Reducing noise from an oil refinery cat cracker

Abstract

This paper concerns the noise from a Cat Cracker exhaust stack, used for the purpose of converting heavy oil into gasoline products. Following the upgrading of the Cat Cracker, there were persistent community complaints of an irregularly varying noise that sounded like an “overflying jet aircraft”. This paper describes a detailed study of the Cat Cracker noise involving: field tests on-plant and in the community; scale model tests in the laboratory; theoretical predictions using thin aerofoil unsteady aerodynamic theory; and a study of atmospheric propagation effects using the Parabolic Equation method. The objective of the study was to i) identify methods of reducing the noise levels, and ii) establish the cause of the irregularity of the noise level in the community. The laboratory tests used a 1/3 scale model to explore qualitatively the nature of any potential interaction between the two principal elements in the stack. The study concluded that the cause of the noise was an interaction between the turbulent flow from a valve and a Multi-Holed Orifice (MHO) downstream of the valve. The irregular variations in the noise where predicted to be atmospheric effects. Following the investigation the valve and the MHO were subsequently replaced by three MHO’s in series which gave a reduction in noise levels at the stack tip of up to 14dB. Noise measurements in the community demonstrated a similar level of noise reduction. In some weather conditions the Cat Cracker noise can still be heard, albeit at a much reduced level. Work is continuing to reduce the noise even further.

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