This project qualifies as containing the largest PLC program that I have written. I based the program on SAMA logic diagrams (developed by Steve Greene) and P&IDs that contained 40 PID controllers, 25 bias stations, numerous velocity limiters & function generators, and several totalizers / moving averages / high-low selectors. To complicate matters further, there were four independent networks in use: Ethernet, ControlNet, DeviceNet (for the MCCs), and Modbus (for talking to the precipitator).
Thankfully this is not the old days (yet) but in yesteryears a PLC could only be programmed using ladder logic. While ideal for discrete programming, ladder logic is awkward for sophisticated process control applications. I and many others have made the best of what we had and so there are thousands upon thousands of process plants around that are well controlled from classic PLCs. Be that as it may, managing ladder logic becomes increasingly difficult as the number of control loops and complexities escalate. At some threshold back in the day one was forced into the DCS realm. These days, however, PLCs have the ability to be programmed in multiple styles or flavors one being function block (another is structured text - see my Gapper Servo project) which is a natural fit for process controls. For my mid-sized boiler project, I utilized these programming enhancements to effectively control what would have previously required a DCS.
This project had a boiler with a capacity of 500K PPH. The fuels for the boiler were bagasse (a bio-fuel byproduct of sugarcane processing) and oil. There was not enough time in the schedule for me to develop the graphics (iFix) so Scott Crider over at Advantage Industrial Automation took that task and did a great job. Steve Greene and I went to Florida and spent the winter of 2004 commissioning the new boiler. I must have enjoyed the weather too much because a few years later the experience of pleasant weather was counter balanced by a project in Iowa during the winter of 2008. Ouch!
The combustion air screen contains 20 air dampers.
The sootblower logic was written to allow the operator a choice of three sequences. Two sequences were fixed while one was configurable by the operator.
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