The figure above shows a P&ID of the relevant installation. The dryer uses steam to dry products. The amount of steam (further referred to as Disturbance Variable, DV) is controlled by another control system that is not considered here, but can be measured. The steam that passes through the dryer, is produced from two heat sources: gas from an electricity generating turbine, and, if those gasses are insufficient, 2 gas fired burners. These heat sources are controlled by valves, further referred to as MV1 and MV2, respectively. These valves need to maintain the boiler pressure at setpoint. Note that MV1 is the bypass valve opening, so that most heat is used for steam when MV1 = 0.
Figure 1: excessive pressure oscillations after a load change.
Figure 2 shows a block-diagram of the old old boiler pressure controller (PC1). It consisted of a PID controller, followed by a split range block. The controller was tuned (wrongly) with fixed settings, since the pressure responds different to MV1 than MV2.
Figure 2: block diagram of the old control system
Figure 3: new control system
The control system was implemented by CONET in a Siemens PCS7 computer control system. They prepared the implementation using the SIMATIC PCS 7. CONET used a simulation model of the relevant process (digital Twin) that acted as an OPC client and communicated via the Siemens OPC server with the SIMATIC PCS7, (just like the PID Tuner).
Figure 4 shows simulation results with the new control system. Initially, the burners are active, since the bypass valve is already completely closed. After the steam load drops at t= 400 seconds, the burners are switched off and the bypass valve is used to control pressure. Clearly, the pressure variations, after a load change, are small, and dampen out quickly.
Figure 4: simulation with new control system (horizontal axis shows time in seconds)