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STEAM TURBINES

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This article discusses the steam turbine and its major components.

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Introduction

Generators driven by steam turbines furnish the bulk of electric power generated today. The turbines are supplied with energy in the form of heat energy in the steam, and they convert this into useful mechanical energy. Because of this they are called "prime movers." Any engine that, in the sequence of energy transformations in the generation and use of power, first converts any form of energy into mechanical energy is called a prime mover. Of all prime movers, the turbine is the most flexible. This is due basically to the fact that it converts heat directly into rotary motion without any intermediate steps. Many other prime movers convert energy first into reciprocating motion and this, in turn, into rotating motion. It is this intermediate step, the reciprocating motion that inherently limits the size of such machines.

All parts of the turbine rotor move constantly and continuously without reversals of direction, thus avoiding the large alternating stresses inherent with the reciprocating masses involved in a diesel or any other reciprocating type of prime mover. This direct conversion of heat energy into rotating motion, without any intermediate motions, is the primary advantage of a turbine.

A second great advantage of a turbine relative to other prime movers is the fact that it delivers a constant and uniform turning force or torque to the shaft. This is important whether driving an electrical generator, centrifugal pump, fan, or axial flow compressor, all of which require constant power input if the speed is to remain constant.

There are two fundamental forms of steam turbines. One form is the impulse turbine (Figure 1), deriving its name from the fact that the rotating member is pushed around by the force of steam impinging on blades or buckets. The second form is the reaction turbine (Figure 2), so called because it is the reactive kick from steam in the rotating element that causes it to rotate. Commercial turbines do not look like these elemental models, but operate on the same basic principles.


Figure 1: Impulse Turbine


Figure 2: Reaction Turbine

If we understand that in an impulse turbine the rotor is maintained in motion by steam striking rotating buckets, that in the reaction turbine the rotating member derives its rotational force from the steam leaving the blades, and that all commercial turbines make use of one or the other of these principles, or a combination of them, we have a sufficient foundation to proceed with a further examination of the turbine. Note the few parts of a turbine. Basically all that is needed is an orifice, or nozzle, through which steam issues, and buckets mounted on the rim of the wheel.

Fundamentally, nothing else is needed for a workable power producing turbine. A casing is added to confine the steam, and valves are added to control the admission of steam to the nozzles. These valves are, in turn, controlled by a governor and more stages may be added to aid in efficiently using the energy in the steam. For various reasons, other modifications may be made, but basically, a turbine consists of only two elements: first, the nozzles, and second, the rotor.

Multistaging does not change the principle of operation (Figure 3). The only reason for adding stages is to increase the efficiency of the turbine at any given speed, and as any stage has its best efficiency under certain conditions of speed and pressures, it is usually necessary to multistage the turbine to obtain the high efficiencies required today. Why a single turbine wheel has its best efficiency under one set of operating conditions may be understood from a study of the elemental turbine (Figure 3).


Figure 3: Elementary Multistage Turbine and Boiler

If it is assumed that this simple turbine has a fixed pressure in its boiler, it follows that a constant flow of steam will issue from its nozzle and this steam will be traveling at a constant velocity. When the paddle wheel (or turbine rotor) is held stationary, the steam issuing from the nozzle strikes stationary buckets. But, under this condition, the rotor is not moving and, hence, no work can be done. It is the condition of maximum torque, zero speed, and zero work.

At the other extreme, consider the case where the speed of the rotor is the same as the speed of the steam. With equal bucket and steam speeds, the steam has no velocity relative to the bucket and can exert no turning effort. This condition, then, is one of maximum speed, zero torque, and zero work.

In between these two extremes, work can be done, for there will always be force exerted by the steam and the rotor will always be in motion. But, as the speed is increased from zero to the maximum, there will be a point where the product of turning effort and speed will result in the greatest work being done. This will be the point of best efficiency for that stage.

In actual practice, turbines are seldom applied to loads where the turbine can seek its most efficient speed. Usually, the turbine speed must be held constant, and this is done by a speed governor that adjusts the steam flow to the load to be carried. Structural limitations prevent turbines being built for usual commercial speeds with a single wheel large enough and efficient enough to use the energy available from most conditions of steam pressures, so another turbine is placed in series.

This is a true multistage turbine. Steam is generated in the boiler at high-pressure, issues from the first stage nozzle, and gives up a portion of its energy to the first stage wheel. The steam in the first stage shell is at a pressure less than boiler pressure, and this pressure will be reduced in each succeeding stage until finally the steam is exhausted. Note, however, that this turbine would be neither practical to be built, or to operate, with its integral boiler and separate shaft for each stage. Figure 4 shows how the turbine would look, redesigned with all wheels on a single shaft, and the boiler divorced from the turbine.


Figure 4: Elementary Multistage Impulse Turbine and Boiler

The final step in making the elemental turbine into a commercial turbine requires multiple nozzles of proper design and a change in the shape of the inefficient paddles to efficient buckets having curved entrances and exits. The resultant steam path may appear as shown in Figure 5.


Figure 5: Cutaway of Nozzles and Buckets of an Impulse Turbine

Turbine Design

Various types of turbines have developed to fit the many desired applications. Figure 6 illustrates these various types.

Basically, all turbines may be divided into two broad classes: condensing units, which operate at back pressures less than atmospheric and non condensing units, with back pressures above atmospheric. This division relates only to pressure at exhaust flange and not to what happens to the steam after it leaves the machine.


Figure 6: Turbine Types

Each class may be subdivided according to whether full throttle flow continues through the machine to exhaust or whether part of the steam is withdrawn from the unit after some expansion. Units of the latter types are referred to as extraction turbines. These may be further classified as simple extraction turbines or automatic extraction units. In the latter, a regulating valve gear is introduced to control the pressure at the extraction flange.

In the simple extraction turbine, sometimes known as a bleeder turbine, one or more stages have openings of fixed size through which steam may be withdrawn. Pressure of this extracted steam varies directly with throttle flow. Each successive stage is separated from the next by nozzles, which, in effect, constitute a series of fixed orifices. At any given steam flow, a definite pressure exists in each stage. At higher flows, stage pressures are higher and at lower flows lower. Introduction of an extraction connection at any stage adds another orifice so that some of the steam entering the stage continues through the turbine and some discharges through the extraction opening. Thus, pressure at the extraction opening is essentially the same as stage pressure, and since stage pressure depends on inlet flow, extraction pressure also varies with inlet or throttle flow (turbine load). For many applications, such as feedwater heating, extraction steam pressure variations can be tolerated. Where constant pressure of extracted steam is important, as in process work, some form of automatic pressure control is required. Machines so equipped are called automatic extraction turbines in contrast to the simple extraction types.

In an automatic-extraction machine, the section following the extraction opening is separated from the section ahead of it and the steam flow between them is regulated by a valve under automatic control. At the extraction stage, a condition exists similar to that in the simple extraction turbine, with total steam flow entering through what amounts to a fixed orifice and leaving through two openings. In this case, however, only one of the openings is fixed in size and the other is variable. The valves controlling steam admission to the stages following the extraction point, being regulated by stage pressure, hold stage and extraction pressure constant over a wide range of throttle flows. In the case of simple extraction units, openings ranging in number from one to four are determined by economic analysis of the overall cycle.

Automatic-extraction turbines may be obtained with either one, two, or three controlled openings, the number and pressure being set by process needs. Under some conditions, it may prove desirable to supply excess low pressure steam to a turbine in addition to the throttle steam. Turbines designed for this service are known as mixed pressure machines. If it is also desired to withdraw low pressure steam at times, an extraction unit is used. A number of other special designs have been developed. Where extremely high throttle pressures are employed for high cycle efficiency, considerable moisture may be present in the last stages unless throttle temperature is also extremely high. Under such conditions it may be advantageous to divide the turbine into two sections, passing steam exhausted from the high pressure section through a reheater to restore the initial temperature before expansion through the low pressure section. This is known as a reheat turbine.

Compounding

Impulse turbines may be compounded in two basic ways: velocity or pressure. If the first stage contains a row of nozzles followed by two rows of buckets, with a set of stationary buckets between them, this is called velocity compounding or Curtis staging. The remaining stages, known as "group stages" are pressure-compounded stages where the pressure drop is divided among a sequence of nozzles, each followed by its row of buckets. One row of nozzles and the row of buckets associated with it is considered a pressure stage. This total arrangement is typical of many straight impulse machines, a velocity compounded first stage followed by a number of pressure or diaphragm stages. The type and number of stages and blade proportions of commercial turbines depend, among other things, on inlet steam pressure and temperature, exhaust pressure, the speed, and the output.

Thus far, mainly the nozzles and buckets have been considered. These are the heart of any turbine, but a number of additional elements are needed to make a complete unit ready for power-plant application. There is a rotor and wheels to carry the buckets, and a casing or shell to confine the steam, support the stationary diaphragms, and provide a structural frame.

Turbine Components

The casing supports the main bearings and the thrust bearing, which maintain the shafts axial position. To minimize and control steam leakage, various seals or glands are needed at the diaphragm bores and at the ends of the casing. A lubrication system must be provided for the moving parts. To control steam admission, a stop valve or throttle valve, a steam chest, steam admission valves, valve gear, and a governor must be provided.For protection against excessive over speed, an overspeed governor and trip mechanism are provided.

Basically, all turbines may be divided into two broad classes: condensing units, which operate at back pressures less than atmospheric, and non condensing units, with back pressures above atmospheric. This division relates only to pressure at exhaust flange and not to what happens to the steam after it leaves the machine.

Each class may be subdivided according to whether full throttle flow continues through the machine to exhaust or whether part of the steam is withdrawn from the unit after some expansion. Units of the latter types are referred to as extraction turbines. These may be further classified as simple extraction turbines or automatic-extraction units. In the latter, a regulating valve gear is introduced to control the pressure at the extraction flange. In the simple extraction turbine, sometimes known as a bleeder turbine, one or more stages have openings of fixed size through which steam may be withdrawn.

Pressure of this extracted steam varies directly with throttle flow. Each successive stage is separated from the next by nozzles, which, in effect, constitute a series of fixed orifices. At any given steam flow, a definite pressure exists in each stage. At higher flows, stage pressures are higher and at lower flows lower. Introduction of an extraction connection at any stage adds another orifice so that some of the steam entering the stage continues through the turbine and some discharges through the extraction opening. Thus, pressure at the extraction opening is essentially the same as stage pressure, and since stage pressure depends on inlet flow, extraction pressure also varies with inlet or throttle flow (turbine load). For many applications, such as feed water heating, extraction steam pressure variations can be tolerated. Where constant pressure of extracted steam is important, as in process work, some form of automatic pressure control is required. Machines so equipped are called automatic-extraction turbines in contrast to the simple extraction types. A number of other special designs have been developed. Where extremely high-throttle pressures are employed for high-cycle efficiency, considerable moisture may be present in the last stages unless throttle temperature is also extremely high. Under such conditions it may be advantageous to divide the turbine into two sections, passing steam exhausted from the high-pressure section through a reheater to restore the initial temperature before expansion through the low-pressure section. This is known as a reheat turbine.

Typical Flow Path

Main steam enters the turbine at the bottom of the high-pressure shell via two separate stop and control valves. The flow of HP steam continues and exits the section via the cold reheat line where it returns to the HRSG (Figure 7).


Figure 7: Typical Flow Path

The reheated, intermediate pressure steam enters the center of the casing via the hot reheat piping and flows through the IP section in the direction opposite that of the HP section. This design results in an even temperature gradient from the center of the casing to the ends, as the highest temperature steam in the system enters at the center of the shell and then gradually reduces its temperature as it flows outward toward the end packing's and bearings. Variability in the steam path design is limited to the high-pressure section, with the HP staging customized for each application.

Staging within the HP and IP sections is based on low reaction design theory, which leads to the use of wheel-and-diaphragm construction. Rows of rotating blades, or buckets, are machined from blocks of 12Cr steel, using a pine tree dovetail design (Figure 8). These buckets are assembled tangentially on a rotor wheel and locked into place by the use of several specially designed closure buckets and by bands or covers.


Figure 8: Pine Tree Dovetail Bucket

Stationary blades, or nozzles, are also machined from 12Cr steel and are assembled in the outer ring and inner web portions of the diaphragm. The diaphragm sections (Figure 9) are then affixed in grooves in the upper and lower halves of the shell. The HP section was designed to accommodate up to 45% additional throttle mass flow based on the site-specific requirements for supplementary firing. Because of the fixed IP steam path and the variable range of reheater pressure drop, the cold reheat pressure varies within a certain range. Hence, this pressure variation requires some customization of HP staging for each application.


Figure 9: Diaphragm Section

With the fixed staging of the IP section, it became possible to closely control the HP/IP rotor design (Figure 10) in terms of forging size and bearing span. Rotor dynamic criteria have been thoroughly analyzed so that the relatively small steam path variations allowed in the high-pressure section do not require re-analysis of the design for each application.


Figure 10: HP/IP Rotor