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

Abstract

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 100dBA 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 inmore » 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. (author)« less

Authors:
 [1];  [2];  [3];  [1]
  1. School of Mechanical Engineering, University of Adelaide, SA 5005 (Australia)
  2. School of Chemical Engineering, University of Adelaide, SA 5005 (Australia)
  3. Shell Refining, Australia, Clyde Refinery, NSW (Australia)
Publication Date:
OSTI Identifier:
20711980
Resource Type:
Journal Article
Resource Relation:
Journal Name: Experimental Thermal and Fluid Science; Journal Volume: 30; Journal Issue: 4; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 02 PETROLEUM; FLARING; PETROLEUM REFINERIES; OSCILLATIONS; FLAMES; RESONANCE; SOUND WAVES; VELOCITY; SCALING LAWS; VORTICES

Citation Formats

Nathan, G.J., Mullinger, P.J., Bridger, D., and Martin, B. Investigation of a combustion driven oscillation in a refinery flare. Part A: Full scale assessment. United States: N. p., 2006. Web. doi:10.1016/j.expthermflusci.2005.05.009.
Nathan, G.J., Mullinger, P.J., Bridger, D., & Martin, B. Investigation of a combustion driven oscillation in a refinery flare. Part A: Full scale assessment. United States. doi:10.1016/j.expthermflusci.2005.05.009.
Nathan, G.J., Mullinger, P.J., Bridger, D., and Martin, B. Wed . "Investigation of a combustion driven oscillation in a refinery flare. Part A: Full scale assessment". United States. doi:10.1016/j.expthermflusci.2005.05.009.
@article{osti_20711980,
title = {Investigation of a combustion driven oscillation in a refinery flare. Part A: Full scale assessment},
author = {Nathan, G.J. and Mullinger, P.J. and Bridger, D. and Martin, B.},
abstractNote = {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 100dBA 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. (author)},
doi = {10.1016/j.expthermflusci.2005.05.009},
journal = {Experimental Thermal and Fluid Science},
number = 4,
volume = 30,
place = {United States},
year = {Wed Mar 01 00:00:00 EST 2006},
month = {Wed Mar 01 00:00:00 EST 2006}
}
  • A flow visualisation study was performed to investigate a periodic flow instability in a bifurcating duct within the tip of the flares at the Shell refinery in Clyde, NSW, to verify the trigger of a combustion-driven oscillation proposed in Part A of this study, and to identify its features. The model study assessed only the flow instability, uncoupled from the acoustic resonance and the combustion that are also present in the actual flare. Three strong, coupled flow oscillations were found to be present in three regions of the fuel line in the flare tip model. A periodic flow separation wasmore » found to occur within the contraction at the inlet to the tip, a coupled, periodic flow oscillation was found in the two transverse ''cross-over ducts'' from the central pipe to the outer annulus and an oscillating flow recirculation was present in the ''end-cap'' region of the central pipe. The dimensionless frequency of these oscillations in the model was found to match that measured in the full-scale plant for high fuel flow rates. This, and the strength of these flow oscillations, gives confidence that they are integral to the full-scale combustion-driven oscillation and most likely the primary trigger. The evidence indicates that the periodic flow instability is initiated by the separation and roll-up of the annular boundary layer at the start of the contraction in the fuel section of the flare tip. The separation generates an annular vortex which interacts with the blind-ended pipe downstream, leading to a pressure wave which propagates back upstream, initiating the next separation event and repeating the cycle. The study also investigated flow control devices with a view to finding a practical approach to mitigate the oscillations. The shape of these devices was constrained to allow installation without removing the tip of the flare. This aspect of the study highlighted the strength and nature of the coupled oscillation, since it proved to be very difficult to mitigate the oscillation in this way. An effective configuration is presented, comprising of three individual components, all three of which were found to be necessary to eliminate the oscillation completely. (author)« less
  • An assessment of the influence of strong combustion-driven oscillations on mixing rates and visible radiation in the flame from a full-scale refinery flare is reported. Importantly, the oscillations were generated naturally, with no external forcing, and at a high Reynolds number of 4 x 10{sup 6}. These conditions differentiate this study from those of previous investigations, which all involved some external forcing and were at a Re too low to ensure fully turbulent flow within the flame. A frame-by-frame analysis of video footage, providing good resolution of the instantaneous edge of each flame, was used to assess flame dimensions, andmore » so to determine a global residence time. Since the flames are in the fast-chemistry regime, the visual imagers can be used to determine a global mixing rate. The analysis reveals a consistent picture that the combustion-driven oscillations do not result in a significant change to the global mixing rate, but do increase the visible radiation. This is in contrast to previous investigations, using externally forced jets, where forcing at the preferred mode has been found to increase mixing rates and reduce radiation. (author)« less
  • The properties of fly ash in coal-fired boilers influence the emission of mercury from power plants into the environment. In this study, seven different bituminous coals were burned in a full-scale 100 MWe pulverized coal combustion boiler and the derived fly ash samples were collected from a mechanical hopper (MH) and an electrostatic precipitator hopper (ESP). The mercury content, specific surface area (SSA), unburned carbon, and elemental composition of the fly ash samples were analyzed to evaluate the correlation between the concentration of particulate-bound mercury and the properties of coal and fly ash. For a given coal, it was foundmore » that the mercury content in the fly ash collected from the ESP was greater than in the fly ash samples collected from the MHP. This phenomenon may be due to a lower temperature of flue gas at the ESP (about 135{sup o}C) compared to the temperature at the air preheater (about 350{sup o}C). Also, a significantly lower SSA observed in MH ash might also contribute to the observation. A comparison of the fly ash samples generated from seven different coals using statistical methods indicates that the mercury adsorbed on ESP fly ashes has a highly positive correlation with the unburned carbon content, manganese content, and SSA of the fly ash. Sulfur content in coal showed a significant negative correlation with the Hg adsorption. Manganese in fly ash is believed to participate in oxidizing volatile elemental mercury (Hg{sup 0}) to ionic mercury (Hg{sup 2+}). The oxidized mercury in flue gas can form a complex with the fly ash and then get removed before the flue gas leaves the stack of the boiler.« less
  • Experimental measurements as well as theoretical models were used to investigate the impact of mineral matter of three coals on ash deposition and heat transfer for pulverized coal fired boilers. The ash deposition experiments were conducted in a pulverized fuel combustion pilot-scale facility and a full-scale unit. A mathematical model with input from computer-controlled scanning electron microscopy analysis of coal minerals was used to predict the effect of ash deposition on heat transfer. The predicted deposit thickness and heat flux from the model are shown to be consistent with the measurements in the test facility. The model differentiates the coalsmore » according to the deposits they form and their effect on heat transfer. The heat transfer predictions in the full-scale unit were found to be most suitable for the water wall under the furnace nose. The study demonstrates that the measurements in a full-scale unit can differ significantly from those in pilot-scale furnaces due to soot-blowing operations. 9 refs., 12 figs., 3 tabs.« less
  • The temperature of water in large water reservoirs remains essentially constant at approximately 60 F throughout the year and therefore is attractive for space-conditioning applications. The energy exchange with the water reservoir is accomplished through a heat exchanger. In the present work, two heat-recovery heat exchangers were designed using a simplified computer model. Design-1, which used smooth tubes, was tested at a flooded abandoned mine vein at Scranton, Pennsylvania. The subsequent design, which used the spirally fluted tubes, was tested at the Ohio State University (OSU) and at a lake in Lackawanna State Park in Pennsylvania. A more detailed thermalmore » hydraulic model was developed for the analysis of these heat-recovery heat exchangers. The data obtained at OSU and Lackawanna were analyzed using the thermal hydraulic model to develop a correlation for natural convection. Finally, using this correlation, it was found that the design with spirally fluted tube clearly offers advantages in terms of reduced size of the heat exchanger for a given heat duty.« less