A Complex 3D Acoustic Induced Vibration of a Low NOx Refinery Boiler

An increasing number of refinery and other large process steam boilers are being installed with or converted to low NOx combustion. The primary result of low NOx combustion is reduced emissions; however, this type of combustion can also lead to unsteady pressures (pulsation), rumbles, and vibrations of boiler components and walls. This paper reports on an investigation of vibrations that were observed on the walls of a new low NOx boiler during its commissioning. Dynamic measurements and 3D acoustic modeling were used to identify and define the fluid force (acoustic) induced vibration problem and to design baffles to eliminate the specific complex acoustic mode that was causing the vibrations. Vibration measurements were recorded at numerous locations on the exterior walls of the boiler so that variations in magnitude and phase of the wall motions at different locations could be determined. The mode shape of the boiler motion was mapped from this data. Pulsation measurements were made on pressure taps at several locations around the boiler. The pulsation data defined the amplitude and phase relationships of the combustion and flow induced acoustic responses within the boiler. A 3D acoustic model of the boiler was created with internal gas properties including the speed of sound based on temperature distribution within the boiler. The model results were compared to measured data and the complex mode of the troublesome acoustic response was identified. Simple symmetric centerline baffles had been attempted by the boiler supplier without eliminating or reducing the vibrations and noise of the boiler. Based on the complex acoustic mode shape of the pulsation within the boiler as identified by the model results that agreed with field data, specific extended baffles were designed and installed to eliminate the vibrations that had been observed. Smooth operation of the low NOx boiler over a wide range of normal operation not previously obtained was achieved with the new baffles installed. The process of dynamic measurements and 3D acoustic modeling to match field data and confirm the mode shape of acoustic responses within a boiler, as described in this paper, can be used to resolve vibration problems including complex situations that are not solvable by other means.