I received a tip from Senior Control Engineering Consultant Eric Ascoli on our Control Performance team. It concerned a recent problem solving mission he had with a sugar mill in the state of Florida in the U.S.
One of the mill tandems at this sugar mill was having serious speed instability problems with all the steam turbine drives. The trouble was that quite frequently the turbines would overspeed during recovery after a bog-down due to abnormal increase in thickness of the bagasse bed when processing the sugar cane. The situation had always been present but it was becoming considerably worse and production was severely impacted due to frequent trips of the turbines.
Eric received a call from plant early and was there soon after a short drive. The Entech Toolkit was connected to process signals and the actuator of the poorest performing turbine. Data were collected while the operator executed tests, both in automatic and manual modes, and several turbine trips were recorded.
Eric’s analysis of the data from these tests indicated that the culprit was poor major loop tuning and an inadequate control strategy to address the range of operating conditions. Contrary to what was believed, the control valve, actuator and positioner were performing really well. With the problem now clearly identified the solution could be achieved. Unfortunately re-tuning and control strategy adjustments could not immediately take place given other operational priorities.
During an upcoming scheduled outage these control changes will be made. Once the mill restarts in September, members of the Control Performance team will perform a dynamic analysis of the whole train and correct the tuning using the Lambda Methodology which has proven to be very successful at stabilizing interacting control loops.

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