Low-frequency pulsation from a package boiler

A high-amplitude, low-frequency pulsation was emitted by a package boiler under certain operating conditions. Preliminary testing showed that the pulsation was at approximately 9 Hz and was only present when the firing rate of the boiler decreased below approximately 50% to 70% capacity. A series of dedicated measurements was made to identify the source of the pulsation, including measurements with microphones, accelerometers, and pressure transducers. Ultimately, the test team concluded that a thermoacoustic resonance was causing a feedback loop that linked the flame output of the boiler to the acoustic cavity in the furnace and duct system. A stack damper was installed to modify the acoustic properties of the duct system, and the pulsation was mitigated.