Enhancing chemical company control system reliability, performance

System integration insights

• Redesigning control systems for any company is a challenge and system integrators often enter projects where they’re dealing with outdated technology that, while perhaps functional, is nowhere near what’s optimal today.
• In this case, much of the focus is on overhauling burner management systems (BMS) and combustion control systems, which have a strong impact on the chemical company’s productivity.
• A system integrator can help improve a company’s operations and make it more automated, but the main challenge is keeping up with these changes and being proactive rather than reactive to maintenance and automation repairs and upgrades.

In 2021, a global chemical company approached Wood’s Industrial Process Automation Team to deal with a dual-cell balance draft hot oil heater, which was disrupting production targets. The burner management systems (BMS) and combustion control systems originally provided by the original equipment manufacturer (OEM) needed an upgrade. The controls provided did not allow the heater to be operated automatically, leaving the customer with manual-only operation. As a result, the chemical company was facing frequent heater trips and an inability to meet design hot oil heating duty.

Creating control system upgrade project objectives

Schedule pressures were intense with a turnaround planned five months after the project start, which had the following objectives:

• Evaluate the design of the BMS and combustion controls relative to established industry best practice standards (e.g., API 556.)
• Evaluate the combustion control design and redesign as necessary to provide stable, automatic, air/fuel ratio control and oxygen trim to provide design heat duty.
• Develop and test the required revisions or replacement for the combustion control and BMS strategies.
• Implement the revised configuration in a scheduled turnaround and commission the controls.
• Provide operator and engineer training, including heater tuning.

Redesigning the BMS and combustion controls

The investigation determined the air/fuel and firing rate controls were not functional. Combustion controls had to take a “new design approach” and be completely redesigned as if it were a new heater design. The heater was complex requiring a delicate balance of automated and manual capabilities. The balanced-draft heater required the burners to be automated while the individual air registers had to be manually set. The original design used dampers and variable frequency drive (VFD) controls on the forced draft (FD) and induced draft (ID) fans. This created a control issue where both pressure-based loops were at odds with each other when trying to accurately control the air/fuel ratio. This would eventually result in heater trips, challenging the client to exclusively operate the dampers in manual control.

The existing complex control design and configuration was abandoned in favor of using a reference library of design templates, API 556 Control Narratives and balanced-draft multi-burner heaters control narratives.

The control design effort was split into two principal complex control strategies: (a) Fan and draft controls and (b) Air/fuel controls. New control schema and narratives were developed based on the base heater and burner design data.

Air/fuel ratio is critical to optimizing combustion efficiency. Too much air leads to lost energy; too little air leads to needless fuel waste. The ideal air/fuel ratio depends on the operating load and type of fuel being burned. To facilitate these needs:

• Combustion engineering and modeling was done for the air/fuel and firing rate controls, based on the provided heater and burner data sheets and fuel gas composition data

• Characterization curves and calculations were developed for implementation in the air/fuel and firing rate controls.

Rethinking the control strategy

Once the proper air/fuel ratio calculations were determined, control schemes were developed to turn the models into a useable application. This was no simple effort. Fan speed controls were decoupled from the combustion air and draft pressure controls using a non-liner gain position control strategy on the primary combustion air and draft control dampers. This allowed for the automatic control of fan speed increases and decreases. Lead/lag control was used, with the draft pressure controls and combustion air flow controls functioning as lead, to provide instantaneous control of draft and air flow, and the VFDs functioning in the lag role. This control scheme effectively decoupled the primary and secondary loops.

Dynamic compensation control of excess air, O₂ and CO was required. This was done through setpoint adaptation based on the number of burners in operation and burners out of service for maintenance. Unlit burners had to be considered, but compensated for separately, because the air flowing into the heater through these burners was unavailable for combustion use.

After completing the controls, the new design was verified as operable prior to installing the new application on the live control system.

Given the complexity of the control strategies, an offline factory acceptance test (FAT) software was considered necessary to verify and validate all the characterizations, calculations and control behavior prior to the site’s mobilization. The new configuration files and graphic modifications were installed during the scheduled turnaround, site acceptance test (SAT) and pre-commissioning checks were performed before the heater was commissioned.

A chemical company contracted Wood to upgrade their burner management systems (BMS) and combustion controls, which were out of date and not performing to required standards. Using Wood’s Standard Reference Library of design templates, API 556 Control Narratives and balanced-draft multi-burner heaters control narratives helped in rethinking the control strategy. Courtesy: Wood

Chemical facility control system installation

After the team was satisfied the heater controls were functional, they moved the configuration to the chemical company’s facility. The heater was lit, operated from pilot-only operation, through below-design turndown firing rates, and eventually to design heat output with the new control strategies operating in cascade and automatic modes. Minor adaptations and tuning changes were made during the staged warm-up and “ramp and hold” process startup, keeping the controls in cascade throughout the plant restart and heat demand ramp-up.

It took a while for the operators to trust the new control scheme because they had grown accustomed to the decoupled controls and frequent control trips without manual intervention. The operators were amazed how they were now able to turn up the heater to previously unachievable heat duty utilizing automatic controls. The heater had never been able to run with all the control in cascade and automatic before.

In spite of the challenges, the project was successfully completed on schedule.

Brad Bonnette is the technical director for Wood’s Applied Intelligence. Wood is a CFE Media and Technology content partner. Edited by Chris Vavra, web content manager, CFE Media and Technology, cvavra@cfemedia.com.

KEYWORDS: Burner management system, chemical manufacturing

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