Steam Systems
In systems that produce steam, the parameters that determine the load are the steam pressure and the steam flow rate. The header steam pressure, that is the pressure at the boiler, is often the master control parameter. The pressure is a direct indicator of the actual energy produced in a boiler. Some systems complement the pressure measurements with the steam flow rate to get a more accurate control signal. The flow rate of steam is used as a feed-forward signal to anticipate changing air and fuel control requirements. The measuring and monitoring of steam pressure also serves as an indication of safe operation, for example by raising alarms if pressure exceeds recom- mended levels in any parts of the boiler (Elliot, 1997).
To further improve the performance of steam boilers, the temperatures of inlet and outlet feedwater is measured. The information from these mea- surements is then used to further improve control calculations, as well as to indicate maintenance issues such as the need to clean heat transfer surfaces (Elliot, 1997).