3.5. Grate control systems, pulverised-fuel, fluidised beds and gas-fired boilers
A well adjusted boiler control system can correct the following situations:
Protective functions:
In the case that the respective technological values are exceeded, the control system warns the operator
with an alarm; if the operators do not manage to intervene, the system puts the boiler out of action.
Control functions:
Remote manual control of individual boiler drives and servo-drives; automatic starting and shutdown of
individual functional units.
Regulatory functions:
Keeping parameters of the output medium at constant set values; overall stabilisation of combustion.
The following regulatory circuits are standard for steam boilers:
- Furnace-draft control
- Water-level control in the boiler cylinder
- Air-fuel ratio control with correction of excess oxygen in waste gases
- Waste gases temperature control at the outlet
- Outlet steam temperature control
- Boiler output regulation to a constant pressure or the amount of steam at the outlet
Boiler control system conception is based on the division of the control system to the operator and the process part.
The process part of the control system is solved by a robust industrial programmable controller (PLC) that implements the protective, control and regulation functions. The PLC does not include any moving components (discs); the program is stored in battery backed RAM memory or in the EEPROM memory. The PLCs used for this part are the basis for the reliability of the whole system and are supplied by leading world manufacturers.
The operator part of the control system carries out acquisition, visualisation and boiler control and archiving of the values measured. The connection between the process and the operator part of the control system is made via a communication line. If requested, the control system can also be connected to a superior system, i.e. company computer network. In this way, the measured values can be made available in real time to other workers, i.e. for the purpose of energy balance.
The operator part is designed with regard to the character and scope of its application. For smaller applications and mainly automatic operations, it is possible to use the operator control panel which can be, with regard to its coverage, placed directly into the industrial workshop. The state of the technology is displayed on a text or graphic display and is controlled by means of buttons. For large applications with a permanent operator presence in the control room, the control and visualisation can be made by means of an ordinary office PC with a monitor and positioning device (mouse). The technology is displayed in the form of coloured mimic diagrams completed with the current measured levels of technological values. The control is made by "clicking" with the positioning device on the individual control elements displayed on the screen. The operator station also archives the levels of technological values, error messages and operators' interventions which make retrospective analysis of a shutdown or another problem much simpler. The possibility to display the levels of technological values in the form of graphs also enables the operators to correctly estimate future trends and to make sound decisions regarding possible regulatory intervention.
The application of the control system contributes to the greater stability of the output medium parameters and the resulting longer service life of the boiler and linked technologies too. What should also not be overlooked is the greater efficiency and smaller amount of emissions achieved by constantly keeping an optimum air-fuel ratio.
