PID controlling gas pressure of feed to burner
This case concerns a PID controller that controls the gas pressure in a pipeline feeding gas to burners. Plant personnel were wondering if this control loop could be improved; they felt that the pressure took a long time to settle to setpoint; as a result, the heat input to the plant (which they control by adjusting the setpoint) was harder to control.
T he PID Tuner was used to check the tuning of this loop. There were concerns by plant personnel that, during tuning, the gas pressure should not rise too much above setpoint, because of explosion risks.
„The new settings were downloaded to the PID controller and they worked out as expected: the pressure returned to setpoint at least a factor 5 faster. .”
Software and hardware setup
The control task was excuted by an Allen Bradley PLC. The PID Tuner software was installed on the same Windows PC that was used to program the PLC. The PID Tuner communicated with the PLC via the OPC server of Rockwell Automation (RSLinx Classic OEM).
Tuning procedure and result
The PID Tuner was set up to execute its tuning in a safe way: the tick box PVmax was used to ensure that control would resume in feedback mode should the pressure exceed a predefined limit. A short step experiment was executed by the PID Tuner, followed by a retuning of the PID controller. The figure below shows the comparison of the closed loop response with the 'old' tuning and the 'optimised' tuning. Clearly, the 'old' tuning was indeed slow compared to what it could be. The new settings were downloaded to the PID controller and they worked out as expected: the pressure returned to setpoint at least a factor 5 faster.
