| Description |
Industrial gas-fired boilers, furnaces and heaters occasionally encounter low-frequency vibrations generated by dynamic feedback between the burner (or burners) and acoustic modes in adjacent cavities in the main combustion chamber or ductwork. Feedback occurs when pressure pulses associated with acoustic resonances propagate to the burner so that they are in phase with combustion rate fluctuations. When the combustion and acoustic fluctuations become sufficiently phase-synchronized, normal sources of dissipation may be insufficient to dampen the combined pressure waves, resulting in amplifications that may reduce thermal efficiency, increase emissions, and eventually cause structural damage. In the literature, such oscillations are referred to as thermoacoustic oscillations or ‘rumble,' and their basic physics have been the subject of numerous investigations for well over a century. Although it occurs relatively infrequently, rumble poses a significant challenge because it is difficult to predict, diagnose, and resolve. The underlying relationships involved are sufficiently complex that it is possible for two apparently identical boilers or furnaces to exhibit completely different rumble tendencies. In previous work, we reviewed common sources of rumble and how nonlinear signal analyses, such as bivariate mutual information and transfer entropy, could be used to locate both its sources and impact in boilers, furnaces, and heaters. Thermoacoustic vibration is more difficult to predict than flow induced vibration due to the complicated resonance coupling of the burner and combustion cavity. Often, vibration concerns can be addressed by minor adjustments to burner settings, damper settings (acoustic damping), or to the furnace enclosure stiffness to reduce vibration amplitudes. However, these changes often can only be identified through a time-consuming trial and error approach. In previous papers, [Flynn (2017), and Flynn (2018)], the current knowledge about the causes of thermoacoustic vibrations in industrial natural-gas-fired furnaces and boilers was summarized, and opportunities for enhancing diagnosis and remediation were identified. Herein, we review pertinent references, discuss the issue of thermoacoustic-induced vibrations in more detail for gas-fired boilers, and apply some of the previously suggested nonlinear analysis techniques, such as bivariate mutual information and time irreversibility. Dare [(2018)] performed analysis on signal data collected from a gas-fired package boiler to characterize thermoacoustic vibration. Whelan [(2019)] suggested a combination of a simplified analysis and practical mitigation techniques to address thermoacoustic vibration. This paper will further describe an improved method to either predict the potential of thermoacoustic vibration or diagnose the root cause on an existing heater or boiler. An approach is suggested to address thermoacoustic vibration on commercial natural gas package boilers using a combination of acoustic finite element analysis (FEA) modeling and analysis of field data. Experience and field data from past episodes of thermoacoustic vibration are integrated into the acoustic FEA modeling. |
