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.

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.

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.

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.

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.