Investigation of a combustion driven oscillation in a refinery flare. Part A: Full scale assessment

An assessment of the cause of an intermittent combustion-driven oscillation in a full-scale refinery flare is reported. When present, ambient sound pressure levels in excess of 100 dBA are generated. The oscillations began after the replacement of an old flare tip with one of a different design which required the use of non-standard components. The assessment included measurements of time resolved and integrated sound pressure in the ambient environment, and pressure in the fuel pipe and the air duct. Images obtained from video recorded from two directions are presented along with relevant process data. A review is also presented of the conditions under which the oscillations are inhibited or enhanced, and a comparison is made with another flare of similar design which does not exhibit an oscillation. The Strouhal numbers of the potential causes of flow oscillation and the wavelength of acoustic resonances in the supply pipes are calculated. The findings are then compared with related investigations found in the literature.The frequency of the oscillation was found to scale with the speed of sound in the fuel and also to match a resonant frequency within three consecutive segments of the fuel supply pipe. In contrast, the speed of sound in air was constant for all tests, while the frequency varied. This shows that the resonance occurs within the fuel system. The frequency also scales approximately with the fuel flow-rate, although more poorly than with the speed of sound, and matches a vortex shedding deduced to be present within the fuel side of the flare tip. In addition the frequency also matches the fundamental vortex shedding frequency of the air jet emerging from the tip. At the same time the visual appearance of the base of the flame was consistent with the air jet being driven at its fundamental mode. This suggests that the oscillations are caused by the coincidence of several coupling mechanisms. The acoustic resonance in the fuel pipe is deduced to control the frequency of oscillation and to amplify the pressure fluctuations. However the root cause of the oscillation is deduced to be a vortex shedding within the fuel supply. This causes fluctuations in the fuel flow rate which are amplified by the heat release. This creates positive feedback in further amplifying the pressure fluctuations in the fuel jet and the acoustic resonance.