Managing Rumbling and Vibration Phenomena in Refinery Flaring Systems

Introduction and Context

??????????? The downstream industry is dealing with increasing demand for reduce their environmental impacts and the players are actively called to improve their ESG policies aiming to not only minimize their environmental footprint as well as to minimize their impact over the adjacent communities of the refining and petrochemical assets.

??????????? One of the most important assets in a crude oil refinery and which have great impact over the adjacent communities is the flare systems. These assets are dedicated to operated like a safety system of the refinery capable to burn, in a safe way, the gases from safety relief systems of the refining processing units, especially when there is operational instability or during planned shutdown events, in other words the flare systems is a safety equipment and is essential for the any refining and petrochemical asset to ensure the stability and safety shutdown of the processing units. The absence of these systems can lead to severe process accidents with great impacts to the environment and people, unfortunately a good example is the Bhopal disaster occurred in a process plant operated by Union Carbide in 1984 where around 2.300 people was died due to relief of a cloud of methyl isocyanate without burn once the flaring system was out of service.

??????????? Nowadays, the evolution of processing technologies lead to reliable and efficient flaring systems but the major concern related with this equipment is to reduce the unnecessary gas flaring due to the carbon emissions. According to IEA (International Energy Agency), in 2022 the global flaring systems was responsible for the emissions of 500 Mton CO2 equivalent. This fact increased the pressure over the downstream players to improve their policies related to the flaring systems operation in order to minimize the unnecessary gas flare as well as lead some players to carry out capital investment to the installation of systems capable to reduce the flowrate of unnecessary gases to flare.

Operating Issues of Flaring Systems

??????????? Like any processing units, the flaring systems present several operating issues, in this article we will keep the focus in two issues which normally lead to claim of the adjacent communities of refining assets which is the pulsation of flare flame and the rumbling phenomena. Also, in this article we will deal with elevated flare only, the discussion about horizontal and ground flare systems will be made in future publications.

Pulsation of Flare Flame

??????????? The flare flame pulsation causes disturbing due to the behavior of the flame which present a frequent pulsation light and sound. Normally this phenomenon is strictly related to the hydraulic seal level of the flare system, this system is dedicated to avoiding a flame return (flash back) from the flare burn, this is especially a concern when the system is operating under low gas flowrates which lead to low velocity of the mixture that can be lower than the flame velocity, this will lead to the burn inside the burner which can lead to explosion. Figure 1 presents a typical arrangement of elevated flaring system of crude oil refineries.

Figure 1 – Typical arrangement for Elevated Flare Systems of Crude Oil Refineries

??????????? The hydraulic seal in elevated flare systems is normally made by a vessel which is filled with a level of water, this seal will also keep a positive pressure over the flaring system avoiding the entry of air to the system which can lead to explosion in the flare system. The pressure of the flaring system can be controlled through the level of the hydraulic seal drum, but for high levels it’s possible to occur the sloshing of the seal drum which lead to a periodic surge in the flare flame which produce noise and instability of the flame.

??????????? To avoid this issue, the level of hydraulic seal drum needs to be controlled in a minimum level sufficient to keep the safety operation of the flaring system. Some references stablish adequate levels between 100 to 200 mm, but this needs to be analyzed case by case. Another possible vibration sources of the flame can be the presence of liquid in the molecular seal of the or the flashback prevention section in Figure 1, in this case it’s necessary to check if the drain line is not obstructed. Molecular seal or a velocity seal, is used at the base of the flare tip, to ensure a minimum continuous flow of purge gas, avoiding air ingress to the flare which can allow the formation for explosive mixture inside the flaring system. The molecular seal principle is based on the difference between the density of air and purge gas, the most common of molecular seal for elevated flare is an inverted can device which is assembly upstream of the flare tip as presented in Figure 2.

Figure 2 – Typical Molecular Seal for Elevated Flares (YOON, B.H. – 2015)

??????????? In case of obstruction of the drain line of the molecular seal can be necessary to shutdown the flare system to clean the drain line, this is especially important to refineries which operates without chemical flaring systems which are dedicated to burn acid and ammoniacal gases from sour water stripping and/or sulfur recovery units. The burning of acid and ammoniacal gases in conventional flaring system can accelerate the salt deposition in the drainage system.

??????????? Another factor which can lead to flame vibrations in flaring systems is an excess of steam flowrate injection. Steam injection is used to reduce the smoke formation during the flare operations, especially when heavy hydrocarbons are relief, and is normal the refiners raise the steam flowrate until reach a transparent flame. Further to be a waste of energy and money, the excess of steam causes pulsation, noise, and instability of the flare flame. To avoid this, the steam flowrate should be reduced to minimum necessary to produce a yellow flame without smoke like presented in the video below.

Rumbling Phenomena in Elevated Flare Systems

??????????? The rumbling phenomena occurs when excess of air is fed to the flare tip, the over aeration of the flame causes their lift from the flare tip followed by a reattachment of the flame once the pilots relight the flame. This periodic and quickly lift and reattachment of the flame lead to a low frequency vibration or rumble which can produce a shock wave propagation which can produce vibration of adjacent structures like windows and doors, this is a special concern for crude oil refineries which are installed in areas with high population density.

??????????? The rumble phenomenon can be avoided through the adequate control of steam flowrate to the flare tip, as discussed above the steam is applied to minimize the smoke formation during the flaring operation, in some flare designs are installed ejectors in the flare tip in order to drag air to the flame promoting air excess to the flame and then reducing the smoke formation, this design is normally applied in refineries with limitation of steam availability. Figure 3 present an example of a device installed in modern flare tips in order to reduce the steam consumption to the flare through the air excess to the flame.

Figure 3 – Device installed in the Modern Flare Tips in order to Reduce Steam Consumption – ZEECO Company, 2023

Despite the advantage of lower steam consumption under smokeless operation of the flare, the presence of the steam ejectors can lead to an excessive air supply to the flame which will lift from the flare tip and starts the rumble phenomena. To avoid the rumble phenomena is recommended to analyze the adequate design of flare to be chosen, especially considering the questions related to noise, lighting and pulsation which can lead to claim of the adjacent communities of the refinery. In installed flare systems the most effective way to prevent the rumble phenomenon is operate the flare under minimum steam flowrate looking for to keep a stable yellow flame as described in the earlier section and exemplified in the video 2.????????

References

American Petroleum Institute, API Standard 537, Flare Details for General Refinery and Petrochemical Service, API Standard 537, Draft 3rd Ed., Sept. 2001.

American Petroleum Institute, Guide for Pressure-relieving and Depressing Systems, API Recommended Practice 521, 6th Ed., January 2014.

John Zink Co., “The John Zink Hamworthy Combustion Handbook,” Vol.3, Applications, Chapter 11: Flares, CRC Press LLC, 2001.

? ????????? YOON, B. H. Optimizing Flare Operation Through Proper Design, Chemical Engineering Magazine, October 2015.

Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in S?o José dos Campos, Brazil. Bachelor’s in chemical engineering from University of Maringa (UEM), Brazil and PhD. in Chemical Engineering from University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).