Improving Set Point Tracking and Process Profitability
Tightening control around Set Point will improve the performance and profitability of most every production process. This is especially true of desuperheater processes that are used by power companies to reduce variability in the temperature of steam that is sent to a plant’s turbines. Oscillations in the process can result in damage to the turbines, cause leaks, and affect heat rate. To address their challenge, this company sought a software solution that could handle the dynamics of a desuperheater. They found LOOP-PRO with its ability to accurately model transitional and oscillatory process data.
By tightening and stabilizing control over their desuperheater process, this power company successfully improved their overall heat rate while both reducing maintenance costs and increasing plant safety. Specific improvements to control touched on everything from Overshoot and Stability to Settling Time and Valve Travel. Each of these correlated directly with bottom-line profitability.
When a Picture Tells a Thousand Words
Desuperheaters – or attemperator – are designed to reduce the temperature of the super-heated steam. By controlling the temperature, the desuperheater protects units that have limited operating temperatures. When the desuperheater fails to temper steam correctly, even a single excursion can damage costly turbines and reduce the turbine’s efficiency.
For this end-user, persistent oscillations of 10-12 degrees within the desuperheater were a major source of concern. Due to concern for the turbines, the plant was unable to operate closer to the process’ upper constraint and to maintain an optimum heat rate.
Shown on the right are process strip charts associated with the end-user’s desuperheater process. As is clear in the graphic at the top, 10-12 degree oscillations of the Process Variable are mirrored by valve swings (i.e. Manipulated Variable) of roughly 30%. The persistent nature of the oscillations made manual tuning extremely difficult, and the lack of a steady-state made tuning nearly impossible for other PID tuning technologies.
Overshoot was eliminated altogether and the process’ Stability was increased 117%. Settling Time was also shortened from 239 minutes to 68 minutes – a 68% reduction. By reducing Valve Travel by 90%, the life of the desuperheater’s final control element was extended considerably.
The end-user performed a bump test and used the dynamic process data to tune the control loop using Control Station’s LOOP-PRO. Even though the bump test was performed in the middle of a transition, LOOP-PRO generated an accurate model of the process and provided much-improved tuning parameters. As is shown in the graphic at the bottom, oscillations of the Process Variable were reduced to an average of 2 degrees – an 80% reduction. The tightened control enabled the end-user to enhance the plant’s heat rate while improving plant safety.