Control system integration projects: Petrochemical
See five lessons learned for a boiler control system upgrades, based on a system integrator’s experiences in the petrochemical industry.
Petrochemical control system integration projects have their own set of challenges. Control Engineering asked a system integrator to share advice from a recent project.
Edward Watkins, project engineer, automation solutions at Maverick Technologies, provided answers on a project ranging from the scope, challenges, and resolutions. He also offered five lessons for a future petrochemical combustion control project.
Control Engineering (CE): Please give a brief description about a recent successful petrochemical industry automation, controls, or instrumentation system integration project for your firm.
Watkins: After a minor safety incident, a chemical manufacturer needed an upgrade to its boiler control system to prevent future safety incidents from happening. The entire project required design, implementation, startup support and in-depth onsite training of the new boiler control system.
CE: What was the scope of the project and goals?
Watkins: The project’s scope was to upgrade the existing parallel positioning combustion control system to a fully-metered, cross-limited control strategy. The goals were to design and implement a combustion control configuration to improve the boiler control system’s reliability to mitigate risk and ensure a more efficient and safer operating environment.
CE: What types of automation, controls, or instrumentation were involved?
Watkins: The distributed control system (DCS) was configured and combustion control strategy diagrams were developed in Scientific Apparatus Makers Association (SAMA) format, along with digital logic diagrams for the combustion control interlocks to make configuring, characterizing and troubleshooting easier for the project team. The fully-metered, cross-limited control strategy with O2 trim ensured an oxygen positive environment for safe combustion and optimum efficiency over the entire load range. Revisions were also made to an operator interface graphic screen for the boiler control system. [Note: SAMA became the Laboratory Products Association in 2011.]
CE: What were the project challenges?
Watkins: There were two main challenges:
- The boiler uses natural gas and a liquid fuel oil waste stream from the chemical process. During normal operations, the fuel oil is base loaded, and the remaining boiler steam demand is made up with natural gas. Natural gas fuel demand had to account for changes in the British thermal unit (BTU) contribution from the fuel oil, particularly during oil-fired starting and stopping. Air and both fuels had to be characterized to ensure an oxygen rich combustion environment over the boiler’s load range to prevent a repeat of the previous safety incident.
- Boiler configuration is demanding so the customer needed help in developing the combustion control system (CCS) SAMA documentation, which was crucial to ensuring the project’s success.
CE: How were those issues resolved?
Watkins: Those challenges were resolved in two ways:
- Generic boiler vendor-supplied combustion control SAMA and Boolean logic diagrams were modified to identify specific control system function blocks by tag name and their function in the control strategy. This added a significant amount of detail to the typical boiler control diagrams but makes for a much more complete documentation package.
- For the SAMA documentation, a control narrative outlining features and functions of the desired upgraded system was created and used as a guide to develop the new control strategies and an organized input/output (I/O) list of the DCS.
CE: Would you share some positive metrics associated with the project?
Watkins: Key project metrics included:
- The boiler was able to perform load swings from 30 to 100% and back while maintaining a surplus oxygen environment.
- Characterizing air to fuel allows the flue gas O2 trim controller to maintain target setpoint based on current boiler steam load for most efficient operation.
CE: What were the resulting lessons learned you’d like to share?
Watkins: Five points of advice follow for a petrochemical combustion control project.
- Industry best practice is to implement a fully metered, cross-limited combustion control strategy for boilers and fired heaters.
- Air flow metering is particularly challenging and proper flow element installation is critical to success. It is common to display air flow as a percentage, but this percentage of total air flow must be based on actual or standard cubic feet per minute for the correct stoichiometric combustion.
- Post-startup combustion testing of the boiler is critical and time consuming. The boiler characterization process is used to establish the air to fuel relationship over the load range of the boiler and results in target O2 curves for the boiler and the fuels being fired. It is important to have an independent combustion analyzer in addition to the boiler flue gas oxygen meter.
- Factory acceptance tests (FAT) and site acceptance tests (SAT) need to be well-planned, documented and executed.
- All involved parties need to understand and accommodate budget, schedule, resource planning and production to allow the boiler to be operated over its entire load range to gather sufficient data to generate good O2 curves.
Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, firstname.lastname@example.org. Maverick Technologies is a CFE Media content partner.
KEYWORDS: Boiler control system upgrades, system integration
A parallel positioning combustion control system can be upgraded to a fully metered, cross-limited control strategy.
Characterization of air and two fuels are needed to reduce risk.
Running the boiler over its full range was needed to generate good boiler flue gas information.
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