Realizing that the boiler is part of a larger plant system, consideration must be given as to how the boiler output will match the load demand placed upon it by the system.
From a control perspective, it is useful to distinguish between electric utility and nonelectric utility (i.e., industrial) boiler applications.
The firing rate of industrial boilers is most often manipulated to control to a constant header pressure. The primary controller that accomplishes this, whose output is firing rate, is usually termed the boiler master.
In contrast, an electric utility boiler normally operates paired off as a single generation unit with a single turbine generator. A simplified structure for load demand control on a typical utility unit is illustrated in following figure:
The frequency input shown is sometimes necessary as an additional correction to prevent feedback control on turbine generator power (MW) or load demand (e.g., firststage pressure) from counteracting the turbine generator’s own power regulation response to frequency variations when connected to the electric power grid.
- The firing rate signal becomes the set point for the fuel and air flow controls that make up the combustion control system.
- Boiler outlet steam pressure is an indication of the balance between the inflow and the outflow of heat between energy supply and load.
- When more than one boiler is operated from the same master controller, the ability to individually bias and take control of each boiler should be provided, in addition to the ability to bias the master controller signal up or down when in automatic as shown in the following figure:
- Situations may arise when it is desirable to have either flow or pressure control. In these cases, a master control arrangement, as shown in the following figure:
- A flow control master can be used for firing rate control on each of multiple boilers
connected to a common header, with the flow controllers cascaded to the header
pressure controller, as shown in the following figure: