Power Plant Control and Instrumentation: the control of boilers and HRSG systems
Intended as a practical guide to the design, installation, operation and maintenance of the systems used for measuring and controlling boilers and heat-recovery steam-generators used in land and marine power plants and in process industries.
Inspec keywords: boilers; combustion; fuel systems; temperature control; power generation control; control equipment
Other keywords: water circuit; refurbishing system; combustion control; feed water control; boiler control; steam circuit; HRSG system; fuel gas circuit; control equipment practice; power plant control; steam temperature control; draught control; steam generation
Subjects: Thermal power stations and plants; Power system control; Thermal variables control; Control of heat systems; Control of electric power systems
- Book DOI: 10.1049/PBCE058E
- Chapter DOI: 10.1049/PBCE058E
- ISBN: 9780852967652
- e-ISBN: 9781849191630
- Page count: 240
- Format: PDF
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Front Matter
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1 The basics of steam generation and use
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Steam power is fundamental to what is by far the largest sector of the electricity-generating industry and without it the face of contemporary society would be dramatically different from its present one. We would be forced to rely on hydro-electric power plant, windmills, batteries, solar cells and fuel cells, all of which are capable of producing only a fraction of the electricity we use. Steam is important, and the safety and efficiency of its generation and use depend on the application of control and instrumentation, often simply referred to as C&I. The objective of this book is to provide a bridge between the discipline of power-plant process engineering and those of electronics, instrumentation and control engineering.
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2 The steam and water circuits
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In a conventional thermal power plant, the heat used for steam generation may be obtained by burning a fossil fuel, or it may be derived from the exhaust of a gas turbine. In a nuclear plant the heat may be derived from the radioactive decay of a nuclear fuel. In this chapter we shall be examining the water and steam circuits of boilers and HRSGs, as well as the steam turbines and the plant that returns the condensed steam to the boiler. In the type of plant being considered in this book, the water is contained in tubes lining the walls of a chamber which, in the case of a simple-cycle plant, is called the furnace or combustion chamber. In a combined-cycle plant the tubes form part of the HRSG. In either case, the application of the heat causes convection currents to form in the water contained in the tubes, causing it to rise up to a vessel called the drum, in which the steam is separated from the water. In some designs of plant the process of natural circulation is augmented by forced circulation, the water being pumped through the evaporative circuit rather than allowed to circulate by convection. This book concentrates on plant where a drum is provided, but it is worth mentioning another type of plant where water passes from the liquid to the vapour stage without the use of such a separation vessel. Such 'once-through' boilers require feed-water and steam-temperature control philosophies that differ quite significantly from those described here, and they are outside the scope of this book.
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3 The fuel, air and flue-gas circuits
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Having looked at the steam and water circuits of boilers and HRSGs, we now move on to examine the plant which is involved in the combustion of fuel in boilers. The heat used for generating the steam is obtained by burning fuel in a furnace, or combustion chamber, but to do this requires the provision of air which is provided by a forced-draught (FD) fan (in larger boilers, two such fans are provided). After the fuel has been burned, the hot products of combustion are extracted from the furnace by another fan, the induced draught (ID) fan, and fed to the chimney. Again, two ID fans are provided on larger boilers. In this chapter we shall examine not only the burners or other equipment used to burn the fuel but also the fans and air heaters. Finally we shall briefly examine how gas turbines are used in combined-cycle plant.
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4 Setting the demand for the steam generator
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In this chapter we have seen how a 'master demand' signal is generated in respect to the nature of the duties that the plant is designed to undertake. This signal is responsible for ensuring that the boiler reacts to changes in demand, and it must also co-ordinate the operation of each of the subsidiary systems. The main areas involved in this process are the combustion and draught systems, the feed-water system and the steam temperature control system.
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5 Combustion and draught control
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When considering fired boilers and heat-recovery steam generators it is clear that in the areas of their steam and water circuits there are many similarities between them (although the HRSG may have two or more pressure systems). But when the systems for controlling the heat input are examined, the two types of plant take on altogether different characteristics. The reason for this is fundamental: within the HRSG, no actual combustion process is involved, since all the heat input is derived from the gas-turbine exhaust (except where supplementary firing is introduced between the gas turbine and the HRSG). The subject of combustion control, which we shall be examining in this chapter, is therefore only relevant to fired plant.
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6 Feed-water control and instrumentation
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The objective of a feed-water control system may seem simple: it is to supply enough water to the boiler to match the evaporation rate. But as is so often the case with boilers, this turns out to be a surprisingly complex mission to accomplish. There are difficulties even in making the basic drum-level measurement on which the control system depends. The design of the control system is then further complicated by the many interactions that occur within the boiler system and by the fact that the effects of some of these interactions are greater or smaller at various points in the boiler's load range. The control-system designer's task is to develop a scheme that provides adequate control under the widest practicable range of operational conditions, and to do so in a manner that is both safe and cost-effective. To do this it is necessary to understand the detailed mechanisms of the feed-water and steam systems and to be fully aware of the operational requirements. In all but the smallest and simplest boilers, each of the interrelated factors has to be taken into account, and it is insufficient to rely on simple responses to the three parameters which seem to be relevant to the supply of feed water: steam flow, feed-water flow and the level of water in the drum.
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7 Steam-temperature control
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The rate at which heat is transferred to the fluid in the tube banks of a boiler or HRSG will depend on the rate of heat input from the fuel or exhaust from the gas turbine. This heat will be used to convert water to steam and then to increase the temperature of the steam in the superheat stages. In aboiler, the temperature of the steam will also be affected by the pattern in which the burners are fired, since some banks of tubes pick up heat by direct radiation from the burners. In both types of plant the temperature of the steam will also be affected by the flow of fluid within the tubes, and by the way in which the hot gases circulate within the boiler.
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8 Control equipment practice
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On an operational plant, the control systems that have so far been examined may be implemented in any of a variety of ways, ranging from pneumatics to advanced computer-based systems, but in all cases it should be possible to identify the various loops within the relevant configuration. These days, most control functions are implemented by means of a computer-based system, so we shall now briefly look at a typical configuration. After that, we shall examine some of the other hardware used in the systems and then consider the environmental factors that influence the selection of control and instrumentation equipment.
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9 Requirements definition and equipment nomenclature
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This almost completes this study of boiler C&I systems. However, updating and refurbishment needs to be discussed. This is a subject that has become important as the pace of electronics development has accelerated. As there is little sign of this trend levelling out, we must look at its implications for power-plant management, users and maintenance staff.
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10 Upgrading and refurbishing systems
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This chapter presents the upgradation and refurbishment of the control systems. Even the newest and most advanced of control systems eventually becomes dated. Up to about the middle of the twentieth century, C&I systems were mainly pneumatic, the pace of development was slow and it was generally possible to keep a system in good functional order, however old it may have been, simply by repairing the equipment or, if the worst came to the worst, buying replacement components from the original manufacturer.
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Back Matter
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