A continuous aluminum sheet is applied with a coating containing flammable liquids, which then enters an oven for heat treating i.e. bake the material on the sheet. The resultant evaporated gas is fed into an incinerator as fuel. If the gas is not enough to maintain temperature in the incinerator, additional fuel is added. While the zones and incinerator are to maintain temperature, the furnace atmosphere must be controlled to maintain the flammable gases below their LEL (lower explosive limit). In addition, to meet environmental requirements, the zones must maintain a negative pressure to prevent fume leaks while the stack pressure must be maintained positive.
Since initial installation, the original objective of the system had been thwarted by the interrelationships of all the contributing factors, which the conventional PID controllers could not handle effectively. The result was that the main incinerator fan was running too fast, more make-up fuel to the incinerator was required than originally planned and product quality was affected, resulting in re-processing. The furnace uses a GE Series 6 PLC with Genius I/O. The LEL detectors are redundant and offer continuous readings (analog values).
The Control Objectives
The control objectives for this application are as listed below:
- Control Temperature in all five zones
- Control Incinerator temperature
- Control Stack Pressure
- Control LEL level
All the above objectives are highly interactive and sometimes conflicting objectives. Controlling the entire operation optimally while working within the given constraints was the biggest challenge in this process.
The Incinerator fan speed should be set to minimum and still maintain -0.2″ water column pressure in ZONE 1, 2, 3. There are eight (8) LEL sensors two in each ZONE 1, 2, 3, one in ZONE 4, and one at the input to Incinerator fan. The maximum LEL reading is used to control LEL level. (subhead) The QuickStudy™ Solution
Before installing the QuickStudy APC (Adaptive Process Controller) the process models were created offline using historical data. The QuickStudy APC connected directly to the PLC and was used to directly control the process.
- One predictive controller Set-Up (PCS) block was configured to control the overall LEL level by manipulating the Incinerator Fan Speed
- One PCS block was configured for each of the 5 temperature zones to control the zone
- temperature by manipulating the damper position
- One PCS block was configured to control the stack pressure by manipulating the stack damper
- One PCS block was configured to control the Incinerator temperature by manipulating the Fuel Valve
Once the controller was put online it was put into control-adapt mode briefly to further update the models. The goodness of the model identified was monitored and the controller was put into control only mode.
The previously cycling valves and temperatures were stabilized with minimum variability. The zone temperatures were controlled within tenths of a degree.
The following benefits were achieved:
- 98% reduction in startup scrap
- 70% reduction in Auxiliary Fuel. The valve went from cycling between 50 to 80% open to a steady 6 % open
- Zone temperatures controlled within +/- 0.1 degree from +/- 15 to 30 Degrees
- Minimum variability on key process parameters