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Show The outcome of the Hazop led to the process and instrumentation diagram (Fig.3) being confirmed as the operating method. The control system is designed to allow the burner to be used only when certain process criteria are met. The controlling function is carried out by a Symax Programmable Controller which not only provides safety interlocking, but also ensures that the Macawber system is kept supplied with pulverised fuel. The pulverised fuel is produced on site using an attritor system and is then transported to the Macawber system using air which is stripped out in the filter silo, leaving pure pulverised fuel in the hopper. Depending on the level of pulverised fuel in the hopper, the control system speeds up or slows down the raw coal feed into the attritor - ideally the rate of feed into the hopper will match the rate of supply to the burner thus keeping the hopper level constant. Due to the requirement not to use the coal filtration plant intermittently, it was decided that all pulverised fuel would be routed through the Macawber system and via the Air Product's control system for both air-pulverised fuel and oxygen-pulverised fuel firing. A nitrogen purge and blanketing facility is included to inert the hopper, silo and pipeline when the system is not in use, even if predominantly empty of pulverised fuel. The nitrogen purge and blanketing sequence will also occur in the event of an increase in silo temperature or an explosion. Unlike many of Air Product's oxygen-fuel systems there is no 'mimic' panel to show the state of various interlocks and alarms. In the place of a 'mimic' panel the control system utilises a Symax 'loader monitor' and printer. Due to the complex nature of the control system - partly controlling the pulverised fuel transport system as well as oxygen, gas and pulverised fuel safety interlocks - a 'mimic' panel would have been approximately 2m by 1.5m with 52 lights showing the state of various interlocks and alarms. It was felt that a display of that nature would be difficult to understand and so an alternative method of informing plant personnel of the system status was sought. The Symax programmable controller can be connected to a 'loader monitor'. This, as it's name implies, enables data to be loaded into the controller and also registers within the controllers memory to be monitored via a keypad and LED display. By using a text processor in conjunction with the 'loader monitor' a certain value within the controllers memory may be output to the display as a message, e.g. 'system running', 'emergency stop pressed'. The messages enable the nature of any faults to be displayed clearly and aid in the quick identification of problem areas. Any faults or unusual occurrences are also printed on a paper roll printer located on the front of the control panel. The date, time and nature of the fault are printed, this enables plant personnel to see clearly the reason for shutdown even if the fault has subsequently righted itself. The system has the ability, with some modification to the control program, to give fully proportionating oxygen-fuel firing. As currently operating the system has two states, high and low fire, the firing rate is controlled by outputting a signal to gas and oxygen control valves or the Macawber screw feeder. The actual flows are Page 7 |