .......................... :............................................................................................... 30 Retaining Rings ......................................................................................................................... 33

4.0

COOLING ............................................................................................................................................. 34

5.0

GENERATOR COMPONENTS ASSOCIATED WITH THE EXCITER ........................................... 35

_1.1 - 8-2 T.


GENERATOR CONSTRUCTION

1.0

INTRODUCTION

This section of the text covers the basic construction of the typical utility AC generator. Different manufacturers may have different names for the same component, and we will deal with these as they occur. All · units have similar construction for the major sub-assemblies and the components, which they contain. The major sub-assemblies of the generator are the stator and rotor. The stator contains all the stationary parts while the rotor carries the field windings, which make the electro-magnetic field . Each of the assemblies has their own components whiCh we will look at in this section. .

2.0

STATOR ASSEMBLY

The stator is the stationary portion of the generator and.contains the stator windings, stator core and the end shields. Figure 8-01 shows a cross section view ofthe generator rotor and stator components. Each of the m~or sections will be discussed separately. Major stator components include:

•

• • • •

Frame Core Armature Windings End Shield HVBushmgs

Stator Core

Retaining Rings

Coolers T-~~_~

~~~~-...;L Collector Rings

~

_ _ _ _-High Voltage Bushings

Figure 8-01 Generator Components (GE Hydrogen Cooled Generator)

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Figure 8-02 represents a cross-sectional drawing of a Westinghouse manufactured generator. The reader should locate those major components identified in the previous schematic.

u:

UJ

...J

g CJ

Figure 8-02 Cross-Sectional Drawing of a Westinghouse Manufactured Generator

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GENERATOR CONSTRUCTION Figure 8-03 represents a large ABB manufactured generator. As we proceed through the materials it will be useful for the reader to draw comparisons of the different units. Again, the reader should carefully note that all the generator stators have the same components: frame, stator core, armature windings, end-windings, cooler, and some form of an end shield. All the rotors are manufactured from a forging (although you may find some where. the rotor is bolted and welded disks) with the field windings wrapped around the forging to create a North and Southpole.

Figure 8-03 Large ABB Manufactured Generator


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Figure 8-04 is a cross-section, for educational purposes of an air-cooled Siemens 4-pole generator. You should immediately notice that all the same components are found within this generator. The component design may be different, but the functions are the same. It is the function of these components that we will be studying.

Cooler

Stator core

C__;;".--~:::J~Winding

Graphic compliments Siemens Power Generation

Figure 8-04 Siemens 4-Pole, Air Cooled Generator

2.1

STATOR FRAME

The stator frame (wrapper or casing) functions to carry the weight of the stator core, windings, and the rotor. In some larger units, this may be a total weight of several hundred tons. It must also be able to withstand the mechanical shocks caused by electrical faults in the power system and withstand the force of an internal hydrogen explosion. In order to meet these functions, the stator frame is cylindrically shaped and fabricated of steel plates, which are welded together. On the inner periphery of the stator frame (Figure 8-05), key bars are located which serve to position the stator core. On some 2-pole units the key bars are attached at the midpoint of the spring bar (see Figure 8-06). The spring bars are attached to the section plates within the frame. This design (typically used only on 2-pole generators) isolates any core vibration from the frame and therefore, the foundation.

_1.1 - 8-6



Figure 8-05 Frame for 2-Pole Generators

Key

Bar o...-.;;;;;~

SpQng Bar

Photograph compliments of Generator Consulting Services, Inc.

Figure 8-06 Core Assembly on Key Bar

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 . The stator frame rests on feet, as shown in Figure 8-07. These feet are bolted at the horizontal centerline of the stator. For those of you interested in mechanical alignment, shims are typically placed under these feet to achieve the desired vertical elevation. The vertical bolting circle seen, is for the attachment of a "trunnion" to use a hydraulic jack or crane for positioning the frame.

Bolting circle for trunnion

Figure 8-07 Generator Feet Located just inside the stator frame are baffle (section) plates and piping which form the gas passages. The hydrogen (or air) coolers are located in the gas passage. The coolers can either be oriented vertically or horizontally. The gas is circulated around the finned tubes in the coolers and the heat is carried away by the cooling water system. Seals are incorporated where the water connections are made to keep hydrogen from leaking out of the frame. See Figure 8-08 for gas passages.

WRAPPER OR CASING

Figure 8-08 Generator Stator Cooling Passages

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•

GENERATOR CONSTRUCTION Many machines are hydrogen cooled. These coolers are either assembled in the four corners or in the dome over the top of the frame. In Figure 8-09 we see a vertical cooler being assembled into the corner of one such generator stator frame.

Figure 8-09 Hydrogen Coolers

Some hydrogen coolers are mounted in the dome, that is the cooler is horizontiilly positioned at the top of the generator frame. Another hydrogen cooler, referred to as an "omega" cooler is in the shape of the greek letter 'omega', Q. The water connection is on one side of the omega (see Figure 8-10), with the exit at the other end.

Graphic compliments of Siemens Power Generation Figure 8-10 Q SIUlpedCooler


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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The power produced in the stator is carried out through the high voltage bushings. These are located on the bottom ofthe stator frame and are connected to the bus ducts, which carry the power to the system. (See Figure 2-11). The high voltage bushings are large porcelain insulators. The insulation is required to prevent the high voltage from jumping to the stator frame . . The high voltage bushings are cooled, usually by both hydrogen and water.

Figure 8-11 High Voltage Bushings N::5I! ; " ' - -

Current Transformers

Connection to bus ductwork

2.2

STATOR CORE

The stator core is mounted along the inner diameter of the generator frame. The core is constructed to provide a low resistance path for the magnetic field and to carry the weight of the stator windings. See Figure 8-01 to locate the stator core inside the frame. The iron used to make the core is a silicon steel with about 90% of the grains lined up along the preferred path for the magnetic field. This makes the resistance of the steel very low and minimizes the losses (and heat generation) in the core metal. The core is made from thin laminations (about 0.015-inch thick) of the special steel. (See Figure 8-12). Each lamination is insulated with an enamel coatingofvamish. On typical units, the laminations are fitted over the key bars one layer at a time. The layers ate staggered so the butt joints of the punchings do not line up from one layer to the next. The layers are periodically compressed (or shook on a shaker table) to squeeze any air pockets out from between the laminations. (See Figure 8-l3). Spacers are also placed in between some layers to form gas passages in the core that help cool the stator windings and rotor. SLOTS FOR ASSEMBLV AND LOCKING

SLOTS FOR STAlUR BARS

Figure 8-12 Typical Stator Core Punching

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Stator Frame

Figure 8-13 Core Assembly

GENERATOR CONSTRUCTION Figure 8-14 illustrates some core laminations with these inside spacers that will, when assembled, create this gap for coolant passage.

Figure 8-14 Laminations w/Spacers Figure 8-15 shows a small stator core removed from the stator frame . In this view you can see the section plates that support the core, within the frame, at the core outer diameter.

Section Plate

Photograph compliments of Aistom Power, Richmond VA

Figure 8-15 Stator Core OD.

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 A picture of the core on a small unit (being repaired in a service shop) is seen in Figure 8-16. On the left you see a core assembled. The inner diameter of the core is viewed on the right. The inside spacer block creates a gap between the layered core assembly. This gap allows cooling gases to flow from/to the air gap through the core.

Photograph oompliments of Aision Powe<. Rittlmond.VA

Photograph oompliments of Alsion Power.Richmond. VA

Figure 8-16 Small Stator Core A regular core lamination is seen in Figure 8-17. Each one of the laminations, once punched, is coated in a varnish such that each lamination is electrically insulated from each adjacent lamination. This is to prevent the existence of eddy currents due to passing rotor magnetic field.

Photograph compliments of Aistom Power. Richmond VA

Figure 8-17 Laminations

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GENERATOR CONSTRUCTION Figure 8-18 illustrates a generator positioned vertically in the factory to facilitate the stacking of the core. Notice the punchings, the key slots on the outer diameter of the punchings, and the key bars along the inside diameter of the frame . Figure 8-19 illustrates the use of outside spacer blocks. These outside spacer blocks help to . keep the core compressed along the fIngers ofthe slots.

Figure 8-18 Core Being Stacked

. Figure 8-19 Outside Spacer Blocks

Westinghouse generator cores are mounted on a 'building bolt' then tightened together using "through bolts" . Figure 8-20 below illustrates a single Westinghouse core punching. Notice the "J" hook at the outer diameter where the punching assembles onto the 'building bolt'. The hole through the center is where the "through bolts" tighten the core axially.

Photograph compliments of AGTServices. Amsterdam. NY

Figure 8-20 W Through Bolt Core Punching


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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Once the core stacking is completed, a clamping ring is bolted on the ends ofthe key bars. This ring helps keep the core compressed. A photograph of the clamping ring and key bar nut is seen in Figure 8-21 below.

Figure 8-21 Clamping Ring

2.3

STATOR WINDINGS

The stator winding (Figure 8-22) is where the AC power is produced in the generator by moving the magnetic field through the conductors, which make up the windings. Recall from the previous section that there are three sets of stator windings, one for each of the three phases. In this figure you also see the liquid connections carrying stator cooling water to/from these windings.

Figure 8-22 Stator Windings

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GENERATOR CONSTRUCTION Each phase of the stator winding is made up of a set number of stator bars, which are connected in series. See Figure 8-23 for a drawing of the stator-winding diagram. The inner section of this drawing diagrams the collector end of the machine, including connection rings (in the very middle) to the High Voltage Bushings. The outer section of this drawing diagrams the turbine end of the generator stator windings. At the center is the stator bar located within its slot along the length of the core. Notice there is an upper and a lower bar. The colors in the figure highlight each of the three phases to aid your reading this schematic. Examine this figure closely and locate T-2 at the end of the red phase. Follow these connections: 1. 2. 3. 4.

On On On On

the the the the

collector end (CE) this red phase becomes the bottom bar of slot 1. turbine end (TE) of slot 1, the red phase is brazed to the top bar of slot 16. CE this bar is brazed to the bottom bar of slot 2. TE this bar (from slot 2) is brazed to the top bar of slot 17 ...... and so on.

Figure 8-23 (GE) Stator Winding Diagram

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STEAM TURBINE-GENERATOR FUNDAMENTALS- TG201 Figure 8-24 shows the collector end of a stator core removed from the frame . Notice the end windings, especially where the connection rings (in this case - upward) route current from the windings toward the connections to external bus duct work.

Figure 8.;.24 CE Connection Rings

---

Photog!Olph oompllmenb of Aislom Power. Richmond VA

Here (Figure 8-25) we see the stator windings contained with the slot of the core. Notice there are two bars per slot (as we seen in the stator winding diagram). The top bar is identified as that bar closest to the air gap. We ' ll look at some of the detail later. For now, notice there are multiple strands of copper that make up the bar; i.e., the stator bar is not solid.

Figure 8-25 Slot Assembly

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GENERATOR CONSTRUCTION The conductors within the bar are not straight, but twisted as shown in Figure 8-26. This "Roebel Transposition" has each conductor occupying all available positions for an equal distance along the bar. With this twisting, every bar is cut by an equal amount of magnetic flux, generates the same current (and, therefore heat) and no single bar is hotte.r than any of the others. The bars are insulated after the twisting occurs and then baked to harden the insulation.

Simple Roebel Bar Showing Transposition of Strands

Photogltlph oompllments of Natlonat Electric Coit, Columbus OH

Figure 8-26 Stator Conductors Figure 8-27 below is a mock-up of these conductors changing positions within the stator bar as the conductors (strands) extend the length of the bar.

Figure 8-27 Roebel Transposition Mock-Up

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The conductors may be hollow, or cooling tubes may be incorporated into the bar between solid conductors. These features are used for internal cooling of the bars. Heat generated within the bar must be removed before it damages the insulation. On larger units the insulation prevents this from being effective from outside the bar. Some GE units use hollow conductors and pump water through the inside of the conductor. Westfughouse will use cooling tubes and direct hydrogen through the tubes. Figure 8-28 illustrates three different stator armature bars. On the left we see a solid strand bar that might . be found on an older or smaller unit. In the middle we have a hydrogen cooled bar, a type used often by Westinghouse. On the right, you will notice the hollow strands that permit water (at 0.2 flMho/cm) to cool the bar. The internal voltage of any of these bars could be 20-kV or higher. Some bars are constructed where every strand is hollow.

Photograph compliments of Natlonat Electric Coli. Columbus OH

Figure 8-28 Water Cooled Stator Bar Once the bars are formed, they are. placed in the slots formed by the fingers of the core laminations, as shown in Figure 8-29. A Resistance Temperature Detector (RTD) is often placed in the slot, usually between the upper and lower bars. Not all slots will have a RTD.

Figure 8-29 Slot Assembly

SIDE·RIPPLE OR "S"SPRING

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WEDGE

GENERATOR CONSTRUCTION Side to side movement of the bars in the slot is often prevented by driving a "S" shaped spring into the slot between the bars and the side wall. This "S" shaped spring is a molded glass type conductive material (carbon is imbedded in this material) as seen in Figure 8-30 below.

Figure 8-30 "S" Spring Insulation is also laid on top of the bar. The bars are pressed into the slot and the use of another "S" spring and wedges keep the bars tight within that slot. The wedges extend full length of the slot: The stator bars extend beyond the end ofthe slot and become end-windings. Examples of these stator wedges are shown in Figure 8-31 and 2-32. A 2° tapered wedge was chosen as an example for no particular reason. The taper, however, helps achieve the desired downward forces acting on the bars within the slot.

Photograph compliments of AGT Services, Amsterdam, NY

Figure 8-31 2° Tapered Stator Wedge

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STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 Figure 8-32 illustrates a top ripple spring and wedge with gage holes. The gage holes allows the compression of the spring to be measured.

Photograph compliments of AGT Services, Amsterdam, NY

Figure 8-32 Ripple Spring & Wedge w/Gage Holes In Figure 8-33 one can see the top bars in position, tied together with the glass roving tie. Some of the bars have been brazed and taped (in the center) and others have brazing in progress.

Figure 8-33 Winding Assembly

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GENERATOR CONSTRUCTION The overhanging bars (the end-windings) are connected together to join our 2-phases. The end windings are brazed together and then wrapped with insulation. Figure 8-34 illustrates an end clip for a GE water cooled stator bar. On the left you see the copper strands within the end clip. On the right you see an end clip (without the bar assembled yet) being bolted to the laminate piece that will be brazed to the end of the series bar.

Photographs provided by AGT Services, Amsterdam NY

Figure 8-34 Emf Clip for GE Water Cooled Stator Bar

Figure 8-35 shows a generator stator in the process of being rewound. Notice how the laminations are laid in place and are brazed to the in-series bar.

Figure 8-35 Brazed Emf Windings

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The end windings are supported by fastening them to the clamping ring as shown in Figure 8-36. Due to the large mechanical forces applied to this area by the rotating magnetic field, the assembly in this area of the generator is critical. The end windings must be able to withstand the vibration caused by the magnetic field during operation and the mechanical forces involved when a fault occurs during operation. This area is .also subjected to the differential expansion between the core metal and the copper conductors. As before, the end winding support system will vary with different manufacturers, and from unit to unit. OUTER WRAPPER PLATE]

II

II

CORE CLAMPING

SPRING AND

d::=l~~~~~

____......-,..J::::;~~.....,;L~KEY

BAR ASSY.

Figure 8-36 End Winding Support Figure 8-37 illustrates an ASB stator bar assembled in the core slot. Notice the spacer, packing and wedge. The intention of these components is to keep the bar tightly positioned in the core slot. Alstomnow constructs a stator bar, such as the one shown here, only the hollow conductors are made of stainless steel.

DIVIDED CONDUCTOR

'W.-..----

PACKING

INTERMEDIATE PACKING

HOLLOW CONDUCTOR

'W.-.£---- CORONA PROTECTION

INTERMEDIATE STRIP

-=:~~III~

FILLER SPACER

Figure 8-37 ABB Stator Bar

MICADUR WV"K'N', _ _

FILLER STRIP ~~"--7'

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2.4

END SHIELD/ BEARING BRACKET

The end of the stator frame is closed by the end shields (GE term) or the bearing bracket (W term). They are similar in construction. This end shield is seen in Figure 8-38. Notice the vertical and horizontal bolting surfaces. The horizontal joint is where the upper half will assemble to the lower half. The vertical joint is where both halves will assemble to the end of the generator frame . This end shield contains the journal bearing to support the generator rotor. On a hydrogen cooled generator, the end shield must also contain the hydrogen seals, whose purpose is to keep the hydrogen gas inside the generator frame. Notice the heavy ribbing that appears on the outside of this generator end shield. These stiffening ribs are welded in place and will support the rotor and is capable of withstanding the force of an internal hydrogen explosion.

Journal Bearing - '__ .-:0 _ J_.~"~

Horizontal

JOlnt~:::'1~~IJ~

End Shield (or Bearing Bracket)

Figure 8-38 Eml Shield

Here (in Figure 8·39) we see an upper half end shield sitting on the floor. Notice the ribbing, vertical and horizontal bolting surfaces.

Figure 8-39 GE Upper Half End Shiehl

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2.5

H YDROGEN SEALS

The horizontal joint of the lower half of a GE end shield is illustrated in Figure 8-40. Notice the location of the journal bearings, hydrogen seals, and the oil deflectors. The bearing supports the generator rotor. Hydrogen seals, shown here, are gapped to the rotor. Oil is routed between the two seals such that it flows along the rotor surface toward the inside and outside seal areas. This oil flow inhibits any oil leakage. The inner oil deflector is . intended to prevent the oil from getting into the generator casing. Look carefully at this figure and you should see grooving along both the horizontal and vertical joint surfaces. This grooving is also to prevent hydrogen leakage.

Photograph compliments of Turbine Services, Inc. , Monaca PA

Figure 8-40 GE End Shields A closer view of the GE Hydrogen sealrings is illustrated in Figure 8-41 , below. Seal oil passes between the rings to seal against H2 leakage along the rotor surface.

Photograph compliments of Turbine Services. Inc., Monaca PA

Figure 8-41 GE Hydrogen Seal Ring

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GENERATOR CONSTRUCTION A cross section of a GE hydrogen seal ring is seen in Figure 8-42. Oil enters from the top middle, flows down between the air side and gas side seal rings. This oil then flows in both directions along the surface of the rotor, affecting a seal against hydrogen leakage.

INNER OIL DEFLECTOR

Figure 8-42 GE Hydroge/1 Seal Rillg

A cross section ofa Westinghouse hydrogen seal ring is seen in Figure 8c43.Westinghouse utilizes a hydrogen side and an air-side oil. . The hydrogen-side oil (shown blue) flows out to the right along the shaft surface. The air-side oil (shown reddish) flows out to the left along the shaft surfaces. If these two oil pressures are the same (and they are supposed to be), then there is no oil mixing that occurs. This ring is contained inside the end bearing bracket. AIR SIDE GLAND SEAL OIL MAINTAINED AT 12 psi ABOVE MACHINE GAS PRESSURE

G..

HYDROGEN SIDE GLAND SEAL OIL MAINTAINED AT SAME PRESSURE AS AIR SIDE OIL

SHAFT .

Figure 8-43 W Hydrogen Seal Rillgs

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BEARINGS

A generator bearing is shown in Figure 8-44. These generator bearings are typically either elliptical or cylindrical, depending upon the OEM. These type bearings are very stable under high load conditions. For those of you familiar with turbine bearings, the major difference is that the generator bearings may be insulated. This insulation is used to prevent any high voltages on the generator shaft from seeking ground potential through the bearings. This action would result in arcing and pitting as the potential arcs across an oil film. By insulating the bearing it is neutral (not grounded), therefore there is no attraction of the rotor high voltage.

BEARING INSULATION

BEARING INSULATION

Figure 8-44 W Generator Bearing Often, only one of the generator bearings is insulated, usually the collector end. This creates some difficulties in checking the integrity of the insulation. If the rotor in place, the rotor provides a ground path, meaning the rotor needs to be isolated from the bearing when meggering the insulation. This can be a big task.

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GENERATOR CONSTRUCTION On the modern GE units they make use of a double insulated bearing ring on the collector end only. This double insulation is illustrated in Figure 8-45 below. This design allows for meggaring of the insulation without having to account for ground paths through the rotor. Notice the center ring in insulated from both the rotor surface and the ground potential of the generator frame . .

Figure 8-45 Double Bearing Insulation

Figure 8-46 shows an exploded view of one of these bearings. Notice the bearing feed near the horizontal joint, in this case the supply pressure is 25-psig through a strainer and an orifice. The orifice meters the oil flow so the volume of oil carefully matches the bearing requirements (on some other designs the pipe size from the header supply to the bearing is determined based upon the oil flow requirements). Drain passage is out the right side of the figure through a sight box. This particular bearing utilizes a lift pump arrangement - notice the life pump supply holes at the bottom centerline of the lower half bearing ring. This is a high pressure oil supply intended to float the rotor during turning gear operation. Collector End Outer Ring (Insu lated) Collector End Inner Ring

Generator Bearing

Rotation

'\

TE

/7 s~\)

~\~

c\.c--;?,

1

011 Feed 25 PSIG

...

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:0( /

Orlftce

}.,----------4-t-'~

~ 7tf

~ Or.ln

CE

Sight Box

~\H~

From LO Header

Cuno

LI ft Pump

1501 VLV

ReliefVLV

Figure 8-46 Exploded View ojGEGenerator Bearing


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GROUNDING BRUSH

On these double insulated units a ground strap rides the turbines end of the generator rotor. See Figure 847 for an illustration of one such ground strap. It is a braided copper strap riding the rotor surface and bolted to the bearing cap.

Figure 8-47 Braided Copper Ground Strap

.Photograph compliments of Alstom Power, Richmond VA

3.0

ROTOR ASSEMBLY

The rotQr serves as a rotating electro-magnetic field whose strength can be varied to control the output of the generator stator windings. In Figure 8-48, you see conductor bars bringing excitation current along the bore of the rotor to the inside of the machine. Connections are then made to the rotor windings. As with the stator, we will discuss the major components that make up the rotor assembly.

LOAD COUPLING

ROTOR IIIIINDINGS

Figure 8-48 Rotor Assembly

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3.1

ROTOR BODY

The rotor body is machined from a solid steel forging, similar to·that of the turbine rotors. The couplings, bearing journals, hydrogen seal areas; collector rings and fan mounting area all machined into the surface of the rotor. The center bore of the rotor will also be machined, as with the turbine rotor. Some foreign manufacturers will use built up rotor bodies made from individual forgings which are then welded together. The couplings and collector rings can be shrunk fit on the rotor but an area must still be machined to accept them. The rotor body will be machined with longitudinal slots which will carry the field windings. The machining of the slots is carefully controlled to minimize the possibility of mechanical unbalance 'in the rotor due to the machining. Slots are machined on opposing sides of the rotor at the same time and, when the slots are completed, the rotor is tunled 180 degrees and the next set of slots are machined. This process evens out the differences between the slots caused by the tool bits. A picture of a rotor body is seen in Figure 8-49.

Figure 8-49 Machi11ed Rotor Forging

The center bore of the rotor is machined to accept the electrical connection between the exciter and the , field windings. (See Figure 8-50). The field windings are connected to the collector rings through terminal studs and copper conductors, which run through the bore (bore copper). The copper conductors are then connected to the field windings with more terminal studs. The terminal studs are sealed where they penetrate the rotor to prevent hydrogen from leaking from the generator out to the collector. The collector rings are shown in the picture to the right. Notice there are two rings; one to bring current in and the other to direct it outward to the excitation system. The rings are labeled as '+' and' -', you should be aware that the orientation is a function of how the circuits are wired. On some plants the polarity is reversed regularly as it is understood the rate of ware on the collector rings is a function of the direction of the current. To the left of the left ring you should notice the terminal stud that connects from the collector ring to the bore copper.

Figure 8-50 Collector End Connectiol1s

GENERATOR ROTOR WINDINGS

TERMINAL STUDS

BORE COPPER

"-"'~'-".

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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 In Figure 8-51, we can see the bore copper inside the bore of the rotor. Notice the plus/minus connections, the two pieces of bore copper are insulated from each other and from the forging itself.

Figure 8-51 Bore Copper

3.2

FIELD WINDINGS

The conductors are connected in loops, which are called turns. Each turn is made of flat copper bars which are around 14" thick and, after being laid in the slot, brazed to the others. In the photograph (Figure 8-52) we see pre-manufactured flat copper bars (turns) suspended above the forging. These turns of copper will be laid into the forging slot.

Photograph compliments of National Electric Coil, Columbus OH

Figure 8-52 Rotor Windings being Installed into Forging

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GENERATOR CONSTRUCTION In Figure 8-53 we see a rotor being wound. We start by insulating the slot with "slot armor (insulation)". This prevents the turns of copper from going to ground with the forging. Next we lay in the first turn of copper. The collector end of this turn is brazed to the terminal stud, connecting back to the excitation supply. Now we lay in a "turn separator", a piece of insulation that prevents the copper·turns from shorting out with each other. A new turn of copper can now be positioned on top of the turn separator. The beginning of this new turn of copper will be brazed to the end of the turn previously installed. We continue until the slot is filled. Once this slot is filled, we start on the next slot out from the pole. The beginning ofthis 2hd slot is brazed to the end of the 1st slot.

Figure 8-53 Coil Stacks

Photogl1lph compliments of AGTServloes. Amsterdam NY

In this Figure 8-54 we see a rotor completely wound. Notice, that the radial lead makes connection to the terminal stud along the rotor surface. The lead travels along the surface of the rotor, and will be brazed to the bottom turn of the I st slot. The turns of copper then make their way to the top of the slot, and in the picture you can see where the top turn of the first slot is brazed to the top turn of the 2nd slot. At the end of the pole, the bottom tum of the last slot is brazed to the bottom turn of the last slot of the opposite pole.

Figure 8-54 End Windings


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STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The coil to coil connection seen here (Figure 8-55 is what is used to braze the two coils together. The pole to pole connection is made out of sight, under the end windings, buy you should be able to visualize the connection. The loops allow for relative movement due to mechanical forces and expansion.

Figure 8-55 Pole to Pole and Coil to Coil Connections The next issue of concern is the containment ofthe rotor end windings. In service, the centrifugal forces . acting on these windings is significant. Notice in the photo (Figure 8-56, how exposed these windings are to centrifugal forces (you may also notice that a couple slots remain to be filled). These windings must be held in position, but in such a manner that expansion is permitted. A wedge is inserted along the rotor slot. An insulator is laid between the wedge and the copper turn. The wedge may be made of steel, but is often aluminum.

Figure8-56 Field Windings Set into the Forging

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Figure 8-57 illustrates a wedge which will be installed in the slots of the rotor forging to hold the generator windings (turns) within their slots.


Figure 8-57 Rotor Wilulillg Wedge

3.3

RETAINING RINGS

The retaining rings prevent the field winding end turns from flying out during rotation due to centrifugal force. Blocking pieces (also known as spacers) placed between the end turns and the retaining ring hold the turns in place. The retaining ring is made of non-magnetic steel and is shrunk fit onto the rotor body or a special collar machined onto the rotor. Since the retaining rings experiences force from the inside, it is usually the most highly stressed component in the unit. The two separate designs are shown in Figure 8-58. FIELD COIL AMORT

DISTANCE BLOCK

DISTANCE

BlOCK ra~~~

BODY MOUNTED

~ETAINING

RING

~ PINDLE

MO,uNTEO RETAINING RING

Figure 8-58 Retaillillg Rillgs


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COOLING

Sometimes we' re asked, "Why is the generator hydrogen cooled?" The answer is illustrated in Table 2-01. L 2. 3.

Hydrogen has the least speCific gravity, i.e., it is the least dense, therefore requires less energy to circulate within the machine. This means circulating hydrogen inside the machine requires less loss. Hydrogen has the best specific heat, meaning it is more capable to carry heat away with it, 17 times more capable than air, and 3 times better than helium. Finally, hydrogen hasthe best thermal conductivity, meaning it is more capallie to pick up heat off of an object.

Speci./k Gravity Spec!ficHeat Thermal Conductivil:J'

Air 1.0 0.24 0.014

Hvdroe:en 0.07 3.42 0.097

Helium 0.14 1.25 0.082

Table 2-01 Advantages of Hydrogen The single disadvantage, of course, is the explosiveness of hydrogen. Mixtures of hydrogen in air between 4.1 % and 74.2% are explosive. One point worthwhile making at this point is that anyone working around a hydrogen cooled generator should always (always!) respect the environment in which you are working. The rrianner in which the rotor windings are cooled varies by OEM, size, and age. Here (Figure 8-59) we see a radial cooled coil slot. It is found on smaller 2-pole and many of the 4-pole units. The hydrogen is blown under the retaining rings from both sides and flows toward the middle in the sub slot shown in the drawing. Hydrogen flows from the sub slot, radially upward toward the air gap, through the rotor turns and wedge. This design has the advantage that it brings the windings in direct contact with the hydrogen. It has a disadvantage in that the machine will be warmer toward the middle.

WEDGE CREEPAGE .BLOCK ...._.:::.....TURN INSULATION SLOT INSULATION COPPER

~f-""=---WIN DINGS

SUB SLOT

Figure 8-59 Radial Cooled Coil Slot

. In this drawing (Figure 8-60) we see an Indirect Cooling system, often referred to as "conventional", and is found on smaller units. There is no flow through the rotor windings, there is flow, however, through a ventilating duct located adjacent to the rotor winding slot. This method cools the surrounding area, cannot cool the rotor windings as well, and also results in a warmer center.

. Figure 8-60 Indirect Cooling System

_1.1 - 8-34 T. HPe

Technical Services

GENERATOR CONSTRUCTION The diagonal cooling system, shown in Figure 8-61, is the most efficient. In this design the wedge "scoops" hydrogen out of the air gap. The cooling gas then flows radially downward in the slot and, at the same time, flows axially downstream. After it gets to the bottom of the slot it comes back out to the air gap in the outlet section. At the outlet section,tbe wedge has a raised leading edge so a negative back pressure (relatively speaking) is developed, pulling the hydrogen back into the air gap.· _---R.;.;;O""TA-T;.;:IO;.:.N_ _

FLUSH INLET SCOOP

INLET REGION

I--~_

SUBDIVIDED FIELD WINDING TURNS

OUTLET REGION

SECTION A · A FROM ABOVE

COPPER CHANNEL INSULATION

--JA

Figure 8-61 Diagonal FlolV Cooling System

5.0

GENERATOR COMPONENTS ASSOCIATED WITH THE EXCITER

On many units the excitation current is from a stationary source. This means we need to transfer the excitation CUlTent from a stationary source to the rotating windings. Collector rings are used for this purpose. The collector rings seen in Figure 8-62 are fit onto the forging and are wired to the bore copper through terminal studs, shown. Notice the collector ring grooves are machined in a helical fashion . Also a fan ring pulls air across the surface of the rings when the unit is rotating. The combination of these rings and the moving air, keeps the rings clean. You would also notice a slight discoloration on the rings, where the brushes ride. This is an oxide film and is good, in the sense that it improves electrical conductivity between the ring surface and the carbon brushes.

Figure 8-62 Collector Rings



T.

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The brushes are made of carbon and are contained in magazines like that shown in Figure 8-63. A number of these magazines fit into the brush holder rigging that surrounds the collector rings. This magazine uses a coil type spring to hold the brushes in contact with the collector rings. These springs are a significant improvement over the more conventional helical springs which are used in older GE and Westinghouse brush holder designs like that shown in Figure 8-64. This is because the coil type spring produces a constant pressure on the brushes regardless of brush wear.

CARBON

S~USf-!ES

Figure 8-63 Brush Magazine

o o Figure 8-64 Collector Brush Holder Rigging (GE)

r _1.1 ~ 8-36


GENERATOR CONSTRUCTION In Figure 8-65 we see the complete circuit.

1. 2. 3. 4. 5. '

Current enters from the + excitation source via the outboard brush rigging onto the collector ring surfa,ce. . ' . ' . . .This current then goes onto the axial lead (or bore copper) and passes inside the machine to an internal terminal stud. The current now goes through the rotor windings. Current exits on the other side through a rotor lead, terminal stud, axial lead (bore copper), terminal stud, to the other collector ring. The excitation culJ'ent then returns via the other set of carbon brushes to the - excitation source.

Figure 8-65 Collector Rings ami Brushes The force developed by the helical spring is dependent upon the spring compression which in turn depends upon the length of the individual brushes. Thus, as the brush wears and becomes shorter and the force developed by the spring is reduced. The helical spring tension must be adjusted to restore the proper spring tension as the brush wears. If the spring tension is allowed to deteriorate below the minimum acceptable, the brushes begin to loose contact with the collector and arcing between the brushes and collector results . This roughens the surface of the collector and results in more loss of contact, more arcing and further deterioration. Another means of getting DC power to the rotor windings is from an rotating exciter as shown in Figure 866. Notice the exciter rotor, it is a rotating armature winding. It rotates inside the path of a stationary field, created by the exciter stator. The AC output is directed to the rotating rectifier wheel shown to the outboard. Once rectified, the DC power is transferred along the rotor bore to the generator, where it connects to the main generator rotor windings.

Figure 8-66 Rotating Rectifier

STEAM TURBINE-GENERATOR FUNDAMENTALS -TG201 The rotating rectifier wheel (on the brushless exciter) is shown in Figure 8-67 below.

Figure 8-67 Rotating Rectifier Wheel used on a Brushless Exciter


.v .8-38 T.


GENERATOR AUXILIARY SYSTEM Chapter 9

TERMINAL OBJECTIVE: To familiarize the reader with generator auxiliary systems, their components and functions.·

ENABLING OBJECTIVE: At the completion of this section, the student should be able to:

1. 2. 3. 4.

Given the identity of a generator auxiliary system, describe the system function. Identify the three functions of the gas control system. Identify the reasons why hydrogen is used as a cooling medium in large AC generators. Identify how stator cooling water is treated to maintain good insulating properties.

© 1999 - TG201J5.0_June09, Printed: l2/14/2010

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 TABLE OF CONTENTS 1.0

INTRODUCTION ..................................................................................................................................... 3

2.0

SYSTEMS ............................................................................................................................................... 3 2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.3 2.4

.1.1 . 9-2 T.

Gas Control System ..................................................................................................................... 3 Generator Shaft Seal System ....................................................................................................... 6 Introduction ............................................................................................................................ 6 Basic Principles ...................................................................................................................... 6 GE System .............................................................................................................................. 7 Westinghouse System ............................................................................................................. 8 Stator Water Cooling System .................................................................................................... 10 Stator Oil Cooling System ......................................................................................................... 11


GENERATOR AUXILIARY SYSTEMS 1.0

INTRODUCTION

In this section we will study four "auxiliary systems" needed for main generator operation. They are the gas control, generator shaft seal, stator oil cooling and stator water cooling systems. Two other systems are also needed; they will be mentioned but not studied here.

2.0

SYSTEMS

One system not studied here is the lube oil system. It also supplies the turbine and is studied in that part of the book. The lube oil system supplies the generator and exciter bearings and the exciter reduction gear ifthere is one. It also supplies oil to the generator shaft seal system. The other system we will not study is the cooling water system. This system is not usually considered part of the turbine-generator unit for maintenance purposes. The cooling water system supplies the hydrogen coolers, the Alterrex generator air coolers (if any) and heat exchangers used to cool generator shaft seal oil and stator water cooling system water.

2.1

GAS CONTROL SYSTEM

As we have already learned, large modem generators are sealed up and filled with hydrogen gas, which is circulated by fans to cool the insides of the machine. The warm hydrogen is cooled by water-filled heat exchangers built inside the generator frame. The purposes of the gas control system are to keep the hydrogen in the generator at the proper pressure, to keep it pure, and to permit removing the hydrogen and replacing it when needed. Hydrogen will bum and is very explosive when mixed with air, so the gas control system must be built to keep hydrogen and air from mixing together at any time. There are three main reasons for using hydrogen for generator cooling: 1. 2. 3.

It has better cooling properties than air or any other gas.

Windage losses are greatly reduced, compared to air cooling. Maintenance costs are reduced.

To point # 1. Hydrogen picks up and carries away heat much better than air or any other gas. A pound of hydrogen will carry fourteen times as much heat as a pound of air for the same temperature change. It also picks up heat from a solid surface 50 percent better than air. The ability of hydrogen to remove heat is also proportional to its density, therefore, the higher the pressure - the more the heat can be removed. To point #2. The density of hydrogen is 1I14th that of air at a given pressure and temperature. Thus, the windage losses ofthe generator rotor are greatly reduced with the use of hydrogen. Also, the power needed to drive the fans on the rotor is greatly reduced. To point #3. Maintenance costs are reduced because the inside of a hydrogen cooled generator stays clean and dry compared to an air cooled unit. The drawback of course is that hydrogen is explosive when mixed with air in the range of 4.1 % through 74.2% hydrogen in air, by volume. Approximately 30% hydrogen in air is the most explosive mixture.


T.

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The generator casing is built strong enough to limit (not eliminate) the destructive effects of an explosion in the generator enclosure. However, with the proper precautions, hydrogen explosions will not occur. The obvious safeguards to the prevention of a hydrogen explosion are: l. - Never permit an explosive mixture to exist, and

2.

any possible source of ignition.

Item 2, the elimination ofsources of ignition, should always be stressed and all corresponding rules should be followed at all times. The part about never permitting an explosive mixture to exist leads us directly to the subject of the Gas Control System. When a new generator comes from the factory or wheri an older one is reassembled after an outage, it is of course filled with air. In order to operate, the air must be replaced with hydrogen. Since air and hydrogen cannot be mixed without danger, we will have to first remove the air and replace it with a gas that is not combustible with air or hydrogen, and do it in such a way that we do not mix the gases as we are replacing them. The gas used is carbon dioxide (C0 2 ). The process is relatively simple. We'll just replace all the air with the CO2 and then we'll replace all the CO 2 with hydrogen. To remove the hydrogen the order will be reversed: replace all the hydrogen with CO 2 and then replace all the CO 2 with air. Of course, there are some important procedures that must be followed and limits that must not be exceeded. Figure 9-01 shows the basic idea of changing from Air to CO 2 . As you can see, there are two distribution pipes located inside the generator casing: one on the bottom and one on the top. Since CO 2 is about II times heavier than air, we'll admit CO 2 very slowly in the bottom distribution pipe while the top pipe is connected to a vent to the atmosphere. We do this slowly to reduce the mixing of gases. We want a rising layer of CO 2 to force out the layer of air. We will continue this until there is no more air in the casing. Once all the air is gone, and this has been confirmed without doubt, we can begin to input hydrogen.

Changing From CO 2 to H ---------- I ~

J •

CO 2

"-

To ~ ....

Vent

......- H

--7 --

Figure 9-02 Changing From CO2 to Hydrogen (H)


Changing From Air to CO2 -

'. :.

J~

• : ..

~:

•• t .. ·1

.:

.. :..

,- -

1

I..

.....

L~

Air To Vent

Figure 9-01 Changing From Air to CO2

Hydrogen is much lighter than CO 2 , (only about one twenty second as heavy) so to apply the same principles we will admit hydrogen from the top and allow the CO 2 to escape from the bottom. This, of course, will require some valve adjustments. Figure 9-02 shows the basic idea of changing from CO 2 to Hydrogen (H). With the necessary valving done, we can begin to input hydrogen, again at a very slow rate, allowing the upper layer of hydrogen to force out the lower layer of CO 2 . Once we have practically pure hydrogen in the generator we can close the vent line and pressurize the generator casing. Hydrogen pressure is always kept above atmospheric pressure so air cannot leak in. The highest rated pressure depends on the unit design but is usually at least 15 psig and may be as high as 75 psig.

GENERATOR AUXILIARY SYSTEMS When the generator is full of hydrogen and we want to remove it, the process is reversed. The hydrogen is released from the upper distribution pipe through the vent to the outside. When the pressure is down to near atmospheric, CO 2 is put in through the lower pipe until all the hydrogen is forced out.

HYDROGEN DISTRIBUTION PIPE ": ' ": ' ,,:_ ,,: . ,': ' ,':' ,': _ ,':_ ,' :, ,0: -

:

GENERATOR .. : .. : .. : : .: .. : .. : .. : .. : .. : . CO2 DISTRIBUTION PIPE ·

VENT

81B

SAFETY SPOOL PIECE

HYDROGEN FEED

74

Figure 9-02 shows the piping and valve arrangement. Valve81B is a plug valve that can have only two positions; the one shown, or connecting 81 A and 82. With a little study you can figure out how the valves would have to be "lined up" for each step of the operation.

People cannot breathe CO 2 ; they will suffocate. This means that CO 2 must be removed from the generator and replaced with air before anyone can work inside. This is done by connecting a dlY air supply to the CO 2 supply piping. There is usually a connection already there, used for leak testing. The gas control system valves are set exactly as they are for replacing air with CO 2 • Dry CO 2 air is blown through the generator for SUPPLY several hours with the vent valve (74) wide open.

ANALYZER

Figure 9-03 Valves ami Piping Arrangemel1t

The manhole covers in the generator end shields may then be taken off (taking care that there is no pressure in the generator). Temporary fans are then set up to blow air in one end of the generator and out the other. The atmosphere inside the generator should be tested before anyone enters, especially in the lower parts such as the high voltage bushing enclosure (sometimes called the "bathtub"). CO 2 weighs about 50% more than air so it tends to settle in the low spots. You might ask - why not put the temporary air supply into the hydrogen distribution tube at the top of the generator and vent the CO 2 out of the bottom, as is done when filling ·with hydrogen? To avoid the possibility of an explosion, air is never allowed to enter the generator through the hydrogen distribution tube. The gas analyzer, more appropriat.ely called the thermal conductivity gas analyzer, is an i.nstrument which measures . the amount of air in CO 2 or hydrogen in CO 2 • It is used to tell the operator what is happening while the gasses are being changed. The scale reads in percent, 0-100% air in CO 2 , and 0-100% H2 in CO 2 . It works on the basis that different gases have different thermal cbnductivities. The thermal conductivity of the mixture bftwo gases varies from the conductivity of one gas to the conductivity of the other gas. The conductivity of the mixture at any moment in time is dependent upon relative quantities of the two gases. Electrical current is simply introduced to a filament that is exposed to the sampled gas. The temperature rise of the filament will be a function of the thermal conductivity of the gas concentration. This sample, of course, is drawn off the vent pipe near the generator as shown in Figure 9-02.

9-5~


'm

STEAM TURBINE-GENERATOR FUNDAMENTALS - TG201 The Hydrogen Control Panel is usually located one or two floor levels below the generator. It looks much like any electrical control panel, about three feet square and seven feet high. Tubing or small diameter pipe runs from it to various parts ofthe gas control system. The hydrogen control panel contains gauges and alarms as listed below. • • • • •

Machine Gas Pressure Gauges Machine Gas - Pressure (High & Low) AlarmPressure Switches Differential Fan Pressure Gauge Gas Analyzer Gas Analyzer Flowmeter Gas Analyzer Filter Dryer

The hydrogen control cabinet also contains signal lamps and alarm horns which are used to indicate trouble. Access to the gauges and the alarm system is through doors in the back of the control cabinet. Pipes lead to and from a gas dryer outside the generator. Hydrogen continuously passes through a drying agent which absorbs any water vapor that may be present. Pipes lead from all the low points in the generator casing to a liquid detector on the floor level below. Because of the sensing line location any liquid in the generator frame will flow to and collect in this detector. A sight glass allows the operator to see anything in the detector. A float switch monitors the level of liquid within the detector and will cause an alarm should liquid collect.

2.2

GENERATOR SHAFT SEAL SYSTEM

2.2.1

Introduction

The purpose of the generator shaft seal system is to seal the clearance space where the generator rotor passes out through the end shields, so that hydrogen cannot leak out. The system is sometimes called the "seal oil system." There are some basic principles that are used on all systems. We will explain these first. Then we will study the GE system and the Westinghouse systems in tum, pointing out how the basic principles are used but with different construction details.

2.2.2

Basic Principles

•

The seal itself is a film of oil that fills the clearance space between the moving rotor shaft and the surrounding stationary part. If the clearance space is full of oil with a pressure greater than the hydrogen pressure, the hydrogen cannot leak out.

•

The clearance is actually between the rotor and stationary "seal rings" which fit accurately around the shaft with an even clearance of several mils (thousandths of an inch) all around. The rings are of various designs but all have a soft metal surface facing the shaft.

•

The seals are a few inches inboard (toward the middle of the generator) from the generator rotor bearings. The bearings are outside the hydrogen space. Since hydrogen pressure can change, the oil pressure must also change, being automatically regulated at a set pressure above hydrogen pressure.

•

Oil flowing to the seals must be very clean because ofthe close clearances.

GENERATOR AUXILIARY SYSTEMS

2.2.3

•

The oil temperature must be kept within limits so that the viscosity is correct.

•

Most of the oil pumped into the clearance flows outward, toward the bearings. This oil has not been in contact with hydrogen and is at atmospheric pressure, so it can mix with the bearing drain oil and drain away with it.

•

The rest of the seal oil flows inward, into the inside of the generator. It is exposed to hydrogen and is under hydrogen pressure. Hydrogen easily dissolves in oil. This means that the "hydrogen side drain oil" must pass out through a drain arrangement that will not allow hydrogen to come out with the oil,· and it must be treated somehow to get most (ifnot all) of the dissolved hydrogen out of it.

•

Air is also easily dissolved in oil, as is hydrogen, and it can also be entrained in oil as foam or bubbles. Oil may also contain water droplets. Any air or water that flows to the gas-side of the seal will get into the hydrogen inside the generator, so that over a long period of time the hydrogen will become contaminated. All systems must have some way to overcome this problem.

•

There must be backup equipment that will work automatically, because a failure of the seal system would allow hydrogen to blowout through the clearance space. This might create a serious danger of Are or explosion. Also, there must be instruments, alarms and controls so the operators will be warned of trouble and can take care of various routine and emergency operating situations.

GE System

The GE system has a single oil supply; all the oil that goes to both sides (hydrogen and air) of both seals (each end of the generator) comes from the same place. On all but the smallest units the seal oil is "vacuum treated" so air or water will not be carried into the generator with the gas-side oil flow. With this background, we can trace through a simplified diagram of the GE system, Figure 9-04. Begin at the vacuum tank. The tank is under vacuum drawn by the seal oil vacuum pump. It is evacuated so there will be no air or water vapor contained in the oil. Oil enters the tank through spray nozzles so the air and water can escape easily. Oil for the sealing system is taken from the vacuum tank.

_~,--->~S""HA",L ~~~;~~:: SIDE

, . . - - ' - - - - - , HYDROGEN DETRAINING SECTION

1----,----' BEARING 011. DRAIN

FLOAT

Now, following the primary oil path only, oil is pumped from' the vacuum tank by the main seal oil pump. The oil flows into a "differential pressure regulating valve" where the oil pressure is regulated automatically, and then through a cooler and a filter. The oil pressure will be maintained about 5 psig above the machine gas pressure and will vary with the generator gas pressure. Continuing on, the oil flows to the shaft.

TRAP

VACUUM VANK

VACUUM PUMP

Figure 9-04 GE System Schematic

9-7

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The shaft seals are stationary with a close clearance to the rotating shaft. The oil flows into the clearance space between the seal rings and the shaft. Since the oil pressure is greater than the machine gas pressure it can flow .through the clearance space toward the inside of the generator casing. As long as there is oil flowing in, the gas cannot flow out. We, therefore, have a seal. The oil flows in two directions along the shaft, toward the gas side and also toward the air side. The gas side oil, after flowing through the seals, leaves the generator and is collected in the hydrogen detraining section. This oil, remember, is under machine gas pressure. The oil then flows to the float trap which allows oil to flow through, but not hydrogen. The air side oil flows into an air detraining section along with the lube oil draining from the bearing which is just outside the seal. This oil is under atmospheric pressiIre. Most of the oil from the air detraining section flows back to the main lube oil tank, but there is also a connection to the float trap tank. Oil from the air detraining tank combines with oil from the hydrogen detraining tank in the float trap tank. Together the two oils return to the vacuum tank. Now, going back to the seal oil pumps, you will see an emergency seal oil pump that will back up the main seal oil pump. The emergency seal oil pump is driven by a DC motor. We can now ensure a seal even with a loss of AC power. The emergency seal oil pump takes suction from the makeup to the vacuum tank. As a further backup, if we should lose both pumps, bearing header oil would pass through check valves permitting a hydrogen seal at low pressures. The operator would need to reduce hydrogen pressure to approximately 8 psig before a seal would be accomplished. .

2.2.4

Westinghouse System

The Westinghouse system has two separate oil supplies, one for each "side" of the seal rings, as shown in Figure 9-05. The hydrogen side oil is recirculated so there is practically no "new" air or water vapor to get inside the generator with the gas side oil flow. For this reason, it is not necessary to "vacuum-treat" any of the oil.

TOH2 SIDE SEALS

TO AIR" SIDE SEALS

COMBINED BEARING otL AND AIR SIDE SEAL OIL DRAIN

.E.~\·~C

~--~~--~~~~~~ SEAL

DRAIN

Begin with the air side supply at the air-side seal oil pump. It takes its suction from the generator bearing oil drain loop seal. The discharge flows through a cooler and a filter, and then continues to the air-side seal. Air-side drain oil flows from the seal ring and into the drain line loop seal we have just mentioned. Now, following the hydrogen-side oil path only, oil is pumped by the hydrogen-side seal .oil pump through a cooler, a filter, and on to the hydrogen-seal seal. Hydrogen-side oil drainage collects in the hydrogen-side drain regulator. It is from this tank that the hydrogenside seal oil pump takes its suction. The hydrogen-side oil pressure will be maintained 12 psi above the hydrogen pressure. . The air-side oil pressure is held at 12 psig above the hydrogen pressure by an automatic


Figure 9-05 Westinghouse System Schematic

GENERATOR AUXILIARY SYSTEMS regulating valve. Then "equalizing valves" (not shown) at each end of the generator automatically adjust the hydrogen-side oil pressure to equal the air-side oil pressure, within a small fraction of one psi. In this way, there is practically no mixing of the two oil supplies. The air-side system is the primary pressure seal. The hydrogen-side system merely separates the air-side. oil from the hydrogen gas. The hydrogen-side oil is constantly recirculated so it does not carry air into the seal. As noted, air-side drain oil mixes with the bearing drain oil, but the hydrogen-side oil needs "special handling." It flows first into a "defoaming tank" just below each bearing, where hydrogen bubbles rise out of the oil and the gas rises back up into the generator. Then the oil drains into a "drain regulator." Here there are two float valves. One opens on rising oil level and lets oil flow to the air-side oil pump suction. The other float valve opens on falling oil level and lets "new" oil in from the air-side pump discharge. In this way the oil level stays constant and seals the drain from the drain regulator, so hydrogen cannot escape. The drain supplies the hydrogen-side oil pump suction.

There is no backup for the hydrogen-side oil pump. If it should stop, oil from the air-side would flow to both sides of the seal. In time, this would cause air and possibly water vapor to get into the generator, but this would take quite a long time; it is not a serious emergency. There is a DC backup for the air-side seal oil pump. It will start automatically if the regular pump stops. Finally, there is a backup from the main oil tank of the lube oil system. The pressure available depends on the lube oil system pump arrangement and operation. One some units it may be necessary at times to reduce hydrogen pressure if operating on the lube oil system backup supply. The seal rings (called "gland seals") are shown in Figure 9-06.

Figure 9-06 Westinghouse Gland Seal Assembly

V .

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STATOR WATER COOLING SYSTEM

As we have already learned, some large modem GE generators have water-cooled stator windings. That is, water is pumped through the hollow stator-winding conductors to cool them. On some units the same water supply cools the silicon diode rectifiers in the Alterrex exciter system. The purpose of the stator cooling water system is to supply this water at the correct temperature, pressure, flow, etc. It is a self-contained system located in the basement .below the generator. The conductors may be carrying up to 20,000 volts, so something must be done to stop the water from causing a short circuit. Most people are surprised to learn that water is a good insulator if it is very pure. In daily life we seldom find water so pure, so we think of it as likely to cause electrical short circuits. The answer then, is to use very pure water in the statOr water cooling system, and to carry it to and from the windings in hoses made of plastic that is also a good insulator. The water is cleaned and kept clean by a "deionizer" that is built into the system. There is a filter and also, all piping, tanks, etc. outside the generator are made of stainless steel. Besides keeping the water clean, the system must cool it to carry away the waste heat, and it must pump the water through the system-at the correct flow and pressure. Also, backup equipment, alarms, instruments and controls are needed. Figure 9-07 is a schematic diagram of the system.

VENT OUTLET HEADER

INLeT HEADER

CE

TE GENERATOR

VENT

STRAINER

STORAGE TANK

TEMPERATURE REGULATOR MAKEUP

PRESSURE

REGULATOR

DElONIZER

Figure 9-07 Water Cooling System

GENERATOR AUXILIARY SYSTEMS One pump is normally running and the other is a backup that will start automatically if needed. The pump draws water from the storage tank and sends it to a pressure control valve (we control flow by controlling inlet pressure). Some water then flows into the coolers and some bypasses the coolers. The cooled water is then mixed with the uncooled water to the correct inlet water temperature. A filter and a strainer clean the water before entering the generator inlet header. Water enters the stator bars on the collector end, and exits via an outlet header also on the collector end. The water then returns to the storage tank. Not shown is a very small connection between the inlet and outlet headers of the generator. It prevents siphoning action, and it equalizes the pressure between the headers when the pumps are off. The vent line, also very small, helps the generator to self-vent. The storage tank vent line allows trapped gases to escape. Its loop seal collects any condensation. Beyond the cooler there is a loop allowing for continuous cleaning of the water. The water flows through a check valve, through a filter, into a deionizer tank, and back to the storage tank. Make-up water may also be added here. There is a control cabinet near the pumping unit that has instruments, alarms and controls to show the operators what is happening, to warn of trouble, and to allow them to control various normal and emergency operations. Some problems cause the turbine governor to "run back" automatically, reducing generator load to a safe value. As noted, there is a backup pump that starts automatically if water flow is too low. If it should also fail the generator load would be automatically cut back and if conditions did not soon return to normal, the turbinegenerator would be shut down.

2.4

STATOR OIL COOLING SYSTEM

Some older GE generators use oil instead of water for direct cooling of the hollow stator windings. The oil used is the same as is often used in transformers and other electrical equipment. It is a good electrical insulator. These systems are very much like the stator water cooling system just studied. The main differences are: Water and foreign material must be kept out of the oil, but extreme high purity is not needed as in the stator system. For this reason, there is no de ionizer and stainless steel piping is not needed. However, there are highquality filters. If oil leaked into the generator it would be very hard to clean up. For this reason, the oil pressure is automatically regulated to stay below the hydrogen pressure. Ifthere is a leak, hydrogen will go into the oil storage tank. A pump, much like the one used in the generator shaft seal system, draws a vacuum on the storage tank and exhausts any hydrogen to the outdoors. The vacuum will also draw off water vapor.

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