Rethink control strategies

Once the appropriate air/fuel ratio is determined, a corresponding control scheme can be developed to translate the model into a usable application. This is not an easy process. The nonlinear gain position control strategy on the main combustion air and pass risk control damper is used to decoupled the fan speed control from the combustion air and flow pressure control. This allows automatic control of increases and decreases in fan speed. Lead/lag control is adopted, in which ventilation pressure control and combustion air flow control play a leading role to control ventilation and air flow in real time, and the inverter plays a lagging role. This control scheme uses effective decoupling of the primary and secondary loops.

A chemical company contracts with Wood to upgrade its burner management system (BMS) and combustion control system. These systems are outdated and do not meet the required standards. Use Wood’s standard reference library of design templates, API 556 control instructions, and Balanced Ventilation multi-burner heater control instructions to help rethink control strategies.

Dynamic compensation control of excess air, oxygen and carbon monoxide is required. Adaptive dynamic compensation is achieved by setting values based on the number of burners in operation and the number of burners out of maintenance. Since the air flowing into the heater through these burners cannot be used for combustion, unignited burners must be considered and compensated separately.

PCI-DIO-32HS

After completing the control scheme, the new design needs to be verified before the new application is installed on the real-time control system.

Given the complexity of the control strategy, it is necessary to use off-line factory acceptance testing (FAT) software to verify and confirm all characteristics, calculations, and control behaviors prior to approach. Install new configuration files during the planned overhaul, implement graphic modifications, perform field acceptance tests (SAT) and pre-commissioning inspections prior to heater commissioning.

Installation of chemical facility control system

After the team was satisfied with the heater control features, they moved the configuration to the chemical plant. The heater is fired, starting with a pilot run, until the ignition rate is lower than the design, and finally the design heat output is achieved through a new control strategy operating in cascade and automatic mode. Minor adjustments were made during the staged warm-up and “ramp and hold” start-up process to maintain cascade control throughout the plant restart and heat demand rise.

It took some time for operators to trust the new control scheme because they were used to knowing about coupling controls and frequent control trips. To the surprise of the operators, they are now able to use automatic control to turn up the heater to a heat load that was not possible before. Previously, heaters have never been able to operate all controls with cascading and automatic controls. Despite the many challenges, the project was successfully completed as planned.